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+*** START OF THE PROJECT GUTENBERG EBOOK 44000 ***
+
+Transcriber's note: A few typographical errors have been corrected: they
+are listed at the end of the text.
+
+Text enclosed by underscores is in italics (_italics_).
+
+A carat character is used to denote superscription. A single character
+following the carat is superscripted (example: X^1). Similarly an
+underscore represents a subscript (_sk_4_ has a subscript 4 and is in
+italics).
+
+Page numbers enclosed by curly braces (example: {25}) have been
+incorporated to facilitate the use of the Index.
+
+       *       *       *       *       *
+
+
+
+
+THE
+
+ORIGIN OF VERTEBRATES
+
+BY
+
+WALTER HOLBROOK GASKELL
+
+M.A., M.D. (CANTAB.), LL.D. (EDIN. AND McGILL UNIV.); F.R.S.; FELLOW OF
+TRINITY HALL AND UNIVERSITY LECTURER IN PHYSIOLOGY, CAMBRIDGE; HONORARY
+FELLOW OF THE ROYAL MEDICAL AND CHIRURGICAL SOCIETY; CORRESPONDING MEMBER
+OF THE IMPERIAL MILITARY ACADEMY OF MEDICINE, ST. PETERSBURG, ETC.
+
+LONGMANS, GREEN, AND CO.
+
+39 PATERNOSTER ROW, LONDON
+
+NEW YORK, BOMBAY, AND CALCUTTA
+
+1908
+
+_All rights reserved_
+
+
+
+
+CONTENTS
+
+
+                                                                       PAGE
+  INTRODUCTION                                                            1
+
+  CHAPTER I
+
+  THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM
+
+  Theories of the origin of vertebrates--Importance of the central
+  nervous system--Evolution of tissues--Evidence of Palæontology--
+  Reasons for choosing Ammocoetes rather than Amphioxus for the
+  investigation of this problem--Importance of larval forms--
+  Comparison of the vertebrate and arthropod central nervous
+  systems--Antagonism between cephalization and alimentation--
+  Life-history of lamprey, not a degenerate animal--Brain of
+  Ammocoetes compared with brain of arthropod--Summary                    8
+
+  CHAPTER II
+
+  THE EVIDENCE OF THE ORGANS OF VISION
+
+  Different kinds of eye--Simple and compound retinas--Upright and
+  inverted retinas--Median eyes--Median or pineal eyes of Ammocoetes
+  and their optic ganglia--Comparison with other median eyes--Lateral
+  eyes of vertebrates compared with lateral eyes of crustaceans--
+  Peculiarities of the lateral eye of the lamprey--Meaning of the
+  optic diverticula--Evolution of vertebrate eyes--Summary               68
+
+  CHAPTER III
+
+  THE EVIDENCE OF THE SKELETON
+
+  The bony and cartilaginous skeleton considered, not the notochord--
+  Nature of the earliest cartilaginous skeleton--The mesosomatic
+  skeleton of Ammocoetes; its topographical arrangement, its
+  structure, its origin in muco-cartilage--The prosomatic skeleton of
+  Ammocoetes; the trabeculæ and parachordals, their structure, their
+  origin in white fibrous tissue--The mesosomatic skeleton of Limulus
+  compared with that of Ammocoetes; similarity of position, of
+  structure, of origin in muco-cartilage--The prosomatic skeleton of
+  Limulus; the entosternite, or plastron, compared with the trabeculæ
+  of Ammocoetes; similarity of position, of structure, of origin in
+  fibrous tissue--Summary                                               119
+
+  CHAPTER IV
+
+  THE EVIDENCE OF THE RESPIRATORY APPARATUS
+
+  Branchiæ considered as internal branchial appendages--Innervation of
+  branchial segments--Cranial region older than spinal--Three-root
+  system of cranial nerves: dorsal, lateral, ventral--Explanation of van
+  Wijhe's segments--Lateral mixed root is appendage-nerve of
+  invertebrate--The branchial chamber of Ammocoetes--The branchial
+  unit, not a pouch but an appendage--The origin of the branchial
+  musculature--The branchial circulation--The branchial heart of the
+  vertebrate--Not homologous with the systemic heart of the arthropod--
+  Its formation from two longitudinal venous sinuses--Summary           148
+
+  CHAPTER V
+
+  THE EVIDENCE OF THE THYROID GLAND
+
+  The value of the appendage-unit in non-branchial segments--The double
+  nature of the hyoid segment--Its branchial part--Its thyroid part--
+  The double nature of the opercular appendage--Its branchial part--Its
+  genital part--Unique character of the thyroid gland of Ammocoetes--
+  Its structure--Its openings--The nature of the thyroid segment--The
+  uterus of the scorpion--Its glands--Comparison with the thyroid
+  gland of Ammocoetes--Cephalic generative glands of Limulus--
+  Interpretation of glandular tissue filling up the brain-case of
+  Ammocoetes--Function of thyroid gland--Relation of thyroid gland
+  to sexual functions--Summary                                          185
+
+  CHAPTER VI
+
+  THE EVIDENCE OF THE OLFACTORY APPARATUS
+
+  Fishes divided into Amphirhinæ and Monorhinæ--Nasal tube of the
+  lamprey--Its termination at the infundibulum--The olfactory organs
+  of the scorpion group--The camerostome--Its formation as a tube--
+  Its derivation from a pair of antennæ--Its termination at the true
+  mouth--Comparison with the olfactory tube of Ammocoetes--Origin of
+  the nasal tube of Ammocoetes from the tube of the hypophysis--
+  Direct comparison of the hypophysial tube with the olfactory tube
+  of the scorpion group--Summary                                        218
+
+  CHAPTER VII
+
+  THE PROSOMATIC SEGMENTS OF LIMULUS AND ITS ALLIES
+
+  Comparison of the trigeminal with the prosomatic region--The
+  prosomatic appendages of the Gigantostraca--Their number and
+  nature--Endognaths and ectognath--The metastoma--The coxal glands--
+  Prosomatic region of Eurypterus compared with that of Ammocoetes--
+  Prosomatic segmentation shown by marks on carapace--Evidence of
+  coelomic cavities in Limulus--Summary                                 233
+
+  CHAPTER VIII
+
+  THE SEGMENTS BELONGING TO THE TRIGEMINAL NERVE-GROUP
+
+  The prosomatic segments of the vertebrate--Number of segments
+  belonging to the trigeminal nerve-group--History of cranial
+  segments--Eye-muscles and their nerves--Comparison with the
+  dorso-ventral somatic muscles of the scorpion--Explanation of the
+  oculomotor nerve and its group of muscles--Explanation of the
+  trochlear nerve and its dorsal crossing--Explanation of the abducens
+  nerve--Number of segments supplied by the trigeminal nerves--
+  Evidence of their motor nuclei--Evidence of their sensory ganglia--
+  Summary                                                               257
+
+  CHAPTER IX
+
+  THE PROSOMATIC SEGMENTS OF AMMOCOETES
+
+  The prosomatic region in Ammocoetes--The suctorial apparatus of the
+  adult Petromyzon--Its origin in Ammocoetes--Its derivation from
+  appendages--The segment of the lower lip or the metastomal segment--
+  The tentacular segments--The tubular muscles--Their segmental
+  arrangement--Their peculiar innervation--Their correspondence with
+  the system of veno-pericardial muscles in Limulus--The old mouth
+  or palæostoma--The pituitary gland--Its comparison with the coxal
+  gland of Limulus--Summary                                             286
+
+  CHAPTER X
+
+  THE RELATIONSHIP OF AMMOCOETES TO THE MOST ANCIENT FISHES--THE
+      OSTRACODERMATA
+
+  The nose of the Osteostraci--Comparison of head-shield of Ammocoetes
+  and of Cephalaspis--Ammocoetes only living representative of these
+  ancient fishes--Formation of cranium--Closure of old mouth--Rohon's
+  primordial cranium--Primordial cranium of Phrynus and Galeodes--
+  Summary                                                               326
+
+  CHAPTER XI
+
+  THE EVIDENCE OF THE AUDITORY APPARATUS AND THE ORGANS OF THE LATERAL LINE
+
+  Lateral line organs--Function of this group of organs--Poriferous
+  sense-organs on the appendages in Limulus--Branchial sense-organs--
+  Prosomatic sense-organs--Flabellum--Its structure and position--
+  Sense-organs of mandibles--Auditory organs of insects and arachnids--
+  Poriferous chordotonal organs--Balancers of Diptera--Resemblance to
+  organs of flabellum--Racquet-organs of Galeodes--Pectens of
+  scorpions--Large size of nerve to all these special sense-organs--
+  Origin of parachordals and auditory capsule--Reason why VIIth nerve
+  passes in and out of capsule--Evidence of Ammocoetes--Intrusion of
+  glandular mass round brain into auditory capsule--Intrusion of
+  generative and hepatic mass round brain into base of flabellum--
+  Summary                                                               355
+
+  CHAPTER XII
+
+  THE REGION OF THE SPINAL CORD
+
+  Difference between cranial and spinal regions--Absence of lateral
+  root--Meristic variation--Segmentation of coelom--Segmental
+  excretory organs--Development of nephric organs; pronephric,
+  mesonephric, metanephric--Excretory organs of Amphioxus--
+  Solenocytes--Excretory organs of Branchipus and Peripatus,
+  appendicular and somatic--Comparison of coelom of Peripatus and
+  of vertebrate--Pronephric organs compared to coxal glands--Origin
+  of vertebrate body-cavity (metacoele)--Segmental duct--Summary of
+  formation of excretory organs--Origin of somatic trunk-musculature--
+  Atrial cavity of Amphioxus--Pleural folds--Ventral growth of
+  pleural folds and somatic musculature--Pleural folds of
+  Cephalaspidæ and of Trilobita--Meaning of the ductless glands--
+  Alteration in structure of excretory organs which have lost their
+  duct in vertebrates and in invertebrates--Formation of lymphatic
+  glands--Segmental coxal glands of arthropods and of vertebrates--
+  Origin of adrenals, pituitary body, thymus, tonsils, thyroid, and
+  other ductless glands--Summary                                        385
+
+  CHAPTER XIII
+
+  THE NOTOCHORD AND ALIMENTARY CANAL
+
+  Relationship between notochord and gut--Position of unsegmented tube
+  of notochord--Origin of notochord from a median groove--Its function
+  as an accessory digestive tube--Formation of notochordal tissue in
+  invertebrates from closed portions of the digestive tube--Digestive
+  power of the skin of Ammocoetes--Formation of new gut in Ammocoetes
+  at transformation--Innervation of the vertebrate gut--The three
+  outflows of efferent nerves belonging to the organic system--The
+  original close contiguity of the respiratory chamber to the cloaca--
+  The elongation of the gut--Conclusion                                 433
+
+  CHAPTER XIV
+
+  THE PRINCIPLES OF EMBRYOLOGY
+
+  The law of recapitulation--Vindication of this law by the theory
+  advanced in this book--The germ-layer theory--Its present position--
+  A physiological not a morphological conception--New fundamental law
+  required--Composition of adult body--Neuro-epithelial syncytium and
+  free-living cells--Meaning of the blastula--Derivation of the
+  Metazoa from the Protozoa--Importance of the central nervous system
+  for Ontogeny as well as for Phylogeny--Derivation of free-living
+  cells from germ-cells--Meaning of coelom--Formation of neural
+  canal--Gastrula of Amphioxus and of Lucifer--Summary                  455
+
+  CHAPTER XV
+
+  FINAL REMARKS
+
+  Problems requiring investigation--
+
+  Giant nerve-cells and giant nerve-fibres; their comparison in fishes
+  and arthropods; blood- and lymph-corpuscles; nature of the skin;
+  origin of system of unstriped muscles; origin of the sympathetic
+  nervous system; biological test of relationship.
+
+  Criticisms of Balanoglossus theory--Theory of parallel development--
+  Importance of the theory advocated in this book for all problems of
+  Evolution                                                             488
+
+  BIBLIOGRAPHY AND INDEX OF AUTHORS                                     501
+
+  GENERAL INDEX                                                         517
+
+
+
+
+  "_GO ON AND PROSPER; THERE IS NOTHING SO
+  USEFUL IN SCIENCE AS ONE OF THOSE EARTHQUAKE
+  HYPOTHESES, WHICH OBLIGE ONE TO FACE
+  THE POSSIBILITY THAT THE SOLIDEST-LOOKING
+  STRUCTURES MAY COLLAPSE._"
+
+                      LETTER FROM PROF. HUXLEY TO
+                      THE AUTHOR. JUNE 2, 1889.
+
+
+
+
+{1}THE
+
+ORIGIN OF VERTEBRATES
+
+
+
+
+_INTRODUCTION_
+
+
+In former days it was possible for a man like Johannes Müller to be a
+leader both in physiology and in comparative anatomy. Nowadays all
+scientific knowledge has increased so largely that specialization is
+inevitable, and every investigator is confined more and more not only to
+one department of science, but as a rule to one small portion of that
+department. In the case of such cognate sciences as physiology and
+comparative anatomy this limiting of the scope of view is especially
+deleterious, for zoology without physiology is dead, and physiology in many
+of its departments without comparative anatomy can advance but little.
+Then, again, the too exclusive study of one subject always tends to force
+the mind into a special groove--into a line of thought so deeply tinged
+with the prevalent teaching of the subject, that any suggestions which
+arise contrary to such teaching are apt to be dismissed at once as
+heretical and not worthy of further thought; whereas the same suggestion
+arising in the mind of one outside this particular line of thought may give
+rise to new and valuable scientific discoveries.
+
+Nothing but good can, in my opinion, result from the incursion of the
+non-specialist into the realm of the specialist, provided that the former
+is in earnest. Over and over again the chemist has given valuable help to
+the physicist, and the physicist to the chemist, so closely allied are the
+two subjects; so also is it with physiology and anatomy, the two subjects
+are so interdependent that a worker in the one may give valuable aid
+towards the solution of some large problem which is the special territory
+of the other.
+
+It has been a matter of surprise to many how it came about that {2}I, a
+worker in the physiological laboratory at Cambridge ever since Foster
+introduced experimental physiology into English-speaking nations, should
+have devoted so much time to the promulgation of a theory of the origin of
+vertebrates--a subject remote from physiology, and one of the larger
+questions appertaining to comparative anatomy. By what process of thought
+was I led to take up the consideration of a subject apparently so remote
+from all my previous work, and so foreign to the atmosphere of a
+physiological laboratory?
+
+It may perhaps be instructive to my readers to see how one investigation
+leads to another, until at last, _nolens volens_, the worker finds himself
+in front of a possible solution to a problem far removed from his original
+investigation, which by the very magnitude and importance of it forces him
+to devote his whole energy and time to seeing whether his theory is good.
+
+In the years 1880-1884 I was engaged in the investigation of the action of
+the heart, and the nature of the nerves which regulate that action. In the
+course of that investigation I was struck by the ease with which it was
+possible to distinguish between the fibres of the vagus and accelerator
+nerves on their way to the heart, owing to the medullation of the former
+and the non-medullation of the latter. This led me to an investigation of
+the accelerator fibres, to find out how far they are non-medullated, and so
+to the discovery that the _rami communicantes_ connecting together the
+central nervous system and the sympathetic are in reality single, not
+double, as had hitherto been thought; for the grey _ramus communicans_ is
+in reality a peripheral nerve which supplies the blood-vessels of the
+spinal cord and its membranes, and is of the same nature as the grey
+accelerators to the heart.
+
+This led to the conclusion that there is no give and take between two
+independent nervous systems, the cerebro-spinal and the sympathetic, as had
+been taught formerly, but only one nervous system, the cerebro-spinal,
+which sends special medullated nerve-fibres, characterized by their
+smallness, to the cells of the sympathetic system, from which fibres pass
+to the periphery, usually non-medullated. These fine medullated nerves form
+the system of white _rami communicantes_, and have since been called by
+Langley the preganglionic nerves. Further investigation showed that such
+white rami are not universally distributed, but are confined to the
+thoracico-lumbar region, where their distribution is easily seen in {3}the
+ventral roots, for the cells of the sympathetic system are entirely
+efferent in nature, not afferent; therefore, the fibres entering into them
+from the central nervous system leave the spinal cord by ventral, not
+dorsal roots.
+
+Following out this clue, I then found that in addition to this
+thoracico-lumbar outflow of efferent ganglionated visceral nerves, there
+are similar outflows in the cranial and sacral regions, belonging in the
+former case especially to the vagus system of nerves, and in the latter to
+the system of nerves which pass from the sacral region of the cord to the
+ganglion-cells of the hypogastric plexus, and from them supply the bladder,
+rectum, etc. To this system of nerves, formerly called the _nervi
+erigentes_, I gave the name pelvic splanchnics, in order to show their
+uniformity with the abdominal splanchnics. These investigations led to the
+conclusion that the organic system of nerves, characterized by the
+possession of efferent nerve-cells situated peripherally, arises from the
+central nervous system by three distinct outflows--cranial,
+thoracico-lumbar, and sacral, respectively. To this system Langley has
+lately given the name 'autonomic.' These three outflows are separated by
+two gaps just where the plexuses for the anterior and posterior extremities
+come in.
+
+This peculiar arrangement of the white _rami communicantes_ set me
+thinking, for the gaps corresponded to an increase of somatic musculature
+to form the muscles of the fore and hind limbs, so that if, as seemed
+probable, the white _rami communicantes_ arise segmentally from the spinal
+cord, then a marked distinction must exist in structure between the spinal
+cord in the thoracic region, where the visceral efferent nerves are large
+in amount and the body musculature scanty, and in the cervical or lumbar
+swellings, where the somatic musculature abounds, and the white _rami
+communicantes_ scarcely exist.
+
+I therefore directed my attention in the next place to the structure of the
+central nervous system in the endeavour to associate the topographical
+arrangement of cell-groups in this system with the outflow of the different
+kinds of nerve-fibres to the peripheral organs.
+
+This investigation forcibly impressed upon my mind the uniformity in the
+arrangement of the central nervous system as far as the centres of origin
+of all the segmental nerves are concerned, {4}both cranial and spinal, and
+also the original segmental character of this part of the nervous system.
+
+I could not, therefore, help being struck by the force of the comparison
+between the central nervous systems of Vertebrata and Appendiculata as put
+forward again and again by the past generation of comparative anatomists,
+and wondered why it had been discredited. There in the infundibulum was the
+old oesophagus, there in the cranial segmental nerves the infraoesophageal
+ganglia, there in the cerebral hemispheres and optic and olfactory nerves
+the supraoesophageal ganglia, there in the spinal cord the ventral chain of
+ganglia. But if the infundibulum was the old oesophagus, what then? The old
+oesophagus was continuous with and led into the cephalic stomach. What
+about the infundibulum? It was continuous with and led into the ventricles
+of the brain, and the whole thing became clear. The ventricles of the brain
+were the old cephalic stomach, and the canal of the spinal cord the long
+straight intestine which led originally to the anus, and still in the
+vertebrate embryo opens out into the anus. Not having been educated in a
+morphological laboratory and taught that the one organ which is homologous
+throughout the animal kingdom is the gut, and that therefore the gut of the
+invertebrate ancestor must continue on as the gut of the vertebrate, the
+conception that the central nervous system has grown round and enclosed the
+original ancestral gut, and that the vertebrate has formed a new gut did
+not seem to me so impossible as to prevent my taking it as a working
+hypothesis, and seeing to what it would lead.
+
+This theory that the so-called central nervous system of the vertebrate is
+in reality composed of two separate parts, of which the one, the segmented
+part, corresponds to the central nervous system of the highest
+invertebrates, while the other, the unsegmented tube, was originally the
+alimentary canal of that same invertebrate, came into my mind in the year
+1887. The following year, on June 23, 1888, I read a paper on the subject
+before the Anatomical Society at Cambridge, which was published in the
+_Journal of Anatomy and Physiology_, vol. 23, and more fully in the
+_Journal of Physiology_, vol. 10. Since that time I have been engaged in
+testing the theory in every possible way, and have published the results of
+my investigations in a series of papers in different journals, a list of
+which I append at the end of this introductory chapter.
+
+{5}It is now twenty years since the theory first came into my mind, and the
+work of those twenty years has convinced me more and more of its truth, and
+yet during the whole time it has been ignored by the morphological world as
+a whole rather than criticized. Whatever may have been the causes for such
+absence of criticism, it is clear that the serial character of its
+publication is a hindrance to criticism of the theory as a whole, and I
+hope, therefore, that the publication of the whole of the twenty years'
+work in book-form will induce those who differ from my conclusions to come
+forward and show me where I am wrong, and why my theory is untenable. Any
+one who has been thinking over any one problem for so long a time becomes
+obsessed with the infallibility of his own views, and is not capable of
+criticizing his own work as thoroughly as others would do. I have been told
+that it is impossible for one man to consider so vast a subject with that
+thoroughness which is necessary, before any theory can be accepted as the
+true solution of the problem. I acknowledge the vastness of the task, and
+feel keenly enough my own shortcomings. For all that, I do feel that it can
+only be of advantage to scientific progress and a help to the solution of
+this great problem, to bring together in one book all the facts which I
+have been able to collect, which appeal to me as having an important
+bearing on this solution.
+
+In this work I have been helped throughout by Miss R. Alcock. It is not too
+much to say that without the assistance she has given me, many an important
+link in the chain of evidence would have been missing. With extraordinary
+patience she has followed, section by section, the smallest nerves to their
+destination, and has largely helped to free the transformation process in
+the lamprey from the mystery which has hitherto enveloped it. She has drawn
+for me very many of the illustrations scattered through the pages in this
+book, and I feel that her aid has been so valuable and so continuous,
+lasting as it does over the whole period of the work, that her name ought
+fittingly to be associated with mine, if perchance the theory of the Origin
+of Vertebrates, advocated in the pages of this book, gains acceptance.
+
+I am also indebted to Mr. J. Stanley Gardiner and to Dr. A. Sheridan Lea
+for valuable assistance in preparing this book for the press. I desire to
+express my grateful thanks to the former for valuable criticism of the
+scientific evidence which I have brought {6}forward in this book, and to
+the latter for his great kindness in undertaking the laborious task of
+collecting the proofs.
+
+
+
+
+LIST OF PREVIOUS PUBLICATIONS BY THE AUTHOR, CONCERNING THE ORIGIN OF
+VERTEBRATES.
+
+  1888.  "Spinal and Cranial Nerves." _Proceedings of the Anatomical
+          Society_, June, 1888. _Journal of Anatomy and Physiology_,
+          vol. xxiii.
+
+  1889.  "On the Relation between the Structure, Function, Distribution,
+          and Origin of the Cranial Nerves; together with a Theory of the
+          Origin of the Nervous System of Vertebrata." _Journal of
+          Physiology_, vol. x., p. 153.
+
+  1889.  "On the Origin of the Central Nervous System of Vertebrates."
+          _Brain_, vol. xii., p. 1.
+
+  1890.  "On the Origin of Vertebrates from a Crustacean-like Ancestor."
+          _Quarterly Journal of Microscopical Science_, vol. xxxi., p. 379.
+
+  1895.  "The Origin of Vertebrates." _Proceedings of the Cambridge
+          Philosophical Society_, vol. ix., p. 19.
+
+  1896.  Presidential Address to Section I. at the meeting of the British
+          Association for the Advancement of Science in Liverpool. _Report
+          of the British Association_, 1896, p. 942.
+
+  1899.  "On the Meaning of the Cranial Nerves." Presidential Address to
+          the Neurological Society for the year 1899. _Brain_, vol. xxii.,
+          p. 329.
+
+A series of papers on "The Origin of Vertebrates, deduced from the study of
+Ammocoetes," in the _Journal of Anatomy and Physiology_, as follows:--
+
+
+  1898.  Part    I. "The Origin of the Brain," vol. xxxii., p. 513.
+
+          "     II. "The Origin of the Vertebrate Cranio-facial Skeleton,"
+                      vol. xxxii., p. 553.
+
+          "    III. "The Origin of the Branchial Segmentation," vol.
+                      xxxiii., p. 154.
+
+  1899.   "     IV. "The Thyroid, or Opercular Segment: the Meaning of the
+                      Facial Nerve," vol. xxxiii., p. 638.
+
+  1900.   "      V. "The Origin of the Pro-otic Segmentation: the Meaning
+                      of the Trigeminal and Eye-muscle Nerves," vol.
+                      xxxiv., p. 465.
+
+  1900.   "     VI. "The Old Mouth and the Olfactory Organ: the Meaning
+                      of the First Nerve," vol. xxxiv., p. 514.
+
+  1900.   "    VII. "The Evidence of Prosomatic Appendages in Ammocoetes,
+                      as given by the Course and Distribution of the
+                      Trigeminal Nerve," vol. xxxiv., p. 537.
+
+  1900.   "   VIII. "The Palæontological Evidence: Ammocoetes a
+                      Cephalaspid," vol. xxxiv., p. 562.
+
+  1901.   "     IX. "The Origin of the Optic Apparatus: the Meaning of the
+                      Optic Nerves," vol. xxxv., p. 224.
+
+  1902.   "      X. "The Origin of the Auditory Organ: the Meaning of the
+                      VIIIth Cranial Nerve," vol. xxxvi., p. 164.
+
+  1903.   "     XI. "The Origin of the Vertebrate Body-cavity and Excretory
+                      Organs: the Meaning of the Somites of the Trunk and
+                      of the Ductless Glands," vol. xxxvii., p. 168.
+
+  1905.   "    XII. "The Principles of Embryology," vol. xxxix., p. 371.
+
+  1906.   "   XIII. "The Origin of the Notochord and Alimentary Canal,"
+                      vol. xl., p. 305.
+
+
+
+
+{8}CHAPTER I
+
+_THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM_
+
+  Theories of the origin of vertebrates.--Importance of the central nervous
+  system.--Evolution of tissues.--Evidence of Palæontology.--Reasons for
+  choosing Ammocoetes rather than Amphioxus.--Importance of larval
+  forms.--Comparison of the vertebrate and arthropod central nervous
+  systems.--Antagonism between cephalization and
+  alimentation.--Life-history of lamprey: not a degenerate animal.--Brain
+  of Ammocoetes compared with brain of arthropod.--Summary.
+
+
+At the present time it is no longer a debatable question whether or no
+Evolution has taken place. Since the time of Darwin the accumulation of
+facts in its support has been so overwhelming that all zoologists look upon
+this question as settled, and desire now to find out the manner in which
+such evolution has taken place. Here two problems offer themselves for
+investigation, which can be and are treated separately--the one dealing
+with the question of those laws of heredity and variation which have
+brought about in the past and are still causing in the present the
+evolution of living beings, _i.e._ the causes of evolution; the other
+concerned with the relationship of animals, or groups of animals, rather
+than with the causes which have brought about such relationship, _i.e._ the
+sequence of evolution.
+
+It is the latter problem with which this book deals, and, indeed, not with
+the whole question at all, but only with that part of it which concerns the
+origin of vertebrates.
+
+This problem of the sequence of evolution is of a twofold character: first,
+the finding out of the steps by which the higher forms in any one group of
+animals have been evolved from the lower; and secondly, the evolution of
+the group itself from a lower group.
+
+In any classification of the animal kingdom, it is clear that large groups
+of animals exist which have so many common characteristics as to
+necessitate their being placed in one larger group or kingdom; {9}thus
+zoologists are able to speak definitely of the Vertebrata, Arthropoda,
+Annelida, Echinodermata, Porifera, Coelenterata, Mollusca, etc. In each of
+these groups affinities can be traced between the members, so that it is
+possible to speak of the progress from lower to higher members of the
+group, and it is conceivable, given time to work out the details, that the
+natural relationships between the members of the whole group will
+ultimately be discovered.
+
+Thus no one can doubt that a sequence of the kind has taken place in the
+Vertebrata as we trace the progress from the lowest fishes to man, and
+already the discoveries of palæontology and anatomy give us a distinct clue
+to the sequence from fish to amphibian, from amphibian to reptile, from
+reptile to mammal on the one hand, and to bird on the other. That the
+different members of the vertebrate group are related to each other in
+orderly sequence is no longer a matter of doubt; the connected problems are
+matters of detail, the solution of which is certain sooner or later. The
+same may be said of the members of any of the other great natural groups,
+such as the Arthropoda, the Annelida, the Echinodermata, etc.
+
+It is different, however, when an attempt is made to connect two of the
+main divisions themselves. It is true enough that there is every reason to
+believe that the arthropod group has been evolved from the segmented
+annelid, and so the whole of the segmented invertebrates may be looked on
+as forming one big division, the Appendiculata, all the members of which
+will some day be arranged in orderly sequence, but the same feeling of
+certainty does not exist in other cases.
+
+In the very case of the origin of the Appendiculata we are confronted with
+one of the large problems of evolution--the origin of segmented from
+non-segmented animals--the solution of which is not yet known.
+
+
+THEORIES OF THE ORIGIN OF VERTEBRATES.
+
+The other large problem, perhaps the most important of all, is the question
+of the relationship of the great kingdom of the Vertebrata: from what
+invertebrate group did the vertebrate arise?
+
+The great difficulty which presents itself in attempting a solution of this
+question is not so much, as used to be thought, the difficulty of deriving
+a group of animals possessing an internal bony and {10}cartilaginous
+skeleton from a group possessing an external skeleton of a calcareous or
+chitinous nature, but rather the difficulty caused by the fundamental
+difference of arrangement of the important internal organs, especially the
+relative positions of the central nervous system and the digestive tube.
+
+[Illustration: FIG. 1.--ARRANGEMENT OF ORGANS IN THE VERTEBRATE (A) AND
+ARTHROPOD (B).
+
+_Al_, gut; _H_, heart; _C.N.S._, central nervous system; V, ventral side;
+D, dorsal side.]
+
+Now, if we take a broad and comprehensive view of the invertebrate kingdom,
+without arguing out each separate case, we find that it bears strongly the
+stamp of a general plan of evolution derived from a coelenterate animal,
+whose central nervous system formed a ring surrounding the mouth. Then when
+the radial symmetry was given up, and an elongated, bilateral, segmented
+form evolved, the central nervous system also became elongated and
+segmented, but, owing to its derivation from an oral ring, it still
+surrounded the mouth-tube, or oesophagus, and thus in its highest forms is
+divided into supra-oesophageal and infra-oesophageal nervous masses. These
+latter {11}nervous masses are of necessity ventral to the digestive tube,
+because the mouth of the coelenterate is on the ventral side. The striking
+characteristic, then, of the invertebrate kingdom is the situation of a
+large portion of the central nervous system ventrally to the alimentary
+canal and the piercing of the nervous system by a tube--the
+oesophagus--leading from the mouth to the alimentary canal. The equally
+striking characteristic of the vertebrate is the dorsal position of the
+central nervous system and the ventral position of the alimentary canal
+combined with the absence of any piercing of the central nervous system by
+the oesophagus.
+
+So fundamentally different is the arrangement of the important organs in
+the two groups that it might well give rise to a feeling of despair of ever
+hoping to solve the problem of the Origin of Vertebrates; and, to my mind,
+this is the prevalent feeling among morphologists at the present time. Two
+attempts at solution have been made. The one is associated with the name of
+Geoffrey St. Hilaire, and is based on the supposition that the vertebrate
+has arisen from the invertebrate by turning over on its back, swimming in
+this position, and so gradually converting an originally dorsal surface
+into a ventral one, and _vice versâ_; at the same time, a new mouth is
+supposed to have been formed on the new ventral side, which opened directly
+into the alimentary canal, while the old mouth, which had now become
+dorsal, was obliterated.
+
+The other attempt at solution is of much more recent date, and is
+especially associated with the name of Bateson. It supposes that
+bilaterally symmetrical, elongated, segmented animals were formed from the
+very first in two distinct ways. In the one case the digestive tube pierced
+the central nervous system, and was situated dorsally to its main mass. In
+the other case the segmented central nervous system was situated from the
+first dorsally to the alimentary canal, and was not pierced by it. In the
+first case the highest result of evolution led to the Arthropoda; in the
+second case to the Vertebrata.
+
+Neither of these views is based on evidence so strong as to cause universal
+acceptance. The great difficulty in the way of accepting the second
+alternative is the complete absence of any evidence, either among animals
+living on the earth at the present day or among those known to have existed
+in the past, of any such chain of intermediate animal forms as must, on
+this hypothesis, have existed in order to link together the lower forms of
+life with the vertebrates.
+
+{12}[Illustration: FIG. 2.--LARVAL BALANOGLOSSUS (from the Royal Natural
+History).]
+
+It has been supposed that the Tunicata and the Enteropneusta
+(_Balanoglossus_) (Fig. 2) are members of this missing chain, and that in
+Amphioxus the vertebrate approaches in organization to these low
+invertebrate forms. The tunicates, indeed, are looked upon as degenerate
+members of an early vertebrate stock, which may give help in picturing the
+nature of the vertebrate ancestor but are not themselves in the direct line
+of descent. Balanoglossus is supposed to have arisen from the
+Echinodermata, or at all events to have affinities with them, so that to
+fill up the enormous gap between the Echinodermata and the Vertebrata on
+this theory there is absolutely nothing living on the earth except
+Balanoglossus, Rhabdopleura, and Cephalodiscus. The characteristics of the
+vertebrate upon which this second theory is based are the notochord, the
+respiratory character of the anterior part of the alimentary canal, and the
+tubular nature of the central nervous system; it is claimed that in
+Balanoglossus the beginnings of a notochord and a tubular central nervous
+system are to be found, while the respiratory portion of the gut is closely
+comparable to that of Amphioxus.
+
+The strength of the first theory is essentially based on the comparison of
+the vertebrate central nervous system with that of the segmented
+invertebrate, annelid or arthropod. In the latter the central nervous
+system is composed of--
+
+1.  The supra-oesophageal ganglia, which give origin to the nerves of the
+eyes and antennules, _i.e._ to the optic and olfactory nerves, for the
+first pair of antennæ are olfactory in function. These are connected with
+the infra-oesophageal ganglia by the oesophageal commissures which encircle
+the oesophagus.
+
+2.  The infra-oesophageal ganglia and the two chains of ventral ganglia,
+which are segmentally-arranged sets of ganglia. Of these, {13}each pair
+gives rise to the nerves of its own segment, and these nerves are not
+nerves of special sense as are the supra-oesophageal nerves, but motor and
+sensory to the segment; nerves by the agency of which food is taken in and
+masticated, respiration is effected, and the animal moves from place to
+place.
+
+In the vertebrate the central nervous system consists of--
+
+1. The brain proper, from which arise only the olfactory and optic nerves.
+
+[Illustration: FIG. 3.--VERTEBRATE CENTRAL NERVOUS SYSTEM COMPARED WITH THE
+CENTRAL NERVOUS SYSTEM AND ALIMENTARY CANAL OF THE ARTHROPOD.
+
+A.  Vertebrate central nervous system. _S. Inf. Br._, supra-infundibular
+brain; _I. Inf. Br._, infra-infundibular brain and cranial segmental
+nerves; _C.Q._, corpora quadrigemina; _Cb._, cerebellum; _C.C._, crura
+cerebri; _C.S._, corpus striatum; _Pn._, pineal gland.
+
+B.  Invertebrate central nervous system. _S. Oes. G._, supra-oesophageal
+ganglia; _I. Oes. G._, infra-oesophageal ganglia; _Oes. Com._, oesophageal
+commissures.]
+
+2. The region of the mid-brain, medulla oblongata, and spinal cord; from
+these arises a series of nerves segmentally arranged, which, as in the
+invertebrate, gives origin to the nerves governing mastication,
+respiration, and locomotion.
+
+Further, the vertebrate central nervous system possesses the peculiarity,
+found nowhere else, of being tubular, and the tube is of a striking
+character. In the spinal region it is a small, simple canal of uniform
+calibre, which at the front end dilates to form the ventricles of the
+region of the brain. From that part of this dilated {14}portion, known as
+the third ventricle, a narrow tube passes to the ventral surface of the
+brain. This tube is called the _infundibulum_, and, extraordinary to
+relate, lies just anteriorly to the exits of the third cranial or
+oculomotor nerves; in other words, it marks the termination of the series
+of spinal and cranial segmental nerves. Further, on each side of this
+infundibular tube are lying the two thick masses of the _crura cerebri_,
+the strands of fibres which connect the higher brain-region proper with the
+lower region of the medulla oblongata and spinal cord. Not only, then, are
+the nerve-masses in the two systems exactly comparable, but in the very
+place where the oesophageal tube is found in the invertebrate, the
+infundibular tube exists in the vertebrate, so that if the words
+infundibular and oesophageal are taken to be interchangable, then in every
+respect the two central nervous systems are comparable. The brain proper of
+the vertebrate, with its olfactory and optic nerves, becomes the direct
+descendant of the supra-oesophageal ganglia; the crura cerebri become the
+oesophageal commissures, and the cranial and spinal segmental nerves are
+respectively the nerves belonging to the infra-oesophageal and ventral
+chain of ganglia.
+
+This overwhelmingly strong evidence has always pointed directly to the
+origin of the vertebrate from some form among the segmented group of
+invertebrates, annelid or arthropod, in which the original oesophagus had
+become converted into the infundibulum, and a new mouth formed. So far, the
+position of this school of anatomists was extremely sound, for it is
+impossible to dispute the facts on which it is based. Still, however, the
+fact remained that the gut of the vertebrate lies ventrally to the nervous
+system, while that of the invertebrate lies dorsally; consequently, since
+the infundibulum was in the position of the invertebrate oesophagus, it
+must originally have entered into the gut, and since the vertebrate gut was
+lying ventrally to it, it could only have opened into that gut in the
+invertebrate stage by the shifting of dorsal and ventral surfaces. From
+this argument it followed that the remains of the original mouth into which
+the infundibulum, _i.e._ oesophagus, opened were to be sought for on the
+dorsal side of the vertebrate brain. Here in all vertebrates there are two
+spots where the roof of the brain is very thin, the one in the region of
+the pineal body, and the other constituting the roof of the fourth
+ventricle. Both of these places have had their advocates as the position of
+the old mouth, the former being upheld by Owen, the latter by Dohrn.
+
+{15}The discovery that the pineal body was originally an eye, or, rather, a
+pair of eyes, has perhaps more than anything else proved the impossibility
+of accepting this reversal of surfaces as an explanation of the genesis of
+the vertebrate from the annelid group. For whereas a pair of eyes close to
+the mid-dorsal line is not only likely enough, but is actually found to
+exist among large numbers of arthropods, both living and extinct, a pair of
+eyes situated close to the mid-ventral line near the mouth is not only
+unheard of in nature, but so improbable as to render impossible the theory
+which necessitates such a position.
+
+Yet this very discovery gives the strongest possible additional support to
+the close identity in the plan of the central nervous system of vertebrate
+and appendiculate.
+
+A truly paradoxical situation! The very discovery which may almost be said
+to prove the truth of the hypothesis, is the very one which has done most
+to discredit it, because in the minds of its authors the only possible
+solution of the transition from the one group to the other was by means of
+the reversal of surfaces.
+
+Still, as already said, even if the theory advanced to explain the facts be
+discredited, the facts remain the same; and still to this day an
+explanation is required as to why such extraordinary resemblances should
+exist between the two nervous systems, unless there is a genetic connection
+between the two groups of animals. An explanation may still be found, and
+must be diligently sought for, which shall take into account the strong
+evidence of this relationship between the two groups, and yet not
+necessitate any reversal of surfaces. It is the object of this book to
+consider the possibility of such an explanation.
+
+What are the lines of investigation most likely to meet with success? Is it
+possible to lay down any laws of evolution? It is instructive to consider
+the nature of the investigations which have led to the two theories just
+mentioned, for the fundamental starting-point is remarkably different in
+the two cases. The one theory is based upon the study of the vertebrate
+itself, and especially of its central nervous system, and its supporters
+and upholders have been and are essentially anatomists, whose chief study
+is that of vertebrate and human anatomy. The other theory is based upon the
+study of the invertebrate, and consists especially of an attempt to find in
+the invertebrate some structure resembling a notochord, such {16}organ
+being considered by them as the great characteristic of the vertebrate;
+indeed, so much is this the case, that a large number of zoologists speak
+now of Chordata rather than of Vertebrata, and in order to emphasize their
+position follow Bateson, and speak of the Tunicata as Uro-chordata, of
+Amphioxus as Cephalo-chordata, of the Enteropneusta as Hemi-chordata, and
+even of Actinotrocha (to use Masterman's term), as Diplo-chordata.
+
+The upholders of this theory lay no stress on the nature of the central
+nervous system in vertebrates, they are essentially zoologists who have
+made a special study of the invertebrate rather than of the vertebrate.
+
+Of these two methods of investigating the problem, it must be conceded that
+the former is more likely to give reliable results. By putting the
+vertebrate to the question in every possible way, by studying its anatomy
+and physiology, both gross and minute, by inquiring into its past history,
+we can reasonably hope to get a clue to its origin, but by no amount of
+investigation can we tell with any certainty what will be its future fate;
+we can only guess and prophesy in an uncertain and hesitating manner. So it
+must be with any theory of the origin of vertebrates, based on the study of
+one or other invertebrate group. Such theory must partake rather of the
+nature of prophecy than of deduction, and can only be placed on a firm
+basis when it so happens that the investigation of the vertebrate points
+irresistibly to its origin from the same group; in fact, "never prophesy
+unless you know."
+
+The first principle, then, I would lay down is this: In order to find out
+the origin of vertebrates, inquire, in the first place, of the vertebrate
+itself.
+
+
+IMPORTANCE OF THE CENTRAL NERVOUS SYSTEM.
+
+Does the history of evolution pick out any particular organ or group of
+organs as more necessary than another for upward progress? If so, it is
+upon that organ or group of organs that special stress must be laid.
+
+Since Darwin wrote the "Origin of Species," and laid down that the law of
+the 'survival of the fittest' is the factor upon which evolution depends,
+it has gradually dawned upon the scientific mind that 'the fittest' may be
+produced in two diametrically opposite ways: {17}either by progress upwards
+to a superior form, or by degeneration to a lower type of animal. The
+principle of degeneration as a factor in the formation of groups of
+animals, which are thereby enabled to survive, is nowadays universally
+admitted. The most striking example is to be found in the widely
+distributed group of Tunicata, which live, in numbers of instances, a
+sedentary life upon the rocks, have the appearance of very low forms of
+animal life, propagate by budding, have lost all the characteristics of
+higher forms, and yet are considered to be derived from an original
+vertebrate stock. Such degenerate forms remain degenerate, and are never
+known to regenerate and again to reach the higher stage of evolution from
+which they arose. Such forms are of considerable interest, but cannot help,
+except negatively, to decide what factor is especially important for upward
+progress.
+
+At the head of the animal race at the present day stands man, and in
+mankind itself some races are recognized as higher than others. Such
+recognition is given essentially on account of their greater brain-power,
+and without doubt the great characteristic which puts man at the head is
+the development of his central nervous system, especially of the region of
+the brain. Not only is this point most manifest in distinguishing man from
+the lower animals, but it applies to the latter as well. By the amount of
+convolution of the brain, the amount of grey matter in the cerebral
+hemispheres, the enlargement and increasing complexity of the higher parts
+of the central nervous system, the anthropoid apes are differentiated from
+the lower forms, and the higher mammals from the lower. In the recent work
+of Elliot Smith, and of Edinger, most conclusive proof is given that the
+upward progress in the vertebrate phylum is correlated with the increase of
+brain-power, and the latter writer shows how steady and remarkable is the
+increase in substance and in complexity of the brain-region as we pass from
+the fishes, through the amphibians and reptiles, to the birds and mammals.
+
+The study of the forms which lived on the earth in past ages confirms and
+emphasizes this conclusion, for it is most striking to see how small is the
+cranium among the gigantic Dinosaurs; how in the great reptilian age the
+denizens of the earth were far inferior in brain-power to the lords of
+creation in after-times.
+
+What applies to the vertebrate phylum applies also to the invertebrate
+groups. Here also an upward progress is recognized as we {18}pass from the
+sponges to the arthropods--a progress which is manifested, first by the
+concentration of nervous material to form a central nervous system, and
+then by the increase in substance and complexity of that nervous system to
+form a higher and a higher type, until the culmination is reached in the
+nervous system of the scorpions and spiders. No upward progress is possible
+with degeneration of the central nervous system, and in all those cases
+where a group owes its existence to degeneration, the central nervous
+system takes part in the degeneration.
+
+This law of the paramount importance of the growth of the central nervous
+system for all upward progress in the evolution of animals receives
+confirmation from the study of the development of individuals, especially
+in those cases where a large portion of the life of the animal is spent in
+a larval condition, and then, by a process of transformation, the larva
+changes into the adult form. Such cases are well known among Arthropoda,
+the familiar instance being the change from the larval caterpillar to the
+adult imago. Among Vertebrata, the change from the tadpole to the frog,
+from the larval form of the lamprey (_Ammocoetes_) to the adult form
+(_Petromyzon_), are well-known instances. In all such cases the larva shows
+signs of having attained a certain stage in evolution, and then a
+remarkable transformation takes place, with the result that an adult animal
+emerges, whose organization reaches a higher stage of evolution than that
+of the larva.
+
+This transformation process is characterized by a very great destruction of
+the larval tissues and a subsequent formation of new adult tissues. Most
+extensive is the destruction in the caterpillar and in the larval lamprey.
+But one organ never shares in this process of histolysis, and that is the
+central nervous system; amidst the ruins of the larva it remains, leading
+and directing the process of re-formation. In the Arthropoda, the larval
+alimentary canal may be entirely destroyed and eaten up by phagocytes, but
+the central nervous system not only remains intact but increases in size,
+and by the concentration and cephalization of its infra-oesophageal ganglia
+forms in the adult a central nervous system of a higher type than that of
+the larva.
+
+So, too, in the transformation of the lamprey, there is not the slightest
+trace of any destruction in the central nervous system, but simply a
+development and increase in nervous material, which {19}results in the
+formation of a brain region more like that of the higher vertebrates than
+exists in Ammocoetes.
+
+In these cases the development is upward--the adult form is of a higher
+type than that of the larva. It is, however, possible for the reverse to
+occur, so that the individual development leads to degeneration, not to a
+higher type. Instances are seen in the Tunicata, and in various parasitic
+arthropod forms, such as Lernæa, etc. In these cases, the transformation
+from the larval to the adult form leads to degradation, and in this
+degradation the central nervous system is always involved.
+
+It is perhaps a truism to state that upward progress is necessarily
+accompanied by increased development of the central nervous system; but it
+is necessary to lay special stress upon the importance of the central
+nervous system in all problems of evolution, because there is, in my
+opinion, a tendency at the present time to ignore this factor to too great
+an extent.
+
+The law of progress is this--The race is not to the swift, nor to the
+strong, but to the wise.
+
+This law carries with it the necessary corollary that the immediate
+ancestor of the vertebrate must have had a central nervous system nearly
+approaching that of the lowest undegenerated vertebrate. Among all the
+animals living on the earth at the present time, the highest invertebrate
+group, the Arthropoda, possesses a central nervous system most closely
+resembling that of the vertebrate.
+
+The law, then, of the paramount importance of a steady development of the
+central nervous system for the upward progress of the animal kingdom,
+points directly to the arthropod as the most probable ancestor of the
+vertebrate.
+
+
+EVOLUTION OF TISSUES.
+
+In the whole scheme of evolution we can recognize, not only an upward
+progress in the organization of the animal as a whole, but also a distinct
+advance in the structure of the tissues composing an individual, which
+accompanies that upward progress. Thus it is possible to speak of an
+evolution of the supporting tissues from the simplest form of connective
+tissue up to cartilage and thence to bone; of the contractile tissues, from
+the simplest contractile protoplasm {20}to unstriped muscle, and thence to
+the highest forms of striated muscle; of the nervous connecting strands,
+from undifferentiated to fine strands, then to thicker, more separated
+ones, resembling non-medullated fibres, and finally to well-differentiated
+separate fibres, each enclosed in a medullated sheath.
+
+In the connective tissue group, bone is confined to the vertebrates,
+cartilage is found among invertebrates, and the closest resemblance to
+vertebrate embryonic or parenchymatous cartilage is found in the cartilage
+of Limulus. Also, as Gegenbaur has pointed out, Limulus, more than any
+other invertebrate, possesses a fibrous connective tissue resembling that
+of vertebrates.
+
+In the muscular group, Biedermann, who has made a special study of the
+physiology of striated muscle, says that among invertebrates the striated
+muscle of the arthropod group resembles most closely that of the
+vertebrate.
+
+In the nervous group the resemblance between the nerve-fibres of Limulus
+and Ammocoetes, both of which are devoid of any marked medullary sheath, is
+very apparent, and Retzius points out that the only evidence of
+medullation, so characteristic of the vertebrates, is found in a species of
+prawn (_Palæmon_). In all these cases the nearest resemblance to the
+vertebrate tissues is to be found in the arthropod.
+
+
+THE EVIDENCE OF PALÆONTOLOGY.
+
+Perhaps the most important of all the clues likely to help in the solution
+of the origin of vertebrates is that afforded by Geology, for although the
+geological record is admittedly so imperfect that we can never hope by its
+means alone to link together the animals at present in existence, yet it
+does undoubtedly point to a sequence in the evolution of animal forms, and
+gives valuable information as to the nature of such sequence. In different
+groups of animals there are times when the group can be spoken of as having
+attained its most flourishing period. During these geological epochs the
+distribution of the group was universal, the numbers were very great, the
+number of species was at the maximum, and some of them had attained a
+maximal size. Such races were at that time dominant, and the struggle for
+existence was essentially among members of the same group. At the present
+time the dominant race is man, and the {21}struggle for existence is
+essentially between the members of that race, and not between them and any
+inferior race.
+
+The effect of such conditions is, as Darwin has pointed out, to cause great
+variation in that group; in consequence of that variation and that
+dominance the evolution of the next higher group is brought about from some
+member of the dominant group. Thus the present age is the outcome of the
+Tertiary period, a time when giant mammals roamed the earth and left as
+their successors the mammals of the present day; a time of dominance of
+quadruped mammals; a time of which the period of maximum development is
+long past, and we now see how the dominance of the biped mammal, man, is
+accompanied by the rapid diminution and approaching extermination of the
+larger mammals. No question can possibly arise as to the immediate ancestor
+of the biped mammal; he undoubtedly arose from one of the dominant
+quadrupedal mammals.
+
+Passing along to the next evidence of the rocks, we find an age of reptiles
+in the Mesozoic period. Here, again, the number and variety is most
+striking; here, again, the size is enormous in comparison with that of the
+present-day members of the group. This was the dominant race at the time
+when the birds and mammals first appeared on the earth, and anatomists
+recognize in these extinct reptilian forms two types; the one bird-like,
+the other more mammalian in character. From some members of the former
+group birds are supposed to have been evolved, and mammals from members of
+the other group. There is no question of their origin directly from lower
+fish-like forms; the time of their appearance on the earth, their
+structure, all point irresistibly to the same conclusion as we have arrived
+at from the consideration of the origin of the biped from the quadruped
+mammal, viz. that birds and mammals arose, in consequence of the struggle
+for existence, from some members of the reptilian race which at that time
+was the dominant one on earth.
+
+Passing down the geological record, we find that when the reptiles first
+appear in the Carboniferous age there is abundant evidence of the existence
+of numbers of amphibian forms. At this time the giant Labyrinthodonts
+flourished. Here among the swamps and marshes of the coal-period the
+prevalent vertebrate was amphibian in structure. Their variety and number
+were very great, and at that period they attained their greatest size.
+Here, again, from the geological record we draw the same conclusion as
+before, that the reptiles arose from the race which was then predominant on
+the earth--the Amphibia.
+
+{22}[Illustration: FIG. 4.--PLAN OF GEOLOGICAL STRATA. (From LANKESTER.)]
+
+{23}Again, another point of great interest is seen here, and that is that
+these Labyrinthodonts, as Huxley has pointed out, possess characters which
+bring them more closely than the amphibians of the present day into
+connection with the fishes; and further, the fish-like characters they
+possessed are those of the Ganoids, the Marsipobranchs, the Dipnoans, and
+the Elasmobranchs, rather than of the Teleosteans.
+
+Now, it is a striking fact that the ancient fishes at the time when the
+amphibians appeared had not reached the teleostean stage. The ganoids and
+elasmobranchs swarmed in the waters of the Devonian and Carboniferous
+times. Dipnoans and marsipobranchs were there, too, in all probability, but
+teleosteans do not appear until the Mesozoic period. The very kinds of
+fish, then, which swarmed in the seas at that time, and were the
+predominant race before the Carboniferous epoch, are those to which the
+amphibians at their first appearance show the closest affinity. Here,
+again, the same law appears; from the predominant race at the time, the
+next higher race arose, and arose by a most striking modification, which
+was the consequence of altering the medium in which it lived. By coming out
+of the water and living on the land, or, rather, being able to live partly
+on land and partly in the water, by the acquisition of air-breathing
+respiratory organs or lungs in addition to, and instead of, water-breathing
+organs or gills, the amphibian not only arose from the fish, but made an
+entirely new departure in the sequence of progressive forms.
+
+This was a most momentous step in the history of evolution--one fraught
+with mighty consequences and full of most important suggestions.
+
+From this time onwards the struggle for existence by which upward progress
+ensued took place on the land, not in the sea, and, as has been pointed
+out, led to the evolution of reptiles from amphibians, birds and
+quadrupedal mammals from reptiles, and man from quadrupeds. In the sea the
+fishes were left to multiply and struggle among themselves, their only
+opponents being the giant cephalopods, which themselves had been evolved
+from a continual succession of the Mollusca. For this reason the struggle
+for existence between the fishes and the higher race evolved from them did
+not {24}take place until some members of that higher race took again to the
+water, and so competed with the fish-tribe in their own element.
+
+Another most important conclusion to be derived from the uprising of the
+Amphibia is that at that time there was no race of animals living on the
+land which had a chance against them. No race of land-living animals had
+been evolved whose organization enabled them to compete with and overcome
+these intruders from the sea in the struggle for existence. For this reason
+that the whole land was their own, and no serious competition could arise
+from their congeners, the fish, they took possession of it, and increased
+mightily in size; losing more and more the habit of going into the water,
+becoming more and more truly terrestrial animals. Henceforth, then, in
+trying to find out the sequence of evolution, we must leave the land and
+examine the nature of the animals living in the sea; the air-breathing
+animals which lived on the land in the Upper Silurian and Devonian times
+cannot have reached a stage of organization comparable with that of the
+fishes, seeing how easily the amphibians became dominant.
+
+We arrive, then, at the conclusion that the ancestors of the fishes must
+have lived in the sea, and applying still the same principles that have
+held good up to this time, the ancestors of the fishes must have arisen
+from some member of the race predominant at the time when they first
+appeared, and also the earliest fishes must have much more closely
+resembled the ancestral form than those found in later times or at the
+present day.
+
+What, then, is the record of the rocks at the time of the first appearance
+of fish-like forms? What kind of fishes were they, and what was the
+predominant race at the time?
+
+We have now reached the Upper Silurian and Lower Devonian times, and most
+instructive and suggestive is the revelation of the rocks. Here, when the
+first vertebrates appeared, the sea was peopled with corals, brachiopods,
+early forms of cephalopods, and other invertebrates; but, above all, with
+the great tribe of trilobites (Fig. 6) and their successors. From the
+trilobites arose, as evidenced by their larval form, the king-crab group,
+called the Xiphosura (Fig. 5). Closely connected with them, and forming
+intermediate stages between trilobites and king-crabs, numerous forms have
+been discovered, known as Belinurus, Prestwichia, Hemiaspis, Bunodes, etc.
+(Fig. 5 and Fig. 12). From them also arose the most striking group {25}of
+animals which existed at this period--the giant sea-scorpions, or
+Gigantostraca. This group was closely associated with the king-crabs, and
+the two groups together are classified under the title Merostomata.
+
+[Illustration: FIG. 5 (from H. WOODWARD).--1. _Limulus polyphemus_ (dorsal
+aspect). 2. _Limulus,_ young, in trilobite stage. 3. _Prestwichia
+rotundata._ 4. _Prestwichia Birtwelli._ 5. _Hemiaspis limuloides._ 6.
+_Pseudoniscus aculeatus._]
+
+
+The appearance of these sea-scorpions is given in Figs. 7 and 8,
+representing Stylonurus, Slimonia, Pterygotus, Eurypterus. They must have
+been in those days the tyrants of the deep, for specimens of Pterygotus
+have been found over six feet in length.
+
+At this time, then, by every criterion hitherto used, by the multitude of
+species, by the size of individual species, which at this period reached
+the maximum, by their subsequent decay and final extinction, we must
+conclude that these forms were in their zenith, that the predominant race
+at this time was to be found in this group of arthropods. Just previously,
+the sea swarmed with trilobites, and right into the period when the
+Gigantostraca flourished, the trilobites {26}are still found of countless
+forms, of great difference in size. The whole period may be spoken of as
+the great trilobite age, just as the Tertiary times form the mammalian age,
+the Mesozoic times the reptilian age, etc. From the trilobites the
+Gigantostraca and Xiphosura arose, as evidenced by the embryology of
+Limulus, and, therefore, in the term trilobite age would be included the
+whole of those peculiar forms which are classified by the names Trilobita,
+Gigantostraca, Xiphosura, etc. Of all these the only member alive at the
+present time is Limulus, or the King-Crab.
+
+[Illustration: FIG. 6.--A TRILOBITE (_Dalmanites_) (after PICTET). Dorsal
+view.]
+
+[Illustration: FIG. 7.--_Eurypterus remipes_ (after NIESKOWSKI). Dorsal
+view.]
+
+As, however, the term 'trilobite' does not include the members of the
+king-crab or sea-scorpion groups, it is advisable to use some other term to
+represent the whole group. They cannot be called crustaceans or arachnids,
+for in all probability they gave origin to both; the nearest approach to
+the Trilobite stage of development at the present time is to be found
+perhaps in Branchipus (Fig. 10) and Apus (Fig. 9), just as the nearest
+approach to the Eurypterid {27}form is Limulus. Crustaceans such as crabs
+and lobsters are of much later origin, and do not occur in any quantity
+until the late Mesozoic period. The earliest found, a kind of prawn, occurs
+in the Carboniferous age.
+
+[Illustration: FIG. 8.--A, _Pterygotus Osiliensis_ (from SCHMIDT). B,
+_Stylonurus Logani_ (from WOODWARD). C, _Slimonia acuminata_ (from
+WOODWARD).]
+
+Korschelt and Heider have accordingly suggested the name _Palæostraca_ for
+this whole group, and _Protostraca_ for the still earlier
+{28}arthropod-like animals which gave origin to the trilobites themselves.
+This name I shall adopt, and speak, therefore, of the _Palæostraca_ as the
+dominant race at the time when vertebrates first appeared.
+
+If, then, there is no break in the law of evolution here, the race which
+was predominant at the time when the vertebrate first appeared must have
+been that from which the first fishes arose, and these fishes must have
+resembled, not the crustacean proper, or the arachnid proper, but a member
+of the palæostracan group. Moreover, just as the Labyrinthodonts show
+special affinities to the fishes which were then living, so we should
+expect that the forms of the earliest fish would resemble the arthropodan
+type dominant at the time more closely than the fish of a later era.
+
+At first sight it seems too great an absurdity even to imagine the
+possibility of any genetic connection between a fish and an arthropod, for
+to the mind's eye there arises immediately the picture of a salmon or a
+shark and a lobster or a spider. So different in appearance are the two
+groups of animals, so different their methods of locomotion, that it is
+apparently only an inmate of a lunatic asylum who could possibly suggest
+such a connection. Much more likely is it that a fish-like form should have
+been developed out of a smooth, wriggling, worm-like animal, and it is
+therefore to the annelids that the upholders of the theory of the reversal
+of surfaces look for the ancestor of the vertebrate.
+
+[Illustration: FIG. 9.--_Apus_ (from the Royal Natural History). Dorsal
+view.]
+
+[Illustration: FIG. 10.--_Branchipus stagnalis._ (From CLAUS.)]
+
+{29}We must endeavour to dismiss from our imagination such forms as the
+salmon and shark as representatives of the fish-tribe, and the lobster and
+spider of the arthropods, and try to picture the kind of animals living in
+the seas in the early Devonian and Upper Silurian times, and then we find,
+to our surprise, that instead of the contrast between fishes and arthropods
+being so striking as to make any comparison between the two seem an
+absurdity, the difficulty in the last century, and even now, is to decide
+in many cases whether a fossil is an arthropod or a fish.
+
+I have shown what kind of animal the palæostracan was like. What
+information is there of the nature of the earliest vertebrate?
+
+The most ancient fishes hitherto discovered have been classified by
+Lankester and Smith Woodward into the three orders, Heterostraci,
+Osteostraci, and Antiarcha. Of these the Heterostraci contain the genera
+Pteraspis and Cyathaspis, and are the very earliest vertebrates yet
+discovered, being found in the Lower Silurian. The Osteostraci are divided
+into the Cephalaspidæ, Tremataspidæ, etc., and are found in the Upper
+Silurian and Devonian beds. The Antiarcha, comprising Pterichthys and
+Bothriolepis, belong to the Devonian and are not found in Silurian
+deposits. This, then, is the order of their appearance--Pteraspis,
+Cephalaspis, and Pterichthys.
+
+In none of these families is there any resemblance to an ordinary fish. In
+no case is there any sign of vertebræ or of jaws. They, like the lampreys,
+were all agnathostomatous. Strange indeed is their appearance, and it is no
+wonder that there should have been a difficulty in deciding whether they
+were fish or arthropod. Their great characteristic is their buckler-plated
+cephalic shield, especially conspicuous on the dorsal side of the head.
+Figs. 11, 14, 15, 16, give the dorsal shields of Pteraspis, Auchenaspis,
+Pterichthys, and Bothriolepis.
+
+In 1904, Drevermann discovered a mass of _Pteraspis Dunensis_ embedded in a
+single stone, showing the same kind of head-shield as _P. rostrata_, but
+the rostrum was longer and the spine at the extremity of the head-shield
+much longer and more conspicuous. The whole shape of the animal as seen in
+this photograph recalls the shape of a Hemiaspid rather than of a fish. It
+is, then, natural enough for the earlier observers to have looked upon such
+a fossil as related to an arthropod rather than a fish.
+
+{30}[Illustration: FIG. 11.--_Pteraspis dunensis_ (from DREVERMANN). Dorsal
+view of body and spine on the right side. Head-end, showing long rostrum on
+the left side.]
+
+[Illustration: FIG. 12.--_Bunodes lunula._ (From SCHMIDT.)]
+
+[Illustration: FIG. 13.--_Auchenaspis (Thyestes) verrucosus_, natural size.
+(From WOODWARD.)]
+
+{31}In Figs. 12 and 13 I have placed side by side two Silurian fossils
+which are found in the same geological horizon. They are both life size and
+possess a general similarity of appearance, yet the one is a Cephalaspidian
+fish known by the name of _Auchenaspis_ or _Thyestes verrucosa_, the other
+a Palæostracan called _Bunodes lunula_.
+
+[Illustration: FIG. 14.--DORSAL HEAD-SHIELD OF _Thyestes (Auchenaspis)
+verrucosus_. (From ROHON.)
+
+_Fro._, narial opening; _l.e._, lateral eyes; _gl._, glabellum or plate
+over brain; _Occ._, occipital region.]
+
+[Illustration: FIG. 15.--_Pterichthys._]
+
+In a later chapter I propose to discuss the peculiarities and the nature of
+the head-shields of these earliest fishes, in connection with the question
+of the affinities of the animals which bore them. At this point of my
+argument I want simply to draw attention to the undoubted fact of the
+striking similarity in appearance between the {32}earliest fishes and
+members of the Palæostraca, the dominant race of arthropods which swarmed
+in the sea at the time: a similarity which could never have been suspected
+by any amount of investigation among living forms, but is immediately
+revealed when the ages themselves are questioned.
+
+[Illustration: FIG. 16.--_Bothriolepis._ (After PATTEN.)
+
+_An._, position of anus.]
+
+I have not reproduced any of the attempted restorations of these old forms,
+as usually given in the text-books, because all such restorations possess a
+large element of fancy, due to the personal bias of the observer. I have
+put in Rohon's idea of the general shape of Tremataspis (Fig. 17) in order
+to draw attention to the lamprey-like appearance of the fish according to
+his researches (_cf._ Fig. 18).
+
+[Illustration: FIG. 17.--RESTORATION OF _Tremataspis_. (After ROHON,
+slightly modified.)]
+
+[Illustration: FIG. 18.--_Ammocoetes._]
+
+The argument, then, from geology, like that from comparative anatomy and
+from the consideration of the importance of the central nervous system in
+the upward development of the animal race, not only points directly to the
+arthropod group as the ancestor of the {33}vertebrate, but also to a
+distinct ancient type of arthropod, the Palæostracan, the only living
+example of which is the King-Crab or Limulus; while the nearest approach to
+the trilobite group among living arthropods are Branchipus and Apus. It
+follows, therefore, that for the following up of this clue, Limulus
+especially must be taken into consideration, while Branchipus and Apus are
+always to be kept in mind.
+
+
+AMMOCOETES RATHER THAN AMPHIOXUS IS THE BEST SUBJECT FOR INVESTIGATION.
+
+It is not, however, Limulus that must be investigated in the first
+instance, but the vertebrate itself; for it can never be insisted on too
+often that in the vertebrate itself its past history will be found, but
+that Limulus cannot reveal the future of its race. What vertebrate must be
+chosen for investigation? Reasons have been given why our attention should
+be fixed upon the king-crab rather than on the lobster on the invertebrate
+side; what is the most likely animal on the vertebrate side?
+
+From the evidence already given it is manifest that the earliest mammal
+belonged to the lowest group of mammals; that the birds on their first
+appearance presented reptilian characteristics, that the earliest reptiles
+belonged to a low type of reptile, that the amphibians at their first
+appearance were nearer in type to the fishes than were the later forms. As
+each of these groups advances in number and power, specialization takes
+place in it, and the latest developed members become further and further
+removed in type from the earliest. So also it must have been with the
+origin of fishes: here too, in the quest for information as to the
+structure and nature of the first-formed fishes, we must look to the lowest
+rather than to the highest living members of the group.
+
+The lowest fish-like animal at present living is Amphioxus, and on this
+ground it is argued that the original vertebrate must have approached in
+organization to that of Amphioxus; it is upon the comparison between the
+structure of Amphioxus and that of Balanoglossus, that the theory of the
+origin of vertebrates from forms like the latter animal is based. For my
+own part, I think that in the first instance, at all events, Amphioxus
+should be put on one side, although of course its structure must always be
+kept in mind, for the following reasons:--
+
+{34}Amphioxus, like the tunicates, does not possess the characteristics of
+other vertebrates. In all vertebrates above these forms the great
+characteristic is a well-defined brain-region from which arise nerves to
+organs of special sense, the eyes and nose. In Amphioxus no eyes exist, for
+the pigmented spot at the anterior extremity of the brain-region is no eye
+but only a mass of pigment, and the so-called olfactory pit is a very
+rudimentary and inferior organ of smell. In connection with the nearly
+complete absence of these two most important sense-organs, the most
+important part of the central nervous system, the region corresponding to
+the cerebral hemispheres, is also nearly completely absent.
+
+Now, the history of the evolution of the central nervous system in the
+animal race points directly to its formation as a concentrated mass of
+nervous material at the anterior extremity of the body, in consequence of
+the formation of special olfactory and visual organs at that extremity. As
+already stated, the concentration of nervous material around the mouth as
+an oral ring was its beginning. In connection with this there arose special
+sense-organs for the guidance of the animal to its food which took the form
+of olfactory and optic organs. With the shifting from the radial to the
+elongated form these sense-organs remained at the anterior or mouth-end of
+the animal, and owing to their immense importance in the struggle for
+existence, that part of the central nervous system with which they were
+connected developed more than any other part, became the leader to which
+the rest of the nervous system was subservient, and from that time onwards
+the development of the brain-region was inevitably associated with the
+upward progress of animal life.
+
+To those who believe in Evolution and the Darwinian theory of the survival
+of the fittest, it is simply inconceivable that a soft-bodied animal living
+in the mud, blind, with a rudimentary brain and rudimentary olfactory
+organs, such as is postulated when we think of Balanoglossus and Amphioxus,
+should hold its own and come victorious out of the struggle for existence
+at a time when the sea was peopled with powerful predaceous scorpion- and
+crab-like armour-plated animals possessing a well-developed brain, good
+eyes and olfactory organs, and powerful means of locomotion. Wherever in
+the scale of animal development Amphioxus may ultimately be placed, it
+cannot be looked upon as the type of the earliest formed fishes such as
+appeared in Silurian times.
+
+{35}The next lowest group of living fishes is the Marsipobranchii which
+include the lampreys and hag-fishes. To these naturally we must turn for a
+clue as to the organization of the earliest fish, for here we find all the
+characteristics of the vertebrates represented: a well-formed brain-region,
+well-developed eyes and nose, cranial nerves directly comparable with those
+of other vertebrates, and even the commencement of vertebræ.
+
+Among these forms the lamprey is by far the best for investigation, not
+only because it is easily obtainable in large quantities, but especially
+because it passes a large portion of its existence in a larval condition,
+from which it emerges into the adult state by a wonderful process of
+transformation, comparable in extent with the transformation of the larval
+caterpillar into the adult imago. So long does the lamprey live in this
+free larval condition, and so different is it in the adult stage, that the
+older anatomists considered that the two states were really different
+species, and gave the name of _Ammocoetes branchialis_ to the larval stage,
+while the adult form was called _Petromyzon planeri_, or _Petromyzon
+fluviatilis_.
+
+This long-continued free-living existence in the larval or Ammocoetes stage
+makes the lamprey, more than any other type of lowly organized fish,
+invaluable for the present investigation, for throughout the animal kingdom
+it is recognized that the larval form approaches nearer to the ancestral
+type than the adult form, whether the latter is progressive or degenerate.
+Not only are the tissues formed during the stages which are passed through
+in a free-living larval form, serviceable tissues comparable to those of
+adult life, but also these stages proceed at so much slower a rate than do
+those in the embryo _in utero_ or in the egg, as to make the larval form
+much more suitable than the embryo for the investigation of ancestral
+problems. It is true enough that the free life of the larva may bring about
+special adaptations which are not of an ancestral character, as may also
+occur during the life of the adult; but the evidence is very strong that
+although some of the peculiarities of the larva may be due to such
+coenogenetic factors, yet on the whole many of them are due to ancestral
+characters, which disappear when transformation takes place, and are not
+found in the adult.
+
+Thus if it be supposed that the amphibian arose from the fish, the tadpole
+presents more resemblance to the fish than the frog. If {36}it be supposed
+that the arthropod arose from the segmented worm, the caterpillar bears out
+the suggestion better than the adult imago. If it be supposed that the
+tunicate arose from a stock allied to the vertebrate, it is because of the
+peculiarities of the larva that such a supposition is entertained. So, too,
+if it be supposed that the fish arose from a member of the arthropod group,
+the larval form of the fish is most likely to give decisive information on
+the point.
+
+For all these reasons the lowest form of fish to be investigated, in the
+hopes of finding out the nature of the earliest formed fish, is not
+Amphioxus, but Ammocoetes, the larval form of the lamprey--a form which, as
+I hope to satisfy my reader after perusal of subsequent pages, more nearly
+resembles the ancient Cephalaspidian fishes than any other living
+vertebrate.
+
+
+COMPARISON OF CENTRAL NERVOUS SYSTEMS OF VERTEBRATE AND ARTHROPOD WITHOUT
+REVERSAL OF SURFACES.
+
+So far different lines of investigation all point to the origin of the
+vertebrate from arthropods, the group of arthropods in question being now
+extinct, the nearest living representative being Limulus; also to the fact
+that of the two theories of the origin of vertebrates, that one which is
+based on the resemblance between the central nervous systems of the
+Vertebrata and the Appendiculata (Arthropoda and Annelida) is more in
+accordance with this evidence than the other, which is based mainly on the
+supposed possession of a notochord among certain animals.
+
+How is it, then, that this theory has been discredited and lost ground?
+Simply, I imagine, because it was thought to necessitate the turning over
+of the animal. Let us, then, again look at the nervous system of the
+vertebrate, and see whether there is any such necessity.
+
+As previously mentioned, the comparison of the two central nervous systems
+showed such close resemblances as to force those anatomists who supported
+this theory to the conclusion that the infundibular tube was in the
+position of the original oesophagus; they therefore looked for the remains
+of a mouth opening in the dorsal roof of the brain, but did not attempt to
+explain the extraordinary fact that the infundibular tube is only a ventral
+offshoot from the tube of the central nervous system. Yet this latter tube
+{37}is one, if not the most striking, of the peculiarities which
+distinguish the vertebrate; a tubular central nervous system such as that
+of the vertebrate is totally unlike any other nervous system, and the very
+fact that the two nervous systems of the vertebrate and arthropod are so
+similar in their nervous arrangements, makes it still more extraordinary
+that the nervous system should be grouped round a tube in the one case and
+not in the other.
+
+Now, in the arthropod the oesophagus leads directly into the stomach, which
+is situated in the head-region, and from this a straight intestine passes
+directly along the length of the body to the anus, where it terminates. The
+relations of mouth, oesophagus, alimentary canal, and nervous system in
+these animals are represented in the diagram (Fig. 3).
+
+Any tube, therefore, such as that of the infundibulum, which would
+represent the oesophagus of such an animal, must have opened into the mouth
+on the ventral side, and into the stomach on the dorsal side, and the
+lining epithelium of such an oesophagus must have been continuous with that
+of the stomach, and so of the whole intestinal tract.
+
+Supposing, then, the animal is not turned over, but that the dorsal side
+still remains dorsal and ventral ventral, then the original mouth-opening
+of the oesophagus must be looked for on the ventral surface of the
+vertebrate brain in the region of the pituitary body or hypophysis, and on
+the dorsal side the tube representing the oesophagus must be continuous
+with a large cephalically dilated tube, which ought to pass into a small
+canal, to run along the length of the body and terminate in the anus.
+
+This is exactly what is found in the vertebrate, for the infundibular tube
+passes into the third ventricle of the brain, which forms, with the other
+ventricles of the brain, the large dilated cephalic portion of the
+so-called nerve tube, and at the junction of the medulla oblongata and
+spinal cord, this dilated anterior part passes into the small, straight,
+central canal of the spinal cord, which in the embryo terminates in the
+anus by way of the neurenteric canal. If the animal is regarded as not
+having been turned over, then the conclusion that the infundibulum was the
+original oesophagus leads immediately to the further conclusion that the
+ventricles of the vertebrate brain represent the original cephalic stomach,
+and the central canal of the spinal cord the straight intestine of the
+arthropod ancestor.
+
+{38}For the first time a logical, straightforward explanation is thus given
+of the peculiarities of the tube of the central nervous system, with its
+extraordinary termination in the anus in the embryo, its smallness in the
+spinal cord, its largeness in the brain region, and its offshoot to the
+ventral side of the brain as the infundibular channel. It is so clear that,
+if the infundibular tube be looked on as the old oesophagus, then its
+lining epithelium is the lining of that oesophagus; and the fact that this
+lining epithelium is continuous with that of the third ventricle, and so
+with the lining of the whole nerve-tube, must be taken into account and not
+entirely ignored as has hitherto been the case. If, then, we look at the
+central nervous system of the vertebrate in the light of the central
+nervous system of the arthropod without turning the animal over, we are led
+immediately to the conclusion that what has hitherto been called the
+vertebrate nervous system is in reality composed of two parts, viz. a
+nervous part comparable in all respects with that of the arthropod
+ancestor, which has grown over and included into itself, to a greater or
+less extent, a tubular part comparable in all respects with the alimentary
+canal of the aforesaid ancestor. If this conclusion is correct, it is
+entirely wrong to speak of the vertebrate central nervous system as being
+tubular, for the tube does not belong to the nervous system, but was
+originally a simple epithelial tube, such as characterizes the oesophagus,
+cephalic stomach, and straight intestine of the arthropod.
+
+Here, then, is the crux of the position--either the so-called nervous tube
+of the vertebrate is composed of two separate factors, consisting of a true
+non-tubular nervous system and a non-nervous epithelial tube, these two
+elements having become closely connected together; or it is composed of one
+factor, an epithelial tube which constitutes the nervous system, its
+elements being all nervous elements.
+
+If this latter hypothesis be accepted, then it is necessary to explain why
+parts of that tube, such as the roof of the fourth ventricle, the choroid
+plexuses of the various ventricles, which are parts of the original roof
+inserted into the ventricles, are not composed of nervous material, but
+form simple single-layered epithelial sheets, which by no possibility can
+be included among functional nervous structures. The upholders of this
+hypothesis can only explain the nature of these thin epithelial parts of
+the nervous tube in one of two ways; either the tube was originally formed
+of nervous {39}material throughout, and for some reason parts of it have
+lost their nervous function and thinned down; or else these thin epithelial
+parts are on their way to become nervous material, are still in an
+embryonic condition, and are of the nature of epiblast-epithelium, from
+which the central nervous system originally arose.
+
+The first explanation is said to be supported by embryology, for at first
+the nerve-tube is formed in a uniform manner, and then later, parts of the
+roof appear to thin out and so form the thin epithelial parts. If this were
+the right explanation, then it ought to be found that in the lowest
+vertebrates there is greater evidence of a uniformly nervous tube than in
+the higher members of the group: while conversely, if, on the contrary, as
+we descend the vertebrate phylum, it is found that more and more of the
+tube presents the appearance of a single layer of epithelium, and the
+nervous material is limited more and more to certain parts of that tube,
+then the evidence is strong that the tubular character of the central
+nervous system is not due to an original nervous tube, but to a non-nervous
+epithelial tube with which the original nervous system has become closely
+connected.
+
+The comparison of the brain region of the different groups of vertebrates
+(Fig. 19) is most instructive, for it demonstrates in the most conclusive
+manner how the roof of the nervous tube in that region loses more and more
+its nervous character, and takes on the appearance of a simple epithelial
+tube, as we descend lower and lower; until at last, in the brain of
+Ammocoetes, as represented in the figures, the whole of the brain-roof,
+from the region of the pineal eye to the commencement of the spinal cord,
+is composed of fold upon fold of a thin epithelial membrane forming an
+epithelial bag, which is constricted in only one place, where the fourth
+cranial nerve crosses over it.
+
+Further, the brain of Ammocoetes (Fig. 20) shows clearly not only that it
+is composed of two parts, an epithelial tube and a nervous system, but also
+that the nerve-masses are arranged in the same relative position with
+respect to this tube as are the nerve-masses in the invertebrate with
+respect to the cephalic stomach and oesophagus. This evidence is so
+striking, so conclusive, that it is impossible to resist the conclusion
+that the tube did not originate as part of the central nervous system, but
+was originally independent of the central nervous system, and has been
+invaded by it.
+
+{40}[Illustration: FIG. 19.--COMPARISON OF VERTEBRATE BRAINS.
+
+_CB._, cerebellum; _PT._, pituitary body; _PN._, pineal body; _C. STR._,
+corpus striatum; _G.H.R._, right ganglion habenulæ. _I._, olfactory; _II._,
+optic nerves.]
+
+{41}[Illustration: FIG. 20.--BRAIN OF AMMOCOETES.
+
+A, dorsal view; B, lateral view; C, ventral view. _C.E.R._, cerebral
+hemispheres; _G.H.R._, right ganglion habenulæ; _PN._, right pineal eye;
+_CH_2_, _CH_3_, choroid plexuses; _I.-XII._ cranial nerves; _C.P._, _Conus
+post-commissuralis_.]
+
+{42}The second explanation is hardly worth serious consideration, for it
+supposes that the nervous system, for no possible reason, was laid down in
+its most important parts--the brain-region--as an epithelial tube with
+latent potential nervous functions; that even up to the highest vertebrate
+yet evolved these nervous functions are still in abeyance over the whole of
+the choroid plexuses and the roof of the fourth ventricle. Further, it
+supposes that this prophetic epithelial tube originally developed into true
+nervous material only in certain parts, and that these parts, curiously
+enough, formed a nervous system absolutely comparable to that of the
+arthropod, while the dormant prophetic epithelial part was formed so as
+just to mimic, in relation to the nervous part, the alimentary canal of
+that same arthropod.
+
+The mere facts of the case are sufficient to show the glaring absurdity of
+such an explanation. This is not the way Nature works; it is not consistent
+with natural selection to suppose that in a low form nervous material can
+be laid down as non-nervous epithelial material in order to provide in some
+future ages for the great increase in the nervous system.
+
+Every method of investigation points to the same conclusion, whether the
+method is embryological, anatomical, or pathological.
+
+First, take the embryological evidence. On the ground that the individual
+development reproduces to a certain extent the phylogenetic development,
+the peculiarities of the formation of the central nervous system in the
+vertebrate embryo ought to receive an appropriate explanation in any theory
+of phylogenetic development. Hitherto such explanation has been totally
+lacking; any suggestion of the manner in which a tubular nervous system may
+have been formed takes no account whatever of the differences between
+different parts of the tube; its dilated cephalic end with its infundibular
+projection ventrally, its small straight spinal part, and its termination
+in the anus. My theory, on the other hand, is in perfect harmony with the
+embryological history, and explains it point by point.
+
+From the very first origin of the central nervous system there is evidence
+of two structures--the one nervous, and the other an epithelial
+surface-layer which ultimately forms a tube; this was first described by
+Scott in Petromyzon, and later by Assheton in the frog. In the latter case
+the external epithelial layer is pigmented, while the underlying nervous
+layer contains no pigment; a marked {43}and conspicuous demarcation exists,
+therefore, between the two layers from the very beginning, and it is easy
+to trace the subsequent fate of the two layers owing to this difference of
+pigmentation. The pigmented cells form the lining cells of the central
+canal, and becoming elongated, stretch out between the cells of the nervous
+layer; while the latter, on their side, invade and press between the
+pigmented cells. In this case, owing to the pigmentation of the epithelial
+layer, embryology points out in the clearest possible manner how the
+central nervous system of the vertebrate is composed of two structures--an
+epithelial non-nervous tube, on the outside of which the central nervous
+system was originally grouped; how, as development proceeds, the elements
+of these two structures invade each other, until at last they become so
+involved together as to give rise to the conception that we are dealing
+with one single nerve tube. It is impossible for embryology to give a
+clearer clue to the past history than it does in this case, for it actually
+shows, step by step, how the amalgamation between the central nervous
+system and the old alimentary canal took place.
+
+Further, consider the shape of the tube when it is first formed, how
+extraordinary and significant that is. It consists of a simple dilated
+anterior end leading into a straight tube, the lumen of which is much
+larger than that of the ultimate spinal canal, and terminates by way of the
+neurenteric canal in the anus.
+
+Why should the tube take this peculiar shape at its first formation? No
+explanation is given or suggested in any text-book of embryology, and yet
+it is so natural, so simple: it is simply the shape of the invertebrate
+alimentary canal with its cephalic stomach and straight intestine ending in
+the anus. Again embryology indicates most unmistakably the past history of
+the race. How are the nervous elements grouped round this tube when it is
+first formed? Here embryology shows that a striking difference exists
+between the part of the tube which forms the spinal cord and the dilated
+cephalic part. Fig. 21, A (2), represents the relation between the nervous
+masses and the epithelial tube in the first instance. At this stage the
+nervous material in the spinal cord lies laterally and ventrally to this
+tube, and at a very early stage the white anterior commissure is formed,
+joining together these two lateral masses; as yet there is no sign of any
+posterior fissure, the tube with its open lumen extends right to the dorsal
+surface.
+
+{44}The interpretation of this stage is that in the invertebrate ancestor
+the nerve-masses were situated laterally and ventrally to the epithelial
+tube, and were connected together by commissures on the ventral side of the
+tube (Fig. 21, A (1)); in other words, the chain of ventral ganglia and
+their transverse commissures lying just ventrally to the intestine, which
+are so characteristic of the arthropod nervous system, is represented at
+this stage.
+
+[Illustration: FIG. 21.--A, METHOD OF FORMATION OF THE VERTEBRATE SPINAL
+CORD FROM THE VENTRAL CHAIN OF GANGLIA AND THE INTESTINE OF AN ARTHROPOD,
+REPRESENTED IN 1; B, METHOD OF FORMATION OF THE VERTEBRATE MEDULLA
+OBLONGATA FROM THE INFRA-OESOPHAGEAL GANGLIA AND THE CEPHALIC STOMACH OF AN
+ARTHROPOD.]
+
+Subsequently, by the growth dorsalwards of nervous material to form the
+posterior columns, the original epithelial tube is compressed dorsally and
+laterally to such an extent that those parts lose all signs of lumen, the
+one becoming the posterior fissure and the others the _substantia
+gelatinosa Rolandi_ on each side. The original tube is thus reduced to a
+small canal formed by its ventral portion only (Fig. 21, A (3)). In this
+way the spinal cord is formed, and the walls of the original epithelial
+tube are finally visible only as the lining of the central canal (Fig. 21,
+A (4)).
+
+When we pass to the brain-region, to the anterior dilated portion of the
+tube, embryology tells a different story. Here, as in the spinal cord, the
+nervous masses are grouped at first laterally and ventrally to the
+epithelial tube, as is seen in Fig. 21, B (2), but owing to the large size
+of its lumen here, the nervous material is not able to enclose it
+completely, as in the case of the spinal cord; {45}consequently there is no
+posterior fissure formed; but, on the contrary, the dorsal roof, not
+enclosed by the nerve-masses, remains epithelial, and so forms the
+membranous roof of the fourth ventricle and of the other ventricles of the
+brain (Fig. 21, B (3)). In the higher animals, owing to the development of
+the cerebrum and cerebellum, this membranous roof becomes pushed into the
+larger brain cavity, and thus forms the choroid plexuses of the third and
+lateral ventricles. In the lower vertebrates, as in Ammocoetes and the
+Dipnoi, it still remains as a dorsal epithelial roof and forms a most
+striking characteristic of such brains.
+
+In this part of the nervous system, then, the nervous material is all
+grouped in its original position on the ventral side of the tube; and yet
+it is the same nervous material as that of the spinal cord, all the
+elements are there, giving origin here to the segmental cranial nerves just
+as lower down they give rise to the segmental spinal nerves, connecting
+together the separate segments each with the other and all with the higher
+brain-centres--the supra-infundibular centres--just as they do in the
+spinal region.
+
+Why should there be this striking difference between the formation of the
+infra-infundibular region of the brain and that of the spinal cord? Do the
+advocates of the origin of vertebrates from Balanoglossus give the
+slightest reason for it? They claim that their view also provides a tubular
+nervous system for the vertebrate, but give not the slightest sign or
+indication as to why the nervous material should be grouped entirely on the
+ventral side of an epithelial tube in the infra-infundibular region and yet
+surround it in the spinal cord region. And the explanation is so natural,
+so simple: embryology does its very best to tell us the past history of the
+race, if only we look at it the right way.
+
+The infra-infundibular nervous mass is naturally confined to the ventral
+side of the epithelial tube, because it represents the infra-oesophageal
+ganglia, situated as they are on the ventral side of the cephalic stomach,
+and, owing to the size of the stomach, they could not enclose it by dorsal
+growth, as they do in the case of the formation of the spinal cord (Fig.
+21, B (1)). Still these nervous masses have grown dorsalwards, have
+commenced to involve the walls of the cephalic stomach even in the lowest
+vertebrate, as is seen in Ammocoetes, in which animal a ventral portion of
+the epithelial bag has been evidently compressed and its lumen finally
+obliterated {46}by the growth of the nerve-masses on each side of it.
+Throughout the whole vertebrate kingdom this obliterated portion still
+leaves its mark as the _raphé_ or seam, which is so characteristic of the
+infra-infundibular portion of the brain.
+
+[Illustration: FIG. 22.--HORIZONTAL SECTION THROUGH THE BRAIN OF
+AMMOCOETES.
+
+_Cr._, membranous cranium; _I_, olfactory nerves; _l.v._, lateral
+ventricles; _gl._, glandular tissue which fills up the cranial cavity.]
+
+Here, again, it is seen how simple is the explanation of a peculiarity
+which has always puzzled anatomists--why should there be this seam in the
+infra-infundibular portion of the brain and not in the supra-infundibular
+or in the spinal cord? The corresponding compression in the upper
+brain-region forms the lateral ventricles, as is seen in the accompanying
+figure of the brain of Ammocoetes (Fig. 22).
+
+[Illustration: FIG. 23.--SECTION THROUGH RHOMBOIDAL SINUS OF BIRD.]
+
+In yet another instance it is seen how markedly the nervous masses are
+arranged in the same position with respect to the central tube as are the
+nerve ganglia with respect to the intestinal tube in the case of the
+invertebrate. Thus in birds a portion of the spinal cord in the
+lumbo-sacral region presents a very different appearance from the rest of
+the cord; it is known as the rhomboidal sinus, and a section of the cord of
+an adult pigeon across this region is given in Fig. 23. As is seen, the
+nervous portions are entirely confined to two masses connected together by
+the white anterior commissures which are situated laterally and ventrally
+to a median gelatinous mass; the small central canal is visible and {47}the
+whole dorsal area of the cord is taken up by a peculiar non-nervous
+wedge-shaped mass of tissue. At its first formation this portion of the
+cord is formed exactly in the same manner as the rest of the cord; instead,
+however, of the nervous material invading the dorsal part of the tube to
+form the posterior fissure, it has been from some cause unable to do so,
+the walls of the original non-nervous tube have become thickened dorsally,
+been transformed into this peculiar tissue, and so caused the peculiar
+appearance of the cord here. The nervous parts have not suffered in their
+development; the mechanism for walking in the bird is as well developed as
+in any other animal; their position only is different, for they still
+retain the original ventro-lateral position, but the non-nervous tube, the
+remains of the old intestine, has undergone a peculiar gelatinous
+degeneration just where it has remained free from invasion by the nervous
+tissue.
+
+Throughout the whole of that part of the nervous system which gives origin
+to the cranial and spinal segmental nerves, the evidence is absolutely
+uniform that the nervous material was originally arranged bilaterally and
+ventrally on each side of the central tube, exactly in the same way as the
+nerve-masses of the infra-oesophageal and ventral chain of ganglia are
+arranged with respect to the cephalic stomach and straight intestine of the
+arthropod. But, in addition, we find in the vertebrate nervous masses, the
+cerebral hemispheres, the corpora quadrigemina and the cerebellum situated
+on the dorsal side of the central tube in the brain-region; this nervous
+material is, however, of a different character to that which gives origin
+to the spinal and cranial segmental nerves. How is the presence of these
+dorsal masses to be explained on the supposition that the dilated anterior
+part of the nerve-tube was originally the cephalic stomach of the arthropod
+ancestor? The cerebral hemispheres are simple enough, for they represent
+the supra-oesophageal ganglia, which of necessity, as they increased in
+size, would grow round the anterior end of the cephalic stomach and become
+more and more dorsal in position.
+
+The difficulty lies rather in the position of the cerebellum and corpora
+quadrigemina, and the solution is as simple as it is conclusive.
+
+Let us again turn to embryology and see what help it gives. In all
+vertebrates the dilated anterior portion of the nerve-tube does not, {48}as
+it grows, increase in size uniformly, but a constriction appears on its
+dorsal surface at one particular place, so as to divide it into an anterior
+and posterior vesicle; then the latter becomes divided into two portions by
+a second constriction. In this way three cerebral vesicles are formed;
+these three primary cerebral vesicles indicate the region of the
+fore-brain, mid-brain, and hind-brain respectively. Subsequently the first
+cerebral vesicle becomes divided into two to form the prosencephalon and
+thalamencephalon, while the third cerebral vesicle is also divided into two
+to form the region of the cerebellum and medulla oblongata.
+
+These constrictions are in the position of commissural bands of nervous
+matter; of these the limiting nervous strands between the thalamencephalon
+and mesencephalon and between the mesencephalon and the hind-brain are of
+primary importance. The first of these commissural bands is in the position
+of the posterior commissure connecting the two optic thalami. In close
+connection with this are found, on the mid-dorsal region, the two pineal
+eyes with their optic ganglia, the so-called _ganglia habenulæ_. From these
+ganglia a peculiar tract of fibre, known as Meynert's bundle, passes on
+each side to the ventral infra-infundibular portion of the brain. In other
+words, the first constriction of the dilated tube is due to the presence
+and growth of nervous material in connection with the median pineal eyes.
+Here in precisely the same spot, as will be fully explained in the next
+chapter, there existed in the arthropod ancestor a pair of median eyes
+situated dorsally to the cephalic stomach, the pre-existence of which
+explains the reason for the first constriction.
+
+The second primary constriction separating the mid-brain from the
+hind-brain is still more interesting, for it is coincident with the
+position of the trochlear or fourth cranial nerve. In all vertebrates
+without exception this nerve takes an extraordinary course; all other
+nerves, whether cranial or spinal, pass ventralwards to reach their
+destination. This nerve passes dorsalwards, crosses its fellow mid-dorsally
+in the valve of Vieussens, where the roof of the brain is thin, and then
+passes out to supply the superior oblique muscle of the eye of the opposite
+side. The two nerves form an arch constricting the dilated tube at this
+place. In the lowest vertebrate (Ammocoetes) the constriction formed by
+this nerve-pair is evident not only in the embryonic condition as in other
+vertebrates, but during the whole larval stage. As Fig. 20, A and B, shows,
+the whole of the dorsal {49}region of the brain up to the region of the
+pineal eye and _ganglion habenulæ_ is one large membranous bag, except for
+the single constriction where the fourth nerve on each side crosses over.
+The explanation of this peculiarity is given in Chapter VII., and follows
+simply from the facts of the arrangement of that musculature in the
+scorpion-group which gave rise to the eye-muscles of the vertebrate.
+
+In Ammocoetes both cerebellum and posterior corpora quadrigemina can hardly
+be said to exist, but upon transformation a growth of nervous material
+takes place in this region, and it is seen that this commencing cerebellum
+and the corpora quadrigemina arise from tissue that is present in
+Ammocoetes along the course of the fourth nerve.
+
+Here, then, again Embryology does its best to tell us how the vertebrate
+arose. The formation of the two primary constrictions in the dilated
+anterior vesicle whereby the brain is divided into fore-brain, mid-brain,
+and hind-brain is simply the representation ontogenetically of the two
+nerve-tracts which crossed over the cephalic stomach in the prevertebrate
+stage, in consequence of the mid-dorsal position of the pineal eyes and of
+the insertion of the original superior oblique muscles.
+
+The subsequent constriction by which the prosencephalon is separated from
+the thalamencephalon is in the position of the anterior commissure, that
+commissure which connects the two supra-infundibular nerve-masses, and is
+one of the first-formed commissures in every vertebrate. This naturally is
+simply the commissure between the two supra-oesophageal ganglia; anterior
+to it, in the middle line, equally naturally, the anterior end of the old
+stomach wall still exists as the _lamina terminalis_.
+
+The other division in the hind-brain region, which separates the region of
+the cerebellum from the medulla oblongata, is due to the growth of the
+cerebellum, and indicates its posterior limit. In such an animal as the
+lamprey, where the cerebellum is only commencing, this constriction does
+not occur in the embryo.
+
+From such simple beginnings as are seen in Ammocoetes, the higher forms of
+brain have been evolved, to culminate in that of man, in which the massive
+cerebrum and cerebellum conceals all sign of the dorsal membranous roof,
+those parts of the simple epithelial tube which still remain being tucked
+away into the cavities to form the various choroid plexuses.
+
+{50}In the whole evolution from the brain of Ammocoetes to that of man, the
+same process is plainly visible, viz. growth and extension of nervous
+material over the epithelial tube; extension dorsally and posteriorly of
+the supra-infundibular nervous masses (as seen in Fig. 19), combined with a
+dorsal growth of parts of the infra-infundibular nervous masses to form the
+cerebellum and posterior corpora quadrigemina.
+
+Especially instructive is the formation of the cerebellum. It consists at
+first of a small mass of nervous tissue accompanying the fourth nerve, then
+by the growth of that mass surrounding and constricting a fold of the
+membranous roof, the _worm_ of the cerebellum is formed, as in the
+dog-fish. This very constriction causes the membrane to be thrown into a
+lateral fold on each side, as seen in Fig. 24, and in the dog-fish the
+nervous material on each side, known as the fimbriæ, is already commencing
+to grow from the ventral mass of the medulla oblongata to surround these
+lateral membranous folds. These _fimbriæ_ develop more and more in higher
+forms, and thus form the cerebellar hemispheres.
+
+Not only does comparative anatomy confirm the teachings of embryology, but
+also pathology gives its quota in the same direction.
+
+[Illustration: FIG. 24.--CEREBELLUM OF DOG-FISH.
+
+_v_, worm of cerebellum; _IV._, membranous roof of fourth ventricle
+continuous with the membranous folds on each side. Through these the
+fimbriæ (_fb._) can be dimly seen.]
+
+One of the striking facts about malformations and disease of the central
+nervous system is the frequency of cystic formations; _spina bifida_ is a
+well-known instance. These cysts are merely epithelial non-nervous cysts
+formed from the epithelium of the central canal, difficult to understand if
+the whole nerve tube is one and entirely nervous, either actually or
+potentially, but natural and easy if we are really dealing with a simple
+epithelial tube on the outside of which the nervous material was originally
+grouped. The cystic formation belongs naturally enough to this tube, not to
+the nervous system.
+
+Again, where animals such as lizards have grown a new tail, owing to the
+breaking off of the original one, it is found that the central canal
+extends into this new tail for some distance, but not {51}the nervous
+material surrounding it; all the nerves supplying the new tail arise from
+the uninjured spinal cord above, the central canal with its lining layer of
+epithelial cells alone grows into the new-formed appendage.
+
+To all intents and purposes the same thing is seen in the termination of
+the spinal cord in a bird-embryo; more and more, as the end of the tail is
+approached, does the nervous matter of the spinal cord grow less and less,
+until at last a naked central canal with its lining epithelium is alone
+left to represent the so-called nerve-tube.
+
+All these different methods of investigation lead irresistibly to the one
+conclusion that the tubular nature of the central nervous system has been
+caused by the central nervous system enclosing to a greater or less extent
+a pre-existing, non-nervous, epithelial tube.
+
+This must always be borne strictly in mind. The problem, therefore, which
+presents itself is the comparison of these two factors separately, in order
+to find out the relationship of the vertebrate to the invertebrate. The
+nervous system without the tube must be compared to other nervous systems,
+and the tube must be considered apart from the nervous system.
+
+
+THE PRINCIPLE OF CONCENTRATION AND CEPHALIZATION.
+
+The central nervous system of the vertebrate resembles that of all the
+Appendiculata in the fact that it is composed of segments joined together
+which give origin to segmental nerves. There is, however, a great
+difference between the two systems: the division into separate segments is
+not obvious to the eye in the vertebrate nervous system, while in the
+invertebrate we can see that it is composed of a series of separate pairs
+of ganglia joined together longitudinally by nervous strands known as
+connectives and transversely by the nerve-commissures. Such a simple
+segmented system is found in the segmented worms, and in the lower
+arthropods, such as Branchipus, no great advance has been made on that of
+the annelid. In the higher forms, however, a greater and greater tendency
+to fusion of separate ganglia exists, especially in the head-region, so
+that the infra-oesophageal ganglia, which, in the lower forms are as
+separate as those of the ventral chain, in the higher forms are fused
+together to form a single nervous mass.
+
+{52}This is the great characteristic of the advancement of the central
+nervous system among the Invertebrata, its concentration in the region of
+the head. It may be called the principle of cephalization, and is
+characteristic not only of higher organization in a group, but also of the
+adult as distinguished from the larval form. Thus in the imago greater
+concentration is found than in the caterpillar.
+
+The segmented annelid type of nervous system consists of a
+supra-oesophageal ganglion, composed of the fused ganglia belonging to the
+pre-oral segments, and an infra-oesophageal chain of separate ganglia. With
+the concentration and modification around the mouth of the most anterior
+locomotor appendages to form organs for prehension and mastication of food,
+a corresponding concentration and fusion of the ganglia belonging to these
+segments takes place, so that finally, in the higher annelids, and in most
+of the great arthropod group, a fusion of a number of the most anterior
+ganglia has taken place to form the infra-oesophageal ganglion-mass.
+
+The infra-oesophageal ganglia which are the first to fuse are those which
+supply the most anterior portion of the animal with nerves, and include
+always those anterior appendages which are modified for mastication
+purposes. To this part the name _prosoma_ has been given; in many cases it
+forms a well-defined, distinct portion of the animal.
+
+Succeeding this prosoma or masticatory region, there occurs in all
+gill-bearing arthropods a respiratory region, in many cases more or less
+distinctly defined, which has received the name of _mesosoma._ The rest of
+the body is called the _metasoma_.
+
+In accordance with this nomenclature the central nervous system of many of
+the Arthropoda may be divided as follows:--
+
+1.  Pre-oral, or supra-oesophageal ganglia.
+
+2.  Infra-oral, or infra-oesophageal ganglia and ventral chain, which
+consist of three groups: prosomatic, mesosomatic, and metasomatic ganglia.
+
+The infra-oesophageal ganglion-mass, then, in most of the Arthropoda may be
+spoken of as formed by the fusion of the prosomatic or mouth-ganglia, the
+mesosomatic and metasomatic remaining separate and distinct. The number of
+ganglia which have fused may be observed by examination of the embryo, in
+which it is easy to see indications of the individual ganglia or
+_neuromeres_, although all such indication has disappeared in the adult;
+thus the {53}infra-oesophageal ganglia of the cray-fish have been shown to
+be constituted of six prosomatic ganglia.
+
+In Fig. 25 I give figures of the central nervous system (with the exception
+of the abdominal or metasomatic ganglia) of Branchipus, Astacus, Limulus,
+Scorpio, Androctonus, Thelyphonus, and Ammocoetes. In all the figures the
+supra-oesophageal ganglia are lined horizontally, and their nerves shown,
+viz. optic (lateral eyes (II) and median eyes (II[prime])), olfactory (I)
+(first antennæ, camerostome, nose); then come the prosomatic ganglia
+(dotted), with their nerves (A) supplying the mouth parts, and the second
+antennæ or cheliceræ; then the mesosomatic (lined horizontally), with their
+nerves (B) supplying respiratory appendages. These figures show that the
+concentrated brain mass around the oesophagus of an arthropod which has
+arrived at the stage of Astacus, is represented by the supra-oesophageal
+ganglia and the fused prosomatic ganglia.
+
+The next stage in the evolution of the brain is seen in the gradual
+inclusion of the mesosomatic ganglia, one after the other, into the
+infra-oesophageal mass of the already fused prosomatic ganglia. With this
+fusion is associated the loss of locomotion in these mesosomatic
+appendages, and their entire subservience to the function of respiration.
+Dana urges that cephalization is a consequence of functional alteration in
+the appendages, from organs of locomotion to those of mastication and
+respiration. Whether this be true or not, it is certainly a fact that in
+Limulus, the ganglion supplying the first mesosomatic appendage has fused
+with the prosomatic, infra-oesophageal mass. It is also a fact that the
+prosomatic appendages are the organs of mastication, their basal parts
+being arranged round the mouth so as to act as foot-jaws, while the
+mesosomatic appendages, though still free to move, have been reduced to
+such an extent as to consist mainly of their basal parts, which are all
+respiratory in function, except in the case of the first pair, where they
+carry the terminal ducts of the genital organs. In the next stage, that, of
+the scorpion, in which the mesosomatic appendages have lost all power of
+free locomotion, and have become internal branchiæ, another mesosomatic
+ganglion has fused with the brain mass, while in Androctonus two of the
+branchial mesosomatic ganglia have fused; and finally, in Thelyphonus and
+Phrynus, all the mesosomatic ganglia have coalesced with the fused
+prosomatic ganglia, while the metasomatic ganglia have themselves fused
+together in the caudal region to form what is known as the caudal brain.
+
+{54}[Illustration: FIG. 25.--COMPARISON OF INVERTEBRATE BRAINS FROM
+BRANCHIPUS TO AMMOCOETES.]
+
+{55}The brain in these animals may be spoken of as composed of three
+parts--(1) the fused supra-oesophageal ganglia, (2) the fused prosomatic
+ganglia, and (3) the fused mesosomatic ganglia. Such a brain is strictly
+homologous with the vertebrate brain, which also is built up of three
+parts--(1) the part in front of the notochord, the prechordal or
+supra-infundibular brain, which consists of the cerebral hemispheres,
+together with the basal and optic ganglia and corresponds, therefore, to
+the supra-oesophageal mass, with its olfactory and optic divisions lying in
+front of the oesophagus; (2 and 3) the epichordal brain, composed of (2) a
+trigeminal and (3) a vagus division, of which the first corresponds
+strictly to the fused prosomatic ganglia, and the second to the fused
+mesosomatic ganglia. Further, just as in the embryo of an arthropod it is
+possible, with more or less accuracy, to see the number of neuromeres or
+original ganglia which have fused to form the supra- and infra-oesophageal
+portions of its brain, so also in the embryo of a vertebrate we are able at
+an early stage to gain an indication, more or less accurate, of the number
+of neuromeres which have built up the vertebrate brain. The further
+consideration of these neuromeres, and the evidence they afford as to the
+number of the prosomatic and mesosomatic ganglia which have formed the
+epichordal part of the vertebrate brain, must be left to the chapter on the
+segmentation of the cranial nerves.
+
+The further continuation of this process of concentration of separate
+segments, together with the fusion of the nervous system with the tube of
+the alimentary canal, leads in the simplest manner to the formation of the
+spinal cord of the vertebrate from the metasomatic ganglia of the ventral
+chain of the arthropod.
+
+
+THE ANTAGONISM BETWEEN CEPHALIZATION AND ALIMENTATION.
+
+This concentration of the nervous system in the head-region, together with
+an actual increase in the bulk of the cephalic nervous masses, constitutes
+the great principle upon which the law of upward progress or evolution in
+the animal kingdom is based, and it illustrates in a striking manner the
+blind way in which natural selection works; for, as already explained, the
+central nervous system arose as a ring round the mouth, in consequence of
+which, with the progressive {56}evolution of the animal kingdom, the
+oesophagus necessarily pierced the central nervous system at the cephalic
+end. At the same time, the very fact that the evolution was progressive
+necessitated the concentration and increase of the nervous masses in this
+very same oesophageal region.
+
+Progress on these lines must result in a crisis, owing to the inevitable
+squeezing out of the food-channel by the increasing nerve-mass; and,
+indeed, the fact that such a crisis had in all probability arisen at the
+time when vertebrates first appeared is apparent when we examine the
+conditions at the present time.
+
+Those invertebrates whose central nervous system is most concentrated at
+the cephalic end belong to the arachnid group, among which are included the
+various living scorpion-like animals, such as Thelyphonus, Androctonus,
+etc.
+
+As already mentioned, the giants of the Palæostracan age were Pterygotus,
+Slimonia, etc., all animals of the scorpion-type--in fact, sea-scorpions.
+Now, all these animals, spiders and scorpions, without exception, are
+blood-suckers, and in all of them the concentrated cephalic mass of nervous
+material surrounds an oesophagus the calibre of which is so small that
+nothing but a fluid pabulum can be taken into the alimentary canal; and
+even for that purpose a special suctorial apparatus has in some species
+been formed on the gastric side of the oesophagus for the purpose of
+drawing blood through this exceedingly narrow tube.
+
+In Fig. 25 this increasing antagonism between brain-power and alimentation,
+as we pass from such a form as Branchipus to the scorpion, is illustrated,
+and in Fig. 26 the relative sizes of the oesophagus and the brain-mass
+surrounding it is shown. The section shows that the food channel is
+surrounded by the white and grey matter of the brain as completely as the
+central canal of the spinal cord of the vertebrate is surrounded by the
+white and grey nervous material.
+
+[Illustration: FIG. 26.--TRANSVERSE SECTION THROUGH THE BRAIN OF A YOUNG
+THELYPHONUS.
+
+_A_, supra-oesophageal ganglia; _B_, infra-oesophageal ganglia; _Al_,
+oesophagus.]
+
+{57}Truly, at the time when vertebrates first appeared, the direction and
+progress of variation in the Arthropoda was leading, owing to the manner in
+which the brain was pierced by the oesophagus, to a terrible
+dilemma--either the capacity for taking in food without sufficient
+intelligence to capture it, or intelligence sufficient to capture food and
+no power to consume it.
+
+Something had to be done--some way had to be found out of this difficulty.
+The atrophy of the brain meant degeneration and the reduction to a lower
+stage of organization, as is seen in the Tunicata. The further development
+of the brain necessitated the establishment of a new method of alimentation
+and the closure of the old oesophagus, its vestiges still remaining as the
+infundibular canal of the vertebrate, meant the enormous upward stride of
+the formation of the vertebrate.
+
+At first sight it might appear too great an assumption even to imagine the
+possibility of the formation of a new gut in an animal so highly organized
+as an arthropod, but a little consideration will, I think, show that such
+is not the case.
+
+In the higher animals we are accustomed to speak of certain organs as vital
+and necessary for the further existence of the animal; these are
+essentially the central nervous system, the respiratory system, the
+circulatory system, and the digestive system. Of these four vital systems
+the first cannot be touched without the chance of degeneration; but that is
+not the case with the second. The passage from the fish to the amphibian,
+from the water-breathing to the air-breathing animal, has actually taken
+place, and was effected by the modification of the swim-bladder to form new
+respiratory organs--the lung; the old respiratory organs--the
+gills--becoming functionless, but still persisting in the embryo as
+vestiges. The necessity arose in consequence of the passage of the animal
+from water to land, and with this necessity nature found a means of
+overcoming the difficulty; air-breathing vertebrates arose, and from the
+very fact of their being able to extend over the land-surfaces, increased
+in numbers and developed in complexity in the manner already sketched out.
+
+For a respiratory system all that is required is an arrangement {58}by
+means of which blood should be brought to the surface, so as to interchange
+its gases with those of the external medium; and it is significant to find
+that of all vertebrates the Amphibia alone are capable of an effective
+respiration by means of the skin.
+
+As to the circulatory system, it is exceedingly easily modified. An animal
+such as Amphioxus has no heart; in some the heart is systemic, in others
+branchial; in some there are more than one heart; in others there are
+contractile veins in addition to a heart. There is no difficulty here in
+altering and modifying the system according to the needs of the individual.
+
+For a digestive system all that is required is an arrangement for the
+digestion and absorption of food, a mechanism which can arise easily if
+some of the cells of the skin possess digestive power. Now Miss Alcock has
+shown that some of the surface-cells of crustaceans secrete a fluid which
+possesses digestive powers, and she has also shown that certain of the
+cells in the skin of Ammocoetes possess digestive power.
+
+The difficulty, then, of forming a new digestive system in the passage from
+the arthropod to the vertebrate is very much the same as the difficulty in
+forming a new respiratory system in the passage from the water-breathing
+fish to the air-breathing amphibian--a change which does not strike us as
+inconceivable, because we know it has taken place.
+
+The whole argument so far leads to the conclusion that vertebrates arose
+from ancient forms of arthropods by the formation of a new alimentary
+canal, and the enclosure of the old canal by the growing central nervous
+system. If this conclusion is true, then it follows that we possess a
+well-defined starting-point from which to compare the separate organs of
+the arthropod with those of the vertebrate, and if, in consequence of such
+working hypothesis, each organ of the arthropod is found in the vertebrate
+in a corresponding position and of similar structure, then the truth of the
+starting-point is proved as fully as can possibly be expected by deductive
+methods. It is, in fact, this method of comparative anatomy which has
+proved the descent of man from the ape, the frog from the fish, etc.
+
+Let us, then, compare all the organs of such a low vertebrate as Ammocoetes
+with those of an arthropod of the ancient type.
+
+
+{59}LIFE HISTORY OF THE LAMPREY--NOT A DEGENERATE ANIMAL.
+
+The striking peculiarity of the lamprey is its life-history. It lives in
+fresh water, spending a large portion of its life in the mud during the
+period of its larval existence: then comes a somewhat sudden
+transformation-stage, characterized, as in the lepidopterous larva, by a
+process of histolysis, by which many of the larval tissues are destroyed
+and new ones formed, with the result that the larval lamprey, or
+Ammocoetes, is transformed into the adult lamprey, or Petromyzon. This
+transformation takes place in August, at all events in the neighbourhood of
+Cambridge, and later in the year the transformed lamprey migrates to the
+sea, grows in size and maturity, and returns to the river the following
+spring up to its spawning beds, where it spawns and forthwith dies. How
+long it lives in the Ammocoetes stage is unknown; I myself have kept some
+without transformation for four years, and probably they live in the rivers
+longer than that before they change from their larval state. It is
+absolutely certain that very much the longest part of the animal's life is
+spent in the larval stage, and that with the maturity of the sexual organs
+and the production of the fertilized ova the life of the individual ends.
+
+Now, the striking point of this transformation is that it produces an
+animal more nearly comparable with higher vertebrates than is the larval
+form; in other words, the transformation from larva to adult is in the
+direction of upward progress, not of degeneration. It is, therefore,
+inaccurate to speak of the adult lamprey as degenerate from a higher race
+of fishes represented by its larval form--Ammocoetes. Its transformation
+does not resemble that of the tunicates, but rather that of the frog, so
+that, just as in the case of the tadpole, the peculiarities of its larval
+form may be expected to afford valuable indications of its immediate
+ancestry. The very peculiarities to which attention must especially be paid
+are those discarded at transformation, and, as will be seen, these are
+essentially characteristic of the invertebrate and are not found in the
+higher vertebrates. In fact, the transformation of the lamprey from the
+Ammocoetes to the Petromyzon stage may be described as the casting off of
+many of its ancestral invertebrate characters and the putting on of the
+characteristics of the vertebrate type. It is this double individuality of
+the lamprey, together with its long-continued existence in the larval form,
+which makes Ammocoetes more {60}valuable than any other living vertebrate
+for the study of the stock from which vertebrates sprang.
+
+Many authorities hold the view that the lamprey, like Amphioxus, must be
+looked upon as degenerate, and therefore as no more suitable for the
+investigation of the problem of vertebrate ancestry than is Amphioxus
+itself. This charge of degeneracy is based on the statement that the
+lamprey is a parasite, and that the eyes in Ammocoetes are under the skin.
+The whole supposition of the degeneracy of the Cyclostomata arose because
+of the prevailing belief of the time that the earliest fishes were
+elasmobranchs, and therefore gnathostomatous. From such gnathostomatous
+fishes the cyclostomes were supposed to have descended, having lost their
+jaws and become suctorial in habit in consequence of their parasitism.
+
+The charge of parasitism is brought against the lamprey because it is said
+to suck on to fishes and so obtain nutriment. It is, however, undoubtedly a
+free-swimming fish; and when we see it coming up the rivers in thousands to
+reach the spawning-beds, and sucking on to the stones on the way in order
+to anchor itself against the current, or holding on tightly during the
+actual process of spawning, it does not seem justifiable to base a charge
+of degeneration upon a parasitic habit, when such so-called habit simply
+consists in holding on to its prey until its desires are satisfied. If, of
+course, its suctorial mouth had arisen from an ancestral gnathostomatous
+mouth, then the argument would have more force.
+
+Dohrn, however, gives absolutely no evidence of a former gnathostomatous
+condition either in Petromyzon or, in its larval state, Ammocoetes. He
+simply assumes that the Cyclostomata are degenerated fishes and then
+proceeds to point out the rudiments of skeleton, etc., which they still
+possess. Every point that Dohrn makes can be turned round; and, with more
+probability, it can be argued that the various structures are the
+commencement of the skeletal and other structures in the higher fishes, and
+not their degenerated remnants. Compare the life-history of the lamprey and
+of the tunicate. In the latter case we look upon the animal as a degenerate
+vertebrate, because the larval stage alone shows vertebrate
+characteristics; when transformation has taken place, and the adult form is
+reached, the vertebrate characteristics have vanished, and the animal,
+instead of reaching a higher grade, has sunk lower in the scale, the
+central nervous system especially having lost all {61}resemblance to that
+of the vertebrate. In the former case a transformation also takes place, a
+marvellous transformation, characterized by two most striking facts. On the
+one hand, the resulting animal is more like a higher vertebrate, for, by
+the formation of new cartilages, its cranial skeleton is now comparable
+with that of the higher forms, and the beginnings of the spinal vertebræ
+appear; by the increased formation of nervous material, its brain increases
+in size and complexity, so as to compare more closely with higher
+vertebrate brains; its eyes become functional, and its branchiæ are so
+modified, simultaneously with the formation of the new alimentary canal in
+the cranial region, that they now surround branchial pouches which are
+directly comparable to those of higher vertebrates. On the other hand, the
+transformation process is equally characterized by the throwing off of
+tissues and organs, one and all of which are comparable in structure and
+function with corresponding structures in the Arthropoda--the thyroid of
+the Ammocoetes, the tentacles, the muco-cartilage, the tubular muscles, all
+these structures, so striking in the Ammocoetes stage, are got rid of at
+transformation. Here is the true clue. Here, in the throwing off of
+invertebrate characters, and the taking on of a higher vertebrate form,
+especially a higher brain, not a lower one, Petromyzon proclaims as clearly
+as is possible that it is not a degenerate elasmobranch, but that it has
+arisen from Ammocoetes-like ancestors, even though Myxine, Amphioxus, and
+the tunicates be all stages on the downward grade from those same
+Ammocoetes-like ancestors.
+
+As to the eyes, they are functional in the adult form and as serviceable as
+in any fish. There is no sign of degeneracy; it is only possible to speak
+of a retarded development which lasts through the larval stage.
+
+
+COMPARISON OF BRAIN OF AMMOCOETES WITH THAT OF AN ARTHROPOD.
+
+Seeing that the steady progress of the development of the central nervous
+system is the most important factor in the evolution of animals, it follows
+that of all organs of the body, the central nervous system must be most
+easily comparable with that of the supposed ancestor. I will, therefore,
+start by comparing the brain of Ammocoetes with that of arthropods,
+especially of Limulus and of the scorpion-group.
+
+{62}The supra-infundibular portion of the brain in vertebrates corresponds
+clearly to the supra-oesophageal portion of the invertebrate brain in so
+far that in both cases here is the seat of the will. Voluntary action is as
+impossible to the arthropod deprived of its supra-oesophageal ganglia as to
+the vertebrate deprived of its cerebrum. It corresponds, also, in that from
+it arise the nerves of sight and smell and no other nerves; this is also
+the case with the supra-oesophageal ganglia, for from a portion of these
+ganglia arise the nerves to the eyes and the nerves to the first antennæ,
+of which the latter are olfactory in function. Thus, in the accompanying
+figure, taken from Bellonci, it is seen that the supra-oesophageal ganglia
+consist of a superior segment corresponding to the cerebrum, a middle
+segment from which arise the nerves to the lateral eyes and to the
+olfactory antennæ, corresponding to the basal ganglia of the brain and the
+optic lobes, and, according to Bellonci, of an inferior segment from which
+arise the nerves to the second pair of antennæ. This last segment is not
+supra-oesophageal in position, but is situated on the oesophageal
+commissures. It has been shown by Lankester and Brauer in Limulus and the
+scorpion to be in reality the first ganglion of the infra-oesophageal
+series, and not to belong to the supra-oesophageal group.
+
+[Illustration: FIG. 27.--THE BRAIN OF _Sphæroma serratum_. (After
+BELLONCI.)
+
+_Ant. I._ and _Ant. II._, nerves to 1st and 2nd antennæ. _f.br.r._,
+terminal fibre layer of retina; _Op. g. I._, first optic ganglion; _Op. g.
+II._, second optic ganglion; _O.n._, optic nerve-fibres forming an optic
+chiasma.]
+
+Further, in Limulus, in the scorpion-group, and in all the extinct
+{63}Eurypteridæ--in fact, in the Palæostraca generally--there are two
+median eyes in addition to the lateral eyes, which were innervated from
+these ganglia.
+
+In Ammocoetes, then, if the supra-infundibular portion of the brain really
+corresponds to the supra-oesophageal of the palæostracan group, we ought to
+find, as indeed is the case, an optic apparatus consisting of two lateral
+eyes and two median eyes, innervated from the supra-infundibular
+brain-mass, and an olfactory apparatus built up on the same lines as in the
+scorpion-group, also innervated from this region. If, in addition, it be
+found that those two median eyes are degenerate eyes of the same type as
+the median eyes of Limulus and the scorpion-group, then the evidence is so
+strong as to amount to a proof of the correctness of the theory. This
+evidence is precisely what has been obtained in recent years, for the
+vertebrate did possess two median eyes in addition to the two lateral ones,
+and these two median eyes are degenerate eyes of the type found in the
+median eyes of arthropods and are not of the vertebrate type. Moreover, as
+ought also to be the case, they are most evident, and one of the pair is
+most nearly functional in the lowest perfect vertebrate, Ammocoetes.
+
+Of all the discoveries made in recent years, the discovery that the pineal
+gland of the vertebrate brain was originally a pair of median eyes is by
+far the most important clue to the ancestry of the vertebrate, for not only
+do they correspond exactly in position with the median eyes of the
+invertebrates, but, being already degenerate and functionless in the lowest
+vertebrate, they must have been functional in a pre-vertebrate stage, thus
+giving the most direct clue possible to the nature of the pre-vertebrate
+stage. It is especially significant that in Limulus they are already
+partially degenerated. What, then, ought to be the structure and relation
+to the brain of the median and lateral eyes of the vertebrate if they
+originated from the corresponding organs of some one or other member of the
+palæostracan group?
+
+This question will form the subject of the next chapter.
+
+
+SUMMARY.
+
+  The object of this book is to attempt to find out from what group of
+  invertebrates the vertebrate arose; no attempt is made to speculate upon
+  the causes of variation by means of which evolution takes place.
+
+  {64}A review of the animal kingdom as a whole leads to the conclusion
+  that the upward development of animals from an original coelenterate
+  stock, in which the central nervous system consists of a ring of nervous
+  material surrounding the mouth, has led, in consequence of the
+  elaboration of the central nervous system, to a general plan among the
+  higher groups of invertebrates in the topographical arrangement of the
+  important organs. The mouth is situated ventrally, and leads by means of
+  the oesophagus into an alimentary canal which is situated dorsally to the
+  central nervous system. Thus the oesophagus pierces the central nervous
+  system and divides it into two parts, the supra-oesophageal ganglia and
+  the infra-oesophageal ganglia. This is an almost universal plan among
+  invertebrates, but apparently does not hold for vertebrates, for in them
+  the central nervous system is always situated dorsally and the alimentary
+  canal ventrally, and there is no piercing of the central nervous system
+  by an oesophagus.
+
+  Yet a remarkable resemblance exists between the central nervous system of
+  the vertebrate and that of the higher invertebrates, of so striking a
+  character as to compel one school of anatomists to attempt the derivation
+  of vertebrates from annelids. Now, the central nervous system of
+  vertebrates forms a hollow tube, and a diverticulum of this hollow tube,
+  known as the infundibulum, passes to the ventral surface of the brain in
+  the very position where the oesophagus would have been if that brain had
+  belonged to an annelid or an arthropod. This school of anatomists
+  therefore concluded that this infundibular tube represented the original
+  invertebrate oesophagus which had become closed and no longer opened into
+  the alimentary canal owing to the formation of a new mouth in the
+  vertebrate. As, however, the alimentary canal of the vertebrate is
+  ventral to the central nervous system, and not dorsal, as in the
+  invertebrate, it follows that the remains of the original invertebrate
+  mouth into which the oesophagus (in the vertebrate the infundibular tube)
+  must have opened must be searched for on the dorsal side of the
+  vertebrate; and so the theory was put forward that the vertebrate had
+  arisen from the annelid by the reversal of surfaces, the back of the one
+  animal becoming the front of the other.
+
+  The difficulties in the way of accepting such reversal of surfaces have
+  proved insuperable, and another school has arisen which suggests that
+  evolution has throughout proceeded on two lines, the one forming groups
+  of animals in which the central nervous system is pierced by the
+  food-channel and the gut therefore lies dorsally to it, the other in
+  which the central nervous system always lies dorsally to the alimentary
+  canal and is not pierced by it. In both cases the highest products of the
+  evolution have become markedly segmented animals, in the former, annelids
+  and arthropods; in the latter, vertebrates. The only evidence on which
+  such theory is based is the existence of low forms of animals, known as
+  the _Enteropneusta_, the best known example of which is called
+  _Balanoglossus_; they are looked upon as aberrant annelid forms by many
+  observers.
+
+  This theory does not attempt to explain the peculiarities of the tube of
+  the vertebrate central nervous system, or to account for the
+  extraordinary resemblance between the structure and arrangement of the
+  central nervous systems of vertebrates and of the highest invertebrate
+  group.
+
+  Neither of these theories is satisfactory or has secured universal
+  acceptance. The problem must be considered entirely anew. What are the
+  guiding principles in this investigation?
+
+  {65}The evolution of animal life on this earth can clearly, on the whole,
+  be described as a process of upward progress culminating in the highest
+  form--man; but it must always be remembered that whole groups of animals
+  such as the Tunicata have been able to survive owing to a reverse process
+  of degeneration.
+
+  If there is one organ more than another which increases in complexity as
+  evolution proceeds, which is the most essential organ for upward
+  progress, surely it is the central nervous system, especially that
+  portion of it called the brain. This consideration points directly to the
+  origin of vertebrates from the most highly organized invertebrate
+  group--the Arthropoda--for among all the groups of animals living on the
+  earth in the present day they alone possess a central nervous system
+  closely comparable with that of vertebrates. Not only has an upward
+  progress taken place in animals as a whole, but also in the tissues
+  themselves a similar evolution is apparent, and the evidence shows that
+  the vertebrate tissues resemble more closely those of the arthropod than
+  of any other invertebrate group.
+
+  The evidence of geology points to the same conclusion, for the evidence
+  of the rocks shows that before the highest mammal--man--appeared, the
+  dominant race was the mammalian quadruped, from whom the highest mammal
+  of all--man--sprung; then comes, in Mesozoic times, the age of reptiles
+  which were dominant when the mammal arose from them. Preceding this era
+  we find in Carboniferous times that the amphibian was dominant, and from
+  them the next higher group--the reptiles--arose. Below the Carboniferous
+  come the Devonian strata with their evidence of the dominance of the
+  fish, from whom the amphibian was directly evolved. The evidence is so
+  clear that each succeeding higher form of vertebrate arose from the
+  highest stage reached at the time, as to compel one to the conclusion
+  that the fishes arose from the race which was dominant at the time when
+  the fishes first appeared. This brings us to the Silurian age, in which
+  the evidence of the rocks points unmistakably to the sea-scorpions,
+  king-crabs, and trilobites as being the dominant race. It was preceded by
+  the great trilobite age, and the whole period, from the first appearance
+  of the trilobite to the time of dwindling away of the sea-scorpions, may
+  be designated the Palæostracan age, using the term Palæostraca to include
+  both trilobites and the higher scorpion and king-crab forms evolved from
+  them. The evidence of geology then points directly and strongly to the
+  origin of vertebrates from the Palæostraca--arthropod forms which were
+  not crustacean and not arachnid, but gave origin both to the modern-day
+  crustaceans and arachnids. The history of the rocks further shows that
+  these ancient fishes, when they first appeared, resembled in a remarkable
+  manner members of the palæostracan group, so that again and again
+  palæontologists have found great difficulty in determining whether a
+  fossil is a fish or an arthropod. Fortunately, there is still alive on
+  the earth one member of this remarkable group--the Limulus, or King-Crab.
+  On the vertebrate side the lowest non-degenerate vertebrate is the
+  lamprey, or Petromyzon, which spends a large portion of its existence in
+  a larval stage, known as the Ammocoetes stage of the lamprey, because it
+  was formerly considered to be a separate species and received the name of
+  Ammocoetes. The larval stages of any animal are most valuable for the
+  study of ancestral problems, so that it is most fortunate for the
+  solution of the ancestry of vertebrates that Limulus on the one side and
+  Ammocoetes on the other are {66}available for thorough investigation and
+  comparison. There are no trilobites still alive, but in Branchipus and
+  Apus we possess the nearest approach to the trilobite organization among
+  living crustaceans.
+
+  So strongly do all these different lines of argument point to the origin
+  of vertebrates from arthropods as to make it imperative to reconsider the
+  position of that school of anatomists who derived vertebrates from
+  annelids by reversing the back and front of the animal. Let us not turn
+  the animal over, but re-consider the position, the infundibular tube of
+  the vertebrate still representing the oesophagus of the invertebrate, the
+  cerebral hemispheres and basal ganglia the supra-oesophageal ganglia, the
+  _crura cerebri_ the oesophageal commissures, and the infra-infundibular
+  part of the brain the infra-oesophageal ganglia. It is immediately
+  apparent that just as the invertebrate oesophagus leads into the large
+  cephalic stomach, so the infundibular tube leads into the large cavity of
+  the brain known as the third ventricle, which, together with the other
+  ventricles, forms in the embryo a large anterior dilated part of the
+  neural tube. In the arthropod this cephalic stomach leads into the
+  straight narrow intestine; in the vertebrate the fourth ventricle leads
+  into the straight narrow canal of the spinal cord. In the arthropod the
+  intestine terminates in the anus; in the vertebrate embryo the canal of
+  the spinal cord terminates in the anus by way of the neurenteric canal.
+  Keep the animal unreversed, and immediately the whole mystery of the
+  tubular nature of the central nervous system is revealed, for it is seen
+  that the nervous matter, which corresponds bit by bit with that of the
+  arthropod, has surrounded to a greater or less extent and amalgamated
+  with the tube of the arthropod alimentary canal, and thus formed the
+  so-called central nervous system of the vertebrate.
+
+  The manner in which the nervous material has invaded the walls of the
+  tube is clearly shown both by the study of the comparative anatomy of the
+  central nervous system in the vertebrate and also by its development in
+  the embryo.
+
+  This theory implies that the vertebrate alimentary canal is a new
+  formation necessitated by the urgency of the case, and, indeed, there was
+  cause for urgency, for the general plan of the evolution of the
+  invertebrate from the coelenterate involved the piercing of the anterior
+  portion of the central nervous system by the oesophagus, while, at the
+  same time, upward progress meant brain-development; brain-development
+  meant concentration of nervous matter at the anterior end of the animal,
+  with the result that in the highest scorpion and spider-like animals the
+  brain-mass has so grown round and compressed the food-tube that nothing
+  but fluid pabulum can pass through into the stomach; the whole group have
+  become blood-suckers. These kinds of animals--the sea-scorpions--were the
+  dominant race when the vertebrates first appeared: here in the natural
+  competition among members of the dominant race the difficulty must have
+  become acute. Further upward evolution demanded a larger and larger brain
+  with the ensuing consequence of a greater and greater difficulty of
+  food-supply. Nature's mistake was rectified and further evolution
+  secured, not by degeneration in the brain-region, for that means
+  degradation not upward progress, but by the formation of a new
+  food-channel, in consequence of which the brain was free to develop to
+  its fullest extent. Thus the great and mighty kingdom of the Vertebrata
+  was evolved with its culminating organism--man--whose massive brain with
+  all its possibilities could never have been evolved if he had still been
+  {67}compelled to pass the whole of his food through the narrow
+  oesophageal tube, still existent in him as the infundibular tube. This,
+  then, is the working hypothesis upon which this book is written. If this
+  view is right, that the Vertebrate was formed from the Palæostracan
+  without any reversal of surfaces, but by the amalgamation of the central
+  nervous system and alimentary canal, then it follows that we have various
+  fixed points of comparison in the central nervous systems of the two
+  groups of animals from which to search for further clues. It further
+  follows that from such starting-point every organ of importance in the
+  body of the arthropod ought to be visible in the corresponding position
+  in the vertebrate, either as a functional or rudimentary organ. The
+  subsequent chapters will deal with this detailed comparison of organs in
+  the arthropod and vertebrate respectively.
+
+
+
+
+{68}CHAPTER II
+
+_THE EVIDENCE OF THE ORGANS OF VISION_
+
+  Different kinds of eye.--Simple and compound retinas.--Upright and
+  inverted retinas.--Median eyes.--Median or pineal eyes of Ammocoetes and
+  their optic ganglia.--Comparison with other median eyes.--Lateral eyes of
+  vertebrates compared with lateral eyes of crustaceans.--Peculiarities of
+  the lateral eye of the lamprey.--Meaning of the optic
+  diverticula.--Evolution of vertebrate eyes.--Summary.
+
+
+THE DIFFERENT KINDS OF EYE.
+
+In all animals the eyes are composed of two parts. 1. A set of special
+sensory cells called the retina. 2. A dioptric apparatus for the purpose of
+forming an image on the sensory cells. The simplest eye is formed from a
+modified patch of the surface-epithelium; certain of the hypodermal cells,
+as they are called, elongate, and their cuticular surface becomes bulged to
+form a simple lens. These elongated cells form the retinal cells, and are
+connected with the central nervous system by nerve-fibres which constitute
+an optic nerve; the cells themselves may contain pigment.
+
+The more complicated eyes are modifications of this type for the purpose of
+making both the retina and the dioptric apparatus more perfect. According
+to a very prevalent view, these modifications have been brought about by
+invaginations of the surface-epithelium. Thus if ABCD (Fig. 28) represents
+a portion of the surface-epithelium, the chitinous cuticle being
+represented by the dark line, with the hypodermal cells beneath, and if the
+part C is modified to form an optic sense-plate, then an invagination
+occurring between A and B will throw the retinal sense-cells with the optic
+nerve further from the surface, and the layers B and A between the retina
+and the source of light will be available for the formation of the dioptric
+apparatus. The lens is now formed from the cuticular surface of A, and the
+{69}hypodermal cells of A elongate to form the layer known by the name of
+corneagen, or vitreogen, the cells of B remaining small and forming the
+pre-retinal layer of cells. The large optic nerve end-cells of the retinal
+layer, C, take up the position shown in the figure, and their cuticular
+surface becomes modified to form rods of varying shape called rhabdites,
+which are attached to the retinal cells. Frequently the rhabdites of
+neighbouring cells form definite groups, each group being called a
+rhabdome. Whatever shape they take it is invariably found that these little
+rods (bacilli), or rhabdites, are modifications of the cuticular surface of
+the cells which form the retinal layer. Also, as must necessarily be the
+case from the method of formation, the optic nerve arises from the nuclear
+end of the retinal cells, never from the bacillary end. As in the case
+first mentioned, so in this case, the light strikes direct upon the
+bacillary end of the retinal cells; such eyes, therefore, are eyes with an
+_upright retina_.
+
+[Illustration: FIG. 28.--DIAGRAM OF FORMATION OF AN UPRIGHT SIMPLE RETINA.]
+
+
+It may happen that the part invaginated is the optic sense-plate itself, as
+would be the case if in the former figure, instead of C, the part B was
+modified to form a sense-plate. This will give rise to an eye of a
+character different from the former (Fig. 29). The optic nerve-fibres now
+lie between the source of light and the retinal end-cells, the layer A as
+before forms the cuticular lens, and its hypodermal cells elongate to form
+the corneagen; there is no pre-retinal layer, but, on the contrary, a
+post-retinal layer, C, called the tapetum, and, as is seen, the light
+passes through the retinal layer to the {70}tapetum. The cuticular surface
+of the retinal cells forming the rods or bacilli is directed towards the
+tapetal layer away from the source of light, and the nuclei of the retinal
+cells are pre-bacillary in position, in contradistinction to the upright
+eye, where they are post-bacillary. The retinal end-cells are devoid of
+pigment, the pigment being in the tapetal layer.
+
+Such an eye, in contradistinction to the former type, is an eye with an
+_inverted retina_; but still the same law holds as in the former case--the
+optic nerve-fibres enter at the nuclear ends of the cells, and the rods are
+formed from the cuticular surface.
+
+In these eyes the pigmented tapetal layer is believed to act as a
+looking-glass; the dioptric apparatus throws the image on to its shiny
+surface, from whence it is reflected directly on to the rods, which are in
+close contact with the tapetum. A similar process has been suggested in the
+case of the mammalian lateral eye, with its inverted retina. Johnson
+describes the post-retinal pigmented layer as being frequently coloured and
+shiny, and imagines that it reflects the image directly back on to the
+rods.
+
+[Illustration: FIG. 29.--DIAGRAM OF FORMATION OF AN INVERTED SIMPLE RETINA.
+
+The arrow shows the direction of the source of light in this as in the
+preceding figure. In both figures the cuticular rhabdites are represented
+by thick black lines.]
+
+Thus we see that eyes can be placed in different categories, _e.g._ those
+with an upright retina and those with an inverted retina; also, according
+to the presence or absence of a tapetum, eyes have been grouped as tapetal
+or non-tapetal. All the eyes considered so far are called simple eyes, or
+ocelli; and a number of ocelli may be {71}contiguous though separate, as in
+the lateral eyes of the scorpion. They may, however, come into close
+contact and form one single, large, compound eye. Such ocelli, in a very
+large number of cases, retain each its own dioptric apparatus, and
+therefore the external appearance of the compound eye represents not a
+single lens, but a large number of facets, as is seen in the eyes of
+insects. Owing to these differences, eyes have been divided into simple and
+compound, and into facetted and non-facetted.
+
+Yet another complication occurs in the formation of eyes, which is,
+perhaps, the most important of all: the retinal portion of the eye, instead
+of consisting of simple retinal cells, with their accompanying rhabdites,
+may include within itself a portion of the central nervous system.
+
+The rationale of such a formation is as follows: The external covering of
+the body is formed by a layer of external epithelial cells--the ectodermal
+cell-layer--and an underlying neural layer, of which the latter gives
+origin to the central nervous system. As development proceeds, this central
+nervous system sinks inwards, leaving as its connection with the ectoderm
+the sensory nerves of the skin. That part of the neural layer which
+underlies the optic plate forms the optic ganglion, and when the central
+nervous system leaves the surface to take up its deeper position, the
+strand of nerve-fibres known as the optic nerve, is left connecting it with
+the retinal cells as seen in Figs. 28, 29. It may, however, happen that
+part of the optic ganglion remains at the surface, in close connection with
+the retinal end-cells, when the rest of the central nervous system sinks
+inwards. The retina of such an eye is composed of the combined optic
+ganglion and retinal end-cells; the strand of nerve-fibres which is left as
+the connection between it and the rest of the brain, which is also called
+the optic nerve, is not a true peripheral nerve, as in the first case, but
+rather a tract of fibres connecting two parts of the brain, of which one
+has remained at the periphery. Such a retina, in contradistinction to the
+first kind, may be called a _compound retina_.
+
+The optic ganglion, as seen in eyes with a simple retina, consists of a
+cortical layer of small, round nerve-cells, and an internal medulla of fine
+nerve-fibres, which form a thick network known as 'Punctsubstanz,' or in
+modern terminology, 'Neuropil.' Fibres which pass into this 'neuropil' from
+other parts of the brain connect them with the optic ganglion.
+
+{72}At the present time, owing to the researches of Golgi, Ramón y Cajal,
+and others, the nervous system is considered to be composed of a number of
+separate nerve-units, called neurones, each neurone consisting of a
+nerve-cell with its various processes; one of these--the
+neuraxon--constitutes the nerve-fibre belonging to that nerve-cell, the
+other processes--the dendrites--establish communication with other
+neurones. The place where these processes come together is called a
+synapse, and the tangle of fine fibres formed at a number of synapses forms
+the 'neuropil.'
+
+[Illustration: FIG. 30.--DIAGRAM OF FORMATION OF AN UPRIGHT COMPOUND
+RETINA.
+
+_ABCD_, as in Fig. 28. _Op. g. I._ and _Op. g. II._, two optic ganglia
+which combine to form the retinal ganglion, _Rt. g._]
+
+When, therefore, a compound retina is formed by the amalgamation of the
+ectodermal part--the retinal cells proper--with the neurodermic part--to
+which the name 'retinal ganglion' may be given,--such a retina consists of
+neuropil substance and nerve-cells, as well as the retinal end-cells. In
+all such compound retinas, the retinal ganglion is not single, but two
+optic ganglia at least are included in it, so that there are two sets of
+nerve-cells and two synapses are always formed; one between the retinal
+end-cells and the neurones of the first optic ganglion, which may be called
+the ganglion of the retina, the other between the first and second ganglia,
+which, seeing that the neuraxons of its cells form the optic nerve, may be
+called the ganglion of the optic nerve. The 'neuropil' formed by these
+synapses forms the molecular layers of the compound retina, and the cells
+themselves form the nuclear layers. Thus an upright compound retina, formed
+in the same way as the upright simple retina, would be illustrated by Fig.
+30.
+
+{73}Further, in precisely the same way as in the case of the simple retina,
+such a compound retina may be upright or inverted. Thus, in the lateral
+eyes of crustaceans and insects, a compound retina of this kind is formed,
+which is upright; while in the vertebrates the compound retina of the
+lateral eyes is inverted.
+
+The compound retina of vertebrates is usually described as composed of a
+series of layers, which may be analyzed into their several components as
+follows:--
+
+  Layer of rods and cones  }
+  External nuclear layer   } retina proper          } Ectodermic part
+  External molecular layer }}
+  Internal nuclear layer    } ganglion of retina    }
+  Internal molecular layer }}                       } retinal } neurodermic
+  Optic nerve-cell layer   } ganglion of optic nerve} ganglion}   part
+  Layer of optic nerve     }
+    fibres
+
+The difference between the development of these two types of eye--those
+with a simple retina and those with a compound retina--has led, in the most
+natural manner, to the conception that the retina is developed, in the
+higher animals, sometimes from the cells of the peripheral epidermis,
+sometimes from the tissue of the brain--two modes of development termed by
+Balfour 'peripheral' and 'cerebral.' An historical survey of the question
+shows most conclusively that all investigators are agreed in ascribing the
+origin of the simple retina to the peripheral method of development, the
+retina being formed from the hypodermal cells by a process of invagination,
+while the cerebral type of development has been described only in the
+development of the compound retina. The natural conclusion from this fact
+is that, in watching the development of the compound retina, it is more
+difficult to differentiate the layers formed from the epidermal retinal
+cells and those formed from the epidermal optic ganglion-cells, than in the
+case of the simple retina, where the latter cells withdraw entirely from
+the surface. This is the conclusion to which Patten has come, and, indeed,
+judging from the text-book of Korschelt and Heider, it is the generally
+received opinion of the day that, as far as the Appendiculata are
+concerned, the retina, in the true sense--the retinal end-cells, with their
+cuticular rods,--is formed, in all cases, from the peripheral cells of the
+hypodermal layer, the cuticular rods being modifications of the general
+cuticular surface of the body. The apparent cerebral development of the
+crustacean {74}retina, as quoted from Bobretsky by Balfour, is therefore in
+reality the development of the retinal ganglion, and not of the retina
+proper.
+
+There is, I imagine, a universal belief that the natural mode of origin of
+a sense-organ, such as the eye, must always have been from the cells
+forming the external surface of the animal, and that direct origin from the
+central nervous system is _a priori_ most improbable. It is, therefore, a
+matter of satisfaction to find that the evidence for the latter origin has
+universally broken down, with the single exception of the eyes of
+vertebrates and their degenerated allies; a fact which points strongly to
+the probability that a reconsideration of the evidence upon which the
+present teaching of the origin of the vertebrate eye is based will show
+that here, too, a confusion has arisen between that part formed from the
+epidermal surface and that from the optic ganglion.
+
+
+THE MEDIAN OR PINEAL EYES.
+
+Undoubtedly, in recent times, the most important clue to the ancestry of
+vertebrates has been given by the discovery that the so-called pineal gland
+in the vertebrate brain is all that remains of a pair of median or pineal
+eyes, the existence of which is manifest in the earliest vertebrates; so
+that the vertebrate, when it first arose, possessed a pair of median eyes
+as well as a pair of lateral eyes. The ancestor of the vertebrate,
+therefore, must also have possessed a pair of median eyes as well as a pair
+of lateral eyes.
+
+Very instructive, indeed, is the evidence with regard to these median eyes,
+for one of the great characteristics of the ancient palæostracan forms is
+the invariable presence of a pair of median eyes as well as a pair of
+lateral eyes. In the living representative of such forms--Limulus--the pair
+of median eyes (Fig. 5) is well shown, and it is significant that here,
+according to Lankester and Bourne, these eyes are already in a condition of
+degeneration; so also in many of the Palæostraca (Fig. 7) the lateral eyes
+are the large, well-developed eyes, while the median eyes resemble those of
+Limulus in their insignificance.
+
+We see, then, that in the dominant arthropod race at the time when the
+fishes first appeared, the type of eyes consisted of a pair of
+well-developed lateral eyes and a pair of insignificant, partially
+degenerated, median eyes. Further, according to all palæontologists, {75}in
+the best-preserved head-shields of the most ancient fishes, especially well
+seen in the Osteostraci, in Cephalaspis, Tremataspis, Auchenaspis,
+Keraspis, a pair of large, prominent lateral eyes existed, between which,
+in the mid-line, are seen a pair of small, insignificant median eyes.
+
+The evidence of the rocks, therefore, proves that the pair of median eyes
+which were originally the principal eyes (Hauptaugen), had already, in the
+dominant arthropod group been supplanted by a pair of lateral eyes, and
+had, in consequence, become small and insignificant, at the time when
+vertebrates first appeared. This dwindling process thus initiated in the
+arthropod itself has steadily continued ever since through the whole
+development of the vertebrates, with the result that, in the highest
+vertebrates, these median or pineal eyes have become converted into the
+pineal gland with its 'brain-sand.'
+
+In the earliest vertebrate these median eyes may have been functional; they
+certainly were more conspicuous than in later forms. Alone among living
+vertebrates the right median eye of Ammocoetes is so perfect and the skin
+covering it so transparent that I have always felt doubtful whether it may
+not be of use to the animal, especially when one takes into consideration
+the undeveloped state of the lateral eyes in this animal, hidden as they
+are under the skin. Thus the one living vertebrate which is comparable with
+these extinct fishes is the one in which one of the pineal eyes is most
+well defined, most nearly functional.
+
+Before passing to the consideration of the structure of the median eyes of
+Ammocoetes, it is advisable to see whether these median eyes in other
+animals, such as arachnids and crustaceans, belong to any particular type
+of eyes, for then assuredly the median eyes of Ammocoetes ought to belong
+to the same type if they are derived from them.
+
+In the specialized crustacean, as in the specialized vertebrate, the median
+eyes have disappeared, at all events in the adult, but still exist in the
+primitive forms, such as Branchipus, which resemble the trilobites in some
+respects. On the other hand, the median eyes have persisted, and are well
+developed in the arachnids, both scorpions and spiders possessing a
+well-developed pair. The characteristics of the median eyes must then be
+especially sought for in the arachnid group.
+
+Both scorpions and spiders possess many eyes, of which two are {76}always
+separate and median in position, while the others form lateral groups; all
+these eyes possess a simple retina and a simple corneal lens. Grenacher was
+the first to point out that in the spiders two very distinct types of eye
+are found. In the one the retina is upright; in the other the retina is
+inverted, and the eye possesses a tapetal layer. The distribution of these
+two types is most suggestive, for the inverted retina is always found in
+the lateral eyes, never in the two median eyes; these always possess a
+simple upright retina.
+
+In the crustaceans, the lateral eyes differ also from the median eyes, but
+not in the same way as in the arachnids; for here both types of eye possess
+an upright retina, but the retina of the lateral eyes is compound, while
+that of the median eyes is simple. In other words, the median eyes are in
+all cases eyes with a simple upright retina and a simple cuticular lens,
+while the retina of the lateral eyes is compound or may be inverted,
+according as the animal in question possesses crustacean or arachnid
+affinities. The lateral eye of the vertebrate, possessing, as it does, an
+inverted compound retina, indicates that the vertebrate arose from a stock
+which was neither arachnid nor crustacean, but gave rise to both groups--in
+fact, was a member of the great palæostracan group. What, then, is the
+nature of the median eyes in the vertebrate?
+
+
+THE MEDIAN EYES OF AMMOCOETES.
+
+The evidence of Ammocoetes is so conclusive that I, for one, cannot
+conceive how it is possible for any zoologist to doubt whether the parietal
+organ, as they insist on calling it, had ever been an eye, or rather a pair
+of eyes.
+
+Anyone who examines the head of the larval lamprey will see on the dorsal
+side, in the median line, first, a somewhat circular orifice--the unpaired
+nasal opening; and then, tailwards to this, a well-marked circular spot,
+where the skin is distinctly more transparent than elsewhere. This spot
+coincides in position with the underlying dorsal pineal eye, which shines
+out conspicuously owing to the glistening whiteness of its pigment. Upon
+opening the brain-case the appearance as in Fig. 20 is seen, and the mass
+of the right _ganglion habenulæ_ (_G.H.R._), as it has been called, stands
+out conspicuously as well as the right or dorsal pineal eye (_Pn._). Both
+eye and ganglion appear at first sight to be one-sided, but further
+examination shows that a left _ganglion habenulæ_ is present, though much
+smaller than on {77}the right side. In connection with this is another
+eye-like organ--the left or ventral pineal eye,--much more aborted, much
+less like an eye than the dorsal one; so also there are two bundles of
+peculiar fibres called Meynert's bundles, which connect this region with
+the infra-infundibular region of the brain; of these, the right Meynert's
+bundle is much larger than the left.
+
+[Illustration: FIG. 31.--ONE OF A SERIES OF HORIZONTAL SECTIONS THROUGH THE
+HEAD OF AMMOCOETES.
+
+_l.m._, upper lip muscles; _m.c._, muco-cartilage; _n._, nose; _na.c._,
+nasal cartilage; _pn._, right pineal eye and nerve; _g.h.r._, right
+_ganglion habenulæ_; _s.m._, somatic muscles; _cr._, membranous wall of
+cranium; _ch._, choroid plexus; _gl._, glandular substance and pigment
+filling up brain-case.]
+
+{78}[Illustration: FIG. 32.--EYE OF ACILIUS LARVA, WITH ITS OPTIC GANGLION.
+
+On the right side the nerve end-cells have been drawn free from pigment.]
+
+[Illustration: FIG. 33.--PINEAL EYE OF AMMOCOETES, WITH ITS _Ganglion
+Habenulæ._
+
+On the left side the eye is drawn as it appeared in the section. On the
+right side I have removed the pigment from the nerve end-cells, and drawn
+the eye as, in my opinion, it would appear if it were functional.]
+
+This difference between right and left indicates a greater degeneration on
+the left side, and points distinctly to a close relationship between the
+nerve-masses known as _ganglia habenulæ_ and the median eyes. In my opinion
+this ganglion is, in part, at all events, the optic ganglion of the median
+eye on each side. It is built up on the same type as the optic ganglia of
+invertebrate simple eyes, with a cortex of small round cells and a medulla
+of fine nerve-fibres. Into this ganglion, on the right side, there passes a
+very well-defined nerve--the nerve of the dorsal eye. The eye itself with
+its nerve, _pn._, and its optic ganglion, _g.h.r._, is beautifully shown by
+means of a horizontal section through the head of Ammocoetes (Fig. 31).
+Originally, as described by Scott, the eye stood vertically {79}above its
+optic ganglion, and presented an appearance remarkably like Fig. 32, which
+represents one of the simple eyes and optic ganglia of a larva of Acilius
+as described by Patten; then, with the forward growth of the upper lip, the
+right pineal eye was dragged forward and its nerve pulled horizontally over
+the _ganglion habenulæ_. For this reason the eye, nerve, and ganglion are
+better shown in a nearly horizontal than in a transverse section.
+
+The optic nerve belonging to this eye is most evident and clearly shown in
+Fig. 31, and in the series of consecutive sections which follow upon this
+section; no doubt can arise as to the structure in question having been the
+nerve of the eye, even though, as is possible, it does not contain any
+functional nerve-fibres.
+
+[Illustration: FIG. 34.--HORIZONTAL SECTION THROUGH BRAIN OF AMMOCOETES, TO
+SHOW THE LEFT, OR VENTRAL PINEAL EYE.
+
+_pn._2_, left or ventral pineal eye; _pn._1_, last remnant of right, or
+dorsal pineal eye; _g.h.r._, right _ganglion habenulæ_; _g.h.l._1_,
+_g.h.l._3_, parts of left _ganglion habenulæ_; _pi._, fold of _pia mater_
+which separates the left _ganglion habenulæ_ from the left pineal eye;
+_f._, strands of nerve-fibres connecting the left eye with its ganglion,
+_g.h.l._3_; _V_3_, third ventricle; _V.aq._, ventricle of aquæduct.]
+
+The second, ventral or left, eye, belonging to the left ganglion habenulæ
+is very different in appearance, being much less evidently an eye. Fig. 34
+is one of the same series of horizontal sections as Fig. 31, _pn._1_ being
+the last remnant of the right, or dorsal, eye, while _pn._2_ shows the
+left, or ventral, eye with its connection with the left _ganglion
+habenulæ_.
+
+{80}In a series of sections I have followed the nerve of the right pineal
+eye to its destination, as described in my paper in the _Quarterly Journal
+of Microscopical Science_, and have found that it enters into the _ganglion
+habenulæ_ just as the nerve to any simple eye enters into its optic
+ganglion. This nerve, as I have shown, is a very distinct, well-defined
+nerve, with no admixture of ganglion-cells or of connective tissue, very
+different indeed to the connection between the left pineal eye and its
+optic ganglion. Here there is no defined nerve at all; but the cells of the
+_ganglion habenulæ_ stretch right up to the remains of the eye itself.
+Seeing, then, that both the eye and ganglion on this side have reached a
+much further grade of degeneration than on the right side, it may be fairly
+concluded that the original condition of these two median eyes is more
+nearly represented by the right eye, with its well-defined nerve and optic
+ganglion, than by the left eye, or by the eyes in lizards and other animals
+which do not show so well-defined a nerve as is possessed by Ammocoetes.
+Quite recently Dendy has examined the two median eyes in the New Zealand
+lamprey _Geotria australis_. In this species the second eye is much better
+defined than in the European lamprey, and its connection with the _ganglion
+habenulæ_ is more nerve-like. In neither eye, however, is the nerve so
+clean cut and isolated as is the nerve of the dorsal, or right, eye in the
+Ammocoetes stage of _Petromyzon Planeri_; in both, cells resembling those
+of the cortex of the _ganglion habenulæ_ and connective tissues are mixed
+up with the nerve-fibres which pass from each eye to its respective optic
+ganglion.
+
+
+THE RIGHT PINEAL EYE OF AMMOCOETES.
+
+The optic fibres of the right median eye of Ammocoetes are connected with a
+well-defined retina, the limits of which are defined by the white pigment
+so characteristic of this eye. This pigment is apparently calcium
+phosphate, which still remains as the 'brain-sand' of the human pineal
+gland. The cells, which are hidden by this pigment, were described by me in
+1890 as the retinal end-cells with large nuclei. In 1893, Studniçka
+examined them more closely, and concluded that the retinal cells are of two
+kinds: the one, nerve end-cells, the sensory cells proper; the other,
+pigmented epithelial cells, which surround the sense-cells. The sense-cells
+may contain some of the white pigment, but not so much as the other cells.
+Similarly, in the {81}median eyes of Limulus, Lankester and Bourne find it
+difficult to determine how far the retinal end-cells contain pigment and
+how far that pigment really is in the cells surrounding these nerve
+end-cells.
+
+The interior of the eye presents the appearance of a cavity in shape like a
+cornucopia, the stalk of which terminates at the place where the nerve
+enters. This cavity is not empty, but the posterior part of it is filled
+with the termination of the nerve end-cells of the retina, as pointed out
+by me and confirmed by Studniçka. These terminations are free from pigment,
+and contain strikingly translucent bodies, which I have described in my
+paper in the _Quarterly Journal_, and called rhabdites, for they present
+the same appearance and are situated in the same position as are many of
+the rhabdites on the terminations of the retinal end-cells of arthropod
+eyes. Studniçka has also seen these appearances, and figures them in his
+second paper on the nerve end-cells of the pineal eye of Ammocoetes.
+
+Up to this point the following conclusions may be drawn:--
+
+  1.  Ammocoetes possesses a pair of median eyes, just as was the case with
+  the most ancient fishes, and with the members of the contemporary
+  palæostracan group.
+
+  2.  The retina of one of these eyes is well-defined and upright, not
+  inverted, and therefore in this respect agrees with that of all median
+  eyes.
+
+  3.  The presence of nerve end-cells, with pigment either in them or in
+  cells around them, to the unpigmented ends of which translucent bodies
+  resembling rhabdites are attached, is another proof that this retina
+  agrees with that of the median eyes of arthropods.
+
+  4.  The simple nature of the nerve with its termination in an optic
+  ganglion closely resembling in structure an arthropod optic ganglion,
+  together with Studniçka's statement that the nerve end-cells pass
+  directly into the nerve, points directly to the conclusion that this
+  retina is a simple, not a compound, retina, and that it therefore in this
+  respect also agrees with the retina of all median eyes.
+
+With respect to this last conclusion, neither I myself nor Studniçka have
+been able to see any definite groups of cells between the nerve end-cells
+and the optic nerve such as a compound retina necessitates.
+
+{82}On the other hand, Dendy describes in the New Zealand lamprey, _Geotria
+australis_, a cavity where the nerve enters into the eye, which he calls
+the atrium. This cavity is distinct from the main cavity of the eye, and is
+separated from it by a mass of cells similar in appearance to those of the
+cortex of the _ganglion habenulæ_. In these two eyes then, groups of cells,
+resembling in appearance those belonging to an optic ganglion, exist in the
+eyes themselves. This atrium is evidently that part of the central cavity
+which I have called the handle of the cornucopia in the European lamprey,
+and the very fact that it is separated from the rest of the central cavity
+is evidence that we are dealing here with a later stage in the history of
+the pineal eyes than in the case of the Ammocoetes of _Petromyzon Planeri_.
+Taking also into consideration the continuity of the mass of small
+ganglion-cells which surround this atrium with the cells of the _ganglion
+habenulæ_ by means of the similar cells scattered along the course of the
+nerve, and also bearing in mind the fact already stated that in the more
+degenerate left eye of Ammocoetes the cells of the _ganglion habenulæ_
+extend right up to the eye itself, it seems more likely than not that these
+cells do not represent the original optic ganglion of a compound retina,
+but rather the subsequent invasion, by way of the pineal nerve, of
+ganglion-cells belonging to a portion of the brain. In the last chapter it
+has been suggested that the presence of the trochlear or fourth cranial
+nerve has given rise to the formation of the cerebellum by a similar
+spreading.
+
+There is certainly no appearance in the least resembling a compound retina
+such as is seen in the vertebrate or crustacean lateral eye. In the median
+eyes of scorpions and of Limulus, cells are found within the capsule of the
+eye among the nerve-fibres and the nerve end-cells. These are especially
+numerous in the median eyes of Limulus, as described by Lankester and
+Bourne, and are called by them intrusive connective tissue cells. The
+meaning of these cells is not, to my mind, yet settled. It is sufficient
+for my purpose to point out that the presence of cells interneural in
+position among the nerve end-cells of the retina of the median eyes of
+Ammocoetes is more probable than not, on the assumption that the retina of
+these eyes is built up on the same plan as that of the median eyes in
+Limulus and the scorpions.
+
+It is further to be borne in mind that these specimens of _Geotria_ worked
+at by Dendy were in the 'Velasia' stage of the New Zealand {83}lamprey, and
+correspond, therefore, more nearly to the Petromyzon than to the Ammocoetes
+stage of the European lamprey.
+
+
+THE DIOPTRIC APPARATUS.
+
+Besides the retina, all eyes possess a dioptric apparatus. What is the
+evidence as to its nature in these vertebrate median eyes? Lankester and
+Bourne have divided the eyes of scorpions and Limulus into two kinds,
+monostichous and diplostichous. In the first the retinal cells are supposed
+to give rise to not only rhabdites but also the cuticular chitinous lens,
+so that the eye is one-layered; in the second the lens is formed by a
+well-marked hypodermal layer, in front of the retina, composed of elongated
+cells, so that these eyes are two-layered or diplostichous. The lateral
+eyes, according to them, are all monostichous, but the median eyes are
+diplostichous.
+
+[Illustration: FIG. 35.--EYE OF ACILIUS LARVÆ. (After PATTEN.)
+
+_l._, chitinous lens; _c._, corneagen; _pr._, pre-retinal layer; _rh._,
+rhabdites; _ret._, retinal end-cells.]
+
+This distinction is not considered valid by other observers. Thus, {84}as
+already indicated, Patten looks on all these eyes as three-layered, and
+states that in all cases a corneagen or vitreogen layer exists, which gives
+origin to the lens. For my own part I agree with Patten, but we are not
+concerned here with the lateral eyes. It is sufficient to note that all
+observers are agreed that the median eyes are characterized by this
+well-marked cell-layer, the so-called vitreous or corneagen layer of cells.
+
+[Illustration: FIG. 36.--EYE OF HYDROPHILUS LARVA, WITH THE PIGMENT OVER
+THE RETINAL END-CELLS.
+
+_l._, chitinous lens; _c._, corneagen; _pr._, pre-retinal layer; _rh._,
+rhabdites; _ret._, retinal end-cells.]
+
+This layer (_c._, Fig. 35) is composed of much-elongated cells of the
+hypodermal layer, in each of which the large nucleus is always situated
+towards the base of the cell. The space between it and the retina contains,
+according to Patten the cells of the pre-retinal layer _(pr.)_. These may
+be so few and insignificant as to give the impression that the vitreous
+layer is immediately adjacent to the retina (_ret._).
+
+Let us turn now to the right pineal eye of Ammocoetes (Fig. 37) and see
+what its further structure is. The anterior part of the eye is free from
+pigment, and is composed, as is seen in hæmatoxylin or carmine specimens,
+of an inner layer of nuclei which are frequently arranged in a wavy line.
+From this nucleated layer, strands of tissue, free from nuclei, pass to the
+anterior edge of the eye.
+
+In the horizontal longitudinal sections it is seen that these strands are
+confined to the middle of the eye. On each side of them the nuclear layer
+reaches the periphery, so that if we consider these strands to represent
+long cylindrical cells, as described by Beard, then the anterior wall may
+be described as consisting of long cylindrical cells, which are flanked on
+either side by shorter cells of a similar kind. The nuclei at the base of
+these cylindrical cells are not all alike. We see, in the first place,
+large nuclei resembling the large nuclei belonging to the nerve end-cells;
+these are the nuclei of {85}the long cylindrical cells. We see also smaller
+nuclei in among these larger ones, which look like nuclei of intrusive
+connective tissue, or may perhaps form a distinct layer of cells, situated
+between the cells of the anterior wall and the terminations of the nerve
+end-cells already referred to.
+
+These elongated cells are in exactly the same position and present the same
+appearance as the cells of the corneagen layer of any median eye. Like the
+latter they are free from pigment and never show with osmic staining any
+sign of the presence of translucent rhabdite-like bodies, such as are seen
+in the termination of the retinal cells, and like the latter their nuclei
+are at the base. The resemblance between this layer and the corneagen cells
+of any median eye is absolute. Between it and the terminations of the
+retinal cells there exists some ill-defined material certainly containing
+cells which may well correspond to Patten's pre-retinal layer of cells.
+
+Retina, corneagen, nerve, optic ganglion, all are there, all in their right
+position, all of the right structure, what more is needed to complete the
+picture?
+
+[Illustration: FIG. 37.--PINEAL EYE OF AMMOCOETES, WITH ITS _Ganglion
+Habenulæ_.]
+
+In order to complete the dioptric apparatus a lens is necessary. Where,
+then, is the lens in these pineal eyes? In all the arachnid eyes, whether
+median or lateral, the lens is a single corneal lens composed of the
+external cuticle, which is thickened over the corneagen cells. This
+thickened cuticle is composed of chitin, and is not cellular, but is dead
+material formed out of the living underlying corneagen cells. Such a lens
+is in marked contrast to the lens of the lateral vertebrate eye, which is
+formed by living cells themselves. This {86}thickening of the cuticular
+layer to form a lens could only exist as long as that layer is absolutely
+external, so that the light strikes immediately upon it; for, if from any
+cause the eye became situated internally, the place of such a lens must be
+filled by the structures situated between it and the surface, and the
+thickened cuticle would no longer be formed.
+
+In all vertebrates these pineal eyes are separated from the external
+surface by a greater or less thickness of tissues; in the case of
+Ammocoetes, as is seen in Fig. 31, the eye lies within the membranous
+cranial wall, and is attached closely to it. The position, then, of the
+cuticular, or corneal lens, as it is often called, on the supposition that
+this is a median eye of the arachnid type, is taken by the membranous
+cranium, and, as I have described in my paper in the _Quarterly Journal_,
+on carefully lifting the eye in the fresh condition from the cranial wall,
+it can be seen under a dissecting microscope that the cranial wall often
+forms at this spot a lens-like bulging, which fits the shallow concavity of
+the surface of the eye, and requires some little force to separate it from
+the eye.
+
+As will appear in a subsequent chapter, this cranial wall has been formed
+by the growth, laterally and dorsally, of a skeletal structure known by the
+name of the _plastron_. The last part of it to be completed would be that
+part in the mid-dorsal line, where apparently, in consequence of the
+insinking of the degenerating eyes, a dermal and subdermal layer already
+intervened between the source of light and the eyes themselves.
+
+When the membranous cranium was completed in the mid-dorsal region, it was
+situated here as elsewhere just internally to the subdermal layer, and
+therefore enclosed the pineal eyes. This, to my mind, is the reason why the
+pineal eyes, which, in all other respects, conform to the type of the
+median eyes of an arachnid-like animal, do not possess a cuticular lens.
+Other observers have attempted to make a lens out of the elongated cells of
+the anterior wall of the eye (my corneagen layer), but without success.
+
+Studniçka, who calls this layer the _pellucida_, does not look upon it as
+the lens, but considers, strangely enough, that the translucent appearances
+at the ends of each nerve end-cell represent a lens for that cell, so that
+every nerve end-cell has its own lens. Still more strange is it that,
+holding this view, he should yet consider these knobs {87}to be joined by
+filaments to the cells in the anterior wall of the eye, a conception fatal
+to the action of such knobs as lenses.
+
+The discovery that the vertebrate possesses, in addition to the lateral
+eyes, a pair of median eyes, which are most conspicuous in the lowest
+living vertebrate, together with the fact that such eyes are built up on
+the same plan as the median eyes of living crustaceans or arachnids, not
+only with respect to the eye itself but also to its nerve and optic
+ganglion, constitutes a fact of the very greatest importance for any theory
+of the origin of vertebrates; especially in view of the further fact, that
+similar eyes in the same position are found not only in all the members of
+the Palæostraca, but also in all those ancient forms (classed as fishes)
+which lived at that time. At one and the same moment it proves the utter
+impossibility of reversing dorsal and ventral surfaces, points in the very
+strongest manner to the origin of the vertebrate from some member or other
+of the palæostracan group, and insists that the advocates of the origin of
+vertebrates from the Hemichordata, etc., should give an explanation of the
+presence of these two median eyes of a more convincing character than that
+given here.
+
+
+THE LATERAL EYES.
+
+Turning now to the consideration of the lateral eyes, we see that these
+eyes in the arachnids often possess an inverted retina, in the crustaceans
+always an upright retina. In the arachnids they possess a simple retina,
+while in the crustaceans their retina is compound; so that in the latter
+case the so-called optic nerve is in reality a tract of fibres connecting
+together the brain-region with a variable number of optic ganglia, which
+have been left at the periphery in close contact with the retinal cells,
+when the brain sunk away from the superficial epithelial covering.
+
+There is, then, this difference between the lateral eyes of crustaceans and
+arachnids, that the retina of the former is compound, but never inverted,
+while that of the latter may be inverted, but is always simple.
+
+The retina of the lateral eyes of the vertebrate resembles both of these,
+for it is compound, as in the crustacean, and inverted as in the arachnid.
+
+It must always be borne in mind that in the palæostracan epoch {88}the
+dominant race was neither crustacean nor arachnid, but partook of the
+characters of both; also, as is characteristic of dominance, there was very
+great variety of form, so that it seems more probable than not that some of
+these forms may have combined the arachnid and crustacean characteristics
+to the extent of possessing lateral eyes with an inverted yet compound
+retina. A certain amount of evidence points in this direction. As already
+stated, the compound retina which characterizes the vertebrate lateral eyes
+is characteristic of all facetted eyes, and in the trilobites facetted
+lateral eyes are commonly found. From this it may be concluded that many of
+the trilobites possessed eyes with a compound retina. There have, however,
+been found in certain species, e.g. _Harpes vittatus_ and _Harpes ungula_,
+lateral eyes which were not facetted, and are believed by Korschelt and
+Heider to be of an arachnid nature. They say, "Palæontologists have
+appropriately described them as ocelli, although, from a zoological point
+of view, they do not deserve this name, having most probably arisen in a
+way similar to that conjectured in connection with the lateral eyes of
+scorpions." If this conjecture is right, then in these forms the retina may
+have been inverted, but because they belonged to the trilobite group, the
+retina was most probably compound, so that here we may have had the
+combination of the arachnid and crustacean characteristics. On the other
+hand, in some forms of Branchipus, and many of the Gammaridæ, a single
+corneal lens is found in conjunction with an eye of the crustacean type, so
+that a non-facetted lateral eye, found in a fossil form, would not
+necessarily imply the arachnid type of eye with the possibility of an
+inverted retina. Whatever may be the ultimate decision upon these
+particular forms, the striking fact remains, that both in the vertebrate
+and in the arachnid the median eyes possess a simple upright retina, while
+the lateral eyes possess an inverted retina, and that both in the
+vertebrate and the crustacean the median eyes possess a simple upright
+retina, while the lateral eyes possess a compound retina.
+
+The resemblance of the retina of the lateral eyes of vertebrates to that of
+the lateral eyes of many arthropods, especially crustaceans, has been
+pointed out by nearly every one who has worked at these invertebrate
+lateral eyes. The foundation of our knowledge of the compound retina is
+Berger's well-known paper, the results of which are summed up by him in the
+following two main conclusions.
+
+{89}1.  The optic ganglion of the Arthropoda consists of two parts, of
+which the one stands in direct inseparable connection with the facetted
+eye, and together with the layer of retinal rods forms the retina of the
+facetted eye, while the other part is connected rather with the brain, and
+is to be considered as an integral part of the brain in the narrower sense
+of the word.
+
+[Illustration: FIG. 38.--THE RETINA OF MUSCA. (After BERGER.)
+
+_Br._, brain; _O.n._, optic nerve; _n.l.o.g._, nuclear layer of ganglion of
+optic nerve; _m.l._, molecular layer (Punktsubstanz); _n.l.r.g.i._ and
+_n.l.r.g.o._, inner and outer nuclear layers of the ganglion of the retina;
+_f.br.r._, terminal fibre-layer of retina; _r._, layer of retinal end-cells
+(indicated only).]
+
+
+2.  In all arthropods examined by him, the retina consists of five layers,
+as follows:--
+
+  (1)  The layer of rods and their nuclei.
+  (2)  The layer of nerve-bundles.
+  (3)  The nuclear layer.
+  (4)  The molecular layer.
+  (5)  The ganglion cell layer.
+
+Berger passes under review the structure and arrangement of the optic
+ganglion in a large number of different groups of arthropods, and concludes
+that in all cases one part of the optic ganglion is always closely attached
+to the visual end-cells, and this combination he calls the retina. On the
+other hand, the nerve-fibres which connect the peripheral part of the optic
+ganglion with the brain, the so-called optic nerve, are by no means
+homologous in the different groups; for in some cases, as in many of the
+stalk-eyed crustaceans, the whole optic ganglion is at the periphery, while
+in others, as in the Diptera, only the retinal ganglion is at the
+periphery, and the nerve-stalk connects this with the rest of the optic
+ganglion, the latter being fused with the main brain-mass. In the Diptera,
+in fact, according to Berger, the optic nerve {90}and retina are most
+nearly comparable to those of the vertebrate. For this reason I give
+Berger's picture of the retina of Musca (Fig. 38), in order to show the
+arrangement there of the retinal layers.
+
+[Illustration: FIG. 39.--THE BRAIN OF _Sphæroma serratum_. (After
+BELLONCI.)
+
+_Ant. I._ and _Ant. II._, nerves to 1st and 2nd antennæ. _f.br.r._,
+terminal fibre-layer of retina; _Op. g. I._, first optic ganglion; _Op. g.
+II._, second optic ganglion; _O.n._, optic nerve-fibres forming an optic
+chiasma.]
+
+In Branchipus and other primitive Crustacea, Berger also finds the same
+retinal layers, but is unable to distinguish in the brain the rest of the
+optic ganglion. Judging from Berger's description of Branchipus, and
+Bellonci's of Sphæroma, it would almost appear as though the cerebral part
+of the retina in the higher forms originated from two ganglionic
+enlargements, an external and internal enlargement, as Bellonci calls them.
+The external ganglion (_Op. g. I._, Fig. 39) may be called the ganglion of
+the retina, the cells of which form the nuclear layer of the higher forms,
+and the internal ganglion (_Op. g. II._, Fig. 39), from which the optic
+nerve-fibres to the brain arise, may therefore be called the ganglion of
+the optic nerve. Bellonci describes how in this latter ganglion cells are
+found very different to the small ones of the external ganglion or ganglion
+of the retina. So also in Branchipus, judging from the pictures of Berger,
+Claus, and from my own observations (_cf._ Fig. 46, in which the double
+nature of the retinal ganglion is indicated), the peripheral part of the
+optic ganglion--_i.e._ the retinal ganglion--may be spoken {91}of as
+composed of two ganglia. The external of these is clearly the ganglion of
+the retina; its cells form the nuclear layer, the striking character of
+which, and close resemblance to the corresponding layer in vertebrates, is
+shown by Claus' picture, which I reproduce (Fig. 40). The internal ganglion
+with which the optic nerve is in connection contains large ganglion cells,
+which, together with smaller ones, form the ganglionic layer of Berger.
+
+The most recent observations of the structure of the compound retina of the
+crustacean eye are those of Parker, who, by the use of the methylene blue
+method, and Golgi's method of staining, has been able to follow out the
+structure of the compound retina in the arthropod on the same lines as had
+already been done for the vertebrate. These two methods have led to the
+conclusion that the arthropod central nervous system and the vertebrate
+central nervous system are built up in the same manner--viz. by means of a
+series of ganglia connected together, each ganglion being composed of
+nerve-cells, nerve-fibres, and a fine reticulated substance called by
+Leydig in arthropods 'Punktsubstanz,' and known in vertebrates and in
+invertebrates at the present time as 'neuropil.' A further analysis
+resolves the whole system into a combination of groups of neurones, the
+cells and fibres of which form the cells and fibres of the ganglia, while
+their dendritic connections with the terminations of other neurones,
+together with the neuroglia-cells form the 'neuropil.' As is natural to
+expect, that part of the central nervous system which helps to form the
+compound retina is built up in the same manner as the rest of the central
+nervous system.
+
+[Illustration: FIG. 40.--BIPOLAR CELLS OF NUCLEAR LAYER IN RETINA OF
+BRANCHIPUS. (After CLAUS.)
+
+_f.br.r._, terminal fibre-layer of retina; _n.l.r.g._, bipolar cells of the
+ganglion of the retina = inner nuclear layer; _m.l._, Punktsubstanz = inner
+molecular layer; _b.m._, basement membrane formed by neurilemma round
+central nervous system.]
+
+Thus, according to Parker, the mass of nervous tissue which occupies the
+central part of the optic stalk in Astacus is composed {92}of four distinct
+ganglia; the retina is connected with the first of these by means of the
+retinal fibres, and the optic nerve extends proximally from the fourth
+ganglion to the brain. Each ganglion consists of ganglion-cells,
+nerve-fibres, and 'neuropil,' and, in addition, supporting cells of a
+neuroglial type. By means of the methylene blue method and the Golgi
+method, it is seen that the retinal end-cells, with their visual rods, are
+connected with the fibres of the optic nerve by means of a system of
+neurones, the synapses of which take place in and help to form the
+'neuropil' of the various ganglia. Thus, an impulse in passing from the
+retina to the brain would ordinarily travel over five neurones, beginning
+with one of the first order and ending with one of the fifth. He makes five
+neurones although there are only four ganglia, because he reckons the
+retinal cell with its elongated fibre as a neurone of the first order, such
+fibre terminating in dendritic processes which form synapses in the
+'neuropil' of the first ganglion with the neurones of the second order.
+
+Similarly the neurones of the second order terminate in the 'neuropil' of
+the second ganglion, and so on, until we reach the neurones of the fifth
+order, which terminate on the one hand in the 'neuropil' of the fourth
+ganglion, and on the other pass to the optic lobes of the brain by their
+long neuraxons--the fibres of the optic nerve.
+
+He compares this arrangement with that of Branchipus, Apus, Estheria,
+Daphnia, etc., and shows that in the more primitive crustaceans the
+peripheral optic apparatus was composed, not of four but of two optic
+ganglia, not, therefore, of five but of three neurones, viz.--
+
+1.  The neurone of the first order--_i.e._ the retinal cell with its fibre
+terminating in the 'neuropil' of the first optic ganglion (ganglion of the
+retina).
+
+2.  The neurone of the second order, which terminates in the 'neuropil' of
+the second ganglion (ganglion of the optic nerve).
+
+3.  The neurone of the third order, which terminates in the optic lobes of
+the brain by means of its neuraxons (the optic nerve).
+
+We see, then, that the most recent researches agree with the older ones of
+Berger, Claus, and Bellonci, in picturing the retina of the primitive
+crustacean forms as formed of two ganglia only, and not of four, as in the
+specialized crustacean group the Malacostraca.
+
+{93}The comparison of the arthropod compound retina with that of the
+vertebrate shows, as one would expect upon the theory of the origin of
+vertebrates put forward in this book, that the latter retina is built up of
+two ganglia, as in the more primitive less specialized crustacean forms.
+The modern description of the vertebrate retina, based upon the Golgi
+method of staining, is exactly Parker's description of the simpler form of
+crustacean retina in which the 'neuropil' of the first ganglion is
+represented by the external molecular layer, and that of the second
+ganglion by the internal molecular layer; the three sets of neurones being,
+according to Parker's terminology:--
+
+1.  The neurones of the first order--viz. the visual cells--the nuclei of
+which form the external nuclear layer, and their long attenuated processes
+form synapses in the external molecular layer with
+
+2.  The neurones of the second order, the cells of which form the internal
+nuclear layer, and their processes form synapses in the internal molecular
+layer with
+
+3.  The neurones of the third order, the cells of which form the ganglionic
+layer and their neuraxons constitute the fibres of the optic nerve which
+end in the optic lobes of the brain.
+
+Strictly speaking, of course, the visual cells with their elongated
+processes have no right to be called neurones: I only use Parker's
+phraseology in order to show how closely the two retinas agree even to the
+formation of synapses between the fine drawn-out processes of the visual
+cells and the neurones of the ganglion of the retina.
+
+
+THE RETINA OF THE LATERAL EYE OF AMMOCOETES.
+
+As in the case of all other organs, it follows that if we are dealing here
+with a true genetic relationship, then the lower we go in the vertebrate
+kingdom the more nearly ought the structure of the retina to approach the
+arthropod type. It is therefore a matter of intense interest to determine
+the nature of the retina in Ammocoetes in order to see whether it differs
+from that of the higher vertebrates, and if so, whether such differences
+are explicable by reference to the structure of the arthropod eye.
+
+Before describing the structure of this retina it is necessary to clear
+away a remarkable misconception, shared among others by {94}Balfour, that
+this eye is an aborted eye, and that it cannot be considered as a primitive
+type. Thus Balfour says: "Considering the degraded character of the
+Ammocoete eye, evidence derived from its structure must be received with
+caution," and later on, "the most interesting cases of partial degeneration
+are those of Myxine and the Ammocoete. The development of such aborted eyes
+has as yet been studied only in the Ammocoete, in which it resembles in
+most important features that of other Vertebrata."
+
+Again and again the aborted character of the eye is stated to be evidence
+of degeneration in the case of the lamprey. What such a statement means,
+why the eye is in any way to be considered as aborted, is to me a matter of
+absolute wonderment: it is true that in the larval form it lies under the
+skin, but it is equally true that at transformation it comes to the
+surface, and is most evidently as perfect an eye as could be desired. There
+is not the slightest sign of any degeneration or abortion, but simply of
+normal development, which takes a longer time than usual, lasting as it
+does throughout the life-time of the larval form.
+
+Kohl, who has especially studied degenerated vertebrate eyes, discusses
+with considerable fulness the question of the Ammocoetes eye, and concludes
+that in aborted eyes a retarded development occurs, and this applies on the
+whole to Ammocoetes, "but with the important difference that in this case
+the period of retarded development is not followed by a stoppage, but on
+the contrary by a period of very highly intensified progressive development
+during the metamorphosis," with the result that "the adult eye of
+_Petromyzon Planeri_ does not diverge from the ordinary type."
+
+Referring in his summing up to this retarded development, he says: "Such
+reminiscences of embryonic conditions are after all present here and there
+in normally developed organs, and by no means entitle us to speak of
+abnormal development."
+
+The evidence, then, is quite clear that the eye of Petromyzon, or, indeed,
+of the full-grown Ammocoetes, is in no sense an abnormal eye, but simply
+that its development is slow during the ammocoete stage. The retina of
+Petromyzon was figured and described by Langerhans in 1873. He describes it
+as composed of the following layers:--
+
+   (1) _Membrana limitans interna._
+   (2) Thick inner molecular layer.
+   (3) Optic fibre layer.
+   (4) Thick inner nuclear layer.
+   (5) Peculiar double-layered ganglionic layer.
+   (6) External molecular layer.
+   (7) External nuclear layer.
+   (8) _Membrana limitans externa._
+   (9) Layer of rods.
+  (10) Pigment-epithelium.
+
+{95}[Illustration: FIG. 41.--RETINA AND OPTIC NERVE OF PETROMYZON. (AFTER
+MÜLLER AND LANGERHANS.)
+
+On the left side the Müllerian fibres and pigment-epithelium are
+represented alone. The retina is divided into an epithelial part, _C_ (the
+layer of visual rod-cells), and a neurodermal or cerebral part which is
+formed of, _A_, the ganglion of the optic nerve and, _B_, the ganglion of
+the retina. 1, int. limiting membrane; 2, int. molecular layer with its two
+layers of cells; 3, layer of optic nerve fibres; 4, int. nuclear layer; 5,
+double row of tangential fulcrum cells; 6, layer of terminal retinal
+fibres; 7, ext. nuclear layer; 8, ext. limiting membrane; 9, layer of rods;
+10, layer of pigment-epithelium. _D_, axial cell layer (Axenstrang) in
+optic nerve. The layer 6 is drawn rather too thick.]
+
+He points out especially the peculiarity of layer (2) (2, Fig. 41), the
+inner molecular, in which two rows of nuclei are arranged with great
+regularity, the one row closely touching the _membrana limitans interna_,
+the other at the inner boundary of the middle third of the {96}molecular
+layer. Of these two rows of nuclei, he describes the innermost as composed
+almost entirely of large nuclei belonging to ganglion cells, while the
+outermost is composed mainly of distinctly smaller nuclei, which in
+staining and appearance appear to belong not to nerve-cells but to the true
+reticular tissue of the molecular layer.
+
+He also draws special attention to the remarkable layer (5) (5, Fig. 41),
+which is not found in the retina of the higher vertebrates, the cells of
+which, in his opinion, are of the nature of ganglion-cells.
+
+W. Müller, in 1874, gave a most careful description of the eye of
+Ammocoetes and Petromyzon, and traced the development of the retina; the
+subsequent paper of Kohl does not add anything new, and his drawings are
+manifestly diagrams, and do not represent the appearances so accurately as
+Müller's illustrations. In the accompanying figure (Fig. 41) I reproduce on
+the right-hand side Müller's picture of the retina of Petromyzon, but have
+drawn it, as in Langerhans' picture, at the place of entry of the optic
+nerve.
+
+From his comparison of this retina with a large number of other vertebrate
+retinas, he comes to the conclusion that the retina of all vertebrates is
+divisible into
+
+  _A._ An ectodermal (epithelial) part consisting of the layer of the
+  visual cells, and
+
+  _B._ A neurodermal (cerebral) part which forms the rest of the retina.
+
+Further, Müller points out that the neuroderm gives origin throughout the
+central nervous system to two totally different structures, on the one hand
+to the true nervous elements, on the other to a system of supporting cells
+and fibres which cannot be classed as connective tissue, for they do not
+arise from mesoblast, and are therefore called by him 'fulcrum-cells.' In
+the retina he recognizes two distinct groups of such supporting
+structures--(1) a system of radial fibres with well-marked elongated
+nuclei, which extend between the two limiting layers, and form at their
+outer ends a membrane-like expansion which was originally the outer limit
+of the retina, but becomes afterwards co-terminous with the _membrana
+limitans externa_, owing to the piercing through it of the external limbs
+of the rods. This system, which is known by the name of the radial
+Müllerian fibres (shown on the left-hand side of Fig. 41), has no
+connection with (2) the spongioblasts and neurospongium, which form a
+framework of neuroglia, in which the terminations of the {97}optic ganglion
+and of the retinal ganglion ramify to form the molecular layers.
+
+It is evident from Fig. 41 that the retina of Ammocoetes and Petromyzon
+differs in a striking manner from the typical vertebrate retina. The
+epithelial part (C) remains the same--viz. the visual rods, the external
+limiting membrane, and the external nuclear layer; but the cerebral part,
+the retinal ganglion (A and B), is remarkably different. It is true, it
+consists in the main of the small-celled mass known as the inner nuclear
+layer, and of the reticulated tissue or 'neuropil' known as the inner
+molecular layer, just as in all other compound retinal eyes; but neither
+the ganglion cell-layer nor the optic fibre-layer is clearly defined as
+separate from this molecular layer; on the contrary, it is matter of
+dispute as to what cells represent the ganglionic layer of higher
+vertebrates, and the optic fibres do not form a distinct innermost layer,
+but pass into the inner molecular layer at its junction with the inner
+nuclear layer. A comparison of this innermost part of the retina (A, Fig.
+41), with the corresponding part in Berger's picture of Musca (_n.l.o.g._,
+Fig. 38), shows a most striking similarity between the two. In both cases
+the fibres of the optic nerve (_O.n._, Fig. 38) which cross at their
+entrance pass into the 'neuropil' of this part of the retinal ganglion, and
+are connected probably (though that is not proved in either case) with the
+cells of the ganglionic layer. In both cases we find two well-marked
+parallel rows of cells in this part of the retina, of which one, the
+innermost, is composed in Ammocoetes of large ganglion-cells, and the other
+mainly of smaller, deeper staining cells apparently supporting in function.
+Similarly, also, in Branchipus, as I conclude from my own observations as
+well as from those of Berger and Claus, the ganglionic layer is composed
+partly of true ganglion-cells and partly of supporting cells arranged in a
+distinct layer. This part, then, of the retina of Ammocoetes is remarkably
+like that of a typical arthropod retina, and forms that part of the retinal
+ganglion which may be called the ganglion of the optic nerve.
+
+Next comes the ganglion of the retina (B, Fig. 41) (Parker's first optic
+ganglion), the cells of which form the small bipolar granule-cells of the
+inner nuclear layer; granule-cells arranged in rows just as they are shown
+in Claus' picture of the same layer in the retina of Branchipus (Fig. 40),
+just as they are found in the cortical layers of the optic ganglion of the
+pineal eye (_ganglion habenulæ_), in the {98}optic lobes and other parts of
+the Ammocoetes brain, or in the cortical layers of the optic ganglia of all
+arthropods.
+
+Between this small-celled nuclear layer (4, Fig. 41) and the layer of
+nuclei of the visual rod cells (7, Fig. 41) (the external nuclear layer),
+we find in the eye of Ammocoetes and Petromyzon two well-marked rows of
+cells of a most striking character--viz. the two remarkably regular rows of
+large epithelial-like cells with large conspicuous nuclei, which give the
+appearance of two opposing rows of limiting epithelium (5, Fig. 41),
+already mentioned in connection with the researches of Langerhans and W.
+Müller. Here, then, is a striking peculiarity of the retina of the lamprey,
+and according to Müller the obliteration of these two layers can be traced
+as we pass upwards in the vertebrate kingdom. Among fishes, they are
+especially well seen in the perch; in the higher vertebrates the whole
+layer is only a rudiment represented, he thinks, by the simple layer of
+round cells which lies close against the inner surface of the layer of
+terminal fibres (Nervenansätze), and is especially evident in birds and
+reptiles. In man and the higher mammals they are probably represented by
+the horizontal cells of the outer part of the inner nuclear layer.
+
+Seeing, then, that they are most evident in Ammocoetes, and become less and
+less marked in the higher vertebrates, it is clear that their origin cannot
+be sought among the animals higher in the scale than Ammocoetes, but must,
+therefore, be searched for in the opposite direction.
+
+Müller describes them as forming a very conspicuous landmark in the
+embryology of the retina, dividing it distinctly into two parts, an outer
+thinner, and an inner somewhat thicker part, the zone formed by them
+standing out conspicuously on account of the size and regularity of the
+cells and their lighter appearance when stained. Thus in his description of
+the retina of an Ammocoetes 95 mm. in length, he says, "The layer of pale
+tangentially elongated cells formed a double layer and produced the
+appearance of a pale, very characteristic zone between the outer and inner
+parts of the retina."
+
+Let us now turn to the retina of the crustacean and see whether there is
+any evidence there that the retina is divisible into an outer and inner
+part, separated by a zone of characteristically pale staining cells with
+conspicuous nuclei. The most elaborate description of the development of
+the retina of Astacus is given by Reichenbach, {99}according to whom the
+earliest sign of the formation of the retina is an ectodermic involution
+(Augen-einstülpung), which soon closes, so that the retinal area appears as
+a thickening. In close contiguity to this thickening, the thickening of the
+optic ganglion arises, so that that part of the optic ganglion which will
+form the retinal ganglion fuses with the thickened optic plate and forms a
+single mass of tissue. Later on a fold (Augen-falte) appears in this mass
+of tissue, in consequence of which it becomes divided into two parts. The
+lining walls of this fold form a double row of cells, the nuclei of which
+are most conspicuous because they are larger and lighter in colour than the
+surrounding nuclei, so that by this fold the retina is divided into an
+outer and an inner wall, the line of demarcation being conspicuous by
+reason of these two rows of large, lightly-staining nuclei.
+
+Reichenbach is unable to say that this secondary fold is coincident with
+the primary involution, and that therefore the junction between the two
+rows of large pale nuclei is the line of junction between the retinal
+ganglion and the retina proper, because all sign of the primary involution
+is lost before the secondary fold appears.
+
+Parker compares the appearances in the lobster with Reichenbach's
+description in the crayfish, and says that he finds only a thickening, no
+primary involution; at the same time he expressly states that in the very
+early stages his material was deficient, and that he had not grounds
+sufficient to warrant the statement that no involution occurs. He also
+finds that in the lobster the ganglionic tissue which arises by
+proliferation is divided into an outer and inner part; the separation is
+effected by a band of large, lightly-staining nuclei, which, in position
+and structure, resemble the band figured by Reichenbach. According to
+Parker, then, the line of separation indicated in the development by
+Reichenbach's outer and inner walls is not the line of junction between the
+retina and the retinal ganglion, as Reichenbach was inclined to think, but
+rather a separation of two rows of large ganglion-cells belonging to the
+retinal ganglion.
+
+The similarity between these conspicuous layers of lightly-staining cells
+in Ammocoetes and in crustaceans is remarkably close, and in both cases
+observers have found the same difficulty in interpreting their meaning. In
+each case one group of observers looks upon them as ganglion-cells, the
+other as supporting structures. Thus in the lamprey, Müller considers them
+to belong to the supporting elements, while Langerhans and Kohl describe
+them as a double {100}layer of ganglion-cells. In the crustacean, Berger in
+Squilla, Grenacher in Mysis, and Parker in Astacus, look upon them as
+supporting elements, while Viallanes in Palinurus considers them to be true
+ganglionic cells.
+
+Whatever the final interpretation of these cells may prove to be, we may,
+it seems to me, represent an ideal compound retina of the crustacean type
+by combining the investigations of Berger, Claus, Reichenbach, and Parker
+in the following figure.
+
+[Illustration: FIG. 42.--IDEAL DIAGRAM OF THE LAYERS IN A CRUSTACEAN EYE.
+
+The retina is divided into an epithelial part, _C_ (the layer of retinular
+cells and rhabdomes), and a neurodermal or cerebral part, which is formed
+of, _A_, the ganglion of the optic nerve, and, _B_, the ganglion of the
+retina. 1, optic nerve fibres which cross at their entrance into the
+retina; 2, int. molecular layer with its two rows of cells; 3, int. nuclear
+layer; 4, Reichenbach's double row of large lightly-staining cells; 5,
+layer of terminal retinal fibres; 6, ext. nuclear layer; 7, ext. limiting
+membrane; 8, layer of crystalline cones; 9, cornea.]
+
+The comparison of this figure (Fig. 42) with that of the Petromyzon retina
+(Fig. 41) shows how great is the similarity of the latter with the
+arthropod type, and how the very points in which it deviates from the
+recognized vertebrate type are explainable by comparison with that of the
+arthropod. The most striking difference between the retinas in the two
+figures is that the layer of terminal nerve fibres (5, Fig. 42), which,
+after all, are only the elongated terminations of the retinal cells
+belonging to Parker's neurones of the first order, is very much longer than
+in Petromyzon or in any vertebrate, for the external molecular layer (6,
+Fig. 41) (Müller's layer of Nervenansätze) is very short and inconspicuous
+(in Fig. 41 it is drawn too thick).
+
+Turning from the retina to the fibres of the optic nerve we again find a
+remarkable resemblance, for in Ammocoetes, as pointed out by
+{101}Langerhans and carefully figured by Kohl, a crossing of the fibres of
+the optic nerve occurs as the nerve leaves the retina, just as is so
+universally the case in all compound retinas. To this crossing Kohl has
+given the name _chiasma nervi optici_, in distinction to the cerebral
+chiasma, which he calls _chiasma nervorum opticorum_. Further, we find that
+even this latter chiasma is well represented in the arthropod brain; thus
+Bellonci in Sphæroma, Berger, Dietl, and Krieger in Astacus, all describe a
+true optic chiasma, the only difference in opinion being, whether the
+crossing of the optic nerves is complete or not. Especially instructive are
+Bellonci's figures and description. He describes the brain of Sphæroma as
+composed of three segments--a superior segment, the cerebrum proper, a
+middle segment, and an inferior segment; the optic fibres, as is seen in
+Fig. 39, after crossing, pass direct into the middle segment, in the
+ganglia of which they terminate. From this segment also arises the nerve to
+the first antenna of that side--_i.e._ the olfactory nerve. The optic part,
+then, of this middle segment is clearly the brain portion of the optic
+ganglionic apparatus, and may be called the optic lobes, in
+contradistinction to the peripheral part, which is usually called the optic
+ganglion, and is composed of two ganglia, Op. g. I. and Op. g. II., as
+already mentioned. These optic lobes are therefore homologous with the
+optic lobes of the vertebrate brain.
+
+The resemblance throughout is so striking as to force one to the conclusion
+that the retina of the vertebrate eye is a compound retina, composed of a
+retina and retinal ganglion of the type found in arthropods. From this it
+follows that the development of the vertebrate retina ought to show the
+formation of (1) an optic plate formed from the peripheral epidermis and
+not from the brain; (2) a part of the brain closely attached to this optic
+plate forming the retinal ganglion, which remains at the surface when the
+rest of the optic ganglion withdraws; (3) an optic nerve formed in
+consequence of this withdrawal, as the connection between the retinal and
+cerebral parts of the optic ganglion.
+
+This appears to me exactly what the developmental process does show
+according to Götte's investigations. He asserts that the retina arises from
+an optic plate, being the optical portion of his 'Sinnes-platte.' At an
+early stage this is separated by a furrow (Furche) from the general mass of
+epidermal cells which ultimately form the brain. This separation then
+vanishes, and the retina and brain-mass {102}become inextricably united
+into a mass of cells, which are still situated at the surface. By the
+closure of the cephalic plate and the withdrawal of the brain away from the
+surface, a retinal mass of cells is left at the surface connected with the
+tubular central nervous system by the hollow optic diverticulum or primary
+optic vesicle. If we regard only the retinal and nervous elements, and for
+the moment pay no attention to the existence of the tube, Götte's
+observation that the true retina has been formed from the optic plate
+(Sinnes-platte) to which the retinal portion of the brain (retinal
+ganglion) has become firmly fixed, and that then the optic nerve has been
+formed by the withdrawal of the rest of the brain (optic lobes), is word
+for word applicable to the description of the development of the compound
+retina of the arthropod eye, as has been already stated.
+
+
+THE SIGNIFICANCE OF THE OPTIC DIVERTICULA.
+
+The origin of the retina from an optic epidermal plate in vertebrates, as
+in all other animals, brings the cephalic eyes of all animals into the same
+category, and leaves the vertebrate eye no longer in an isolated and
+unnatural position. In one point the retina of the vertebrate eye differs
+from that of a compound retina of an invertebrate; in the former, a
+striking supporting tissue exists, known as Müller's fibres, which is
+absent in the latter. This difference of structure is closely associated
+with another of the same character as in the central nervous system, viz.
+the apparent development of the nervous part from a tube. We see, in fact,
+that the retinal and nervous arrangements of the vertebrate eye are
+comparable with those of the arthropod eye, in precisely the same way and
+to the same extent as the nervous matter of the brain of the vertebrate is
+comparable with the brain of the arthropod. In both cases the nervous
+matter is, in structure, position, and function, absolutely homologous; in
+both cases there is found in the vertebrate something extra which is not
+found in the invertebrate--viz. a hollow tube, the walls of which, in the
+case of the brain, are utilized as supporting tissues for the nerve
+structures. The explanation of this difference in the case of the brain is
+the fundamental idea of my whole theory, namely, that the hollow tube is in
+reality the cephalic stomach of the invertebrate, around which the nervous
+brain-matter was originally grouped in precisely the same manner as in the
+invertebrate. What, then, are the optic diverticula?
+
+{103}"The formation of the eye," as taught by Balfour, "commences with the
+appearance of a pair of hollow outgrowths from the anterior cerebral
+vesicle. These outgrowths, known as the optic vesicles, at first open
+freely into the cavity of the anterior cerebral vesicle. From this they
+soon, however, become partially constricted, and form vesicles united to
+the base of the brain by comparatively narrow, hollow stalks, the rudiments
+of the optic nerves."
+
+"After the establishment of the optic nerves, there takes place (1) the
+formation of the lens, and (2) the formation of the optic cup from the
+walls of the primary optic vesicle."
+
+He then goes on to explain how the formation of the lens forms the optic
+cup with its double walls from the primary optic vesicle, and says--
+
+"Of its double walls, the inner, or anterior, is formed from the front
+portion, the outer, or posterior, from the hind portion of the wall of the
+primary optic vesicle. The inner, or anterior, which very speedily becomes
+thicker than the other, is converted into the retina; in the outer, or
+posterior, which remains thin, pigment is eventually deposited, and it
+ultimately becomes the tesselated pigment-layer of the choroid."
+
+The difficulties in connection with this view of the origin of the eye are
+exceedingly great, so great as to have caused Balfour to discuss seriously
+Lankester's suggestion that the eye must have been at one time within the
+brain, and that the ancestor of the vertebrate was therefore a transparent
+animal, so that light might get to the eye through the outer covering and
+the brain-mass; a suggestion, the unsatisfactory nature of which Balfour
+himself confessed. Is there really evidence of any part of either retina or
+optic nerve being formed from the epithelial lining of the tube?
+
+This tube is formed as a direct continuation of the tube of the central
+nervous system, and we can therefore apply to it the same arguments as have
+been used in the discussion of the meaning of the latter tube. Now, the
+striking point in the latter case is the fact that the lining membrane of
+the central canal is in so many parts absolutely free from nervous matter,
+and so shows, as in the so-called choroid plexuses, its simple, non-nervous
+epithelial structure. This also we find in the optic diverticulum. Where
+there is no evidence of any invasion of the tube by nervous elements, there
+it retains its simple non-nervous character of a tube composed of a single
+layer of {104}epithelial cells--viz. in that part of the tube which, as
+Balfour says, remains thin, in which pigment is eventually deposited, and
+which ultimately becomes the tesselated pigment-layer of the choroid.
+Nobody has ever suggested that this pigment-layer is nervous matter, or
+ever was, or ever will be, nervous matter; it is in precisely the same
+category as the membranous roof of the brain in Ammocoetes, which never
+was, and never will be, nervous matter. Yet, according to the old
+embryology both in the case of the eye and the brain, the pigment-layer and
+the so-called choroid plexuses are a part of the tubular nervous system.
+
+Turning now to the optic nerve, Balfour describes it as derived from the
+hollow stalk of the optic vesicle. He says--
+
+"At first the optic nerve is equally continuous with both walls of the
+optic cup, as must of necessity be the case, since the interval which
+primarily exists between the two walls is continuous with the cavity of the
+stalk. When the cavity within the optic nerve vanishes, and the fibres of
+the optic nerve appear, all connection is ruptured between the outer wall
+of the optic cup and the optic nerve, and the optic nerve simply perforates
+the outer wall, and becomes continuous with the inner one."
+
+In this description Balfour, because he derived the optic nerve fibres from
+the epithelial wall of the optic stalk, of necessity supposed that such
+fibres originally supplied both the outer and inner walls of the optic cup
+and, therefore, seeing that when the fibres of the optic nerve appear they
+do not supply the outer wall, he supposes that their original connection
+with the outer wall is ruptured, because a discontinuity of the epithelial
+lining takes place coincidently with the appearance of the optic
+nerve-fibres, and, according to him, the optic nerve simply perforates the
+outer wall and becomes continuous with the inner one. This last statement
+is very difficult to understand. I presume he meant that some of the fibres
+of the optic nerve supplied from the beginning the inner wall of the optic
+cup, but that others which originally supplied the outer wall were first
+ruptured, then perforated the outer wall, and finally completed the supply
+to the inner wall or retina.
+
+This statement of Balfour's is the necessary consequence of his belief,
+that the epithelial cells of the optic stalk gave rise to the fibres of the
+optic nerve. If, instead of this, we follow Kölliker and His, who state
+that the optic nerve-fibres are formed outside the {105}epithelial walls of
+the optic stalk, and that the cells of the latter form supporting
+structures for the nerve-fibres, then the position of the optic nerve
+becomes perfectly simple and satisfactory without any rupturing of its
+connection with the outer wall and subsequent perforation, for the optic
+nerve-fibres from their very first appearance pass directly to supply the
+retina--_i.e._ the inner wall of the optic cup and nothing else.
+
+They pass, as is well known, without any perforation by way of the
+choroidal slit to the inner surface of the inner wall (retina) of the optic
+cup; then, when the choroidal slit becomes closed by the expansion of the
+optic cup, the optic nerve naturally becomes situated in the centre of the
+base of the cup and spreads over its inner surface as that surface expands.
+
+A section across the optic cup at an early stage at the junction of the
+optic stalk and optic cup would be represented by the upper diagram in Fig.
+43; at a later stage, when the choroidal slit is closed, by the lower
+diagram.
+
+[Illustration: FIG. 43.--DIAGRAM OF THE RELATION OF THE OPTIC NERVE TO THE
+OPTIC CUP.
+
+The upper diagram represents a stage before the formation of the choroidal
+slit, the lower one the stage of closure of the choroidal slit. _R._,
+retina; _O.n._, optic nerve; _p._, pigment epithelium.]
+
+The evident truth of this manner of looking at the origin of the optic
+nerve is demonstrated by the appearance of the optic nerve in Ammocoetes
+and Petromyzon. In the latter, although the development is complete, and
+the eye, and consequently also the optic nerve-fibres, are fully
+functional, there is still present in the axial core of the nerve a row of
+epithelial cells (Axenstrang) which are altered so as to form supporting
+structures, in the same way as a row of epithelial cells in the retina is
+altered to form the system of supporting cells known by the name of the
+Müllerian fibres.
+
+The origin of this axial core of cells is perfectly clear, as has been
+pointed out by W. Müller. He says--
+
+"The development of the optic nerve shows peculiarities in {106}Petromyzon
+of such a character as to make this animal one of the most valuable objects
+for deciding the various controversial questions connected with the genesis
+of its elements. The lumen of the stalk of the primary optic vesicle is
+obliterated quite early by a proliferation of its lining epithelium. Also
+the original continuity of this epithelium with that of the pigment-layer
+is at an early period interrupted at the point of attachment of the optic
+stalk. This interruption occurs at the time when the fibres of the optic
+nerve first become visible."
+
+Further on he says--
+
+"The epithelium of the optic stalk develops entirely into supporting cells,
+which in Petromyzon fill up the original lumen and so form an axial core
+(Axenstrang) to the nerve-fibres which are formed entirely outside them;
+the projections of these supporting cells are directed towards the
+periphery, and so separate the bundles of the optic nerve-fibres. The
+mesodermal coat of the optic stalk takes no part in this separation; it
+simply forms the connective tissue sheath of the optic nerve. The
+development of the optic nerve in the higher vertebrates also obeys the
+same law, as I am bound to conclude from my own observations."
+
+The evidence, then, of Ammocoetes is very conclusive. Originally a tube
+composed of a single layer of epithelial cells became expanded at the
+anterior end to form a bulb. On the outside of this tube or stalk the
+fibres of the optic nerve make their appearance, arising from the
+ganglion-cell layer of the retina, and, passing over the surface of the
+epithelial tube at the choroidal fissure, proceed to the brain by way of
+the optic chiasma. Owing to the large number of fibres, their crossing at
+the junction of the stalk with the bulb, and the narrowness at this neck,
+the obliteration of the lumen of the tube which takes place in the stalk is
+carried out to a still greater extent at this narrow part. The result of
+this is that all continuity of the cell-layers of the original tube of the
+optic stalk with those of both the inner and outer walls of the bulb is
+interrupted, and all that remains in this spot of the original continuous
+line of cells which connected the tube of the stalk with that of the bulb
+are possibly some of the groups of cells which are found scattered among
+the fibres of the optic nerve at their entrance into the retina. Such
+separation of the originally continuous elements of the epithelial wall of
+the optic stalk, which is apparent only at this neck of the nerve in
+Petromyzon, takes place {107}along the whole of the optic nerve in the
+higher vertebrates, so that no continuous axial core of cells exist, but
+only scattered supporting cells.
+
+If further proof in support of this view be wanted, it is given by the
+evidence of physiology, which shows that the fibres of the optic nerve are
+not different from other nerve-fibres of the central nervous system, but
+that they degenerate when separated from their nerve-cell, and that the
+nerve-cell of which the optic nerve-fibre is a process is the large
+ganglion-cell of the ganglionic layer of the retina. The origin of the
+ganglionic layer of the retina cannot therefore be separated from that of
+the optic nerve-fibres. If the one is outside the epithelial tube, so is
+the other, and what holds true of the ganglionic layer must hold good of
+the rest of the retinal ganglion and, from all that has been said, of the
+retina itself. We therefore come to the conclusion that the evidence is
+distinctly in favour of the view, that the retina and optic nerve in the
+true sense are structures which originally were outside a non-nervous tube,
+but, just like the central nervous system as a whole, have amalgamated so
+closely with the elements of this tube as to utilize them for supporting
+structures. One part of this non-nervous tube, its dorsal wall, like the
+corresponding part of the brain-tube, still retains its original character,
+and by the deposition of pigment has been pressed into the service of the
+eye to form the pigmented epithelial layer.
+
+We can, however, go further than this, for we know definitely in the case
+of the retina what the fate of the epithelial cells lining this tube has
+been. They have become the system of supporting structures known as
+Müllerian fibres.
+
+The epithelial layer of the primary optic vesicle can be traced into direct
+continuity with the lining epithelium of the brain cavity, as a single
+layer of epithelial cells in the core of the optic nerve, forming the optic
+stalk, which, in consequence of close contact, becomes the well-known axial
+layer of supporting cells. This epithelial layer of the optic stalk then
+expands to form the optic bulb, the outer or dorsal wall of which still
+remains as a single layer of epithelium and becomes the layer of pigment
+epithelium. This layer of epithelium becomes doubled on itself by the
+approximation of the inner or ventral wall of the optic cup to the outer or
+dorsal wall in consequence of the presence of the lens, and still remaining
+a single layer, forms the _pars ciliaris retinæ_; then suddenly, at the
+_ora {108}serrata_, the single epithelial layer vanishes, and the layers of
+the retina take its place. It has long been known, however, that even
+throughout the retina this single epithelial layer still continues, being
+known as the fibres of Müller. This is how the fact is described in the
+last edition of Foster's "Text-book of Physiology," p. 1308--
+
+"Stretching radially from the inner to the outer limiting membrane in all
+regions of the retina are certain peculiar-shaped bodies known as the
+radial fibres of Müller. Each fibre is the outcome of the changes undergone
+by what was at first a simple columnar epithelial cell. The changes are, in
+the main, that the columnar form is elongated into that of a more or less
+prismatic fibre, the edges of which become variously branched, and that
+while the nucleus is retained the cell substance becomes converted into
+neuro-keratin. And, indeed, at the _ora serrata_ the fibres of Müller may
+be seen suddenly to lose their peculiar features and to pass into the
+ordinary columnar cells which form the _pars ciliaris retinæ_."
+
+[Illustration: FIG. 44.--DIAGRAM REPRESENTING THE SINGLE-LAYERED EPITHELIAL
+TUBE OF THE VERTEBRATE EYE AFTER REMOVAL OF THE NERVOUS AND RETINAL
+ELEMENTS.
+
+_O.n._, axial core of cells in optic nerve; _p._, pigment epithelium;
+_p.c.r., pars ciliaris retinæ_; _m.f._, Müllerian fibres; _l._, lens.]
+
+It is then absolutely clear that the essential parts of the eye may be
+considered as composed of two parts--
+
+1. A tube or diverticulum from the tube of the central nervous system,
+composed throughout of a single layer of epithelium, which forms the
+supporting axial cells in the optic nerve, the pigment epithelium and the
+Müllerian fibres of the retina. Such a tube would be represented by the
+accompanying Fig. 44, and the left side of Fig. 41.
+
+2. The retina proper with the retinal ganglion and the optic nerve-fibres
+as already described. In this part supporting elements are found, just as
+in any other compound retina, of the nature of neuroglia, which are
+independent of the Müllerian fibres.
+
+{109}Of these two parts we have already seen that the second is to all
+intents and purposes a compound retina of a crustacean eye, and seeing that
+the single-layered epithelial tube is continuous with the single-layered
+epithelial tube of the central nervous system--_i.e._ with the cephalic
+part of the gut of the arthropod ancestor--it follows with certainty that
+the ancestor of the vertebrates must have possessed two anterior
+diverticula of the gut, with the wall of which, near the anterior
+extremity, the compound retina has amalgamated on either side, just as the
+infra-oesophageal ganglia have amalgamated with the ventral wall of the
+main gut-tube. In this way, and in this way alone, does the interpretation
+of the structure of the vertebrate lateral eye harmonize in the most
+perfect manner with the rest of the conclusions already arrived at.
+
+The question therefore arises:--Have we any grounds for believing that the
+ancient forms of primitive crustaceans and primitive arachnids, which were
+so abundant in the time when the Cephalaspids appeared, possessed two
+anterior diverticula of the stomach, such as the consideration of the
+vertebrate eye strongly indicates must have been the case?
+
+The beautiful pictures of Blanchard, and his description, show how, on the
+arachnid side, paired diverticula of the stomach are nearly universal in
+the group. Thus, although they are not present in the scorpions, still, in
+the Thelyphonidæ, Phrynidæ, Solpugidæ, Mygalidæ, the most marked
+characteristic of the stomach-region is the presence of four pairs of
+coecal diverticula, which spread laterally over the prosomatic region. In
+the spiders the number of such diverticula increases, and the whole
+prosomatic region becomes filled up with these tubes. Blanchard considers
+that they form nutrient tubes for the direct nutrition of the organs in the
+prosoma, especially the important brain-region of the central nervous
+system. He points out that these animals are blood-suckers, and that,
+therefore, their food is already in a suitable form for purposes of
+nutrition when it is taken in by them, so that, as it were, the anterior
+part of the gut is transformed into a series of vessels or diverticula
+conveying blood directly to the important organs in the prosoma, by means
+of which they obtain nourishment in addition to their own blood-supply.
+
+The universality of such diverticula among the arachnids makes it highly
+probable that their progenitors did possess an alimentary canal with one or
+more pairs of anterior diverticula. In the {110}vertebrate, however, the
+paired diverticula are associated with a compound retina, a combination
+which does not occur among living arachnids; we must, therefore, examine
+the crustacean group for the desired combination, and naturally the most
+likely group to examine is the Phyllopoda, especially such primitive forms
+as Branchipus and Artemia, for it is universally acknowledged that these
+forms are the nearest living representatives of the trilobites. If,
+therefore, it be found that the retina and optic nerve in Artemia is in
+specially close connection with an anterior diverticulum of the gut on each
+side, then it is almost certain that such a combination existed also in the
+trilobites.
+
+[Illustration: FIG. 45.--SECTION THROUGH ONE OF THE TWO ANTERIOR
+DIVERTICULA OF THE GUT IN ARTEMIA AND THE RETINAL GANGLION.
+
+The section is through the extreme anterior end of the diverticulum, thus
+cutting through many of the columnar cells at right angles to their axis.
+_Al._, gut diverticulum; _rt. gl._, retinal ganglion.]
+
+{111}[Illustration: FIG. 46.--THE BRAIN, EYES, AND ANTERIOR TERMINATION OF
+THE ALIMENTARY CANAL OF ARTEMIA, VIEWED FROM THE DORSAL ASPECT.
+
+_Br._, brain; _l.e._, lateral eyes; _c.e._, median eyes; _Al._, alimentary
+canal.]
+
+[Illustration: FIG. 47.--A, THE FORMATION OF THE RETINA OF THE EYE OF
+AMMOCOETES (after SCOTT); B, THE FORMATION OF THE RETINA OF THE EYE OF
+AMMOCOETES, ON MY THEORY.
+
+_R._, retina; _l._, lens; _O.n._, optic nerve fibres; _Al._, cephalic end
+of invertebrate alimentary canal; _V._, cavity of ventricles of brain;
+_Al.d._, anterior diverticulum of alimentary canal; _op.d._, optic
+diverticulum.]
+
+My friend Mr. W. B. Hardy has especially investigated the nervous system of
+Artemia. In the course of his work he cut serial sections through the whole
+animal, and, as mentioned in my paper in the _Journal of Anatomy and
+Physiology_, he discovered that the retinal ganglion of each lateral eye is
+so closely attached to the end of the corresponding diverticulum of the gut
+that the lining cells of the ventral part of the diverticulum form a lining
+to the retinal ganglion (Fig. 45). In this animal there are only two
+gut-diverticula, which are situated most anteriorly. I have plotted out
+this series of sections by means of a camera lucida, with the result that
+the retina appears as a bulging attached ventro-laterally to the extremity
+of each gut-diverticulum, as is shown in Fig. 46. It is instructive to
+compare with this figure Scott's picture of the developing eye in
+Ammocoetes, where he figures the retina as {112}a bulging attached
+ventrally to the extremity of the narrow tube of the optic diverticulum. In
+Fig. 47, A, I reproduce this figure of Scott, and by the side of it, Fig.
+47, B, I have represented the origin of the vertebrate eye as I believe it
+to have occurred.
+
+We see, then, this very striking fact, that in the most primitive of the
+Crustacea, not only are there two anterior diverticula of the gut, but also
+the retinal ganglion of the lateral eye is in specially close connection
+with the end of the diverticulum on each side. In fact, we find in the
+nearest living representative of the trilobites a retina and retinal
+ganglion and optic nerve, closely resembling that of the vertebrate, in
+close connection with an epithelial tube which has nothing to do with the
+organ of sight, but is one of a pair of anterior gut-diverticula. It is
+impossible to obtain more decisive evidence that the trilobites possessed a
+pair of gut-diverticula surrounded to a greater or less extent by the
+retina and optic nerve of each lateral eye.
+
+Such anterior diverticula are commonly found in the lower Crustacea; they
+are usually known by the name of liver-diverticula, but as they take no
+part in digestion, and, on the contrary, represent that part of the gut
+which is most active in absorption, the term liver is not appropriate, and
+it is therefore better to call them simply the pair of anterior
+diverticula. Our knowledge of their function in Daphnia is given in a paper
+by Hardy and M^cDougall, which does not appear to be widely known. Hardy
+succeeded in feeding Daphnia with yolk of egg in which carmine grains were
+mixed, and was able in the living animal to watch the whole process of
+deglutition, digestion, and absorption. The food, which is made into a
+bolus, is moved down to the middle region of the gut, and there digestion
+takes place. Then by an antiperistaltic movement the more fluid products of
+the digestion-process are sent right forward into the two anterior
+diverticula, where the single layer of columnar cells lining these
+diverticula absorbs these products, the cells becoming thickly studded with
+fat-drops after a feed of yolk of egg. The carmine particles, which were
+driven forward with the proteid- and fat-particles, are not absorbed, but
+are at intervals driven back by contractions of the anterior diverticula to
+the middle region of the gut.
+
+These observations prove most clearly that the anterior diverticula have a
+special nutrient function, being the main channels by which new nutrient
+material is brought into the body, and, as {113}pointed out by the authors,
+it is a remarkable exception in the animal kingdom that absorption should
+occur in that portion of the gut which is anterior to the part in which
+digestion occurs. In all these animals the two anterior diverticula extend
+forwards over the brain, and, as we have seen in Artemia, the anterior
+extremity of each one is so intimately related to a part of the brain--viz.
+the retinal ganglion--as to form a lining membrane to that mass of
+nerve-cells. It follows, therefore, that the nutrient fluid absorbed by the
+cells of this part of the gut-diverticulum must be primarily for the
+service of the retinal ganglion. In fact, the relations of this anterior
+portion of the gut to the brain as a whole suggest strongly that the marked
+absorptive function of this anterior portion of the gut exists in order to
+supply nutrient material in the first place to the most vital, most
+important organ in the animal--the brain and its sense-organs. This
+conclusion is borne out by the fact that in these lower crustaceans the
+circulation of blood is of a very inefficient character, so that the
+tissues are mainly dependent for their nutrition on the fluid immediately
+surrounding them. It stands to reason that the establishment of the
+anterior portion of the gut as a nutrient tube to the brain would
+necessitate a closer and closer application of the brain to that tube, so
+that the process of amalgamation of the brain with the single layer of
+columnar epithelial cells which constitutes the wall of the gut (which we
+see in its initial stage in the retinal ganglion of Artemia), would tend
+rapidly to increase as more and more demands were made upon the brain,
+until at last both the supra- and infra-oesophageal ganglia, as well as the
+retinal ganglia and optic nerves, were in such close intimate connection
+with the ventral wall of the anterior portion of the gut and its
+diverticula as to form a brain and retina closely resembling that of
+Ammocoetes.
+
+Such an origin for the lateral eyes of the vertebrate explains in a simple
+and satisfactory manner why the vertebrate retina is a compound retina, and
+why both retina and optic nerve have an apparent tubular development.
+
+At the same time one discrepancy still exists which requires
+consideration--viz. in no arthropod eye possessing a compound retina is the
+retina inverted. All the known cases of inversion among arthropods occur in
+eyes, the retina of which is simple, and are all natural consequences of
+the process of invagination by which {114}the retina is formed. On the
+other hand, eyes with an inverted compound retina are not entirely unknown
+among invertebrates, for the eyes of Pecten and of Spondylus possess a
+retina which is inverted after the vertebrate fashion and still may be
+spoken of as compound rather than simple. It is clear that an invagination,
+the effect of which is an inversion of the retinal layer, would lead to the
+same result, whether the retinal optic nerves were short or long, whether,
+in fact, a retinal ganglion existed or not. Undoubtedly the presence of the
+retinal ganglion tends greatly to obscure any process of invagination, so
+that, as already mentioned, many observers, with Parker, consider the
+retina of the crustacean lateral eye to be formed by a thickening only,
+without any invagination, while Reichenbach says an obscure invagination
+does take place at a very early stage. So in the vertebrate eye most
+observers speak only of a thickening to form the retina, but Götte's
+observation points to an invagination of the optic plate at an early stage.
+So also in the eye of Pecten, Korschelt and Heider consider that the
+thickening, by which the retina is formed according to Patten, in reality
+hides an invagination process by means of which, as Bütschli suggests, an
+optic vesicle is formed in the usual manner. The retina is formed from the
+anterior wall of this vesicle, and is therefore inverted.
+
+The origin of the inverted retina of the vertebrate eye does not seem to me
+to present any great difficulty, especially when one takes into
+consideration the fact that the retina is inverted in the arachnid group,
+only in the lateral eyes. The inversion is usually regarded as associated
+with the tubular formation of the vertebrate retina, and it is possible to
+suppose that the retina became inverted in consequence of the involvement
+of the eye with the gut-diverticulum. I do not myself think any such
+explanation is at all probable, because I cannot conceive such a process
+taking place without a temporary derangement--to say the least of it--of
+the power of vision, and as I do not believe that evolution was brought
+about by sudden, startling changes, but by gradual, orderly adaptations,
+and as I also believe in the paramount importance of the organs of vision
+for the evolution of all the higher types of the animal kingdom, I must
+believe that in the evolution from the Arthropod to the Cephalaspid, the
+lateral eyes remained throughout functional. I therefore, for my own part,
+would say that the inversion of the {115}retina took place before the
+complete amalgamation with the gut-diverticulum, that, in fact, among the
+proto-crustacean, proto-arachnid forms there were some sufficiently
+arachnid to have an inverted retina, and at the same time sufficiently
+crustacean to possess a compound retina, and therefore a compound inverted
+retina after the vertebrate fashion existed in combination with the
+anterior gut-diverticula. Thus, when the eye and optic nerve sank into and
+amalgamated with the gut-diverticulum, neither the dioptric apparatus nor
+the nervous arrangements would suffer any alteration, and the animal
+throughout the whole process would possess organs of vision as good as
+before or after the period of transition.
+
+Further, not only the retina but also the dioptric apparatus of the
+vertebrate eye point to its origin from a type that combined the
+peculiarities of the arachnids and the crustaceans. In the former it is
+difficult to speak of a true lens, the function of a lens being undertaken
+by the cuticular surface of the cells of the corneagen (Mark's 'lentigen'),
+while in the latter, in addition to the corneal covering, a true lens
+exists in the shape of the crystalline cones. Further, these crustacean
+lenses are true lenses in the vertebrate sense, in that they are formed by
+modified hypodermal cells, and not bulgings of the cuticle, as in the
+arachnid. We see, in fact, that in the compound crustacean eye an extra
+layer of hypodermal cells has become inserted between the cornea and the
+retina to form a lens. So also in the vertebrate eye the lens is formed by
+an extra layer of the epidermal cells between the cornea and the retina.
+The fact that the vertebrate eye possesses a single lens, though its retina
+is composed of a number of ommatidia, while the crustacean eye possesses a
+lens to each ommatidium, may well be a consequence of the inversion of the
+vertebrate retina. It is most probable, as Korschelt and Heider have
+pointed out, that the retina of the arachnid eyes is composed of a number
+of ommatidia, just as in the crustacean eyes and in the inverted eyes it is
+probable that the image is focussed on to the pigmented tapetal layer, and
+thence reflected on to the percipient visual rods. In such a method of
+vision a single lens is a necessity, and so it must also be if, as I
+suppose, eyes existed with an inverted compound retina. Owing to the
+crustacean affinities of such eyes, a lens would be formed and the retina
+would be compound: owing to the arachnid affinities, the retina would be
+inverted and the hypodermal cells which formed the lens would be massed
+{116}together to form a single lens, instead of being collected in groups
+of four to form a series of crystalline cones.
+
+To sum up: The study of the vertebrate eyes, both median and lateral, leads
+to most important conclusions as to the origin of the vertebrates, for it
+shows clearly that whereas, as pointed out in this and subsequent chapters,
+their ancestors possessed distinct arachnid characteristics, yet that they
+cannot have been specialized arachnids, such as our present-day forms, but
+rather they were of a primitive arachnid type, with distinct crustacean
+characteristics: animals that were both crustacean and arachnid, but not
+yet specialized in either direction: animals, in fact, of precisely the
+kind which swarmed in the seas at the time when the vertebrates first made
+their appearance. In the opinion of the present day, the ancestral forms of
+the Crustacea, which were directly derived from the Annelida, may be
+classed as an hypothetical group the Protostraca, the nearest approach to
+which is a primitive Phyllopod.
+
+"Starting from the Protostraca," say Korschelt and Heider, "according to
+the present condition of our knowledge, we may, as has been already
+remarked, assume three great series of development of the Arthropodan
+stock, by the side of which a number of smaller independent branches have
+been retained. One of these series leads through the hypothetical primitive
+Phyllopod to the Crustacea; the second through the Palæostraca (Trilobita,
+Gigantostraca, Xiphosura) to the Arachnida; the third through forms
+resembling Peripatus to the Myriapoda and the Insecta. The Pantapoda and
+the Tardigrada must probably be regarded as smaller independent branches of
+the Arthropodan stock."
+
+To these "three great series of development of the Arthropodan stock" the
+evidence of Ammocoetes shows that a fourth must be added, which, starting
+also from the Protostraca, and closely connected with the second,
+palæostracan branch, leads through the Cephalaspidæ to the great kingdom of
+the Vertebrata. Such a direct linking of the earliest vertebrates with the
+Annelida through the Protostraca is of the utmost importance, as will be
+shown later in the explanation of the origin of the vertebrate coelom and
+urinary apparatus.
+
+
+{117}SUMMARY.
+
+  The most important discovery of recent years which gives a direct clue to
+  the ancestry of the vertebrates is undoubtedly the discovery that the
+  pineal gland is all that remains of a pair of median eyes which must have
+  been functional in the immediate ancestor of the vertebrate, seeing how
+  perfect one of them still is in Ammocoetes. The vertebrate ancestor,
+  then, possessed two pairs of eyes, one pair situated laterally, the other
+  median. In striking confirmation of the origin of the vertebrate from
+  Palæostracans it is universally admitted that all the Eurypterids and
+  such-like forms resembled Limulus in the possession of a pair of median
+  eyes, as well as of a pair of lateral eyes. Moreover, the ancient mailed
+  fishes the Ostracodermata, which are the earliest fishes known, are all
+  said to show the presence of a pair of median eyes as well as of a pair
+  of lateral eyes. This evidence directly suggests that the structure of
+  both the median and lateral vertebrate eyes ought to be very similar to
+  that of the median and lateral arthropod eyes. Such is, indeed, found to
+  be the case.
+
+  The retina of the simplest form of eye is formed from a group of the
+  superficial epidermal cells, and the rods or rhabdites are formed from
+  the cuticular covering of these cells; the optic nerve passes from these
+  cells to the deeper-lying brain. This kind of retina may be called a
+  simple retina, and characterizes the eyes, both median and lateral, of
+  the scorpion group.
+
+  In other cases a portion of the optic ganglion remains at the surface,
+  when the brain sinks inwards, in close contiguity to the epidermal
+  sense-cells which form the retina; a tract of fibres connects this optic
+  ganglion with the underlying brain, and is known as the optic nerve. Such
+  a retina may be called a compound retina and characterizes the lateral
+  eyes of both crustaceans and vertebrates. Also, owing to the method of
+  formation of the retina by invagination, the cuticular surface of the
+  retinal sense-cells, from which the rods are formed, may be directed
+  towards the source of light or away from it. In the first case the retina
+  may be called upright, in the second inverted.
+
+  Such inverted retinas are found in the vertebrate lateral eyes and in the
+  lateral eyes of the arachnids, but not of the crustaceans.
+
+  The evidence shows that all the invertebrate median eyes possess a simple
+  upright retina, and in structure are remarkably like the right median or
+  pineal eye of Ammocoetes; while the lateral eyes possess, as in the
+  crustaceans, an upright compound retina, or, as in many of the arachnids,
+  a simple inverted retina. The lateral eyes of the vertebrates alone
+  possess a compound inverted retina.
+
+  This retina, however, is extraordinarily similar in its structure to the
+  compound crustacean retina, and these similarities are more accentuated
+  in the retina of the lateral eye of Petromyzon than that of the higher
+  vertebrates.
+
+  The evidence afforded by the lateral eye of the vertebrate points
+  unmistakably to the conclusion that the ancestor of the vertebrate
+  possessed both crustacean and arachnid characters--belonged, therefore,
+  to a group of animals which gave rise to both the crustacean and arachnid
+  groups. This is precisely the position of the Palæostracan group, which
+  is regarded as the ancestor of both the crustaceans and arachnids.
+  {118}In two respects the retina of the lateral eyes of vertebrates
+  differs from that of all arthropods, for it possesses a special
+  supporting structure, the Müllerian fibres, which do not exist in the
+  latter, and it is developed in connection with a tube, the optic
+  diverticulum, which is connected on each side with the main tube of the
+  central nervous system. These two differences are in reality one and the
+  same, for the Müllerian fibres are the altered lining cells of the optic
+  diverticulum, and this tube has the same significance as the rest of the
+  tube of the nervous system; it is something which has nothing to do with
+  the nervous portion of the retina but has become closely amalgamated with
+  it. The explanation is, word for word, the same as for the tubular
+  nervous system, and shows that the ancestor of the vertebrate possessed
+  two anterior diverticula of its alimentary canal which were in close
+  relationship to the optic ganglion and nerve of the lateral eye on each
+  side. It is again a striking coincidence to find that Artemia, which with
+  Branchipus represents a group of living crustaceans most nearly allied to
+  the trilobites, does possess two anterior diverticula of the gut which
+  are in extraordinarily close relationship with the optic ganglia of the
+  retina of the lateral eyes on each side.
+
+  The evidence of the optic apparatus of the vertebrate points most
+  remarkably to the derivation of the Vertebrata from the Palæostraca.
+
+
+
+
+{119}CHAPTER III
+
+_THE EVIDENCE OF THE SKELETON_
+
+  The bony and cartilaginous skeleton considered, not the
+  notochord.--Nature of the earliest cartilaginous skeleton.--The
+  mesosomatic skeleton of Ammocoetes; its topographical arrangement, its
+  structure, its origin in muco-cartilage.--The prosomatic skeleton of
+  Ammocoetes; the trabeculæ and parachordals, their structure, their origin
+  in white fibrous tissue.--The mesosomatic skeleton of Limulus compared
+  with that of Ammocoetes; similarity of position, of structure, of origin
+  in muco-cartilage.--The prosomatic skeleton of Limulus; the entosternite
+  or plastron compared with the trabeculæ of Ammocoetes; similarity of
+  position, of structure, of origin in fibrous tissue.--Summary.
+
+
+The explanation of the two optic diverticula given in the last chapter
+accounts in the same harmonious manner for every other part of the tube
+around which the central nervous system of the vertebrate has been grouped.
+The tube conforms in all respects to the simple epithelial tube which
+formed the alimentary canal of the ancient type of marine arthropods such
+as were dominant in the seas when the vertebrates first appeared. The whole
+evidence so far is so uniform and points so strongly in the direction of
+the origin of vertebrates from these ancient arthropods, as to make it an
+imperative duty to proceed further and to compare one by one the other
+parts of the central nervous system, together with their outgoing nerves in
+the two groups of animals.
+
+Before proceeding to do this, it is advisable first to consider the
+question of the origin of the vertebrate skeletal tissues, for this is the
+second of the great difficulties in the way of deriving vertebrates from
+arthropods, the one skeleton being an endo-skeleton composed of cartilage
+and bone, and the other an exo-skeleton composed of chitin. Here is a
+problem of a totally different kind to that we have just been considering,
+but of so fundamental a character that it must, if possible, be solved
+before passing on to the consideration of the cranial nerves and the organs
+they supply.
+
+{120}Is there any evidence which makes it possible to conceive the method
+by which the vertebrate skeleton may have arisen from the skeletal tissues
+of an arthropod? By the vertebrate skeleton I mean the bony and
+cartilaginous structures which form the backbone, the cranio-facial
+skeleton, the pectoral and pelvic girdles, and the bones of the limbs. I do
+not include the notochord in these skeletal tissues, because there is not
+the slightest evidence that the notochord played any part in the formation
+of these structures; the notochordal tissue is something _sui generis_, and
+never gives rise to cartilage or bone. The notochord happens to lie in the
+middle line of the body and is very conspicuous in the lowest vertebrate;
+with the development of the backbone the notochord becomes obliterated more
+and more, until at last it is visible in the higher vertebrates only in the
+embryo; but that obliteration is the result of the encroachment of the
+growing bone-masses, not the cause of their growth. Although, then, the
+notochord may in a sense be spoken of as the original supporting axial rod
+of the vertebrate, it is so different to the rest of the endo-skeleton, has
+so little to do with it, that the consideration of its origin is a thing
+apart, and must be treated by itself without reference to the origin of the
+cartilaginous and bony skeleton.
+
+
+THE COMMENCEMENT OF THE BONY SKELETON IN THE VERTEBRATE.
+
+What is the teaching of the vertebrate? What evidence is there as to the
+origin of the bony skeleton in the vertebrate phylum itself?
+
+The axial bony skeleton of the higher Mammalia consists of two parts, (1)
+the vertebral column with its attached bony parts, and (2) the
+cranio-facial skeleton. Of these two parts, the bony tissue of the first
+arises in the embryo from cartilage, of the second partly from cartilage,
+partly from membrane.
+
+In strict accordance with their embryonic origin is their phylogenetic
+origin: as we pass from the higher vertebrates to the lower these
+structures can be traced back to a cartilaginous and membranous condition,
+so that, as Parker has shown, the cranio-facial bony skeleton of the higher
+vertebrates can be derived directly from a non-bony cartilaginous skeleton,
+such as is seen in Petromyzon and the cartilaginous fishes.
+
+Balfour, in his "Comparative Embryology," states that the {121}primitive
+cartilaginous cranium is always composed of the following parts:--
+
+1. A pair of cartilaginous plates on each side of the cephalic section of
+the notochord known as the parachordals (_pa.ch._, Fig. 49; _iv._, Fig.
+48). These plates, together with the notochord (_ch._) enclosed between
+them, form a floor for the hind and mid-brain.
+
+[Illustration: FIG. 48.--EMBRYO PIG, TWO-THIRDS OF AN INCH LONG (from
+PARKER), ELEMENTS OF SKULL SEEN FROM BELOW.
+
+_ch._, notochord; _iv._, parachordals; _au._, auditory capsule; _py._,
+pituitary body; _tr._, trabecula; _ctr._, trabecular cornu; _pn._,
+pre-nasal cartilage; _ppg._, palato-pterygoid tract; _mn._, mandibular
+arch; _th.h._, first branchial arch; _VII.-XII._, cranial nerves.]
+
+[Illustration: FIG. 49.--HEAD OF EMBRYO DOG-FISH (from PARKER), BASAL VIEW
+OF CRANIUM FROM ABOVE.
+
+_ol._, olfactory sacs; _au._, auditory capsule; _py._, pituitary body;
+_pa.ch._, parachordal cartilage; _tr._, trabecula; _inf._, infundibulum;
+_pt.s._, pituitary space; _e._, eye.]
+
+2. A pair of bars forming the floor for the fore-brain, known as the
+trabeculæ (_tr_). These bars are continued forward from the parachordals.
+They meet posteriorly and embrace the front end of the notochord, and after
+separating for some distance bend in again in such a way as to enclose a
+space--the pituitary space (_pt.s._). In {122}front of this space they
+remain in contact, and generally unite. They extend forward into the nasal
+region (_pn._).
+
+3. The cartilaginous capsules of the sense organs. Of these the auditory
+(_au._) and the olfactory capsules (_ol._) unite more or less intimately
+with the cranial walls; while the optic capsules, forming the usually
+cartilaginous sclerotics, remain distinct.
+
+The parachordals and notochord form together the basilar plate, which forms
+the floor for that section of the brain belonging to the primitive postoral
+part of the head, and its extent corresponds roughly to that of the
+basioccipital of the adult skull.
+
+The trabeculæ, so far as their mere anatomical relations are concerned,
+play the same part in forming the floor for the front cerebral vesicle as
+do the parachordals for the mid- and hind-brain. They differ, however, from
+the parachordals in one important feature, viz. that except at their hinder
+end they do not embrace the notochord. The notochord always terminates at
+the infundibulum, and the trabeculæ always enclose a pituitary space, in
+which lies the infundibulum (_inf._) and the pituitary body (_py._).
+
+In the majority of the lower forms the trabeculæ arise quite independently
+of the parachordals, though the two sets of elements soon unite.
+
+The trabeculæ are usually somewhat lyre-shaped, meeting in front and
+behind, and leaving a large pituitary space between their middle parts.
+Into this space the whole base of the fore-brain primitively projects, but
+the space itself gradually becomes narrowed until it usually contains only
+the pituitary body.
+
+The trabecular floor of the brain does not long remain simple. Its sides
+grow vertically upwards, forming a lateral wall for the brain, in which in
+the higher types, two regions may be distinguished, viz. an alisphenoidal
+region behind, growing out from what is known as the basisphenoidal region
+of the primitive trabeculæ, and an orbito-sphenoidal region in front,
+growing out from the presphenoidal region of the trabeculæ. These plates
+form at first a continuous lateral wall of the cranium. The cartilaginous
+walls which grow up from the trabecular floor of the cranium generally
+extend upwards so as to form a roof, though almost always an imperfect
+roof, for the cranial cavity.
+
+The basi-cranial cartilaginous skeleton reduces itself always into
+trabeculæ and parachordals with olfactory and auditory cartilaginous
+capsules.
+
+{123}In addition, a branchial skeleton exists, which consists of a series
+of bars known as the branchial bars, so situated as to afford support to
+the successive branchial pouches. An anterior arch known as the mandibular
+arch (Fig. 50, _Mn._), placed in front of the hyo-mandibular cleft, and a
+second arch, known as the hyoid arch (_Hy._), placed in front of the
+hyo-branchial cleft, are developed in all types; the succeeding arches are
+known as the true branchial arches (_Br._), and are only fully developed in
+the Ichthyopsida. In all cases of jaw-bearing (gnathostomatous) vertebrates
+the first arch has become a supporting skeleton for the mouth (Fig. 51),
+and in the higher vertebrates in combination with the second or hyoid arch
+takes part in the formation of the ear-bones.
+
+[Illustration: FIG. 50.--HEAD OF EMBRYO DOG-FISH, ELEVEN LINES LONG. (From
+PARKER.)
+
+_Tr._, trabecula; _Mn._, mandibular cartilage; _Hy._, hyoid arch; _Br_1._,
+first branchial arch; _Na._, olfactory sac; _E._, eye; _Au._, auditory
+capsule; _Hm._, hemisphere; _C_1_, _C_2_, _C_3_, cerebral vesicles.]
+
+[Illustration: FIG. 51.--SKULL OF ADULT DOG-FISH, SIDE VIEW. (From PARKER.)
+
+_cr._, cranium; _Br._, branchial arches; _Mn._ + _Hy._, mandibular and
+hyoid arches.]
+
+The true branchial arches persist, to a certain extent, in the Amphibia,
+and become still more degenerated in the Amniota (reptiles, birds, and
+mammals) in correlation with the total disappearance of a branchial
+respiration at all periods of their life. {124}Their remnants become more
+or less important parts of the hyoid bone, and are employed solely in
+support of the tongue.
+
+In no single animal is there any evidence that the foremost arch, the
+mandibular, is a true branchial arch. As low down as the Elasmobranchs it
+becomes divided into two elements which form respectively the upper and
+lower jaws; the hyoid arch, on the other hand, although it has altered its
+form and acquired the secondary function of supporting the mandibular arch,
+still retains its respiratory function.
+
+The evidence afforded by the mode of formation of the skeletal tissues of
+vertebrates down to the Elasmobranchs indicates that the primitive cranial
+skeleton arose from two paired basal cartilages, the parachordals and
+trabeculæ, to which were attached respectively cartilaginous cases
+enclosing the organs of hearing and smell. In addition, the branchial
+portion of the cranial region was provided with cartilaginous bars arranged
+serially for the support of the branchiæ, with the exception of the
+foremost, the mandibular bar, which formed supporting tissues for the
+mouth--the upper and lower jaws.
+
+Just as in past times the spinal nerves and the segments they supplied were
+supposed to represent the type on which the original vertebrate was built,
+so also the spinal vertebræ afforded the type of the segmented skeleton,
+and the anatomists of those days strove hard to resolve the cranio-facial
+skeleton into a series of modified vertebræ. Owing especially to the
+labours of Huxley, who showed that the segmentation in the head-region was
+essentially a segmentation due to the presence of branchial bars, this
+conception was finally laid to rest and nowadays it is admitted to be
+hopeless to resolve the cranium into vertebral segments. Still, however,
+the vertebrate is a segmented animal and its segmented nature is visible in
+the cranial region, so far as the skeletal tissues are concerned, in the
+shape of the series of branchial and visceral bars.
+
+To this segmentation the name of 'branchiomeric' has been given, while that
+due to the presence of vertebræ is called 'mesomeric.'
+
+As we have seen, the internal bony skeleton of the vertebrate commences as
+a cartilaginous and membranous skeleton. For this reason the preservation
+of such skeletons is impossible in the fossil form, unless the cartilage
+has become impregnated with lime salts, so that there is but little hope of
+ever obtaining traces of such {125}structures in the fossils of the
+Silurian age either among the vertebrate or invertebrate remains.
+Fortunately for this investigation there are still living on the earth two
+representatives of that age; on the invertebrate side Limulus, and on the
+vertebrate side Ammocoetes.
+
+The Elasmobranchs represent the most primitive of the gnathostomatous
+vertebrates. Below them come the Agnatha, known as the cyclostomatous
+fishes or Marsipobranchii, the lampreys (Petromyzon) and the hag-fishes
+(Myxine).
+
+The skeleton of Petromyzon (Fig. 52) consists of a cranio-facial skeleton
+composed of a cartilaginous unsegmented cranium, with the basal trabeculæ
+and parachordals and a series of branchial and visceral cartilaginous bars
+forming the so-called branchial basket-work; to these must be added
+auditory and nasal capsules. In contradistinction to this elaborate
+cranio-facial skeleton, the spinal vertebral skeleton is represented only
+by segmentally arranged small pieces of cartilage formed in the connective
+tissue dissepiments between segmented sheets of body-muscles (myotomes).
+
+[Illustration: FIG. 52.--SKELETON OF PETROMYZON. (From PARKER.)
+
+_na._, nasal capsule; _au._, auditory capsule; _nc._, notochord.]
+
+But Petromyzon is derived from Ammocoetes by a remarkable process of
+transformation, and a most important part of that transformation is the
+formation of new cartilaginous structures. Thus we see that in Ammocoetes
+there is no sign of a cartilaginous vertebral column; at transformation the
+rudimentary vertebræ of Petromyzon are formed. In Ammocoetes the brain-case
+is a simple fibrous membranous covering; at transformation this becomes
+cartilaginous. In Ammocoetes there are no cartilaginous structures
+corresponding to the sub-ocular arches; these are all formed at
+transformation. It follows, that we can trace back the bony skeleton of the
+vertebrate head to the skeleton of Ammocoetes, and we may therefore
+conclude {126}that the primitive cartilaginous skeleton of the vertebrate
+consisted of the following structures (Fig. 53, B), viz. the branchial bars
+forming a basket-work, the trabeculæ and parachordals, the auditory and
+nasal capsules--a clear proof that the cranial skeleton is older than the
+spinal. Of these structures the branchial bars are the only evidently
+segmented parts.
+
+[Illustration: FIG. 53.--COMPARISON OF CARTILAGINOUS SKELETON OF LIMULUS
+AND AMMOCOETES.
+
+A, Diagram of cartilaginous skeleton of Limulus. _Soft cartilage_,
+entapophysial ligaments, deep black; branchial bars simply hatched; _hard
+cartilage_, lateral trabeculæ of entosternite, netted; _Ph._, position of
+pharynx.
+
+B, Diagram of cartilaginous skeleton of Ammocoetes. _Soft cartilage_,
+sub-chordal cartilaginous bands, deep black; branchial basket-work (first
+formed part), simply hatched; _hard cartilage_, cranio-facial skeleton,
+trabeculæ, parachordals and auditory capsules, netted; _Inf._, position of
+tube of infundibulum (old oesophagus).]
+
+
+THE SOFT CARTILAGE OF THE BRANCHIAL SKELETON OF AMMOCOETES.
+
+The study of Ammocoetes gives yet another clue to the nature of the
+earliest skeleton, for these two marked groups of cartilage--the branchial
+and basi-cranial--are characterized by a difference in structure as well as
+a difference in topographical position. J. Müller was the first to point
+out that these two sets of cartilages differ in appearance and
+constitution, and he gave to them the name of yellow and grey cartilage.
+Parker has described them fully under the terms soft and hard cartilage,
+terms which Schaffer has also used, and I shall also make use of them here.
+The whole of the branchial cartilaginous skeleton is composed of soft
+cartilage, while the basi-cranial skeleton, consisting of trabeculæ,
+parachordals, and auditory capsule, is composed {127}of hard cartilage, the
+only soft cartilage in this region being that which forms the nasal
+capsule, not represented in Fig. 53, B.
+
+These two groups of cartilage arise independently, so that at first the
+basi-cranial system is quite separate from the branchial, and only late in
+the history of the animal is a junction effected between the branchial
+system and the trabeculæ and parachordals, an initial separation which is
+especially striking when we consider that in this animal all the
+cartilaginous structures of any one system are continuous: there is no sign
+of anything in the nature of joints.
+
+Of these two main groups, the branchial cartilages are formed first in the
+embryo, a fact which suggests that they are the most primitive of the
+vertebrate cartilages, and that, therefore, the first true formation of
+cartilage in the invertebrate ancestor may be looked for in the shape of
+bars supporting the branchial mechanism. The evidence of the origin of the
+cartilaginous structures in Ammocoetes is given by Shipley in the following
+words:--
+
+"The branchial bases are the first part of the skeleton to appear. They
+arise about the 24th day as straight bars of cartilage, lying external and
+slightly posterior to the branchial vessel.
+
+"The first traces of the basi-cranial skeleton appear on the 30th day as
+two rods of cartilage--the trabeculæ."
+
+Our attention must, in the first place, be directed to this branchial
+basket-work of Ammocoetes.
+
+Underlying the skin of Ammocoetes in the branchial region is situated the
+sheet of longitudinal body-muscles, divided into a series of segments or
+myotomes, which forms the somatic muscles so characteristic of all fishes.
+This muscular sheet is depicted on the left-hand side of Fig. 54. It does
+not extend over the lower lip or over that part in the middle line where
+the thyroid gland is situated. In these parts a sheet of peculiar tissue
+known by the name of muco-cartilage lies immediately under the skin,
+covering over the thyroid gland and lower lip. The somatic muscular sheet
+with the superjacent skin can be stripped off very easily owing to the
+vascularity and looseness of the tissue immediately underlying it. When
+this is done the branchial basket-work comes beautifully into view as is
+seen on the right-hand side of Fig. 54. It forms a cage within which the
+branchiæ and their muscles lie entirely concealed.
+
+This is the great characteristic of this most primitive form of the
+branchial cartilaginous bars and distinguishes it from the branchial
+{128}bars of other higher fishes, in that it forms a system of cartilages
+which lie external to the branchiæ--an extra-branchial system.
+
+This branchial basket-work is simpler in Ammocoetes than in Petromyzon, and
+its actual starting-point consists of a main transverse bar corresponding
+to each branchial segment; from this transverse bar the system of
+longitudinal bars by which the basket-work is formed has sprung. These
+transverse bars arise from a cartilaginous longitudinal rod, situated close
+against the notochord on each side. These rods may be called the subchordal
+cartilaginous bands (Fig. 53), and, according to the observations of
+Schneider and others, each subchordal band does not form at first a
+continuous cartilaginous rod, but the cartilage is conspicuous only at the
+places where the transverse bars arise. In the youngest Ammocoetes examined
+by Schaffer, he could find no absolute discontinuity of the cartilage
+except between the first two transverse bars, but he says that the thinning
+between the transverse bars was so marked as to make it highly probable
+that at an earlier stage there was discontinuity. The whole system of
+branchial bars and subchordal rods is at first absolutely disconnected from
+the cranial system of trabeculæ and parachordals, and only later do the two
+systems join.
+
+[Illustration: FIG. 54.--VENTRAL VIEW OF HEAD REGION OF AMMOCOETES.
+
+_Th._, thyroid gland; _M._, lower lip, with its muscles.]
+
+These observations on Ammocoetes lead most definitely to the conclusion
+that the starting-point of the whole cartilaginous skeleton of the
+vertebrate consisted of a series of transverse cartilaginous bars, for the
+purpose of supporting branchial segments; these were connected with two
+axial longitudinal cartilaginous rods, which at first contained cartilage
+only near the places of junction of the branchial {129}bars. This system
+may be called the mesosomatic skeleton, as it is entirely confined to the
+branchial or mesosomatic region.
+
+In addition to this primitive cartilaginous framework, which was formed for
+the support of the mesosomatic or respiratory segments, but at a slightly
+later period in the phylogenetic history, a separate cartilaginous system
+was formed for the support of the prosomatic segments, viz. the trabeculæ
+and parachordals with the auditory capsules: a system which was at first
+entirely separated from the mesosomatic, and, as we shall see, is more
+advanced in structure than the branchial system. Later still, the story is
+completed at the time of transformation to Petromyzon by the formation of
+the simple cartilaginous skull and the rudimentary vertebræ, the structure
+of which is also of a more advanced type.
+
+
+THE STRUCTURE OF THE SOFT BRANCHIAL CARTILAGE.
+
+Having considered the topographical position of the primitive branchial
+cartilaginous skeleton, we may now inquire, What was its structure and how
+was it formed?
+
+In the higher vertebrates various forms of cartilage are described, viz.
+hyaline, fibro-cartilage, elastic cartilage, and parenchymatous cartilage.
+Of these, the parenchymatous cartilage is looked upon as the most primitive
+form, because it preserves without modification the characters of embryonic
+cartilage.
+
+Embryology, then, would lead to the belief that the earliest form of
+cartilage in the vertebrate kingdom ought to be of this type, viz. large
+cells, each of which is enclosed in a simple capsule, so that the capsules
+of the cells form the whole of the matrix, and thus form a simple
+homogeneous honeycomb-structure, in the alveoli of which the
+cartilage-cells lie singly. If, then, the branchial cartilages of
+Ammocoetes are, as has just been argued, the representatives of the
+cartilaginous skeleton of the primitive vertebrate, it is reasonable to
+suppose that they should resemble in structure this embryonic cartilage.
+Such is undoubtedly the case: all observers who have described the
+branchial basket-work of Ammocoetes or Petromyzon have been struck with the
+extremely primitive character of the cartilage, and the last observer
+(Schaffer) describes it as composed of thin walls of homogeneous material,
+in which there are no lines of separation, which form a simple
+honeycomb-structure, in the alveoli {130}of which the separate cells lie
+singly. These branchial cartilages are each surrounded by a layer of
+perichondrium, and in Fig. 55, A, I give a picture of a section of a
+portion of one of the bars.
+
+[Illustration: FIG. 55.--A, BRANCHIAL CARTILAGE OF AMMOCOETES, STAINED WITH
+THIONIN. B, BRANCHIAL CARTILAGE OF LIMULUS, STAINED WITH THIONIN.]
+
+Hence we see that structurally as well as topographically the branchial
+bars of Ammocoetes justify their claim to be considered as the origin of
+the vertebrate cartilaginous framework.
+
+
+ON THE STRUCTURE OF THE MUCO-CARTILAGE IN AMMOCOETES.
+
+We can, however, go further than this, and ask how this cartilage itself is
+formed in Ammocoetes? The answer is most definite, most instructive and
+suggestive, for in all cases this particular kind of cartilage is formed
+from, or at all events in, a peculiar fibrous tissue, which was called by
+Schneider "Schleim-Knorpel," or muco-cartilage, a tissue which is
+distinguishable from other connective tissues, not only by its structural
+peculiarities, but also by its strong affinity for all dyes which
+differentiate mucoid or chondro-mucoid substances.
+
+This muco-cartilage is thus described by Schneider:--The perichondrium in
+Ammocoetes is not confined to the true cartilaginous structures, but
+extends itself in the form of thin plates in definite directions. Between
+these plates of perichondrium a peculiar tissue (Fig. 56)--the
+muco-cartilage--exists, consisting of fibrillæ, whose direction is mainly
+at right angles to the planes of the perichondrial plates, with star-shaped
+cells in among them, and with the spaces between the fibrillæ filled up
+with a semi-fluid mass.
+
+{131}From this tissue all the primitive cartilages which resemble the
+branchial bars are formed, either by the invasion of chondroblasts from the
+surrounding perichondrium, or by the proliferation and encapsulation of the
+cells of the muco-cartilage itself.
+
+[Illustration: FIG. 56.--SECTION OF MUCO-CARTILAGE FROM DORSAL HEAD-PLATE
+OF AMMOCOETES.]
+
+This very distinctive tissue--the muco-cartilage--is of very great
+importance in all questions of the origin of the skeletal tissues. In all
+descriptions of the skeletal tissues it has been practically disregarded
+until recent years when, besides my own observations, its distribution has
+been mapped out by Schaffer. Thus Parker, in his well-known description of
+the skeleton of the marsipobranch fishes, does not even mention its
+existence. Its importance is shown by its absolute disappearance at
+transformation and its non-occurrence in any of the higher vertebrates. It
+is entirely confined to the head-region, and its distribution there is most
+suggestive, for, as will be described fully later on, it forms a skeleton
+which both in structure and position resembles very closely the
+head-shields of cephalaspidian fishes. At the present part of my argument
+its more immediate interest lies in the method of tracing this tissue. For
+this purpose I made use of the micro-chemical reaction of thionin, a dye
+which, as shown by Hoyer, stains all mucin-containing substances a bright
+purple. Schaffer made use of a corresponding basophil stain, hæmalum. When
+stained with thionin, the matrix, or ground-substance of the branchial
+cartilages as well as the matrix or semi-fluid substance in which the
+fibrils of the muco-cartilaginous cells are embedded take on a deep purple
+colour, while the fibrous material of the cranial walls and other
+connective tissue strands, such as the perichondrium, are coloured light
+blue. Muco-cartilage, then, may be described as a peculiar form of
+connective tissue which differs from other connective tissue not only in
+its appearance but in {132}its chemical composition, for unlike white
+fibrous tissue it contains a large amount of mucin, and this tissue is the
+forerunner of the earliest cartilaginous vertebrate skeleton, the branchial
+bars of Ammocoetes.
+
+The conclusions to which we are led by the study of the structure,
+position, and mode of origin of these primitive cartilages of Ammocoetes
+may be thus summed up:--
+
+1.  The immediate ancestor of the vertebrate must have possessed a peculiar
+fibrous tissue--the ground-substance of which stained deep purple with
+thionin--in which cartilage arose.
+
+2.  The cartilage so formed was not like hyaline cartilage, but resembled
+in a striking manner parenchymatous cartilage.
+
+3.  This cartilage was situated partly in two axial longitudinal bands,
+partly as transverse bars, which supported the branchial apparatus.
+
+
+THE PROSOMATIC OR BASI-CRANIAL SKELETON OF AMMOCOETES.
+
+Before searching for any evidence of a similar tissue in any invertebrate
+group, it is advisable to consider the other portion of the cartilaginous
+skeleton of Ammocoetes, which consists of the trabeculæ, parachordals and
+auditory capsules--the basi-cranial skeleton--and is composed of hard, not
+soft cartilage.
+
+This basi-cranial skeleton represented in Fig. 53, B, is confined to the
+region of the notochord, the cranial walls being composed entirely of a
+white fibrous membrane. It is separated at first entirely from the
+sub-chordal portion of the branchial basket-work, and is composed of a
+foremost part, the trabeculæ (_Tr._), and of a hindermost part, the
+parachordals (_Pr.ch._), which are characterized by the attachment on each
+side of the large auditory capsule (_Au._). In Ammocoetes the trabecular
+bars are continuous with the parachordals, the junction being marked by a
+small lateral projection on each side, which at transformation is seen to
+play an important part in the formation of the sub-ocular arch. The
+trabecular bar lies close against the notochord on each side up to its
+termination; it then bends away from the middle line and curves round until
+it meets its fellow on the opposite side, thus forming, as it were, the
+head of a racquet of which the notochord forms the splice in the handle.
+The strings of the racquet are represented by a thin membrane, in the
+centre of which the position of the infundibulum (_Inf._) of the {133}brain
+can be clearly seen. In an earlier stage of Ammocoetes the two trabecular
+horns do not meet, but are separated by connective tissue, which afterwards
+becomes cartilaginous.
+
+As far, then, as the topography of this basi-cranial skeleton is concerned,
+the striking points are--the shape of the trabecular portion, diverging as
+it does around the infundibulum, and the presence on the parachordal
+portion of the two large auditory capsules.
+
+These two points indicate, on the hypothesis that infundibulum and
+oesophagus are convertible terms, that two supporting structures of a
+cartilaginous nature must have existed in the ancestor of the vertebrate,
+the first of which surrounded the oesophagus, and the second was in
+connection with its auditory apparatus.
+
+[Illustration: FIG. 57.--A, CARTILAGE OF TRABECULÆ OF AMMOCOETES, STAINED
+WITH HÆMATOXYLIN AND PICRIC ACID. B, NESTS OF CARTILAGE CELLS IN
+ENTOSTERNITE OF HYPOCTONUS, STAINED WITH HÆMATOXYLIN AND PICRIC ACID.]
+
+
+STRUCTURE OF THE HARD CARTILAGES.
+
+The structure of this hard cartilage of the trabeculæ and auditory capsules
+resembles that of the soft, in so far that it consists of large cells with
+a comparatively small amount of intercellular substance. Schaffer, who has
+described it lately, considers that it is a nearer approach to hyaline
+cartilage than the soft, but yet cannot be called hyaline cartilage in the
+usual sense of the term. Its peculiarities and its differences from the
+soft are especially well seen by its staining reactions. I have myself been
+particularly struck with the effect of picrocarmine or combined hæmatoxylin
+and picric acid {134}staining (Fig. 57). In the case of the soft cartilage
+the capsular substance stains respectively a brilliant red or blue, while
+that of the hard cartilage is coloured a deep yellow, so that the junction
+between the parachordals and the branchial cartilages is beautifully marked
+out. Then, again, with thionin, which gives so marked a reaction in the
+case of the soft cartilage, the hard cartilage of the auditory capsule is
+not stained at all, and in the trabeculæ the deep purple colour is confined
+to the mucoid cement-substance between the capsules, just as Schaffer has
+stated. The same kinds of reactions have been described by Schaffer: thus
+by double staining with hæmalum-eosin the hard cartilage stains red, the
+soft blue; and he points out that even with over-staining by hæmalum the
+auditory capsule remains colourless, just as I have noticed with thionin.
+He infers, precisely as I have done from the thionin reaction, that
+chondro-mucoid, which is so marked a constituent of the soft cartilage and
+of the muco-cartilage, is absent or present in but slight quantities in the
+hard cartilage. Similarly, he points out that double staining with
+tropoeolin-methyl-violet stains the hard cartilage a bright orange colour,
+and the soft cartilage a violet.
+
+The evidence, then, shows clearly that a marked chemical difference exists
+between these two cartilages, which may be expressed by saying that the one
+contains very largely a basophil substance, which we may speak of as
+belonging to the class of chondro-mucoid substances, while the other
+contains mainly an oxyphil substance, probably a chondro-gelatine
+substance.
+
+We may perhaps go further and attribute this difference of composition to a
+difference of origin; for whereas the soft cartilage is invariably formed
+in a special tissue, the muco-cartilage, which shows by its reaction how
+largely it is composed of a mucoid substance, the hard cartilage is
+certainly, in the case of the cartilage of the cranium where its origin has
+been clearly made out, formed in the membranous tissue of the cranium of
+Ammocoetes--_i.e._ in a tissue which stains light blue with thionin, and
+contains a gelatinous rather than a mucoid substratum.
+
+The best opportunity of finding out the mode of origin of the hard
+cartilage is afforded at the time of transformation, when so much of this
+kind of cartilage is formed anew. Unfortunately, it is very difficult to
+obtain the early transformation stages, consequently we cannot be said to
+possess any really exhaustive and {135}definite account of how the new
+cartilages are formed. Bujor, Kaensche, and Schaffer all profess to give a
+more or less definite account of their formation, and the one striking
+impression left on the mind of the reader is how their descriptions vary.
+In one point only are they agreed, and in that I also agree with them, viz.
+the manner in which the new cranial walls are formed. Schaffer describes
+the process as the invasion of chondroblasts into the homogeneous fibrous
+tissue of the cranial walls. Such chondroblasts not only form the
+cartilaginous framework, but also assimilate the fibrous tissue which they
+invade, so that finally all that remains of the original fibrous matrix in
+which the cartilage was formed are these lines of cement-substance between
+the groups of cartilage cells, which, containing some basophil material,
+are marked out, as already mentioned (Fig. 57).
+
+We may therefore conclude, from the investigation of Ammocoetes, that the
+front part of the basi-cranial skeleton arose as two trabecular bars, to
+which muscles were attached, situated bilaterally with respect to the
+central nervous system. These bars were composed of tendinous material with
+a gelatinous rather than a mucoid substratum, in which nests of
+cartilage-cells were formed, the cartilaginous material formed by these
+cells being of the hard variety, not staining with thionin, and staining
+yellow with picro-carmine, etc. By the increase of such nests and the
+assimilation of the intermediate fibrous material, the original
+fibro-cartilage was converted into the close-set semi-hyaline cartilage of
+the trabeculæ and auditory capsules, in which the fibrous material still
+marks out by its staining-reaction the limits of the cell-clusters.
+
+Such I gather to be Schaffer's conclusions, and they are certainly borne
+out by my own and Miss Alcock's observations. As far as we have had an
+opportunity of observing at present, the first process at transformation
+appears to consist of the invasion of the fibrous tissue of the cranial
+wall by groups of cells which form nests of cells between the fibrous
+strands. These nests of cells form round themselves capsular material, and
+thus form cell-territories of cartilage, which squeeze out and assimilate
+the surrounding fibrous tissue, until at last all that remains of the
+original fibrous matrix is the lines of cement-substance which mark out the
+limits of the various cell-groups.
+
+At present I am inclined to think that both soft and hard cartilage
+originate in a very similar manner, viz. by the formation of capsular
+{136}material around the invading chondroblasts, and that the difference in
+the resulting cartilage is mainly due to the difference in chemical
+composition of the matrix of the connective tissue which is invaded. Thus
+the difference may be formulated as follows:--
+
+The hard cartilage is formed by the invasion of chondroblasts into a
+fibrous tissue, which contains a gelatinous rather than a mucoid
+substratum, in contradistinction to the soft cartilage which is formed,
+probably also by the invasion of chondroblasts, in a tissue--the
+muco-cartilage--which contains a specially mucoid substratum.
+
+Such, then, is the very clearly defined starting-point of the vertebrate
+skeleton--two distinct formations of different histological and chemical
+structure,--the one forming a segmented branchial skeleton, the other a
+non-segmented basi-cranial skeleton.
+
+
+THE CARTILAGINOUS SKELETON OF LIMULUS.
+
+Among the whole of the invertebrates at present living on the earth, is
+there any sign of an internal cartilaginous skeleton that will give a
+direct clue to the origin of the primitive vertebrate skeleton? The answer
+to this question is most significant: only one animal among all those at
+present known possesses a cartilaginous skeleton, which is directly
+comparable with that of Ammocoetes, and here the comparison is very
+close--only one animal among the thousands of living invertebrate forms,
+and that animal is the only representative still surviving of the
+palæostracan group, which was the dominant race when the vertebrate first
+made its appearance. The Limulus, or king-crab, possesses a segmented
+branchial internal cartilaginous skeleton (Fig. 53, A), made up of the same
+kind of cartilage as the branchial skeleton of Ammocoetes, confined to the
+mesosomatic or branchial region, just as in Ammocoetes, forming, as in
+Ammocoetes, cartilaginous bars supporting the branchiæ, and these bars are
+situated externally to the branchiæ, as in Ammocoetes. In addition this
+animal possesses a basi-cranial internal semi-cartilaginous unsegmented
+plate known as the entosternite or plastron situated, with respect to the
+oesophagus, similarly to the position of the trabeculæ with respect to the
+infundibulum in Ammocoetes. Moreover, the cartilaginous cells in this
+tissue differ from those in the branchial region, in precisely the same
+manner as the hard cartilage differs from the soft in Ammocoetes.
+
+{137}This plastron, it is true, is found in other animals, all of which are
+members of the scorpion tribe, except in one instance, and this, strikingly
+enough, is the crustacean Apus--a strange primitive form, which is
+acknowledged to be the nearest representative of the Trilobita still living
+on the earth. None of these forms, however, possess any sign of an internal
+cartilaginous branchial skeleton, such as is possessed by Limulus.
+Scorpions, Apus, Limulus, are all surviving types of the stage of
+organization which had been reached in the animal world when the vertebrate
+first appeared.
+
+
+THE MESOSOMATIC OR RESPIRATORY SKELETON OF LIMULUS, COMPOSED OF SOFT
+CARTILAGE.
+
+Searching through the literature of the histology of the cartilaginous
+tissues in invertebrate animals, to see whether any cartilage had been
+described similar to that seen in the branchial cartilages of Ammocoetes,
+and whether such cartilage, if found, arose in a fibrous tissue resembling
+muco-cartilage, I was speedily rewarded by finding, in Ray Lankester's
+article on the tropho-skeletal tissues of Limulus, a picture of the
+cartilage of Limulus, which would have passed muster for a drawing of the
+branchial cartilage of Ammocoetes. This clue I followed out in the manner
+described in my former paper in the _Journal of Anatomy and Physiology_,
+and mapped out the topography of this remarkable tissue.
+
+Limulus, like other water-dwelling arthropods, breathes by means of gills
+attached to its appendages. These gill-bearing appendages are confined to
+the mesosomatic region, as is seen in Fig. 59; and these appendages are
+very different to the ordinary locomotor appendages, which are confined to
+the prosomatic region. Each appendage, as is seen in Fig. 58, consists
+mainly of a broad, basal part, which carries the gill-book on its under
+surface; the distal parts of the appendage have dwindled to mere rudiments
+and still exist, not for locomotor purposes, but because they carry on each
+segment organs of special importance to the animal (see Chapter XI.). As is
+seen in Fig. 58, the basal parts of each pair of appendages form a broad,
+flattened paddle, by means of which the animal is able to swim in a clumsy
+fashion. Very striking and suggestive is the difference between these
+gill-bearing mesosomatic appendages and the non-gill-bearing locomotor
+appendages of the prosoma.
+
+{138}[Illustration: FIG. 58.--TRANSVERSE SECTION THROUGH THE MESOSOMA OF
+LIMULUS, TO SHOW THE ANTERIOR (A) AND THE POSTERIOR (B) SURFACES OF A
+MESOSOMATIC OR BRANCHIAL APPENDAGE.
+
+In each figure the branchial cartilaginous bar, _Br.C._, has been exposed
+by dissection on one side. _Ent._, entapophysis; _Ent.l._, entapophysial
+ligament cut across; _Br.C._, branchial cartilaginous bar, which springs
+from the entapophysis; _H._, heart; _P._, pericardium; _Al._, alimentary
+canal; _N._, nerve cord; _L.V.S._, longitudinal venous sinus; _Dv._,
+dorso-ventral somatic muscle; _Vp._, veno-pericardial muscle.]
+
+At the base of each of these appendages, where it is attached to the body
+of the animal, the external chitinous surface is characterized by a
+peculiar stumpy, rod-like marking, and upon removing the chitinous
+covering, this surface-appearance is seen to correspond to a well-marked
+rod of cartilage (_Br.C._), which extends from the body {139}of the animal
+well into each appendage. This bar of cartilage arises on each side from
+the corresponding entapophysis (_Ent._), which is the name given to a
+chitinous spur which projects a short distance (Fig. 58, B) into the animal
+from the dorsal side, for the purpose of giving attachment to various
+segmental muscles. These entapophyses are formed by an invagination of the
+chitinous surface on the dorsal side and are confined to the mesosomatic
+region, so that the mesosomatic carapace indicates, by the number of
+entapophyses, the number of segments in that region, in contradistinction
+to the prosomatic carapace, which gives no indication on its surface of the
+number of its components.
+
+Each entapophysis is hollow and its walls are composed of chitin; but from
+the apex of each spur there stretches from spur to spur a band of tissue,
+called by Lankester the entapophysial ligament (_Ent.l._) (Fig. 58), and in
+this tissue cartilage is formed. Isolated cartilaginous cells, or rather
+groups of cells, are found here and there, but a concentration of such
+groups always takes place at each entapophysis, forming here a solid mass
+of cartilage, from which the massive cartilaginous bar of each branchial
+appendage arises.
+
+Further, not only is this cartilage exactly similar to parenchymatous
+cartilage, as it occurs in the branchial cartilages of Ammocoetes, but also
+its matrix stains a brilliant purple with thionin in striking contrast to
+the exceedingly slight light-blue colour of the surrounding perichondrium.
+In its chemical composition it shows, as might be expected, that it is a
+cartilage containing a very large amount of some mucin-body.
+
+
+THE MUCO-CARTILAGE OF LIMULUS.
+
+The resemblance between this structure and that of the branchial bars of
+Ammocoetes does not end even here, for, as already mentioned, the cartilage
+originates in a peculiar connective tissue band, the entapophysial
+ligament, and this tissue bears the same relation in its chemical reactions
+to the ordinary connective tissue of Limulus, as muco-cartilage does to the
+white fibrous tissue of Ammocoetes. The white connective tissue of Limulus,
+as already stated, resembles that of the vertebrate more than does the
+connective tissue of any other invertebrate, and, similarly to that of
+Ammocoetes, does not stain, or gives only a light-blue tinge with thionin.
+The tissue of {140}the entapophysial ligament, on the contrary, just like
+muco-cartilage, takes on an intense purple colour when stained with
+thionin. It possesses a mucoid substratum, just as does muco-cartilage, and
+in both cases a perfectly similar soft cartilage is born from it.
+
+[Illustration: FIG. 59.--DIAGRAM OF LIMULUS, TO SHOW THE NERVES TO THE
+APPENDAGES (1-13) AND THE BRANCHIAL CARTILAGES.
+
+The branchial cartilages and the entapophysial ligaments are coloured blue,
+the branchiæ red. _gl._, generative and hepatic glands surrounding the
+central nervous system and passing into the base of the flabellum (_fl._).]
+
+One difference, however, exists between the branchial cartilages of these
+two animals; the innermost axial layer of the branchial bar of Limulus is
+very apt to contain a specially hard substance, apparently chalky in
+nature, so that it breaks up in sections, and gives the appearance of a
+broken-down spongy mass; if, however, the tissue is first placed in a
+solution of hydrochloric acid, it then cuts easily, and the whole tissue is
+seen to be of the same structure throughout, the main difference being that
+the capsular spaces in the axial region are much larger and much more free
+from cell-protoplasm than are those of the smaller younger cells near the
+periphery.
+
+{141}I have attempted in Fig. 53 to represent this close resemblance
+between the segmented branchial skeleton of Limulus and of Ammocoetes, a
+resemblance so close as to reach even to minute details, such as the
+thinning out of the cartilage in the subchordal bands and entapophysial
+ligaments respectively between the places where the branchial bars come
+off.
+
+[Illustration: FIG. 60.--DIAGRAM OF AMMOCOETES CUT OPEN TO SHOW THE LATERAL
+SYSTEM OF CRANIAL NERVES _V., VII., IX., X._, AND THE BRANCHIAL CARTILAGES.
+
+The branchial cartilages and sub-chordal ligaments are coloured blue, the
+branchiæ red. _gl._, glandular substance surrounding the central nervous
+system and passing into the auditory capsule with the auditory nerve
+(_VIII._).]
+
+In Fig. 59 I have shown the prosoma and mesosoma of Limulus, and indicated
+the nerves to the appendages together with the mesosomatic cartilaginous
+skeleton.
+
+In Fig. 60 I have drawn a corresponding picture of the prosomatic and
+mesosomatic region of Ammocoetes with the corresponding nerves {142}and
+cartilages. In this figure the animal is supposed to be slit open along the
+ventral mid-line and the central nervous system exposed.
+
+
+THE PROSOMATIC SKELETON OF LIMULUS, COMPOSED OF HARD CARTILAGE.
+
+The rest of the primitive vertebrate skeleton arose in the prosomatic
+region, and formed a support for the base of the brain. This skeleton was
+composed of hard cartilage, and arose in white fibrous tissue containing
+gelatin rather than mucin.
+
+Is there, then, any peculiar tissue of a cartilaginous nature in Limulus
+and its allies, situated in the prosomatic region, which is entirely
+separate from the branchial cartilaginous skeleton, which acts as a
+supporting internal framework, and contains a gelatinous rather than a
+mucoid substratum?
+
+It is a striking fact, common to the whole of the group of animals to which
+our inquiries, deduced from the consideration of the structure of
+Ammocoetes, have, in every case, led us in our search for the vertebrate
+ancestor, that they do possess a remarkable internal semi-cartilaginous
+skeleton in the prosomatic region, called the entosternite or plastron,
+which gives support to a large number of the muscles of that region; which
+is entirely independent of the branchial skeleton, and differs markedly in
+its chemical reactions from that cartilage, in that it contains a
+gelatinous rather than a mucoid substratum.
+
+In Limulus it is a large, tough, median plate, fibrous in character, in
+which are situated rows and nests of cartilage-cells. The same structure is
+seen in the plastron of Hypoctonus, of Thelyphonus, and to a certainty in
+all the members of the scorpion group. Very different is the behaviour of
+this tissue to staining from that of the branchial region. No part of the
+plastron stains purple with thionin; it hardly stains at all, or gives only
+a very slight blue colour. In its chemical composition there is a marked
+preponderance of gelatin with only a slight amount of a mucin-body. In some
+cases, as in Hypoctonus (Fig. 57, B) and Mygale, the capsules of the
+cartilage-cells stain a deep yellow with hæmatoxylin and picric acid, while
+the fibres between the cell-nests stain a blue-brown colour, partly from
+the hæmatoxylin, partly from the picric acid.
+
+All the evidence points to the plastron as resembling the basi-cranial
+skeleton of Ammocoetes in its composition and in the origin {143}of its
+cells in a white fibrous tissue. What, then, is its topographical position?
+It is in all cases a median structure lying between the cephalic stomach
+and the infra-oesophageal portion of the central nervous system, and in all
+cases it possesses two anterior horns which pass around the oesophagus and
+the nerve-masses which immediately enclose the oesophagus (Fig. 61, A).
+These lateral horns, then, which lie laterally and slightly ventral to the
+central nervous system, and are called by Ray Lankester and Benham the
+sub-neural portion of the entosternite, are very nearly in exactly the
+position of the racquet-shaped head of the trabeculæ in Ammocoetes. It is
+easy to see that, with a more extensive growth of the nervous material
+dorsally, such lateral horns might be caused to take up a still more
+ventral position. Now, these two lateral horns of the plastron of Limulus
+are continued along its whole length so as to form two thickened lateral
+ridges, which are conspicuous on the flat surface of the rest of this
+median plate. In other cases, as in the Thelyphonidæ, the plastron consists
+mainly of these two lateral ridges or trabeculæ, as they might be called,
+and Schimkéwitsch, who more than any one else has made a comparative study
+of the entosternite, describes it as composed in these animals of two
+lateral trabeculæ crossed by three transverse trabeculæ. I myself can
+confirm his description, and give in Fig. 61, B, the appearance of the
+entosternite of Thelyphonus or of Hypoctonus. The supra-oesophageal ganglia
+and part of the infra-oesophageal ganglia fill up the space _Ph._;
+stretching over the rest of the infra-oesophageal mass is a transverse
+trabecula, which is very thin; then comes a space in which is seen the rest
+of the infra-oesophageal mass, and then the posterior part of the plastron,
+ventrally to which lies the commencement of the ventral nerve-cord.
+
+[Illustration: FIG. 61.--A, ENTOSTERNITE OF LIMULUS; B, ENTOSTERNITE OF
+THELYPHONUS.
+
+_Ph._, position of pharynx.]
+
+{144}In these forms, in which the central nervous system is more
+concentrated towards the cephalic end than in Limulus, the whole of the
+concentrated brain-mass is separated from the gut only by this thin
+transverse band of tissue. Judging, then, from the entosternite of
+Thelyphonus, it is not difficult to suppose that a continuation of the same
+growth of the brain-region of the central nervous system would cause the
+entosternite to be separated into two lateral trabeculæ, which would then
+take up the ventro-lateral position of the two trabeculæ of Ammocoetes.
+
+On the other hand, it might be that two lateral trabeculæ, similar to those
+of Thelyphonus and situated on each side of the central nervous system,
+were the original form from which, by the addition of transverse fibres
+running between the gut and nervous system, the entosternite of Thelyphonus
+and of the scorpions, etc., was formed. From an extensive consideration of
+the entosternite in different animals, Schimkéwitsch has come to the
+conclusion that this latter explanation is the true one. He points out that
+the lateral trabeculæ can be distinguished from the transverse by their
+structure, being much more cellular and less fibrous, and the cell-cavities
+more rounded, or, as I should express it, the two lateral trabeculæ are
+more cartilaginous, while the transverse are more fibrous. Schimkéwitsch,
+from observations of structure and from embryological investigations, comes
+to the conclusion that the entosternite was originally composed of two
+parts--
+
+1. A transverse muscle corresponding to the adductor muscle of the shell of
+certain crustaceans, such as Nebalia.
+
+2. A pair of longitudinal mesodermic tendons, which may have been formed
+originally out of a number of segmentally arranged mesodermic tendons, and
+are crossed by the fibrils of the transverse muscular bundles.
+
+These paired tendons of the entosternite he considers to correspond to the
+intermuscular tendons, situated lengthways, which are found in the ventral
+longitudinal muscles of most arthropods.
+
+It is clear from these observations of Schimkéwitsch, that the essential
+part of the entosternite consists of two lateral trabeculæ, which were
+originally tendinous in nature and have become of the nature of
+cartilaginous tissue by the increase of cellular elements in the matrix of
+the tissue: these two trabeculæ function as supports for the attachment of
+muscles, which are specially attached at certain places. At these places
+transverse fibres belonging to some {145}of the muscular attachments cross
+between the two longitudinal trabeculæ, and so form the transverse
+trabeculæ.
+
+I entirely agree with Schimkéwitsch that the nests of cartilage-cells are
+much more extensive in, and indeed nearly entirely confined to, these two
+lateral trabeculæ in the entosternite of Hypoctonus. Ray Lankester
+describes in the entosternite of Mygale peculiar cell-nests strongly
+resembling those of Hypoctonus, and he also states that they are confined
+to the lateral portions of the entosternite.
+
+From this evidence it is easy to see that that portion of the basi-cranial
+skeleton known as the trabeculæ may have originated from the formation of
+cartilage in the plastron or entosternite of a palæostracan animal. Such an
+hypothesis immediately suggests valuable clues as to the origin of the
+cranium and of the rest of the basi-cranial skeleton--the parachordals and
+the auditory capsules. The former would naturally be a dorsal extension of
+the more membranous portion of the plastron, in which, equally naturally,
+cartilaginous tissue would subsequently develop; and the reason why it is
+impossible to reduce the cranium into a series of segments would be
+self-evident, for even though, as Schimkéwitsch thinks, the plastron may
+have been originally segmented, it has long lost all sign of segmentation.
+The latter would be derived from a second entosternite of the same nature
+as the plastron, but especially connected with the auditory apparatus of
+the invertebrate ancestor. The following out of these two clues will be the
+subject of a future chapter.
+
+In our search, then, for a clue to the origin of the skeletal tissues of
+the vertebrate we see again that we are led directly to the palæostracan
+stock on the invertebrate side and to the Cyclostomata on that of the
+vertebrate; for in Limulus, the only living representative of the
+Palæostraca, and in Limulus alone, we find a skeleton marvellously similar
+to the earliest vertebrate skeleton--that found in Ammocoetes. Later on I
+shall give reasons for the belief that the earliest fishes so far found,
+the Cephalaspidæ, etc., were built up on the same plan as Ammocoetes, so
+that, in my opinion, in Limulus and in Ammocoetes we actually possess
+living examples allied to the ancient fauna of the Silurian times.
+
+
+{146}SUMMARY.
+
+  The skeleton considered in this chapter is not the notochord, but that
+  composed of cartilage. The tracing downwards of the vertebrate bony and
+  cartilaginous skeleton to its earliest beginnings leads straight to the
+  skeleton of the larval lamprey (Ammocoetes), in which vertebræ are not
+  yet formed, but the cranial and branchial skeleton is well marked.
+
+  The embryological and phylogenetic histories are in complete unison to
+  show that the cranial skeleton is older than the spinal, and this
+  primitive branchial skeleton is also in harmony with the laws of
+  evolution, in that its structure, even in the adult lamprey (Petromyzon),
+  never gets beyond the stage characteristic of embryonic cartilage in the
+  higher vertebrates.
+
+  The simplest and most primitive skeleton is that found in Ammocoetes and
+  consists of two parts: (1) a prosomatic, (2) a mesosomatic skeleton.
+
+  The prosomatic skeleton forms a non-segmented basi-cranial skeleton of
+  the simplest kind--the trabeculæ and the parachordals with their attached
+  auditory capsules, just as the embryology of the higher vertebrates
+  teaches us must be the case. There in the free-living, still-existent
+  Ammocoetes we find the manifest natural outcome of the embryological
+  history in the shape of simple trabeculæ and parachordals, from which the
+  whole complicated basi-cranial skeleton of the higher vertebrates arose.
+
+  The mesosomatic skeleton, which is formed before the prosomatic,
+  consisted, in the first instance, of simple branchial bars segmentally
+  arranged, which were connected together by a longitudinal subchordal bar,
+  situated laterally on each side of the notochord. These simple branchial
+  bars later on form the branchial basket-work, which forms an open-work
+  cage within which the branchiæ are situated.
+
+  The cartilages which compose these two skeletons respectively are
+  markedly different in chemical constitution, in that the first (hard
+  cartilage) is mainly composed of chondro-gelatin, the second (soft
+  cartilage) of chondro-mucoid material.
+
+  The same kind of difference is seen in the two kinds of connective tissue
+  which are the forerunners of these two kinds of cartilage. Thus, the
+  cranial walls in Ammocoetes are formed of white fibrous tissue, an
+  essentially gelatin-containing tissue; at transformation these are
+  invaded by chondro-blasts and the cartilaginous cranium, formed of hard
+  cartilage, results. On the other hand, the forerunner of the branchial
+  soft cartilage is a very striking and peculiar kind of connective tissue
+  loaded with mucoid material, to which the name muco-cartilage has been
+  given.
+
+  The enormous interest of this muco-cartilage consists in the fact that it
+  forms very well-defined plates of tissue, entirely confined to the
+  head-region, which are not found in any higher vertebrate, not even in
+  the adult form Petromyzon, for every scrap of the tissue as such
+  disappears at transformation.
+
+  It is this evidence of primitive non-vertebrate tissues, which occur in
+  the larval but not in the adult form, which makes Ammocoetes so valuable
+  for the investigation of the origin of vertebrates.
+
+  The evidence, then, is extraordinarily clear as to the beginnings of the
+  vertebrate skeletal tissues.
+
+  {147}In the invertebrate kingdom true cartilage occurs but scantily.
+  There is a cartilaginous covering of the brain of cephalopods. It is
+  never found in crabs, lobsters, bees, wasps, centipedes, butterflies,
+  flies, or any of the great group of Arthropoda, except, to a slight
+  extent, in some members of the scorpion group, and more fully in one
+  single animal, the King-crab or Limulus: a fact significant of itself,
+  but still more so when the nature of the cartilage and its position in
+  the animal is taken into consideration, for the identity both in
+  structure and position of this internal cartilaginous skeleton with that
+  of Ammocoetes is extraordinarily great.
+
+  Here, in Limulus, just as in Ammocoetes, an internal cartilaginous
+  skeleton is found, composed of two distinct parts: (1) prosomatic, (2)
+  mesosomatic. As in Ammocoetes, the latter consists of simple branchial
+  bars, segmentally arranged, which are connected together on each side by
+  a longitudinal ligament containing cartilage--the entapophysial ligament.
+  This cartilage is identical in structure and in chemical composition with
+  the soft cartilage of Ammocoetes, and, as in the latter case, arises in a
+  markedly mucoid connective tissue. The former, as in Ammocoetes, consists
+  of a non-segmental skeleton, the plastron, composed of a white fibrous
+  connective tissue matrix, an essentially gelatin-containing tissue, in
+  which are found nests of cartilage cells of the hard cartilage variety.
+
+  This remarkable discovery of the branchial cartilaginous bars of Limulus,
+  together with that of the internal prosomatic plastron, causes the
+  original difficulty of deriving an animal such as the vertebrate from an
+  animal resembling an arthropod to vanish into thin air, for it shows that
+  in the past ages when the vertebrates first appeared on the earth, the
+  dominant arthropod race at that time, the members of which resembled
+  Limulus, had solved the question; for, in addition to their external
+  chitinous covering, they had manufactured an internal cartilaginous
+  skeleton. Not only so, but that skeleton had arrived, both in structure
+  and position, exactly at the stage at which the vertebrate skeleton
+  starts.
+
+  What the precise steps are by which chitin-formation gives place to
+  chondrin-formation are not yet fully known, but Schmiedeberg has shown
+  that a substance, glycosamine, is derivable from both these skeletal
+  tissues, and he concludes his observations in the following words: "Thus,
+  by means of glycosamine, the bridge is formed which connects together the
+  chitin of the lower animals with the cartilage of the more highly
+  organized creations."
+
+  The evidence of the origin of the cartilaginous skeleton of the
+  vertebrate points directly to the origin of the vertebrate from the
+  Palæostraca, and is of so strong a character that, taken alone, it may
+  almost be considered as proof of such origin.
+
+
+
+
+{148}CHAPTER IV
+
+_THE EVIDENCE OF THE RESPIRATORY APPARATUS_
+
+  Branchiæ considered as internal branchial appendages.--Innervation of
+  branchial segments.--Cranial region older than spinal.--Three-root system
+  of cranial nerves, dorsal, lateral, ventral.--Explanation of van Wijhe's
+  segments.--Lateral mixed root is appendage-nerve of invertebrate.--The
+  branchial chamber of Ammocoetes.--The branchial unit, not a pouch but an
+  appendage.--The origin of the branchial musculature.--The branchial
+  circulation.--The branchial heart of the vertebrate.--Not homologous with
+  the systemic heart of the arthropod.--Its formation from two longitudinal
+  venous sinuses.--Summary.
+
+
+The respiratory apparatus in all the terrestrial vertebrates is of the same
+kind--one single pair of lungs. These lungs originate as a diverticulum of
+the alimentary canal. On the other hand, the aquatic vertebrates breathe by
+means of a series of branchiæ, or gills, which are arranged segmentally,
+being supported by the segmental branchial cartilaginous bars, as already
+mentioned in the last chapter.
+
+The transition from the gill-bearing to the lung-bearing vertebrates is
+most interesting, for it has been proved that the lungs are formed by the
+modification of the swim-bladder of fishes; and in a group of fishes, the
+Dipnoi, or lung-fishes, of which three representatives still exist on the
+earth, the mode of transition from the fish to the amphibian is plainly
+visible, for they possess both lungs and gills, and yet are not amphibians,
+but true fishes. But for the fortunate existence of Ceratodus in Australia,
+Lepidosiren in South America, and Protopterus in Africa, it would have been
+impossible from the fossil remains to have asserted that any fish had ever
+existed which possessed at the same moment of time the two kinds of
+respiratory organs, although from our knowledge of the development of the
+amphibian we might have felt sure that such a transitional stage must have
+existed. Unfortunately, there is at present no likelihood of any
+corresponding transitional stage being discovered {149}living on the earth
+in which both the dorsal arthropod alimentary canal and the ventral
+vertebrate one should simultaneously exist in a functional condition; still
+it seems to me that even if Ceratodus, Lepidosiren, and Protopterus had
+ceased to exist on the earth, yet the facts of comparative anatomy,
+together with our conception of evolution as portrayed in the theory of
+natural selection, would have forced us to conclude rightly that the
+amphibian stage in the evolution of the vertebrate phylum was preceded by
+fishes which possessed simultaneously lungs and gills.
+
+In the preceding chapter the primitive cartilaginous vertebrate skeleton,
+as found in Ammocoetes, was shown to correspond in a marvellous manner to
+the cartilaginous skeleton of Limulus. In a later chapter I will deal with
+the formation of the cranium from the prosomatic skeleton; in this chapter
+it is the mesosomatic skeleton which is of interest, and the consideration
+of the necessary consequences which logically follow upon the supposition
+that the branchial cartilaginous bars of Limulus are homologous with the
+branchial basket-work of Ammocoetes.
+
+
+INTERNAL BRANCHIAL APPENDAGES.
+
+Seeing that in both cases the cartilaginous bars of Limulus and Ammocoetes
+are confined to the branchial region, their homology of necessity implies
+an homology of the two branchial regions, and leads directly to the
+conclusion that the branchiæ of the vertebrate were derived from the
+branchiæ of the arthropod, a conclusion which, according to the generally
+accepted view of the origin of the respiratory region in the vertebrate, is
+extremely difficult to accept; for the branchiæ of Limulus and of the
+Arthropoda in general are part of the mesosomatic appendages, while the
+branchiæ of vertebrates are derived from the anterior part of the
+alimentary canal. This conclusion, therefore, implies that the vertebrate
+has utilized in the formation of the anterior portion of its new alimentary
+canal the branchial appendages of the palæostracan ancestor.
+
+{150}[Illustration: FIG. 62.--_Eurypterus._
+
+The segments and appendages on the right are numbered in correspondence
+with the cranial system of lateral nerve-roots as found in vertebrates.
+_M._, metastoma. The surface ornamentation is represented on the first
+segment posterior to the branchial segments. The opercular appendage is
+marked out by dots.]
+
+Let us consider dispassionately whether such a suggestion is _a priori_ so
+impossible as it at first appears. One of the principles of evolution is
+that any change which is supposed to have taken place in the process of
+formation of one animal or group of animals from a lower group must be in
+harmony with changes which are known to have occurred in that lower group.
+On the assumption, therefore, that the vertebrate branchiæ represent the
+branchial portion of the arthropod mesosomatic appendages which have sunk
+in and so become internal, we ought to find that in members of this very
+group such inclusion of branchial appendages has taken place. This, indeed,
+is exactly what we do find, for in all the scorpion tribe, which is
+acknowledged to be closely related to Limulus, there are no external
+mesosomatic appendages, but in all cases these appendages have sunk into
+the body, have disappeared as such, and retained only the vital part of
+them--the branchiæ. In this way the so-called lung-books of the scorpion
+are formed, which are in all respects homologous with the branchiæ or
+gill-books of Limulus. Now, as already mentioned, the lords of creation in
+the palæostracan times were the sea-scorpions, which, as is seen in Fig.
+62, resembled the land-scorpions of the present day in the entire absence
+of any external appendages on the segments of the mesosomatic region. As
+they lived in the sea, they must have breathed with gills, and those
+branchial appendages must have been internal, just as in the land-scorpions
+of the present time. Indeed, markings have been found on the internal side
+of the segments 1-5, Fig. 62, which are supposed to indicate branchiæ, and
+these segments are therefore supposed to have borne the branchiæ. Up to the
+present time no indication of gill-slits has been found, and we cannot say
+with certainty how these animals breathed. Further, in the Upper Silurian
+of Lesmahago, Lanarkshire, a scorpion (_Palæophonus Hunteri_), closely
+resembling the modern scorpion, has been found, which, as Lankester states,
+was in all probability aquatic, and not terrestrial in its habits. How it
+{151}breathed is unknown; it shows no signs of stigmata, such as exist in
+the scorpion of to-day.
+
+Although we possess as yet no certain knowledge of the position of the
+gill-openings in these ancient scorpion-like forms, what we can say with
+certainty--and that is the important fact--is, that at the time when the
+vertebrates appeared, a very large number of the dominant arthropod race
+possessed internally-situated branchiæ, which had been directly derived
+from the branchiæ-bearing appendages of their Limulus-like kinsfolk.
+
+This abolition of the branchiæ-bearing appendages as external organs of
+locomotion, with the retention of the important branchial portion of the
+appendage as internal branchiæ, is a very important suggestion in any
+discussion of the way vertebrates have arisen from arthropods; for, if the
+same principle is of universal application, it leads directly to the
+conclusion that whenever an appendage possesses an organ of vital
+importance to the animal, that organ will remain, even though the appendage
+as such completely vanishes. Thus, as will be shown later, special
+sense-organs such as the olfactory remain, though the animal no longer
+possesses antennæ; the important excretory organs, the coxal glands, and
+important respiratory organs, the branchiæ, are still present in the
+vertebrate, although the appendages to which they originally belonged have
+dwindled away, or, at all events, are no longer recognizable as arthropod
+appendages.
+
+
+INNERVATION OF BRANCHIAL SEGMENTS.
+
+Passing from _a priori_ considerations to actual facts, it is advisable to
+commence with the innervation of the branchial segments; for, seeing that
+the foundation of the whole of this comparative study of the vertebrate and
+the arthropod is based upon the similarity of the two central nervous
+systems, it follows that we must look in the first instance to the
+innervation of any organ or group of organs in order to find out their
+relationship in the two groups of animals.
+
+The great characteristic of the vertebrate branchial organs is their
+segmental arrangement and their innervation by the vagus group of nerves,
+_i.e._ by the hindermost group of the cranial segmental nerves. These
+cranial nerves are divided by Gegenbaur into two great groups--an anterior
+group, the trigeminal, which supplies the muscles of mastication, and a
+posterior group, the vagus, which is essentially {152}respiratory in
+function. Of these two groups, I will consider the latter group first.
+
+In Limulus the great characteristic of the branchial region is its
+pronounced segmental arrangement, each pair of branchial appendages
+belonging to a separate segment. This group of segments forms the mesosoma,
+and these branchial appendages are the mesosomatic appendages. Anterior to
+them are the segments of the prosoma, which bear the prosomatic or
+locomotor appendages. The latter are provided at their base with gnathites
+or masticating apparatus, so that the prosomatic group of nerves, like the
+trigeminal group in the vertebrate, comprises essentially the nerves
+subserving the important function of mastication. As already pointed out,
+the brain-region of the vertebrate is comparable to the supra-oesophageal
+and infra-oesophageal ganglia of the invertebrate, and it has been shown
+(p. 54) how, by a process of concentration and cephalization, the foremost
+region of the infra-oesophageal ganglia becomes the prosomatic region, and
+is directly comparable to the trigeminal region in the vertebrate; while
+the hindermost region is formed from the concentration of the mesosomatic
+ganglia, and is directly comparable to the medulla oblongata, _i.e._ to the
+vagus region of the vertebrate brain.
+
+As far, then, as concerns the centres of origin of these two groups of
+nerves and their exits from the central nervous system, they are markedly
+homologous in the two groups of animals.
+
+
+COMPARISON OF THE CRANIAL AND SPINAL SEGMENTAL NERVES.
+
+It has often been held that the arrangements of the vertebrate nervous
+system differ from those of other segmented animals in one important
+particular. The characteristic of the vertebrate is the origin of every
+segmental nerve from two roots, of which one contains the efferent fibres,
+while the other possesses a sensory ganglion, and contains only afferent
+fibres. This arrangement, which is found along the whole spinal cord of all
+vertebrates, is not found in the segmental nerves of the invertebrates; and
+as it is supposed that the simpler arrangement of the spinal cord was the
+primitive arrangement from which the vertebrate central nervous system was
+built up, it is often concluded that the animal from which the vertebrate
+arose must have possessed a series of nerve-segments, from each of which
+there arose bilaterally ventral (efferent) and dorsal (afferent) roots.
+
+{153}Now, the striking fact of the vertebrate segmental nerves consists in
+this, that, as far as their structure and the tissues which they innervate
+are concerned, the cranial segmental nerves are built up on the same plan
+as the spinal; but as far as concerns their exit from the central nervous
+system they are markedly different. A large amount of ingenuity, it is
+true, has been spent in the endeavour to force the cranial nerves into a
+series of segmental nerves, which arise in the same way as the spinal by
+two roots, of which the ventral series ought to be efferent and the dorsal
+series afferent, but without success. We must, therefore, consider the
+arrangement of the cranial segmental nerves by itself, separately from that
+of the spinal nerves, and the problem of the origin of the vertebrate
+segmental nerves admits of two solutions--either the cranial arrangement
+has arisen from a modification of the spinal, or the spinal from a
+simplification of the cranial. The first solution implies that the spinal
+cord arrangement is older than the cranial, the second that the cranial is
+the oldest.
+
+In my opinion, the evidence of the greater antiquity of the cranial region
+is overwhelming.
+
+The evidence of embryology points directly to the greater phylogenetic
+antiquity of the cranial region, for we see how, quite early in the
+development, the head is folded off, and the organs in that region thereby
+completed at a time when the spinal region is only at an early stage of
+development. We see how the first of the trunk somites is formed just
+posteriorly to the head region, and then more and more somites are formed
+by the addition of fresh segments posteriorly to the one first formed. We
+see how, in Ammocoetes, the first formed parts of the skeleton are the
+branchial bars and the basi-cranial system, while the rudiments of the
+vertebræ do not appear until the Petromyzon stage. We see how, with the
+elongation of the animal by the later addition of more and more spinal
+segments, organs, such as the heart, which were originally in the head,
+travel down, and the vagus and lateral-line nerves reach their ultimate
+destination. Again, we see that, whereas the cranial nerves, viz. the
+ocular motor, the trigeminal, facial, auditory, glossopharyngeal, and vagus
+nerves, are wonderfully fixed and constant in all vertebrates, the only
+shifting being in the spino-occipital region, in fact, at the junction of
+the cranial and spinal region, the spinal nerves, on the other hand, are
+not only remarkably variable in number in different {154}groups of animals,
+but that even in the same animal great variations are found, especially in
+the manner of formation of the limb-plexuses. Such marked meristic
+variation in the spinal nerves, in contrast to the fixed character of the
+cranial nerves, certainly points to a more recent formation of the former
+nerves.
+
+Also the observations of Assheton on the primitive streak of the rabbit,
+and on the growth in length of the frog embryo, have led him to the
+conclusion that, as in the rabbit so in the frog, there is evidence to show
+that the embryo is derived from two definite centres of growth: the first,
+phylogenetically the oldest, being a protoplasmic activity, which gives
+rise to the anterior end of the embryo; the second, one which gives rise to
+the growth in length of the embryo. This secondary area of proliferation
+coincides with the area of the primitive streak, and he has shown, in a
+subsequent paper, by means of the insertion of sable hairs into the
+unincubated blastoderm of the chick, that a hair inserted into the centre
+of the blastoderm appears at the anterior end of the primitive streak, and
+subsequently is found at the level of the most anterior pair of somites.
+
+He then goes on to say--
+
+"From these specimens it seems clear that all those parts in front of the
+first pair of mesoblastic somites--that is to say, the heart, the brain and
+medulla oblongata, the olfactory, optic, auditory organs and foregut--are
+developed from that portion of the unincubated blastoderm which lies
+anterior to the centre of the blastoderm, and that all the rest of the
+embryo is formed by the activity of the primitive streak area."
+
+In other words, the secondary area of growth, _i.e._ the primitive streak
+area, includes the whole of the spinal cord region, while the older primary
+centre of growth is coincident with the cranial region.
+
+In searching, then, for the origin of the segmental nerves, we must
+consider the type on which the cranial nerves are arranged rather than that
+of the spinal nerves.
+
+The first striking fact occurs at the spino-occipital region, where the
+spinal cord merges into the medulla oblongata, for here in the cervical
+region we find each spinal segment gives origin to three distinct roots,
+not two--a dorsal root, a ventral root, and a lateral root. This third root
+gives origin to the spinal accessory nerve, and in the region of the
+medulla oblongata these lateral roots merge directly into the roots of the
+vagus nerve; more anteriorly the same system {155}continues as the roots of
+the glossopharyngeal nerve, as the roots of the facial nerve, and as a
+portion, especially the motor portion, of the trigeminal nerve. Now, all
+these nerves belong to a well-defined system of nerves, as Charles Bell[1]
+pointed out in 1830, a system of nerves concerned with respiration and
+allied mechanisms, such as laughing, sneezing, mastication, deglutition,
+etc., nerves innervating a set of muscles of very different kind from the
+ordinary body-muscles concerned with locomotion and equilibration. Also the
+centres from which these motor nerves arise are well defined, and form
+cell-masses in the central nervous system, quite separate from those which
+give origin to somatic muscles.
+
+This original idea of Charles Bell, after having been ignored for so long a
+time, is now seen to be a very right one, and it is an extraordinary thing
+that his enunciation of the dual nature of the spinal roots, which was, to
+his mind, of subordinate importance, should so entirely have overshadowed
+his suggestion, that in addition to the dorsal and ventral roots, a lateral
+system of nerves existed, which were not exclusively sensory or exclusively
+motor, but formed a separate system of respiratory nerves.
+
+Further, anatomists divide the striated muscles of the body into two great
+natural groups, characterized by a difference of origin and largely by a
+difference of appearance. The one set is concerned with the movements of
+internal organs, and is called visceral, the other is derived from the
+longitudinal sheet of musculature which forms the myotomes of the fish, and
+has been called parietal or somatic. The motor nerves of these two sets of
+muscles correspond with the lateral or respiratory and ventral roots
+respectively.
+
+Finally, it has been shown that the segments of which a vertebrate is
+composed are recognizable in the embryo by the segmented manner in which
+the musculature is laid down, and van Wijhe has shown that in the cranial
+region two sets of muscles are laid down segmentally, thus forming a dorsal
+and ventral series of commencing muscular segments. Of these the anterior
+segments of the dorsal series give origin to the striated muscles of the
+eye which are innervated by the IIIrd (oculomotor), IVth (trochlearis), and
+VIth (abducens) nerves, while the posterior segments give origin to the
+{156}muscles from the cranium to the shoulder-girdle, innervated by the
+XIIth (hypoglossal) nerve. The ventral series of segments give origin to
+the musculature supplied by the trigeminal, facial, glossopharyngeal, and
+vagus nerves.
+
+Also, the afferent or sensory nerves of the skin over the whole of this
+head-region are supplied by the trigeminal nerve, while the afferent nerves
+to the visceral surfaces are supplied by the vagus, glossopharyngeal and
+facial nerves.
+
+In van Wijhe's original paper he arranged the segments belonging to the
+cranial nerves in the following table:--
+
+  ---------+---------------------------------
+  Segments.| Ventral nerve-roots and muscles
+           |      derived from myotomes.
+  ---------+---------+-----------------------
+      1    |  III.   |M. rectus superior,
+           |         | m. rectus
+           |         | internus, m.
+           |         | rectus inferior,
+           |         | m. obliquus inferior
+      2    |   IV.   |M. obliquus
+           |         | superior
+      3    |   VI.   |M. rectus externus
+      4    |   --    |      --
+      5    |   --    |      --
+      6    |   --    |      --
+      7    |   XII.  |} Muscles from
+      8    |   XII.  |}  cranium to
+      9    |   XII.  |}shoulder-girdle
+  ---------+---------+-----------------------
+
+  ---------+------------------+--------------------------------
+  Segments.|  Visceral clefts.| Dorsal nerve-roots and muscles.
+           |                  |
+  ---------+------------------+--------------+-----------------
+      1    |                  |  V. N. op-   |
+           |                  |  thalmicus   |
+           |                  |  profundus   |
+      2    |                  |     V.       |   Masticating
+           | 1st Mandibular   |              |     muscles.
+      3    |                  |     VII._1   |{Facial muscles
+           |                  |              |{(VIII. is dorsal
+           | 2nd {Hyoid_1     |     VII._2   |{branch of VII.)
+      4    |     {Hyoid_2     |              |
+      5    |3rd 1st Branchial |    IX.       |
+      6    |4th 2nd    "      |     X._1     |
+      7    |{5th 3rd   "      |     X._2     |Branchial and
+      8    |{6th 4th   "      |     X._3}    |visceral muscles.
+      9    |{7th 5th   "      |     X._4     |
+  ---------+------------------+--------------+-----------------
+
+As is seen in the table, van Wijhe attempts to arrange the cranial
+segmental nerves into dorsal and ventral roots, in accordance with the
+arrangement in the spinal region. In order to do this he calls the Vth,
+VIIth, IXth, and Xth nerves dorsal roots, although they are not purely
+sensory nerves, but contain motor fibres as well.
+
+It is not accidental that he should have picked out for his dorsal roots
+the very nerves which form Charles Bell's lateral series of roots, inasmuch
+as this system of lateral roots, apart from dorsal and ventral roots,
+really is, as Charles Bell thought, an important separate system, dependent
+upon a separate segmentation in the embryo of the musculature supplied by
+these roots. This segmentation may receive the name of _visceral_ or
+_splanchnic_ in contradistinction to _somatic_, since all the muscles
+without exception belong to the visceral group of striated muscles.
+
+{157}These observations of van Wijhe lead directly to the following
+conclusion. In the cranial region there is evidence of a double set of
+segments, which may be called somatic and splanchnic. The somatic segments,
+consisting of the outer skin and the body musculature, are _doubly_
+innervated as are those of the spinal cord by a series of ventral motor
+roots, the oculomotor or IIIrd nerve, the trochlear or IVth nerve, the
+abducens or VIth nerve, and the hypoglossal or XIIth nerve, and by a series
+of dorsal sensory roots, the sensory part of the trigeminal or Vth nerve.
+But the splanchnic segments are innervated by _single_ roots, the vagus or
+Xth nerve, glossopharyngeal or IXth nerve, facial or VIIth nerve, and
+trigeminal or Vth nerve, which are mixed, containing both sensory and motor
+fibres, thus differing markedly from the arrangement of the spinal nerves.
+
+From this sketch it follows that the arrangement seen in the spinal cord,
+would result from the cranial arrangement if this third system of lateral
+roots were left out. Further, since the cranial system is the oldest, we
+must search in the invertebrate ancestor for a tripartite rather than a
+dual system of nerve-roots for each segment; a system composed of a dorsal
+root supplying only the sensory nerves of the skin-surfaces, a lateral
+mixed root supplying the system connected with respiration with both
+sensory and motor fibres, and a ventral root supplying the motor nerves to
+the body-musculature.
+
+
+COMPARISON OF THE APPENDAGE NERVES OF LIMULUS AND BRANCHIPUS TO THE LATERAL
+ROOT SYSTEM OF THE VERTEBRATE.
+
+If the argument used so far is correct, and this tripartite system of
+nerve-roots, as seen in the cranial nerves of the vertebrate, really
+represents the original scheme of innervation in the palæostracan ancestor,
+then it follows that each segment of Limulus ought to be supplied by three
+nerves--(1), a sensory nerve supplying its own portion of the skin-surface
+of the prosomatic and mesosomatic carapaces; (2), a lateral mixed nerve
+supplying exclusively the appendage of the segment, for the appendages
+carry the respiratory organs; and (3), a motor nerve supplying the
+body-muscles of the segment.
+
+It is a striking fact that Milne-Edwards describes the nerve-roots in
+exactly this manner. The great characteristic of the nerve-roots {158}in
+Limulus as in other arthropods is the large appendage-nerve, which is
+always a mixed nerve; in addition, there is a system of sensory nerves to
+the prosomatic and mesosomatic carapaces, called by him the epimeral
+nerves, which are purely sensory, and a third set of roots which are motor
+to the body-muscles, and possibly also sensory to the ventral surface
+between the appendages.
+
+Moreover, just as in the vertebrate central nervous system the centres of
+origin of the motor nerves of the branchial segmentation are distinct from
+those of the somatic segmentation, so we find, from the researches of
+Hardy, that a similar well-marked separation exists between the centres of
+origin of the motor nerves of the appendages and those of the somatic
+muscles in the central nervous system of Branchipus and Astacus.
+
+In the first place, he points out that the nervous system of Branchipus is
+of a very primitive arthropod type; that it is, in fact, as good an example
+of an ancient type as we are likely to find in the present day; a matter of
+some importance in connection with my argument, since the arthropod
+ancestor of the vertebrate, such as I am deducing from the study of
+Ammocoetes, must undoubtedly have been of an ancient type, more nearly
+connected with the strange forms of the trilobite era than with the crabs
+and spiders of the present day.
+
+His conclusions with respect to Branchipus may be tabulated as follows:--
+
+1.  Each ganglion of the ventral chain is formed mainly for the innervation
+of the appendages.
+
+2.  Each ganglion is divided into an anterior and posterior division, which
+are connected respectively with the motor and sensory nerves of the
+appendages.
+
+3.  The motor nerves of the appendages arise as well-defined axis-cylinder
+processes of nerve-cells, which are arranged in well-defined groups in the
+anterior division of the ganglion.
+
+4.  A separate innervation exists for the muscles and sensory surfaces of
+the trunk. The trunk-muscles consist of long bundles, from which slips pass
+off to the skin in each segment; they are thus imperfectly segmented. In
+accordance with this, a diffuse system of nerve-fibres passes to them from
+certain cells on the dorsal surface of each lateral half of the ganglion.
+These cell-groups are therefore very distinct from those which give origin
+to the motor {159}appendage-nerves, and, moreover, are not confined to the
+ganglion, but extend for some distance into the interganglionic region of
+the nerve-cords which connect together the ganglia of the ventral chain.
+
+Hardy's observations, therefore, combined with those of Milne-Edwards, lead
+to the conclusion that in such a primitive arthropod type as my theory
+postulates, each segment was supplied with separate sensory and motor
+somatic nerves, and with a pair of nerves of mixed function, devoted
+entirely to the innervation of the pair of appendages; that also, in the
+central nervous system, the motor nerve-centres were arranged in accordance
+with a double set of segmented muscles in two separate groups of
+nerve-cells. These nerve-cells in the one case were aggregated into
+well-defined groups, which formed the centres for the motor nerves of the
+markedly segmented muscles of the appendages, and in the other case formed
+a system of more diffused cells, less markedly aggregated into distinct
+groups, which formed the centres for the imperfectly segmented somatic
+muscles.
+
+Such an arrangement suggests that in the ancient arthropod type a double
+segmentation existed, viz. a segmentation of the body, and a segmentation
+due to the appendages. Undoubtedly, the segments originally corresponded
+absolutely as in Branchipus, and every appendage was attached to a
+well-defined separate body-segment. In, however, such an ancient type as
+Limulus, though the segmentation may be spoken of as twofold, yet the
+number of segments in the prosomatic and mesosomatic regions are much more
+clearly marked out by the appendages than by the divisions of the soma;
+for, in the prosomatic region such a fusion of somatic segments to form the
+tergal prosomatic carapace has taken place that the segments of which it is
+composed are visible only in the young condition, while in the mesosomatic
+region the separate somatic segments, though fused to form the mesosomatic
+carapace, are still indicated by the entapophysial indentations.
+
+Clearly, then, if the mesosomatic branchial appendages of forms related to
+Limulus were reduced to the branchial portion of the appendage, and that
+branchial portion became internal, just as is known to be the case in the
+scorpion group, we should obtain an animal in which the _mesosomatic
+region_ would be characterized by a segmentation predominantly branchial,
+which might be termed, as in vertebrates, the _branchiomeric segmentation_,
+but yet would show {160}indications of a corresponding somatic or
+_mesomeric segmentation_. The nerve supply to these segments would consist
+of--
+
+1.  The epimeral purely sensory nerves to the somatic surface, equivalent
+in the vertebrate to the ascending root of the trigeminal.
+
+2.  The mixed nerves to the internal branchial segments, equivalent in the
+vertebrate to the vagus, glossopharyngeal, and facial.
+
+3.  The motor nerves to the somatic muscles, equivalent in the vertebrate
+to the original nerve-supply to the somatic muscles belonging to these
+segments, _i.e._ to the muscles derived from van Wijhe's 4th, 5th, and 6th
+somites.
+
+Further, the centres of origin of these appendage-nerves would form centres
+in the central nervous system separate from the centres of the motor nerves
+to the somatic muscles, just as the centres of origin of the motor parts of
+the facial, vagus, and glossopharyngeal nerves form groups of cells quite
+distinct from the centres for the hypoglossal, abducens, trochlear, and
+oculomotor nerves.
+
+In fact, if the vertebrate branchial nerves are looked upon as the
+descendants of nerves which originally supplied branchial appendages, then
+every question connected with the branchial segmentation, with the origin
+and distribution of these nerves, receives a simple and adequate
+solution--a solution in exact agreement with the conclusion that the
+vertebrate arose from a palæostracan ancestor.
+
+It would, therefore, be natural to expect that the earliest fishes breathed
+by means of branchial appendages situated internally, and that the evidence
+for such appendages would be much stronger in them than in more recent
+fishes.
+
+Although we know nothing of the nature of the respiratory apparatus in the
+extinct fishes of Silurian times, we have still living, in the shape of
+Ammocoetes, a possible representative of such types. If, then, we find, as
+is the case, that the respiratory apparatus of Ammocoetes differs markedly
+from that of the rest of the fishes, and, indeed, from that of the adult
+form or Petromyzon, and that that very difference consists in a greater
+resemblance to internal branchial appendages in the case of Ammocoetes,
+then we may feel that the proof of the origin of the branchial apparatus of
+the vertebrate from the internal branchial appendages of the invertebrate
+has gained enormously.
+
+
+{161}THE RESPIRATORY CHAMBER OF AMMOCOETES.
+
+In order to make clear the nature of the branchial segments in Ammocoetes,
+I have divided the head-part of the animal by means of a longitudinal
+horizontal section into halves--ventral and dorsal--as shown in Figs. 63
+and 64. These figures are each a combination of a section and a solid
+drawing. The animal was slit open by a longitudinal section in the
+neighbourhood of the gill-slits, and each half was slightly flattened out,
+so as to expose the ventral and dorsal internal surfaces respectively. The
+structures in the cut surface were drawn from one of a series of horizontal
+longitudinal sections taken through the head of the animal. These figures
+show that the head-region of Ammocoetes consists of two chambers, the
+contents of which are different. In front, an oral or stomodæal chamber,
+which contains the velum and tentacles, is enclosed by the upper and lower
+lips, and was originally separated by a septum from the larger respiratory
+chamber, which contains the separate pairs of branchiæ. A glance at the two
+drawings shows clearly that Rathke's original description of this chamber
+is the natural one, for he at that time, looking upon _Ammocoetes
+branchialis_ as a separate species, described the branchial chamber as
+containing a series of paired gills, with the gill-openings between
+consecutive gills. His branchial unit or gill, therefore, was represented
+by each of the so-called diaphragms, which, as seen in Figs. 63, 64, are
+all exactly alike, except the first and the last. Any one of these is
+represented in section in Fig. 65, and represents a branchial unit in
+Rathke's view and in mine. Clearly, it may be described as a branchial
+appendage which projects into an open pharyngeal chamber, so that the
+series of such appendages divides the chamber into a series of
+compartments, each of which communicates with the exterior by means of a
+gill-slit, and with each other by means of the open space between opposing
+appendages.
+
+Each of these appendages possesses its own cartilaginous bar (_Br. cart._),
+as explained in Chapter III.; each possesses its own branchial or visceral
+muscles (coloured blue in Figs. 63 and 64), separated absolutely from the
+longitudinal somatic muscles (coloured dark red in Figs. 63 and 64) by a
+space (_Sp._) containing blood and peculiar fat-cells, etc. Each possesses
+its own afferent branchial blood-vessel from the ventral aorta, and its own
+efferent vessel to the dorsal aorta (Fig. 65, _a. br._ and _v. br._). Each
+possesses its own segmental nerve, which supplies its own branchial muscles
+and no others with motor fibres, and sends sensory fibres to the general
+surface of each appendage, as also to the special sense-organs in the shape
+of the epithelial pits (_S._, Fig. 65) arranged along the free edges of the
+diaphragms; each of these nerves possesses its own ganglion--the
+epibranchial ganglion.
+
+{162}[Illustration: FIG. 63.--VENTRAL HALF OF HEAD-REGION OF AMMOCOETES.
+
+Somatic muscles coloured red. Branchial and visceral muscles coloured blue.
+Tubular constrictor muscles distinguished from striated constrictor muscles
+by simple hatching. _Tent._, tentacles; _Tent. m.c._, muco-cartilage of
+tentacles; _Vel. m.c._, muco-cartilage of the velum; _Hy. m.c._,
+muco-cartilage of the hyoid segment; _Ps. br._, pseudo-branchial groove;
+_Br. cart._, branchial cartilages; _Sp._, space between somatic and
+splanchnic muscles; _Th. op._, orifice of thyroid; _H._, heart.]
+
+{163}[Illustration: FIG. 64.--DORSAL HALF OF HEAD-REGION OF AMMOCOETES.
+
+_Tr._, trabeculæ; _Pit._, pituitary space; _Inf._, infundibulum; _Ser._,
+median serrated flange of velar folds.]
+
+{164}[Illustration: FIG. 65.--SECTION THROUGH BRANCHIAL APPENDAGE OF
+AMMOCOETES.
+
+_br. cart._, branchial cartilage; _v. br._, branchial vein; _a. br._,
+branchial artery; _b.s._, blood-spaces; _p._, pigment; _S._, sense-organ;
+_c._, ciliated band; _E., I._, external and internal borders; _m. add._,
+adductor muscle; _m.c.s._, striated constrictor muscle; _m.c.t._, tubular
+constrictor muscle; _m._ and _m.v._, muscles of valve.]
+
+[Illustration: FIG. 66.--SECTION THROUGH BRANCHIAL APPENDAGE OF LIMULUS.
+
+_br. cart._, branchial cartilage; _v.br._, branchial vein; _b.s._,
+blood-spaces formed by branchial artery; _P._, pigment; _m_1_, posterior
+entapophysio-branchial muscle; _m_2_, anterior entapophysio-branchial
+muscle; _m_3_, external branchial muscle.]
+
+The work of Miss Alcock has shown that the segmental branchial nerve
+supplies solely and absolutely such an appendage or branchial {165}segment,
+and does not supply any portion of the neighbouring branchial segments. The
+nerve-supply in Ammocoetes gives no countenance to the view that the
+original unit was a branchial pouch, the two sides of which each nerve
+supplied, but is strong evidence that the original unit was a branchial
+appendage, which was supplied by a _single_ nerve with both motor and
+sensory fibres.
+
+Any observer having before him only this picture of the respiratory chamber
+of Ammocoetes, upon which to base his view of a vertebrate respiratory
+chamber, would naturally look upon the branchial unit of a vertebrate as a
+gilled appendage projecting into the open cavity of the anterior part of
+the alimentary canal or pharynx. This is not, however, the usual
+conception. The branchial unit is ordinarily described as a gill-pouch,
+which possesses two openings or slits, an internal one into the lumen of
+the alimentary canal, and an external one into the surrounding medium. This
+view is based upon embryological evidence of the following character:--
+
+The alimentary canal of all vertebrates forms a tube stretching the whole
+length of the animal; the anterior part of this tube becomes pouched on
+each side at regular intervals, and the walls of each pouch becoming folded
+form the respiratory surfaces or gills. The openings of these separate
+pouches into the central lumen of the gut form the internal gill-pouch
+openings; the other extremity of the pouch approaches the external surface
+of the animal, and finally breaks through to form a series of external
+gill-pouch openings.
+
+From the mesoblastic tissue, between each gill-pouch, there is formed a
+supporting cartilaginous bar, to which are attached a system of branchial
+muscles, with their nerves and blood-vessels. These cartilaginous bars, in
+all fishes above the Cyclostomata, form a supporting framework for the
+internal gill-slit, so that the gills are situated externally to them; the
+more primitive arrangement is, as already mentioned, a system of
+cartilaginous bars, extra-branchial in position, so that the gills are
+situated internally to them.
+
+From this description of the mode of formation of the respiratory apparatus
+in water-breathing vertebrates the conception has arisen of the gill-pouch
+as the branchial unit, a conception which is absolutely removed from all
+idea of a branchial unit such as is found in an arthropod, viz. an
+appendage.
+
+This conception of spaces as units pervades the whole of embryology, and is
+the outcome of the gastrula theory--a theory which {166}teaches that all
+animals above the Protozoa are derived from a form which by invagination of
+its external surface formed an internal cavity or primitive gut. From
+pouches of this gut other cavities were said to be formed, called coelomic
+cavities, and thus arose the group of coelomatous animals. To speak of the
+developmental history of animals in terms of spaces; to speak of the
+atrophy of a cavity as though such a thing were possible, is, to my mind,
+the wrong way of looking at the facts of anatomy. It resembles the
+description of a net as a number of holes tied together with string, which
+is not usually considered the best method of description.
+
+There are two ways in which a series of pouches can be formed from a simple
+tube without folding, either by a thinning at regular intervals of the
+original tissue surrounding the tube, or by the ingrowth into the tube of
+the surrounding tissue at regular intervals, thus--
+
+[Illustration: FIG. 67.--DIAGRAMS TO SHOW THE TWO METHODS OF
+POUCH-FORMATION.
+
+A, by the thinning of the mesoblast at intervals. B, by the ingrowth of
+mesoblast at intervals. _Ep._, epiblast; _Mes._, mesoblast; _Hy._,
+hypoblast.]
+
+In the first case (A) the formation of a pouch is the significant act, and
+therefore the branchial segments might be expressed in terms of pouches. In
+the second case (B) the formation of a pouch is {167}brought about in
+consequence of the ingrowth of the mesoblastic tissues at intervals; here,
+although the end-result is the same as in the first case, the
+pouch-formation is only secondary, the true branchial unit is the
+mesoblastic ingrowth.
+
+The evidence all points directly to the second method of formation. Thus
+Shipley, in his description of the development of the lamprey, says--
+
+"The gill-slits appear to me to be the result of the ventral downgrowth of
+mesoblast taking place only at certain places, these forming the gill-bars.
+Between each downgrowth the hypoblastic lining of the alimentary canal
+remains in contact with the epiblast; here the gill-opening subsequently
+appears about the twenty-second day."
+
+Dohrn describes and gives excellent pictures of the growth of the
+diaphragms, as the Ammocoetes grows in size, pictures which are distinctly
+reminiscent of the corresponding illustrations given by Brauer of the
+growth of the internal gills in the scorpion embryo.
+
+Another piece of evidence confirmatory of the view that the branchial
+segments are really of the nature of internal appendages, as the result of
+which gill-pouches are formed, is given by the presence in each of these
+branchial bars or diaphragms of a separate coelomic cavity. From the walls
+of this cavity the branchial muscles and cartilaginous bar are formed.
+
+Now, from an embryological point of view, the vertebrate shows that it is a
+segmented animal by the formation of somites, which consist of a series of
+divisions of the coelom, of which the walls form a series of muscular and
+skeletal segments. In the head-region, as already mentioned, such coelomic
+divisions form two rows--a dorsal and a ventral set. From the walls of the
+dorsal set the somatic musculature is formed. From those of the ventral set
+the branchial musculature. From the latter also the branchial cartilaginous
+bars are formed. Thus Shipley, in his description of the development of the
+lamprey, says: "The mesoblast between the gills arranges itself into
+head-cavities, and the walls of these cavities ultimately form the skeleton
+of the gill-arches."
+
+Similarly, in the arthropod, the segments in the embryo are marked out by a
+series of coelomic cavities and Kishinouye has described in Limulus a
+separate coelomic cavity for every one of the mesosomatic or branchial
+segments, and he states that in Arachnida {168}the segmental coelomic
+cavities extend into the limbs. These cavities both in the vertebrate and
+in the arthropod disappear before the adult condition is reached.
+
+The whole evidence thus points strongly to the conclusion that the true
+branchial segmental units are the branchial bars or diaphragms, not the
+pouches between them.
+
+It is possible to understand why such prominence has been given to the
+conception of the branchial unit as a gill-pouch rather than as a
+gill-appendage, when the extraordinary change of appearance in the
+respiratory chamber of the lamprey which occurs at transformation, is taken
+into consideration. This change is of a very far-reaching character, and
+consists essentially of the formation of a new alimentary canal in this
+region, whereby the pharyngeal chamber of Ammocoetes is cut off posteriorly
+from the alimentary canal, and is confined entirely to respiratory
+purposes, its original lumen now forming a tube called the bronchus, which
+opens into the mouth and into a series of branchial pouches.
+
+In Fig. 68 I give diagrammatic illustrations taken from Nestler's paper to
+show the striking change which takes place at transformation, (A)
+representing three branchial segments of Ammocoetes, and (B) the
+corresponding three segments of Petromyzon. The corresponding parts in the
+two diagrams are shown by the cartilages (_br. cart._), the sense-organs
+(S), and the branchial veins (_V. br._); the corresponding diaphragms are
+marked by the figures 1, 2, 3 respectively. As is clearly seen, it is
+perfectly possible in the latter case to describe the respiratory chamber,
+as Nestler has done, as divided into a series of separate smaller
+chambers--the gill-pouches--by means of a series of diaphragms or branchial
+bars. The surface of these gill-pouches is in part thrown into folds for
+respiratory purposes, and each gill-pouch opens, on the one hand, into the
+bronchus (_Bro._), and, on the other, to the exterior by means of the
+gill-slit. The branchial unit in Petromyzon is, therefore, according to
+Nestler and other morphologists, the folded opposed surfaces of two
+contiguous diaphragms, and each one of the diaphragms is intersegmental
+between two gill-pouches.
+
+Nestler then goes on to describe the arrangement in Ammocoetes in the same
+terms, although there is no bronchus or gill-pouch, but only an open
+chamber into which these gill-bearing diaphragms project, which open
+chamber serves both for the passage of food and {169}of the water for
+respiration. This is manifestly the wrong way to look at the matter: the
+adult form is derived from the larval, not _vice versâ_, and the
+transformation process shows exactly how the gills, in Rathke's sense, come
+together to form the bronchus and so make the gill-pouches of Petromyzon.
+
+When we bear in mind that almost all observers consider that the internal
+branchiæ of the scorpion group are directly derived from branchial
+appendages of a kind similar to those of Limulus, it is evident that a
+branchial appendage such as that of Ammocoetes might also have arisen from
+such an appendage, because in various respects it is easier to compare the
+branchial appendage of Ammocoetes, than that of the scorpion group, with
+that of Limulus.
+
+[Illustration: FIG. 68.--DIAGRAM OF THREE BRANCHIAL SEGMENTS OF AMMOCOETES
+(A) COMPARED WITH THREE BRANCHIAL SEGMENTS AFTER TRANSFORMATION (B) TO SHOW
+HOW THE BRANCHIAL APPENDAGES OF AMMOCOETES FORM THE BRANCHIAL POUCHES OF
+PETROMYZON. (After NESTLER.)
+
+In both figures the branchial cartilages (_br. cart._), the branchial view
+(_V. br._), and the sense-organs (_S_), are marked out in order to show
+corresponding points. The muscles, blood-spaces, branchial arteries, etc.,
+of each branchial segment are not distinguished, being represented a
+uniform black colour. _Bro._, the bronchus into which each gill-pouch
+opens.]
+
+In the case of the scorpions, various suggestions have been made as to the
+manner in which such a conversion may have taken place. The most probable
+explanation is that given by Macleod, in which {170}each of the branchiæ of
+the scorpion group is directly compared with the branchial part of the
+Limulus appendage which has sunk into and amalgamated with the ventral
+surface.
+
+According to this view, the modification which has taken place in
+transforming the branchial Limulus-appendage into the branchial
+scorpion-appendage is a further stage of the process by which the Limulus
+branchial appendage itself has been formed, viz. the getting rid of the
+free locomotor segments of the original appendage, thus confining the
+appendage more and more to the basal branchial portion. So far has this
+process been carried in the scorpion that all the free part of the
+appendage has disappeared; apparently, also, the intrinsic muscles of the
+appendage have vanished, with the possible exception of the post-stigmatic
+muscle, so that any direct comparison between the branchial appendages of
+Limulus and the scorpions is limited to the comparison of their branchiæ,
+their nerves, and their afferent and efferent blood-vessels.
+
+In the case of Ammocoetes the comparison must be made not with
+air-breathing but with water-breathing scorpions, such as existed in past
+ages in the forms of Eurypterus, Pterygotus, Slimonia, and with the crowd
+of trilobite and Limulus-like forms which were in past ages so predominant
+in the sea; forms in some of which the branchial appendages had already
+become internal, but which, from the very fact of these forms being
+water-breathers, probably resembled, in respect of their respiratory
+apparatus, Limulus rather than the present-day scorpion.
+
+On the assumption that the branchial appendages of Ammocoetes, like the
+branchial appendages of the scorpion group, are to a certain extent
+comparable with those of Limulus, it becomes a matter of great interest to
+inquire whether the mode in which respiration is effected in Ammocoetes
+resembles most that of Limulus or of the scorpion.
+
+
+THE ORIGIN OF THE BRANCHIAL MUSCULATURE.
+
+The difference between the movements of respiration in Limulus and those of
+the scorpions consists in the fact that, although in both cases respiration
+is effected mainly by dorso-ventral muscles, these muscles are not
+homologous in the two cases: in the former, the dorso-ventral
+appendage-muscles are mainly concerned, in the latter, the dorso-ventral
+somatic muscles.
+
+{171}The paper by Benham gives a full description of the musculature of
+Limulus, and according to his arrangement the muscles are divided into two
+sets, longitudinal and dorso-ventral. Of the latter, each mesosomatic
+segment possesses a pair of dorso-ventral muscles, attached to the
+mid-ventral mesosomatic entochondrite, and to the tergal surface (Fig. 58,
+_Dv._). These muscles are called by Benham the vertical mesosomatic
+muscles. I shall call them the somatic dorso-ventral muscles, in
+contradistinction to the dorso-ventral muscles of the branchial appendages.
+Of the latter, the two chief are the external branchial (Fig. 66, _m_3_)
+and the posterior entapophysio-branchial (Fig. 66, _m_1_); a third muscle
+is the anterior entapophysio-branchial (Fig. 66, _m_2_). Of these muscles,
+the posterior entapophysio-branchial (_m_1_) is closely attached along the
+branchial cartilaginous bar up to its round-headed termination on the
+anterior surface of the appendage. The anterior entapophysio-branchial
+muscle (_m_2_) is attached to the branchial cartilage near the
+entapophysis.
+
+In the case of the scorpion, as described by Miss Beck, the branchial
+appendage has become reduced to the branchiæ, and the intrinsic
+appendage-muscles have entirely disappeared, with the possible exception of
+the small post-stigmatic muscle; on the other hand, the dorso-ventral
+somatic muscles, which are clearly homologous with the corresponding
+muscles of Limulus, have remained, and become the essential respiratory
+muscles.
+
+Of these two possible types of respiratory movement it is quite conceivable
+that in the water-breathing scorpions of olden times and in their allies,
+the dorso-ventral muscles of their branchial appendages may have continued
+their _rôle_ of respiratory muscles, and so have given origin to the
+respiratory muscles of the ancestors of Ammocoetes.
+
+The respiratory muscles of Ammocoetes are three in number, and have been
+described by Nestler and Miss Alcock as the adductor muscle, the striated
+constrictor muscle, and the tubular constrictor muscle (Fig. 65, _m. add._,
+_m.c.s._, and _m.c.t._). Of these, the constrictor muscle (Fig. 71, _m.
+con. str._) is in close contact with its cartilaginous bar, while the
+adductor (Fig. 71, _m. add._) is attached to the cartilage only at its
+origin and insertion, and the tubular muscles (Fig. 71, _m. con. tub._)
+have nothing whatever to do with the cartilage at all, being attached
+ventrally to the connective tissue in the neighbourhood {172}of the ventral
+aorta (_V.A._), and dorsally to the mid-line between the dorsal aorta
+(_D.A._) and the notochord.
+
+The close relationship of the constrictor muscle to the cartilaginous
+branchial bar does not favour the surmise that this muscle is homologous
+with the dorso-ventral somatic muscle of the scorpion. It is, however,
+directly in accordance with the view that this muscle is homologous with
+one of the dorso-ventral appendage-muscles, such as the posterior
+entapophysio-branchial muscle (_m_1_, Fig. 66) of the Limulus appendage,
+especially when the homology of the Ammocoetes branchial cartilage with the
+Limulus branchial cartilage is borne in mind. I am, therefore, inclined to
+look upon the constrictor and adductor muscles of the Ammocoetes branchial
+segment as more likely to have been derived from dorso-ventral muscles
+which belonged originally to a branchial appendage, such as we see in
+Limulus, than from dorso-ventral somatic muscles, such as the vertical
+mesosomatic muscles which are found both in Limulus and scorpion. In other
+words, I am inclined to hold the view that the somatic dorso-ventral
+muscles have disappeared in this region in Ammocoetes, while dorso-ventral
+appendage-muscles have been retained, _i.e._ the exact reverse to what has
+taken place in the air-breathing scorpion.
+
+I am especially inclined to this view because of the manner in which it
+fits in with and explains van Wijhe's results. Ever since Schneider divided
+the striated muscles of vertebrates into parietal and visceral, such a
+division has received general acceptance and, as far as the head-region is
+concerned, has received an explanation in van Wijhe's work; for Schneider's
+grouping corresponds exactly to the two segmentations of the
+head-mesoblast, discovered by van Wijhe, _i.e._ to the somatic and
+splanchnic striated muscles according to my nomenclature. Of these two
+groups the splanchnic or visceral striated musculature, innervated by the
+Vth, VIIth, IXth, and Xth nerves, which ought on this theory to be derived
+from the musculature of the corresponding appendages, is, speaking
+generally, dorso-ventral in direction in Ammocoetes and of the same
+character throughout; the somatic musculature, on the other hand, is
+clearly divisible, in the head region, into two sets--a spinal and a
+cranial set. The somatic muscles innervated by the spinal set of nerves,
+including in this term the spino-occipital or so-called hypoglossal nerves,
+are in Ammocoetes most sharply defined from all the other muscles of the
+body. They form the great dorsal and ventral longitudinal
+{173}body-muscles, which extend dorsally as far forward as the nose and are
+developed embryologically quite distinctly from the others, being formed as
+muscle-plates (Kästchen). On the other hand, the cranial somatic muscles
+are the eye-muscles, the formation of which resembles that of the visceral
+muscles, and not of the spinal somatic. Their direction is not
+longitudinal, but dorso-ventral; they cannot, in my opinion, be referred to
+the somatic trunk-muscles, and must, therefore, form a separate group to
+themselves. Thus the striated musculature of the Ammocoetes must be divided
+into (1) the visceral muscles; (2) the longitudinal somatic muscles; and
+(3) the dorso-ventral somatic muscles. Of these the 1st, on the view just
+stated, represent the original appendage-muscles; the 2nd belong to the
+spinal region, and will be considered with that region; the 3rd represent
+the original segmental dorso-ventral somatic muscles, which are so
+conspicuous in the musculature of the Limulus and the scorpion group.
+
+The discussion of this last statement will be given when I come to deal
+with the prosomatic segments of Ammocoetes. I wish, here, simply to point
+out that van Wijhe has shown that the eye-muscles develop from his 1st,
+2nd, and 3rd dorsal mesoblastic segments, and therefore represent the
+somatic muscles belonging to those segments, while no development of any
+corresponding muscles takes place in the 4th, 5th, and 6th segments; so
+that if the eye-muscles represent a group of dorso-ventral somatic muscles,
+such muscles have been lost in the 4th, 5th, and 6th segments. The latter
+segments are, however, the glossopharyngeal and vagus segments, the
+branchial musculature of which is derived from the ventral segments of the
+mesoderm. In other words, van Wijhe's observations mean that the
+dorso-ventral somatic musculature has been lost in the branchial or
+mesosomatic region, while the dorso-ventral appendage musculature has been
+retained, and that, therefore, the mode of respiration in Ammocoetes more
+closely resembles that of Limulus than of Scorpio.
+
+In addition to these branchial muscles, another and very striking set of
+muscles is found in the respiratory region of Ammocoetes--the so-called
+tubular muscles. These muscles are of great interest, but as they are
+especially connected with the VIIth nerve, their consideration is best
+postponed to the chapter dealing with that nerve.
+
+Also, in connection with the vagus group of nerves, special sense-organs
+are found in the skin covering this mesosomatic region, the so-called
+epithelial pit-organs (_Ep. pit._, Fig. 71). They, too, are of {174}great
+interest, but their consideration may also better be deferred to the
+chapter dealing with those special sense-systems known as the lateral line
+and auditory systems.
+
+
+COMPARISON OF THE BRANCHIAL CIRCULATION IN AMMOCOETES AND LIMULUS.
+
+Closely bound up with the respiratory system is the nature of the
+circulation of blood through the gills. Before, therefore, proceeding to
+the consideration of the segments in front of those which carry branchiæ,
+it is worth while to compare the circulation of the blood in the gills of
+Limulus and of Ammocoetes respectively.
+
+In all the higher vertebrates the blood circulates in a closed system of
+capillaries, which unite the arterial with the venous systems. In all the
+higher invertebrates this capillary system can hardly be said to exist; the
+blood is pumped from the arterial system into blood spaces or lacunæ, and
+thus comes into immediate contact with the tissues. From these it is
+collected into veins, and so returned to the heart. There is, in fact, no
+separate lymph-system in the higher invertebrates; the blood-system and
+lymph-system are not yet differentiated from each other. This also is the
+case in Ammocoetes; here, too, in many places the blood is poured into a
+lacunar space, and collected thence by the venous system; a capillary
+system is only in its commencement and a lymph-system does not yet exist.
+In this part of its vascular system Ammocoetes again resembles the higher
+invertebrates more than the higher vertebrates.
+
+This resemblance is still more striking when the circulation in the
+respiratory organs of the two animals is compared. A branchial appendage is
+essentially an appendage whose vascular system is arranged for the special
+purpose of aerating blood. In the higher vertebrates such a purpose is
+attained by the pulmonary capillaries, in Limulus by the division of the
+posterior surface of the basal part of the appendage into thin lamellar
+plates, the interior of each of which is filled with blood. The two
+surfaces of each lamella are kept parallel to each other by means of
+fibrous or cellular strands forming little pillars at intervals, called by
+Macleod "colonettes." A precisely similar arrangement is found in the
+scorpion gill-lamella, as seen in Fig. 69, A, taken from Macleod. In
+Ammocoetes there are no well-defined branchial capillaries, but the blood
+circulates, as in {175}the invertebrate gill, in a lamellar space; here,
+also, as Nestler has shown, the opposing walls of the gill-lamella are held
+in position by little pillar-like cells, as seen in Fig. 69, B, taken from
+his paper.
+
+In this representative of the earliest vertebrates the method of
+manufacturing an efficient gill out of a lacunar blood-space is precisely
+the same as that which existed in Limulus and the scorpion, and, therefore,
+as that which existed in the dominant invertebrate group at the time when
+vertebrates first appeared. This similarity indicates a close resemblance
+between the circulatory systems of the two groups of animals, and
+therefore, to the superficial inquirer, would indicate an homology between
+the heart of the vertebrate and the heart of the higher invertebrate; but
+the former is situated ventrally to the gut and the nervous system, while
+the latter is composed of a long vessel which lies in the mid-dorsal line
+immediately under the external dorsal covering. Indeed, this ventral
+position of the heart in the one group of animals and its dorsal position
+in the other, combined with the corresponding positions of the central
+nervous system, is one of the principal reasons why all the advocates of
+the origin of vertebrates from the Appendiculata, with the single exception
+of myself, feel compelled to reverse the dorsal and ventral surfaces in
+deriving the vertebrate from the invertebrate. But there is one most
+important fact which ought to make us hesitate before accepting the
+homology of the dorsal heart of the arthropod with the ventral heart of the
+vertebrate--The heart in all invertebrates is a systemic heart, _i.e._
+drives the arterial blood to the different organs of the body, and then the
+veins carry it back to the respiratory organ, from whence it passes to the
+heart.
+
+[Illustration: FIG. 69.--COMPARISON OF BRANCHIAL LAMELLÆ OF LIMULUS AND
+SCORPIO WITH BRANCHIAL LAMELLÆ OF AMMOCOETES.
+
+A, Branchial lamellæ of Scorpio (after Macleod); B, Branchial lamellæ of
+Ammocoetes (after Nestler).]
+
+The only exception to this scheme is found in the vertebrate where the
+heart is essentially a branchial heart, the blood being {176}driven from
+the heart to the ventral aorta, from which by the branchial arteries it is
+carried to the gills, and then, after aeration, is collected into the
+dorsal aorta, whence it is distributed over the body. The distributing
+systemic vessel is the dorsal aorta, not the heart which belongs
+essentially to the ventral venous system. This constitutes a very strong
+reason for believing that the systemic heart of the invertebrate is not
+homologous with the heart of the vertebrate. How, then, did the vertebrate
+heart arise?
+
+Let us first see how the blood is supplied to the gills in Limulus.
+
+[Illustration: FIG. 70.--LONGITUDINAL DIAGRAMMATIC SECTION THROUGH THE
+MESOSOMATIC REGION OF LIMULUS, TO SHOW THE ORIGIN OF THE BRANCHIAL
+ARTERIES. (After BENHAM.)
+
+_L.V.S._, longitudinal venous sinus, or collecting sinus; _a. br._,
+branchial arteries; _V.p._, veno-pericardial muscles; _P._, pericardium.]
+
+In Limulus the blood flows into the lamellæ from sinuses or blood-spaces
+(_b.s._, Fig. 66) at the base of each of the lamellæ, which sinuses are
+filled by a vessel which may be called the branchial artery, since it is
+the afferent branchial vessel. On each side of the middle line of the
+ventral surface of the body a large longitudinal venous sinus exists,
+called by Milne-Edwards the venous collecting sinus, _L.V.S._, (Fig. 70 and
+Fig. 58), which gives off to each of the branchial appendages on that side
+a well-defined afferent branchial vessel--the branchial artery (_a. br._).
+The blood of the branchial artery flows into the blood-spaces between the
+anterior and posterior laminæ of the appendage and thence into the
+gill-lamellæ, from which it is collected into an efferent vessel or
+branchial vein, termed by Milne-Edwards the branchio-cardiac canal, which
+carries it back to the dorsal heart. The position of the branchial artery
+and vein is shown in Fig. 66, which represents a section through the
+branchial appendage of Limulus at right angles to the cartilaginous
+branchial bar (_br. cart._), just as Fig. 65 represents a section through
+the {177}branchial appendage of Ammocoetes at right angles to the
+cartilaginous branchial bar.
+
+Further, the observations of Blanchard, Milne-Edwards, Ray Lankester, and
+Benham concur in showing that in both Limulus and the scorpion group a
+striking and most useful connection exists between the heart and these two
+collecting venous sinuses, in the shape of a segmentally arranged series of
+muscular bands (_V.p._, Fig. 70 and Fig. 58), attached, on the one hand, to
+the pericardium, and on the other to the venous collecting sinus on each
+side. These muscular bands, to which Lankester and Benham have given the
+name of 'veno-pericardial muscles,' are so different in appearance from the
+rest of the muscular substance, that Milne-Edwards did not recognize them
+as muscular, but called them 'brides transparentes.' Blanchard speaks of
+them in the scorpion as 'ligaments contractiles,' and considers that they
+play an important part in assisting the pulmonary circulation; for, he
+says, "en mettant a nu une portion du coeur, on remarque que ces battements
+se font sentir sur les ligaments contractiles, et determinent sur les
+poches pulmonaires une pression qui fait aussitot refluer et remonter le
+sang dans les vaisseaux pneumocardiaques." Lankester, in discussing the
+veno-pericardial muscles of Limulus and of the scorpions, says that these
+muscles probably contract simultaneously with the heart and are of great
+importance in assisting the flow through the pulmonary system. More
+recently Carlson has investigated the action of these muscles in the living
+Limulus and found that they act simultaneously with the muscles of
+respiration.
+
+Precisely the same arrangement of veno-pericardial muscles and of
+longitudinal venous collecting sinuses occurs in the scorpions. It is one
+of the fundamental characters of the group, and we may fairly assume that a
+similar arrangement existed in the extinct forms from which I imagine the
+vertebrate to have arisen. The further consideration of this group of
+muscles will be given in Chapter IX.
+
+Passing now to the condition of the branchial blood-vessels of Ammocoetes,
+we see that the blood passes into the gill-lamellæ from a blood-space in
+the appendage, which can hardly be dignified by the name of a blood-vessel.
+This blood-space is supplied by the branchial artery which arises
+segmentally from the ventral aorta (_V.A._), as seen in Fig. 71 (taken from
+Miss Alcock's paper). From the gill-lamellæ the blood is collected into an
+efferent or branchial vein (_v. br._), which {178}runs, as seen in Fig. 65,
+along the free edge of the diaphragm, and terminates in the dorsal aorta.
+
+The ventral aorta is a single vessel near the heart, but at the
+commencement of the thyroid it divides into two, and so forms two ventral
+longitudinal vessels, from which the branchial arteries arise segmentally.
+
+[Illustration: FIG. 71.--DIAGRAM CONSTRUCTED FROM A SERIES OF TRANSVERSE
+SECTIONS THROUGH A BRANCHIAL SEGMENT, SHOWING THE ARRANGEMENT AND RELATIVE
+POSITIONS OF THE CARTILAGE, MUSCLES, NERVES, AND BLOOD-VESSELS.
+
+Nerves coloured red are the motor nerves to the branchial muscles. Nerves
+coloured blue are the internal sensory nerves to the diaphragms and the
+external sensory nerves to the sense-organs of the lateral line system.
+_Br. cart._, branchial cartilage; _M. con. str._, striated constrictor
+muscles; _M. con. tub._, tubular constrictor muscles; _M. add._, adductor
+muscle; _D.A._, dorsal aorta; _V.A._, ventral aorta; _S._, sense-organs on
+diaphragm; _n. Lat._, lateral line nerve; _X._, epibranchial ganglia of
+vagus; _R. br. prof. VII._, _ramus branchialis profundus_ of facial;
+_J.v._, jugular vein; _Ep. pit._, epithelial pit.]
+
+From this description it is clear that the vascular supply of the branchial
+segment of Ammocoetes would resemble most closely the vascular supply of
+the Limulus branchial appendage, if the ventral aorta of the former was
+derived from two longitudinal veins, homologous with the paired
+longitudinal venous sinuses of the latter.
+
+{179}_A priori_, such a derivation seems highly improbable; and yet it is
+precisely the manner in which embryology teaches us that the heart and
+ventral aorta of the vertebrate have arisen.
+
+
+THE ORIGIN OF THE INVERTEBRATE HEART AND THE ORIGIN OF THE VERTEBRATE
+HEART.
+
+Not only does the vertebrate heart differ from that of the invertebrate, in
+that it is branchial while the latter is systemic, but also it is unique in
+its mode of formation in the embryo. In the Appendiculata the heart is
+formed as a single organ in the mid-dorsal line by the growth of the two
+lateral plates of mesoblast dorsalwards, the heart being formed where they
+meet. In Mammalia and Aves, the heart and ventral aorta commence as a pair
+of longitudinal veins, one on each side of the commencing notochord.
+
+If the embryo be removed from the yolk, the surface of the embryo covering
+these two venous trunks can be spoken of as the ventral surface of the
+embryo at that stage, and indeed we find that in the present day there is
+an increasing tendency to speak of this surface as the ventral surface of
+the embryo. Thus, Mitsukuri, in his studies of chelonian embryos, lays
+great stress on the importance of surface views and when the embryo has
+been removed from the yolk, figures and speaks of its ventral surface. So,
+also, Locy and Neal find that the best method of seeing the early segments
+of the embryo is to remove the embryo from the yolk, and examine what they
+speak of as a ventral view. At the period, then, before the formation of
+the throat, we may say that on the ventral surface of the embryo a pair of
+longitudinal venous sinuses are found, one on each side of the mid-ventral
+line, which are in the same position with respect to the mid-axis of the
+embryo as are the longitudinal venous sinuses in Limulus.
+
+The next step is the formation of the throat by the extension of the layers
+of the embryo laterally to meet in the mid-line and so form the pharynx,
+with the consequence that a new ventral surface is formed; these two veins,
+as is well known, travel round also, and, meeting together in the new
+mid-ventral line, form the subintestinal vein, the heart, and the ventral
+aorta.
+
+What is true of Mammalia and Aves, has been shown by P. Mayer to be true
+universally among vertebrates, so that in all cases the heart and ventral
+aorta have arisen by the coalescence in the new mid-ventral {180}line of
+two longitudinal venous channels, which were originally situated one on
+each side of the notochord, in what was then the ventral surface of this
+part of the embryo. This history is especially instructive in showing how
+the pharyngeal region is formed by the growing round of the lateral
+mesoblast, _i.e._ the muscular and other mesoblastic tissues of the
+branchial segments, and how the two longitudinal veins take part in this
+process. The phylogenetic interpretation of this embryological fact seems
+to be, that the new ventral surface of the vertebrate in this region is
+formed, not only by the branchial appendages, but also by the growth
+ventrally of that part of the original ventral surface which covered each
+longitudinal venous sinus.
+
+The following out of the consecutive clues, which one after the other arise
+in harmonious succession as the necessary sequence of the original working
+hypothesis, brings even now into view the manner in which the respiratory
+portion of the alimentary canal arose, and gives strong hints as to the
+position of that part of the arthropod which gave origin to the notochord.
+Here I will say no more at present, for the origin of the new alimentary
+canal of the vertebrate and of the notochord will be more fittingly
+discussed as a whole, after all the other organs of the vertebrate have
+been compared with the corresponding organs of the arthropod.
+
+[Illustration: FIG. 72.--DIAGRAM (UPPER HALF OF FIGURE) OF THE ORIGINAL
+POSITION OF VEINS (H) WHICH COME TOGETHER TO FORM THE HEART OF A
+VERTEBRATE.
+
+_C.N.S._, central nervous system; _nc._, notochord; _m._, myotome.
+
+The lower half of figure shows comparative position of the longitudinal
+venous sinus (_L.V.S._) in Limulus. _C.N.S._, central nervous system;
+_Al._, alimentary canal; _H._, heart; _m._, body-muscles.]
+
+The strong evidence that the vertebrate heart was formed from a pair of
+longitudinal venous sinuses on the ventral side of the central canal,
+carries with it the conclusion that the original single median dorsal heart
+of the arthropod is not represented in the vertebrate, {181}for the dorsal
+aorta cannot by any possibility represent that heart.
+
+Although it is not now functional the original existence of so important an
+organ as a dorsal heart may have left traces of its former presence; if so,
+such traces would be most likely to be visible in the lowest vertebrates,
+just as the median eyes are much more evident in them than in the higher
+forms. In Fig. 58 the position of the dorsal heart is shown in Limulus, and
+in Fig. 70 the shape and extent of this dorsal heart is shown. It extends
+slightly into the prosomatic region, and thins down to a point there, runs
+along the length of the animal and finally thins down to a point at the
+caudal end.
+
+The heart is surrounded by a pericardium, from which at regular intervals a
+number of dorso-ventral muscles pass, to be inserted into the longitudinal
+venous sinus on each side. These veno-pericardial muscles are absolutely
+segmental with the mesosomatic segments, and are confined to that region,
+with the exception of two pairs in the prosomatic region. Their homologies
+will be discussed later.
+
+Any trace of a heart such as we have just described must be sought for in
+Ammocoetes between the central nervous system and the mid-line dorsally.
+Now, in this very position a large striking mass of tissue is found,
+represented in section in Fig. 73, _f_. It forms a column of similar tissue
+along the whole mid-dorsal region, except at the two extremities; it tapers
+away in the caudal region, and headwards grows thinner and thinner, so that
+no trace of it is seen anterior to the commencement of the branchial
+region. It resembles in its dorsal position, in its shape, and in its size
+a dorsal heart-tube such as is seen in Limulus and elsewhere, but it
+differs from such a tube in its extension headwards. The heart-tube of
+Limulus ceases at the anterior end of the mesosomatic region, this
+fat-column of Ammocoetes at the posterior end. In its structure there is
+not the slightest sign of anything of the nature of a heart; it is a solid
+mass of closely compacted cells, and the cells are all very full of fat,
+staining intensely black with osmic acid. Nowhere else in the whole body of
+Ammocoetes is such a column of fat to be found. It is not skeletogenous
+tissue with cells of the nature of cartilage-cells, as Gegenbaur thought
+and as Balfour has depicted (Vol. II., Fig. 315) in his 'Comparative
+Embryology,' as though this tissue were a part of the vertebral column, but
+is simply fat-cells, such as might easily have taken the place of some
+other previously existing organ.
+
+{182}I do not know how to decide the question which thus arises. Supposing,
+for the sake of argument, that this column of fat-cells has really taken
+the place of the original dorsal heart, what criterion would there be as to
+this? The heart _ex hypothesi_ having ceased to function, the muscular
+tissue would not remain, and the space would be filled up, presumably with
+some form of connective tissue. As likely as not, the connective tissue
+might take the form of fatty tissue, the storage of fat being a
+physiological necessity to an animal, while at the same time no special
+organ has been developed for such a purpose, but fat is being laid down in
+all manner of places in the body.
+
+This dorsal fat-column, as it is seen in Ammocoetes, is not found in the
+higher vertebrates, so that it possesses, at all events, the significance
+of being a peculiarity of ancient times before the vertebrate skeletal
+column was formed.
+
+I mention it here in connection with my view as to the origin of
+vertebrates, because there it is, in the very place where the dorsal heart
+ought to have been. For my own part, I should not have expected that a
+muscular organ such as the heart would leave any trace of itself if it
+disappeared, so that its absence in the dorsal region of the vertebrate
+does not seem to me in the slightest degree to invalidate my theory.
+
+[Illustration: FIG. 73.--SECTION THROUGH THE NOTOCHORD (_NC._), THE SPINAL
+CANAL AND THE FAT-COLUMN (_F._), OF AMMOCOETES, DRAWN FROM AN OSMIC
+PREPARATION.
+
+_sp. c._, spinal cord; _gl._, glandular tissue filling the spinal canal;
+_sk._, Gegenbaur's skeletogenous cells; _p._, pigment.]
+
+
+{183}SUMMARY.
+
+  From the close similarity of structure and position between the branchial
+  skeleton of Limulus and of Ammocoetes, as given in the preceding chapter,
+  it logically follows that the branchiæ of Ammocoetes must be homologous
+  with the branchiæ of Limulus. But the respiratory apparatus of Limulus
+  consists of branchial appendages. It follows, therefore, that the
+  branchiæ of Ammocoetes, and consequently of the vertebrates, must have
+  been derived from branchial appendages, and as they are internal, not
+  external, such branchial appendages must have been of the nature of
+  'sunk-in' branchial appendages. Such internal appendages are
+  characteristic of the scorpion tribe, and of, perhaps, the majority of
+  the Palæostraca, for no external respiratory appendages have been
+  discovered in any of the sea-scorpions.
+
+  In the vertebrates--and it is especially well shown in Ammocoetes--a
+  double segmentation exists in the head-region, a body or somatic
+  segmentation, and a branchial or splanchnic segmentation, respectively
+  expressed by the terms mesomeric and branchiomeric segmentations. The
+  nerves which supply the latter segments form a very well-marked group
+  (Charles Bell's system of lateral or respiratory nerves) which do not
+  conform to the system of spinal nerves, for they do not arise from
+  separate motor and sensory roots, but are mixed nerves from the very
+  beginning.
+
+  The system of cranial segmental nerves is older than the spinal system,
+  and cannot, therefore, be derived from it, but can be arranged as a
+  system supplying two segments, somatic and splanchnic, which differ in
+  the following way: Each somatic segment is supplied by two roots, motor
+  and sensory respectively, as in the spinal cord segments, while each
+  splanchnic segment possesses only one root, which is mixed in function.
+
+  The peculiarities of the grouping of the cranial segmental nerves, which
+  have hitherto been unexplained, immediately receive a straightforward and
+  satisfactory explanation if the splanchnic or branchiomeric segments owe
+  their origin to a system of appendages after the style of those of
+  Limulus.
+
+  In Limulus and all the Arthropoda, the segmentation is double, being
+  composed of (1) somatic or body-segments, constituting the mesomeric
+  segmentation; (2) appendage-segments, which, seeing that they carry the
+  branchiæ, constitute a branchiomeric segmentation. Similarly to the
+  cranial region of the vertebrate, the nerves which supply the somatic
+  segments arise from separate sensory and motor roots, while the single
+  nerve which supplies each appendage contains all the fibres for the
+  appendage, both motor and sensory.
+
+  It follows from this that the branchial segments supplied by the vagus
+  and glossopharyngeal nerves ought to have arisen from appendages bearing
+  branchiæ.
+
+  Although the evidence of such appendages has entirely disappeared in the
+  higher vertebrates, together with the disappearance of branchiæ, and is
+  not strikingly apparent in the higher gill-bearing fishes, yet in
+  Ammocoetes, so great is the difference here from all other fishes, it is
+  natural to describe the pharyngeal or respiratory chamber as a chamber
+  into which a symmetrical series of respiratory appendages, the so-called
+  diaphragms, are dependent. Each of these appendages possesses its own
+  mixed nerve, glossopharyngeal or vagus, {184}its own cartilage, its own
+  set of visceral muscles, its own sense-organs, just as do the respiratory
+  appendages of Limulus.
+
+  The branchial unit in the vertebrate is not the gill-pouch, but the
+  branchial bar or appendage between the pouches. Embryology shows how each
+  such appendage grows inwards, how a coelomic cavity is formed in it,
+  similarly to the ingrowing of the branchial appendage in scorpions.
+
+  We do not know how the palæostracan sea-scorpions breathed; they resemble
+  the scorpion of the present day somewhat in form, but they are in many
+  respects closely allied to Limulus. The present-day scorpion is a land
+  animal, and the muscles by which he breathes are dorso-ventral somatic
+  muscles, while those of Limulus are the appendage-muscles.
+
+  The old sea-scorpions very probably used their appendage-muscles after
+  the Limulus fashion, being water-breathers, even although their
+  respiratory appendages were no longer free but sunk in below the surface
+  of the body. The probability that such was the case is increased after
+  consideration of the method of breathing in Ammocoetes, for the
+  respiratory muscles of the latter animal are directly comparable with the
+  muscles of the respiratory appendages of Limulus, and are not somatic.
+  Even the gills themselves of Ammocoetes are built up in the same fashion
+  as are those of Limulus and the scorpions. The conception of the
+  branchial unit as a gill-bearing appendage, not a gill-pouch, immediately
+  explains the formation of the vertebrate heart, which is so strikingly
+  different from that of all invertebrate hearts, in that it originates as
+  a branchial and not as a systemic heart, and is formed by the coalescence
+  of two longitudinal veins.
+
+  The origin of these two longitudinal veins is immediately apparent if the
+  vertebrate arose from a palæostracan, for in Limulus and the whole
+  scorpion tribe, in which the heart is a systemic heart, the branchiæ are
+  supplied with blood from two large longitudinal venous sinuses, situated
+  on each side of the middle line of the animal in an exactly corresponding
+  position to that of the two longitudinal veins, which come together to
+  form the heart and ventral aorta of the vertebrate. The consideration of
+  the respiratory apparatus and of its blood-supply in the vertebrate still
+  further points to the origin of vertebrates from the Palæostraca.
+
+
+
+
+{185}CHAPTER V
+
+_THE EVIDENCE OF THE THYROID GLAND_
+
+  The value of the appendage-unit in non-branchial segments.--The double
+  nature of the hyoid segment.--Its branchial part.--Its thyroid part.--The
+  double nature of the opercular appendage.--Its branchial part.--Its
+  genital part.--Unique character of the thyroid gland of Ammocoetes--Its
+  structure.--Its openings.--The nature of the thyroid segment.--The uterus
+  of the scorpion.--Its glands.--Comparison with the thyroid gland of
+  Ammocoetes.--Cephalic genital glands of Limulus.--Interpretation of
+  glandular tissue filling up the brain-case of Ammocoetes.--Function of
+  thyroid gland.--Relation of thyroid gland to sexual functions.--Summary.
+
+
+I have now given my reasons why I consider that the glossopharyngeal and
+vagus nerves were originally the nerves belonging to a series of
+mesosomatic branchial appendages, each of which is still traceable in the
+respiratory chamber of Ammocoetes, and gives the type-form from which to
+search for other serially homologous, although it may be specially
+modified, segments.
+
+As long as the branchial unit consisted of the gill-pouch the segments of
+the head-region were always referred to such units, hence we find Dohrn and
+Marshall picturing to themselves the ancestor of vertebrates as possessing
+a series of branchial pouches right up to the anterior end of the body.
+Marshall speaks of olfactory organs as branchial sense-organs; Dohrn of the
+mouth as formed by the coalescence of gill-slits, of the trigeminal nerve
+as supplying modified branchial segments, etc.; thus a picture of an animal
+is formed such as never lived on this earth, or could be reasonably
+imagined to have lived on it. Yet Dohrn's conceptions of the segmentation
+were sound, his interpretation only was in fault, because he was obliged to
+express his segments in terms of the gill-pouch unit. Once abandon that
+point of view and take as the unit a branchial appendage, then immediately
+we see that in the region in front of the branchiæ we may still have
+segments {186}homologous to the branchial segments, originally
+characterized by the presence of appendages, but that such appendages need
+never have carried branchiæ. The new mouth may have been formed by such
+appendages, which would express Dohrn's suggestion of its formation by
+coalesced gill-slits; the olfactory organ may have been the sense-organ
+belonging to an antennal appendage, which would be what Marshall really
+meant in calling it a branchial sense-organ.
+
+
+THE FACIAL NERVE AND THE FOREMOST RESPIRATORY SEGMENT.
+
+This simple alteration of the branchiomeric unit from a gill-pouch to an
+appendage, which may or may not bear branchiæ, immediately sheds a flood of
+light on the segmentation of the head-region, and brings to harmony the
+chaos previously existing. Let us, then, follow out its further teachings.
+Next anteriorly to the glossopharyngeal and vagus nerves comes the facial
+nerve; a nerve which supplies the hyoid segment, or, rather, according to
+van Wijhe the two hyoid segments, for embryologically there is evidence of
+two segments. As already mentioned, the facial nerve is usually included in
+the trigeminal or pro-otic group of nerves, the opisthotic group being
+confined to the glossopharyngeal and vagus. This inclusion of the facial
+nerve into the pro-otic group of nerves forms one of the main reasons why
+this group has been supposed to have originally supplied gill-pouch
+segments, for the hyoid segment is clearly associated with branchiæ.
+
+When, however, we examine Ammocoetes (_cf._ Figs. 63 and 64) it is clear
+that the foremost of the segments forming the respiratory chamber, which
+must be classed with the rest of the mesosomatic or opisthotic segments, is
+that supplied by the facial nerves.
+
+An examination of this respiratory chamber shows clearly that there are six
+pairs of branchial appendages or diaphragms, which are all exactly similar
+to each other. These are those already considered, the foremost of which
+are supplied by the IXth or glossopharyngeal nerves. Immediately anterior
+to this glossopharyngeal segment is seen in the figures the segment
+supplied by the VIIth or facial nerves. It is so much like the segments
+belonging to the glossopharyngeal and vagus nerves as to make it certain
+that we are dealing here with a branchial segment, composed of a pair of
+branchial appendages similar to those in the other cases, except that the
+cartilaginous bar is here replaced by a bar of muco-cartilage and the
+branchiæ are confined to the posterior part of each appendage. The anterior
+portion is, as is seen in Fig. 74, largely occupied by blood-spaces, but in
+addition carries the ciliated groove (_ps. br._) called by Dohrn
+'pseudo-branchiale Rinne.' This groove leads directly into the thyroid
+gland, which is a large bilateral organ situated in the middle line, as
+seen in Fig. 80 and Fig. 85. As shown by Miss Alcock, the facial nerve
+supplies this thyroid gland, as well as the posterior hyoid branchial
+segment, and, as pointed out by Dohrn, there is every reason to consider
+this thyroid gland as indicative of a separate segment, especially when van
+Wijhe's statement that the hyoid segment is in reality double is taken into
+account.
+
+{187}[Illustration: FIG. 74.--VENTRAL HALF OF HEAD-REGION OF AMMOCOETES.
+
+Somatic muscles coloured red. Branchial and visceral muscles coloured blue.
+Tubular constrictor muscles distinguished from striated constrictor muscles
+by simple hatching. _Tent._, tentacles; _Tent. m.c._, muco-cartilage of
+tentacles; _Vel. m.c._, muco-cartilage of the velum; _Hy. m.c._,
+muco-cartilage of the hyoid segment; _Ps. br._, pseudo-branchial groove;
+_Br. cart._, branchial cartilages; _Sp._, space between somatic and
+splanchnic muscles; _Th. op._, orifice of thyroid; _H._, heart.]
+
+{188}The evidence, then, of Ammocoetes points directly to this conclusion:
+The facial nerves represent the foremost of the mesosomatic group of
+nerves, and supply two segments, which have amalgamated with each other.
+The most posterior of these, the hyoid segment, is a branchial segment of
+the same character as those supplied by the vagus and glossopharyngeal
+nerves; represents, therefore, the foremost pair of branchial appendages.
+The anterior or thyroid segment, on the other hand, differs from the rest
+in that, instead of branchiæ, it carries the thyroid gland with its two
+ciliated grooves. If this segment, which is the foremost of the mesosomatic
+segments, also indicates a pair of appendages which carry the thyroid gland
+instead of branchiæ, then it follows that this pair of appendages has
+joined together in the mid-line ventrally and thus formed a single median
+organ--the thyroid gland. If, then, we find that the foremost of the
+mesosomatic appendages in the Palæostraca was really composed of two pairs
+of appendages, of which the most posterior carried branchiæ, while the
+anterior pair had amalgamated in the mid-line ventrally, and carried some
+special organ instead of branchiæ, then the accumulation of coincidences is
+becoming so strong as to amount to proof of the correctness of our line of
+investigation.
+
+
+THE FIRST MESOSOMATIC SEGMENT IN LIMULUS AND ITS ALLIES.
+
+What, then, is the nature of the foremost pair of mesosomatic appendages in
+Limulus. They differ from the rest of the mesosomatic appendages in that
+they do not carry branchiæ, and instead of being {189}separate are joined
+together in the mid-line ventrally to form a single terminal plate-like
+appendage known as the operculum. On its posterior surface the operculum
+carries the genital duct on each side.
+
+So also in the scorpion group, the operculum is always found and always
+carries the genital ducts.
+
+A survey of the nature of the opercular appendage demonstrates the
+existence of three different types--
+
+1.  That of Limulus, in which the operculum is free, and carries only the
+terminations of the genital ducts. In this type the duct on each side opens
+to the exterior separately (Fig. 75).
+
+[Illustration: FIG. 75.--OPERCULUM OF LIMULUS TO SHOW THE TWO SEPARATE
+GENITAL DUCTS.]
+
+[Illustration: FIG. 76.--OPERCULUM OF MALE SCORPION.
+
+_Ut._, terminal chamber, or uterus.]
+
+2.  The type of Scorpio, Androctonus, Buthus, etc., in which the operculum
+is not free, but forms part of the ventral surface of the body-wall, but,
+like Limulus, carries only the terminations of the genital ducts. In this
+type the duct on each side terminates in a common chamber (vagina or
+uterus), which communicates with the exterior by a single external median
+opening. This common chamber, or uterus (_Ut._), extends the whole breadth
+of the operculum (as seen in Fig. 76), and is limited to that segment.
+
+3. The type of Thelyphonus, Hypoctonus, Phrynus, and other members of the
+Pedipalpi, in which the operculum forms a part of the ventral surface of
+the body wall, but no longer covers only the termination of the genital
+apparatus. It really consists of two parts, a median anterior, which covers
+the terminal genital apparatus, {190}and a lateral posterior, which covers
+the first pair of gills, or lung-books, as they are called. In this type
+(Fig. 77) the genital ducts terminate in a common chamber or uterus, the
+nature of which will be further considered.
+
+As has been pointed out by Blanchard, the terminal genital organs of the
+scorpions and the Pedipalpi vary considerably in the different genera,
+especially the male genital organs. The general type of structure is the
+same, and consists in both male and female of vasa deferentia, which come
+together to form a common chamber before the actual opening to the
+exterior. This common chamber has been called in the female scorpion the
+vagina, or in Thelyphonus the uterus. I shall use the latter term, in
+accordance with Tarnani's work, and the corresponding chamber in the male
+will be the _uterus masculinus_.
+
+A considerable discussion has taken place about the method of action of the
+external genital organs in the members of the scorpion tribe, into which it
+is hardly necessary to enter here. The evidence points to the conclusion
+that in all these forms the operculum covers a median single chamber or
+uterus, into which the genital ducts open on each side, the main channels
+of emission being provided with a massive chitinous internal framework. We
+may feel certain that in the old extinct sea-scorpions, Eurypterus, etc., a
+similar arrangement existed, and that therefore in them also the median
+portion of the operculum covered a median chamber or uterus composed of the
+amalgamation of the terminations of the two genital ducts, which were
+originally separate, as in Limulus.
+
+[Illustration: FIG. 77.--OPERCULUM AND FOLLOWING SEGMENTS OF MALE
+THELYPHONUS.
+
+Opercular segment is marked out by thick black line. _Ut. Masc._, uterus
+masculinus; _Int. Op._, internal opening of uterus into genital chamber;
+_Ext. Op._, common external opening to genital chamber (_Gen. Ch._) and
+pulmonary chamber.]
+
+The observations of Schmidt, Zittel, and others show that the
+{191}operculum in the old extinct sea-scorpions, Eurypterus, Pterygotus,
+etc., belonged to the type of Thelyphonus, rather than to that of Limulus
+or Scorpio. In Fig. 78 I give a picture from Schmidt of the ventral aspect
+of Eurypterus, and by the side of it a picture of the isolated operculum.
+Schmidt considers that there were five branchiæ-bearing segments
+constituting the mesosoma, the foremost of which formed the operculum. Such
+operculum is often found isolated, and is clearly composed of two lateral
+appendages fused together in the middle line, of such a nature as to form a
+median elongated tongue, which lies between and separates the first three
+pairs of branchial segments. This median tongue, together with the anterior
+and median portion of the operculum, concealed, in all probability,
+according to Schmidt, the terminal parts of the genital organs, just as the
+median part of the operculum in Phrynus and Thelyphonus conceals the
+complicated terminal portions of the genital organs. The posterior part of
+the operculum, like that of Phrynus and Thelyphonus, carried the first pair
+of branchiæ, so Schmidt thinks from the evidence of markings on some
+specimens.
+
+[Illustration: FIG. 78.--_Eurypterus._
+
+The segments and appendages on the right are numbered in correspondence
+with the cranial system of lateral nerve-roots as found in vertebrates.
+_M._, metastoma. The surface ornamentation is represented on the first
+segment posterior to the branchial segments. The opercular appendage is
+marked out by dots.]
+
+Apparently an opercular appendage of this kind is in reality the result of
+a fusion of the genital operculum with the first branchial appendage in
+forms such as the scorpion; for, in order that the tergal plates may
+correspond in number with the sternal in Eurypterus, etc., it is necessary
+to consider that the operculum is composed of two sternites joined
+together. Similarly in Thelyphonus, Phrynus, etc., this numerical
+correspondence is only observed if the operculum is looked upon as double.
+
+A restoration of the mesosomatic region of Eurypterus, viewed {192}from the
+internal surface, might be represented by Fig. 79, in which the thick line
+represents the outline of the opercular segment, and the fainter lines the
+succeeding branchial segments. The middle and anterior part of the
+opercular segment carried the terminations of the genital organs; these I
+have represented, in accordance with our knowledge of the nature of these
+organs in the present-day scorpions, as a median elongated uterus,
+bilaterally formed, from which the genital ducts passed, probably as in
+Limulus, towards a mass of generative gland in the cephalic region, and not
+as in Scorpio or Thelyphonus, tailwards to the abdominal region.
+
+[Illustration: FIG. 79.--DIAGRAM TO INDICATE THE PROBABLE NATURE OF THE
+MESOSOMATIC SEGMENTS OF EURYPTERUS.
+
+The opercular segment is marked out by the thick black line. The segments
+_II.-VI._ bear branchiæ, and segment _I._ is supposed in the male to carry
+the uterus masculinus (_Ut. Masc._) and the genital ducts.]
+
+It is possible that in Holm's representation of Eurypterus, Fig. 104, the
+genital duct on each side is indicated.
+
+
+THE THYROID GLAND OF AMMOCOETES.
+
+If we compare this mesosomatic region of Eurypterus with that of
+Ammocoetes, the resemblance is most striking, and gives a meaning to the
+facial nerve which is in absolute accordance with the interpretation
+already given of the glossopharyngeal and vagus nerves. In both cases the
+foremost respiratory or mesosomatic segment is double, the posterior
+lateral part alone bearing the branchiæ, while the median and anterior part
+bore in the one animal the uterus and genital ducts, in the other the
+thyroid gland and ciliated grooves. We are driven, therefore, to the
+conclusion that this extraordinary and unique organ, the so-called thyroid
+gland of Ammocoetes, which exists only in the larval condition and is got
+rid of as soon as the adult sexual organs are formed, shows the very form
+and position of the uterus of this invertebrate ancestor of Ammocoetes.
+What, then, is the nature of the thyroid gland in Ammocoetes?
+
+{193}Throughout the vertebrate kingdom it is possible to compare the
+thyroid gland of one group of animals with that of another without coming
+across any very marked difference of structure right down to and including
+Petromyzon. When, however, we examine Ammocoetes, we find that the thyroid
+has suddenly become an organ of much more complicated structure, covering a
+much larger space, and bearing no resemblance to the thyroid glands of the
+higher forms. At transformation the thyroid of Ammocoetes is largely
+destroyed, and what remains of the gland in Petromyzon becomes limited to a
+few follicles resembling those of other fishes. The structure and position
+of this gland in Ammocoetes is so well known that it is unnecessary to
+describe it in detail. For the purpose, however, of making my points clear,
+I give in Fig. 80 the position and appearance of the thyroid gland (_Th._)
+when the skin and underlying laminated layer has been removed by the action
+of hypochlorite of soda. On the one side the ventral somatic muscles have
+been removed to show the branchial cartilaginous basket-work.
+
+[Illustration: FIG. 80.--VENTRAL VIEW OF HEAD REGION OF AMMOCOETES.
+
+_Th._, thyroid gland; _M._, lower lip, with its muscles.]
+
+The series of transverse sections in Fig. 81 represents the nature of the
+organ at different levels in front of and behind the opening into the
+respiratory chamber; and in Fig. 82 I have sketched the appearance of the
+whole gland, viewed so as to show its opening into the respiratory chamber
+and its posterior curled-up termination.
+
+{194}[Illustration: FIG. 81.--SAMPLES FROM A COMPLETE SERIES OF TRANSVERSE
+SECTIONS THROUGH THE THYROID GLAND OF AMMOCOETES.
+
+Sections 1 and 2 are anterior to the thyroid opening, _Th. o._; sections 3,
+4, and 5 are through the thyroid opening; and section 6 is posterior to the
+thyroid opening before the commencement of the curled portion.]
+
+{195}The series of transverse sections (1-6, Fig. 81) show that we are
+dealing here with a central glandular chamber, C (Fig. 81 (6) and Fig. 82),
+which opens by the thyroid duct (_Th. o._) into the pharyngeal chamber, and
+is curled upon itself in its more posterior part. This central chamber
+divides, anteriorly to the thyroid orifice, into two portions, A, A[prime]
+(Fig. 82), giving origin to two tubes, B, B[prime], which lie close
+alongside of, and extend further back than, the posterior limit of the
+curled portion of the central chamber, C. The structure of the central
+chamber, C, and, therefore, of the separate coils, is given in both
+Schneider's and Dohrn's pictures, and is represented in Fig. 81 (6), which
+shows the peculiar arrangement and character of the glandular cells typical
+of this organ, and also the nature of the central cavity, with the
+arrangement of the ciliated epithelium. The structure of each of the
+lateral tubes, B, is different from that of the central chamber, in that
+only half the central chamber is present in them, as is seen by the
+comparison of the tube B with the tube C in Fig. 81 (5 and 6), so that we
+may look upon the central chamber, C, as formed of two tubes, similar in
+structure to the tubes B, which have come together to form a single chamber
+by the partial absorption of their walls, the remains of the wall being
+still visible as the septum, which partially divides the chamber, C, into
+halves.
+
+In the walls of each of these tubes is situated a continuous glandular
+line, the structure of the glandular elements being specially characterized
+by the length of the cells, by the large spherical nucleus situated at the
+very base of each cell, and by the way in which the cells form a
+wedge-shaped group, the thin points of all the wedge-shaped cells coming
+together so as to form a continuous line along the chamber wall. This free
+termination of the cells of the gland in the lumen of the chamber
+constitutes the whole method for the secretion of the gland; there is no
+duct, no alveolus, nothing but this free termination of the cells.
+
+Moreover, sections through the portion A, A[prime] (Fig. 82) show that
+here, as in the central chamber, C, four of these glandular lines open into
+a common chamber, but they are not the same four as in the case of the
+central chamber, for if we name these glandular lines on the left side _a
+b, a[prime] b[prime]_ (Fig. 81), and on the right side _c d, c[prime]
+d[prime]_, then the central chamber has opening into it the glands _a b, c
+d_, while the chambers of A and A[prime] have opening into them
+respectively _a b, a[prime] b[prime]_, and _c d, c[prime] d[prime]_.
+Further, the same series of sections shows that the glands _a_ and _b_ are
+continuous with the glands _a[prime]_ and _b[prime]_ respectively across
+the apex of A, and similarly on the other side, so that the two glandular
+rows _a b_ are continuous with the two glandular rows _a[prime] b[prime]_,
+and we see that the {196}cavity of the portion A or A[prime] is formed by
+the bending over of the tube or horn, B or B[prime], with the partial
+absorption of the septum so formed between the tube and its bent-over part.
+If, then, we uncoil the curled-up part of C, and separate the portion, B,
+on each side from the chamber, C, we see that the so-called thyroid of
+Ammocoetes may be represented as in Fig. 83, _i.e._ it consists of a long,
+common chamber, C, which, for reasons apparent afterwards, I will call the
+_palæo-hysteron_, which opens, by means of a large orifice, into the
+respiratory or pharyngeal chamber. The anterior end of this chamber
+terminates in two tubes, or horns, B, B[prime], the structure of which
+shows that the median chamber, C, is the result of the amalgamation of two
+such tubes, and consequently in this chamber, or _palæo-hysteron_, the
+glandular lines are symmetrically situated on each side.
+
+[Illustration: FIG. 82.--DIAGRAMMATIC REPRESENTATION OF THE SO-CALLED
+THYROID GLAND OF AMMOCOETES.
+
+_C_, central chamber; _A, A[prime]_, anterior extremity; _B, B[prime]_,
+posterior extremity; _Th. o._, thyroid opening into respiratory chamber;
+_Ps. br., Ps. br[prime]._, ciliated grooves, Dohrn's pseudo-branchial
+grooves.]
+
+[Illustration: FIG. 83.--THYROID GLAND AS IT WOULD APPEAR IF THE CENTRAL
+CHAMBER WERE UNCURLED AND THE TWO HORNS, _B_, _B[prime]_, SEPARATED FROM
+THE CENTRAL CHAMBER.]
+
+Any explanation, then, of the thyroid gland of Ammocoetes, must {197}take
+into account the clear evidence that it is composed of two tubes, which
+have in part fused together to form an elongated central chamber, in part
+remain as horns to that chamber, and that in its walls there exist lines of
+gland-cells of a striking and characteristic nature.
+
+Further, this central chamber, with its horns, is not a closed chamber, but
+is in communication with the pharyngeal or respiratory chamber by three
+ways. In the first place, the central chamber, as is well known, opens into
+the respiratory chamber by a funnel-shaped opening--the so-called thyroid
+duct (_Th. o._). In the second place, there exist two ciliated grooves
+(_Ps. br._, _Ps. br[prime]._), the pseudo-branchial grooves of Dohrn, which
+have direct communication with the thyroid chamber. The manner in which
+these grooves communicate with the thyroid chamber has never, to my
+knowledge, been described previously to my description in the _Journal of
+Physiology and Anatomy_; it is very instructive, for, as I have there
+shown, each groove enters into the corresponding lateral horn, so that, in
+reality, there are three openings into the thyroid chamber or
+palæo-hysteron--a median opening into the central chamber, and a separate
+opening into each lateral horn.
+
+The system of ciliated grooves on the inner ventral surface of the
+respiratory chamber of Ammocoetes was originally described by Schneider as
+consisting of a single median groove, which extends from the opening of the
+thyroid to the posterior extremity of the branchial chamber, and a pair of
+grooves, or semi-canals, which, starting from the region of the thyroid
+orifice, run headwards and diverge from each other, becoming more and more
+lateral, and more and more dorsal, till they come together in the
+mid-dorsal pharyngeal line below the auditory capsules. The latter are the
+pseudo-branchial grooves of Dohrn, of which I have already spoken.
+Schneider looked upon the whole of this system as a single system, for he
+speaks of "a ciliated groove, which extends from the orifice of the stomach
+(_i.e._ anterior intestine) to the orifice of the thyroid, then divides
+into two, and runs forward right and left of the median ridge, etc." Dohrn
+rightly separates the median ciliated groove posterior to the thyroid
+orifice (seen in Fig. 81 (6)) from the paired pseudo-branchial grooves; the
+former is a shallow depression which opens into the rim of the thyroid
+orifice, while the latter has a much more intimate connection with the
+thyroid gland itself.
+
+{198}A series of sections, such as is given in Fig. 81, shows the relation
+of this pair of ciliated grooves to the thyroid better than any elaborate
+description. In the first place, it is clear that they remain separate up
+to their termination--they do not join in the middle line to open into the
+thyroid duct; in the second place, they are separate from the thyroid
+orifice--they do not terminate at the rim of the orifice, as is the case
+with the median groove just mentioned, but continue on each side on the
+wall of the thyroid duct (Fig. 81 (2)), gradually moving further and
+further away from the actual opening of the duct into the pharyngeal
+chamber. During the whole of their course on the wall of the funnel-shaped
+duct they retain the character of grooves, and are therefore open to the
+lumen of the duct. The direction of the groove (_Ps. br._) shifts as it
+passes deeper and deeper towards the thyroid, until at last, as seen in
+Fig. 81 (3 and 4), it is continuous with the narrow diverticulum of the
+turned-down single part of the thyroid (B), or turned-down horn, as I have
+called it. In other words, the median chamber opens into the pharyngeal or
+respiratory chamber by a single large, funnel-shaped opening, and, in
+addition, the two ciliated grooves terminate in the lateral horns on each
+side, and only indirectly into the central chamber, owing to their being
+semi-canals, and not complete canals. If they were originally canals, and
+not grooves, then the thyroid of Ammocoetes would be derived from an organ
+composed of a large, common glandular chamber, which opened into the
+respiratory chamber by means of an extensive median orifice, and possessed
+anteriorly two horns, from each of which a canal or duct passed headwards
+to terminate somewhere in the region of the auditory capsule.
+
+Dohrn has pointed out that a somewhat similar structure and topographical
+arrangement is found in Amphioxus and the Tunicata, the gland-cells being
+here arranged along the hypobranchial groove to form the endostyle and not
+shut off to form a closed organ, as in the thyroid of Ammocoetes. Dohrn
+concludes, in my opinion rightly, that the endostyle in the Tunicata and in
+Amphioxus represents the remnants of the more elaborate organ in
+Ammocoetes, and that, therefore, in order to explain the meaning of these
+organs in the former animals, we must first find out their meaning in
+Ammocoetes. Dohrn, however, goes further than this; for just as he
+considers Amphioxus and the Tunicata to have arisen by degeneration from an
+Ammocoetes-like form, so he considers Ammocoetes to have arisen {199}from a
+degenerated Selachian; therefore, in order to be logical, he ought to show
+that the thyroid of Ammocoetes is an intermediate downward step between the
+thyroid of Selachians and that of Amphioxus and the Tunicates. Here, it
+seems to me, his argument utterly breaks down; it is so clear that the
+thyroid of Petromyzon links on to that of the higher fishes, and that the
+Ammocoetes thyroid is so immeasurably more complicated and elaborate a
+structure than is that of Petromyzon, as to make it impossible to believe
+that the Ammocoetes thyroid has been derived by a process of degeneration
+from that of the Selachian. On the contrary, the manner in which it is
+eaten up at transformation and absolutely disappears in its original form
+is, like the other instances mentioned, strong evidence that we are dealing
+here with an ancestral organ, which is confined to the larval form, and
+disappears when the change to the higher adult condition takes place.
+Dohrn's evidence, then, points strongly to the conclusion that the
+starting-point of the thyroid gland in the vertebrate series is to be found
+in the thyroid of Ammocoetes, which has given rise, on the one hand, to the
+endostyle of Amphioxus and the Tunicata, and on the other, to the thyroid
+gland of Petromyzon and the rest of the Vertebrata.
+
+The evidence which I have just given of the intimate connection of the two
+pseudo-branchial grooves with the thyroid chamber shows, to my mind,
+clearly that Dohrn is right in supposing that morphologically these two
+grooves and the thyroid must be considered together. His explanation is
+that the whole system represents a modified pair of branchial segments
+distinct from those belonging to the VIIth and IXth nerves. The cavity of
+the thyroid and the pseudo-branchial grooves are, therefore, according to
+him, the remains of the gill-pouches of this fused pair of branchial
+segments, which no longer open to the surface, and the glandular tissue of
+the thyroid is derived from the modified gill-epithelium. This view of
+Dohrn's, which he has urged most strongly in various papers, is, I think,
+right in so far as the separateness of the thyroid segment is concerned,
+but is not right, and is not proven, in so far as concerns the view that
+the thyroid gland is a modified pair of gills.
+
+We may distinctly, on my view, look upon the thyroid segment, with its
+ciliated grooves and its covering plate of muco-cartilage, as a distinct
+paired segment, homologous with the branchial segments, without any
+necessity of deriving the thyroid gland from a pair of gills.
+
+{200}The evidence that such a median segment has been interpolated
+ventrally between the foremost pairs of branchial segments is remarkably
+clear, for the limits ventrally of the branchial segments are marked out on
+each side by the ventral border of the cartilaginous basket-work; and it is
+well known, as seen in Fig. 80, that whereas this cartilaginous framework
+on the two sides meets together in the middle ventral line in the posterior
+branchial region, it diverges in the anterior region so as to form a
+tongue-shaped space between the branchial segments on the two sides. This
+space is covered over with a plate of muco-cartilage which bears on its
+inner surface the thyroid gland.
+
+[Illustration: FIG. 84.--DIAGRAM OF (A) VENTRAL SURFACE AND (B) LATERAL
+SURFACE OF AMMOCOETES, SHOWING THE ARRANGEMENT OF THE EPITHELIAL PITS ON
+THE BRANCHIAL REGION, AND THEIR INNERVATION BY _VII._, THE FACIAL, _IX._,
+THE GLOSSOPHARYNGEAL, AND _X^1_-_X^6_, THE VAGUS NERVES.]
+
+In addition to this evidence that we are dealing here with a ventral
+tongue-like segment belonging to the facial nerve which is interpolated
+between the foremost branchial segments, we find the most striking fact
+that at transformation the whole of this muco-cartilaginous plate
+disappears, the remarkable thyroid gland of the {201}Ammocoetes is eaten
+up, and nothing is left except a small, totally different glandular mass;
+and now the cartilaginous basket-work meets together in the middle line in
+this region as well as in the more posterior region. In other words, the
+striking characteristic of transformation here is the destruction of this
+interpolated segment, and the resulting necessary drawing together
+ventrally of the branchial segments on each side.
+
+[Illustration: FIG. 85.--FACIAL SEGMENT OF AMMOCOETES MARKED OUT BY
+SHADING.
+
+_VII._ 1, thyroid part of segment; _VII._ 2, hyoid or branchial part; 3-9,
+succeeding branchial segments belonging to IXth and Xth nerves; _V_, the
+velar folds; _Ps. br._, Dohrn's pseudo-branchial groove; _Th. o._, thyroid
+opening; _C_, curled portion of thyroid.]
+
+Moreover, another most instructive piece of evidence pointing in the same
+direction is afforded by the behaviour of the ventral epithelial {202}pits,
+as determined by Miss Alcock. Although there is no indication on the
+ventral surface of the skin of any difference between the anterior and
+posterior portions of the respiratory region, yet when the ventral rows of
+the epithelial pits supplied by each branchial nerve are mapped out, we see
+how the most anterior ones diverge more and more from the mid-ventral line,
+following out exactly the limits of the underlying muco-cartilaginous
+thyroid plate (Fig. 84).
+
+The whole evidence strongly leads to the conclusion that the thyroid
+portion of the facial segment was inserted as a median tongue between the
+foremost branchial segments on each side, and that, therefore, the whole
+facial segment, consisting as it does of a thyroid part and a hyoid or
+branchial part, may be represented as in Fig. 85, which is obtained by
+splitting an Ammocoetes longitudinally along the mid-dorsal line, so as to
+open out the pharyngeal chamber and expose the whole internal surface. The
+facial segment is marked out by shading lines, the glosso-pharyngeal and
+vagus segments and the last of the trigeminal segments being indicated
+faintly. The position of the thyroid gland is indicated by oblique lines, C
+being the curled portion.
+
+
+THE UTERUS OF THE SCORPION GROUP.
+
+Seeing how striking is the arrangement and the structure of the glandular
+tissue of this thyroid, how large the organ is and how absolutely it is
+confined to Ammocoetes, disappearing entirely as such at transformation, we
+may feel perfectly certain that a corresponding, probably very similar,
+organ existed in the invertebrate ancestor of the vertebrate; for the
+transformation process consists essentially of the discarding of
+invertebrate characteristics and the putting on of more vertebrate
+characters; also, so elaborate an organ cannot possibly have been evolved
+as a larval adaptation during the life of Ammocoetes. We may therefore
+assert with considerable confidence that the thyroid gland was the
+_palæo-hysteron_, and was derived from the uterus of the ancient
+palæostracan forms. If, then, it be found that a glandular organ of this
+very peculiar structure and arrangement is characteristic of the uterus of
+any living member of the scorpion group, then the confidence of this
+assertion is greatly increased.
+
+In Limulus, as already stated, the genital ducts open separately {203}on
+each side of the operculum, and do not combine to form a uterus; I have
+examined them and was unable to find any glandular structure at all
+resembling that of the thyroid gland of Ammocoetes. I then turned my
+attention to the organs of the scorpion, in which the two ducts have fused
+to form a single uterus.
+
+[Illustration: FIG. 86.--SECTION THROUGH THE TERMINAL CHAMBER OR UTERUS OF
+THE MALE SCORPION.
+
+_C_, cavity of chamber. A portion of the epithelial lining of the channels
+of emission is drawn above the section of the uterus.]
+
+{204}[Illustration: FIG. 87.--LONGITUDINAL SECTION THROUGH THREE OF THE
+CONES OF THE UTERINE GLANDS OF THE SCORPION.]
+
+[Illustration: FIG. 88.--SAGITTAL SECTION THROUGH THE UTERINE GLAND OF
+SCORPION, SHOWING THE INTERNAL CHITINOUS SURFACE (_b_) AND THE GLANDULAR
+CONES (_a_) CUT THROUGH AT VARIOUS DISTANCES FROM THE INTERNAL SURFACE.]
+
+I there found that both in the male and in the female the genital ducts on
+each side terminate in a common chamber or uterus, which underlies the
+whole length of the operculum, and opens to the exterior in the middle
+line, as shown in Fig. 76. In transverse section, this uterus has the
+appearance shown in Fig. 86, _i.e._ it is a large tube, evidently
+expansible, lined with a chitinous layer and epithelial cells belonging to
+the chitinogenous layer, except in two symmetrical places, where the
+uniformity of the uterine wall is interrupted by two large, remarkable
+glandular structures. The structure of these glands is better shown by
+means of sagittal sections. They are composed of very long, wedge-shaped
+cells, each of which possesses a large, round nucleus at the basal end of
+the cell (Fig. 87). These cells are arranged in bundles of about eight to
+ten, which are separated from each other by connective tissue, the apex of
+each conical bundle being directed into the cavity of the uterus; where
+this brush-like termination of the cells reaches the surface, the chitinous
+layer is absent, so that this layer is, on surface view, seen (Fig. 88
+(_b_)) to be pitted with round holes over that part of the internal surface
+of the uterus where these glands are situated. Each of these holes
+represents the termination of one of these cone-shaped wedges of cells. If
+the section is cut across at right angles to the axis of these cones, then
+its appearance is represented in Fig. 88 (_a_), and shows well the
+arrangement of the blocks of cells, separated from each other by connective
+tissue. When the section passes through the basal part of the cones, and
+only in that case, then the nuclei of the cells appear, often in
+considerable numbers in one section, as {205}is seen in Fig. 89. In Fig. 88
+the section shows at _b_ the holes in the chitin in which the cones
+terminate, and then a series of layers of sections through the cones
+further and further away from their apices.
+
+These conical groups of long cells, represented in Fig. 87, form on each
+side of the uterus a gland, which is continuous along its whole length, and
+thus forms a line of secreting surface on each side, just as in the
+corresponding arrangement of the glandular structures in the thyroid of
+Ammocoetes. This uterus and glandular arrangement is found in both sexes;
+the gland is, however, more developed in the male than in the female
+scorpion.
+
+[Illustration: FIG. 89.--TRANSVERSE SECTION THROUGH THE BASAL PART OF THE
+UTERINE GLANDS OF THE SCORPION.]
+
+The resemblance between the structure of the thyroid of Ammocoetes and the
+uterus of the scorpion is most striking, except in two respects, viz. the
+nature of the lining of the non-glandular part of the cavity--in the one
+case ciliated, in the other chitinous--and the place of exit of the cavity,
+the thyroid of Ammocoetes opening into the respiratory chamber, while the
+uterus of Scorpio opens direct to the exterior.
+
+[Illustration: FIG. 90.--SECTION OF CENTRAL CHAMBER OF THYROID OF
+AMMOCOETES AND SECTION OF UTERUS OF SCORPION.]
+
+With respect to the first difference, the same difficulty is met {206}with
+in the comparison of the ciliated lining of the tube in the central nervous
+system of vertebrates with the chitinous lining of the intestine in the
+arthropod. Such a difference does not seem to me either unlikely or
+unreasonable, seeing that cilia are found instead of chitin in the
+intestine of the primitive arthropod Peripatus. Also the worm-like
+ancestors of the arthropods almost certainly possessed a ciliated
+intestine. Finally, the researches of Hardy and McDougall on the intestine
+of Daphnia point directly to the presence of a ciliated rather than a
+chitinous epithelial lining of the intestine in this animal--all evidence
+pointing to the probability that in the ancient arthropod forms, derived as
+they were from the annelids, the intestine was originally ciliated and not
+chitinous. It is from such forms that I suppose vertebrates to have sprung,
+and not from forms like the living king-crabs, scorpions, Apus, Branchipus,
+etc. I only use them as illustrations, because they are the only living
+representatives of the great archaic group, from which the Crustacea,
+Arachnida, and Vertebrata all took origin.
+
+The second difference is more important, and is at first sight fatal to any
+comparison between the two organs. How is it possible to compare the uterus
+of the scorpion, which opens on the surface by an _external_ genital
+opening, with the thyroid of Ammocoetes, which opens by an _internal_
+opening into the respiratory chamber? However close may be the histological
+resemblance of structure in the two cases, surely such a difference is too
+great to be accounted for.
+
+It is, however, to be remembered that the operculum of Scorpio covers only
+the terminal genital apparatus, and does not, therefore, resemble the
+operculum of the presumed ancestor of Ammocoetes, which, as already argued,
+must have resembled the operculum of Thelyphonus with its conjoint
+branchial and genital apparatus, rather than that of Scorpio. Before,
+therefore, making too sure of the insuperable character of this difficulty,
+we must examine the uterus of the Pedipalpi, and see the nature of its
+opening.
+
+The nature of the terminal genital organs in Thelyphonus has been described
+to some extent by Blanchard, and more recently by Tarnani. The ducts of the
+generative organs terminate, according to the latter observer, in the large
+uterus, which is found both in the male and female; he describes the walls
+of the uterus in the female as formed of elongated glandular epithelium,
+with a strongly-developed porous, chitinized intima. In the male, he says
+that the {207}epithelium of the uterus masculinus and its processes is
+extraordinarily elongated, the chitin covering being thick. In these
+animals, then, the common chamber or uterus into which the genital ducts
+empty, which, like the corresponding chamber in the scorpion, occupies the
+middle region of the operculum, is a large and conspicuous organ. Further,
+and this is a most striking fact, the _uterus masculinus_ does not open
+direct to the exterior, but into the genital cavity, "which lies above the
+uterus, so that the latter is situated between the lower wall of the
+genital cavity and the outer integument." The opening, therefore, of the
+uterus is not external but _internal_, into the large internal space known
+as the genital cavity. The arrangement is shown in Fig. 91, taken from
+Tarnani's paper, which represents a diagrammatic sagittal section through
+the exit of the male genital duct. Yet another most striking fact is
+described by Tarnani. This genital cavity is continuous with the pulmonary
+or gill cavities on each side, so that instead of a single opening for the
+genital products and one on each side for each gill-pouch, as would be the
+case if the arrangement was of the same kind as in the scorpion, there is a
+single large chamber, the genital chamber, common to both respiratory and
+genital organs.
+
+[Illustration: FIG. 91.--SAGITTAL MEDIAN DIAGRAMMATIC SECTION THROUGH THE
+OPERCULUM OF THE MALE THELYPHONUS. (From TARNANI.)
+
+The thick line is the operculum, composed of two segments, _I._ and _II._
+_Ut. Masc._, uterus masculinus; _Gen. Ch._, genital chamber; _Int. Op._,
+internal opening; _Ext. Op._, external opening common to the genital and
+respiratory organs.]
+
+This genital chamber, according to Tarnani, opens to the exterior by a
+single median opening between the operculum and the succeeding segment;
+similarly, a communication from side to side exists between the second pair
+of gill-pouches. I have been able to examine _Hypoctonus formosus_ and
+_Thelyphonus caudatus_, and in both cases, in both male and female, the
+opening to the exterior of the common chamber for respiration and for the
+genital products was {208}not a single opening, as described by Tarnani in
+_Thelyphonus asperatus_, but on each side of the middle line, a round
+orifice closed by a lid, like the nest of the trapdoor spider, led into the
+common genital chamber (_Gen. Ch._) into which both uterus and gills
+opened. In Fig. 77 I have endeavoured to represent the arrangement of the
+genital and respiratory organs in the male Thelyphonus according to
+Tarnani's and my own observations.
+
+If we may take Thelyphonus as a sample of the arrangement in those
+scorpions in which the operculum was fused with the first branchial
+appendage, among which must be included the old sea-scorpions, then it is
+most significant that their uterus should open internally into a cavity
+which was continuous with the respiratory cavity. Thus not only the
+structure of the gland, but also the arrangement of the internal opening
+into the respiratory, or, as it became later, the pharyngeal cavity, is in
+accordance with the suggestion that the thyroid of Ammocoetes represents
+the uterus of the extinct Eurypterus-like ancestor.
+
+Into this uterus the products of the generative organs were poured by means
+of the _vasa deferentia_, so that there was not a single median opening or
+duct in connection with it, but also two side openings, the terminations of
+the _vasa deferentia_. These are described by Tarnani in Thelyphonus as
+opening into the two horns of the uterus, which thus shows its bilateral
+character, although the body of the organ is median and single; these ducts
+then pass within the body of the animal, dorsal to the uterus, towards the
+testes or ovaries as the case may be, organs which are situated in these
+animals, as in other scorpions, in the abdomen, so that the direction of
+the ducts from the generative glands to the uterus is headwards. If,
+however, we examine the condition of affairs in Limulus, we find that the
+main mass of the generative material is cephalic, forming with the liver
+that dense glandular mass which is packed round the supra-oesophageal and
+prosomatic ganglia, and round the stomach and muscles of the head-region.
+From this cephalic region the duct passes out on each side at the junction
+of the prosomatic and mesosomatic carapace to open separately on the
+posterior surface of the operculum, near the middle line, as is indicated
+in Fig. 75.
+
+We have, therefore, two distinct possible positions for the genital ducts
+among the group of extinct scorpion-like animals, the one from the cephalic
+region to the operculum, and the other from the abdominal region to the
+operculum.
+
+
+{209}THE GENERATIVE GLANDS OF LIMULUS AND ITS ALLIES.
+
+The whole argument, so far, has in every case ended with the conclusion
+that the original scorpion-like form with which I have been comparing
+Ammocoetes resembled in many respects Limulus rather than the present-day
+scorpions, and therefore in the case also of the generative organs, with
+which the thyroid gland or palæo-hysteron was in connection, it is more
+probable that they were cephalic in position rather than abdominal. If this
+were so, then the duct on each side, starting from the median ventral
+uterus, would take a lateral and dorsal course to reach the huge mass of
+generative gland lying within the prosomatic carapace, just as I have
+represented in the figure of Eurypterus (Fig. 79), a course which would
+take much the same direction as the ciliated groove in Ammocoetes.
+
+We ought, therefore, on this supposition, to expect to find the remains of
+the invertebrate generative tissue, the ducts of which terminated in the
+thyroid, in the head-region, and not in the abdomen.
+
+Upon removal of the prosomatic carapace of Limulus, a large brownish
+glandular-looking mass is seen, in which, if it happens to be a female,
+masses of ova are very conspicuous. This mass is composed of two separate
+glands, the generative glands and the hepatico-pancreatic glands--the
+so-called liver--and surrounds closely the central nervous system and the
+alimentary canal. From the generative glands proceed the genital ducts to
+terminate on the posterior surface of the operculum. From the liver ducts
+pass to the pyloric end of the cephalic stomach, and carry the fluid by
+means of which the food is digested, for, in all these animals, the active
+digesting juices are formed in the so-called liver, and not in the cells of
+the stomach or intestine.
+
+It is a very striking fact that the brain of Ammocoetes is much too small
+for the brain-case, and that the space between brain and brain-case is
+filled up with a very peculiar glandular-looking tissue, which is found in
+Ammocoetes and not elsewhere. Further, it is also striking that in the
+brain of Ammocoetes there should still exist the remains of a tube
+extending from the IVth ventricle to the surface at the _conus
+post-commissuralis_, which can actually be traced right into this tissue on
+the outside of the brain (see Fig. 13, _a-e_, Pl. XXVI., in my paper in the
+_Quarterly Journal of Microscopical Science_). {210}This, in my opinion, is
+the last remnant of one of the old liver-ducts which extended from the
+original stomach and intestine into the cephalic liver-mass. This
+glandular-looking material is shown surrounding the pineal eye and its
+nerve, in Fig. 31, also in Fig. 22, and separately in Fig. 92. It is
+composed of large cells, with a badly staining nucleus, closely packed
+together with lines of pigment here and there between the cells; this
+pigment is especially congregated at the spot where the so-called
+liver-duct loses itself in this tissue. The protoplasm in these large cells
+does not stain well, and with osmic acid gives no sign of fat, so that
+Ahlborn's description of this tissue as a peculiar arachnoideal fat-tissue
+is not true; peculiar it certainly is, but fatty it is not.
+
+[Illustration: FIG. 92.--DRAWING OF THE TISSUE WHICH SURROUNDS THE BRAIN OF
+AMMOCOETES.]
+
+This tissue has been largely described as a peculiar kind of connective
+tissue, which is there as packing material, for the purpose of steadying a
+brain too small for its case. On the face of it such an explanation is
+unscientific; certainly for all those who really believe in evolution, it
+is out of the question to suppose that a brain-case has been laid down in
+the first instance too large for the brain, in order to provide room for a
+subsequent increase of brain; just as it is out of the question to suppose
+that the nervous system was laid down originally as an epithelial tube in
+order to provide for the further development of the nervous system by the
+conversion of more and more of that tube into nervous matter. Yet this
+latter proposition has been seriously put forward by professed believers in
+evolution and in natural selection.
+
+This tissue bears no resemblance whatever to any form of connective tissue,
+either fatty or otherwise. By every test this tissue tells as plainly as
+possible that it is a vestige of some former organ, presumably glandular,
+which existed in that position; that it is not there as packing material
+because the brain happened to be too small for its case, but that, on the
+contrary, the brain is too small for its case, because the case, when it
+was formed, included this organ as well as the brain; in other words, this
+tissue {211}is there because it is the remnant of the great glandular mass
+which so closely surrounds the brain and alimentary canal in animals such
+as Limulus. In my paper in the _Quarterly Journal of Microscopical
+Science_, in which I was comparing the tube of the vertebrate nervous
+system with the alimentary canal of the invertebrate, I spoke of this
+tissue as being the remnant of the invertebrate liver. At the same time the
+whole point of my argument was that the glandular material surrounding the
+brain of Limulus was made up of two glands--liver and generative gland--so
+that this tissue might be the remnant of either one or the other, or both.
+All I desired, at that time, was to point out the glandular appearance of
+this so-called packing tissue, which surrounded the brain-region of
+Ammocoetes, in connection with the fact that the brain and alimentary canal
+of Limulus were closely surrounded with a glandular mass composed partly of
+liver, partly of the generative gland. At present, I think these large
+cells found round the brain in Ammocoetes are much more likely to be the
+remnant of the generative gland than of the liver; the size of the cells
+and their arrangement recalls Owen's picture of the generative gland in
+Limulus, and seeing how important all generative glands are in their
+capacity of internal secreting glands, apart entirely from the extrusion of
+the ripe generative products, and how unimportant is an hepato-pancreas
+when the alimentary canal is closed, it is much more likely that of the two
+glands the former would persist longer than the latter. It may be that all
+that is left of the old hepato-pancreas consists of the pigment so markedly
+found in between these cells, especially at the place where the old
+liver-duct reaches the surface of the brain; just as the only remnant of
+the two pineal eyes in the higher vertebrates is the remains of the
+pigment, known as brain-sand, which still exists in the pineal gland of
+even the highest vertebrate. This, however, is a mere speculation of no
+importance. What is important is the recognition of this tissue round the
+brain as the remnant of the glandular mass round the brain of animals such
+as Limulus. Still further confirmation of the truth of this comparison will
+be given when the origin of the auditory organ comes up for discussion.
+
+I conclude, therefore, from the evidence of Ammocoetes, that the generative
+glands in the ancestral form were situated largely in the cephalic region,
+and suggest that the course and direction of the ciliated pseudo-branchial
+grooves on each side indicate the direction of the {212}original opercular
+ducts by which the generative products were conveyed to the uterine
+chamber, i.e. to the chamber of the thyroid gland, and thence to the common
+genital and respiratory cavity, and so to the exterior.
+
+It is easy to picture the sequence of events. First, the generative glands,
+chiefly confined to the cephalic region, communicating with the exterior by
+separate ducts on the inner surface of the operculum as in Limulus. Then,
+in connection with the viviparous habit, these two oviducts fused together
+to form a single chamber, covered by the operculum, which opened out to the
+exterior by a single opening as in Scorpio: or, in forms such as
+Eurypterus, in which the operculum had amalgamated with the first branchial
+appendage and possessed a long, tongue-like ventral projection, the
+amalgamated ducts formed a long uterine chamber which opened internally
+into the genital chamber--a chamber which, as in Thelyphonus, was common
+with that of the two gill-chambers, while at the same time the genital
+ducts from the cephalic generative material opened into two uterine horns
+which arose from the anterior part of the uterus, as in Thelyphonus.
+
+Such an arrangement would lead directly to the condition found in
+Ammocoetes, if the generative material around the brain lost its function,
+owing to a new exit for generative products being formed in the posterior
+part of the body. The connection of the genital duct with this cephalic
+gland being then closed and cut off by the brain-case, the position of the
+oviducts would still be shown by the ciliated grooves opening into the
+folded-down thyroid tube, _i.e._ the folded-down horns of the uterus; the
+uterus itself would remain as the main body of the thyroid and still open
+by a conspicuous orifice into the common respiratory chamber. Next, in the
+degeneration process, we may suppose that not only the oviducts opened out
+to form the ciliated groove, but that the uterine chamber itself also
+opened out, and thus formed the endostyle of Amphioxus and of the Tunicata.
+
+It might seem at first sight improbable that a closed tube should become an
+open groove, although the reverse phenomenon is common enough; the
+difficulty, however, is clearly not considered great, for it is precisely
+what Dohrn imagines to have taken place in the conversion of the thyroid of
+Ammocoetes into the endostyle of Amphioxus and the Tunicata; it is only
+carrying on the same idea a stage further to see in the open, ciliated
+groove of Ammocoetes the remains of the closed genital duct of Limulus and
+its allies.
+
+{213}Such is the conclusion to which the study of the thyroid gland in
+Ammocoetes seems to me to lead, and one cannot help wondering why such an
+unused and rudimentary organ should have remained after its original
+function had gone. Is it possible to find out its function in Ammocoetes?
+
+
+THE FUNCTION OF THE THYROID GLAND IN AMMOCOETES.
+
+The thyroid gland has been supposed to secrete mucus into the respiratory
+chamber for the purpose of entangling the particles of food, and so aiding
+in digestion. I see no sign of any such function; neither by the thionin
+method, nor by any other test, have Miss Alcock and myself ever been able
+to see any trace of mucous secretion in the thyroid, and, indeed, the
+thyroid duct is always remarkably free from any sign of any secretion
+whatever. Not only is there no evidence of any mucous secretion in the
+thyroid of the fully developed Ammocoetes, but also no necessity for such
+secretion from Dohrn's point of view, for so copious a supply of mucus is
+poured out by the glands of the branchiæ, along the whole pharyngeal tract,
+especially from the cells of the foremost or hyoid gills, as to mix up with
+the food as thoroughly as can possibly be needed. Further, too, the
+ciliated pharyngeal bands described by Schneider are amply sufficient to
+move this mixed mass along in the way required by Dohrn. Finally, the
+evidence given by Miss Alcock is absolutely against the view that the
+thyroid takes any part in the process of digestion, while, on the other
+hand, her evidence directly favours the view that these glandular
+_branchial_ mucus-secreting cells play a most important part in the
+digestive process.
+
+In Fig. 93, A is a representation of the respiratory tissue of a normal
+gill; B is the corresponding portion of the first or hyoid gill, in which,
+as is seen, the whole of the respiratory epithelium is converted into
+gland-tissue of the nature of mucous cells.
+
+To sum up, the evidence is clear and conclusive that the Ammocoetes
+possesses in its pharyngeal chamber mucus-secreting glands, which take an
+active part in the digestive process, which do not in the least resemble
+either in structure or arrangement the remarkable cells of the thyroid
+gland, and that the experimental evidence that the latter cells either
+secrete mucus or take any part in digestion is so far absolutely negative.
+It is, of course, possible, that they {214}may contain mucin in the younger
+developmental stages, and therefore possible that they might at that stage
+secrete it; they certainly, however, show no sign of doing so in their more
+adult condition, and cannot be compared in the very faintest degree to the
+glandular cells of the pharyngeal region. It is also perfectly possible for
+gland-cells belonging to a retrograde organ to become mucus-secreting, and
+so to give rise to the cells of Amphioxus and the Tunicata.
+
+[Illustration: Fig. 93.--A, PORTION OF A GILL OF AMMOCOETES WITH ORDINARY
+RESPIRATORY EPITHELIUM; B, CORRESPONDING PORTION OF THE FIRST OR HYOID
+GILL.]
+
+If, then, these cells were not retained for digestive purposes, what was
+their function? To answer this question we must first know the function of
+the corresponding gland-cells in the uterus of the scorpion, which
+undoubtedly secreted into the cavity of the uterus and took some part in
+connection with the generative act, and certainly not with digestion. What
+the function of these cells is or in what way they act I am unable at
+present to say. I can only suppose that the reason why the thyroid gland
+has persisted throughout the vertebrate kingdom, after the generative
+tissues had found a new outlet for their products in the body-cavity of the
+posterior region, is because it possessed some important function in
+addition to that connected with the exit of the products of the generative
+organs; a function which was essential to the well-being, or even to the
+life of the animal. We do not know its function in the scorpion, or the
+nature of its secretion in that animal. We know only that physiology at the
+present day has demonstrated clearly that the actual external secretion of
+a gland may be by no means its most important function; in addition, glands
+possess what is called an internal secretion, viz. a {215}secretion into
+the blood and lymph, and this latter secretion may be of the most vital
+importance. Now, the striking fact forces itself prominently forward, that
+the thyroid gland of the higher vertebrates is the most conspicuous example
+of the importance of such internal secretion. Here, although ductless, we
+have a gland which cannot be removed without fatal consequences. Here, in
+the importance of its internal secretion, we have a reason for the
+continued existence of this organ; an organ which remains much the same
+throughout the Vertebrata down to and including Petromyzon, but, as is seen
+at transformation, is all that remains of the more elaborate, more
+extensive organ of Ammocoetes. Surely we may argue that it is this second
+function which has led to the persistence of the thyroid, and that its
+original form, without its original function, is seen in Ammocoetes,
+because that is a larval form, and not a fully-developed animal. As soon as
+the generative organs of Petromyzon are developed at transformation, all
+trace of its connection with a genital duct vanishes, and presumably its
+internal secretory function alone remains.
+
+Yet, strange to say, a mysterious connection continues to exist between the
+thyroid gland and the generative organs, even up to the highest vertebrate.
+That the thyroid gland, situated as it is in the neck, should have any
+sympathy with sexual functions if it was originally a gland concerned with
+digestion is, to say the least of it, extremely unlikely, but, on the
+contrary, likely enough if it originated from a glandular organ in
+connection with the sexual organs of the palæostracan ancestor of the
+vertebrate.
+
+Freund has shown, and shown conclusively, that there is an intimate
+connection between the condition of the thyroid gland and the state of the
+sexual organs, not only in human beings, but also in numerous animals, such
+as dogs, sheep, goats, pigs, and deer. He points out that the swelling of
+the gland, which occurs in consequence of sexual excitement (a fact
+mentioned both in folk-lore tales and in poetical literature), and also the
+swelling at the time of puberty, may both lead to a true goitrous
+enlargement; that most of the permanent goitres commence during a menstrual
+period; that during pregnancy swelling of the thyroid is almost universal,
+and may become so extreme as to threaten suffocation, or even cause death;
+that the period of puberty and the climacteric period are the two maximal
+periods for the onset of goitre, and that exophthalmic goitre especially is
+associated with a special disease connected with the uterus.
+
+
+{216}SUMMARY.
+
+  Step by step in the preceding chapters the evidence is accumulating in
+  favour of the origin of vertebrates from a member of the palæostracan
+  group. In a continuously complete and harmonious manner the evidence has
+  throughout been most convincing when the vertebrate chosen for the
+  purpose of my arguments has been Ammocoetes.
+
+  So many fixed points have been firmly established as to enable us to
+  proceed further with very great confidence, in the full expectation of
+  being able ultimately to homologize the Vertebrata with the Palæostraca
+  even to minute details.
+
+  Perhaps the most striking and unexpected result of such a comparison is
+  the discovery that the thyroid gland is derived from the uterus of the
+  palæostracan ancestor. Yet so clear is the evidence that it is difficult
+  to see how the homology can be denied.
+
+  In the one animal (Palæostraca) the foremost pair of mesosomatic
+  appendages forms the operculum, which always bears the terminal
+  generative organs and is fused in the middle line. In many forms,
+  essentially in Eurypterus and the ancient sea-scorpions, the operculum
+  was composed of two segments fused together: an anterior one which
+  carried the uterus, and a posterior one which carried the first pair of
+  branchiæ.
+
+  In the other animal (Ammocoetes) the foremost segments of the mesosomatic
+  or respiratory region, immediately in front of the glossopharyngeal
+  segments, are supplied by the facial nerve, and are markedly different
+  from those supplied by the vagus and glossopharyngeal, for the facial
+  supplies two segments fused together; the anterior one, the thyroid
+  segment, carrying the thyroid gland, the posterior one, the hyoid
+  segment, carrying the first pair of branchiæ.
+
+  Just as in Eurypterus the fused segment, carrying the uterus on its
+  internal surface, forms a long median tongue which separates the most
+  anterior branchial segments on each side, so also the fused segment
+  carrying the thyroid forms in Ammocoetes a long median tongue, which
+  separates the most anterior branchial segments on each side.
+
+  Finally, and this is the most conclusive evidence of all, this thyroid
+  gland of Ammocoetes is totally unlike that of any of the higher
+  vertebrates, and, indeed, of the adult form Petromyzon itself, but it
+  forms an elaborate complicated organ, which is directly comparable with
+  the uterus and genital ducts of animals such as scorpions. Not only is
+  such a comparison valid with respect to its shape, but also with respect
+  to its structure, which is absolutely unique among vertebrates, and very
+  different to that of any other vertebrate gland, but resembles in a
+  striking manner a glandular structure found in the uterus, both of male
+  and female scorpions.
+
+  The generative glands in Limulus, together with the liver-glands, form a
+  large glandular mass, situated in the head-region closely surrounding the
+  central nervous system, so that the genital ducts pass from the
+  head-region tailwards to the operculum. In the scorpion they lie in the
+  abdominal region, so that their ducts pass headwards to the operculum.
+
+  Probably in the Palæostraca the generative mass was situated in the
+  cephalic region as in Limulus, and it is probable that the remnant of it
+  still exists in {217}Ammocoetes in the shape of the peculiar large cells
+  packed together, with pigment masses in between them, which form such a
+  characteristic feature of the glandular-looking material, which fills up
+  the space between the cranial walls and the central nervous system.
+
+  Finally, the relationship which has been known from time immemorial to
+  exist between the sexual organs and the thyroid in man and other animals,
+  and has hitherto been a mystery without any explanation, may possibly be
+  the last reminiscence of a time when the thyroid glands were the uterine
+  glands of the palæostracan ancestor.
+
+  The consideration of the facial nerve, and the segments it supplies,
+  still further points to the origin of the Vertebrata from the
+  Palæostraca.
+
+
+
+
+{218}CHAPTER VI
+
+_THE EVIDENCE OF THE OLFACTORY APPARATUS_
+
+  Fishes divided into Amphirhinæ and Monorhinæ.--Nasal tube of the
+  lamprey.--Its termination at the infundibulum.--The olfactory organs of
+  the scorpion group.--The camerostome.--Its formation as a tube.--Its
+  derivation from a pair of antennæ.--Its termination at the true
+  mouth.--Comparison with the olfactory tube of Ammocoetes.--Origin of the
+  nasal tube of Ammocoetes from the tube of the hypophysis.--Direct
+  comparison of the hypophysial tube with the olfactory tube of the
+  scorpion group--Summary.
+
+
+In the last chapter I finished the evidence given by the consideration of
+the mesosomatic or opisthotic nerves, and the segments they supplied. The
+evidence is strongly in accordance with that of previous chapters, and not
+only confirms the conclusion that vertebrates arose from some member of the
+Palæostraca, but helps still further to delimit the nature of that member.
+It is almost startling to see how the hypothesis put forward in the second
+chapter, suggested by the consideration of the nature of the vertebrate
+central nervous system and of the geological record, has received stronger
+and stronger confirmation from the consideration of the vertebrate optic
+apparatus, the vertebrate skeleton, the respiratory apparatus, and,
+finally, the thyroid gland. All fit naturally into a harmonious whole, and
+give a feeling of confidence that a similar harmony will be found upon
+consideration of the rest of the vertebrate organs.
+
+Following naturally upon the segments supplied by the opisthotic
+(mesosomatic) cranial nerves, we ought to consider now the segments
+supplied by the pro-otic (prosomatic) cranial nerves, i.e. the segments
+belonging to the trigeminal nerve-group in the vertebrate, and in the
+invertebrate the segments of the prosoma with their characteristic
+appendages. There are, however, in all vertebrates in this foremost cranial
+region, in addition to the optic nerves, two other well-marked nerves of
+special sense, the olfactory and the auditory. Of these, the former are in
+the same class as the optic nerves, for they arise {219}in the vertebrate
+from the supra-infundibular nerve-mass, and in the invertebrate from the
+supra-oesophageal ganglia. The latter arise in the vertebrate from the
+infra-infundibular nerve-mass, and, as the name implies, are situated in
+the region where the pro-otic nerves are contiguous to the opisthotic,
+_i.e._ at the junction of the prosomatic and mesosomatic nerve-regions.
+
+The chapter dealing with the evidence given by the olfactory nerves and the
+olfactory apparatus ought logically to have followed immediately upon the
+one dealing with the optic apparatus, seeing that both these special
+sense-nerves belong to the supra-infundibular segments in the vertebrate
+and to the supra-oesophageal in the invertebrate.
+
+I did not deal with them in that logical sequence because it was necessary
+for their understanding to introduce first the conception of modified
+appendages as important factors in any consideration of vertebrate
+segments; a conception which followed naturally after the evidence afforded
+by the skeleton in Chapter III., and by the branchial segments in Chapter
+IV. So, too, now, although the discussion of the prosomatic segmentation
+ought logically to follow immediately on that of the mesosomatic
+segmentation, I have determined to devote this chapter to the evidence of
+the olfactory organs, because the arguments as to the segments belonging to
+the trigeminal nerve-group are so much easier to understand if the position
+of the olfactory apparatus is first made clear.
+
+
+
+
+In all vertebrates the nose is double and opens into the pharynx, until we
+descend to the fishes, where the whole group Pisces has been divided into
+two subsidiary groups, Monorhinæ and Amphirhinæ, according as they possess
+a median unpaired olfactory opening, or a paired opening. The Monorhinæ
+include only the Cyclostomata--the lampreys and hag-fishes.
+
+In the lampreys the single olfactory tube ends blindly, while in the
+hag-fishes it opens into the pharynx. In the lamprey, both in Petromyzon
+and Ammocoetes, the opening of this nasal tube is a conspicuous object on
+the dorsal surface of the head in front of the transparent spot which
+indicates the position of the right median eye. It is especially
+significant, as showing the primitive nature of this median olfactory
+passage, that a perfectly similar opening in the {220}same position is
+always found in the dorsal head-shields of all the Cephalaspidæ and
+Tremataspidæ, as will be explained more fully in Chapter X.
+
+All the evidence points to the conclusion that the olfactory apparatus of
+the vertebrate originated as a single median tube, containing the special
+olfactory sense-epithelium, which, although median and single, was
+innervated by the olfactory nerve of each side. The external opening of
+this tube in the lamprey is dorsal. How does it terminate ventrally?
+
+The ventral termination of this tube is most instructive and suggestive. It
+terminates blindly at the very spot where the infundibular tube terminates
+blindly and the notochord ends. After transformation, when the Ammocoete
+becomes the Petromyzon, the tube still ends blindly, and does not open into
+the pharynx as in Myxine; it, however, no longer terminates at the
+infundibulum, but extends beyond it towards the pharynx.
+
+This position of the nasal tube suggests that it may originally have opened
+into the tube of the central nervous system by way of the infundibular
+tube. This suggestion is greatly enhanced in value by the fact that in the
+larval Amphioxus the tube of the central nervous system is open to the
+exterior, its opening being known as the anterior neuropore, and this
+anterior neuropore is situated at the base of a pit, known as the olfactory
+pit because it is supposed to represent the olfactory organ of other
+fishes.
+
+Following the same lines of argument as in previous chapters, this
+suggestion indicates that the special olfactory organs of the invertebrate
+ancestor of the vertebrates consisted of a single median olfactory tube or
+passage, which led directly into the oesophagus and was innervated, though
+single and median, by a pair of olfactory nerves which arose from the
+supra-oesophageal ganglia. Let us see what is the nature of the olfactory
+organs among arthropods, and whether such a suggestion possesses any
+probability.
+
+
+THE OLFACTORY ORGANS OF THE SCORPION GROUP.
+
+At first sight the answer appears to be distinctly adverse, for it is well
+known that in all the Insecta, Crustacea, and the large majority of
+Arthropoda, the first pair of antennæ, often called the antennules, are
+olfactory in function, and these are free-moving, bilaterally
+{221}situated, independent appendages. Still, even here there is the
+striking fact that the nerves of these olfactory organs always arise from
+the supra-oesophageal ganglia, although those to the second pair of antennæ
+arise from the infra-oesophageal ganglia, just as the olfactory nerves of
+the vertebrate arise from the supra-infundibular brain-mass. Not only is
+there this similarity of position, but also a similarity of structure in
+the olfactive lobes of the brain itself of so striking a character as to
+cause Bellonci to sum up his investigations as follows:--
+
+"The structure and connections of the olfactive lobes present the same
+fundamental plan in the higher arthropods and in the vertebrates. In the
+one, as in the other, the olfactory fibres form, with the connecting fibres
+of the olfactory lobes, a fine meshwork, which, consisting as it does of
+separate groups, may each one be called an olfactory glomerulus."
+
+He attributes this remarkable resemblance to a physiological necessity that
+similarity of function necessitates similarity of structure, for he
+considers it out of the question to suppose any near relationship between
+arthropods and vertebrates.
+
+Truly an interesting remark, with the one fallacy that relationship is out
+of the question.
+
+The evidence so far has consistently pointed to some member of the
+palæostracan group as the ancestor of the vertebrates--a group which had
+affinities both to the crustaceans and the arachnids; indeed, many of its
+members resembled scorpions much more than they resemble crustaceans. The
+olfactory organs of the scorpions and their allies are, therefore, more
+likely than any others to give a clue to the position of the desired
+olfactory organs. In these animals and their allies paired olfactory
+antennæ are not present, either in the living land-forms or the extinct
+sea-scorpions, for all the antennæ-like, frequently chelate, appendages
+seen in Pterygotus, etc. (Fig. 8), represent the cheliceræ, and correspond,
+therefore, to the second pair of antennæ in the crustaceans.
+
+What, then, represents the olfactory antennæ in the scorpions? The answer
+to this question has been given by Croneberg, and very striking it is. The
+two olfactory antennæ of the crustacean have combined together to form a
+hollow tube at the base of which the mouth of the animal is situated, so
+that the food passes along this olfactory passage before it reaches the
+mouth. This organ is often called after Latreille, the camerostome,
+sometimes the rostrum; it is naturally median in position and appears,
+therefore, to be an unpaired organ; its paired {222}character is, of
+course, evident enough, for it is innervated by a pair of nerves, and these
+nerves, as ought to be the case, arise from the supra-oesophageal ganglia.
+In Galeodes it is a conspicuously paired antennæ-like organ (Fig. 94).
+
+Croneberg has also shown that this rostrum, or camerostome, arises
+embryologically as a pair of appendages similar to the other appendages.
+This last observation of Croneberg has been confirmed by Brauer in 1894,
+who describes the origin of the upper lip, as he calls it, in very similar
+terms, without, however, referring to Croneberg's paper. Croneberg further
+shows that this foremost pair of antennæ not only forms the so-called upper
+lip or camerostome, but also a lower lip, for from the basal part of the
+camerostome there projects on each side of the pharynx a dependent
+accessory portion, which in some cases fuses in the middle line, and forms,
+as it were, a lower lip. The entosclerite belonging to this dependent
+portion is apparently the post-oral entosclerite of Lankester and Miss
+Beck.
+
+[Illustration: FIG. 94.--DORSAL VIEW OF BRAIN AND CAMEROSTOME OF GALEODES.
+
+_cam._, camerostome; _pr. ent._, pre-oral entosclerite; _l.l._, dependent
+portion of camerostome; _ph._, pharynx; _al._, alimentary canal; _n. op._,
+median optic nerves; _pl._, plastron; _v.c._, ventral nerve chain; 2, 3,
+second and third appendages.]
+
+At the base of the tubular passage formed by this modified first pair of
+antennæ the true mouth is found opening directly into the dilated pharynx,
+the muscles of which enable the act of suction to be carried out. The
+narrow oesophagus leads out from the pharynx and is completely surrounded
+by the supra- and infra-oesophageal nerve masses.
+
+Huxley also describes the mouth of the scorpion in precisely the same
+position (_cf. o_, Fig. 96).
+
+{223}In order to convey to my readers the antennæ-like character of the
+camerostome in Galeodes (Fig. 101), and its position, I give a figure (Fig.
+94) of the organ from its dorsal aspect, after removal of the cheliceræ and
+their muscles. A side view of the same organ is given in Fig. 95 to show
+the feathered termination of the camerostome, and the position of the
+dependent accessory portion (_l.l._) (Croneberg's 'untere Anhang') with its
+single long antenna-like feather. In both figures the alimentary canal
+(_al._) is seen issuing from the conjoined supra- and infra-oesophageal
+mass.
+
+As is seen in the figures, the bilateral character of the rostrum, as
+Croneberg calls it, is apparent not only in its feathered extremity but
+also in its chitinous covering, the softer median dorsal part (left white
+in figure) being bounded by two lateral plates of hard chitin, which meet
+in the middle line near the extremity of the organ. In all the members of
+the scorpion group, as is clearly shown in Croneberg's figures, the rostrum
+or camerostome is built up on the same plan as in Galeodes, though the
+antenna-like character may not be so evident.
+
+[Illustration: FIG. 95.--LATERAL VIEW OF BRAIN AND CAMEROSTOME OF GALEODES.
+
+_gl. supr. oes._, supra-oesophageal ganglion; _gl. infr. oes._,
+infra-oesophageal ganglion. The rest of the lettering same as in Fig. 94.]
+
+When we consider that the first pair of antennæ in the crustaceans are
+olfactory in function, Croneberg's observations amount to this--
+
+In the arachnids and their allies the first pair of antennæ form a pre-oral
+passage or tube, olfactory in function; the small mouth, which opens
+directly into the pharynx, being situated at the end of this olfactory
+passage.
+
+{224}Croneberg's observations and conclusions are distinctly of very great
+importance in bringing the arachnids into line with the crustaceans, and it
+is therefore most surprising that they are absolutely ignored by Lankester
+and Miss Beck in their paper published in 1883, in which Latreille only is
+mentioned with respect to this organ, and his term "camerostome," or upper
+lip, is used throughout, in accordance with the terminology in Lankester's
+previous paper. That this organ is not only a movable lip or tongue, but
+essentially a sense-organ, almost certainly of smell and taste, as follows
+from Croneberg's conclusions, is shown by the series of sections which I
+have made through a number of young Thelyphonus (Fig. 102).
+
+[Illustration: FIG. 96.--MEDIAN SAGITTAL SECTION THROUGH A YOUNG
+THELYPHONUS.]
+
+I give in Fig. 96 a sagittal median section through the head-end of the
+animal, which shows clearly the nature of Croneberg's conception. At the
+front end of the body is seen the median eye (_ce._), _o_ is the mouth,
+_Ph._ the pharynx, _oes._ the narrow oesophagus, compressed between the
+supra-oesophageal (_supr. oes._) and infra-oesophageal (_infr. oes._) brain
+mass, which opens into the large alimentary canal (_Al._); _Olf. pass._ is
+the olfactory passage to the mouth, lined with thick-set, very fine hairs,
+which spring from the hypostome (_Hyp._) as well as from the large
+conspicuous camerostome (_Cam._), which limits this tube anteriorly. The
+space between the camerostome and the median eye is filled up by the
+massive cheliceræ, which are not shown in this section, as they begin to
+appear in the {225}sections on each side of the median one. The muscles of
+the pharynx and the muscles of the camerostome are attached to the pre-oral
+entosclerite (_pr. ent._). The post-oral entosclerite is shown in section
+as _post. ent._ The dorsal blood-vessel, or heart, is indicated at _H._
+
+In Fig. 97 I give a transverse section through another specimen of the same
+litter, to show the nature of this olfactory tube when cut across. Both
+sections show most clearly that we are dealing here with an elaborate
+sense-organ, the surface of which is partly covered with very fine long
+hairs, partly, as is seen in the figure, is composed of long, separate,
+closely-set sense-rods (_bat._), well protected by the long hairs which
+project on every side in front of them, which recall to mind Bellonci's
+figure of the 'batonnets olfactives' on the antennæ of Sphæroma. Finally,
+we have the observation of Blanchard quoted by Huxley, to the effect that
+this camerostome is innervated by nerves from the supra-oesophageal ganglia
+which are clearly bilateral, seeing that they arise from the ganglion on
+each side and then unite to pass into the camerostome; in other words,
+paired olfactory nerves from the supra-oesophageal ganglia.
+
+These facts demonstrate with wonderful clearness that in one group of the
+Arthropoda the olfactory antennæ have been so modified as to form an
+olfactory tube or passage, which leads directly into the mouth and so to
+the oesophagus of the animal, and, strikingly enough, this group, the
+Arachnida, is the very one to which the scorpions belong.
+
+If for any cause the mouth _o_ in Fig. 96 were to be closed, then the
+olfactory tube (_olf. pass._) might still remain, owing to its importance
+as the organ of smell, and the olfactory tube would terminate blindly at
+the very spot where the corresponding tube does terminate in the
+vertebrate, according to the theory put forward in this book.
+
+
+THE OLFACTORY TUBE OF AMMOCOETES.
+
+In all cases where there is similarity of topographical position in the
+organs of the vertebrate and arthropod we may expect also to find
+similarity of structure. At first sight it would appear as though such
+similarity fails us here, for a cross-section of the olfactory tube in
+Petromyzon represents an elaborate organ such as is shown in Fig. 98, very
+different in appearance to the section across the olfactory passage of a
+young Thelyphonus given in Fig. 97.
+
+{226}[Illustration: FIG. 97.--TRANSVERSE SECTION THROUGH THE OLFACTORY
+PASSAGE OF A YOUNG THELYPHONUS.
+
+1 and 2, sections of first and second appendages.]
+
+[Illustration: FIG. 98.--TRANSVERSE SECTION THROUGH THE OLFACTORY PASSAGE
+OF PETROMYZON.
+
+_cart._, nasal cartilage.]
+
+{227}As is seen, it is difficult to see any connection between these folds
+of olfactory epithelium and the simple tube of the scorpion. But in the
+nose, as in all other parts of the head-region of the lamprey, remarkable
+changes take place at transformation, and examination of the same tube in
+Ammocoetes demonstrates that the elaborate structure of the adult olfactory
+organ is actually derived from a much simpler form of organ, represented in
+Fig. 99. Here, in Ammocoetes, the section is no longer strikingly different
+from that of the Thelyphonus organ, but, instead, most strikingly similar
+to it. Thus, again, it is shown that this larval form of the lamprey gives
+more valuable information as to vertebrate ancestry than all the rest of
+the vertebrates put together.
+
+[Illustration: FIG. 99.--TRANSVERSE SECTION THROUGH THE OLFACTORY PASSAGE
+OF AMMOCOETES.
+
+_cart._, nasal cartilage.]
+
+Still, even now the similarity between the two organs is not complete, for
+the tube in the lamprey opens on to the exterior on the dorsal surface of
+the head, while in the scorpion tribe it is situated ventrally, being the
+passage to the mouth and alimentary canal. In accordance with this there is
+no sign of any opening on the dorsal carapace of any of the extinct
+sea-scorpions or of the living land-scorpions, such as is so universally
+found in the cephalaspids, tremataspids, and lampreys. Here is a
+discrepancy of an apparently serious character, yet so wonderfully does the
+development of the individual recapitulate the development of the race,
+that this very discrepancy becomes converted into a triumphant vindication
+of the {228}correctness of the theory advocated in this book, as soon as we
+turn our attention to the development of this nasal tube in the lamprey.
+
+We must always remember not only the great importance of a larval stage for
+the unriddling of problems of ancestry, but also the great advantage of
+being able to follow more favourably any clues as to past history afforded
+by the development of the larva itself, owing to the greater slowness in
+the development of the larva than of the embryo. Such a clue is especially
+well marked in the course of development of Ammocoetes according to
+Kupffer's researches, for he finds that when the young Ammocoetes is from 5
+to 7 mm. in length, some time after it has left the egg, when it is living
+a free larval life, a remarkable series of changes takes place with
+considerable rapidity, so that we may regard the transformation which takes
+place at this stage, as in some degree comparable with the great
+transformation which occurs when the Ammocoetes becomes a Petromyzon.
+
+All the evidence emphasizes the fact that the latter transformation
+indicates the passage from a lower into a higher form of vertebrate, and is
+to be interpreted phylogenetically as an indication of the passage from the
+Cephalaspidian towards the Dipnoan style of fish. If, then, the former
+transformation is of the same character, it would indicate the passage from
+the Palæostracan to the Cephalaspid.
+
+What is the nature of this transformation process as described by Kupffer?
+
+It is characterized by two most important events. In the first place, up to
+this time the oral chamber has been cut off from the respiratory chamber by
+a septum--the velum--so that no food could pass from the mouth to the
+alimentary canal. At this stage the septum is broken through, the oral
+chamber communicates with the respiratory chamber, and the velar folds of
+the more adult Ammocoetes are left as the remains of the original septum.
+The other striking change is the growth of the upper lip, by which the
+orifice of the nasal tube is transferred from a ventral to a dorsal
+position. Fig. 100, taken from Kupffer's paper, represents a sagittal
+section through an Ammocoetes 4 mm. long; _l.l._ is the lower lip, _u.l._
+the upper lip, and, as is seen, the short oral chamber is closed by the
+septum, _vel._ Opening ventrally is a tube called the tube of the
+hypophysis, _Hy._, which extends close up to the termination of the
+infundibulum. On the anterior surface of this tube is the projection called
+by Kupffer the olfactory plakode. At this stage the upper lip grows with
+great {229}rapidity and thickens considerably, thus forcing the opening of
+the hypophysial tube more and more dorsalwards, until at last, in the
+full-grown Ammocoetes, it becomes the dorsal opening of the nasal tube, as
+already described. Here, then, in the hypophysial tube we have the original
+position of the olfactory tube of the vertebrate ancestor, and it is
+significant, as showing the importance of this organ, to find that such a
+hypophysial tube is characteristic of the embryological development of
+every vertebrate, whatever may be the ultimate form of the external nasal
+orifices.
+
+The single median position of the olfactory organ in the Cyclostomata, in
+contradistinction to its paired character in the rest of the vertebrates,
+has always been a stumbling-block in the way of those who desired to
+consider the Cyclostomata as degenerated Selachians, for the origin of the
+olfactory protuberance, as a single median plakode, seemed to indicate that
+the nose arose as a single organ and not as a paired organ.
+
+[Illustration: FIG. 100.--GANGLIA OF THE CRANIAL NERVES OF AN AMMOCOETES, 4
+MM. IN LENGTH, PROJECTED ON TO THE MEDIAN PLANE. (After KUPFFER.)
+
+_A-B_, the line of epibranchial ganglia; _au._, auditory capsule; _nc._,
+notochord; _Hy._, tube of hypophysis; _Or._, oral cavity; _u.l._, upper
+lip; _l.l._, lower lip; _vel._, septum between oral and respiratory
+cavities; _V._, _VII._, _IX._, _X._, cranial nerves; _x._, nerve with four
+epibranchial ganglia.]
+
+On the other hand, the two olfactory nerves of Ammocoetes compare
+absolutely with the olfactory nerves of other vertebrates, and force one to
+the conclusion that this median organ of Ammocoetes arose from a pair of
+bilateral organs, which have fused in the middle line.
+
+{230}[Illustration: FIG. 101.--_Galeodes._ (From the Royal Natural
+History.)]
+
+{231}The comparison of this olfactory organ with the camerostome gives a
+satisfactory reason for its appearance in the lowest vertebrates as an
+unpaired median organ; equally so, the history of the camerostome itself
+supplies the reason why the olfactory nerves are double, why the organ is
+in reality a paired organ and not a single median one. Thus, in a sense,
+the grouping of the fishes into Monorhinæ and Amphirhinæ has not much
+meaning, seeing that the olfactory organ is in all cases double.
+
+[Illustration: FIG. 102.--_Thelyphonus._ (From the Royal Natural History.)]
+
+The evidence of the olfactory organs in the vertebrate not only confirms,
+in a most striking manner, the theory of the origin of the {232}vertebrate
+from the Palæostracan, but points indubitably to an origin from a
+scorpion-like rather than a crustacean-like stock. To complete the
+evidence, it ought to be shown that the ancient sea-scorpions did possess
+an olfactory passage similar to the modern land-scorpions. The evidence on
+this question will come best in the next chapter, where I propose to deal
+with the prosomatic appendages of the Palæostracan group.
+
+
+SUMMARY.
+
+  The vertebrate olfactory apparatus commences as a single median tube
+  which terminates dorsally in the lamprey, and is supplied by the two
+  olfactory nerves which arise from the supra-infundibular portion of the
+  brain. It is a long, tapering tube which passes ventrally and terminates
+  blindly at the infundibulum in Ammocoetes. The dorsal position of the
+  nasal opening is not the original one, but is brought about by the growth
+  of the upper lip. The nasal tube originally opened ventrally, and was at
+  that period of development known as the tube of the hypophysis.
+
+  The evidence of Ammocoetes thus goes to show that the olfactory apparatus
+  started as an olfactory tube on the ventral side of the animal, which led
+  directly up to, and probably into, the oesophagus of the original
+  alimentary canal of the palæostracan ancestor.
+
+  Strikingly enough, although in the crustaceans the first pair of antennæ
+  form the olfactory organs, no such free antennæ are found in the
+  arachnids, but they have amalgamated to form a tube or olfactory passage,
+  which leads directly into the mouth and oesophagus of the animal.
+
+  This olfactory passage is very conspicuous in all members of the scorpion
+  group, and, like the olfactory tube of the vertebrate, is innervated by a
+  pair of nerves, which resemble those supplying the first pair of antennæ
+  in crustaceans as to their origin from the supra-oesophageal ganglia.
+
+  This nasal passage, or tube of the hypophysis, corresponds in structure
+  and in position most closely with the olfactory tube of the scorpion
+  group, the only difference being that in the latter case it opens
+  directly into the oesophagus, while in the former, owing to the closure
+  of the old mouth, it cannot open into the infundibulum.
+
+  The evidence of the olfactory apparatus, combined with that of the optic
+  apparatus, is most interesting, for, whereas the former points
+  indubitably to an ancestor having scorpion-like affinities, the structure
+  of the lateral eyes points distinctly to crustacean, as well as arachnid,
+  affinities.
+
+  Taking the two together the evidence is extraordinarily strong that the
+  vertebrate arose from a member of the palæostracan group with marked
+  scorpion-like affinities.
+
+
+
+
+{233}CHAPTER VII
+
+_THE PROSOMATIC SEGMENTS OF LIMULUS AND ITS ALLIES_
+
+  Comparison of the trigeminal with the prosomatic region.--The prosomatic
+  appendages of the Gigantostraca.--Their number and nature.--Endognaths
+  and ectognath.--The metastoma.--The coxal glands.--Prosomatic region of
+  Eurypterus compared with that of Ammocoetes.--Prosomatic segmentation
+  shown by muscular markings on carapace.--Evidence of coelomic cavities in
+  Limulus.--Summary.
+
+
+The derivation of the olfactory organs of the vertebrate from the olfactory
+antennæ of the arthropod in the last chapter is confirmatory proof of the
+soundness of the proposition put forward in Chapter IV., that the
+segmentation in the cranial region of the vertebrate was derived from that
+of the prosomatic and mesosomatic regions of the palæostracan ancestor.
+Such a segmentation implies a definite series of body-segments,
+corresponding to the mesomeric segmentation of the vertebrate, and a
+definite series of appendages corresponding to the splanchnic segmentation
+of the vertebrate.
+
+Of the foremost segments belonging to the supra-oesophageal region
+characterized by the presence of the median eyes, of the lateral eyes, and
+of the olfactory organs, a wonderfully exact replica has been shown to
+exist in the pineal eyes, the lateral eyes, and the olfactory organ of the
+vertebrate, belonging, as they all do, to the supra-infundibular region.
+
+Of the infra-oesophageal segments belonging to the prosoma and mesosoma
+respectively, the correspondence between the mesosomatic segments carrying
+the branchial appendages and the uterus, with those in the vertebrate
+carrying the branchiæ and the thyroid gland respectively, has been fully
+proved in previous chapters.
+
+There remain, then, only the segments of the prosomatic region to be
+considered, a region which, both in the vertebrate and invertebrate, is
+never respiratory in function but always masticatory, such {234}mastication
+being performed in Limulus and its allies by the muscles which move the
+foot-jaws or gnathites, which are portions of the prosomatic appendages
+specially modified for that purpose, and in the vertebrates by the
+masticatory muscles, which are always innervated by the trigeminal or Vth
+cranial nerve. This comparison implies that the motor part of the
+trigeminal nerve originally supplied the prosomatic appendages.
+
+The investigations of van Wijhe and of all observers since the publication
+of his paper prove that in this trigeminal region, as in the vagus region,
+a double segmentation exists, of which the ventral or splanchnic segments,
+corresponding to the appendages in the invertebrate, are supplied by the
+trigeminal nerves, while the dorsal or somatic segments, corresponding to
+the somatic segments in the invertebrate, are supplied by the IIIrd or
+oculomotor and the IVth or trochlear nerves--nerves which supply muscles
+moving the lateral eyes.
+
+In accordance, then, with the evidence afforded by the nerves of the
+branchial segments, it follows that the muscles supplied by the motor part
+of the trigeminal ought originally to have moved the appendages belonging
+to a series of prosomatic segments. On the other hand, the eye-muscles
+ought to have belonged to the body-part of the prosomatic segments, and
+must therefore have been grouped originally in a segmental series
+corresponding to the prosomatic appendages.
+
+The evidence for and against this conclusion will be the subject of
+consideration in this and the succeeding chapters. At the outset it is
+evident that any such comparison necessitates an accurate knowledge of the
+number of the prosomatic segments in the Gigantostraca and of the nature of
+the corresponding appendages.
+
+In all this group of animals, the evidence as to the number of segments in
+either the prosomatic or mesosomatic regions is given by--
+
+1.  The number of appendages.
+
+2.  The segmental arrangement of the muscles of the prosoma or mesosoma
+respectively.
+
+3.  The segmental arrangement of the coelomic or head-cavities.
+
+4.  The divisions of the central nervous system, or neuromeres, together
+with their outgoing segmental nerves.
+
+It follows, therefore, that if from any cause the appendages are not
+apparent, as is the case in many fossil remains, or have dwindled {235}away
+and become insignificant, we still have the muscular, coelomic, and nervous
+arrangements left to us as evidence of segmentation in these animals, just
+as in vertebrates.
+
+In this prosomatic region, we find in Limulus the same tripartite division
+of the nerves as in the mesosomatic region, so that the nerves to each
+segment may be classed as (1) appendage-nerve; (2) sensory or dorsal
+somatic nerve, supplying the prosomatic carapace; (3) motor or ventral
+somatic nerve, supplying the muscles of the prosoma, and containing
+possibly some sensory fibres. The main difference between these two regions
+in Limulus consists in the closer aggregation of the prosomatic nerves,
+corresponding to the concentration of the separate ganglia of origin in the
+prosomatic region of the brain.
+
+The number of prosomatic segments in Limulus is not evident by examination
+of the prosomatic carapace, so that the most reliable guide to the
+segmentation of this region is given by the appendages, of which one pair
+corresponds to each prosomatic segment.
+
+The number of such segments, according to present opinion, is seven,
+viz.:--
+
+(1) The foremost segment, which bears the cheliceræ.
+
+(2, 3, 4, 5, 6) The next five segments, which carry the paired locomotor
+appendages; and
+
+(7) The last segment, to which belongs a small abortive pair of appendages,
+known by the name of the chilaria, situated between the last pair of
+locomotor appendages and the operculum or first pair of mesosomatic
+appendages. These appendages are numbered from 1-7 in the accompanying
+drawing (Fig. 103).
+
+Of these seven pairs of appendages, the significance of the first and the
+last has been matter of dispute. With respect to the first pair, or the
+cheliceræ, the question has arisen whether their nerves belong to the
+infra-oesophageal group, or are in reality supra-oesophageal.
+
+It is instructive to observe the nature and the anterior position of this
+pair of appendages in the allied sea-scorpions, especially in Pterygotus,
+where the only chelate organs are found in these long, antennæ-like
+cheliceræ. In Slimonia and in Stylonurus they are supposed by Woodward to
+be represented by the small non-chelate antennæ seen in Fig. 8, B and C (p.
+27), taken from Woodward. If such is the case, then these figures show that
+a pair of appendages is missing in each {236}of these forms, for they
+possess only five free prosomatic appendages instead of six, as in Limulus
+and in Pterygotus. Similarly, Woodward only allowed five appendages for
+Pterygotus, so that his restorations were throughout consistent. Schmidt,
+in _Pterygotus osiliensis_ has shown that the true number was six, not
+five, as seen in his restoration given in Fig. 8, A (p. 27).
+
+[Illustration: FIG. 103.--VENTRAL SURFACE OF LIMULUS. (Taken from
+KISHINOUYE.)
+
+The gnathic bases of the appendages have been separated from those of the
+other side to show the promesosternite or endostoma (_End._).]
+
+With respect to Eurypterus, Schmidt figures an exceedingly minute pair of
+antennæ between the coxal joints of the first pair of appendages, thus
+making six pairs of appendages. Gerhard Holm, however, in his recent
+beautiful preparations from Schmidt's specimens and others collected at
+Rootziküll, has proved most conclusively that the cheliceræ of Eurypterus
+were of the same kind as those of Limulus. I reproduce his figure (Fig.
+104) showing the small chelate cheliceræ (1) overhanging the mouth orifice,
+just as in Limulus or in Scorpio.
+
+{237}So, also, since Woodward's monograph, Laurie has discovered in
+_Slimonia acuminata_ a small median pair of chelate appendages exactly
+corresponding to the cheliceræ of Limulus, or of Eurypterus, or of Scorpio.
+We may, therefore, take it for granted that such was also the case in
+Stylonurus, and that the foremost pair of prosomatic appendages in all
+these extinct sea-scorpions were in the same position and of the same
+character as the cheliceræ of the scorpions.
+
+[Illustration: FIG. 104.--_Eurypterus Fischeri._ (From HOLM.)]
+
+In the living scorpion and in Limulus the nerves to this pair of appendages
+undoubtedly arise from the foremost prosomatic ganglia, and the reason why
+they appear to belong to the supra-oesophageal brain-mass has been made
+clear by Brauer's investigations on the embryology of Scorpio; for he has
+shown that the cheliceral ganglia shift from the ventral to the dorsal side
+of the oesophagus during development, thus becoming
+pseudo-supra-oesophageal, though in reality belonging to the
+infra-oesophageal ganglia. This cheliceral pair of appendages is, in all
+probability, homologous with the second pair of antennæ in the crustacea.
+
+{238}I conclude, then, that the cheliceræ must truly be included in the
+prosomatic group, but that they stand in a somewhat different category to
+the rest of the prosomatic appendages, inasmuch as they take up a very
+median anterior and somewhat dorsal position, and their ganglia of origin
+are also exceptional in position.
+
+Next for consideration come the chilaria (7 in Fig. 103), which Lankester
+did not consider to belong to appendages at all, but to be a peculiar pair
+of sternites. Yet their very appearance, with their spinous hairs
+corresponding to those of the other gnathites and their separate
+nerve-supply, all point distinctly to their being a modified pair of
+appendages, and, indeed, the matter has been placed beyond doubt by the
+observations of Kishinouye, who has found embryologically that they arise
+in the same way as the rest of the prosomatic appendages, and belong to a
+distinct prosomatic segment, viz. the seventh segment. In accordance with
+this, Brauer has found that in the scorpion there is in the embryo a
+segment, whose appendages degenerate, which is situated between the segment
+bearing the last pair of thoracic appendages and the genital operculum--a
+segment, therefore, comparable in position to the chilarial segment of
+Limulus.
+
+Coming now to the five locomotor appendages, we find that they resemble
+each other to a considerable extent in most cases, with, however, certain
+striking differences. Thus in Limulus they are chelate, with their basal
+joints formed as gnathites, except in the case of the fifth appendage, in
+which the extremity is modified for the purpose of digging in the sand. In
+Pterygotus, Slimonia, Eurypterus, the first four of these appendages are
+very similar, and are called by Huxley and Woodward endognaths; in all
+cases they possess a basal part or sterno-coxal process, which acts as a
+gnathite or foot-jaw, and a non-chelate tactile part, which possesses no
+prehensile power, and in most cases could have had no appreciable share in
+locomotion, called by Huxley and Woodward the palpus. These small palps
+were probably retractile, and capable of being withdrawn entirely under the
+hood. The fifth appendage is usually different, being a large swimming
+organ in Pterygotus, Eurypterus, and Slimonia (Figs. 8 and 104), and is
+known as the ectognath.
+
+Finally, in _Drepanopterus Bembycoides_, as stated by Laurie, all five
+locomotor appendages are built up after the same fashion, the last one not
+being formed as a paddle-shaped organ or elongated as {239}in Stylonurus,
+but all five possess no special locomotor or prehensile power. According to
+Laurie this is a specially primitive form of the group.
+
+It is significant to notice from this sketch that with the absence of
+special prehensile terminations such as chelæ, or the absence of special
+locomotor functions such as walking or swimming, these appendages tend to
+dwindle and become insignificant, taking up the position of mere feelers
+round the mouth, and at the same time are concentrated and pressed closely
+together, so that their appendage-nerves must also be close together.
+
+This sketch therefore shows us that--
+
+Of the six foremost prosomatic appendages, the cheliceræ and the four
+endognaths were, at the time when the vertebrates first appeared, in very
+many cases dwindling away; the latter especially no longer functioned as
+locomotor appendages, but were becoming more and more mere palps or
+tentacles situated round the mouth, which could by no possibility afford
+any help to locomotion.
+
+On the contrary, the sixth pair of appendages--the ectognaths--remained
+powerful, being modified in many cases into large oar-like limbs by which
+the animal propelled itself through the water.
+
+It is a striking coincidence that those ancient fishes, Pterichthys and
+Bothriolepis, should have possessed a pair of large oar-like appendages.
+
+At this time, then, in strong contrast to the endognaths, the ectognaths,
+or sixth pair of appendages, remained strong and vigorous. What about the
+seventh pair, the chilaria of Limulus?
+
+Of all the prosomatic appendages these are the most interesting from the
+point of view of my theory, for whereas in the scorpion of the present day
+they have dwindled away and left no trace except in the embryo, in the
+sea-scorpions of old, far from dwindling, they had developed and become a
+much more important organ than the chilaria of Limulus.
+
+In all these animals a peculiarly striking and unique structure is found in
+this region known by the name of the metastoma, or lip-plate (Figs. 8 and
+104 (7)); it is universally considered to be formed by the fusion of the
+two chilarial appendages.
+
+All observers are agreed that this lip-plate was freely movable. Nieskowski
+considers that the movement of the metastoma was entirely in a vertical
+direction, whereby the cleft which is seen {240}between the basal joints of
+all the pairs of locomotor appendages could be closed from behind. Woodward
+says it no doubt represents the labium, and served more effectually to
+enclose the posterior part of the buccal orifice, being found exteriorly to
+the toothed edges of the ectognaths or maxillipedes. Schmidt agrees with
+Nieskowski, and looks on the mestasoma as forming a lower lip within which
+the bases of the ectognaths worked.
+
+[Illustration: FIG. 105.--DIAGRAM OF SAGITTAL MEDIAN SECTION THROUGH A,
+LIMULUS, B, EURYPTERUS.]
+
+Quite recently Gerhard Holm has worked over again the very numerous
+specimens of _Eurypterus Fischeri_, which are obtainable at Rootziküll, and
+has thrown new light on the relation of the metastoma to the mouth-parts.
+His preparations show clearly that the true lower lip of Eurypterus was not
+the metastoma, for when the metastoma is removed another plate (_End._,
+Fig. 105, B) situated {241}internally to it is disclosed, which, in his
+view, corresponds to the sternite between the bases of the pro-somatic
+appendages in Limulus, _i.e._ to the sternite called by Lankester, the
+pro-mesosternite (_End._, Fig. 103). This inner plate formed with the
+metastoma ((7) Fig. 105) and the ectognaths (6) a chamber closed
+posteriorly, within which the bases of the ectognaths worked. In other
+words, the removal of the metastoma discloses in Eurypterus the true
+anterior ventral surface of the animal which corresponds to that of
+Limulus, or of the scorpion group, with its pro-mesosternite and laterally
+attached gnathites or sterno-coxal processes. To this inner plate or
+pro-mesosternite Holm gives the name of _endostoma_.
+
+To the anterior edge of the endostoma a thinner membrane is attached which
+passes inwards in the direction of the throat, and forms, therefore, the
+lower lip (_Hyp._, Fig. 105, B) of the passage of the mouth (_olf. p._).
+This membrane bears upon its surface a tuft of hairs, which he thought were
+probably olfactory in function. Consequently, in his preliminary
+communication, he describes this lower lip as forming, in all probability,
+an olfactory organ; in his full communication he repudiates this
+suggestion, because he thinks it unlikely that such an organ would be
+situated within the mouth. I feel sure that if Holm had referred to
+Croneberg's paper, and seen how the true mouth in all the scorpion group is
+situated at the base of an olfactory passage, he would have recognized that
+his first suggestion is in striking accordance with the nature of the
+entrance to the mouth in other scorpions.
+
+That Eurypterus also possessed a camerostome (_cam._) seems to follow of
+necessity from its evident affinities both with Limulus and the scorpions.
+We see, in fact, that the mouth of these old sea-scorpions was formed after
+the fashion of Limulus, surrounded by masticatory organs in the shape of
+foot-jaws, and yet foreshadowed that of the scorpion, so that an ideal
+sagittal section of one of these old palæostracan forms would be obtained
+by the combination of actual sagittal sections through Limulus and a member
+of the scorpion group, with, at the same time, a due recognition of Holm's
+researches. Such a section is represented in Fig. 105, B, in which I have
+drawn the central nervous system and its nerves, the median eyes (_C.E._),
+the olfactory organs (_Cam._), the pharynx (_Ph._), oesophagus (_oes._),
+and alimentary canal (_Al._), but have not tried to indicate the lateral
+eyes. I have represented the prosomatic appendages by numbers (1-7), and
+{242}the foremost mesosomatic segments by numbers (8-13). I have placed the
+four endognaths and the nerves going to them close together, and made them
+small, mere tentacles, in recognition of the character of these appendages
+in Eurypterus, and have indicated the position and size of the large
+ectognath, with its separate nerve, by (6). If among the ancient
+Eurypterus-like forms, which were living at the time when vertebrates first
+appeared, there were some in which the ectognaths also had dwindled to a
+pair of tentacles, then such animals would possess a prosomatic chamber
+formed by a metastoma or accessory lip, within which were situated five
+pairs of short tactile appendages or tentacles. If the vertebrate were
+derived from such an animal, then the trigeminal nerve, as the
+representative of these prosomatic appendage-nerves, ought to be found to
+supply the muscles of this accessory lip and of these five pairs of
+tentacles in the lowest vertebrate.
+
+This prosomatic or oral chamber, as it might be called, was limited
+posteriorly by the fused metastoma (7) and operculum (8), so that if in the
+same imaginary animal one imagines that the gill-chambers, instead of being
+separate, are united to form one large respiratory chamber, then, in such
+an animal, a prosomatic oral chamber, in which the prosomatic appendages
+worked, would be separated from a mesosomatic respiratory chamber by a
+septum composed of the conjoined basal portions of the mesosomatic
+operculum and the prosomatic metastoma, as indicated in the diagram. In
+this septum the nerves to the last prosomatic appendage (equivalent to the
+last part of the trigeminal in the vertebrate) and to the first mesosomatic
+(equivalent to the thyroid part of the facial) would run, as shown in the
+figure, close together in the first part of their course, and would
+separate when the ventral surface was reached, to pass headwards and
+tailwards respectively.
+
+
+THE COXAL GLANDS.
+
+One more characteristic of these appendages requires mention, and that is
+the excretory glands situated at the base of the four endognaths known as
+the coxal glands. These glands are the main excretory organs in Limulus and
+the scorpions, and extend into the basal segments or coxæ of the four
+endognaths, not into those of the ectognaths or the chilaria (or
+metastoma). Hence their name, coxal {243}glands; and, seeing the importance
+of the excretory function, it is likely enough that they would remain, even
+when the appendages themselves had dwindled away. With the concentration
+and dwindling of the endognaths these coxal glands would also be
+concentrated, so that in the diagram (Fig. 105) they would rightly be
+grouped together in the position indicated (_cox. gl._).
+
+Such a diagram indicates the position of all the important organs of the
+head-region except the special organs for taste and hearing. These, for the
+sake of convenience, I propose to take separately, in order at this stage
+of my argument not to overburden the simplicity of the comparison I desire
+to make with too much unavoidable detail.
+
+
+THE PROSOMATIC REGION OF AMMOCOETES.
+
+Let us now compare this diagram with that of the corresponding region in
+Ammocoetes and see whether or no any points of similarity exist.
+
+With respect to this region, as in so many other instances already
+mentioned, Ammocoetes occupies an almost unique position among vertebrates,
+for the region supplied by the trigeminal nerve--the prosomatic
+region--consists of a large oral chamber which was separated from the
+respiratory chamber in the very young stage by a septum which is
+subsequently broken through, and so the two chambers communicate.
+
+This chamber is bounded by the lower lip ventrally, the upper lip and
+trabecular region dorsally, and the remains of the septum or velum
+laterally and posteriorly. It contains a number of tentacles arranged in
+pairs within the chamber so as to form a sieve-like fringe inside the
+circular mouth; of these, the ventral pair are large, fused together, and
+attached to the lower lip.
+
+All the muscles belonging to this oral chamber are of the visceral type,
+and are innervated by the trigeminal nerve. In accordance with the evidence
+obtained up to this point this means that such an oral chamber was formed
+by the prosomatic appendages of the invertebrate ancestor, similarly to the
+oral chamber just figured for Eurypterus.
+
+This chamber in the full-grown Ammocoetes is not only open to the
+respiratory chamber, but is bounded by the large upper lip (_U.L._, Fig.
+106, D). On the dorsal surface of this region, in front of the {244}pineal
+eye (_C.E._), is the most conspicuous opening of the olfactory tube
+(_Na._), which olfactory tube passes from the dorsal region to the ventral
+side to terminate blindly at the very spot where the infundibulum comes to
+the surface of the brain. Here, also, is situated that extraordinary
+glandular organ known as the pituitary body (_Pit._). A sagittal section,
+then, in diagram form, of the position of parts in the full-grown
+Ammocoetes, would be represented as in Fig. 106, D.
+
+But, as argued out in the last chapter, the diagram of the adult Ammocoetes
+must be compared with that of a cephalaspidian fish; the diagram of the
+palæostracan must be compared with the larval condition of Ammocoetes. In
+other words, Fig. 106, B, must be compared with Fig. 106, C, which
+represents a section through the larval Ammocoetes as it would appear if it
+reached the adult condition without any forward growth of the upper lip or
+any breaking through of the septum between the oral and respiratory
+chambers. The striking similarity between this diagram and that of
+Eurypterus becomes immediately manifest even to the smallest details. The
+only difference between the two, except, of course, the notochord, consists
+in the closure of the mouth opening (_o_), in Fig. 106, B, by which the
+olfactory passage (_olf. p._) of the scorpion becomes converted into the
+hypophysial tube (_Hy._), Fig. 106, C, and later into the nasal tube
+(_Na._), Fig. 106, D, of the full-grown Ammocoetes. That single closure of
+the old mouth is absolutely all that is required to convert the Eurypterus
+diagram into the Ammocoetes diagram.
+
+Such a comparison immediately explains in the simplest manner a number of
+anatomical peculiarities which have hitherto been among the great mysteries
+of the vertebrate organization. For not only do the median eyes (_C.E._)
+correspond in position in the two diagrams, and the infundibular tube
+(_Inf._) and the ventricles of the brain (_C.C._) correspond to the
+oesophagus (_oes._) and the cephalic stomach (_Al._), as already fully
+discussed; but even in the very place where the narrow oesophagus opened
+into the wider chamber of the pharynx (_Ph._), there, in all the lower
+vertebrates, the narrow infundibular tube opens into the wider chamber of
+the membranous _saccus vasculosus_ (_sac. vasc._). This is the last portion
+of the membranous part of the tube of the central nervous system which has
+not received explanation in the previous chapters, and now it is seen how
+simple its explanation is, how natural its presence--it represents the old
+pharyngeal chamber of the palæostracan ancestor.
+
+{245}[Illustration: FIG. 106.--DIAGRAM OF SAGITTAL MEDIAN SECTION THROUGH
+B, EURYPTERUS; C, LARVAL AMMOCOETES; D, FULL-GROWN AMMOCOETES.]
+
+{246}Next among the mysteries requiring explanation is the pituitary body,
+that strange glandular organ always found so closely attached to the brain
+in the infundibular region that when it is detached in taking out the brain
+it leaves the infundibular canal patent right into the IIIrd ventricle. A
+comparison of the two diagrams indicates that such a glandular organ
+(_Pit._), Fig. 106, C, was there because the coxal excretory glands (_cox.
+gl._), Fig. 106, B, were in a similar position in the palæostracan
+ancestor--that, indeed, the pituitary body is the descendant of the coxal
+glands.
+
+Finally, the diagrams not only indicate how the mesosomatic
+appendage-nerves supplying in the one case the operculum and the
+respiratory appendages correspond to the respiratory group of nerves, VII.,
+IX., X., supplying in the other case the thyroid, hyoid, and branchial
+segments, but also that a similar correspondence exists between the
+prosomatic appendage-nerves in the one case and the trigeminal nerve in the
+other; a correspondence which supplies the reason why in the vertebrate a
+septum originally existed between an oral and respiratory chamber.
+
+Such a comparison, then, leads directly to the suggestion that the
+trigeminal nerve originally supplied the prosomatic appendages, such
+appendages being: 1. The metastoma, which has become in Ammocoetes the
+lower lip supplied by the velar or mandibular branch of the trigeminal
+nerve (7); 2. The ectognath, which has become the large median ventral
+tentacle, called by Rathke the tongue, supplied by the tongue nerve (6); 3.
+The endognaths, which have been reduced to tentacles and are supplied by
+the tentacular branch of the trigeminal nerve (2, 3, 4, 5).
+
+I have purposely put these two diagrams of the larval Ammocoetes and of
+Eurypterus before the minds of my readers at this early stage of my
+argument, so as to make what follows more understandable. I propose now to
+consider fully each one of these suggestive comparisons, and to see whether
+or no they are in accordance with the results of modern research.
+
+In the first instance, the diagrams suggest that the trigeminal nerve
+originally supplied the prosomatic appendages of the palæostracan ancestor,
+while the eye-muscle nerves supplied the body-muscles of the prosoma.
+
+{247}As these appendages did not carry any vital organs such as branchiæ,
+but were mainly locomotor and masticatory in function, it follows that
+their disappearance as such would be much more complete than that of the
+mesosomatic branchial appendages. Most probably, then, in the higher
+vertebrates no trace of such appendages might be left; consequently the
+segmentation due to their presence would be very obscure, so that in this
+region the very reverse of what is found in the region of the vagus nerve
+would be the rule. There branchiomeric segmentation is especially evident,
+owing to the persistence of the branchial part of the branchial appendages;
+here, owing to the disappearance of the appendages, the segmentation is no
+longer branchiomeric, but essentially mesomeric in consequence of the
+persistence of the somatic eye-muscles.
+
+In addition to the evidence of the appendages themselves, the number of
+prosomatic segments is well marked out in all the members of the scorpion
+group by the divisions of the central nervous system into well-defined
+neuromeres in accordance with the appendages, a segmentation the
+reminiscence of which may still persist after the appendages themselves
+have dwindled or disappeared. In accordance with this possibility we see
+that one of the most recent discoveries in favour of a number of segments
+in the head-region of the vertebrate is the discovery in the early embryo
+of a number of partial divisions in the brain-mass, forming a system of
+cephalic neuromeres which may well be the rudiments of the well-defined
+cephalic neuromeres of animals such as the scorpion.
+
+
+THE EVIDENCE OF THE PROSOMATIC MUSCULATURE.
+
+Even if the appendages as such become obscure, yet their muscles might
+remain and show evidence of their presence. The most persistent of all the
+appendage-muscles are the basal muscles which pass from coxa to carapace
+and are known by the name of tergo-coxal muscles. They are large, well
+marked, segmentally arranged muscles, dorso-ventral in direction, and,
+owing to their connecting the limb with the carapace, are likely to be
+retained even if the appendage dwindles away.
+
+The muscular system of Limulus and Scorpio has been investigated by Benham
+and Miss Beck under Lankester's direction, and the conclusions to which
+Lankester comes are these--
+
+{248}The simple musculature of the primitive animal from which both Limulus
+and the scorpions arose consisted of--
+
+  1. A series of paired longitudinal dorsal muscles passing from tergite to
+  tergite of each successive segment.
+
+  2. A similar series of paired longitudinal ventral muscles.
+
+  3. A pair of dorso-ventral muscles passing from tergite to sternite in
+  each segment.
+
+  4. A set of dorso-ventral muscles moving the coxa of each limb in its
+  socket.
+
+  5. A pair of veno-pericardial muscles in each segment.
+
+Of these groups of muscles, any one of which would indicate the number of
+segments, Groups 1 and 2 do not extend into the prosomatic region, and
+Group 5 extends only as far as the heart extends in the case of both
+Limulus and the Scorpion group; so that we may safely conclude that in the
+Palæostraca the evidence of somatic segmentation in the prosomatic region
+would be given, as far as the musculature is concerned, by the
+dorso-ventral somatic muscles (Group 3), and of segmentation due to the
+appendages by the dorso-ventral appendage musculature (Group 4).
+
+Therefore, if, as the evidence so far indicates, the vertebrate has arisen
+from a palæostracan stock, we should expect to find that the musculature of
+the somatic segments in the region of the trigeminal nerve did not resemble
+the segmental muscles of the spinal region, was not, therefore, the
+continuation of the longitudinal musculature of the body, but was
+dorso-ventral in position, and that the musculature of the splanchic
+segments resembled that of the vagus region, where, as pointed out in
+Chapter IV., the respiratory muscles arose from the dorso-ventral muscles
+of the mesosomatic appendages. This is, of course, exactly what is found
+for the muscles which move the lateral eyes of the vertebrate; these
+muscles, innervated by the IIIrd, IVth, and VIth nerves, afford one of the
+main evidences of segmentation in this region, are always grouped in line
+with the somatic muscles of spinal segments, and yet cannot be classed as
+longitudinal muscles. They are dorso-ventral in direction, and yet belong
+to the somatic system; they are exactly what one ought to find if they
+represent Group 3--the dorso-ventral body-muscles of the prosomatic
+segments of the invertebrate ancestor.
+
+The interpretation of these muscles will be given immediately; at present I
+want to pass in review all the different kinds of evidence {249}of
+segmentation in this region afforded by the examination of the
+invertebrate, whether living or fossil, so as to see what clues are left if
+the evidence of appendages fails us. I will take in the first instance the
+evidence of segmentation afforded by the presence of the musculature of
+Group 4, even when, as in the case of many fossils, no appendages have yet
+been found. In such animals as Mygale and Phrynus the prosomatic carapace
+is seen to be marked out into a series of elevations and depressions, and
+upon removing the carapace we see that these elevations correspond with and
+are due to the large tergo-coxal muscles of the appendages; so that if such
+carapace alone were found fossilized we could say with certainty: this
+animal possessed prosomatic appendages the number of which can be guessed
+with more or less certainty by these indications of segments on the
+carapace.
+
+In those forms, then, which are only known to us in the fossil condition,
+in which no prosomatic appendages have been found, but which possess, more
+or less clearly, radial markings on the prosomatic carapace resembling
+those of Phrynus or Mygale, such radial markings may be interpreted as due
+to the presence of prosomatic appendages, which are either entirely
+concealed by the prosomatic carapace or dorsal head-plate, or were of such
+a nature as not to have been capable of fossilization.
+
+The group of animals in question forms the great group of animals, chiefly
+extinct, classified by H. Woodward under the order of Merostomata. They are
+divided by him into the sub-order of Eurypteridæ, which includes--(1)
+Pterygotus, (2) Slimonia, (3) Stylonurus, (4) Eurypterus, (5)
+Adelophthalmus, (6) Bunodes, (7) Arthropleura, (8) Hemiaspis, (9)
+Exapinurus, (10) Pseudoniscus; and the sub-order Xiphosura, which
+includes--(1) Belinurus, (2) Prestwichia, (3) Limulus.
+
+{250}[Illustration: FIG. 107.--_Phrynus Margine-Maculata._
+
+_Ce._, median eyes; _le._, lateral eyes; _glab._, median plate over brain;
+_Fo._, fovea.]
+
+[Illustration: FIG. 108.--_Phrynus sp._ (?). CARAPACE REMOVED.
+
+_cam._, camerostome; _pl._, plastron.]
+
+{251}The evidence of the Xiphosura and of the Hemiaspidæ conclusively
+shows, in Woodward's opinion, that the Merostomata are closely related to
+the Trilobita, and the Hemiaspidæ especially are supposed to be
+intermediate between the trilobites and the king-crabs. They are
+characterized, as also Belinurus and Prestwichia, by the absence of any
+prosomatic appendages, so that in these cases, as is seen in Fig. 12 (p.
+30), representing _Bunodes lunula_, found in the Eurypterus layer at
+Rootziküll, we have an animal somewhat resembling Limulus in which the
+prosomatic appendages have either dwindled away and are completely hidden
+by the prosomatic carapace, or became so soft as not to be preserved in the
+fossilized condition. The appearance of the prosomatic carapace is, to my
+mind, suggestive of the presence of such appendages, for it is marked out
+radially, as is seen in the figure, in a manner resembling somewhat the
+markings on the prosomatic carapace of Mygale or Phrynus; the latter
+markings, as already mentioned, are due to the aponeuroses between the
+tergo-coxal muscles of the prosomatic appendages which lie underneath and
+are attached to the carapace.
+
+A very similar radial marking is shown by Woodward in his picture of
+_Hemiaspis limuloides_, reproduced in Fig. 109, found in the Lower Ludlow
+beds at Leintwardine. This species has yielded the most perfect specimens
+of the genus Hemiaspis, which is recognized as differing from Bunodes by
+the possession of a telson.
+
+It is striking to find that similar indications of segments have been found
+on the dorsal surface of the head-region in many of the most ancient
+extinct fishes, as will be fully discussed later on.
+
+[Illustration: FIG.109.--_Hemiaspis limuloides._ (From WOODWARD.)
+
+_gl._, glabellum.]
+
+
+THE EVIDENCE OF COELOMIC CAVITIES.
+
+In the head-region of the vertebrate, morphologists depend largely upon the
+embryonic divisions of the mesoderm for the estimation of the number of
+segments, and, therefore, upon the number of coelomic cavities in this
+region, the walls of which give origin to the striated muscles of the head,
+so that the question of the number of segments depends very largely upon
+the origin of the muscles from the walls of these head-cavities. It is
+therefore interesting to examine whether a similar criterion of
+segmentation holds good in such a segmented {252}animal as Limulus, or in
+the members of the scorpion group, in which the number of segments are
+known definitely by the presence of the appendages. In Limulus we know,
+from the observations of Kishinouye, that a series of coelomic cavities are
+formed embryologically in the various segments of the mesosoma and prosoma,
+in a manner exceedingly similar to their mode of formation in the
+head-region of the vertebrate, and he has shown that in the mesosoma a
+separate coelomic cavity exists for each segment, so that just as the
+dorso-ventral somatic muscles are regularly segmentally arranged in this
+region, so are the coelomic cavities, and we should be right in our
+estimation of the number of segments in this region by the consideration of
+the numerical correspondence of these cavities with the mesomatic
+appendages. Similarly, in the vertebrate, we find every reason to believe
+that a single, separate head-cavity corresponds to each of the branchial
+segments in the opisthotic region, and therefore we should estimate rightly
+the number of segments by the division of the mesoderm in this region.
+
+In the prosomatic region of Limulus, the dorso-ventral muscles are not
+arranged with such absolute segmental regularity as in the mesosomatic
+region, and Kishinouye's observations show that the coelomic cavities in
+this region do not correspond absolutely with the number of prosomatic
+appendages. His words are:--
+
+A pair of coelomic cavities appears in every segment except the segments of
+the 2nd, 3rd, and 4th appendages, in which coelomic cavities do not appear
+at all. At least eleven pairs of these cavities are produced. The eleventh
+pair belongs to the seventh abdominal segment.
+
+The first pair of coelomic cavities is common to the cephalic lobe and the
+segment of the first appendage (_i.e._ the cheliceræ).
+
+The second coelomic cavity belongs to the segment of the fifth appendage.
+It is well developed.
+
+The ventral portion of the second coelomic cavity remains as the coxal
+gland.
+
+*       *       *       *       *       *
+
+Consequently, if we were to estimate the number of segments in this region
+by the number of coelomic cavities we should not judge rightly, for we
+should find only four cavities and seven appendages, as is seen in the
+following table:--
+
+  Key:                                                                {253}
+  A Prosomatic.
+  B Mesosomatic.
+  C.c. Coelomic cavities.
+
+  ---------------------------------------------------------+---------------
+                             LIMULUS.                      |  VERTEBRATE.
+  ---------+----------------+-----------------------+------+---------------
+  Segments.|   Appendages.  | Eurypterid appendages.| C.c. |   Coelomic
+           |                |                       |      |   cavities.
+  ---+-----+----------------+-----------------------+------+---------------
+     |  1  | Cheliceræ or   |  Cheliceræ            |   1  |  Anterior
+     |     |  1st locomotor.|                       |      |
+     |  2  | 2nd locomotor  |}                      |      |
+   A |  3  | 3rd     "      |} Endognaths           |   2  |  Premandibular
+     |  4  | 4th     "      |}                      |      |
+     |  5  | 5th     "      |}                      |      |
+     |  6  | 6th     "      |  Ectognath            |   3  |} Mandibular
+     |  7  | Chilaria       |  Metastoma            |   4  |}
+  ---+-----+----------------+-----------------------+------+---------------
+     |  8  | Operculum      |} Operculum (Genital)  |   5  |} Hyoid
+     |  9  | 1st branchial  |}    " (1st branchial) |   6  |}
+     | 10  | 2nd    "       |  2nd branchial        |   7  |  1st branchial
+   B | 11  | 3rd    "       |  3rd     "            |   8  |  2nd    "
+     | 12  | 4th    "       |  4th     "            |   9  |  3rd    "
+     | 13  | 5th    "       |  5th     "            |  10  |  4th    "
+     | 14  | 6th    "       |                       |  11  |
+  ---+-----+----------------+-----------------------+------+---------------
+
+The second cavity would in reality represent four segments belonging to the
+2nd, 3rd, 4th, 5th locomotor appendages, _i.e._ the very four segments
+which in the Eurypteridæ are concentrated together to form the endognaths,
+and we should be justified in putting this interpretation on it, because,
+according to Kishinouye, its ventral portion forms the coxal gland, and,
+according to Lankester, the coxal gland sends prolongations into the coxa
+of the 2nd, 3rd, 4th, 5th locomotor appendages. Similarly in the
+vertebrate, we find three head-cavities in the region which corresponds, on
+my theory, to the prosomatic region of Limulus, (1) the anterior cavity
+discovered by Miss Platt, (2) the premandibular cavity, and (3) the
+mandibular cavity, which, if they corresponded with the prosomatic coelomic
+cavities of Limulus, would represent not three segments but seven segments,
+as follows:--the anterior cavity would correspond to the first coelomic
+cavity, _i.e._ the cavity of the cheliceral segments in both Limulus and
+the Eurypteridæ; the premandibular, to the second coelomic cavity,
+representing, therefore, the 2nd, 3rd, 4th, 5th prosomatic segments in
+Limulus and the endognathal segments in the Eurypteridæ; and the mandibular
+to the 3rd and 4th coelomic cavities, representing the last locomotor and
+chilarial segments in Limulus, _i.e._ the ectognathal and metastomal
+segments in the Eurypteridæ.
+
+{254}It is worthy of note that, in respect to their coelomic cavities, as
+in the position and origin of their nerves in the central nervous system,
+the first pair of appendages, the cheliceræ, retain a unique position,
+differing from the rest of the prosomatic appendages.
+
+In the table I have shown how the vertebrate coelomic cavities may be
+compared with those of Limulus. The next question to consider is the
+evidence obtained by morphologists and anatomists as to the number of
+segments supplied by the trigeminal nerve-group; this question will be
+considered in the next chapter.
+
+
+SUMMARY.
+
+  In Chapters IV. and V. I have dealt with the opisthotic segments of the
+  vertebrate, including therein the segments supplied by the facial nerve,
+  and shown that they correspond to the mesosomatic segments of the
+  palæostracan; consequently the facial (VII.), glossopharyngeal (IX.), and
+  vagus (X.) nerves originally supplied the branchial and opercular
+  appendages.
+
+  In this chapter the consideration of the pro-otic segments is commenced,
+  that is, the segments supplied by the trigeminal (V.) and the eye-muscle
+  nerves (III., IV., VI.). I have considered the VIth nerve with the rest
+  of the eye-muscle nerves for convenience' sake, though in reality it
+  belongs to the same segment as the facial. Of these, that part of the
+  trigeminal which innervates the muscles of mastication corresponds to the
+  splanchnic segments, while the eye-muscle nerves belong to the
+  corresponding somatic segments; but the pro-otic segments of the
+  vertebrate ought to correspond to the prosomatic segments of the
+  invertebrate, just as the opisthotic correspond to the mesosomatic.
+  Therefore the motor part of the trigeminal ought to supply muscles which
+  originally moved the prosomatic appendages, while the eye-muscles ought
+  to have belonged to the somatic part of the same segments.
+
+  The first question considered is the number of segments which ought to be
+  found in this region. In Limulus, the Eurypteridæ, and the scorpions
+  there are seven prosomatic segments which carry (1) the cheliceræ, (2, 3,
+  4, 5) the four first locomotor appendages--the endognaths, (6) the large
+  special appendage--the ectognath--and (7) the appendages, which in
+  Limulus are known as the chilaria, and are small and insignificant, but
+  in Eurypterus and other forms grow forwards, fuse together, and form a
+  single median lip to an accessory oral chamber, which lip is known as the
+  metastoma. Of these appendages the cheliceræ and endognaths tend to
+  dwindle away and become mere tentacles, while the large swimming
+  ectognath and metastoma remain strong and vigorous.
+
+  In this, the prosomatic region, the somatic segmentation is not
+  characterized by the presence of the longitudinal muscle segments, for
+  they do not extend into this head-region, but only by the presence of the
+  segmental somatic {255}ventro-dorsal muscles. Among the muscles of the
+  appendages the system of large tergo-coxal muscles is especially
+  apparent.
+
+  From these considerations it follows that the number of segments in this
+  region in the vertebrate ought to be seven; that the musculature supplied
+  by the trigeminal nerve ought to represent seven ventral or splanchnic
+  segments, of which only the last two are likely to be conspicuous; and
+  that the musculature supplied by the eye-muscle nerves ought to be
+  dorso-ventral in direction, which it is, and represent seven dorsal or
+  somatic segments.
+
+  A further peculiarity of this region, both in Limulus and the scorpions,
+  is found in the excretory organs which are known by the name of coxal
+  glands, because they extend into the basal joint, or coxa, of certain of
+  the prosomatic limbs. The appendages so characterized are always the four
+  endognaths, and it follows that if these four endognaths lose their
+  locomotor power, become reduced in size, and concentrated together to
+  form mere tentacles, then of necessity the coxal glands will be
+  concentrated together, and tend to form a glandular mass in the region of
+  the mouth; in fact, take up a position corresponding to that of the
+  pituitary body in vertebrates.
+
+  Taking all these facts into consideration, it is possible to construct a
+  drawing of a sagittal section through the head-region of Eurypterus,
+  which will represent, with considerable probability, the arrangement of
+  parts in that animal. This can be compared with the corresponding section
+  through the head of Ammocoetes.
+
+  Now, as pointed out in the last chapter, the early stage of Ammocoetes is
+  remarkably different from the more advanced stage; at that time the
+  septum between the oral and respiratory chambers has not yet broken
+  through, and the olfactory or nasal tube, known at this stage as the tube
+  of the hypophysis, is directed ventrally, not dorsally.
+
+  The comparison of the diagram of Eurypterus with that of the early stage
+  of Ammocoetes is remarkably close, and immediately suggests not only that
+  the single nose of the former is derived from the corresponding organ in
+  the palæostracan, but that the pituitary body is derived from the
+  concentrated coxal glands, and the lower lip from the metastoma. The
+  further working out of these homologies will be discussed in the next
+  chapter.
+
+  In addition to the evidence of segmentation afforded by the appendages,
+  there are in this region, in Limulus and the scorpion group, three other
+  criteria of segmentation available to us, if from any cause the evidence
+  of appendages fails us. These are--
+
+  1. The number of neuromeres are marked out in this region of the brain
+  more or less plainly, especially in the young animal, just as they are
+  also in the embryo of the vertebrate.
+
+  2. The segmentation is represented here, just as in the mesosomatic
+  region, by two sets of muscle-segments; the one _somatic_, consisting of
+  the segmentally arranged dorso-ventral muscles, the continuation of the
+  group already discussed in connection with the mesosomatic segmentation,
+  and the other _appendicular_ characterized by the tergo-coxal muscles.
+  These latter segmental muscles are especially valuable, for in such forms
+  as Mygale, Phrynus, etc., their presence is indicated externally by
+  markings on the prosomatic carapace, and thus corresponding markings
+  found on fossil carapaces or on dorsal head-shields can be
+  {256}interpreted. These two sets of muscle-segments correspond in the
+  vertebrate to the somatic and splanchnic segmentations.
+
+  3. In the vertebrate the segmentation in this region is indicated by the
+  coelomic or head-cavities, which are cavities formed in the mesoderm of
+  the embryo, the walls of which give origin to the striated muscles of the
+  head. In Limulus corresponding coelomic cavities are found, which are
+  directly comparable with those found in the vertebrate.
+
+
+
+
+{257}CHAPTER VIII
+
+_THE SEGMENTS BELONGING TO THE TRIGEMINAL NERVE-GROUP_
+
+  The prosomatic segments of the vertebrate.--Number of segments belonging
+  to the trigeminal nerve-group.--History of cranial segments.--Eye-muscles
+  and their nerves.--Comparison with the dorso-ventral somatic muscles of
+  the scorpion.--Explanation of the oculomotor nerve and its group of
+  muscles.--Explanation of the trochlearis nerve and its dorsal
+  crossing.--Explanation of the abducens nerve.--Number of segments
+  supplied by the trigeminal nerves.--Evidence of their motor
+  nuclei.--Evidence of their sensory ganglia.--Summary.
+
+
+From the evidence given in the last chapter, combined with that given in
+Chapter IV., the probability of the theory that the trigeminal group of
+nerves of the vertebrate have been derived from the prosomatic group of
+nerves of the invertebrate can be put to the test by the answers to the
+following morphological and anatomical questions:--
+
+1. Do we find in the vertebrate two segmentations in this region
+corresponding to the two segmentations in the branchial region, _i.e._ a
+somatic or dorsal series of segments, and a splanchnic or ventral series of
+segments? The latter would not be branchial, but rather of the nature of
+free tactile appendages; so that it is useless to look for or talk about
+gill-slits, although such appendages, being serially homologous with the
+branchial mesosomatic appendages, would readily give rise to the conception
+of branchial segments.
+
+2. Is there morphological evidence that the trigeminal nerve is not the
+nerve belonging to a single segment, or even to two segments, but is really
+a concentration of at least six, probably seven, segmental nerves?
+
+3. Is there morphological evidence that the oculomotor and trochlear
+nerves, which on all sides are regarded as belonging to the trigeminal
+segments, are not single nerves corresponding each {258}to a single
+segment, but are the somatic motor roots belonging to the same segments as
+those to which the trigeminal supplies the splanchnic roots?
+
+4. Do the mesoderm segments, which give origin to the eye-muscles, and
+therefore do the head-cavities of this region, correspond with the
+trigeminal segments? Considering the concentration of parts in this region
+and the difficulty already presented by the want of numerical agreement
+between the prosomatic appendages and the prosomatic coelomic cavities in
+Limulus, it may very probably be difficult to determine the actual number
+of the mesoderm segments.
+
+5. Is there anatomical evidence that the ganglion of origin of the motor
+part of the trigeminal nerve is not a single ganglion, but a representative
+of many, probably seven?
+
+6. Is there anatomical evidence that the ganglia of origin of the
+oculomotor and trochlear nerves represent many ganglia?
+
+7. Is there any evidence that the organs originally supplied by the motor
+part of the trigeminal nerve are directly comparable with prosomatic
+appendages?
+
+It is agreed on all sides that in this region of the head there is distinct
+evidence of double segmentation, the dorsal mesoderm segments giving origin
+to the eye-muscles, and the ventral segments to the musculature innervated
+by the trigeminal nerve. Originally, according to the scheme of van Wijhe,
+two segments only were recognized, the dorsal parts of which were
+innervated by the IIIrd and IVth nerves respectively. Since his paper, the
+tendency has been to increase the number of segments in this region, as is
+seen in the following sketch, taken from Rabl, of the history of cranial
+segmentation.
+
+
+HISTORY OF CRANIAL SEGMENTATION.
+
+The first attempt to deal with this question was made by Goethe and Oken.
+They considered that the cranial skeleton was composed of a series of
+vertebræ, but as early as 1842 Vogt pointed out that only the occipital
+segments could be reduced to vertebræ. In 1869, Huxley showed that vertebræ
+were insufficient to explain the cranial segmentation, and that the nerves
+must be specially considered. The olfactory and optic nerves he regarded as
+parts of the brain, not true segmental nerves; the rest of the cranial
+nerves {259}were segmental, with special reference to branchial arches and
+clefts, the facial, glossopharyngeal, and separate vagus branches supplying
+the walls of the various branchial pouches. In a similar manner, the supra-
+and infra-maxillary branches of the trigeminal were arranged on each side
+of the mouth, and the inner and outer twigs of the first (ophthalmic)
+branch of the trigeminal on each side of the orbito-nasal cleft, the
+trabecular and the supra-maxillary arches being those on each side of this
+cleft. Thus Huxley considered that there was evidence of a series of pairs
+of ventral arches belonging to the skull, viz. the trabecular and maxillary
+in front of the mouth, the mandibular, hyoid, and branchial arches behind,
+and that the Vth, VIIth, IXth, and Xth nerves were segmental in relation to
+these arches and clefts. Gegenbaur, in 1871 and 1872, considered that the
+branchial arches represented the lower arches of cranial vertebræ, and
+therefore corresponded to lower arches in the spinal region, _i.e._ the
+skull was composed of as many vertebræ as there are branchial arches. These
+vertebræ were confined to the notochordal part of the skull, the prechordal
+part having arisen secondarily from the vertebral part, while the number of
+vertebræ are at least nine, possibly more. The nerves which could be
+homologized with spinal nerves were, he thought, divisible into two great
+groups--(1) the trigeminal group, which included the eye-muscle nerves, the
+facial, and its dorsal branch, the auditory; (2) the vagus group, which
+included the glossopharyngeal and vagus.
+
+Such was the outcome of the purely comparative anatomical work of Huxley
+and Gegenbaur--work that has profoundly influenced all the views of
+segmentation up to the present day.
+
+Now came the investigations of the embryologists, of whom I will take, in
+the first instance, Balfour, whose observations on the embryology of the
+Selachians led him to the conclusion that besides the evidence of
+segmentation to be found in the cranial nerves and in the branchial clefts,
+further evidence was afforded by the existence of head-cavities, the walls
+of which formed muscles just as they do in the spinal region. He came to
+the conclusion that the first head-cavity belonged to one or more pre-oral
+segments, of which the nerves were the oculomotor, trochlearis, and
+possibly abducens; while there were seven post-oral segments, each with its
+head-cavity and its visceral arch, of which the trigeminal, facial,
+glossopharyngeal, and the four parts of the vagus were the respective
+nerves.
+
+{260}Marshall, in 1882, considered that the cranial segments were all
+originally respiratory, and that all the segmental nerves are arranged
+uniformly with respect to a series of gill-clefts which have become
+modified anteriorly and have been lost, to a certain extent, posteriorly.
+He included the olfactory nerves among the segmental nerves, and looked
+upon the olfactory pit, the orbito-nasal lacrymal duct, the mouth, and the
+spiracle as all modified gill-slits, so that he reckoned three pre-oral and
+oral segments belonging to the Ist, IIIrd, IVth, and Vth nerves, and eight
+post-oral segments belonging respectively to the VIIth and VIth nerves, and
+to the IXth nerve, and six segments belonging to the Xth nerve. He pointed
+out that muscles supplied by the oculomotor nerve develop from the outer
+wall of the first head-cavity; not, however, the _obliquus superior_ and
+_rectus externus_, the latter originating probably from the walls of the
+third cavity.
+
+In the same year, 1882, came van Wijhe's well-known paper, in which he
+showed that the mesoderm of the head in the selachian divided into two sets
+of segments, dorsal and ventral; that the dorsal segments were continuous
+with the body-somites, and that the ventral segments formed the lateral
+plates of mesoblast between each of the visceral and branchial pouches. He
+concluded that the dorsal somites were originally nine in number, that each
+was supplied with a ventral nerve-root, in the same way as the somites in
+the trunk, and that to each one a visceral pouch corresponded, whose walls
+were supplied by the corresponding dorsal nerve-root; of these nine
+segments, the ventral nerve-roots of the first three segments were
+respectively the oculomotor, trochlearis, and abducens nerves. The next
+three segments possessed no definable ventral root or muscles, and the
+seventh, eighth, and ninth segments possessed as ventral roots the
+hypoglossal nerve, with its muscular supply. The corresponding dorsal
+nerve-roots were the trigeminal, facial, auditory, glossopharyngeal and
+vagus nerves, the difference between cranial and spinal dorsal roots being
+that the former contain motor fibres.
+
+Ahlborn, in 1884, drew a sharp distinction between the segments of the
+mesoderm and those of the endoderm. The former segmentation he called
+mesomeric, the latter branchiomeric. He considered the two segmentations to
+be independent, and concluded that the branchiomeric was secondary to the
+mesomeric, and therefore not of {261}segmental value. As to the segments of
+the mesoderm in the head, the three hindmost or occipital in Petromyzontidæ
+remain permanently, and correspond to the three last segments in the
+selachian head. Of the anterior mesoderm segments, he considered that there
+were originally six, and that there are six typical eye-muscles in all
+Craniota, which have been compressed into three segments, as in Selachia.
+
+Froriep (1885) showed in sheep-embryos and in chicks that the hypoglossal
+nerve belongs to three proto-vertebræ posterior to the vagus region, which
+were true spinal segments. He therefore modified Gegenbaur's conceptions to
+this extent: that portion of the skull designated by Gegenbaur as vertebral
+must be divided into two parts--a hind or occipital region, which is
+clearly composed of modified vertebræ and is the region of the hypoglossal
+nerves, and a front region, extending from the oculomotor to the
+accessorius nerves, which is characterized segmentally by the formation of
+branchial arches, but in which there is no evidence that proto-vertebræ
+were ever formed. He therefore divides the head-skeleton into three parts--
+
+1. Gegenbaur's evertebral part--the region of the olfactory and optic
+nerves--which cannot be referred to any metameric segmentation.
+
+2. The pseudo-vertebral, pre-spinal, or branchial part, clearly shown to be
+segmented from the consideration of the nerves and branchial arches, but
+not referable to proto-vertebræ--the region of the trigeminal and vagus
+nerves.
+
+3. The vertebral spinal part--the region of the hypoglossal nerves.
+
+He further showed that the ganglia of the specially branchial nerves, the
+facial, glossopharyngeal, and vagus, are at one stage in connection with
+the epidermis, so that these parts of the epidermis represent sense-organs
+which do not develop; these organs probably belonged to the lateral line
+system. As the connection takes place at the dorsal edge of the gill-slits,
+they may also be called rudimentary branchial sense-organs.
+
+Since this paper of Froriep's, it has been generally recognized, and
+Gegenbaur has accepted Froriep's view, that the three hindmost metameres,
+which distinctly show the characteristics of vertebræ, belong to the spinal
+and not to the cranial region, so that the metameric segmentation of the
+cranial region proper has become {262}more and more associated with the
+branchial segmentation. Froriep's discovery of the rudimentary branchial
+sense-organs as a factor in the segmentation question has led Beard to the
+conclusion that the olfactory and auditory organs represent in a permanent
+form two of these rudimentary branchial sense-organs. He therefore includes
+both the olfactory and auditory nerves in his list of cranial segmental
+nerves, and makes eleven cranial branchial segments in front of the spinal
+segments represented by the hypoglossal.
+
+A still larger number of cranial segments is supposed to exist, according
+to the researches of Dohrn and Killian, in the embryos of _Torpedo
+ocellata_. The former, holding to the view that vertebrates arose from
+annelids, considered that the head was formed of a series of metameres, to
+each one of which a mesoderm-segment, a gill-arch, a gill-cleft, a
+segmental nerve and vessel belonged. He found in the front head-region of a
+Torpedo embryo, corresponding to van Wijhe's first four somites, no less
+than twelve to fifteen mesoderm segments, and concluded, therefore, that
+the eye-muscle nerves, especially the oculomotor, represented many
+segmental nerves, and were not the nerves of single segments; so, also,
+that the inferior maxillary part of the trigeminal and the hyoid nerve of
+the facial are probably not single nerves, but a fusion of several. Killian
+comes to much the same conclusion as Dohrn, for he finds seventeen to
+eighteen separate mesoderm segments in the head, of which twelve belong to
+the trigeminal and facial region.
+
+Since Rabl's paper, a number of papers have appeared, especially from
+America, dealing with yet another criterion of the original segmentation of
+the head, viz. a series of divisions of the central nervous system itself,
+which are seen at a very early stage of development, and are called
+neuromeres; the divisions in the cranial region being known as
+encephalomeres, and those of the spinal region as myomeres. Locy's paper
+has especially brought these divisions into prominence as a factor in the
+question of segmentation. They are essentially segments of the epiblast and
+not of the mesoblast; they are conspicuous in very early stages, and appear
+to be in relation with the cranial nerves, according to Locy. He recognizes
+in _Squalus acanthias_, in front of the spino-occipital region, fourteen
+pairs of such encephalomeres and a median unsegmented termination, which
+may represent one more pair fused in the middle line, making at least
+fifteen. He distributes these fifteen segments as follows: {263}fore-brain
+three and unsegmented termination, mid-brain two, and hind-brain nine.
+
+Again, Kupffer, in his recent papers on the embryology of Ammocoetes,
+asserts that especial information as to the number of primitive segments is
+afforded by the appearance in the early stages of a series of epibranchial
+ganglia in connection with the cranial nerves, which remain permanently in
+the case of the vagus nerves, but disappear in the case of pro-otic nerves.
+He considers that the evidence points to the number of segments in the mid-
+and hind-brain region as being primitively fifteen, viz. six segments
+belonging to the trigeminal and abducens group, three segments belonging
+respectively to the facial, auditory, and glossopharyngeal, and six to the
+vagus.
+
+From this sketch we see that the modern tendency is to make six segments at
+least out of the region of the trigeminal nerves rather than two. In this
+region, as already mentioned, the evidence of segmentation is based more
+clearly on the somatic than on the splanchnic segments. We ought,
+therefore, in the first place, to consider the teaching of the eye-muscles
+and their nerves and the coelomic cavities in connection with them, and see
+whether the hypothesis that such muscles represent the original
+dorso-ventral somatic muscles of the palæostracan ancestor is in harmony
+with and explains the facts of modern research.
+
+
+EYE-MUSCLES AND THEIR NERVES.
+
+The only universally recognized somatic nerves belonging to these segments
+which exist in the adult are the nerves to the eye-muscles, of which,
+according to van Wijhe, the oculomotor is the nerve of the 1st segment, the
+trochlearis of the 2nd, and the abducens of the 3rd; while the nerves and
+muscles belonging to the 4th and 5th segments, _i.e._ the 2nd facial and
+glossopharyngeal segments respectively, show only the merest rudiments, and
+do not exist in the adult. One significant fact appears in this statement
+of van Wijhe, and is accepted by all those who follow him, viz. that the
+oculomotor nerve has equal segmental value with the trochlearis and the
+abducens, although it supplies a number of muscles, each of which, on the
+face of it, has the same anatomical value as the superior oblique or
+external rectus. Dohrn alone, as far as I know, as already pointed out,
+insists upon the multiple character of the oculomotor nerve.
+
+{264}As far as the anatomist is concerned, the evidence is becoming clearer
+and clearer that the nucleus of the IIIrd nerve is a composite ganglion
+composed of a number of nuclei, each similar to that of the trochlearis, so
+that if the trochlearis nucleus is a segmental motor nucleus, then the
+oculomotor nucleus is a combined nucleus belonging to at least four
+segmental nerves, each of which has the same value as that of the
+trochlearis.
+
+The investigations of a number of anatomists, among whom may be mentioned
+Gudden, Obersteiner, Edinger, Kölliker, Gehuchten, all lead directly to the
+conclusion that this oculomotor nucleus is composed of a number of separate
+nuclei, of which the most anterior as also the Edinger-Westphal nucleus
+contains small cells, while the others contain large cells. Thus Edinger
+divides the origin of the oculomotor nerve into a small-celled anterior
+part and a larger posterior part, of which the cells are larger and
+distinctly arranged in three groups--(1) dorsal, (2) ventral, and (3)
+median. Between the anterior and posterior groups lies the Edinger-Westphal
+nucleus, which is small-celled; naturally, the large-celled group is that
+which gives origin to the motor nerves of the eye-muscles, the small-celled
+being possibly concerned with the motor nerves of the pupillary and ciliary
+muscles. I may mention that Kölliker considers that the anterior lateral
+nucleus has nothing to do with the oculomotor nerve, but is a group of
+cells in which the fibres of the posterior longitudinal bundle and of the
+deep part of the posterior commissure terminate.
+
+These conclusions of Edinger are the outcome of work done in his laboratory
+by Perlia, who says that in new-born animals the nucleus of origin of the
+oculomotor nerve is made up of a number of groups quite distinct from each
+other, each group being of the same character as that of the trochlearis.
+He finds the same arrangement in various mammals and birds. Further, he
+finds that some of the fibres arise from the nucleus of the opposite side,
+thus crossing, as in the trochlearis; these crossing fibres belong to the
+most posterior of the dorsal group of nuclei, _i.e._ to the nerve to the
+inferior oblique muscle.
+
+The evidence, therefore, points to the conclusion that the oculomotor
+nucleus is a multiple nucleus, each part of which gives origin to one of
+the nerves of one of the eye-muscles.
+
+Edinger says that such an array of clinical observations exists, {265}and
+of facts derived from post-mortem dissections, that one may venture to
+designate the portion of the nucleus from which the innervation of each
+individual ocular muscle comes. He gives Starr's table, the latest of these
+numerous attempts, begun by Pick. According to Starr, the nuclei of the
+nerves to the individual muscles are arranged from before backward, thus--
+
+  _m. sphincter iridis._    _m. ciliaris._
+  _m. levator palpebræ._    _m. rectus internus._
+  _m. rectus superior._     _m. rectus inferior._
+  _m. obliquus inferior._
+
+Further, the evidence of the well-known physiological experiments of Hensen
+and Völckers that the terminal branches of the oculomotor nerve arise from
+a series of segments of the nucleus, arranged more or less one behind the
+other in a longitudinal row, leads them to the conclusion that the nuclei
+of origin are arranged as follows, proceeding from head to tail:--
+
+  Nearest brain.   1. _m. ciliaris._
+                   2. _m. sphincter iridis._
+                   3. _m. rectus internus._
+                   4. _m. rectus superior._
+                   5. _m. levator palpebræ._
+                   6. _m. rectus inferior._
+  Most posterior.  7. _m. obliquus inferior._
+
+It is instructive to compare this arrangement of Hensen and Völckers with
+the arrangement of the origin of these muscles from the premandibular
+cavity as given by Miss Platt.
+
+Thus she states that the most posterior part of the premandibular cavity is
+cut off so as to form a separate cavity, resembling, except in position,
+the anterior cavity; this separate, most posterior part gives origin to the
+inferior oblique muscle. She then goes on to describe how the dorsal wall
+of the remainder of the premandibular cavity becomes thickened, to form
+posteriorly the rudiment of the inferior rectus and anteriorly the
+rudiments of the superior and internal recti, a slight depression in the
+wall of the cavity separating these rudiments. The internal rectus is the
+more median of the two anterior muscles. In other words, her evidence
+points not only to a fusion of somites to form the premandibular cavity,
+but also to the arrangement of these somites as follows, from head to tail:
+(1) internal rectus, (2) superior rectus, (3) inferior rectus, (4) inferior
+{266}oblique--an order precisely the same as that of Hensen and Völckers,
+and of Starr.
+
+I conclude, from the agreement between the anatomical, physiological, and
+morphological evidence, that the IIIrd and IVth nerves contain the motor
+somatic nerves belonging to the same segments as the motor trigeminal, in
+other words, to the prosomatic segments, so that the eye-muscles,
+innervated by III. and IV., represent segmental muscles belonging to the
+prosoma. Further, I conclude that originally there were seven prosomatic
+segments, the first of which is represented by the anterior cavity
+described by Miss Platt, and does not form any permanent muscles; that the
+next four belong to the premandibular cavity, and the muscles formed are
+the superior rectus, internal rectus, inferior rectus, and inferior
+oblique; and that the last two belong to the mandibular cavity, the muscles
+formed being Miss Platt's mandibular muscle and the superior oblique. It
+is, to say the least of it, a striking coincidence that such an arrangement
+of the coelomic cavities as here given should be so closely mimicked by the
+arrangement in the prosomatic region of Limulus as already mentioned; it
+suggests inevitably that the head-cavities of the vertebrate are nothing
+more than the prosomatic and mesosomatic segmental coelomic cavities, as
+found in animals such as Limulus. In the table on p. 253, I have inserted
+the segments in the vertebrate for comparison with those of Limulus.
+
+Before we can come to any conclusion as to the original position of these
+eye-muscles, it is necessary to consider the VIth nerve and the external
+rectus muscle. This nerve and this muscle belong to van Wijhe's 4th
+segment. The muscle is, therefore, the somatic segmental muscle belonging
+to the same segment as the facial and is, in fact, a segmental muscle
+belonging not to the prosoma, but to the mesosoma. Neal comes to the
+conclusion that the existing abducens is the only root which remains
+permanent among a whole series of corresponding ventral roots belonging to
+the opisthotic segments, and further points out that the external rectus
+was originally an opisthotic muscle which has taken up a pro-otic position,
+or, translating this statement into the language of Limulus, etc., it is a
+mesosomatic muscle which has taken up a prosomatic position.
+
+There is, however, another muscle--the _Retractor oculi_--belonging to the
+same group which is innervated by the VIth nerve. Quite recently Edgeworth
+has shown that in birds and reptiles this muscle {267}belongs to the hyoid
+segment; so that in this respect also the hyoid segment proclaims its
+double nature.
+
+With respect to the external rectus muscle, Miss Platt has shown that the
+mandibular muscle is formed close alongside the external rectus, so that
+the two are in close relationship as long as the former exists.
+
+Further, as already mentioned, the eye-muscles in Ammocoetes must be
+considered by themselves; they do not belong in structure or position to
+the longitudinal somatic muscles innervated by the spinal nerves; their
+structure is not the same as that of the tubular constrictor or branchial
+muscles, but resembles that structure somewhat; their position is
+dorso-ventral rather than longitudinal; they may be looked upon as a
+primitive type of somatic muscles segmentally arranged, the direction of
+which was dorso-ventral.
+
+Anderson also has shown that the time of medullation of the nerves
+supplying these muscles is much earlier than that of the nerves belonging
+to the somatic trunk-muscles, their medullation taking place at the same
+time as that of the motor nerves supplying the striated visceral muscles;
+and Sherrington has observed that these muscles do not possess
+muscle-spindles, while all somatic trunk-muscles do. Both these
+observations are strong confirmation of the view that the eye-muscles must
+be classified in a different category to the ordinary somatic trunk muscle
+group.
+
+What, then, is the interpretation of these various embryological and
+anatomical facts?
+
+Remembering the tripartite division of each segmental nerve-group in
+Limulus into (1) dorsal or sensory somatic nerve, (2) appendage-nerve, and
+(3) ventral somatic nerve, I venture to suggest that the three nerves--the
+_oculomotorius_, the _trochlearis_, and the _abducens_--represent the
+ventral somatic nerves of the prosoma, and partly also of the mesosoma;
+that they are nerves, therefore, which may have originally contained
+sensory fibres, and which still contain the sensory fibres of the
+eye-muscles themselves, as stated by Sherrington. According to this
+suggestion, the eye-muscles are the sole survivors of the segmental
+dorso-ventral somatic muscles, so characteristic of the group from which I
+imagine the vertebrates to have sprung. In the mesosomatic region the
+dorso-ventral muscles which were retained were those of the appendages and
+not of the mesosoma itself, because the presumed ancestor breathed after
+the fashion of the water-breathing Limulus, by means of the dorso-ventral
+muscles of its {268}branchial appendages, and not after the fashion of the
+air-breathing scorpion, by means of the dorso-ventral muscles of the
+mesosoma. The only mesosomatic dorso-ventral muscles which were retained
+were those of the foremost mesosomatic segments, _i.e._ those supplied by
+the VIth nerve, which were preserved owing to their having taken on a
+prosomatic position and become utilized to assist in the movements of the
+lateral eyes.
+
+Let us turn now to the consideration of the corresponding musculature in
+Limulus and in the scorpion group. These muscles constitute the markedly
+segmental muscles to which I have given the name 'dorso-ventral somatic
+muscles.' They are most markedly segmental in the mesosomatic region, both
+in Limulus and in Scorpio, each mesosomatic segment possessing a single
+pair of these vertical mesosomatic muscles, as Benham calls them (_cf._
+Fig. 58 (_Dv._)). In the prosomatic region the corresponding muscles are
+not so clearly defined in Limulus; they are apparently attached to the
+plastron forming the group of plastro-tergal muscles. From Benham's
+description it is sufficiently evident that they formed originally a single
+pair to each prosomatic segment.
+
+In Scorpio, according to Miss Beck, the dorso-ventral prosomatic muscles
+are situated near the middle line on each side and form the following
+well-marked series of pairs of muscles, shown in Fig. 110, A, taken from
+her paper, and thus described by her:--
+
+1. The dorso-cheliceral-sternal muscle (61) is the most anterior of the
+dorso-ventral muscles. It is very small, and is attached to the carapace
+near the median line anteriorly to the central eyes.
+
+2. The median dorso-preoral-entosclerite muscle (62) is a large muscle,
+between which and its fellow of the opposite side the eyes are situated. It
+is attached dorsally to the carapace and ventrally to the pre-oral
+entosclerite.
+
+3. The anterior dorso-plastron muscle (63) is attached dorsally to the
+carapace in the middle line, being joined to its fellow of the opposite
+side. They separate, and are attached ventrally to the plastron. Through
+the arch thus formed the alimentary canal and the dorsal vessel pass.
+
+4. The median dorso-plastron muscle (64) is attached dorsally to the
+posterior part of the carapace. It runs forward on the anterior surface of
+the posterior flap of the plastron to the body of the plastron, to which it
+is attached.
+
+{269}[Illustration] A.
+
+DORSO-VENTRAL MUSCLES ON CARAPACE OF SCORPION. (From MISS BECK.)
+
+
+[Illustration] B.
+
+SIMILAR MUSCLES ON CARAPACE OF EURYPTERUS.
+
+
+[Illustration] C.
+
+SIMILAR MUSCLES ON HEAD-SHIELD OF A CEPHALASPID.
+
+_l.e._, lateral eyes; _c.e._, central eyes; _Fro._, narial opening.
+
+62-65 refer to Miss Beck's catalogue of the scorpion muscles.
+
+
+FIG. 110.
+
+
+{270}To these may be added, owing to its attachment to the plastron,
+
+5. The posterior dorso-plastron muscle (65). This is the first of the
+dorso-ventral muscles attached to the mesosomatic tergites, being attached
+to the tergite of the first segment of the mesosoma.
+
+This muscle is of interest, in connection with the prosomatic dorso-ventral
+muscles, because it is attached to the plastron, and runs a course in close
+contact with the muscle (64), the two muscles being attached dorsally close
+together, on each side of the middle line, the one at the very posterior
+edge of the prosomatic carapace, and the other at the very anterior edge of
+the mesosomatic carapace.
+
+Taking these muscles separately into consideration, it may be remarked with
+respect to (61) that the cheliceral segment in its paired dorso-ventral
+muscles, as in its tergo-coxal muscles, takes up a separate position
+isolated from the rest of the prosomatic segments.
+
+Next comes (62) the median dorso-preoral-entosclerite muscle, which is
+strikingly different from all the other dorso-ventral muscles in its large
+size and the extent of its attachment to the dorsal carapace, according to
+Miss Beck's figures. The reason of its large size is clearly seen upon
+dissection of the muscles in _Buthus_, for I find that, strictly speaking,
+it is not a single muscle, but is composed of a series of muscle-bundles,
+separated from each other by connective tissue. There are certainly three
+separate muscles included in this large muscle, which are attached in a
+distinct series along the pre-oral entosclerite, and present the appearance
+given in Fig. 110, A, at their attachment to the prosomatic carapace. Of
+this muscle-group the most anterior and the most posterior bundle are
+distinctly separate muscles; I am not, however, clear whether the middle
+bundle represents one or two muscles.
+
+This division of Miss Beck's muscle (62) into three or four muscles brings
+the prosomatic region of the scorpion into line with the mesosomatic, and
+enables us to feel sure that a single pair of dorso-ventral somatic muscles
+belongs to each prosomatic segment just as to each mesosomatic, and,
+conversely, that each such single pair of muscles possesses segmental value
+in this region as much as in the mesosomatic.
+
+It is very striking to see how in all the Scorpionidæ, in which the two
+median eyes are the principal eyes, this muscle group (62) on the two sides
+closely surrounds these two eyes, so that with a fixed {271}pre-oral
+entosclerite, a slight movement of the eyes, laterally or anteriorly, owing
+to the flexibility of the carapace, might result as the consequence of
+their contraction. But this cannot be the main object of these muscles. The
+pre-oral entosclerite is firmly fixed to the camerostome, as is seen in
+Fig. 94, _pr. ent._, so that the main object of these muscles is, as Huxley
+has pointed out, the movement of this organ.
+
+In order to avoid repetition of the long name given to this muscle group
+(62) by Miss Beck, because of their position, and for other reasons which
+will appear in the sequel, I will call this group of muscles the group of
+recti muscles. These recti muscles belong clearly to the segments posterior
+to the first prosomatic or cheliceral segment, and represent certainly
+three, probably four, of these segments, _i.e._ belong to the segments
+corresponding to the second, third, fourth, and fifth prosomatic locomotor
+appendages--the endognaths of the old Eurypterids.
+
+The next pair of muscles is the pair of anterior dorso-plastron muscles
+(63). This muscle-pair evidently belongs to a segment posterior to the
+segments represented by the group already discussed, and belongs,
+therefore, in all probability to the same segment as the sixth pair of
+prosomatic appendages--the ectognaths of the old Eurypterids. This can be
+settled by considering either the nerve-supply or the embryological
+development. In the Eurypteridæ it seems most highly probable that the
+dorso-ventral muscles of each half of the segments belonging to the
+endognaths should be compressed together and separate from the
+dorso-ventral muscle belonging to the ectognathal segment, on account of
+the evident concentration and small size of the endognathal segments in
+contradistinction to the separateness and large size of the ectognathal
+segment.
+
+The striking peculiarity of this muscle-pair, which distinguishes it from
+all other muscles in the scorpion, is the common attachment of the muscles
+of the two sides in the mid-dorsal line, so that the pair of muscles forms
+an arch through which the alimentary canal and dorsal blood-vessel pass.
+
+The same dorso-ventral muscles are present in _Phrynus_, and in this animal
+the fibres of this pair of muscles (63) actually interlace before the
+attachment to the prosomatic carapace, so that the attachment of the muscle
+on each side overpasses the mid-dorsal line, and a true crossing occurs. In
+Fig. 108 the position of this pair of {272}muscles is shown just
+posteriorly to the brain-mass. This muscle I will call the oblique muscle.
+
+Finally we come to the muscles (64) and (65), the median and posterior
+dorso-plastron muscles, which run close together. Both muscles are attached
+to the plastron, and, therefore, to that extent belong to the prosomatic
+region; they are attached dorsally close to the junction of the prosoma and
+mesosoma. This position of the first mesosomatic dorso-ventral muscle
+belonging to the opercular segment may be compared with the position of the
+first mesosomatic dorso-ventral muscle in Limulus which has become attached
+to the prosomatic carapace; in both cases we see an indication that the
+foremost pair of mesosomatic dorso-ventral somatic muscles tend to take up
+a prosomatic position.
+
+As to the pair of small muscles (64), I believe that they represent the
+dorso-ventral muscles of the seventh prosomatic segment (if the pair of
+muscles (63) belongs to the segment of the sixth locomotor prosomatic
+appendages), _i.e._ they belong to the chilarial segment or metastoma.
+
+I desire to draw especial attention to the fact that the dorso-ventral
+muscle (64), which represents the seventh segment, always runs close
+alongside the dorso-ventral muscle (65), which represents the first
+mesosomatic or opercular segment.
+
+The comparison, then, of these two sets of facts leads to the following
+conclusions:--
+
+The foremost prosomatic or trigeminal segment stood separate and apart,
+being situated most anteriorly; the musculature of this segment does not
+develop, so that the only evidence of its presence is given by the anterior
+coelomic cavity. This corresponds, according to my scheme, with the first
+or anterior coelomic cavity of Limulus, and therefore represents, as far as
+the prosomatic appendages are concerned, the first prosomatic
+appendage-pair, or the cheliceræ; the appendage-muscles being the muscles
+of the cheliceræ, and the dorso-ventral somatic muscles the pair of
+dorso-cheliceral sternal muscles (61) in the scorpion. Both these sets of
+muscles, therefore, dwindle and disappear in the vertebrate.
+
+Then came four segments fused together to form the premandibular segment,
+the characteristic of which is the apparent non-formation of any permanent
+musculature from the ventral mesoderm-segments, and the formation of the
+eye-muscles innervated by the {273}oculomotor nerve from the dorsal
+mesoderm segments. These four segments have been so fused together that van
+Wijhe looked upon them as a single segment, and the premandibular cavity as
+the cavity of a single segment. They represent, according to my scheme, the
+segments belonging to the endognaths, _i.e._ the second, third, fourth,
+fifth pairs of prosomatic appendages; the premandibular cavity, therefore,
+represents the second coelomic cavity in Limulus, which, according to
+Kishinouye, is the sole representative of the coelomic cavities of the
+second, third, fourth, fifth prosomatic segments. The muscles derived from
+the ventral mesoderm-segments represent the muscles of these appendages,
+which therefore dwindle and disappear in the vertebrate, with the possible
+exception of the muscles innervated by the descending root of the
+trigeminal. The muscles derived from the dorsal mesoderm-segments, _i.e._
+the eye-muscles supplied by the oculomotor nerve, represent the
+dorso-ventral somatic muscles of these four segments, muscles which are
+represented in the scorpion by the recti group of muscles, _i.e._ the
+median dorso-preoral-entosclerite muscles (62).
+
+Then came two segments, the mandibular, in which muscles are formed both
+from the ventral and from the dorsal mesoderm-segments. From the former
+arose the main mass of muscles innervated by the motor root of the
+trigeminal, from the latter the superior oblique muscle and the mandibular
+muscle of Miss Platt, of which the former alone survives in the adult
+condition. These two segments are looked upon as a single segment by van
+Wijhe, of which the mandibular cavity is the coelomic cavity. They
+represent, according to my scheme, the segments belonging to the sixth pair
+of prosomatic appendages or ectognaths, and the seventh pair, _i.e._ the
+chilaria or metastoma.
+
+The first part, then, of the mandibular cavity represents the third
+coelomic cavity in Limulus and the muscles derived from the ventral
+mesoderm, in all probability the muscles of the tongue in the lamprey
+(_cf._ Chap. IX.), which represents the ectognaths or sixth pair of
+prosomatic appendages, while the muscles derived from the dorsal mesoderm,
+_i.e._ the superior oblique muscles, represent the dorso-ventral somatic
+muscles of this segment, muscles which are represented in the scorpion
+group by the pair of anterior dorso-plastron or oblique muscles (63).
+
+The second part of the mandibular cavity represents the 4th {274}coelomic
+cavity in Limulus and the muscles derived from the ventral mesoderm, in all
+probability the muscles of the lower lip in the lamprey (_cf._ Chap. IX.),
+which represents the metastoma; while the muscles derived from the dorsal
+mesoderm, _i.e._ Miss Platt's pair of mandibular muscles, represent the
+dorso-ventral somatic muscles of this segment, muscles which are
+represented in the scorpion group by the pair of median dorso-plastron
+muscles (64).
+
+In connection with this last pair of muscles we find that the external
+rectus in the vertebrate represents the first dorso-ventral mesosomatic
+muscle in the scorpion, _i.e._ the posterior dorso-plastron muscle (65),
+and, as already mentioned (p. 267), that it always lies closely alongside
+the mandibular muscle, just as in the scorpion group muscle (65) always
+lies alongside muscle (64).
+
+In the invertebrate as well as in the vertebrate this muscle is a
+mesosomatic muscle which has taken up a prosomatic position.
+
+The question naturally arises, what explanation can be given of the fact
+that these dorso-ventral muscles attached on each side of the mid-dorsal
+line to the prosomatic carapace became converted into the muscles moving
+the eyeballs of the two lateral eyes? An explanation which must take into
+account not only the isolated position of the abducens nerve, but also the
+extraordinary course of the trochlearis. The natural and straightforward
+answer to this question appears to me quite satisfactory, and I therefore
+venture to commend it to my readers.
+
+I have argued the case out to myself as follows: The lateral eyes must have
+been originally situated externally to the group of muscles innervated by
+the oculomotor nerve, for a sheet of muscle representing the superior
+_internal_ and inferior rectus muscles could only wrap round the internal
+surface of each lateral eye; _i.e._ the arrangement of the muscle-sheet, as
+in the scorpion, about two median eyes, is in the wrong position, for if
+those two eyes, which are the main eyes in the scorpion, were to move
+outwards to become two lateral eyes, then such a muscle-group would form a
+superior _external_ and inferior rectus group. The evidence, however, of
+Eurypterus and similar forms is to the effect that the lateral eyes became
+big and the median eyes insignificant and degenerate. If, then, with the
+degeneration of the one and the increasing importance of the other, these
+lateral eyes came near the middle line, then the muscular group (62), which
+I have called the recti group, would naturally be pressed into their
+{275}service, and would form an internal and not an external group of
+eye-muscles.
+
+In Fig. 110, A, taken from Miss Beck's paper, I have shown the relative
+position of the eyes and the segmental dorso-ventral prosomatic muscles on
+the carapace of the scorpion. In Fig. 110, B, I have drawn the prosomatic
+carapace of _Eurypterus Scouleri_, taken from Woodward's paper, with the
+eyes as represented there; in this I have inserted the segmental
+dorso-ventral muscles as met with in the scorpion, thereby demonstrating
+how, with the degeneration of the median eyes and the large size of the
+lateral eyes, the recti muscles of the scorpion would approach the position
+of an internal recti group to the lateral eyes, and so give origin to the
+group of muscles innervated by the oculomotor nerve. In the Eurypterus
+these large eyes are large single eyes, not separate ocelli, as in the
+scorpion.
+
+All, then, that is required is that in the first formed fishes, which still
+possessed the dorso-ventral muscles of their Eurypterid ancestors, the
+lateral eyes should be the important organs of sight, large and near the
+mid-dorsal line. Such, indeed, is found to be the case. In amongst the
+masses of Eurypterids found in the upper Silurian deposits at Oesel, as
+described by Rohon, numbers of the most ancient forms of fish are found
+belonging to the genera Thyestes and Tremataspis. The nature of the dorsal
+head-shields of these fishes is shown in Fig. 14, which represents the
+dorsal head-shield of _Thyestes verrucosus_, and Fig. 111 that of
+_Tremataspis Mickwitzi_. They show how the two lateral eyes were situated
+close on each side of the mid-dorsal line in these Eurypterus-like fishes,
+in the very position where they must have been if the eye-muscles were
+derived from the dorso-ventral somatic muscles of a Eurypterid ancestor.
+
+[Illustration: FIG. 111.--DORSAL HEAD-SHIELD OF _Tremataspis Mickwitzi_.
+(From ROHON.)
+
+_Fro._, narial opening; _l.e._, lateral eyes; _gl._, glabellum plate over
+brain; _Occ._, occipital spine.]
+
+In Lankester's words, one of the characteristics of the Osteostraci
+(Cephalaspis, Auchenaspis, etc.), as distinguished from the Heterostraci
+(Pteraspis), are the large orbits placed near the centre of the shield. The
+apparent exception of Thyestes mentioned by him is no {276}exception, for
+orbits of the same character have since been discovered, as is seen in
+Rohon's figure (Fig. 14). In Fig. 110, C, I give an outline of the frontal
+part of the head-shield of a Cephalaspid, in which I have drawn the
+eye-muscles as in the other two figures.
+
+Although all the members of the Osteostraci possess large lateral eyes
+towards the centre of the head-shield, the other group of ancient fishes,
+the Heterostraci, are characterized by the presence of lateral eyes far
+apart, situated on the margin of the head-shield on each side (_cf._ Fig.
+142, _o_, p. 350).
+
+So, also, on the invertebrate side, the lateral eyes of Pterygotus and
+Slimonia are situated on the margin of the prosomatic carapace, while those
+of Eurypterus and Stylonurus are situated much nearer the middle line of
+the prosomatic carapace.
+
+Next comes the question of the superior oblique muscle and the trochlearis
+nerve. Why does this nerve (_n.IV._ in Fig. 106, C and D) alone of all the
+nerves in the body take the peculiar position it always does take? The only
+suggestion that I know of which sounds reasonable and worth consideration
+is that put forward by Fürbringer, which is an elaboration of the original
+suggestion of Hoffmann. Hoffmann suggested in 1889 that the trochlearis
+nerve represented originally a nerve for a protecting organ of the pineal
+eye, which became secondarily a motor nerve for the lateral eye as the
+pineal eye degenerated. Fürbringer differs from Hoffmann in that he
+considers that the nerve was originally a motor nerve, and was not
+transformed from sensory to motor, yet thinks Hoffmann's suggestion is in
+the right direction.
+
+He points out that the crossing of the trochlearis is not a crossing of
+fibres between two centres in the central nervous system, but may be
+explained by the shifting of the peripheral organ, _i.e._ the muscle, from
+one side to the other, and the nerve following this shift. Consequently,
+says Fürbringer, the course of the nerve indicates the original position of
+the muscle, and therefore he imagines that the ancestor of the superior
+oblique muscle was a muscle the fibres of which were attached in the
+mid-dorsal line, and interlaced with those of the other side, the two
+muscles thus forming an arch through which the nervous system with its
+central canal passed. Then, for the sake of getting a more efficient pull,
+the crossing muscle-fibres became more definitely attached to the opposite
+side of the middle line, and finally obtained a new attachment on the
+opposite side, with the {277}obliteration of the muscular arch; the nerve
+on each side, following the shifts of the muscle, naturally took up the
+position of the original muscular arch, and so formed the trochlear nerve,
+with its dorsal crossing. This explanation of Fürbringer's was associated
+by him with movements of the median pineal eyes, the length of their nerve,
+according to him, even yet indicating their previous mobility. This
+assumption is not, it seems to me, necessary. The length of the nerve is
+certainly no indication of mobility, for in Limulus and the scorpion group
+the nerve to each median eye is remarkably long, yet these eyes are
+immovably fixed in the carapace. All that is required is a pair of
+dorso-ventral muscles belonging to the segment immediately following the
+group of segments represented by the oculomotor nerves, the fibres of which
+should cross the mid-dorsal line at their attachment; for, seeing that the
+lateral eyes were originally so near this position, it follows that such
+muscles might form part of the muscular group belonging to the lateral eye
+without having previously moved the pineal eyes. In fact, Fürbringer's
+explanation requires as starting-point that the pair of muscles which
+ultimately become the superior oblique should have the exact position of
+the pair of dorso-ventral muscles in the scorpion, called by Miss Beck the
+anterior dorso-plastron muscles (63), which I have named the oblique
+muscles. Here, and here only, do we find an interlacement, across the
+mid-dorsal line, of the fibres of attachment of the muscles on the two
+sides, in consequence of which this pair of muscles is described by her as
+forming an arch encircling the alimentary canal and dorsal vessel. If,
+then, as I have previously argued, the primitive plastron formed a pair of
+trabeculæ, and the nervous system grew round the alimentary canal, such an
+arch would encircle the tubular central nervous system of the vertebrate.
+
+Still more striking is this pair of muscles (63) in Phrynus (Fig. 108),
+where we see how the arch formed by them almost touches the posterior
+extremity of the supra-oesophageal brain-mass, crossing, therefore, over
+the beginning of the stomach region of the animal. The angle formed by the
+arch is much more obtuse than that formed in Scorpio, so that an actual
+crossing of the muscle-fibres has taken place at the point of attachment to
+the carapace. Also, only the part nearest the carapace is muscular, the
+rest forming a long tendinous prolongation of the plastron wall (the
+primordial cranium), as seen in the figure.
+
+{278}This muscle-pair is, as it should be, the pair of dorso-ventral
+muscles belonging to the segment immediately following on the group of
+segments represented by the recti muscles, _i.e._ according to previous
+argument, the segment belonging to the sixth pair of locomotor appendages
+or ectognaths; a muscle, therefore, which would arise in the vertebrate
+from the mandibular, and not from the premandibular cavity. A similar
+muscle probably existed in Eurypterus (_M.obl._ in Fig. 106, B), and, as in
+the case of the formation of the oculomotor group, derived from the recti
+group of the scorpion, would form the commencement of the superior oblique
+muscle in Thyestes and Tremataspis.
+
+[Illustration: FIG. 112.--A, DIAGRAM OF POSITION OF OBLIQUE MUSCLE IN
+SCORPION; B, DIAGRAM OF TRANSITION STAGE; C, DIAGRAM OF SUPERIOR OBLIQUE
+MUSCLE IN VERTEBRATE.
+
+_l.e._, lateral eyes; _c.e._, central eyes; _C.N._, central nervous system;
+_Al._, alimentary canal; _c._, _aqueductus Sylvii_.]
+
+It is instructive to notice that the original position of attachment of
+this muscle is naturally posterior to that of the oculomotor group of
+muscles, and that Fürbringer, in his description of the eye-muscles of
+Petromyzon, asserts that this muscle in this primitive vertebrate {279}form
+is not attached as in other vertebrates, but is posterior to the other
+muscles, so that he calls it the posterior rather than the superior
+oblique. The nature of the change by which the muscle known in the scorpion
+as the anterior dorso-plastron muscle (63) was probably converted into the
+superior oblique muscle of the vertebrate, is represented in the drawings
+Fig. 112, in which also are indicated the dwindling of the median eyes, and
+the progressive superiority of the lateral eyes, as well as the
+transformation of the recti muscle-group of the scorpion into the muscles
+supplied by the oculomotor nerve of the vertebrate.
+
+With respect to the external rectus muscle, it follows naturally that if
+the muscles (64) and (65) are to follow suit with the rest of the group and
+become attached to the lateral eyes, they must take up an external
+position. These two muscles, which always run together, as seen in Fig.
+110, A, the one belonging to the prosoma and the other to the mesosoma, are
+represented by the mandibular muscle of Miss Platt and the external rectus,
+the former derived from the walls of the last pro-otic head-cavity, the
+latter from the foremost of the opisthotic head-cavities.
+
+Such, then, is the simple explanation of the origin of the eye-muscles
+which follows from my theory, and we see that the successive alterations of
+the position of the orbit, and, therefore, of the globe of the eye with its
+muscles, as we pass from Thyestes to man, is the natural consequence of the
+growth of the frontal bone, _i.e._ of the brain.
+
+
+THE TRIGEMINAL NERVES AND THE MUSCLES SUPPLIED BY THEM.
+
+Turning now to the evidence as to the number of ventral segments, _i.e._
+the motor and sensory supply to the prosomatic appendages afforded by the
+trigeminal nerve, we must, I think, come to the same conclusion as Dohrn,
+viz. that if there were originally seven dorsal or somatic segments in this
+region represented by: 1, Anterior cavity, muscle lost; 2, 3, 4, 5, muscles
+of the premandibular cavity, _sup. rectus_, _inf. rectus_, _int. rectus_,
+_inf. oblique_, supplied by IIIrd nerve; 6, 7, muscles of the mandibular
+cavity, _sup. oblique_, supplied by IVth nerve and muscle lost, there must
+have been also seven corresponding ventral or splanchnic segments supplied
+by the trigeminal. At present the evidence for such segments is nothing
+like so strong as for the corresponding somatic ones; there are, however,
+certain suggestive {280}facts which point distinctly in this direction in
+connection with both the motor and sensory parts of the trigeminal. The
+origin of the trigeminal motor fibres in the central nervous system is most
+striking. We may take it for granted that a nucleus of cells giving origin
+to one or more segmental motor nerves will possess a greater or less
+longitudinal extension in the central nervous system, according to the
+number of fused separate segmental centres it represents. Thus a nucleus
+such as that of the IVth nerve or of the facial is small and compact in
+comparison to the extensive conjoint nucleus of the vagus and cranial
+accessory.
+
+Upon examination of the motor nucleus of the trigeminal, we find a compact
+or well-defined nucleus, the _nucl. masticatorius_, the nerves of which
+supply the masseter, temporal, and other muscles, so that the anatomical
+evidence at first sight appears to bear out van Wijhe's conclusion that the
+motor trigeminal supplies at most two segments. Further examination,
+however, shows that this is not all, for the extraordinary so-called
+descending root of the Vth must be taken into consideration in any question
+of the origin of the motor elements, just as the equally striking ascending
+root enters into the consideration of the meaning of the sensory elements
+of the Vth.
+
+It is not necessary here to discuss the controversy as to whether this
+descending root is motor or sensory. It is universally considered at
+present to be motor, and is believed to supply, as Kölliker suggested,
+among other muscles, the _m. tensor tympani_ and the _m. tensor veli
+palati_. It is thus described by Obersteiner--
+
+"From the region of the mid-brain the motor root receives an important
+addition of thick fibres, which form the cerebral or descending root. The
+large, round vesicular cells from which the fibres of the descending root
+arise form no single compact group, but are partly single, partly arranged
+like little bunches of grapes, as far as the region of the anterior corpora
+quadrigemina. The further we go brainwards, the smaller is the number of
+fibres. In the region of the anterior corpora quadrigemina, the few cells
+of origin are found more and more median; so that the uppermost trigeminal
+fibres descend in curves almost from the mid-line, as is shown by the
+exceptional occurrence of one or more of the characteristic cells above the
+aqueduct. At the height of the posterior commissure one finds the last of
+these trigeminal cells."
+
+{281}The anatomy of the Vth nerve reveals, then, three most striking
+facts:--
+
+1. The motor nucleus of the Vth extends from the very commencement of the
+infra-infundibular region to nearly the commencement of the nucleus of the
+VIIth; in other words, the motor nucleus of the Vth extends through the
+whole prosomatic region, just as it must have done originally if its motor
+nerves supplied the muscles of the prosomatic appendages. Such an extended
+range of origin is indicative of the remains of an equally extended series
+of segmental centres or ganglia.
+
+2. Of these centres the caudalmost have alone remained large and vigorous,
+constituting the _nucleus masticatorius_, which in the fish is divided into
+an anterior and posterior group, thus indicating a double rather than a
+single nucleus; while the foremost ones have dwindled away until they are
+represented only by the cells of the descending root, the muscles of these
+segments being still represented by possibly the _tensor veli palati_ and
+the other muscles innervated from these cells.
+
+3. The headmost of these cells takes up actually a position dorso-lateral
+to the central canal, so that the groups on each side nearly come together
+in the mid-dorsal line; a very unique and extraordinary position for a
+motor cell-group, but not improbable when we recall to mind Brauer's
+assertion as to the shifting of the foremost prosomatic ganglion-cells of
+the scorpion from the ventral to the dorsal side of the alimentary canal.
+
+On the sensory side the evidence is also suggestive, the question here
+being not so much the distribution of the sensory nerves as the number of
+ganglia belonging to each of the cranial nerves.
+
+With respect to this question, morphologists have come to the conclusion
+that there is a marked difference between spinal and cranial nerves, in
+that whereas the posterior root-ganglia of the spinal nerves arise from the
+central nervous system itself, _i.e._ from the neural crest, the ganglia of
+the cranial nerves arise partly from the neural crest, partly from the
+proliferation of cells on the surface of the animal; and because of the
+situation of these proliferating epidermal patches over the gill-clefts in
+the case of the vagus and glossopharyngeal nerves, they have been called by
+Froriep and Beard branchial sense-organs. Beard divides the cranial ganglia
+into two sets, one connected with the neural ridges, called the neural
+ganglia, {282}and the other connected with the surface-cells, which he
+calls the lateral ganglia. This second set corresponds to Kupffer's
+epibranchial ganglia. Now it is clear that in the case of the vagus nerve,
+where, as is well shown in Ammocoetes, the nerve is not a single segmental
+nerve, but is in reality made up of a number of nerves going to separate
+branchial segments, the indication of such segments is not given by the
+main vagus ganglion or neural ganglion, but by the series of lateral
+ganglia. So also it is argued in the case of the trigeminal, that if in
+addition to the ganglion-cells arising from the neural crest separate
+ganglion-masses are found in the course of development, in connection with
+proliferating patches of the surface (plakodes, Kupffer calls them), then
+such isolated lateral ganglia are indications of separate segments, just as
+in the case of the vagus, even though the separate segments do not show
+themselves in the adult. So far the argument appears to me just, but the
+further conclusion that the presence of such plakodes shows the previous
+existence of _branchial_ sense-organs, and, therefore, that such ganglia
+are _epibranchial_ ganglia, indicating the position of a lost gill-slit, is
+not justified by the premises. If, as I suppose, the trigeminal nerve
+supplied a series of non-branchial appendages serially homologous with the
+branchial appendages supplied by the vagus, then it is highly probable that
+the trigeminal should behave with respect to its sensory ganglia similarly
+to the vagus nerve, without having anything to do with branchiæ.
+
+Such plakodal ganglia, then, may give valuable indication of non-branchial
+segments as well as of branchial segments. The researches of Kupffer on the
+formation of the trigeminal ganglia in Ammocoetes are the chief attempt to
+find out from the side of the sensory ganglia the number of segments
+originally belonging to the trigeminal. The nature and result of these
+researches is described in my previous paper (_Journal of Anatomy and
+Physiology_, vol. xxxiv.), and it will suffice here to state that he
+himself concludes that the trigeminal originally supplied five at least,
+probably six, segments. As I have stated there, the evidence as given by
+him seems to me to indicate even as many as seven segments.
+
+In the full-grown Ammocoetes, as is well known, there are two distinct
+ganglia belonging to the trigeminal, the one the ganglion of the _ramus
+ophthalmicus_, the other the main ganglion.
+
+According to Kupffer the larval Ammocoetes possesses three sets of ganglia,
+not two, for between the foremost and hindmost ganglion {283}he describes a
+nerve (_x._, Fig. 113), with four epibranchial ganglia, which do not
+persist as separate ganglia, but either disappear or are absorbed into the
+two main ganglia (Fig. 113). This discovery of Kupffer's is very
+suggestive, for, as already stated, a transformation takes place when the
+Ammocoetes is 5 mm. long, so that the arrangement of the parts before that
+period is distinctly more indicative of the ancestral arrangement than any
+later one.
+
+If we use the name plakodal ganglia to represent that part of these ganglia
+which was originally connected with the skin, then Kupffer's researches
+assert that in the larval Ammocoetes there were seven such plakodal
+ganglia, one in front belonging to the foremost trigeminal ganglion, two
+behind, parts of the hindmost ganglion, and four in between, which do not
+exist later as separate ganglia.
+
+[Illustration: FIG. 113.--GANGLIA OF THE CRANIAL NERVES OF AN AMMOCOETES, 4
+MM. IN LENGTH, PROJECTED ON TO THE MEDIAN PLANE. (After KUPFFER.)
+
+_A-B_, the line of epibranchial ganglia; _au._, auditory capsule; _nc._,
+notochord; _Hy._, tube of hypophysis; _Or._, oral cavity; _u.l._, upper
+lip; _l.l._ lower lip; _vel._, septum between oral and respiratory
+cavities; _V._, _VII._, _IX._, _X._, cranial nerves; _x._, nerve with four
+epibranchial ganglia.]
+
+In accordance with the views put forward in this book, a possible
+interpretation of these plakodal ganglia would be given as follows:--
+
+Beard, who, after Froriep, drew attention to this relation of the cranial
+ganglia to special skin-patches, has compared them with the parapodial
+ganglia of annelids, _i.e_. ganglia in connection with annelidan
+appendages; whether we are here obtaining a glimpse of the far-off
+annelidan ancestry of both arthropods and vertebrates it would be premature
+at present to say. It is natural enough to expect, on my view, to find
+evidence of annelidan ancestry in {284}vertebrate embryology (as has been
+so often asserted to be the case), seeing that undoubtedly the Arthropoda
+are an advanced stage of Annelida; and, indeed, the way is not a long one
+when we consider Beecher's evidence that the Trilobita belong to the
+Phyllopoda, certainly a primitive crustacean group, which Bernard derives
+directly from the annelid group Chætopoda. If, then, these plakodal ganglia
+indicate the former presence of appendages, we obtain this result:--The
+foremost ganglion on each side possesses one plakodal ganglion, and
+therefore indicates an anterior pair of appendages, possibly the cheliceræ.
+Then comes the peculiar nerve with four plakodal ganglia indicating on each
+side four appendages close together, possibly the endognaths. Then,
+finally, on each side, the second large ganglion with two plakodal ganglia,
+indicating two pairs of appendages, possibly the ectognaths and the
+metastoma.
+
+
+SUMMARY.
+
+  The consideration of the history of the cranial segmentation shows that
+  whereas, from the commencement of that history, the evidence for two
+  ventral segments supplied by the trigeminal nerve is clear and
+  unmistakable, later observers have tended more and more to increase the
+  number of these segments, until at the present time the evidence is in
+  favour of at least six, probably seven, as the number of segments
+  supplied by the motor part of the trigeminal.
+
+  So, also, the original evidence for the number of dorsal or somatic
+  segments limits the number to three, innervated respectively by the
+  oculomotor (III.), trochlear (IV.), and abducens (VI.) nerves, or rather
+  two, since the last nerve belongs to the facial segment. The muscles
+  which these three nerves supply are derived respectively from the walls
+  of the premandibular, mandibular, and hyoid coelomic cavities.
+
+  Later evidence points strongly to the conclusion that the oculomotor
+  nerve and the premandibular cavity represent not one segment but the
+  fusion of four, while the mandibular cavity represents two segments. In
+  addition to these, Miss Platt has discovered a still more anterior
+  head-cavity, which she has named the anterior cavity, so that the
+  pro-otic segments on this reckoning are seven in number, viz.: (1) the
+  anterior cavity, (2, 3, 4, 5) the premandibular cavity, (6, 7) the
+  mandibular cavity. The somatic muscles belonging to these dorsal segments
+  are the eye-muscles, which are all dorso-ventral in position, and are not
+  the same as the longitudinal somatic muscles, but belong to a distinct
+  dorso-ventral segmental group, the only representative of which at
+  present known in the mesosomatic region is the external rectus innervated
+  by the VIth nerve.
+
+  These head-cavities, and these muscles of the vertebrate, resemble the
+  corresponding cavities and muscles of the invertebrate to an
+  extraordinary {285}degree, so that it becomes easy to see how the
+  dorso-ventral muscles of the prosomatic segments of the latter have
+  become converted into the eye-musculature of the former. The most
+  powerful proof of all that such a conversion has taken place is that a
+  natural and simple explanation is at once given of the extraordinary
+  course taken by the IVth or trochlear nerve. Ever since neurology began,
+  the course of this nerve has arrested the attention of anatomists. Why
+  should just this one pair of nerve-roots of all those in the whole body
+  be directed dorsalwards instead of ventralwards, and cross each other in
+  the valve of Vieussens, each to supply a simple eye-muscle (the superior
+  oblique) belonging to the other side? For generations anatomists have
+  wondered and found no solution, and yet, without any straining of
+  hypotheses, in consequence simply of the investigation of the anatomy of
+  the corresponding pair of muscles in the scorpion group, the solution is
+  immediately apparent.
+
+  This pair of muscles alone, of all the musculature attached to the
+  carapace, crosses the mid-dorsal line to be attached to the other side,
+  thus carrying its nerve with it to the other side; by a continuation of
+  the same process the relation of the trochlear to the superior oblique
+  muscle can be explained.
+
+  The comparison of the eye-muscles of the vertebrate with the
+  dorso-ventral segmented muscles of the invertebrate makes the number and
+  nature of the pro-otic segments much clearer.
+
+
+
+
+{286}CHAPTER IX
+
+_THE PROSOMATIC SEGMENTS OF AMMOCOETES_
+
+  The prosomatic region in Ammocoetes.--The suctorial apparatus of the
+  adult Petromyzon.--Its origin in Ammocoetes.--Its derivation from
+  appendages.--The segment of the lower lip or metastomal segment.--The
+  tentacular segments.--The tubular muscles.--Their segmental
+  arrangement.--Their peculiar innervation.--Their correspondence with the
+  system of veno-pericardial muscles in Limulus.--The old mouth or
+  palæostoma.--The pituitary gland.--Its comparison with the coxal gland of
+  Limulus.--Summary.
+
+
+In the last chapter it was seen not to be incompatible with both the
+anatomical and morphological evidence to look upon the trigeminal nerves as
+having originally supplied the seven prosomatic pairs of appendages of the
+invertebrate ancestor, the foremost of which, the cheliceræ, and the four
+pairs of endognaths dwindled away and became insignificant, leaving as
+trace of their former presence the descending root of the Vth nerve; while
+the two hindmost pairs, the ectognaths and the chilaria, or metastoma,
+remained vigorous and developed, leaving as proof of their presence the
+_nucleus masticatorius_. Evidence in favour of this suggestion and of the
+nature of the dwindling process is afforded when we examine what the
+trigeminus does supply in Ammocoetes. In all vertebrates this nerve
+supplies the great muscles of mastication which, in all gnathostomatous
+fishes, move the jaws. The lowest fishes, the cyclostomes, possess no jaws;
+they take in their food by attaching themselves to their prey and by means
+of rasping teeth situated in serried rows within the circular mouth,
+combined with a powerful suctorial apparatus, they suck the juices of the
+fish they feed upon. Not possessing jaws, they feed by suction on the
+living animal, a method of feeding which gives them no more claim to be
+classed as parasitic animals than the whole group of spiders which feed in
+a similar manner on living flies.
+
+
+{287}THE ORIGIN OF THE SUCTORIAL APPARATUS OF PETROMYZON.
+
+This powerful suctorial apparatus is innervated entirely by the trigeminal
+nerve, so that here in its muscular arrangements any original segmental
+arrangement of the muscles of mastication might be expected to be visible.
+It consists of a large rod or piston, to which are attached powerful
+longitudinal muscles; a large muscle, the basilar muscle, which assists the
+piston in producing a vacuum, and annular muscles around the circular lip.
+
+Turn now to the full-grown larval form, Ammocoetes, an animal in the case
+of _Petromyzon Planeri_ as large as the full-grown Petromyzon, and seek for
+this musculature. There is, apparently, no sign of it, no suctorial
+apparatus whatever, only, as already mentioned, an oral chamber bounded by
+the lower and upper lips and the remains of the septum between it and the
+respiratory chamber--the velar folds. Attached to its walls a number of
+tentacles are situated, which form a fringe around and within the mouth.
+Most extraordinary is the contrast here between the larval and the adult
+stages; in the former, no sign of the suctorial apparatus, but simply
+tentacles and velar folds; in the latter, no sign of tentacles or of velar
+folds, but a massive suctorial apparatus.
+
+In order, then, to understand the origin of the muscles of mastication, it
+is necessary to study the changes which occur at transformation, and thus
+to find out how the suctorial apparatus of the adult arises. This most
+important investigation has been undertaken by Miss Alcock, and owing to
+the kindness of Mr. Millington, of Thetford, we have been able to obtain a
+better series in the transformation process than has ever been obtained
+before. Miss Alcock has not yet published her researches, but has allowed
+me to make use of some of her facts.
+
+An enormous proliferation of muscular tissue takes place with great
+rapidity during this transformation, which causes the disappearance of the
+tentacles, and gives origin to the suctorial apparatus. The starting point
+of this proliferation can be traced back in all cases to little groups of
+embryonic tissue found below the epithelial lining of the oral chamber in
+Ammocoetes. Of these groups the most conspicuous one is situated at the
+base of the large median ventral tentacles. Others are situated at the base
+of the tentacular ridge. Further, although this extraordinary change takes
+place in the {288}peripheral organ, no marked difference occurs in the
+arrangement of the nerves issuing from the trigeminal motor centre, no new
+nerves are formed to supply the new muscles, but every motor nerve-fibre
+and the motor cell from which it arises increases enormously in size, and
+these giant nerve-fibres thus formed split into innumerable filaments
+corresponding with the proliferation of the muscular elements.
+
+The clue, then, to the origin of the suctorial apparatus and of the nature
+of the original organs supplied by the trigeminal is afforded in this case,
+as in all other similar inquiries, by the central nervous system and its
+outgoing nerves. Here is always the citadel, the fixed seat of government,
+here is 'headquarters,' from which the answers to all our inquiries must
+originate.
+
+[Illustration: FIG. 114.--DISTRIBUTION OF TRIGEMINAL NERVE IN AMMOCOETES.
+
+_ps. br._, pseudo-branchial groove; _met._, nerve to lower lip, or
+metastomal nerve; _t._, nerve to tongue; _tent._, nerve to tentacles. The
+mandibular and internal maxillary nerves are coloured red; the purely
+sensory nerves to the external surface are coloured black.]
+
+
+THE TRIGEMINAL NERVE OF AMMOCOETES.
+
+Striking is the answer. In Fig. 114, Miss Alcock has drawn the distribution
+of the trigeminal nerve as traced by her through a series of sections. It
+arises, as is well known, from two separate ganglia, of which the foremost
+gives rise to a purely cutaneous nerve, the ophthalmic nerve, and the
+hindmost to three nerves, the most posterior of which is purely cutaneous
+and passes tailwards over the ventral branchial region, as shown in the
+figure; the other two nerves, both {289}of which contain motor fibres, are
+called by Hatschek the mandibular and maxillary nerves. Of these the
+mandibular or velar nerve (_met._) is a large, conspicuous nerve, which
+arises so separately from the rest of the trigeminal as almost to deserve
+the title of a separate nerve. When it leaves the large posterior ganglion,
+it passes into the anterior part of the velum, runs along with the tubular
+muscles, which it supplies, to the ventral surface as far as the junction
+of the lower lip with the thyroid plate, and has not been followed further
+by Hatschek. Miss Alcock, however, by means of serial sections, has traced
+it further, and shown that at this point it turns abruptly headwards to
+terminate in the muscles of the lower lip. If, then, as suggested, the
+lower lip represents the metastoma--the last pair of prosomatic
+appendages--then this mandibular or velar nerve represents that segmental
+nerve.
+
+The other nerve--the maxillary nerve of Hatschek--which constitutes the
+larger part of the trigeminal, passes forwards from the ganglion, and at a
+point somewhere about the anterior region of the eyeball, divides into two,
+an external (_black_ in Fig. 114) and an internal (_red_ in Fig. 114)
+nerve. The external branch is apparently entirely sensory, and supplies the
+external surfaces of the upper and lower lips. The internal branch is
+mainly motor, and supplies the muscles of the upper lip; it contains also
+the nerves of the tentacles.
+
+The nerve to the median ventral tentacle (_t._) or tongue leaves the
+internal division of the maxillary immediately after its separation from
+the external; it runs ventralwards, and at the same time passes internally
+until it reaches a position between the muco-cartilage and the epithelium
+lining the cavity of the throat. It then turns, and passing posteriorly
+(towards the tail) to the point where the median ventral tentacle is
+attached to the lower lip, it supplies some very rudimentary-looking
+muscles which run from the tentacle to the adjoining surface, and no doubt
+serve to move the tentacle from side to side. A portion of the nerve still
+continues to run along the side of the median ventral ridge, as far back as
+the point where the muscles of the hyoid segment pass round to the ventral
+side between the velum and the thyroid; in fact, this small nerve passes
+along the whole length of the median ventral ridge.
+
+This description shows that the trigeminal nerve divides itself into two
+groups: the one represented black in the figure, which is purely cutaneous
+and sensory, corresponding, in the main, according {290}to my theory, to
+the epimeral nerves of Limulus; the other coloured red, which supplies
+muscles belonging to the visceral or splanchnic muscle-group, and contains
+also the nerves to the tentacles.
+
+This latter group, which is formed by two distinct well-defined nerves,
+viz. the mandibular and the internal branch of the maxillary, corresponds,
+according to my theory, to the amalgamated nerves of the prosomatic
+appendages, and is clearly divisible into three distinct nerves--
+
+1. The lower lip-nerve or the metastomal nerve (_met._).
+
+2. The tongue-nerve (_t._).
+
+3. The nerve (_tent._) to the upper lip and tentacles.
+
+Of these three pairs of nerves it is suggested that the first pair were
+derived from the nerves to the metastomal appendage. The second pair of
+nerves ought, on this theory, originally to have supplied the pair of
+appendages immediately in front of the metastoma--that is, the pair of
+ectognaths, and therefore the ventral pair of tentacles, known as the
+tongue, would represent the last remnant of these ectognaths. Similarly,
+the other tentacles would represent the endognaths, and therefore the third
+pair of nerves would represent the fused nerves to these concentrated
+endognaths, which, in the Eurypterids, stand aloof from the ectognaths.
+
+Let us consider these three propositions separately. In the first place,
+have we any right to attribute segmental value to the mandibular nerve?
+What evidence is there of segments in this region in Ammocoetes?
+
+
+THE SEGMENT OF THE LOWER LIP, OR METASTOMAL SEGMENT.
+
+We have seen that in the branchial or mesosomatic region the segments
+corresponding to the mesosomatic appendages were mapped out by means of
+their supporting or skeletal structures, their segmental muscles, and their
+nervous arrangements, as well as by the arrangement of the branchiæ.
+Similarly, the segments in front of the branchial region, corresponding to
+the prosomatic appendages, ought to be definable by the same means,
+although, owing to the absence of branchiæ and the greater concentration in
+this region, the separate segments would probably not be so conspicuous.
+
+The last segment considered was the segment belonging to the VIIth nerve
+corresponding to the opercular appendages of the {291}Eurypterid. The
+segment immediately in front of this is the next for consideration, viz.
+that corresponding to the chilarial appendages or metastoma; and as the
+basal part of this pair of appendages was fused with the basal part of the
+operculum, the one cannot be discussed without the other; therefore, the
+segment to which the lower lip belongs must be considered in connection
+with and not apart from the thyro-hyoid segments already dealt with.
+
+In Chapter V., p. 188, I stated that the supporting bars of the foremost
+mesosomatic segments, the thyro-hyoid segments, differed from the
+cartilaginous bars of the branchial segments, in that they were composed of
+muco-cartilage. Also in addition to the muco-cartilaginous skeletal bars, a
+ventral plate of muco-cartilage exists in Ammocoetes which covers over the
+thyroid gland.
+
+Similarly in the prosomatic segments the skeletal bars are composed of
+muco-cartilage and the ventral plate of muco-cartilage continues forward as
+the plate of the lower lip. It is of special interest, in connection with
+the segments indicated by such supporting structures, to find that this
+special tissue is entirely confined to the head-region, and disappears
+absolutely at transformation, thus indicating the ancestral nature of the
+segments marked out by its presence.
+
+This muco-cartilaginous skeleton is the key to the whole position, and
+requires, therefore, to be understood. It is of great importance, not only
+because it demonstrates the position of the segments in Ammocoetes which
+characterized its invertebrate ancestor, but also because it possesses a
+structure remarkably similar to that found in the head-plates of the most
+ancient fishes. For the present I will confine myself to the consideration
+of this muco-cartilaginous skeleton as evidence of the relationship of
+Ammocoetes to the Eurypterids, and in the next chapter will show how
+absolutely the same skeleton corresponds to that of the Cephalaspidæ, so
+that Ammocoetes is really a slightly modified Cephalaspid, the larval form
+of which was Eurypterid in character.
+
+{292}[Illustration: FIG. 115.--DORSAL HALF OF HEAD-REGION OF AMMOCOETES.
+
+_Tr._, trabeculæ; _Pit._, pituitary space; _Inf._, infundibulum; _Ser._,
+median serrated flange of velar folds.] {293}[Illustration: FIG.
+116.--HORIZONTAL SECTION THROUGH THE ANTERIOR PART OF AMMOCOETES,
+IMMEDIATELY VENTRALLY TO THE AUDITORY CAPSULE.
+
+_sk_1_-_sk_5_, skeletal bars; _m_1_-_m_5_, striated visceral muscles;
+_mt_1_-_mt_4_, tubular muscles; _br_1_-_br_3_, branchiæ; _tr._, trabeculæ;
+_inf._, infundibulum; _ped._, pedicle; _V._, trigeminal nerve.
+Muco-cartilage, _red_; soft cartilage, blue; hard cartilage, purple.]
+
+{294}[Illustration: FIG. 117.--SAGITTAL LATERAL SECTION THROUGH THE
+ANTERIOR PART OF AMMOCOETES.
+
+Lettering and colouring same as in Fig. 116. _aud._, auditory capsule;
+_j.v._, jugular vein.]
+
+In Chapter IV., Figs. 63, 64, I have given a representation of the ventral
+and dorsal views of an Ammocoetes cut in half horizontally. Such a section
+shows with great clearness the series of branchial appendages with their
+segmental muscles and cartilaginous bars which form the branchial segments
+innervated by the IXth and Xth nerves, according to my view of the
+branchial unit. As is seen (Fig. 64 or 115), the skeletal bar of the hyoid
+or opercular appendage, which is clearly serially homologous with the other
+branchial bars, is composed of muco-cartilage, and not of cartilage. If we
+follow this series of horizontal sections nearer to the origin of the
+cartilaginous bars from the sub-chordal cartilaginous rod on each side of
+the notochord, we obtain a picture, as in Fig. 116, in which each branchial
+segment is defined by the section of the branchial cartilaginous bar
+(_sk_4_, _sk_5_), by the section of the separate branchiæ (_br_2_, _br_3_),
+and by the separate segmental muscles arranged round each bar, these
+muscles being partly ordinary striated (_m_4_, _m_5_), partly tubular
+(_mt_3_, _mt_4_). The uppermost of these branchial segments shows the same
+arrangement; (_sk_3_) is the branchial skeletal bar, which is now composed
+of muco-cartilage, not cartilage; (_br_1_) is the branchiæ in the same
+situation as the others, but here composed of glandular rather than of
+respiratory epithelium, while the ordinary striated branchial muscles of
+this segment are marked as (_m_3_), being separated from the tubular
+muscles of the segment (_mt_2_), owing to the large size of the blood-space
+in which these latter muscles are lying. In front of this segment so
+defined we see again another well-marked skeletal bar (_sk_2_) of
+muco-cartilage, evidently indicating a similar segment anterior to the
+hyoid segment. In connection with this bar there are no branchiæ, but again
+we see two sets of visceral muscles, the one ordinary striated, marked
+(_m_2_), and the other tubular, marked (_mt_1_). Here, then, the section
+indicates the existence of a segment of the same character as the
+posteriorly situated branchial segments but belonging to a non-branchial
+region--a segment which would represent a non-branchial appendage, the
+last, therefore, of the prosomatic appendages. Let us, then, follow
+{295}out these two segmental muco-cartilaginous bars and their attendant
+muscles, and see to what sort of segments their investigation leads.
+
+The bar which comes first for consideration (_sk_3_) arises immediately
+behind the auditory capsule from the first branchial cartilage very soon
+after it leaves the sub-chordal cartilaginous ligament; the soft cartilage
+of the sub-chordal ligament ceases abruptly in its extension along the
+notochord at the place where the hard cartilage of the parachordal joins
+it, and in a sense it may be said to leave the notochord at this place and
+pass into the basal part of the first branchial bar. The most anterior
+continuation of this branchial system is this muco-cartilaginous bar
+(_sk_3_), which passes forward and ventralwards, being separated from the
+axial line by the auditory capsule (_cf._ Fig. 118, A, B, C). Its position
+is well seen in a sagittal section, such as Fig. 117. It follows absolutely
+the line of the pseudo-branchial groove (_ps. br._, Fig. 114), and
+ventrally joins the plate of muco-cartilage which covers the thyroid gland.
+It forms a thickened border to this plate anteriorly, just as the branchial
+cartilaginous bars border it posteriorly. In fact, it behaves with respect
+to the hyoid segment in a manner similar to the rest of the cartilaginous
+bars with respect to their respective segments.
+
+It represents, although composed of muco-cartilage, the cartilaginous bar
+of the operculum in Limulus, which also forms the termination of the
+branchial cartilaginous system, as fully explained in Chapter III.; it may
+therefore be called the opercular bar.
+
+The next bar (_sk_2_) is extremely interesting, as we are now out of the
+branchial or mesosomatic region, and into the region corresponding to the
+prosoma. It starts from a cartilaginous projection made of hard cartilage,
+just in front of the auditory capsule, called by Parker the 'pedicle of the
+pterygoid'--a projection (_ped._) which defines the posterior limit of the
+trabeculæ on each side, where they join on to the parachordals,--and
+winding round and below the auditory capsule, joins the opercular bar
+(_cf._ Fig. 118), to pass thence into and form part of the
+muco-cartilaginous plate of the lower lip. In the section figured (Fig.
+116), this projection of hard cartilage is not directly continuous with
+(_sk_2_), owing to a slight curvature in the bar; the next few sections
+show clearly the connection between (_ped._) and (_sk_2_), and consequently
+the complete separation by means of this bar of the hyoid segment from the
+segment in front.
+
+{296}[Illustration: FIG. 118.--SKELETON OF HEAD-REGION OF AMMOCOETES. A,
+LATERAL VIEW; B, VENTRAL VIEW; C, DORSAL VIEW.
+
+Muco-cartilage, _red_; soft cartilage, _blue_; hard cartilage, _purple_.
+_sk_1_, _sk_2_, _sk_3_, skeletal bars; _c.e._, position of pineal eye; _na.
+cart._, nasal cartilage; _ped._, pedicle; _cr._, cranium; _nc._,
+notochord.]
+
+{297}In the figures, the hard cartilage is coloured purple, the soft
+cartilage blue, and the muco-cartilage red, so that the position of this
+bar is well shown. This bar may be looked upon as bearing the same relation
+to the muco-cartilaginous plate of the lower lip as the opercular bar does
+to the muco-cartilaginous plate over the thyroid; and seeing that these two
+plates form one continuous ventral head-shield of muco-cartilage (Fig. 118,
+B), and also that this bar fuses with the opercular bar, we may conclude
+that the segment represented by the lower lip is closely connected with the
+hyoid or opercular segments. In other words, if the lower lip arose from
+the metastoma, then this pair of skeletal bars might be called the
+metastomal bars, which formed the supporting skeleton of the last pair of
+prosomatic appendages and, as is likely enough, arose in connection with
+the posterior lateral horns of the plastron; these posterior lateral horns,
+like the rest of the plastron, would give rise to hard cartilage, and so
+form in Ammocoetes the two lateral so-called pterygoid projections.
+
+In the branchial region the muscles which marked out each branchial segment
+were of two kinds--ordinary striated visceral muscles and tubular muscles.
+Of these the former represented the dorso-ventral muscles of the branchial
+appendages, while the latter formed a separate group of dorso-ventral
+muscles with a separate innervation which may have been originally the
+segmental veno-pericardial muscles so characteristic of Limulus and the
+scorpions. In Figs. 116, 117, the grouping of these muscles in each
+branchial segment is well shown, and it is immediately seen that the hyoid
+segment possesses its group of striated visceral muscles (_m_3_) supplied
+by the VIIth nerve in the same manner as the posterior groups, as has
+already been pointed out by Miss Alcock in her previous paper. Passing to
+the segment in front, Fig. 116 shows that the group of visceral muscles
+(_m_2_) corresponds in relative position with respect to the metastomal bar
+to the hyoid muscles with respect to the opercular bar or to the branchial
+visceral muscles with respect to each branchial bar. What, then, is this
+muscular group? The series of sections show that these are the
+dorso-ventral muscles belonging to the lower lip, which, as seen in Fig.
+119 (_M._), form a well-marked muscular sheet, whose fibres interlace
+across the mid-ventral line of the lower lip. This group of lower
+lip-muscles is very suggestive, for these muscles arise, not from the
+trabeculæ, but from the front dorsal region of the cranium, just in front
+of the two lateral {298}eyes. In Fig. 117 the dorsal part is seen cut
+across on its way to its dorsal attachment. Such an origin is reminiscent
+of the tergo-coxal group of muscles, arising, as they do, from the
+primordial cranium and the tergal carapace, and suggests at once that when
+the chilarial appendages expanded to form a metastoma, their tergo-coxal
+muscles formed a sheet of muscles similar to those of the lower lip of
+Ammocoetes, by which the movements of the metastoma were effected. The
+posterior limit of these muscles ventrally marks out the junction of the
+segment of the lower lip with that of the thyroid; in other words,
+indicates where the metastoma had fused ventrally with the operculum (Fig.
+117).
+
+[Illustration: FIG. 119.--VENTRAL VIEW OF HEAD-REGION OF AMMOCOETES.
+
+_Th._, thyroid gland; _M._, lower lip, with its muscles.]
+
+Besides the striated visceral muscles, each branchial segment possesses its
+own tubular muscles, shown in Fig. 116 (_mt_3_) and (_mt_4_). As the
+section shows, there is clearly a group of tubular muscle-fibres belonging
+to the hyoid segment (_mt_2_), and also another group belonging to the
+segment in front of the hyoid (_mt_1_); so that, judging from this section,
+each of these segments possesses its own tubular musculature just as do the
+branchial segments, the difference being that the tubular muscles are more
+separated from the striated visceral group than in the true branchial
+segments, owing to the size of the blood-spaces surrounding them. What,
+then, are these two groups of muscles? Tracing them in the series of
+sections, both groups are seen to belong to the system of velar muscles,
+forming an anterior and a posterior group respectively; and we see,
+further, that there is not the slightest trace of any tubular muscles
+anterior to these muscles of the velum.
+
+In the living Ammocoetes the velar folds on each side can be seen {299}to
+move synchronously with the movements of respiration, contracting at each
+expiration, and thus closing the slit by which the oral and respiratory
+chambers communicate, and so forcing the waters of respiration through the
+gill-slits, as described by Schneider. Such a fact is clear evidence that
+these tubular muscles of the velar folds belong to the same series as the
+tubular muscles of the branchial segments, so that if, as I have already
+suggested, the latter muscles were originally the veno-pericardial muscles
+of segments corresponding to the branchial appendages, then the former
+would represent the veno-pericardial muscles of the segments corresponding
+to the opercular and metastomal appendages. What, then, are these velar
+folds, and how is it that the tubular muscles of these two segments become
+the velar muscles? I will consider, in the first instance, the posterior
+group of muscles (_mt_2_) in Fig. 116.
+
+It has already been pointed out that the tubular muscles of the branchial
+segments are dorso-ventral, but do not run with the ordinary constrictors,
+having separate attachments and running part of their course internally to
+and part externally to the ordinary constrictors. At first sight, as is
+usually stated, the hyoid segment does not appear to possess tubular
+muscles at all. If, however, we follow the posterior group of velar muscles
+(_mt_2_), we see (Fig. 117) that they pass between the auditory capsule and
+the opercular bar (_sk_3_) of muco-cartilage to reach the region of the
+jugular vein (_j.v._) posteriorly to the auditory capsule, so that their
+dorsal origin bears the same relation to the hyoid segment as the dorsal
+attachment of the rest of the tubular muscles to their respective segments.
+Further, these muscles run along the length of the velar fold, and are
+attached ventrally on each side of the thyroid gland, so that their ventral
+attachment also corresponds in position, as regards the hyoid segment, with
+the ventral attachment of the rest of the tubular muscles as regards their
+respective segments.
+
+This ventral attachment is shown in Fig. 119 on each side of the thyroid,
+and in Fig. 120 (_mt_2_); while in Fig. 117 the fibres are seen converging
+to this ventral position. In other words, this large posterior muscle of
+the velar folds is a dorso-ventral muscle, and would actually take the same
+position in the hyoid segment as the dorso-ventral tubular muscles in the
+other branchial segments, if the velum were put back into its original
+position as the septum terminating the branchial chamber. Conversely, the
+presence of these {300}hyoid tubular muscles in the velum gives evidence
+that the opercular segment takes part in the formation of the septum, as
+already suggested.
+
+Miss Alcock, in her paper, speaks of tubular muscles belonging to the hyoid
+segment, which are attached to the muco-cartilage. Schaffer also speaks of
+certain tubular muscles belonging to the velar group as piercing the
+muco-cartilage (_h. r. s._) in his figures 24 and 25, _i.e._ the metastomal
+bar, near its junction with the opercular bar. In my specimens there is a
+distinct group of tubular muscles which pierce the opercular bar of
+muco-cartilage at its junction with the metastomal bar, and pass into the
+posterior group of velar muscles. They clearly belong to the hyoid segment,
+as Miss Alcock supposed, but are not attached to the muco-cartilage. It is
+possible that they represent a different group to those already considered,
+and suggest the possibility that this opercular or thyro-hyoid segment is
+double with respect to its original veno-pericardial muscles as well as in
+other respects.
+
+The anterior group of tubular muscles (_mt_1_, Figs. 116, 117) belonging to
+the same segment as the metastomal bar must now be taken into
+consideration. Very different is their origin to that of the posterior
+group: they arise close up against the eye, and have given rise to
+Kupffer's and Hatschek's misconception that the superior oblique muscle of
+the eye arises from a part of the velar musculature. Naturally, as Neal has
+pointed out, they have nothing to do with the eye-muscles; the superior
+oblique muscle is plainly in its true place entirely apart from these velar
+muscles, which form the foremost group of the segmental tubular muscles.
+They pass into the anterior part of the velar folds and run round to the
+ventral side just in the same way as does the posterior group. This
+anterior group of tubular muscles represents the veno-pericardial muscle of
+the segment immediately in front of the opercular, _i.e._ the metastomal
+segment, and is the foremost of these veno-pericardial muscles. Its
+presence shows that the velar folds, formed as they were by the breaking
+down of the septum, are in reality part of two segments, viz. the opercular
+and the metastomal, which have fused together in their basal parts, and by
+such fusion have caused the inter-relationship between the VIIth and Vth
+nerves, so apparent in the anatomy of the vertebrate cranial nerves.
+
+A further piece of evidence that this anterior portion of the velum
+{301}belongs to the same segment as the lower lip is the fact that in
+addition to the tubular muscles a single ordinary striated muscle is found
+in the velum which, like the muscles of the lower lip, is innervated by
+this same mandibular nerve.
+
+This muscle is attached laterally to the muco-cartilage of the metastomal
+bar (_sk_2_) at its junction with the muco-cartilage of the lower lip, and
+spreads out into a number of strands which are attached at intervals along
+the whole length of the free anterior edge of the velum. It is the only
+non-tubular muscle belonging to the velum, and by its contraction it draws
+the anterior portions of the velar folds apart from each other, and so
+opens the slit between them, through which the food and mud must pass.
+Clearly from its position it does not belong to the original tergo-coxal
+group of muscles as do those of the lower lip; it must have been one of the
+intrinsic muscles of the metastoma itself.
+
+This anterior portion of the velar folds affords yet another striking hint
+of the correctness of my comparison of the lower lip segment of Ammocoetes
+with the chilaria of Limulus or the metastoma of Eurypterus; for the most
+dorsal anterior portion, which at its attachment possesses a wedge of
+muco-cartilage, forms a separate, well-defined, rounded basal projection
+marked _Ser._ in Fig. 115, and _B_ in the accompanying Fig. 120. This is
+that part of the velar folds which comes together in the middle line and
+closes the entrance into the respiratory chamber. The epithelial surface
+here is most striking and suggestive, for it is markedly serrated, being
+covered with a large number of closely-set projections or serræ. The
+serration of the surface here is of so marked a character that Langerhans
+considered this part of the velar folds to act as a masticating organ,
+grinding and rasping the food and mud which passed through the narrow slit.
+In fact, Langerhans supposed that this portion of the velum acted in a
+manner closely resembling the action of the gnatho-bases of the prosomatic
+appendages in Limulus or the Eurypteridæ.
+
+This suggestion of Langerhans is surely most significant, considering that
+this somewhat separate portion of the velum, to which he assigns such a
+function, is in the very place where the gnathite portion of the metastomal
+appendages would have been situated if it were true that the lower lip and
+anterior portion of the velum of Ammocoetes were derived from the
+metastoma.
+
+In addition to this marked serrated edge the whole surface of {302}the
+anterior portion of the velum is covered over with a scale-like or
+tubercular pattern remarkably like the surface-ornamentation seen in many
+of the members of the ancient group Eurypteridæ. In Fig. 121 I give a
+picture of this surface-marking of the velum. It is striking to see that
+just as in the case of the invertebrate this marking and these serræ are
+formed simply by the cuticular surface of the epithelial cells; a surface
+which, according to Wolff, possibly contains chitin. The interpretation
+which I would give of the velar folds is therefore as follows:--
+
+They represent the fused basal parts of the opercular and metastomal
+appendages, the gnatho-bases of the latter still retaining in a reduced
+degree their rasping surfaces, because, owing to their position on each
+side of the opening into the respiratory chamber they were still able to
+manipulate the food as it passed by them after the closure of the old
+mouth.
+
+[Illustration: FIG. 120.--AMMOCOETES CUT OPEN IN MID-VENTRAL LINE TO SHOW
+POSITION OF VELUM; VELAR FOLDS REMOVED ON ONE SIDE.
+
+_tr._, trabeculæ; vel., velum; _B._, anterior gnathic portion of velum;
+_ps. br._, pseudo-branchial groove; _m_2_, muscles of lower lip segment;
+_m_3_, muscles of thyro-hyoid segment; _mt_2_, insertion of tubular muscles
+of velum near thyroid.]
+
+[Illustration: FIG. 121.--SURFACE VIEW OF ANTERIOR SURFACE OF VELUM.]
+
+The whole evidence points irresistibly to the conclusion that the
+mandibular or velar nerve of the trigeminal does supply a splanchnic
+{303}segment which is, in all respects, comparable with the segments
+supplied by the facial, glossopharyngeal, and vagus nerves, except that it
+does not possess branchiæ. This simply means that the appendages which
+these nerves originally supplied were prosomatic, not mesosomatic, and
+corresponded, therefore, to the chilarial or metastomal appendages.
+
+A comparison of the ventral surface of Slimonia, as given in Fig. 8, p. 27,
+with that of Ammocoetes (Fig. 119), when the thyroid gland and lower lip
+muscles have been exposed to view, enables the reader to recognize at a
+glance the correctness of this conclusion.
+
+
+THE TENTACULAR SEGMENTS AND THE UPPER LIP.
+
+Anterior to this metastomal segment, Fig. 116 shows a group of visceral
+muscles, _m_1_, and yet again a muco-cartilaginous bar, _sk_1_, but, as
+already stated, no tubular muscles. These visceral muscles indicate the
+presence in front of the lower lip-segment of one or more segments of the
+nature of appendages. The muscles in question (_m_1_) are the muscles of
+the upper lip, the skeletal elements form a pair of large bars of
+muco-cartilage (_sk_1_), which start from the termination of the trabeculæ,
+and pass ventralwards to fuse with the muco-cartilaginous plate of the
+lower lip (Figs. 117 and 118). This large bar forms the tentacular ridge on
+each side, and gives small projections of muco-cartilage into each
+tentacle. In addition to this tentacular bar, a special bar of
+muco-cartilage exists for the fused pair of median tentacles, the so-called
+tongue, which extends in the middle line along the whole length of the
+lower lip, being separated from the muco-cartilaginous plate of the lower
+lip by the muscles of the lower lip. This tongue bar of muco-cartilage
+joins with the muco-cartilage of the lower lip at its junction with the
+thyroid plate, and also with the tentacular bar just before the latter
+joins the muco-cartilaginous plate of the lower lip. This arrangement of
+the skeletal tissue suggests that the pair of tentacles known as the tongue
+stand in a category apart from the rest of the tentacles; a suggestion
+which is strongly confirmed by the separate character of its nerve-supply,
+as already mentioned.
+
+For three reasons, viz. the separateness both of their nerve-supply and of
+their skeletal tissue, and the importance they assume at transformation,
+this pair of ventral tentacles must, it seems to me, be put {304}into a
+separate category from the rest of the tentacles. On the other hand, the
+innervation of the rest of the tentacles by a single nerve which sends off
+a branch as it passes each one, together with the concentration of their
+skeletal elements into a single bar, with projections into each tentacle,
+points directly to the conclusion that these tentacles must be considered
+as a group, and not singly.
+
+I suggest that these tentacles are the remains of the ectognaths and
+endognaths; the tongue representing the two ectognaths, and the four
+tentacles on each side the four pairs of endognaths.
+
+As we see, this method of interpretation attributes segmental value to the
+tentacles, a conclusion which is opposed to the general opinion of
+morphologists, who regard them as having no special morphological
+importance, and certainly no segmental value. On the other hand, the
+importance of the pair of ventral tentacles, the 'tongue' of Rathke, which
+lie in the mid-line of the lower lip, has been shown by Kaensche, Bujor,
+and others, all of whom are unanimous in asserting that at transformation
+they are converted into that large and important organ the piston or tongue
+of the adult Petromyzon. It is supposed that the rest of the tentacles
+vanish at transformation, being absorbed; they appear to me rather to take
+part in the formation of the sucking-disc, so that I am strongly inclined
+to believe that the whole of the remarkable suctorial apparatus of
+Petromyzon is derived from the tentacles of Ammocoetes. In other words, on
+my view, a conversion of the prosomatic appendages into a suctorial
+apparatus takes place at transformation, just as is frequently the case
+among the Arthropoda.
+
+It is to the arrangement of the muscles that we look for evidence of
+segmental value. As long as it was possible to look upon these tentacles as
+mere sensory feelers round the mouth entrance, it was natural to deny
+segmental value to them. Matters are now, however, totally different since
+Miss Alcock's discovery of the rudimentary muscles at the base of the
+tentacles and their development at transformation. If these muscles
+represent some of the appendage muscles belonging to the foremost
+prosomatic segments just as the ocular muscles represent the dorso-ventral
+somatic muscles of those same segments, then we may expect ultimately to be
+able to give as good evidence of segmentation in their case as I have been
+able to give in the case of these latter muscles; for the two sets of
+muscles are curiously alike, seeing that the eye-muscles do not develop
+until {305}transformation, but throughout the Ammocoetes stage remain in
+almost as rudimentary a condition as the tentacular muscles.
+
+Another difficulty with respect to the tentacles is the determination of
+the number of them, owing to the fact that in addition to what may be
+called well-defined tentacles a large number of smaller tactile projections
+are found on the surface of the upper lip, as is seen in Fig. 115. In the
+very young condition, 7 or 8 mm. in length, it is easier to make sure on
+this point. At this stage they may be spoken of as arranged in two groups:
+an anterior small group and a posterior larger group. The anterior group
+consists of a pair of very small tentacles and a very small median
+tentacle, all three situated quite dorsally in the front part of the upper
+lip. The posterior group, which is separate from the anterior, consists of
+five pairs of much larger tentacles, the most ventral pair in the mid-line
+ventrally on the lower lip being fused together to form the large ventral
+median tentacle or tongue already mentioned. This pair, according to
+Shipley, is markedly larger than the others. There are, therefore, five
+conspicuous tentacles on each side, and in front of them a smaller pair and
+a small median dorsal one. In the very young condition the accessory
+projections above-mentioned are not present, or at all events are not
+conspicuous, and the tentacles are also markedly larger in comparison to
+the size of the animal than in the older condition, where they have
+distinctly dwindled.
+
+This posterior group of five conspicuous tentacles is the one which I
+suggest represents the four endognaths and one ectognath. What the
+significance of the small anterior group is, I know not. It is possible
+that the cheliceræ are represented here, for they are situated distinctly
+anterior to the other group; I know, however, of no sign of a markedly
+separate innervation to these most dorsal tentacles such as I should have
+expected to find if they represented the cheliceræ.
+
+The muscles of the upper lip, which distinctly belong to the visceral and
+not to the somatic musculature, form part of the foremost segments, and in
+these muscles the tentacular nerve reaches its final destination. From
+their innervation, then, they must have belonged to the same appendages as
+the tentacles supplied by the tentacular nerve, _i.e._ to the endognaths.
+What conclusion can we form as to the probable origin of the upper lip of
+Ammocoetes? Since the oral chamber was formed by the forward growth of the
+metastoma, _i.e._ the lower lip of Ammocoetes, it follows that the upper
+{306}lip is the continuation forwards of the original ventral surface of
+such an animal as Limulus or a member of the scorpion group, where there is
+no metastoma, and corresponds to the endostoma, as Holm calls it, of
+Eurypterus. This termination of the ventral surface in all these animals is
+made up of two parts: (1) Of sternites composing the true median ventral
+surface of the body, called by Lankester the pro- and meso-sternites; and
+(2) of the sterno-coxal processes of the foremost prosomatic appendages,
+called in the case of Limulus gnathites, because they are the main agents
+in triturating the food previously to its passage into the mouth. In
+Limulus, a conjoined pro-mesosternite forms the median ventral wall to
+which the sterno-coxal processes are attached on each side, and in Phrynus
+and Mygale a well-marked pro-sternite and meso-sternite are present,
+forming the posterior limit of the olfactory opening. In Buthus and the
+true scorpions the sterno-coxal processes of the 2nd, 3rd, and 4th
+prosomatic appendages take part in surrounding the olfactory tubular
+passage; in Thelyphonus only the processes of the 2nd pair of prosomatic
+appendages play such a part, the pro-sternite not being present (_cf._ Fig.
+97).
+
+Seeing, then, what a large share the sterno-coxal processes of one or more
+of these prosomatic appendages plays in the formation of this endostoma,
+and seeing also that the nerve which supplies the upper lip-muscles in
+Ammocoetes is the same as that supplying the tentacles which are attached
+to the upper lip, it appears to me more probable than not that the muscles
+in question are the vestiges of the sterno-coxal muscles. These muscles
+differ markedly in their attachments from the muscles of the lower lip, for
+whereas the latter resemble the tergo-coxal group in their extreme dorsal
+attachment, the former resemble the sterno-coxal group in their attachment
+to what corresponds to the endostoma.
+
+This interpretation of the meaning of the transformation process is in
+accordance with all the previous evidence both from the side of the
+palæostracan as from the side of the vertebrate, for it signifies that a
+dwindling process has taken place in the foremost of the original
+prosomatic appendages--the cheliceræ and the endognaths; while, on the
+contrary, the ectognath and the metastoma have continued to increase in
+importance right into the vertebrate stage. This process is simply a
+continuation of what was already going on in the invertebrate stage, for
+whereas in Eurypterus and other cases {307}the cheliceræ and endognaths had
+dwindled down to mere tentacles, the ectognath was the large swimming
+appendage, and the metastoma was on the upward grade from the two
+insignificant chilaria of Limulus.
+
+The transformation of these foremost appendages into a suctorial apparatus
+is very common among the arthropods, as is seen in the transformation of
+the caterpillar into the butterfly, and it is in accordance with the
+evidence that the main mass of that suctorial apparatus should be formed
+from appendages corresponding to the ectognath and metastoma rather than
+from the four endognaths. In all probability the _nucleus masticatorius_ of
+the trigeminal nerve with its innervation of the great muscles of
+mastication is evidence of the continued development of the musculature of
+these two last prosomatic appendages, just as the descending root of the
+Vth demonstrates the further disappearance of all that belongs to the
+foremost prosomatic appendages. As yet, however, as far as I know, the
+musculature of the head-region of Petromyzon has not been brought into line
+with that of other vertebrates, and until that comparative study has been
+completed it is premature to discuss the exact position of the masticating
+muscles of the higher vertebrates.
+
+The analysis of these tentacular segments belonging to the trigeminal nerve
+presents greater difficulties than that of any of the other cranial
+segments, owing to the deficiency of our knowledge of what occurs at
+transformation. Light is required not only on the origin of the new muscles
+but also on the origin of the new and elaborate cartilages which are newly
+formed at this time.
+
+Miss Alcock has not yet worked out the origin of all these cartilages and
+muscles, so that we are not yet in a position to analyze the trigeminal
+supply in Petromyzon into its component appendage elements, an analysis
+which ought ultimately to enable us to determine from which
+appendage-muscles the masticating muscles in the higher vertebrates have
+arisen. As far as the muscles are concerned, she gives me the following
+information:--
+
+The tongue-nerve supplies in Ammocoetes the rudimentary muscles which pass
+laterally from the base of the large ventral tentacle to the wall of the
+throat, and even in Ammocoetes must possess some power of moving that
+tentacle.
+
+At transformation these muscles proliferate and develop enormously, and
+form the bulk of the large basilar muscle which {308}surrounds the throat
+ventrally and laterally, and is the most bulky muscle in the suctorial
+apparatus.
+
+The velar or mandibular nerve supplies in Ammocoetes the muscles of the
+lower lip. In Petromyzon it supplies also the longitudinal muscles of the
+tongue. The tongue-cartilage first develops in the region of the median
+ventral tentacle, and there the longitudinal tongue-muscles first begin to
+develop, not from the rudimentary muscles in the tongue but from those in
+the lower lip region.
+
+In Ammocoetes the tentacular nerve supplies the rudimentary muscles in the
+tentacles and the muscles of the upper lip. The latter disappear entirely
+at transformation, and in Petromyzon the tentacular nerve supplies the
+circular, pharyngeal, and annular muscles, which are derived from the
+rudimentary tentacular muscles.
+
+For the convenience of my reader I append here a table showing my
+conception of the manner in which the endognathal and ectognathal segments
+of the Palæostracan are represented in Ammocoetes. It shows well the
+uniform manner in which all the individual segmental factors have been
+fused together to represent the appearance of a single segment (van Wijhe's
+first segment) in the case of the four endognathal segments, but have
+retained their individuality in the case of the ectognathal segment.
+
+  +----------+----------+---------------------------+----------+----------+
+  |          |          |      Appendages.          |          |          |
+  |V. Wijhe's|Eurypterid+-------------+-------------+ Appendage| Skeletal |
+  |segments. |segments. | Eurypterid. | Ammocoetes. |  nerves. | elements.|
+  |          |          |             |             |          |          |
+  +----------+----------+-------------+-------------+----------+----------+
+  |          |          |             |             |          |          |
+  |          |     2}   |             |             |    1     |    1     |
+  |     1    |     3}   |      4      |      4      |Tentacular|Tentacular|
+  |          |     4}   |  Endognaths |  Tentacles  |   to 4   | bar to 4 |
+  |          |     5}   |             |             | tentacles| tentacles|
+  |          |          |             |             |          |          |
+  +----------+----------+-------------+-------------+----------+----------+
+  |          |          |             |             |          |          |
+  |     2    |     6    |      1      |      1      | 1 Tongue | 1 Tongue |
+  |          |          |  Ectognath  |   Tongue    |   nerve  |    bar   |
+  |          |          |             |             |          |          |
+  +----------+----------+-------------+-------------+----------+----------+
+
+  +----------+----------+---------+---------+----------+---------+
+  |          |          |         | Dorso-  |          |         |
+  |V. Wijhe's|Eurypterid| Somatic | ventral |Coelomic|  Coxal  |
+  |segments. |segments. |  motor  |segmental| cavities.| glands. |
+  |          |          | nerves. | muscles.|          |         |
+  +----------+----------+---------+---------+----------+---------+
+  |          |          |         |         |          |    1    |
+  |          |     2}   | 1 Oculo-|  Sup.   |  1 Pre-  |Pituitary|
+  |     1    |     3}   |  motor  |inf. int.|mandibular|  body;  |
+  |          |     4}   |supplying| rectus  |  fusion  |fusion of|
+  |          |     5}   |    4    | and inf.|   of 4   | 4 coxal |
+  |          |          | muscles | oblique |          |  glands |
+  +----------+----------+---------+---------+----------+---------+
+  |          |          | 1 Troch-|         |          |         |
+  |     2    |     6    |  learis |   Sup.  |    1     |         |
+  |          |          |supplying| oblique |Mandibular|         |
+  |          |          | 1 muscle|         |          |         |
+  +----------+----------+---------+---------+----------+---------+
+
+
+{309}THE TUBULAR MUSCLES.
+
+The only musculature innervated by the trigeminal nerve which remains for
+further discussion, consists of those peculiar muscles found in the velum,
+known by the name of striated tubular muscles. This group of muscles has
+already been referred to in Chapter IV., dealing with respiration and the
+origin of the heart.
+
+It is a muscular group of extraordinary interest in seeking an answer to
+the question of vertebrate ancestry, for, like the thyroid gland, it bears
+all the characteristics of a survival from a prevertebrate form, which is
+especially well marked in Ammocoetes. I have already suggested in this
+chapter that the homologues of these muscles are represented in Limulus by
+the veno-pericardial group of muscles. I will now proceed to deal with the
+evidence for this suggestion.
+
+The structure of the muscle-fibres is peculiar and very characteristic, so
+that wherever they occur they are easily recognized. Each fibre consists of
+a core of granular protoplasm, in the centre of which the nuclei are
+arranged in a single row. This core is surrounded by a margin of striated
+fibrillæ, as is seen in Fig. 122. Such a structure is characteristic of
+various forms of striated muscle found in various invertebrates, such as
+the muscle-fibre of mollusca. It is, as far as I know, found nowhere in the
+vertebrate kingdom, except in Ammocoetes. At transformation these muscles
+entirely disappear, becoming fattily degenerated and then absorbed.
+
+[Illustration: FIG. 122.--A TUBULAR MUSCLE-FIBRE OF AMMOCOETES.
+
+A, portion of fibre seen longitudinally; B, transverse section of fibre
+(osmic preparation); the black dots are fat-globules.]
+
+For all these reasons they bear the stamp of a survival from a
+prevertebrate form. This alone would not make this tissue of any great
+importance, but when in addition these muscles are found to be arranged
+absolutely segmentally throughout the whole of the branchial region, then
+this tissue becomes a clue of the highest importance.
+
+As mentioned in Chapter IV., the segmental muscles of respiration consist
+of the adductor muscle and the two constrictor muscles--the {310}striated
+constrictor and the tubular constrictor. Of these muscles, both the muscles
+possessing ordinary striation are attached to the branchial cartilaginous
+skeleton, whereas the tubular constrictors have nothing to do with the
+cartilaginous basket-work, but are attached ventrally in the neighbourhood
+of the ventral aorta.
+
+These segmental tubular muscles are found also in the velar folds--the
+remains of the septum or velum which originally separated the oral from the
+respiratory chamber. In the branchial region they act with the other
+constrictors as expiratory muscles, forcing the water out of the
+respiratory chamber. In the living Ammocoetes, the velar folds on each side
+can be seen to move synchronously with the movements of respiration,
+contracting at each expiration; they thus close the slit by which the oral
+and respiratory chambers communicate, and therefore, in conjunction with
+the respiratory muscles, force the water of respiration to flow out through
+the gill-slits, as described by Schneider.
+
+These tubular muscles thus form a dorso-ventral system of muscles
+essentially connected with respiration; they belong to each one of the
+respiratory segments, and are also found in the velum; anterior to this
+limit they are not to be found. What, then, are these tubular muscles in
+the velar folds? Miss Alcock has worked out their topography by means of
+serial sections, and, as already fully explained, has shown that they form
+exactly similar dorso-ventral groups, which belong to the two segments
+anterior to the purely branchial segments, _i.e._ to the facial or hyoid
+segments and the lower lip-segment of the trigeminal nerve. If the velar
+folds could be put back into their original position as a septum, then the
+hyoid or facial group of tubular muscles would take up exactly the same
+position as those belonging to each branchial segment.
+
+The presence of these two so clearly segmental groups of muscles in the
+velum--the one belonging to the region of the trigeminal, the other to the
+region of the facial--is strong confirmation of my contention that this
+septum between the oral and respiratory chambers was caused by the fusion
+of the last prosomatic and the first mesosomatic appendages, represented in
+Limulus by the chilaria and the operculum.
+
+Yet another clue to the meaning of these muscles is to be found in their
+innervation, which is very extraordinary and unexpected. Throughout the
+branchial region the striated muscles of each segment {311}are strictly
+supplied by the nerve of that segment, and, as already described, each
+segment is as carefully mapped out in its innervation as it is in any
+arthropod appendage. One exception occurs to this orderly, symmetrical
+arrangement: a nerve arises in connection with the facial nerve, and passes
+tailwards throughout the whole of the branchial region, giving off a branch
+to each segment as it passes. This nerve (_Br. prof._, Fig. 123) is known
+by the name of the _ramus branchialis profundus_ of the facial, and its
+extraordinary course has always aroused great curiosity in the minds of
+vertebrate anatomists. Miss Alcock, by the laborious method of following
+its course throughout a complete series of sections, finds that each of the
+segmental branches which is given off, passes into the tubular muscles of
+that segment (Fig. 124). The tubular muscles which belong to the velum,
+_i.e._ those belonging to the lower lip-segment and to the hyoid segments,
+receive their innervation from the velar or mandibular nerve, and belong,
+therefore, to the trigeminal, not to the facial, system.
+
+[Illustration: FIG. 123.--DIAGRAM SHOWING THE DISTRIBUTION OF THE FACIAL
+NERVE.
+
+Motor branches, _red_; sensory branches, blue.]
+
+The evidence presented by these muscles is as follows:--
+
+In the ancestor of the vertebrate there must have existed a segmentally
+arranged set of dorso-ventral muscles of peculiar structure, concerned with
+respiration, and confined to the mesosomatic segments and to the last
+prosomatic segment, yet differing from the other dorso-ventral muscles of
+respiration in their innervation and their attachment.
+
+Interpreting these facts with the aid of my theory of the origin of
+vertebrates, and remembering that the homologue of the vertebrate ventral
+aorta in such a palæostracan as Limulus is the longitudinal {312}venous
+sinus, while the opercular and chilarial segments are respectively the
+foremost mesosomatic and the last prosomatic segments; they signify that
+the palæostracan ancestor must have possessed a separate set of segmental
+dorso-ventral muscles confined to the branchial, opercular and chilarial or
+metastomal segments, which, on the one hand, were respiratory in function,
+and on the other were attached to the longitudinal venous sinus. Further,
+these muscles must all have received a nerve-supply from the neuromeres
+belonging to the chilarial and opercular segments, an unsymmetrical
+arrangement of nerves, on the face of it, very unlikely to occur in an
+arthropod.
+
+[Illustration: FIG. 124.--DIAGRAM CONSTRUCTED FROM A SERIES OF TRANSVERSE
+SECTIONS THROUGH A BRANCHIAL SEGMENT, SHOWING THE ARRANGEMENT AND RELATIVE
+POSITIONS OF THE CARTILAGE, MUSCLES, NERVES, AND BLOOD-VESSELS.
+
+Nerves coloured red are the motor nerves to the branchial muscles. Nerves
+coloured blue are the internal sensory nerves to the diaphragms and the
+external sensory nerves to the sense-organs of the lateral line system.
+_Br. cart._, branchial cartilage; _M. con. str._, striated constrictor
+muscles; _M. con. tub._, tubular constrictor muscles; _M. add._, adductor
+muscle; _D.A._, dorsal aorta; _V.A._, ventral aorta; _S._, sense-organs on
+diaphragm; _n. Lat._, lateral line nerve; _X._, epibranchial ganglia of
+vagus; _R. br. prof. VII._, _ramus branchialis profundus_ of facial;
+_J.v._, jugular vein; _Ep. pit._, epithelial pit.]
+
+{313}Is this prophecy borne out by the examination of Limulus? In the first
+place, these muscles were dorso-ventral and segmental, and, referring back
+to Chapter VII., Lankester arranges the segmental dorso-ventral muscles in
+three groups: (1) The dorso-ventral somatic muscles; (2) the dorso-ventral
+appendage muscles; and (3) the veno-pericardial muscles. Of these the first
+group is represented in the vertebrate by the muscles which move the eye,
+the second group by the striated constrictor and adductor muscles and the
+muscles for the lower lip. There is, then, the possibility of the third
+group for this system of tubular muscles.
+
+Looking first at the structure of these muscles as previously described, so
+different are they in appearance from the ordinary muscles of Limulus, that
+Milne-Edwards, as already stated, called them "brides transparentes," and
+did not recognize their muscular character, while Blanchard called them in
+the scorpion, "ligaments contractils."
+
+Consider their attachment and their function. They are attached to the
+longitudinal sinus, according to Lankester's observation, in such a way
+that the muscle-fibres form a hollow cone filled with blood; when they
+contract they force this blood towards the gills, and thus act as accessory
+or branchial hearts. According to Blanchard, in the scorpion they contract
+synchronously with the heart; according to Carlson, in Limulus they
+contract with the respiratory muscles. In Ammocoetes, where the respiration
+is effected after the fashion of Limulus, not of Scorpio, the tubular
+muscles are respiratory in function.
+
+Look at their limits. The veno-pericardial muscles in Limulus are limited
+by the extent of the heart, they do not extend beyond the anterior limit of
+the heart. In Fig. 70 (p. 176) two of these muscles are seen in front of
+the branchial region also attached to the longitudinal venous sinus,
+although in front of the gill-region. In Ammocoetes the upper limit of the
+tubular muscles is the group found in the velum; this most anterior group
+belongs to a region in front of the branchial region--that of the
+trigeminal.
+
+Moreover, the supposition that the segmental tubular muscles belong
+throughout to the veno-pericardial group gives an adequate reason why they
+do not occur in front of the velum; for, as their existence is dependent
+upon the longitudinal collecting sinus in Limulus and Scorpio, which is
+represented by the ventral aorta in {314}Ammocoetes, they cannot extend
+beyond its limits. Now, Dohrn asserts that the ventral aorta terminates in
+the spiracular artery, which exists only for a short time; and, in another
+place, speaking of this same termination of the ventral aorta, he states:
+"Dass je eine vorderste Arterie aus den beiden primären Aesten des Conus
+arteriosus hervorgeht, die erste Anlage der Thyroidea umfasst, in der
+Mesodermfalte des späteren Velums in die Höhe steigt um in die Aorta der
+betreffenden Seite einzumunden." These observations show that the vessel
+which in Ammocoetes represents the longitudinal collecting sinus in the
+Merostomata does not extend further forwards than the velum, and in
+consequence the representatives of the veno-pericardial muscles cannot
+extend into the segments anterior to the velum. One of the extraordinary
+characteristics of these tubular muscles which distinguishes them from
+other muscles, but brings them into close relationship with the
+veno-pericardial group, is the manner in which the bundles of muscle-fibres
+are always found lying freely in a blood-space; this is clearly seen in the
+branchial region, but most strikingly in the velum, the interior of which,
+apart from its muco-cartilage, is simply a large lacunar blood-space
+traversed by these tubular muscles.
+
+All these reasons point to the same conclusion: the tubular muscles in
+Ammocoetes are the successors of the veno-pericardial system of muscles.
+
+If this is so, then this homology ought to throw light on the extraordinary
+innervation of these tubular muscles by the _branchialis profundus_ branch
+of the facial nerve and the velar branch of the trigeminal. We ought, in
+fact, to find in Limulus a nerve arising exclusively from the ganglia
+belonging to the chilarial and opercular segments, which, instead of being
+confined to those segments, traverses the whole branchial region on each
+side, and gives off a branch to each branchial segment; this branch should
+supply the veno-pericardial muscle of that side.
+
+Patten and Redenbaugh have traced out the distribution of the peripheral
+nerves in Limulus, and have found that from each mesosomatic ganglion a
+segmental cardiac nerve arises which passes to the heart and there joins
+the cardiac median nerve, or rather the median heart-ganglion, for this
+so-called nerve is really a mass of ganglion-cells. In all the branchial
+segments the same plan exists, each cardiac nerve belonging to that
+neuromere is strictly segmental. {315}Upon reaching the opercular and
+chilarial neuromeres an extraordinary exception is found; the cardiac
+nerves of these two neuromeres are fused together, run dorsally, and then
+form a single nerve called the pericardial nerve, which runs outside the
+pericardium along the whole length of the mesosomatic region, and gives off
+a branch to each of the cardiac nerves of the branchial neuromeres as it
+passes them.
+
+This observation of Patten and Redenbaugh shows that the pericardial nerve
+of Limulus agrees with the very nerve postulated by the theory, as far as
+concerns its origin from the chilarial and opercular neuromeres, its
+remarkable course along the whole branchial region, and its segmental
+branches to each branchial segment.
+
+At present the comparison goes no further; there is no evidence available
+to show what is the destination of these segmental branches of the
+pericardial nerve, and so far all evidence of their having any connection
+with the veno-pericardial muscles is wanting. Carlson, at my request,
+endeavoured in the living Limulus to see whether stimulation of the
+pericardial nerve caused contraction of the veno-pericardial muscles, but
+was unable to find any such effect. On the contrary, his experimental work
+indicated that each veno-pericardial muscle received its motor supply from
+the corresponding mesosomatic ganglion. This is not absolutely conclusive,
+for if, as Blanchard asserts in the case of the scorpion, a close
+connection exists between the action of these muscles and of the heart, it
+is highly probable that their innervation conforms to that of the heart.
+Now Carlson has shown that this cardiac nerve from the opercular and
+chilarial neuromeres is an inhibitory nerve to the heart, while the
+segmental cardiac nerves belonging to the branchial ganglia are the
+augmentor nerves of the heart.
+
+His experiments, then, show that the motor nerves of the heart and of the
+veno-pericardial muscles run together in the same nerves, but he says
+nothing of the inhibitory nerves to the latter muscles. If they exist and
+if they are in accordance with those to the heart, then they ought to run
+in the pericardial nerve, and would naturally reach the veno-pericardial
+muscles by the segmental branches of the pericardial nerve.
+
+Moreover, inhibitory nerves are, in certain cases, curiously associated
+with sensory fibres; so that the nerve which corresponds {316}to the
+pericardial nerve, viz. the _branchialis profundus_ of the facial, may be
+an inhibitory and sensory nerve, and not motor at all. Miss Alcock's
+observations are purely histological; no physiological experiments have
+been made.
+
+At present, then, it does not seem to me possible to say that Carlson's
+experiments have disproved _any_ connection of the pericardial nerve with
+the veno-pericardial muscles. We do not know what is the destination of its
+segmental branches; they may still supply the veno-pericardial muscles even
+if they do not cause them to contract; they certainly do not appear to pass
+directly into them, for they pass into the segmental cardiac nerves, and
+can only reach the muscles in conjunction with their motor nerves. Such a
+course would not be improbable when it is borne in mind how, in the frog,
+the augmentor nerves run with the inhibitory along the whole length of the
+vagus nerve.
+
+Until further evidence is given both as to the function of the segmental
+branches of the pericardial nerve in the Limulus, and of the _branchialis
+profundus_ in Ammocoetes, it is impossible, I think, to consider that the
+phylogenetic origin of these tubular muscles is as firmly established as is
+that of most of the other organs already considered. I must say, my own
+bias is strongly in favour of looking upon them as the last trace of the
+veno-pericardial system of muscles, a view which is distinctly strengthened
+by Carlson's statement that the latter system contracts synchronously with
+the respiratory movements, for undoubtedly in Ammocoetes their function is
+entirely respiratory. Then again, although at present there is no evidence
+to connect the pericardial nerve in Limulus with this veno-pericardial
+system of muscles, yet it is extraordinarily significant that in such
+animals as Limulus and Ammocoetes, in both of which the mesosomatic or
+respiratory region is so markedly segmental, an intrusive nerve should, in
+each case, extend through the whole region, giving off branches to each
+segment. Still more striking is it that this nerve should arise from the
+foremost mesosomatic and the last prosomatic neuromeres in Limulus--the
+opercular and chilarial segments--precisely the same neuromeres which give
+origin to the corresponding nerve in Ammocoetes, for according to my theory
+of the origin of vertebrates, the nerves which supplied the opercular and
+metastomal appendages have become the facial nerve and the lower lip-branch
+of the trigeminal nerve.
+
+{317}With the formation of the vertebrate heart from the two longitudinal
+venous sinuses and the abolition of the dorsal invertebrate heart, the
+function of these tubular muscles as branchial hearts was no longer needed,
+and their respiratory function alone remained. The last remnant of this is
+seen in Ammocoetes, for the ordinary striated muscles were always more
+efficient for the respiratory act, and so at transformation the inferior
+tubular musculature was got rid of, there being no longer any need for its
+continued existence.
+
+
+THE PALÆOSTOMA, OR OLD MOUTH.
+
+The arrangement of the oral chamber in Ammocoetes is peculiar among
+vertebrates, and, upon my theory, is explicable by its comparison with the
+accessory oral chamber which apparently existed in Eurypterus. According to
+this explanation, the lower lip of the original vertebrate mouth was formed
+by the coalescence of the most posterior pair of the prosomatic
+appendages--the chilaria; from which it follows that the vertebrate mouth
+was not the original mouth, but a new structure due to such a formation of
+the lower lip.
+
+It is very suggestive that the direct following out of the original working
+hypothesis should lead to this conclusion, for it is universally agreed by
+all morphologists that the present mouth is a new formation, and Dohrn has
+argued strongly in favour of the mouth being formed by the coalescence of a
+pair of gill-slits. Interpret this in the language of my theory, and
+immediately we see, as already explained, gill-slits must mean in this
+region the spaces between appendages which did not carry gills; the mouth,
+therefore, was formed by the coalescence of a pair of appendages to form a
+lower lip just as I have pointed out.
+
+Where, then, must we look for the palæostoma, or original mouth? Clearly,
+as already suggested, it was situated at the base of the olfactory passage,
+and the olfactory passage or nasal tube of Ammocoetes was originally the
+tube of the hypophysis, so that the following out of the theory points
+directly to the tube of the hypophysis as the place where the palæostoma
+must be looked for.
+
+This conclusion is not only not at variance with the opinions of
+morphologists, but gives a straightforward, simple explanation why the
+palæostoma was situated in the very place where they are most inclined to
+locate it. Thus, if we trace the history of the question, {318}we see that
+Dohrn's original view of the comparison of the vertebrate and the annelid
+led him to the conception that the vertebrate mouth was formed by the
+coalescence of a pair of gill-slits, and that the original mouth was
+situated somewhere on the dorsal surface and opened into the gut by way of
+the infundibulum and the tube of the hypophysis. This, also, was
+Cunningham's view as far as the tube of the hypophysis was concerned.
+Beard, in 1888, holding the view that the vertebrates were derived from
+annelids which had lost their supra-oesophageal ganglia, and that,
+therefore, there was no question of an oesophageal tube piercing the
+central nervous system of the vertebrate, explained the close connection of
+the infundibulum with the hypophysis by the comparison of the tube of the
+hypophysis with the annelidan mouth, so that the infundibular or so-called
+nervous portion was a special nervous innervation for the original throat,
+just as Kleinenberg had shown to be the case in many annelids. Beard
+therefore called this opening of the hypophysial tube the old mouth, or
+palæostoma. Recently, in 1893, Kupffer has also put forward the view that
+the hypophysial opening is the palæostoma. basing this view largely upon
+his observations on Ammocoetes and Acipenser.
+
+[Illustration: FIG. 125.--DIAGRAM TO SHOW THE MEETING OF THE FOUR TUBES IN
+SUCH A VERTEBRATE AS THE LAMPREY.
+
+_Nc._, neural canal with its infundibular termination; _Nch._, notochord;
+_Al._, alimentary canal with its anterior diverticulum; _Hy._, hypophysial
+or nasal tube; _Or._, oral chamber closed by septum.]
+
+As is seen in Fig. 125, the position of this palæostoma is a very
+suggestive one. At this single point in Ammocoetes, four separate tubes
+terminate; here is the end of the notochordal tube, the termination of the
+infundibulum, the blind end of the nasal tube or tube {319}of the
+hypophysis, and the pre-oral elongation of the alimentary canal.
+
+It is perfectly simple and easy for the olfactory tube to open into any one
+of the other three. By opening into the infundibulum it reproduces the
+condition of affairs seen in the scorpion; by opening into the gut it
+produces the actual condition of things seen in Myxine and other
+vertebrates; by opening into the notochordal tube it would produce a
+transitional condition between the other two.
+
+The view held by Kupffer is that this nasal tube (tube of the hypophysis)
+opened into the anterior diverticulum of the vertebrate gut, and was for
+this reason the original mouth-tube; then a new mouth was formed, and this
+connection was closed, being subsequently reopened as in Myxine. My view is
+that this tube originally opened into the infundibulum, in other words,
+into the original gut of the palæostracan ancestor, and was for this reason
+the original mouth-tube, in the same sense as the olfactory passage of the
+scorpion may be, and often is, called the mouth-tube. When, with the
+breaking through of the septum between the oral and respiratory chambers,
+the external opening of the oral chamber became a new mouth, the old mouth
+was closed but the olfactory tube still remained, owing to the importance
+of the sense of smell. Subsequently, as in Myxine and the higher
+vertebrates, it opened into the pharynx, and so formed the nose of the
+higher vertebrates.
+
+It is not, to my mind, at all improbable that during the transition stage,
+between its connection with the old alimentary canal, as in Eurypterus or
+the scorpions, and its blind ending, as in Ammocoetes, the nasal tube
+opened into the tube of the notochord. This question will be discussed
+later on when the probable significance of the notochord is considered.
+
+
+THE PITUITARY GLAND.
+
+Turning back to the comparison of Fig. 106, B, and Fig. 106, C, which
+represent respectively an imaginary sagittal section through an
+Eurypterus-like animal and through Ammocoetes at a larval stage, all the
+points for comparison mentioned on p. 244 have now been discussed with the
+exception of the suggested homology between the coxal glands of the one
+animal and the pituitary body of the other.
+
+{320}This latter gland undoubtedly arises posteriorly to the hypophysial
+tube, or Rathke's pouch (as it is sometimes called), and, as already
+mentioned, is supposed by Kupffer to be formed from the posterior wall of
+this pouch. More recently, as pointed out in Haller's paper, Nusbaum, who
+has investigated this matter, finds that the glandular hypophysis is not
+formed from the walls of Rathke's pouch, but from the tissue of the
+rudimentary connection or stalk between the two premandibular cavities,
+which becomes closely connected with the posterior wall of Rathke's pouch,
+and becoming cut off from the rest of the premandibular cavity on each
+side, becomes permanently a part of the 'Hypophysis Anlage.'
+
+The importance of Nusbaum's investigation consists in this, that he derives
+the glandular hypophysis from the connecting stalk between the two
+premandibular cavities, and therefore from the walls of the ventral
+continuation of this cavity on each side.
+
+This may be expressed as follows:--
+
+The coelomic cavity, known as the premandibular cavity, divides into a
+dorsal and a ventral part; the walls of the dorsal part give origin to the
+somatic muscles belonging to the oculomotor nerve, while the walls of the
+ventral part on each side form the connecting stalk between the two
+cavities, and give origin to the glandular hypophysis.
+
+Now, as already pointed out, the premandibular cavity is homologous with
+the 2nd prosomatic coelomic cavity of Limulus, and this 2nd prosomatic
+coelomic cavity divides, according to Kishinouye, into a dorsal and a
+ventral part; and, further, the walls of this ventral part form the coxal
+gland. Both in the vertebrate, then, and in Limulus, we find a marked
+glandular tissue in a corresponding position, and the conclusion is forced
+upon us that the glandular hypophysis was originally the coxal gland of the
+invertebrate ancestor. As in all other cases already considered, when the
+facts of topographical anatomy, of morphology and of embryology, all
+combine to the same conclusion as to the derivation of the vertebrate organ
+from that of the invertebrate, then there must be also a structural
+similarity between the two. What, then, is the nature of the coxal gland in
+the scorpions and Limulus? Lankester's paper gives us full information on
+this point as far as the scorpion and Limulus are concerned, and he shows
+that the coxal gland of Limulus differs markedly from that of Scorpio in
+the size of the cells and in the {321}arrangement of the tubes. In Fig.
+126, A, I give a picture of a piece of the coxal gland of Limulus taken
+from Lankester's paper.
+
+Turning now to the vertebrate, Bela Haller's paper gives us a number of
+pictures of the glandular hypophysis from various vertebrates, and he
+especially points out the tubular nature of the gland and its
+solidification in the course of development in some cases. In Fig. 126, B,
+I give his picture of the gland in Ammocoetes.
+
+The striking likeness between Haller's picture and Lankester's picture is
+apparent on the face of it, and shows clearly that the histological
+structure of the glands in the two cases confirms the deductions drawn from
+their anatomical and morphological positions.
+
+[Illustration: FIG. 126.--A, SECTION OF COXAL GLAND OF LIMULUS (from
+LANKESTER); B, SECTION OF PITUITARY BODY OF AMMOCOETES (from BELA HALLER).
+
+_n.a._, termination of nasal passage.]
+
+The sequence of events which gave rise to the pituitary body of the
+vertebrate was in all probability somewhat as follows:--
+
+Starting with the excretory glands of the Phyllopoda, known as
+shell-glands, which existed almost certainly in the phyllopod Trilobite, we
+pass to the coxal gland of the Merostomata. Judging from Limulus, these
+were coextensive with the coxæ of the 2nd, 3rd, 4th, and 5th locomotor
+appendages. When these appendages became reduced in size and purely tactile
+they were compressed and concentrated round the mouth region, forming the
+endognaths of the Merostomata; as a necessary consequence of the
+concentration of the coxæ of the endognaths, the coxal gland also became
+concentrated, {322}and took up a situation close against the pharynx, as
+represented in Fig. 106, B. When, then, the old mouth closed, and the
+pharynx became the _saccus vasculosus_, the coxal gland remained in close
+contact with the _saccus vasculosus_, and became the pituitary body, thus
+giving the reason why there is always so close a connection between the
+pituitary body and the infundibular region.
+
+Whatever was the condition of the digestive tracts at the transition stage
+between the arthropod and the vertebrate, the original mouth-opening at the
+base of the olfactory tube was ultimately closed. The method of its closure
+was exceedingly simple and evident. The membranous cranium was in process
+of formation by the extension of the plastron laterally and dorsally; a
+slight growth of the same tissue in the region of the mouth would suffice
+to close it and thus separate the infundibulum from the olfactory tube. As
+evidence that such was the method of closure, it is instructive to see how
+in Ammocoetes the glandular tissue of the pituitary body is embedded in and
+mixed up with the tissue of this cranial wall; how the termination of the
+nasal tube is embedded in this same thickened mass of the cranial
+wall--how, in fact, both coxal gland and olfactory tube have become
+involved in the growth of the tissue of the plastron, by means of which the
+mouth was closed.
+
+I have now passed in review the nature of the evidence which justifies a
+comparison between the segments supplied by the cranial nerves of the
+vertebrate and the prosomatic and mesosomatic segments of the palæostracan.
+For the convenience of my readers I have put these conclusions into tabular
+form (see p. 323), for all the segments as far as that supplied by the
+glossopharyngeal nerves. In both vertebrate and invertebrate this is a
+fixed position, for in the former, however variable may be the number of
+branchial segments which the vagus supplies, the second branchial segment
+is always supplied by a separate nerve, the glossopharyngeal, and in the
+latter, though the number of segments bearing branchiæ varies, the minimum
+number of such segments (as seen in the Pedipalpi) is never less than two.
+
+{323}TABLE OF COMPARISON OF CORRESPONDING SEGMENTS IN THE EURYPTERIDS AND
+IN AMMOCOETES (_i.e._ IN CEPHALASPIDS).
+
+  Key:
+  So. = Supra-oesophageal.
+  Si. = Supra-infundibular.
+  Io. = Infra-oesophageal.
+  Ii. = Infra-infundibular.
+  Ps. = Prosomatic.
+  Ms. = Mesosomatic.
+
+  +---+--------------------------------------------------------+
+  |   |                      Median Eyes.                      |
+  |   +--------------------------------------------------------+
+  |So.|                      Lateral Eyes.                     |
+  |   +--------------------------------------------------------+
+  |   |                      Camerostome.                      |
+  +---+--------------------------------------------------------+
+  |   |         Invertebrate (Limulus or Eurypterid).          |
+  +---+---+---------+-----------------------------+------------+
+  |   |   |         |        Appendages.          |  Coelomic  |
+  |   |   |Segments.+-------------+---------------+  Cavities. |
+  |   |   |         |  Limulus.   | Eurypterid.   |            |
+  |   |   +---------+-------------+---------------+------------+
+  |   |   |    1    |  Cheliceræ  | Cheliceræ     |     1      |
+  |   |   +---------+-------------+---------------+------------+
+  |   |   |    2    |1st Locomotor|}              |            |
+  |   |   +---------+-------------+}              |     2      |
+  |   |   |    3    | 2nd    "    |}              |Ventral part|
+  |   |Ps.+---------+-------------+}4 Endognaths  | forms coxal|
+  |   |   |    4    | 3rd    "    |}              |   gland.   |
+  |   |   +---------+-------------+}              |            |
+  |   |   |    5    | 4th    "    |}              |            |
+  |   |   +---------+-------------+---------------+------------+
+  |Io.|   |    6    | 5th    "    |   Ectognath   |     3      |
+  |   |   |         |             |               |            |
+  |   |   +---------+-------------+---------------+------------+
+  |   |   |    7    | Chilaria    |   Metastoma   |     4      |
+  |   |   |         |             |               |            |
+  |   +---+---------+-------------+---------------+------------+
+  |   |   |    8    |  Operculum  |Genital  }     |     5      |
+  |   |   |---------+-------------+         }Oper-+------------+
+  |   |   |    9    |1st Branchial|         }culum|     6      |
+  |   |   |         |             |Branchial}     |            |
+  |   |Ms.+---------+-------------+---------------+------------+
+  |   |   |   10    | 2nd    "    | 2nd Branchial |     7      |
+  +---+---+---------+-------------+---------------+------------+
+
+  +-----------------------------------------------------+------+
+  |                Pineal Eyes.                         |      |
+  +-----------------------------------------------------+      |
+  |                Lateral Eyes.                        |  Si. |
+  +-----------------------------------------------------+      |
+  |               Olfactory Organ.                      |      |
+  +-----------------------------------------------------+------+
+  |        Vertebrate (Ammocoetes or Cephalaspid).      |      |
+  +-------------+---------------+-------------+---------+------+
+  |             |  Splanchnic   |   Somatic   | Somatic |      |
+  | Appendages. |    Nerves.    |  Segmental  | Nerves. |      |
+  |             |               |   Muscles.  |         |      |
+  +-------------+---------------+-------------+---------+      |
+  |      ...    |      ...      |     ...     |   ...   |      |
+  +-------------+---------------+-------------+---------+      |
+  |             |               |             |         |      |
+  |             |      V        |   Muscles   |         |      |
+  |4 tentacles  |  Tentacular   | supplied by |   III   |      |
+  | and upper   |  and upper    |  oculomotor |         |      |
+  |    lip.     |  lip nerve.   |   nerve.    |         |      |
+  |             |               |             |         |      |
+  |             |               |             |         |      |
+  +-------------+---------------+-------------+---------+      |
+  |   Tongue    |       V       | Sup.oblique |   IV    |  Ii. |
+  |             | Tongue nerve  |             |         |      |
+  +-------------+---------------+-------------+---------+      |
+  |  Lower lip  |       V       |             |         |      |
+  |             |Lower lip nerve|             |         |      |
+  +-------------+---------------+-------------+---------+      |
+  |   Thyroid   |}              |             |         |      |
+  +-------------+}     VII      | Ext. rectus |    VI   |      |
+  |    Hyoid or |}              |Retract oculi|         |      |
+  |1st Branchial|}              |             |         |      |
+  +-------------+---------------+-------------+---------+      |
+  |2nd Branchial|       IX      |     ...     |   ...   |      |
+  +-------------+---------------+-------------+---------+------+
+
+  +-------------------------+------+
+  |        Pineal Nerve.    |      |
+  +-------------------------+      |
+  |          II             |  Si. |
+  +-------------------------+      |
+  |           I             |      |
+  +-------------------------+------+
+  |        Vertebrate.      |      |
+  +-------------+-----------+------+
+  |   Coelomic  |V. Wijhe's |      |
+  |   Cavities. | Segments. |      |
+  |             |           |      |
+  +-------------+-----------+      |
+  |  Anterior.  |    ...    |      |
+  +-------------+-----------+      |
+  |  Premandi-  |           |      |
+  |    bular    |           |      |
+  |   Ventral   |           |      |
+  |  part forms |     1     |      |
+  |   pituitary |           |      |
+  |   body.     |           |      |
+  |   pituitary |           |      |
+  +-------------+-----------+      |
+  |  Mandibular |}          |  Ii. |
+  |             |}          |      |
+  +-------------+}    2     |      |
+  |  Mandibular |}          |      |
+  |             |}          |      |
+  +-------------+-----------+      |
+  |   Hyoid_1   |     3     |      |
+  +-------------+-----------+      |
+  |   Hyoid_2   |     4     |      |
+  |             |           |      |
+  +-------------+-----------+      |
+  |2nd Branchial|     5     |      |
+  +-------------+-----------+------+
+
+{324}SUMMARY.
+
+  The general consideration of the evidence of the number of segments, and
+  their nature in the pro-otic region of the vertebrate, as given in the
+  last chapter, is not incompatible with the view that the trigeminal nerve
+  originally supplied seven appendages, which appendages did not carry
+  branchiæ, but were originally used for purposes of locomotion as well as
+  of mastication.
+
+  Such appendages clearly no longer exist in the higher vertebrates, the
+  muscles of mastication only remaining; but in the earliest fish-forms
+  they must have existed, as, indeed, is seen in Pterichthys and
+  Bothriolepis. Judging from all the previous evidence some signs of their
+  existence may reasonably be expected still to remain in Ammocoetes. Such
+  is indeed the case.
+
+  In the adult Petromyzon the trigeminal nerve innervates specially a
+  massive suctorial apparatus, by means of which it holds on to other
+  fishes, or to stones in the bottom of the stream. There is here no
+  apparent sign of appendages. Very great, however, is the difference in
+  the oral chamber of Ammocoetes; here there is no sign of any suctorial
+  apparatus, but instead, a system of tentacles, together with the remains
+  of the septum or velum, which originally closed off the oral from the
+  respiratory chamber. These tentacles are the last remnants of the
+  original foremost prosomatic appendages of the palæostracan ancestor.
+  Like the lateral eyes they do not develop until the transformation comes,
+  but during the whole larval condition their musculature remains in an
+  embryonic condition, and then from these embryonic muscles the whole
+  massive musculature of the suctorial apparatus develops; a sucking
+  apparatus derived from the modification of appendages, as so frequently
+  occurs in the arthropods.
+
+  The study of Ammocoetes indicates that the velum and lower lip correspond
+  to the metastoma of the Eurypterid, _i.e._ the chilaria of Limulus, while
+  the large ventral pair of tentacles, called the tongue, correspond to the
+  ectognaths of the Eurypterids, and probably to the oar-like appendages of
+  Pterichthys and Bothriolepis. From these two splanchnic segments the
+  suctorial apparatus in the main arises; the motor supply of these two
+  segments forms the mass of the trigeminal nerve-supply, and the nerves
+  supplying them, the velar nerve and the tongue-nerve, are markedly
+  separate from the rest of the trigeminal nerve.
+
+  The rest of the tentacles present much less the sign of independent
+  segments. In their nerves, their muco-cartilaginous skeleton, and their
+  rudimentary muscles, they indicate a concentration and amalgamation, such
+  as might be expected from the concentrated endognaths. The continuation
+  of the dwindling process, already initiated in the Eurypterid, would
+  easily result in the tentacles of Ammocoetes.
+
+  The nasal tube of Ammocoetes, which originates in the hypophysial tube,
+  corresponds absolutely in position and in its original structure, to the
+  olfactory tube of a scorpion-like animal. From this homology two
+  conclusions of importance follow: (1) the old mouth, or palæostoma, of
+  the vertebrate was situated at the end of this tube, therefore, at the
+  termination of the infundibulum; (2) the upper lip, which by its growth,
+  brings the olfactory tube from a ventral to a dorsal position, was
+  originally formed by the foremost sternites or endostoma, or else by the
+  sterno-coxal processes of the second pair of prosomatic appendages of the
+  palæostracan ancestor.
+
+  In strict accordance with the rest of the comparisons made in this
+  region, the pituitary body shows by similarity of structure, as well as
+  of position, that it arose from the coxal glands, which were situated at
+  the base of the four endognaths.
+
+  {325}One after another, when once the clue has been found, all these
+  mysterious organs of the vertebrate, such as the pituitary and thyroid
+  glands, fall harmoniously into their place as the remnants of
+  corresponding important organs in the palæostraca.
+
+  Yet another clue is afforded by the tubular muscles of Ammocoetes, that
+  strange set of non-vertebrate striated muscles, which are so markedly
+  arranged in a segmental manner, which disappear at transformation, and
+  are never found in any of the higher vertebrates, for the limits of their
+  distribution correspond to the veno-pericardial muscles of Limulus.
+
+  Their nerve-supply in Ammocoetes is most extraordinary; for, although
+  they are segmentally arranged throughout the whole respiratory region,
+  which is segmentally supplied by the VIIth, IXth, and Xth nerves, and are
+  found in front of this region only in one segment, that of the lower lip,
+  which is supplied by the velar branch of the Vth nerve, yet they are not
+  supplied segmentally, but only by the velar nerve and a branch of the
+  VIIth, the _ramus branchialis profundus_. This latter nerve extends
+  throughout the respiratory region, and gives off segmental branches to
+  supply these muscles.
+
+  It is also a curious coincidence that in such a markedly segmented animal
+  as Limulus, a nerve--the pericardial nerve--which arises from the nerves
+  of the chilarial and opercular segments, should pass along the whole
+  respiratory region and give off branches to each mesosomatic segment. It
+  is strange, to say the least of it, that the chilarial or metastomal and
+  the opercular segments of Limulus should, on the theory advocated in this
+  book, correspond to the lower lip and hyoid segments of the vertebrate.
+  At present the homology suggested is not complete, for there is no
+  evidence as yet that the veno-pericardial muscles have anything to do
+  with the pericardial nerve.
+
+
+
+
+{326}CHAPTER X
+
+_THE RELATIONSHIP OF AMMOCOETES TO THE MOST ANCIENT FISHES--THE
+OSTRACODERMATA_
+
+  The nose of the Osteostraci.--Comparison of head-shield of Ammocoetes and
+  of Cephalaspis.--Ammocoetes the only living representative of these
+  ancient fishes.--Formation of cranium.--Closure of old mouth.--Rohon's
+  primordial cranium.--Primordial cranium of Phrynus and
+  Galeodes.--Summary.
+
+
+The shifting of the orifice of the olfactory passage, which led to the old
+mouth, from the ventral to the dorsal side, as seen in the transformation
+of the ventrally situated hypophysial tube of the young Ammocoetes, to the
+dorsally situated nasal tube of the full-grown Ammocoetes, affords one of
+the most important clues in the whole of this story of the origin of
+vertebrates; for, if Ammocoetes is the nearest living representative of the
+first-formed fishes, then we ought to expect to find that the dorsal
+head-shield of such fishes is differentiated from that of the contemporary
+Palæostraca by the presence of a median frontal opening anterior to the
+eyes. Conversely, if such median nasal orifice is found to be a marked
+characteristic of the group, in combination with lateral and median eyes,
+as in Ammocoetes, then we have strong reasons for interpreting these
+head-shields by reference to the head of Ammocoetes.
+
+The oldest known fishes belong to a large group of strange forms which
+inhabited the Silurian and Devonian seas, classed together by Smith
+Woodward under the name of Ostracodermi. These are divided into three
+orders: (1) the Heterostraci, including one family, the Pteraspidæ, to
+which Pteraspis and Cyathaspis belong; (2) the Osteostraci, divisible into
+two families, the Cephalaspidæ and Tremataspidæ, which include Cephalaspis,
+Eukeraspis, Auchenaspis or Thyestes, and Tremataspis; and (3) the
+Antiarcha, with one family, the Astrolepidæ, including Astrolepis,
+Pterichthys, and Bothriolepis. {327}Of these, the first two orders belong
+to the Upper Silurian, while the third is Devonian.
+
+
+THE DORSAL HEAD-SHIELD OF THE OSTEOSTRACI.
+
+Of the three orders above-named, the Heterostraci and Osteostraci are the
+oldest, and among them the Cephalaspidæ have afforded the most numerous and
+best worked-out specimens. At Rootziküll, in the island of Oesel, the form
+known as _Thyestes (Auchenaspis) verrucosus_ is especially plentiful, being
+found thickly present in among the masses of Eurypterid remains, which give
+the name to the deposit. Of late years this species has been especially
+worked at by Rohon, and many beautiful specimens have been figured by him,
+so that a considerable advance has been made in our knowledge since Pander,
+Eichwald, Huxley, Lankester, and Schmidt studied these most interesting
+primitive forms.
+
+All observers agree that the head-region of these fishes was covered by a
+dorsal and ventral head-shield, while the body-region was in most cases
+unknown, or, as in Eichwald's specimens, and in the specimens figured in
+Lankester and Smith Woodward's memoirs, was made up of segments which were
+not vertebral in character, but formed an aponeurotic skeleton, being the
+hardened aponeuroses between the body-muscles. This body-skeleton, which
+possesses its exact counterpart in Ammocoetes, will be considered more
+fully when I discuss the origin of the spinal region of the vertebrate.
+
+Of the two head-shields, ventral and dorsal, the latter is best known and
+characterizes the group. It consists of a dorsal plate, with characteristic
+horns, which in _Thyestes verrucosus_ (Fig. 128), as described by Rohon, is
+composed of two parts, a frontal part and an occipital part (_occ._), the
+occipital part being composed of segments, and possessing a median
+ridge--the _crista occipitalis_. In Lankester's memoir and in Smith
+Woodward's catalogue, a large number of known forms are described and
+delineated, and we may perhaps say that in some of the forms, such as
+_Eukeraspis pustuliferus_ (Fig. 127, B), the frontal part of the shield
+only is capable of preservation as a fossil, while in Cephalaspis (Fig.
+127, A) not only the frontal part but a portion of the occipital region is
+preserved, the latter being small in extent when compared with the
+occipital region of Auchenaspis (Thyestes). Finally, in Tremataspis and
+Didymaspis, the whole of both frontal {328}and occipital region is capable
+of preservation, the line of demarcation between these two regions being
+well marked in the latter species.
+
+[Illustration: FIG. 127.--A, DORSAL HEAD-SHIELD OF CEPHALASPIS (from
+LANKESTER); B, DORSAL HEAD-SHIELD OF KERASPIS (from LANKESTER).]
+
+In the best preserved specimens of all this group of fishes a frontal
+median orifice is always present; it appears in some specimens obscurely
+partially divided into two parts. Perhaps the best specimen of all was
+obtained by Rohon at Rootziküll, and is thus described by him:--
+
+The frontal part of the dorsal head-plate carried (Fig. 128) the two orbits
+for the lateral eyes (_l.e._), a marked frontal organ (_fro._), and a
+median depression (_gl._), to which he gives the name parietal organ. The
+occipital part (_occ._) was clearly segmented, and carried, he thinks, the
+branchiæ. I reproduce Rohon's figure of the frontal organ in Thyestes (Fig.
+129); he describes it as a deeply sunk pit, divided in the middle by a
+slit, which leads deeper in, he supposes, towards the central nervous
+system.
+
+[Illustration: FIG. 128.--DORSAL HEAD-SHIELD OF _Thyestes (Auchenaspis)
+verrucosus_. (From ROHON.)
+
+_Fro._, narial opening; _l.e._, lateral eyes; _gl._, glabellum or plate
+over brain; _Occ._, occipital region.]
+
+{329}A similar organ was described by Schmidt in Tremataspis, and
+considered by him to be a median nose. Such also is the view of Jaekel, who
+points out that a median pineal eye exists between the two lateral eyes in
+this animal, as in all other of these ancient fishes, so that this frontal
+organ does not, as Patten thinks, represent the pineal eye. The whole of
+this group of fishes, then, is characterized by the following striking
+characteristics:--
+
+1.  Two well-marked lateral eyes near the middle line.
+
+2.  Between the lateral eyes, well-marked median eyes, very small.
+
+3.  In front of the eye-region a median orifice, single.
+
+In addition, behind the eye-region a median plate is always found,
+frequently different in structure to the rest of the head-shield, being
+harder in texture--the so-called post-orbital plate.
+
+[Illustration: FIG. 129.--NARIAL OPENING AND LATERAL ORBITS OF _Thyestes
+Verrucosus_. (From ROHON.)]
+
+
+STRUCTURE OF HEAD-SHIELD OF CEPHALASPIS COMPARED WITH THAT OF AMMOCOETES.
+
+What is the structure of this head-shield? It has been spoken of as formed
+of bone because it possesses cells, being thus unlike the layers of chitin,
+which are formed by underlying cells but are not themselves cellular. At
+the same time, it is recognized on all sides that it has no resemblance to
+bone-structure as seen in fossil remains of higher vertebrates. The latest
+and best figure of the structure of this so-called bone is given in Rohon's
+paper already referred to. It is, so he describes, clearly composed of
+fibrillæ and star-shaped cells, arranged more or less in regular layers,
+with other sets of similar cells and fibrillæ arranged at right angles to
+the first set, or at varying angles. The groundwork of this tissue, in
+which these cells and fibrils are embedded, contained calcium salts, and so
+the whole tissue was preserved. In places, spaces are found in it, in the
+deepest layer large medullary spaces; more superficially, ramifying spaces
+which he considers to be vascular, and calls Haversian canals; the
+{330}star-like cells, however, are not arranged concentrically around these
+spaces, as in true Haversian canals.
+
+This structure is therefore a calcareous infiltration of a tissue with
+cells in it. Where is there anything like it?
+
+As soon as I saw Rohon's picture (Fig. 130), I was astounded at its
+startling resemblance to the structure of muco-cartilage as is seen in Fig.
+131, taken from Ammocoetes. If such muco-cartilage were infiltrated with
+lime salts, then the muco-cartilaginous skeleton of Ammocoetes would be
+preserved in the fossil condition, and be comparable with that of
+Cephalaspis, etc.
+
+[Illustration: FIG. 130.--SECTION OF A HEAD-PLATE OF A CEPHALASPID. (From
+ROHON.)]
+
+[Illustration: FIG. 131.--SECTION OF MUCO-CARTILAGE FROM DORSAL HEAD-PLATE
+OF AMMOCOETES.]
+
+The whole structure is clearly remarkably like Rohon's picture of a section
+of the head-plate of a Cephalaspid (Fig. 130). In the latter case the
+matrix contains calcium salts, in the former it is composed of the peculiar
+homogeneous mucoid tissue which stains so characteristically with thionin.
+With respect to this calcification, it is instructive to recall the
+calcification in the interior of the branchial cartilages of Limulus, as
+described in Chapter III., for this example shows how easy it is to obtain
+a calcification in this chondro-mucoid material. With respect to the
+medullary spaces and smaller spaces in this tissue, as described by Rohon,
+I would venture to suggest that they need not all necessarily indicate
+blood-vessels, for similar spaces would appear in the head-shield of
+Ammocoetes if its muco-cartilage alone {331}were preserved. Of these, some
+would indicate the position of blood-vessels, such, for instance, as of the
+external carotid which traverses this structure; but the largest and most
+internal spaces, resembling Rohon's medullary spaces, would represent
+muscles, being filled up with bundles of the upper lip-muscles.
+
+
+THE MUCO-CARTILAGINOUS HEAD-SHIELD OF AMMOCOETES.
+
+The resemblance between the structure of the head-shield of Thyestes and
+the muco-cartilage of Ammocoetes, is most valuable, for muco-cartilage is
+unique, occurs in no other vertebrate, and every trace of it vanishes at
+transformation; it is essentially a characteristic of the larval form, and
+must, therefore, in accordance with all that has gone before, be the
+remnant of an ancestral skeletal tissue. The whole story deduced from the
+study of Ammocoetes would be incomplete without some idea of the meaning of
+this tissue. So also, as already mentioned, the skeleton of Ammocoetes is
+incomplete without taking this tissue into account. It is confined entirely
+to the head-region; no trace of it exists posteriorly to the branchial
+basket-work. It consists essentially of dorsal and ventral head-shields,
+connected together by the tentacular, metastomal, and thyroid bars, as
+already described. The ventral shield forms the muco-cartilaginous plate of
+the lower lip and the plate over the thyroid gland, so that the skeleton
+ventrally is represented by Fig. 118, B, which shows how the cartilaginous
+bars of the branchial basket-work are separated from each other by this
+thyroid plate. At transformation, with the disappearance of this
+muco-cartilaginous plate, the bars come together in the middle line, as in
+the more posterior portion of the branchial basket-work.
+
+The dorsal head-shield of muco-cartilage covers over the upper lip, sends a
+median prolongation over the median pineal eyes and a lateral prolongation
+on each side as far as the auditory capsules, giving the shape of the
+head-shield of muco-cartilage, as in Fig. 118, C.
+
+Not only then is the structure of the head-shield of a Cephalaspid
+remarkably like the muco-cartilage of Ammocoetes, but also its general
+distribution strangely resembles that of the Ammocoetes muco-cartilage.
+
+Now, these head-shields in the Cephalaspidæ and Tremataspidæ {332}vary very
+much in shape, as is seen by the comparison of Tremataspis and Auchenaspis
+with Cephalaspis and Eukeraspis, and yet, undoubtedly, all these forms
+belong to a single group, the Osteostraci.
+
+The conception that Ammocoetes is the solitary living form allied to this
+group affords a clue to the meaning of this variation of shape, which
+appears to me to be possible, if not indeed probable. There is a certain
+amount of evidence given in the development of Ammocoetes which indicates
+that the branchial region of its ancestors was covered with plates of
+muco-cartilage as well as the prosomatic region.
+
+The evidence is as follows:--
+
+The somatic muscles of Ammocoetes form a continuous longitudinal sheet of
+muscles along the length of the body, which are divided up by connective
+tissue bands into a series of imperfect segments or myotomes. This simple
+muscular sheet can be dissected off along the whole of the head-region of
+the animal, with the exception of the most anterior part, without
+interfering with the attachments or arrangements of the splanchnic muscular
+system in the least. The reason why this separation can be so easily
+effected is to be found in the fact that the two sets of muscles are not
+attached to the same fascia. The sheet of fascia to which the somatic
+muscles are attached is separated from the fascia which encloses the
+branchial cavity by a space (_cf._ Figs. 63 and 64) filled with
+blood-spaces and cells containing fat, in which space is also situated the
+cartilaginous branchial basket-work. These branchial bars are closely
+connected with the branchial sheet of fascia, and have no connection with
+the somatic fascia, their perichondrium forming part of the former sheet.
+Upon examination, this space is seen to be mainly vascular, the
+blood-spaces being large and frequently marked with pigment; but it also
+possesses a tissue of its own, recognized as fat-tissue by all observers.
+The peculiarity of the cells of this tissue is their arrangement; they are
+elongated cells arranged at right angles to the plates of fascia, just as
+the fibres of the muco-cartilage are largely arranged at right angles to
+their limiting plates of perichondrium. These cells do not necessarily
+contain fat; and when they do, the fat is found in the centre of each cell,
+and does not push the protoplasm of the cell to the periphery, as in
+ordinary fat cells.
+
+{333}In Fig. 132, B, I give a specimen of this tissue stained by osmic
+acid; in Fig. 132, A, I give a drawing of ordinary muco-cartilage taken
+from the plate of the lower lip; and in Fig. 133, A, a modification of the
+muco-cartilage taken from the velum, which shows the formation of a tissue
+intermediate between ordinary muco-cartilage and this branchial fat-tissue.
+
+Further, in fully-grown specimens of Ammocoetes, in the region of undoubted
+muco-cartilage, a fatty degeneration of the cells frequently appears,
+together with an increase in the blood spaces,--the precursor, in fact, of
+the great change which overtakes this tissue soon afterwards, at the time
+of transformation, when it is invaded by blood, and swept away, except in
+those places where new cartilage is formed. I conclude, then, that the
+tissue of this vascular space was originally muco-cartilage, which has
+degenerated during the life of the Ammocoetes. The fact that in most cases
+undoubted muco-cartilage is to be found here and there in this space, is
+strong confirmation of the truth of this conclusion.
+
+[Illustration: FIG. 132.--A, MUCO-CARTILAGE OF LOWER LIP (_Mc._); _m.ph._,
+muscle of lower lip; _m.sm._, somatic muscle; _Cor._, laminated layer of
+skin. B, DEGENERATED MUCO-CARTILAGE OF BRANCHIAL REGION. _F._, fat layer;
+_P._, pigment; _Bl._, blood-space; _N._, somatic nerve; _m.br._, branchial
+muscle; _m.sm._, somatic muscle.]
+
+If this conclusion is correct, we may expect that it would be confirmed by
+the embryological history of the tissue, and we ought to find that in much
+younger stages a homogeneous tissue of the same nature as muco-cartilage
+fills up the spaces in the branchial {334}region, where in the Ammocoetes
+only blood and fat-containing cells are present. For this purpose Shipley
+kindly allowed me to examine his series of sections through the embryo at
+various ages. These specimens are very instructive, especially those
+stained by osmic acid, which preserves the natural thickness of this space
+better than other staining methods. At an age when the branchial cartilages
+are seen to be formed, when no fat-cells are present, a distinctive tissue
+(Fig. 133, B) is plainly visible in the velum and at the base of the
+tentacles, in the very position where in the more advanced Ammocoetes
+muco-cartilage exists. Taking, then, this tissue as our guide, the
+specimens show that the space between the skin and the visceral muscles in
+which the cartilaginous basket-work lies is filled with a similar material.
+At this stage a sheet of embryonic tissue occupies the position where,
+later on, blood-spaces and fat-cells are found, and this tissue resembles
+that seen in the velum and other places where muco-cartilage is afterwards
+found.
+
+[Illustration: FIG. 133.--A, MUCO-CARTILAGE OF VELUM; B, EMBRYONIC
+MUCO-CARTILAGE OF TENTACULAR BAR.]
+
+I conclude, therefore, that originally the branchial or mesosomatic region
+was covered with a dorsal plate of muco-cartilage, which carried on its
+under surface the dorsal part of the branchial basket-work, and sprang from
+the central core of skeletogenous tissue around the notochord; this plate
+was separated from the plate which covered this region ventrally by the
+lateral grove in which the gill-slits are situated. The ventral plate
+carried on its under surface the ventral part of the branchial basket-work,
+and was originally continuous with the plate over the thyroid gland.
+
+{335}[Illustration: FIG. 134.--SKELETON OF HEAD-REGION OF AMMOCOETES. A,
+LATERAL VIEW; B, VENTRAL VIEW; C, DORSAL VIEW.
+
+Muco-cartilage, _red_; soft cartilage, _blue_; hard cartilage, _purple_.
+_sk_1_, _sk_2_, _sk_3_, skeletal bars; _c.e._, position of pineal eye; _na.
+cart._, nasal cartilage; _ped._, pedicle; _cr._, cranium; _nc._,
+notochord.]
+
+{336}In Fig. 134, A, B, C, the cranial skeleton of Ammocoetes is
+represented from the dorsal, ventral, and lateral aspects. The
+muco-cartilage is coloured red, the branchial or soft cartilage blue, and
+the hard cartilage purple. The degenerated muco-cartilage of the branchial
+region is represented as an uncoloured plate, on which the branchial
+basket-work stands in relief. If it were restored to its original condition
+of muco-cartilage, it would represent a uniform plate, on the _under_
+surface of which the basket-work would be situated; and if it were
+calcified and made solid, the branchial basket-work would not show at all
+in these figures.
+
+Is it possible to find the reason why this skeletal covering has
+degenerated so early before transformation, and why the thyroid plate
+remains intact until transformation? We see that all that part which has
+degenerated is covered over by the somatic muscles,--by, in fact, muscles
+which, being innervated by the foremost spinal nerves, belong naturally to
+the region immediately following the branchial. I suggest, therefore, that
+the original skeletal covering of muco-cartilage has remained intact only
+where it has not been invaded and covered over by somatic muscles, but has
+been invaded by blood and undergone the same kind of degenerative change as
+overtakes the great mass of this tissue at transformation wherever the
+somatic muscles have overgrown it.
+
+The covering somatic muscles in the branchial region form a dorsal and
+ventral group, of which the latter is formed in the embryo much later than
+the former, the line of separation between the two groups being the lateral
+groove, with its row of branchial openings. This groove ends at the first
+branchial opening, but the ventral and dorsal somatic muscles continue
+further headwards. It is instructive to see that, although the lateral
+groove terminates, the separation between the two groups of muscles is
+still marked out by a ridge of muco-cartilage, represented in Fig. 134, A,
+which terminates anteriorly in the opercular bar.
+
+Passing now to the prosomatic region, we find that here, too, the
+muco-cartilaginous external covering is divisible into a dorsal and a
+ventral head-plate, the ventral head-plate being the plate of the lower
+lip, and the dorsal head-plate the plate of muco-cartilage over the front
+part of the head. The staining reaction with thionin maps out this dorsal
+head-plate in a most beautiful manner, and shows that the whole of the
+upper lip-region in front of the nasal orifice is one large plate of
+muco-cartilage, obscured largely by the invasion of the crossing muscles of
+the upper lip, but left pure and uninvaded all around the nasal orifice,
+and where the upper and lower lips come together. In addition to this
+foremost plate, a median tongue of muco-cartilage covers over the pineal
+eye and fills up the {337}median depression between the two median dorsal
+somatic muscles. Also, two lateral cornua pass caudalwards from the main
+frontal mass of muco-cartilage over the lateral eyes, forming the
+well-known wedge which separates the dorsal and lateral portions of the
+dorso-lateral somatic muscle. In fact, similarly to what we find in the
+branchial region, the muco-cartilaginous covering can be traced with
+greater or less completeness only in those parts which are not covered by
+somatic muscles.
+
+In Fig. 134, A, B, C, this striking muco-cartilaginous head-shield, both
+dorsal and ventral, is shown. Seeing that the upper lip wraps round the
+lower one on each side, and that this most ventral edge of the upper lip
+contains muco-cartilage, as is seen in Fig. 117, the dorsal head-shield of
+muco-cartilage ought, strictly speaking, to extend more ventrally in the
+drawings. I have curtailed it in order not to interfere with the
+representation of the lower lip and tentacular muco-cartilages.
+
+From what has been said, it follows that the past history of the skeletal
+covering of the whole head-region of Ammocoetes, both frontal and
+occipital, can be conjectured by means of the ontogenetic history of the
+foremost myomeres.
+
+Dohrn and all other observers are agreed that during the development of
+this animal a striking forward growth of the foremost somatic myomeres
+takes place, so that, as Dohrn puts it, the body-musculature has extended
+forwards over the gill-region, and at the same time the gill-region has
+extended backwards. It is therefore probable that in the ancestral form the
+myotomes, innervated by the first spinal nerves, immediately succeeded the
+branchial region. Judging from Ammocoetes, the forward growth was at first
+confined to the dorsal region, and therefore invaded the dorsal head-plate,
+the ventral musculature being distinctly a later growth. With respect to
+this dorsal part of the myotomes, the first myotome is originally situated
+some distance behind the auditory capsule, and then grows forward towards
+the nasal opening; the lateral part, according to Hatschek, grows forward
+more quickly than the dorsal part, and splits itself above and below the
+eye into a dorso-lateral part, which extends up to the olfactory capsule,
+and a ventro-lateral part (_m. lateralis capitis_ anterior, superior, and
+inferior), thus giving rise to the characteristic appearance of the
+muco-cartilaginous head-shield of Ammocoetes.
+
+According, then, to the extent of the growth of these somatic {338}muscles,
+the shape of the muco-cartilaginous head-shield will vary, and if it were
+calcified and then fossilized we should obtain fossil head-shields of
+widely differing configuration, although such fossils might be closely
+allied to each other. This is just what is found in this group. Let the
+muco-cartilage extend over the whole of the branchial region of Ammocoetes,
+the resulting head-shield would be as in Fig. 135, A; the branchial bars
+below the muco-cartilaginous shield might or might not be evident, and the
+line between the branchial and the trigeminal region might or might not be
+indicated. Such a head-shield would closely resemble those of Didymaspis
+and Tremataspis respectively. Now suppose the somatic musculature to
+encroach slightly on the branchial region and also laterally to the end of
+the anterior branchial region, then we should obtain a shape resembling
+that of Thyestes (Fig. 135, B). Continue the same process further, the
+lateral muscle always encroaching further than the median masses, until the
+whole or nearly the whole branchial region is invested, and we get the
+head-shield of Cephalaspis (Fig. 135, C); further still, that of Keraspis,
+and yet still further, that of Ammocoetes (Fig. 135, D).
+
+[Illustration: FIG. 135.--DIAGRAMS TO SHOW THE DIFFERENT SHAPES OF
+HEAD-SHIELDS DUE TO THE FORWARD GROWTH OF THE SOMATIC MUSCULATURE.
+
+A, Didymaspis; B, Auchenaspis; C, Cephalaspis; D, Ammocoetes.]
+
+So close is this similarity, from the comparative point of view, between
+the dorsal head-shield of the Osteostraci and the dorsal cephalic region of
+Ammocoetes that it justifies us in taking Ammocoetes as the nearest living
+representative of such types; it is justifiable, therefore, to interpret by
+means of Ammocoetes the position of other organs in these forms. First and
+foremost is the hard plate {339}known as the post-orbital plate, so
+invariably found. In Fig. 134, C, I have inserted (_cr._) the position of
+the membranous cranium of Ammocoetes, and it is immediately evident that
+the primordial cranium of the Osteostraci must occupy the exact position
+indicated by this median hard plate. For this very reason this median plate
+would be harder than the rest in order to afford a better protection to the
+brain underneath. This plate, because of its position, may well receive the
+same name as the similar plate in the trilobite and various palæostracans
+and be called the glabellum.
+
+
+EVIDENCE OF SEGMENTATION IN THE HEAD-SHIELD--FORMATION OF CRANIUM.
+
+We may thus conceive the position of the nose, lateral eyes, median eyes,
+and cranium in these old fishes. In addition, other indications of a
+segmentation in this head-region have been found. The most striking of all
+the specimens hitherto discovered are some of _Thyestes verrucosus_,
+discovered by Rohon, in which the dorsal shield has been removed, and so we
+are able to see what that dorsal shield covered.
+
+In Fig. 136, I reproduce his drawing of one of his specimens from the
+dorsal and lateral aspects. These drawings show that the frontal part of
+the shield covered a markedly segmented part of the animal; five distinct
+segments are visible apart from the median most anterior region. This
+segmented region is entirely confined to the prosomatic region, _i.e._ to
+the region innervated by the trigeminal nerve. An indication of similar
+markings is given in Lankester's figure of _Eukeraspis pustuliferus_ (see
+Fig. 127, B), and, indeed, evidence of a segmentation under the
+antero-lateral border of the head-shield is recognized at the present time,
+not only in the Cephalaspidæ, but also in the Pteraspidæ, as was pointed
+out to me by Smith Woodward in the specimens at the British Museum. Also,
+in _Cyathaspis_, Jaekel has drawn attention to markings of a similar
+segmental nature (Fig. 137).
+
+There seems, then, little doubt but that these primitive fishes possessed
+something in this region which was of a segmental character, and indicated
+at least five segments, probably more.
+
+Rohon entitles his discovery 'the segmentation of the primordial cranium.'
+It would, I think, be better to call it the segmentation of {340}the
+anterior region of the head, for that is in reality what his figures show,
+not the segmentation of the primordial cranium, which, to judge from
+Ammocoetes, was confined to the region of the glabellum.
+
+What is the interpretation of this appearance?
+
+[Illustration: FIG. 136.--LATERAL AND DORSAL VIEWS OF THE FRONTAL AND
+OCCIPITAL REGIONS OF THE HEAD-SHIELD OF THYESTES, AFTER REMOVAL OF THE
+OUTER SURFACE. (From ROHON.)]
+
+[Illustration: FIG. 137.--UNDER SURFACE OF HEAD-SHIELD OF CYATHASPIS. (From
+JAEKEL.)
+
+_A._, lateral eyes; _Ep._, median eyes.]
+
+Any segmentation in the head-region must be indicative of segments
+belonging to the trigeminal or prosomatic region, or of segments belonging
+to the vagus or mesosomatic region. Many palæontologists, looking upon
+segmentation as indicative of gills and gill-slits, have attempted to
+interpret such markings as branchial segments, regardless of their
+position. As the figures show, they extend in front of the eyes and reach
+round to the front middle line, a position which is simply impossible for
+gills, but points directly to a segmentation connected with the trigeminal
+nerve. Comparison with Ammocoetes makes it plain enough that the markings
+in question are prosomatic in position, and that the gill-region must be
+sought for in the place {341}where Schmidt and Rohon located it in
+Thyestes, viz. the so-called occipital region.
+
+This discovery of Rohon's is, in my opinion, of immense importance, for it
+indicates that, in these early fishes, the prosomatic segmentation,
+associated with the trigeminal nerve, was much more well-marked than in any
+fishes living in the present day. Why should it be more well-marked?
+Turning to the palæostracan, it is very suggestive to compare the markings
+on their prosomatic carapace with these markings. Again and again we find
+indications of segmentation in these fossils similar to those seen in the
+ancient fishes. Thus in Fig. 138 I have put side by side the palæostracan
+_Bunodes_ and the fish _Thyestes_, both life size. In the latter I have
+indicated Rohon's segments; in the former the markings usually seen.
+
+From the evidence of Phrynus, Mygale, etc., as already pointed out, such
+markings in the palæostracan fossils would indicate the position of the
+tergo-coxal muscles of the prosomatic appendages, even though such
+appendages have not yet been discovered, and it is significant that in all
+these cases there is a distinct indication of a median plate or glabellum
+in addition to the segmental markings. Especially instructive is the
+evidence of Phrynus, as is seen by a comparison of Figs. 107 and 108, which
+shows clearly that this median plate (_glab._) covered the brain-region, a
+brain-region which is isolated and protected from the tergo-coxal muscles
+by the growth dorsalwards of the flanges of the plastron. In this way an
+incipient cranium of a membranous character is formed, which helps to give
+attachment to these tergo-coxal muscles. As such cranium is derived
+directly from the plastron, it is natural that it should ultimately become
+cartilaginous, just as occurs when Ammocoetes becomes Petromyzon and the
+cartilaginous cranium of the latter arises from the membranous cranium of
+the former. In Galeodes also the growth dorsalwards of the lateral flanges
+of the plastron to form an incipient cranium in which the brain lies is
+very apparent.
+
+[Illustration: FIG. 138.--A, OUTLINE OF _Thyestes Verrucosus_ WITH ROHON'S
+SEGMENTS INDICATED; B, OUTLINE OF _Bunodes Lunula_ WITH LATERAL EYES
+INSERTED.
+
+Both figures natural size.]
+
+{342}I venture, then, to suggest that in the Osteostraci the median hard
+plate or glabellum protected a brain which was enclosed in a membranous
+cranium, very probably not yet complete in the dorsal region--certainly not
+complete if the median pineal eyes so universally found in these ancient
+fishes were functional--a cranium derived from the basal trabeculæ, in
+precisely the same manner as we see it already in its commencement in
+Phrynus and other scorpions. With the completion of this cranium and its
+conversion into cartilage, and subsequently into bone, an efficient
+protection was afforded to the most vital part of the animal, and thus the
+hard head-shield of the Palæostraca and of the earliest fishes was
+gradually supplanted by the protecting bony cranium of the higher
+vertebrates.
+
+Step by step it is easy to follow in the mind's eye the evolution of the
+vertebrate cranium, and because it was evolved direct from the plastron,
+the impossibility of resolving it into segments is at once manifest; for
+although the plastron was probably originally segmented, as Schimkéwitsch
+thinks, all sign of such segmentation had in all probability ceased, before
+ever the vertebrates first made their appearance on the earth.
+
+It follows further, from the comparison here made, that those
+antero-lateral markings indicative of segments, found so frequently in
+these primitive fishes, must be interpreted as due not to gills but to
+aponeuroses, due to the presence of muscles which moved prosomatic
+appendages, muscles which arose from the dorsal region in very much the
+same position as do the muscles of the lower lip in Ammocoetes; the latter,
+as already argued, represent the tergo-coxal muscles of the last pair of
+prosomatic appendages--the chilaria or metastoma. Such an interpretation of
+these markings signifies that the first-formed fishes must have possessed
+prosomatic appendages of a more definite character than the tentacles of
+Ammocoetes, something intermediate between those of the palæostracan and
+Ammocoetes.
+
+For my part I should not be in the least surprised were I to hear that
+something of the nature of appendages in this region had been found,
+especially in view of the well-known existence of the pair of appendages in
+the members of the Asterolepidæ--large, oar-like appendages which may well
+represent the ectognaths.
+
+
+{343}THE RELATIONSHIP OF THE OSTRACODERMS.
+
+Of the three groups of fishes--the Heterostraci, the Osteostraci, and the
+Antiarcha--the last is Devonian, and therefore the latest in time of the
+three, while the earliest is the first group, as both Pteraspis and
+Cyathaspis have been found in lower levels of the Silurian age than any of
+the Osteostraci, and, indeed, Cyathaspis has been discovered in Sweden in
+the lower Silurian. This, the earliest of all groups of fishes, is confined
+to two forms only--Pteraspis and Cyathaspis,--for Scaphaspis is now
+recognized to be the ventral shield of Pteraspis.
+
+Hitherto a strong tendency has existed in the minds both of the comparative
+anatomist and the palæontologist to look on the elasmobranchs as the
+earliest fishes, and to force, therefore, these strange forms of fish into
+the elasmobranch ranks. For this purpose the same device is often used as
+has been utilized in order to account for the existence of the
+Cyclostomata, viz. that of degeneration. The evidence I have put forward is
+very strongly in favour of a connection between the cyclostomes and the
+cephalaspids, and agrees therefore with all the rest of the evidence that
+the jawless fishes are more ancient than those which bore jaws--the
+Gnathostomata.
+
+This is no new view. It was urged by Cope, who classified the Heterostraci,
+Osteostraci, and Antiarcha under one big group--the Agnatha--from which
+subsequently the Gnathostomata arose. Cope's arguments have not prevailed
+up to the present time, as is seen in the writings of Traquair, one of the
+chief authorities on the subject in Great Britain. He is still an advocate
+of the elasmobranch origin of all these earliest fishes, and claims that
+the latest discoveries of the Silurian deposits (_Thelodus Pagei_) and
+other members of the Coelolepidæ confirm this view of the question.
+
+This view may be summed up somewhat as follows:--
+
+Cartilaginous jaws would not fossilize, and the Ostracoderms may have
+possessed them.
+
+They may have degenerated from elasmobranchs just as the cyclostomes are
+supposed to have degenerated.
+
+Seeing that bone succeeds cartilage, the presence of bony shields in
+Cephalaspis, etc., indicates that their precursors were cartilaginous,
+presumably elasmobranch fishes.
+
+Of these arguments the strongest is based on the supposed bony
+{344}covering of the Osteostraci, with the consequent supposition that
+their ancestors possessed a cartilaginous covering. This argument is
+entirely upset, if, as I have pointed out, the structure of the cephalaspid
+shield is that of muco-cartilage and not of bone. If these plates are a
+calcified muco-cartilage, then the whole argument for their ancestry from
+animals with a cartilaginous skeleton falls to the ground, for
+muco-cartilage is the precursor not only of bone, but also of cartilage
+itself.
+
+The evidence, then, points strongly in favour of Cope's view that the most
+primitive fishes were Agnatha, after the fashion of cyclostomes, as is also
+believed by Smith Woodward, Bashford Dean, and Jaekel.
+
+Among living animals, as I have shown, the Limulus is the sole survivor of
+the palæostracan type, and Ammocoetes alone gives a clue to the nature of
+the cephalaspid, _i.e._ the osteostracan fish. Older than the latter is the
+heterostracan, Pteraspis, and Cyathaspis. Is it possible from their
+structure to obtain any clue as to the actual passage from the palæostracan
+to the vertebrate?
+
+Here again, as in the case of the Osteostraci, a relationship to the
+elasmobranch has been supposed, for the following reasons:--
+
+The latest discoveries in the Silurian and Devonian deposits have brought
+to light strange forms such as Thelodus and Drepanaspis, of which the
+latter from the Devonian must, according to Traquair, be included in the
+Heterostraci. It possessed, as seen in Fig. 139, large plates, after the
+fashion of Pteraspis, and also many smaller ones.
+
+The former, from the upper Silurian, belongs to the Coelolepidæ, and was
+covered over with shagreen composed of small scutes, after the fashion of
+an elasmobranch. Traquair suggests that Thelodus arose from the original
+elasmobranch stock; that by the fusion of scutes such a form as Drepanaspis
+occurred, and, with still further fusion, Pteraspis.
+
+There are always two ways of looking at a question, and it seems to me
+possible and more probable to turn the matter round and to argue that the
+original condition of the surface-covering was that of large plates, as in
+Pteraspis. By the subsequent splitting up of such plates, Drepanaspis was
+formed, and later on, by further splitting, the elasmobranch, Thelodus
+being a stage on the way to the formation of an elasmobranch, and not a
+backward stage from the elasmobranch towards Pteraspis.
+
+{345}This method of looking at the problem seems to me to be more in
+consonance with the facts than the reverse; for, as pointed out by Jaekel,
+the fishes with large plates are the oldest, and in Cyathaspis, the very
+oldest of all, the size of the plates is most conspicuous; he considers,
+therefore, this preconceived view that large plates are formed by the
+fusion of small ones must give way to the opposite belief.
+
+[Illustration: FIG. 139.--DREPANASPIS. VENTRAL AND DORSAL ASPECTS. (After
+LANKESTER.)
+
+_A._, anus; _E._, lateral eyes.]
+
+So also Rohon, as quoted by Traquair, who, in his first paper accepted
+Lankester's view that the ridges of the pteraspidian shield were formed by
+the fusion of a linear arrangement of numbers of placoid scales, suggests
+in his second paper that these ridges may have been the most primitive
+condition of the dermal skeleton of the vertebrate, out of which, by
+differentiation, the dermal denticles (placoid scales) of the selachian, as
+well as their modifications in the ganoids, teleosteans, and amphibians,
+have arisen.
+
+One thing is agreed upon on all sides; no sign of bone-corpuscles is to be
+found in this dermal covering of Pteraspis. In the deeper layers are large
+spaces, the so-called pulp-cavities leading into narrow canaliculi, the
+so-called dentine canals. The structure is {346}looked upon as similar to
+that of the pulp and dentine canals of many fish-scales.
+
+On the other hand, this dermal covering of Pteraspis has been compared by
+Patten with the arrangement of the chitinous structure of certain parts of
+the external covering of Limulus, a comparison which to my mind presents a
+great difficulty. The chitin-layers in Limulus are _external_ to the
+epidermal cells, being formed by them; the layers in Pteraspis which look
+like chitin must have been _internal_ to the epidermal layer; for each
+vascular canal which passes from a pulp-cavity on its way to be distributed
+into the dentine canals of the ridge gives off short side branches, which
+open directly into the groove between the ridges. If these canals were
+filled with blood they could not possibly open directly into the open
+grooves between the ridges; these openings must, therefore, have been
+covered over with an epithelial layer which covered over the surface of the
+animal, and consequently the chitin-like structure must have been internal
+to the epidermis, and not external, as on Patten's view. The comparison of
+this structure with the dentine of fish-scales signifies the same thing,
+for in the latter the epidermis is external to the dentine-plates, the hard
+skeletal structure is in the position of the cutis, not of the cuticle.
+
+The position appears to me to be this: the dermal cranial skeleton of
+vertebrates, whether it takes the form of a bony skull or of the dorsal
+plates of a cephalaspid or a pteraspid is, in all cases, not cuticular,
+_i.e._ is not an external formation of the epidermal cells, but is formed
+in tissue of the nature of connective tissue underlying the epidermis. On
+the contrary, the hard part of the head-carapace of the palæostracan is an
+external formation of the epidermal cells.
+
+If, then, this tissue of Pteraspis is not to be looked upon as chitin, how
+can we imagine its formation? It is certainly not bone, for there are no
+bone-corpuscles; it is a very regular laminated structure resembling in
+appearance chitin rather than anything else.
+
+As in all cases of difficulty, turn to Ammocoetes and let us see what clue
+there is to be found there. The skin of Ammocoetes is peculiar among
+vertebrates in many respects. It consists of a number of epidermal cells,
+as in Fig. 140, the varying function of which need not be considered here,
+covered over with a cuticular layer which is extraordinarily thick for the
+cuticle of a vertebrate skin; this cuticular layer is perforated with fine
+canaliculi, through which the {347}secretion of the underlying cells
+passes, as is seen in Fig. 140, A and B. This cuticle corresponds to the
+chitinous covering of the arthropod, and like it is perforated with
+canaliculi, and, according to Lwoff, possibly contains chitin. The
+epidermal cells rest on a thick layer of most striking appearance (Fig.
+141), for it resembles, in an extraordinary degree, when examined
+superficially, a layer of chitin; it is called the laminated layer, and is
+characterized by the extreme regularity of the laminæ. This appearance is
+due, as the observations of Miss Alcock show, to alternate layers of
+connective tissue fibres arranged at right angles to each other, each fibre
+running a straight course and possessing its own nucleus. Although the
+fibres in each layer are packed close together, they are sufficiently apart
+to form with the fibres of the alternate layers a meshwork rather than a
+homogeneous structure, and thus the surface view of this layer shows a
+regular network of very fine spaces through which nerve-fibres and fluid
+pass. This layer is easily dissolved in a solution of hypochlorite of soda,
+a fluid which dissolves chitin. Any one looking at Ammocoetes would say
+that the only part of its skin which resembles chitin is this laminated
+layer, and therefore the only part of its skin which would afford an
+indication of the nature of the skeleton of Pteraspis is this laminated
+layer, which belongs to the cutis, and not to the cuticle. Yet another
+significant peculiarity of this layer is its entire disappearance at
+transformation. Miss Alcock, in a research not yet published, has shown
+that this layer is completely broken up and absorbed at transformation; the
+cutis of Petromyzon is formed entirely anew, and no longer presents any
+regular laminated character, but resembles rather the sub-epidermal
+connective tissue layer of the skin of higher vertebrates. This laminated
+layer, then, just like the muco-cartilage, shows, by its complete
+disappearance at transformation, its ancestral character.
+
+[Illustration: FIG. 140.--EPITHELIAL CELLS OF AMMOCOETES TO SHOW THE
+CANALICULI IN THE THICK CUTICLE (B). A, TRANSVERSE SECTION THROUGH THE
+CUTICLE.]
+
+Very suggestive is the arrangement of the different skeletal {348}tissues
+in the head-region of Ammocoetes. Fig. 141 represents a section through the
+head near the pineal eye. Most internally is _a_, a section of the
+membranous cranium, then comes _b_, the muco-cartilaginous skeleton, then
+_c_, the laminated layer, and finally _d_, the external cuticle. If in
+Ammocoetes we possess an epitome of the history of the vertebrate, how
+would these layers be represented in the past ages, supposing they could be
+fossilized?
+
+[Illustration: FIG. 141.--SECTION OF SKIN AND UNDERLYING TISSUES IN THE
+HEAD-REGION OF AMMOCOETES.
+
+_a_, cranial wall; _b_, muco-cartilage; _c_, laminated layer; _d_, external
+cuticular layer.]
+
+The most internal layer _a_, by the formation of cartilage and then bone,
+represents the great mass of vertebrate fossils; the next layer _b_, by a
+process of calcification, as previously argued, represents the head-shield
+of the Osteostracan fishes; while the cuticular layer _d_, no longer thin,
+is the remnant of the Palæostracan head-carapace. Between these two layers,
+_b_ and _d_, lies the laminated layer _c_. Intermediate to the Palæostracan
+and the Osteostracan comes the Heterostracan, with its peculiar
+head-shield--a head-shield whose origin is more easily conceivable as
+arising from something of the nature of the laminated layer than from any
+other structure represented in Ammocoetes.
+
+My present suggestion, then, is this: the transition from the skeletal
+covering of the Palæostracan to that of the highest vertebrates was brought
+about by the calcification of successive layers from without inwards, all
+of which still remain in Ammocoetes and show how the external chitinous
+covering of the arthropod was gradually replaced by the deep-lying internal
+bony cranium of the higher vertebrates.
+
+In Ammocoetes the layer which represents the covering of the
+{349}Palæostracan has already almost disappeared. At transformation the
+layers representing the stage arrived at by the Heterostracan and the
+Osteostracan disappear; but the stage representing the higher vertebrates,
+far from disappearing, by the formation of cartilage reaches a higher stage
+and prepares the way for the ultimate stage of all--the formation of the
+bony cranium.
+
+So much for the evidence as to the nature of the structure of the
+head-shield of the Pteraspidæ.
+
+It suggests that these fishes were covered anteriorly with armoured plates
+derived from the cutis layer of the skin, a layer which was specially
+thickened and very vascular, apparently, to enable respiration to be very
+largely, if not entirely, effected by the surface of the body. It is
+difficult to understand how the sea-scorpions breathed, and it is easy to
+see how the formation of ventral and dorsal plates enclosing the
+mesosomatic appendages may at the outset have hindered the action of the
+branchiæ. The respiratory chamber, according to my view, had at first the
+double function of respiration and digestion. A new digestive apparatus was
+the pressing need at the time; it would, therefore, be of distinct
+advantage to remove, as much as possible, the burden of respiration from
+this incipient alimentary canal.
+
+What can be said as to the shape of these ancient forms of fishes? Certain
+parts of them are absolutely known, other parts are guesswork. They are
+known to have possessed a dorsal shield, a ventral shield formerly looked
+upon as belonging to a separate species, called Scaphaspis, and a spine
+attached to the dorsal shield. The rest of their configuration, as given in
+Smith Woodward's restoration (Fig. 142) is guesswork; the fish-like body
+with its scales, the heterocercal tail, is based on the most insufficient
+evidence of something of the nature of scales having being found near the
+head-plates.
+
+The dorsal shield is characterized by a pair of lateral eyes situated on
+the edge of the shield, not as in Cephalaspis near the middle line. In the
+middle line, where the rostrum meets the large dorsal plate, median eyes
+were situated. But the slightest sign of any median single nasal opening,
+such as is so characteristic of the head-shield of the Osteostraci and of
+Ammocoetes has never been discovered. The olfactory organ must have been
+situated on the ventral side as in the larval stage of Ammocoetes, or in
+the Palæostraca. Many of these head-shields are remarkably well preserved,
+{350}and it is difficult to believe that an olfactory opening would not be
+seen if any such had existed, as it does in Thyestes.
+
+[Illustration: FIG. 142.--RESTORATION OF PTERASPIS. (After SMITH
+WOODWARD.)]
+
+The difficulty of interpreting these types is the difficulty of
+understanding their method of locomotion; that is largely the reason why
+the spine has been placed as if projecting from the back, and a fish-like
+body with a heterocercal tail-fin added. If, on the contrary, the spine is
+a terminal tail-spine, then, as far as the fossilized remains indicate, the
+animal consisted of a dorsal shield, a ventral shield, and a tail-spine, to
+which must be added two apparently lateral pieces and a few scales. If the
+animal did not possess a flexible body with a tail-fin, but terminated in a
+rigid spike after the fashion of a Limulus-like animal, then it must have
+moved by means of {351}appendages. At present we have not sufficient
+evidence to decide this question.
+
+That the animal crawled about in the mud by means of free appendages is by
+no means an impossible view, seeing how difficult it is to find the remains
+of appendages in the fossils of this far-back time, even when we are sure
+that they existed. Thus, for many generations, the appendages of
+trilobites, which occur in such countless numbers, and in such great
+variety of form, were absolutely unknown, until at last, in consequence of
+a fortunate infiltration by pyrites, they were found by Beecher preserved
+down to the minutest detail. Even to this day no trace of appendages has
+been found in such forms as Hemiaspis, Bunodes, Belinurus, Prestwichia.
+
+The whole question of the evidence of any prosomatic appendages in these
+ancient fishes is one of very great interest, and of late years has been
+investigated by Patten. It has long been known that forms such as
+Pterichthys and Bothriolepis possessed two large, jointed locomotor
+appendages, and Patten has lately obtained better specimens of Bothriolepis
+than have ever been found before, which show not only the general
+configuration of the fish, but also the presence of mandibles or gnathites
+in the mouth-region resembling those of an arthropod. These mandibles had
+been seen before (Smith Woodward), but Patten's specimens are more perfect
+than any previously described, and cause him to conclude that these ancient
+fish were of the nature of arthropods rather than of vertebrates.
+
+Patten has also been able to obtain some excellent specimens of the under
+surface of the head of Tremataspis, which, as evident in Fig. 143, show the
+presence of a series of holes, ranging on each side from the mouth-opening,
+in a semicircular fashion towards the middle line. He considers that these
+openings indicate the attachments of appendages, in opposition to other
+observers, such as Jaekel, who look upon them as gill-slits. To my mind,
+they are not in the right position for gill-slits; they are certainly in a
+prosomatic rather than in a mesosomatic position, and I should not be at
+all surprised if further research justified Patten's position. So convinced
+is he of the presence of appendages in all these old forms, that he
+considers them to be arthropods rather than vertebrates, although, at the
+same time, he looks upon them as indicating the origin of vertebrates from
+arthropods. Here, perhaps, it is advisable to say a few words on Patten's
+attitude towards this question.
+
+{352}Two years after I had put forward my theory of the derivation of
+vertebrates from arthropods, Patten published, in the _Quarterly Journal of
+Microscopical Science_, simultaneously with my paper in that journal, a
+paper entitled "The Origin of Vertebrates from Arachnids." In this paper he
+made no reference to my former publications, but he made it clear that
+there was an absolutely fundamental difference between our treatment of the
+problem; for he took the old view that of necessity there must be a
+reversal of surfaces in order that the internal organs should be in the
+same relative positions in the vertebrate and in the invertebrate. He
+simply, therefore, substituted Arachnid for Annelid in the old theory.
+Because of this necessity for the reversal of surfaces he discarded the
+terms dorsal and ventral as indicative of the surfaces of an animal, and
+substituted hæmal and neural, thereby hopelessly confusing the issue and
+making it often very difficult to understand his meaning.
+
+[Illustration: FIG. 143.--UNDER-SURFACE OF HEAD-REGION IN TREMATASPIS.
+(After PATTEN.)]
+
+He still holds to his original opinion, and I am still waiting to find out
+when the reversal of surfaces took place, for his investigations lead him,
+as must naturally be the case, to compare the dorsal (or, as he would call
+it, the hæmal) surface of Bothriolepis, of the Cephalaspidæ, and of the
+Pteraspidæ with the dorsal surface of the Palæostraca.
+
+All these ancient fishes are, according to him, still in the arthropod
+stage, have not yet turned over, though in a peculiarly unscientific manner
+he argues elaborately that they must have swum on their back rather than on
+their front, and so indicated the coming reversal. Because they were
+arthropods they cannot have had a {353}frontal nose-organ; therefore,
+Patten looks upon the nose and the two lateral eyes of the Osteostraci as a
+complex median eye, regardless of the fact that the median eyes already
+existed.
+
+Every atom of evidence Patten has brought forward, every new fact he has
+discovered, confirms my position and makes his still more hopelessly
+confused. Keep the animal the right side uppermost, and the evidence of the
+rocks confirms the transition from the Palæostracan to the Cyclostome;
+reverse the surfaces, and the attempt to derive the vertebrate from the
+palæostracan becomes so confused and hopelessly muddled as to throw
+discredit on any theory of the origin of vertebrates from arthropods. For
+my own part, I fully expect that appendages will be found not only in the
+Cephalaspidæ but also in the Pteraspidæ, and I hope Patten will continue
+his researches with increasing success. I feel sure, however, his task will
+be much simplified if he abandons his present position and views the
+question from my standpoint.
+
+
+SUMMARY.
+
+  The shifting of the nasal tube from a ventral to a dorsal position, as
+  seen in Ammocoetes, is, perhaps, the most important of all clues in
+  connection with the comparison of Ammocoetes to the Palæostracan on the
+  one hand, and to the Cephalaspid on the other; for, whereas the exact
+  counterpart of the opening of such a tube is always found on the dorsal
+  head-shield in all members of the latter group, nothing of the kind is
+  ever found on the dorsal carapace of the former group.
+
+  The reason for this difference is made immediately evident in the
+  development of Ammocoetes itself, for the olfactory tube originates as a
+  ventral tube--the tube of the hypophysis--in exactly the same position as
+  the olfactory tube of the Palæostracan, and later on in its development
+  takes up a dorsal position.
+
+  In fact, Ammocoetes in its development indicates how the Palæostracan
+  head-shield became transformed into that of the Cephalaspid.
+
+  In another most important character Ammocoetes indicates its relationship
+  to the Cephalaspidæ, for it possesses an external skeleton or head-shield
+  composed of muco-cartilage, which is the exact counterpart of the
+  so-called bony head-shield of the latter group; and still more strikingly
+  the structure of the cephalaspidian head-shield is remarkably like that
+  of muco-cartilage. In the one case, by the deposition of calcium salts, a
+  hard external skeleton, capable of being preserved as a fossil, has been
+  formed; in the other, by the absence of the calcium salts, a soft
+  chondro-mucoid matrix, in which the characteristic cells and fibrils are
+  embedded, distinguishes the tissue.
+
+  The recognition that the head-shields of these most primitive fishes were
+  not composed of bone, but of muco-cartilage, the precursor of both
+  cartilage and bone, immediately clears up in the most satisfactory manner
+  the whole {354}question of their derivation from elasmobranch fishes; for
+  the main argument in favour of the latter derivation is the exceedingly
+  strong one that bone succeeds cartilage--not _vice versâ_--therefore,
+  these forms, since their head-shield is bony, must have arisen from some
+  other fishes with a cartilaginous skeleton, most probably of an
+  elasmobranch nature. Seeing, however, that the structure of their shields
+  resembles muco-cartilage much more closely than bone, and that Ammocoetes
+  forms a head-shield of muco-cartilage closely resembling theirs, there is
+  no longer any necessity to derive the jawless fishes from the
+  gnathostomatous; but, on the contrary, we may look with certainty upon
+  the Agnatha as the most primitive group from which the others have been
+  derived.
+
+  The history of the rocks shows that the group of fishes, Pteraspis and
+  Cyathaspis, are older than the Cephalaspidæ--come, therefore,
+  phylogenetically between the Palæostraca and the latter group. In this
+  group the head-shields are of a very different character, without any
+  sign of any structure comparable with that of bone, and although they
+  possessed both lateral and median eyes, there is never in any case any
+  trace of a dorsal nasal orifice. Their olfactory passage, like that of
+  the Palæostraca, must have been ventral.
+
+  The remarkable comparison which exists between the head-shields of
+  Ammocoetes and Cephalaspis, enables us to locate the position of the
+  brain and cranium of the latter with considerable accuracy, and so to
+  compare the segmental markings found in many of these fossils with the
+  corresponding markings, found either in fossil Palæostraca or on the
+  head-carapaces of living scorpions and spiders, such as Phrynus and
+  Mygale. In all cases the cranial region was covered with a median plate,
+  often especially hard, which corresponded to the glabellum of the
+  trilobite; the growth of the cranium can be traced from its beginnings as
+  the upturned lateral flanges of the plastron to the membranous cranium of
+  Ammocoetes.
+
+  From such a comparison it follows that the segments, found in the
+  antero-lateral region of the head-shield, were not segments of the
+  cranium, but of parts beyond the region of the cranium, and from their
+  position must have been segments supplied by the trigeminal nerve, and
+  not by the vagus group; segments, therefore, which did not indicate gills
+  and gill-slits, but muscles, innervated by the trigeminal nerve; muscles
+  which, as indicated by the corresponding markings on the carapace of
+  Phrynus, Mygale, etc., were the tergo-coxal muscles of the prosomatic
+  appendages.
+
+  The discovery of the nature of these appendages in the Pteraspidæ and
+  Cephalaspidæ, as well as in the Asterolepidæ (Pterichthys and
+  Bothriolepis), is a problem of the future, though in the latter, not only
+  have the well-known oar-like appendages been long since discovered, but
+  Patten has recently found specimens of Bothriolepis which throw light on
+  the anterior masticating gnathite-like appendages which these ancient
+  forms possessed.
+
+
+
+
+{355}CHAPTER XI
+
+_THE EVIDENCE OF THE AUDITORY APPARATUS AND THE ORGANS OF THE LATERAL LINE_
+
+  Lateral line organs.--Function of this group of organs.--Poriferous
+  sense-organs on the appendages in Limulus.--Branchial
+  sense-organs.--Prosomatic sense organs.--Flabellum.--Its structure and
+  position.--Sense-organs of mandibles.--Auditory organs of insects and
+  arachnids.--Poriferous chordotonal organs.--Balancers of
+  Diptera.--Resemblance to organs of flabellum.--Racquet-organs of
+  Galeodes.--Pectens of scorpions.--Large size of nerve to all these
+  special sense-organs.--Origin of parachordals and auditory
+  capsule.--Reason why VIIth nerve passes in and out of capsule.--Evidence
+  of Ammocoetes.--Intrusion of glandular mass round brain into auditory
+  capsule.--Intrusion of generative and hepatic mass round brain into base
+  of flabellum.--Summary.
+
+
+When speaking of the tripartite arrangement of the cranial nerves, an
+arrangement which gave the clue to the meaning of the cranial segments, I
+spoke of the trigeminal as supplying the sensory nerves to the skin in the
+head-region, and I compared this dorsal system of afferent nerves to the
+system of epimeral nerves in Limulus which supply the prosomatic and
+mesosomatic carapaces of Limulus with sensory fibres. I compared the
+ventral system of eye-muscle nerves with the system of nerves supplying the
+segmental dorso-ventral somatic muscles of the prosomatic region, and I
+compared the lateral system of mixed nerves with the nerves supplying the
+prosomatic and mesosomatic appendages of Limulus. I compared, also, the
+optic nerves and the olfactory nerves with the corresponding nerves in the
+same invertebrate group. My readers will see at once that one well-marked
+group of nerves--the auditory and lateral line system--has been entirely
+omitted up to the present, it has not even been mentioned in the scheme of
+the cranial segments; I have purposely reserved its consideration until
+now, because the organs these nerves supply, though situated in the skin,
+are of such a special character {356}as to form a category by themselves.
+These nerves cannot be classed among the afferent nerves of the skin any
+more than the nerves of the optic and olfactory apparatus; they require
+separate consideration. A very extensive literature has grown up on the
+subject of this system of lateral line sense-organs and their innervation,
+the outcome of which is decisively in favour of this system being classed
+with the sense-organs supplied by the auditory nerve, so that in
+endeavouring to understand the position of the auditory nerve, we must
+always bear in mind that any theory as to its origin must apply to the
+system of lateral line nerves as well.
+
+Now, although the auditory apparatus is common to all vertebrates, the
+lateral line system is not found in any land-dwelling animals; it belongs
+essentially to the fishes, and is, therefore, an old system so far as
+concerns the vertebrate group. Its sense-organs are arranged along the
+lateral line of the fish, and, in addition, on the head-region in three
+well-marked lines known as the supra-orbital, infra-orbital, and mandibular
+line systems. These sense-organs lie in the skin in a system of canals, and
+are innervated by a special nervous system different to that innervating
+adjacent skin-areas. The great peculiarity of their innervation consists in
+the fact that their nerves all belong to the branchial system of nerves; no
+fibres arise in connection with the trigeminal, but all of them in
+connection with the facial, glossopharyngeal and vagus nerves. In other
+words, although organs in the skin, their nerve-supply belongs to the
+lateral nervous system which supplies splanchnic and not somatic segments,
+a system which, according to the theory advanced in this book, originated
+in the nerves supplying appendages. The conclusion, therefore, is that in
+order to obtain some clue as to the origin of the sense-organs of this
+system in the assumed palæostracan ancestor, we must examine the
+mesosomatic appendages and see whether they possess any special
+sense-organs of similar function.
+
+Further, considering that the auditory organ is to be regarded as a
+specially developed member of this system, we must especially look for an
+exceptionally developed organ in the region supplied by the auditory nerve.
+
+The question of the origin of this system of lateral line sense-organs
+possesses a special interest for all those who attempt to obtain a solution
+of the origin of vertebrates, for the upholders of the view that the
+vertebrates have descended from annelids have always {357}found its
+strongest support in the similarity of two sets of segmental organs found
+in annelids and vertebrates. On the one hand, great stress was laid upon
+the similarity of the segmental excretory organs in the two groups of
+animals, as will be discussed later; on the other, of the similarity of the
+segmentally arranged lateral sense-organs.
+
+These lateral sense-organs of the annelids have been specially described by
+Eisig in the Capitellidæ, and, according to Lang, "there are many reasons
+for considering these lateral organs to be homologous with the dorsal cirri
+of the ventral parapodia of other Polychæta, and in the family of the
+Glyceridæ we can follow, almost step by step, the transformation of the
+cirri into lateral organs." Eisig describes them in the thoracic
+prebranchial region as slightly different from those in the abdominal
+branchial region; in the latter region, the ventral parapodia are
+gill-bearing, so that these lateral organs are in the branchial region
+closely connected with the branchiæ, just as is also the case in the
+vertebrates. It is but a small step from the gill-bearing ventral parapodia
+of the annelid to the gill-bearing appendages of the phyllopod-like
+protostracan; so that if we assume that this is the correct line along
+which to search for the origin of the vertebrate auditory apparatus, then,
+on my theory of the origin of the vertebrates from a group resembling the
+Protostraca, it follows that special sense-organs must have existed either
+on or in close connection with the branchial and prebranchial appendages of
+the protostracan ancestor of the vertebrates, which would form an
+intermediate link between the lateral organs of the annelids and the
+lateral and auditory organs of the vertebrates.
+
+Further, these special sense-organs could not have been mere tactile hairs,
+but must have possessed some special function, and their structure must
+have been compatible with that function. Can we obtain any clear conception
+of the original function of this whole system of sense-organs?
+
+A large amount of experimental work has been done to determine the function
+of the lateral line organs in fishes, and they have been thought at one
+time or another to be supplementary organs for equilibration, organs for
+estimating pressure, etc. The latest experimental work done by Parker
+points directly to their being organs for estimating slow vibrations in
+water in contradistinction to the quicker vibrations constituting sound. He
+concludes that surface wave-movements, whether produced by air moving on
+the water or {358}solid bodies falling into the water, are accompanied by
+disturbances which are stimuli for the lateral line organs.
+
+One of these segmental organs has become especially important and exists
+throughout the whole vertebrate group, whether the animal lives on land or
+in water--this is the auditory organ. Throughout, the auditory organ has a
+double function--the function of hearing and the function of equilibration.
+If, then, this is, as is generally supposed, a specialized member of the
+group, it follows that the less specialized members must possess the
+commencement of both these functions, just as the experimental evidence
+suggests.
+
+In our search, then, for the origin of the auditory organ of vertebrates,
+we must look for special organs for the estimation of vibrations and for
+the maintenance of the equilibrium of the animal, situated on the
+appendages, especially the branchial or mesosomatic appendages; and,
+further, we must specially look for an exceptional development of such
+segmental organs at the junction of the prosomatic and mesosomatic regions.
+
+Throughout this book the evidence which I have put forward has in all cases
+pointed to the same conclusion, viz. that the vertebrate arose by way of
+the Cephalaspidæ from some arthropod, either belonging to, or closely
+allied to, the group called Palæostraca, of which the only living
+representative is Limulus. If, then, my argument so far is sound, the
+appendages of Limulus, both prosomatic and mesosomatic, ought to possess
+special sense-organs which are concerned in equilibration or the
+appreciation of the depth of the water, or in some modification of such
+function, and among these we might expect to find that somewhere at the
+junction of the prosoma and mesosoma such sense-organs were specially
+developed to form the beginning of the auditory organ.
+
+Now, it is a striking fact that the appendages of Limulus do possess
+special sense-organs of a remarkable character, which are clearly not
+simply tactile. Thus Gegenbaur, as already stated, has drawn attention to
+the remarkable branchial sense-organs of Limulus; and Patten has pointed
+out that special organs, which he considers to be gustatory in function,
+are present on the mandibles of the prosomatic appendages. I myself, as
+mentioned in my address to the British Association at Liverpool in 1896,
+searched for some special sense-organ at the junction of the prosoma and
+mesosoma, and was rewarded by finding that that extraordinary adjunct to
+the {359}last locomotor appendage, known as the flabellum, was an elaborate
+sense-organ. I now propose to show that all these special sense-organs are
+constructed on a somewhat similar plan; that the structure of the branchial
+sense-organs suggests that they are organs for the estimation of water
+pressures; that among air-breathing arthropods sense-organs, built up on a
+somewhat similar plan, are universally found, and are considered to be of
+the nature of auditory and equilibration organs; and, what is especially of
+importance, in view of the fact that the most prominent members of the
+Palæostraca were the sea-scorpions, that the remarkable sense-organs of the
+scorpions known as the pectens belong apparently to the same group.
+
+
+THE PORIFEROUS SENSE-ORGANS OF THE APPENDAGES IN LIMULUS.
+
+On all the branchial appendages in Limulus, special sense-organs are found
+of a most conspicuous character. They form in the living animal bluish
+convex circular patches, the situation of which on the appendages is shown
+in Fig. 58. These organs are not found on the non-branchial operculum.
+Gegenbaur, who was the first to describe them, has pointed out how the
+surface of the organ is closely set with chitinous goblets shaped as seen
+in Fig. 144, A, which do not necessarily project free on the surface, but
+are extruded on the slightest pressure. Each goblet fits into a socket in
+the chitinous covering, and is apparently easily protruded by variations of
+pressure from within. The whole surface of the organ on the appendage is
+slightly bulged in the living condition, and the chitin is markedly softer
+here than in the surrounding part of the limb. Each of these organs is
+surrounded by a thick protection of strongly branching spines. On the
+surface of the organ itself no spines are found, only these goblets, so
+that the surface-view presents an appearance as in Fig. 144, B. Each goblet
+possesses a central pore, which is the termination of a very fine, very
+tortuous, very brittle chitinous tubule (_ch.t._), which passes from the
+goblet through the layers of the chitin into the subjacent tissue. The
+goblets vary considerably in size, a few very large ones being scattered
+here and there. The fine chitinous tubule is especially conspicuous in
+connection with these largest goblets. In the smaller ones there is the
+same appearance of a pore and a commencing tube, but I have not been able
+to trace the tube through the chitinous layers, as in the case of the
+larger goblets.
+
+{360}[Illustration: FIG. 144.--A, A GOBLET FROM ONE OF THE BRANCHIAL
+SENSE-ORGANS OF LIMULUS (_ch.t._, chitinous tubule); B, SURFACE VIEW OF A
+PORTION OF A BRANCHIAL SENSE-ORGAN.]
+
+[Illustration: FIG. 145.--THE ENDOGNATHS OF LIMULUS PUSHED OUT OF THE WAY
+ON ONE SIDE IN ORDER TO SHOW THE POSITION OF THE FLABELLUM (_fl._)
+PROJECTING TOWARDS THE CRACK BETWEEN THE PROSOMATIC AND MESOSOMATIC
+CARAPACES.]
+
+Gegenbaur, in his picture, draws a straight tubule passing from every
+goblet among the fine canaliculi of the chitin. He says they are difficult
+to see, except in the case of the larger goblets. The tubule from the
+larger goblets is most conspicuous, and is in my sections always tortuous,
+never straight, as represented by Gegenbaur. A special branch of the
+appendage-nerve passes to these organs, and upon the fine branches of this
+nerve groups of ganglion-cells are seen, very similar in appearance to the
+groups described by Patten on the terminal branches of the nerves which
+supply the mandibular organs. At present I can see no mechanism by which
+the goblets are extruded or returned into place. In the case of the
+Capitellidæ, Eisig describes retractor muscles by means of which the
+lateral sense-organs are {361}brought below the level of the surface, and
+he imagines that the protrusion is effected by hydraulic means, by the aid
+of the vascular system. In the branchial sense-organs of Limulus there are
+no retractor muscles, and it seems to me that both retraction and
+protrusion must be brought about by alterations of pressure in the vascular
+fluids. Certainly the cavity of the organ is very vascular. If this be so,
+it seems likely enough that such an organ should be a very delicate organ
+for estimating changes in the pressure of the external medium, for the
+position of the goblets would depend on the relation between the pressure
+of the fluid inside the organ and that on the surface of the appendage.
+Whether the chitinous tubule contains a nerve-terminal or not I am unable
+to decide from my specimens, but, judging from Patten's description of the
+similar chitinous tubules belonging to the mandibular organs, it is most
+highly probable that these tubules also contain a fine terminal
+nerve-fibre.
+
+These organs, then, represent segmental branchial sense-organs, of which it
+can be said their structure suggests that they may be pressure-organs; but
+the experimental evidence is at present wanting.
+
+Passing now from the branchial to the prosomatic region, the first thing
+that struck me was the presence of that most conspicuous projection at the
+base of the last locomotor appendage, which is usually called the
+flabellum, and has been described by Lankester as an exopodite of this
+appendage. It is jointed on to the most basal portion of the limb (_cf._
+Fig. 155), and projects dorsally from the limb into the open slit between
+the prosomatic and mesosomatic carapace, as is seen in Fig. 145 (_fl._). Of
+its two surfaces, the undermost is very convex and the uppermost nearly
+flat from side to side, the whole organ being bent, so that when the animal
+is lying half buried in sand, entirely covered over by the prosomatic and
+mesosomatic carapaces except along this slit between the two, the upper
+flat or slightly convex surface of the flabellum is exposed to any movement
+of water through this slit, and owing to its possessing a joint, the
+direction of the whole organ can be altered to a limited extent. The whole
+of this flat upper surface is one large sense-organ of a striking
+character, thus forming a great contrast to the convex under surface, which
+is remarkably free from tactile spines or special sense-organs.
+
+The nerve going to the flabellum is very large, almost as large as the
+nerve to the rest of the appendage, and the very large majority {362}of the
+nerve-fibres turn towards the flat, uppermost side, where the sense-organ
+is situated. Between the nerve-fibres (_n._) and the chitinous surface
+containing the special sense-tubes masses of cells (_gl._) are seen, as in
+Fig. 146, apparently nerve-cells, which form a broad border between the
+nerve-fibres and the pigmented chitinogenous layer (_p._). On the opposite
+side, nothing of the sort intervenes between the pigmented layer and the
+blood-spaces and nerve-fibres which constitute the central mass of the
+flabellum.
+
+[Illustration: FIG. 146.--SECTION THROUGH FLABELLUM.
+
+_ch._, chitinous layers; _s.o._, sense-organs; _sp._, spike-organ; _p._,
+pigment layer; _gl._, ganglion cell layer; _bl._ and _n._, blood-spaces and
+nerves.]
+
+[Illustration: FIG. 147.--SECTION PARALLEL TO THE SURFACE OF FLABELLUM,
+SHOWING THE POROUS TERMINATIONS OF THE SENSE-ORGANS AND THE ARRANGEMENT OF
+THE CANALICULI ROUND THEM.]
+
+At present I am inclined to look upon this mass of cells as constituting a
+large ganglion, which extends over the whole length and breadth of the
+upper surface of the flabellum. At the same {363}time, my preparations are
+not sufficiently clear to enable me to trace out the connections of these
+cells, especially their connections with the special sense-organs.
+
+[Illustration: FIG. 148.--SECTION THROUGH THE THREE SENSE-ORGANS OF
+FLABELLUM.
+
+_bl._, blood-spaces; _n._, nerve; _gl._, layer of ganglion-cells; _p._,
+pigment layer; _ch._, 1, 2, 3, the three layers of chitin; _ch.t._,
+chitinous tubule in large tube of sense-organ; _cap._, capitellum or
+swollen extremity of large tube; _can._, very fine porous canals or
+canaliculi of chitin.]
+
+In Fig. 148 I give a magnified representation of a section through three of
+these flabellar sense-organs. As is seen, the section divides itself into
+four zones: (1) the chitinous layer (_ch._); (2) the layer of pigment
+(_p._) and hypodermal cells; (3) the layer of ganglion-cells (_gl._); and
+(4) the layer of nerve-fibres (_n._) and blood-spaces (_bl._). The
+chitinous layer is composed of the usual three zones of the Limulus
+surface--externally (Fig. 148), a thin homogeneous layer, followed by a
+thick layer of chitin (3), in which the fine vertical tubules or canaliculi
+are well marked; the external portion (2) of this layer is differentiated
+from the rest by the presence of well-marked horizontal layers in addition
+to the canaliculi.
+
+In this chitinous layer the special sense-organs are found. They consist of
+a large tube which passes through all the layers of the chitin except the
+thin homogeneous most external layer. {364}This tube is conical in shape,
+its base, which rests on the pigmented layer, being so large and the organs
+so crowded together that a section of the chitin across the base of the
+tubes gives the appearance of a honeycomb, the septa of which is all that
+remains of the chitin. This large tube narrows down to a thin elongated
+neck as it passes through the chitin, and then, at its termination, bulges
+out again into an oval swelling (_cap._) situated always beneath the
+homogeneous most external layer of chitin. Within this tube a fine
+chitinous tubule (_ch. t._) is situated similar to that seen in the
+branchial sense-organs; it lies apparently free in the tube, not straight,
+but sinuous, and it passes right through all the chitinous layers to open
+at the surface as a pore; in the last part of its course, where it passes
+through the most external layer (1) of chitin, it lies always at right
+angles to the surface.
+
+If the flabellum be stained with methylene blue and acid fuchsin, then all
+the canaliculi in the chitin show up as fine red lines, and present the
+appearance given in Fig. 148, and it is seen that each of the terminations
+of the tubules is surrounded in the homogeneous layer of chitin by a
+thick-set circular patch of canaliculi which pass to the very surface of
+the chitin, while the canaliculi in other parts terminate at the
+commencement of the homogeneous layer and do not reach the surface.
+Further, the contents of the oval swelling, and, indeed, of the tube as a
+whole, are stained blue, the chitinous tubule being either unstained or
+slightly pink in colour. We see, then, that the chitinous tubule alone
+reaches the surface, while the large tube, which contains the tubule,
+terminates in an oval swelling, which often presents a folded or wrinkled
+appearance, as in Fig. 149 (see also Patten's Fig. 1, Plate I.). This
+terminal bulging of the tube is reminiscent of the bulging in the chitinous
+tubes of the lyriform organs of the Arachnida, as described by Gaubert, and
+of the poriferous chordotonal organs in insects, as described by Graber
+(see Fig. 150). This terminal swelling is filled with a homogeneous
+refringent mass staining blue with methylene blue, in which I have seen no
+trace of a nucleus; through this the chitinous tubule makes its way without
+any sign of bulging on its part. Patten, in his description of the
+sense-organs on the mandibles of Limulus, which are evidently the same in
+structure as those on the flabellum, refers to this homogeneous mass as a
+coagulum. I doubt whether this is an adequate description; it appears to me
+to stain rather more {365}readily than a blood-coagulum, yet in the sense
+of being structureless it resembles a coagulum.
+
+The enormous number of these organs crowded together over the whole flat
+surface of the flabellum produces a very striking appearance when viewed on
+the surface. Such a view presents an appearance resembling that of the
+surface-view of the branchial sense-organs; in both cases the surface is
+covered with a great number of closely set circular plaques, in the centre
+of each of which is seen a well-marked pore. The circular plaques in the
+case of the flabellum are much smaller than those of the branchial
+sense-organs, and clearly are not protrusible as in the latter organs, the
+appearance as of a plaque being due to the ring of thickly-set canaliculi
+round the central tubule, as already described. When stained with methylene
+blue, the surface view of the flabellum under a low power presents an
+appearance of innumerable circular blue masses, from each of which springs
+a fine bent hair, terminating in a pore at the surface. The blue masses are
+the homogeneous substance (_cap._) of the bulgings seen through the
+transparent external layer of chitin, and the hairs are the terminal part
+of the chitinous tubules. Patten has represented their appearance in the
+mandibles in his Fig. 2, Plate I.
+
+The large tubes in the chitin alter in shape according to their position.
+Those in the middle of the sensory surface of the flabellum, in their
+course through the chitinous layers, are hardly bent at all; as they
+approach the two lateral edges of this surface, their long thin neck
+becomes bent more and more, the bending always being directed towards the
+middle of the surface (see Fig. 146); in this way the chitinous tubules
+increase more or less regularly in length from the centre of the organ to
+the periphery. The large basal part of the conical tube contains, besides
+the chitinous tubule, a number of nuclei which are confined to this part of
+the tube; some of these nuclei look like those belonging to nerve-fibres,
+others are apparently the nuclei of the chitinogenous membrane lining the
+tube. I have never seen any sign of nerve-cells in the tube itself.
+
+The only other kind of sense-organ I have found in connection with these
+sense-organs are a few spike-like projections, the appearance of which is
+given in Fig. 149. I have always seen these in the position given in Fig.
+146 (_sp._), _i.e._ at the junction of the surface which contains the
+sense-organs and the surface which is free from them. They are, so far as I
+have seen, not very numerous; I have {366}not, however, attempted to
+examine the whole sense-organ for the purpose of estimating their number
+and arrangement.
+
+As is seen in Fig. 149, they possess a fine tubule of the same character as
+that of the neighbouring sense-organs, which apparently terminates at the
+apex of the projecting spike. They appear to belong to the same group as
+the other poriferous sense-organs, and are of special interest, because in
+their appearance they form a link between the latter and the poriferous
+sense-organs which characterize the pecten of the scorpion (_cf._ Fig. 152,
+C).
+
+[Illustration: FIG. 149.--SPIKE-ORGAN OF FLABELLUM.
+
+_ch.t._, chitinous tubule.]
+
+Such, then, is the structure of this remarkable sense-organ of the
+flabellum, as far as I have been able to work it out with the materials at
+my disposal. It is evident that the flabellar organs, apart from the
+spike-organs, are of the same kind as those described by Patten on the
+mandibles and chelæ of Limulus, and therefore it is most probable that the
+nerve-terminals in the chitinous tubules, and the origin of the latter, are
+similar in the two sets of organs.
+
+These organs, as Patten has described them, are situated in lines on the
+spines of the mandibles of the prosomatic locomotor appendages, and are
+grouped closely together to form a compact sense-organ on the surface of
+the inner mandible (Lankester's epicoxite) (_i.m._ in Fig. 155), so that a
+surface-view of the organ here gives the characteristic appearance of these
+poriferous sense-patches. Precisely similar organs are found on the
+chilaria, which are, in function at all events, simply isolated mandibles,
+to use Patten's terminology.
+
+On the digging appendage (ectognath), as the comparison of Fig. 155, A and
+C, shows, the mandibular spines are almost non-existent, and the inner
+mandible or epicoxite is not present, so that {367}the special sense-organ
+of this appendage is represented solely by the flabellum.
+
+This sketch of the special sense-organs of Limulus shows that all the
+appendages of Limulus possess special sense-organs, with the exception of
+the operculum. All these sense-organs are formed on the same plan, in that
+they possess a fine chitinous tubule passing through the layers of chitin
+into the underlying hypodermal and nervous tissues, which terminates on the
+surface in a pore. The surface of the chitin where these pores are situated
+is perfectly smooth, although, in the case of the branchial sense-organs,
+the goblet-shaped masses of chitin, each of which contains a pore, are able
+to be pressed out beyond the level of the surface.
+
+As to their functions, we unfortunately do not know much that is definite.
+Patten considers that he has evidence of a gustatory function in the case
+of the mandibular organs, and suggests also a temperature-sense in the case
+of some of these organs. The large organ of the flabellum and the branchial
+organs he has not taken into consideration. The situation of these organs
+puts the suggestion of any gustatory function, as far as they are
+concerned, out of the question; and I do not think it probable that such
+large specialized organs would exist only for the estimation of
+temperature, when one sees how, in the higher animals, the
+temperature-nerves and the nerves of common sensation are universally
+distributed over the body. As already stated, the structure of the
+branchial organs seems to me to point to organs for estimating varying
+pressures more than anything else, and I am strongly inclined to look upon
+the whole set of organs as the derivatives of the lateral sense-organs of
+annelids, such as are described by Eisig in the Capitellidæ. This is
+Patten's opinion with respect to the mandibular organs; and from what I
+have shown, these organs cannot be separated in type of structure from
+those of the flabellum and the branchial sense-organs.
+
+In our search, then, for the origin of the vertebrate auditory organ in
+Limulus and its allies, we see so far the following indications:--
+
+1.  The auditory organ of the vertebrate is regarded as a special organ
+belonging to a segmentally arranged set of lateral sense-organs, whose
+original function was co-ordination and equilibration.
+
+2.  Such a set of segmentally arranged lateral sense-organs is found in
+annelids in connection with the dorsal cirri of the ventral parapodia.
+
+{368}3.  If, as has been supposed, there is a genetic connection between
+(1) and (2) and if, as I suppose, the vertebrates did not arise from the
+annelids directly, but from a protostracan group, then it follows that the
+lateral sense-organs, one of which gave rise to the auditory organ, must
+have been situated on the protostracan appendages.
+
+4.  In Limulus, which is the sole surviving representative of the
+palæostracan group, such special sense-organs are found on both the
+prosomatic and mesosomatic appendages, and therefore may be expected to
+give a direct clue to the origin of the vertebrate auditory organ.
+
+5.  Both from its position, its size, and its specialization, the
+flabellum, _i.e._ an organ corresponding to the flabellum, must be looked
+upon as more likely to give a direct clue to the origin of the auditory
+organ than the sense-organs of the branchial appendages, or the so-called
+gustatory organs of the mandibles.
+
+
+THE AUDITORY ORGANS OF ARACHNIDS AND INSECTS.
+
+The difficulty of the investigating these organs consists in the fact that
+so little is known about them in those Arthropoda which live in the water;
+the only instance of any organ apparently of the nature of an auditory
+organ, is the pair of so-called auditory sacs at the base of the antennæ in
+various decapods. We are in a slightly better position when we turn to the
+land-living arthropods; here the presence of stridulating organs in so many
+instances carries with it the necessity of an organ for appreciating sound.
+It has now been shown that such stridulating organs are not confined to the
+Insecta, but are present also in the scorpion group, and I myself have
+added to their number by the discovery of a distinct stridulating apparatus
+in various members of the Phrynidæ. We may then take it for granted that
+arachnids as well as insects hear. Where is the auditory organ?
+
+Many observers believe that certain surface-organs found universally among
+the spiders, to which Gaubert has given the name of lyriform organs, are
+auditory in function. His investigations show that they are universally
+present on the limbs and pro-meso-sternite of all spiders; that they are
+present singly, not in groups, on the limbs of Thelyphonus, and that a
+group of them exists on the second segment of each limb in the members of
+the Phrynus tribe. In the latter case this organ is the most elaborate of
+all described by him.
+
+{369}It is especially noticeable that they do not exist in Galeodes or in
+the scorpions, but in the former special sense-organs are found in the
+shape of the so-called 'racquet-organs,' on the basal segments of the most
+posterior pair of appendages, and also, according to Gaubert, on the
+extremity of the palps and the first pair of feet, while in the latter they
+occur in the shape of the pectens.
+
+This observation of Gaubert suggests that the place of the lyriform organs
+in other arachnids is taken in Galeodes by the racquet-organs, and in the
+scorpions by the pectens. Bertkau, Schimkéwitsch, and Wagner, as quoted by
+Gaubert, all suggest that the lyriform organs of the arachnids belong to
+the same group of sense-organs as the porous chordotonal organs of the
+Insecta, sense-organs which have been found in every group of Insecta, and
+are generally regarded as auditory organs. Gaubert does not agree with
+this, and considers the lyriform organs to be concerned with the
+temperature-sense rather than with audition.
+
+The chordotonal organs of insects have been specially studied by Graber. He
+divides them into two groups, the poriferous and the non-poriferous, the
+former being characterized by the presence of pores on the surface arranged
+in groups or lines. These poriferous chordotonal organs are remarkably
+constant in position, being found only at the base of the wings on the
+subcostal ridge, in marked contrast to the other group of chordotonal
+organs which are found chiefly on the appendages in various regions. The
+striking character of this fixity of position of these organs and the
+universality of their presence in the whole group, led Graber to the
+conclusion that in these poriferous chordotonal organs we are studying a
+form of auditory apparatus which characterized the ancestor of the
+insect-group. These organs are always well developed on the hind wings, and
+in the large group of Diptera the auditory apparatus has usurped the whole
+of the function of the wing; for the balancers or 'halteres,' as they are
+called, are the sole representatives of the hind wings, and they are
+usually considered to be of the nature of auditory organs. It is
+instructive to find that such an auditory organ serves not only for the
+purpose of audition, but also as an organ of equilibration; thus Lowne
+gives the evidence of various observers, and confirms it himself, that
+removal of the balancers destroys the power of orderly flight in the
+animal.
+
+A striking peculiarity of these organs in the Insecta, as described {370}by
+Graber, is the bulging of the porous canal near its termination (Fig. 150,
+C). This bulging is filled with a homogeneous, highly refractive material,
+from which, according to Lowne, a chordotonal thread passes, to be
+connected with a ganglion-cell and nerve. This sphere of refractive
+material he calls the 'capitellum' of the chordotonal thread. The presence
+of this material produces in a surface view an appearance as of a halo
+around the terminal plaque with its central pore; Graber has attempted to
+represent this by the white area round the central area (in Fig. 150, B). A
+very similar appearance is presented by the surface view of the flabellum
+in those parts where the tube runs straight to the surface, so that the
+refractive material which fills the oval bulging shines through the
+overlying chitin and appears to surround the terminal plaque with a
+translucent halo.
+
+[Illustration: FIG. 150 (from GRABER).--A, SECTION OF SUBCOSTAL NERVURE OF
+HIND WING OF DYTISCUS TO SHOW PATCH OF PORIFEROUS ORGANS (_s.o._). B,
+SURFACE VIEW OF PORIFEROUS ORGANS; THE WHITE SPACE ROUND EACH ORGAN
+INDICATES THE DEEPER LYING REFRINGENT BODY WHICH FILLS THE BULGING OF THE
+CANAL SEEN IN TRANSVERSE SECTION IN C.]
+
+Such a peculiarity must have a very definite meaning, and suggests that the
+canals in the flabellum of Limulus and in the hind wings of insects belong
+to the same class of organ, the chitinous tubule with its nerve-terminal in
+the former corresponding to the chordotonal thread in the latter. One
+wonders whether this sphere of refractive material or 'capitellum' (to use
+Lowne's phraseology) is so universally present in order to act as a damper
+upon the vibrations of the chordotonal thread in the one case and of the
+{371}chitinous tubule in the other, just as the _membrana tectoria_ and the
+otoliths act in the case of the vertebrate ear.
+
+Patten says that the only organs which seem to him to be comparable with
+the gustatory porous organs of Limulus are the sense-organs in the
+extremities of the palps and of the first pair of legs of Galeodes, as
+described by Gaubert. I imagine that he was thinking only of arachnids, for
+the comparison of his drawings with those of Graber show what a strong
+family resemblance exists between the poriferous sense-organs of Limulus
+and those of the insects. On the course of the terminal nerve-fibres,
+between the nerve-cell and their entrance into the porous chitinous canal,
+Graber describes the existence of rods or scolophores. On the course of the
+terminal fibres in the Limulus organ, between the nerve-cells and their
+entrance into the porous chitinous canal, Patten describes a spindle-shaped
+swelling, containing a number of rod-like thickenings among the fibrils in
+the spindle, which present an appearance reminiscent of the rods described
+by Graber.
+
+It appears as though a type of sense-organ, characterized by the presence
+of pores on the surface and a fine chitinous canal which opens at these
+pores, was largely distributed among the Arthropoda. According to Graber,
+this kind of organ represents a primitive type of sense-organ, which was
+probably concerned with audition and equilibration, and he expresses
+surprise that similar organs have not been discovered among the Crustacea.
+It is, therefore, a matter of great interest to find that so ancient a type
+of animal as Limulus, closely allied to the primitive crustacean stock,
+_does_ possess poriferous sense-organs upon its appendages which are
+directly comparable with these poriferous chordotonal organs of the
+Insecta.
+
+
+THE PECTENS OF SCORPIONS.
+
+Among special sense-organs such as those with which I am now dealing, the
+pectens of scorpions and the 'racquet-organs' of Galeodes must, in all
+probability, be classed. I have given my reasons for this conclusion in my
+former paper.[2] At present such reasons are based entirely upon the
+structure of the organs; experimental {372}evidence as to their function is
+entirely wanting. With respect to the pectens of the scorpion (Fig. 151),
+it has been suggested that they are of the nature of copulatory organs, a
+suggestion which may be dismissed without hesitation, for they are not
+constructed after the fashion of claspers, but are simply elaborate
+sense-organs, and, as such, are found equally in male or female. The only
+observer who has hitherto specially studied the structure of the
+sense-organs in the pecten is, as far as I know, Gaubert, and he describes
+their structure together with that of the sense-organs of the racquets of
+Galeodes, in connection with the lyriform organs of arachnids, as though he
+recognized a family resemblance between the three sets of organs.
+
+[Illustration: FIG. 151.--UNDER SURFACE OF SCORPION (ANDROCTONUS).
+
+The operculum is marked out with dots, and on each side of it is seen one
+of the pectens.]
+
+The pecten of the scorpions is an elaborate sense-organ, or rather group of
+sense-organs, the special organ being developed on each tooth of the comb;
+its surface, which is frequently flattened, being directed backwards and
+inwards, when the axis of the pecten is horizontal at right angles to the
+length of the body. The surface view of this part of the tooth resembles
+that of the branchial organs or of the flabellum in Limulus, in that it is
+thickly covered with circular patches, in the centre of which an
+ill-defined appearance as of a fine pore is seen. In Fig. 152, B, I give a
+sketch of the surface view of a part of the organ.
+
+Transverse sections of a tooth of the comb of _Scorpio Europæus_ present
+the appearance given in Fig. 152, A, and show that each of these circular
+patches is the surface-view of a goblet-shaped chitinous organ, Fig. 152,
+C, from the centre of which a short, somewhat cylindrical chitinous spike
+projects. Within this spike, and running through the goblet into the
+subjacent tissue, is a fine tubule. The series of goblets gives rise to the
+appearance of the circular plaques on the surface-view, while the spike
+with its tubule {373}is the cause of the ill-defined appearance of the
+central pore, just as the terminal pore is much less conspicuous on
+surface-view in the spike-organs of the flabellum than in the purely
+poriferous organs, no part of which projects beyond the level of the
+chitinous surface.
+
+[Illustration: FIG. 152.--A, SECTION THROUGH TOOTH OF PECTEN OF SCORPION;
+B, SURFACE VIEW OF SENSE-ORGANS; C, GOBLET OF SENSE-ORGAN MORE HIGHLY
+MAGNIFIED.
+
+_bl._ and _n._, region of blood-spaces and nerves; _gl._, ganglion-cell
+layer; _ch._, modified chitinous layer; _s.o._, sense-organ.]
+
+The fine tubule is soon lost in the thickened but soft modification of the
+chitinous layer (_ch._) which is characteristic of the sense-organ; at all
+events, I have not succeeded in tracing it through this layer with any more
+success than in the corresponding case of the tubules belonging to the
+smaller goblets of the branchial sense-organ of Limulus already described.
+
+At the base of the modified chitinous layer a series of cells is seen,
+many, if not all, of which belong to the chitinogenous layer. Next to these
+is the marked layer of ganglion-cells (_gl._), similar to those seen in the
+flabellum of Limulus. The rest of the space in the section of the tooth is
+filled up with nerves (_n._) and blood-spaces (_bl._) just as in the
+section, Fig. 146, of the flabellum of Limulus.
+
+Gaubert does not appear to have seen the goblets at all clearly; {374}he
+describes them simply as conical eminences, and states that they
+"recouvrent un pore analogue a celui des poils mais plus petit; il est
+rempli par le protoplasma de la couche hypodermique." From the ganglion,
+according to him, nervous prolongations pass, which traverse the
+chitinogenous layer and terminate at the base of the conical eminences.
+Each of these prolongations "présente sur son trajet, mais un peu plus près
+du ganglion que de sa terminaison périphérique, une cellule nerveuse
+fusiforme (_g._) offrant, comme celles du ganglion, un gros noyau." He
+illustrates his description with the following, Fig. 153, taken from his
+paper.
+
+[Illustration: FIG. 153 (from GAUBERT).--SECTION OF A TOOTH OF PECTEN OF
+SCORPION.
+
+_n._, nerve; _gl._, ganglion.]
+
+I have not been able to obtain any evidence of a fusiform nerve-cell on the
+course of the terminal nerve-fibres as depicted by him; fusiform cells
+there are in plenty, as depicted in my drawing, but none with a large
+nucleus resembling those of the main ganglion. In no case, either in the
+flabellum or in the branchial organs of Limulus, or in the pecten-organs,
+have I ever seen a ganglion-cell within the chitin-layer; all the nuclei
+seen there resemble those of the cells of the hypodermis or else the
+elongated nuclei characteristic of the presence of nerve-fibres. Gaubert's
+drawing is a striking one, and I have looked through my specimens to see
+whether there was anything similar, but have hitherto failed to obtain any
+definite evidence of anything of the kind.
+
+I feel, myself, that an exhaustive examination of the structure and
+function of the pecten of scorpions ought to be undertaken. At present I
+can only draw the attention of my readers to the similarity of the
+arrangement of parts, and of the nature of the end-organs, in the
+sense-organs of the flabellum of Limulus and of the pecten of the scorpion.
+In both cases the special nerve-fibres terminate in a massive ganglion,
+situated just below the chitinogenous layer. In both cases the terminal
+fibres from these ganglion-cells pass through the modified chitinous layer
+to supply end-organs of a striking character; and although the end-organ of
+the pecten of the scorpion does {375}not closely resemble the majority of
+the end-organs of the flabellum, yet it does resemble, on the one hand, the
+isolated poriferous spikes found on the flabellum (Fig. 149) and, on the
+other, the poriferous goblets found on the sense-patches of the branchial
+appendages of Limulus (Fig. 144, A), so that a combination of these two
+end-organs would give an appearance very closely resembling that of the
+pecten of the scorpion.
+
+Finally, the special so-called 'racquet-organs' of Galeodes, which are
+found on the most basal segments of the last pair of prosomatic appendages,
+ought also to be considered here. Gaubert has described their structure,
+and shown how the nerve-trunk in the handle of the racquet splits up into a
+great number of separate bundles, which spread out fan-shaped to the free
+edge of the racquet; each of these separate bundles supplies a special
+sense-organ, which terminates as a conical eminence on the floor of a deep
+groove, running round the whole free edge of the racquet. This groove is
+almost converted into a canal, owing to the projection of its two sides.
+Gaubert imagines that the sense-organs are pushed forward out of the groove
+to the exterior by the turgescence of the whole organ; each of the
+nerve-fibres forming a bundle is, according to Gaubert, connected with a
+nerve-cell before it reaches its termination.
+
+This sketch of the special sense-organs on the appendages of Limulus, of
+the scorpions, of Galeodes, and other arachnids, and their comparison with
+the porous chordotonal organs of insects, affords reason for the belief
+that we are dealing here with a common group of organs, which, although
+their nature is not definitely known, have largely been accredited with the
+functions of equilibration and audition, a group of organs among which the
+origin of the auditory organ of vertebrates must be sought for, upon any
+theory of the origin of vertebrates from arthropods.
+
+Whenever in any animal these organs are concentrated together to form a
+special organ, it is invariably found that the nerve going to this organ is
+very large, out of all proportion to the size of the organ, and also that
+the nerve possesses, close to its termination in the organ, large masses of
+nerve-cells. Thus, although the whole hind wing in the blow-fly has been
+reduced to the insignificant balancers or 'halteres,' yet, as Lowne states,
+the nerves to them are the largest in the body.
+
+The pectinal nerve in the scorpion is remarkable for its size, and {376}so,
+also, is the nerve to the flabellum in Limulus, while the large size of the
+auditory nerve in the vertebrate, in distinction to the size of the
+auditory apparatus, has always aroused the attention of anatomists.
+
+Throughout this book my attention has been especially directed to both
+Limulus and the scorpion group in endeavouring to picture to myself the
+ancestor of the earliest vertebrates, because the Eurypteridæ possessed
+such marked scorpion-like characteristics; so that in considering the
+origin of a special sense-organ, such as the vertebrate auditory organ near
+the junction of the prosoma and mesosoma, it seems to me that the presence
+of such marked special sense-organs as the flabellum on the one hand and
+the pecten on the other, must both be taken into account, even although the
+former is an adjunct to a prosomatic appendage, while the latter
+represents, according to present ideas, the whole of a mesosomatic
+appendage.
+
+From the point of view that the VIIIth nerve represents a segment
+immediately posterior to that of the VIIth, it is evident that an organ in
+the situation of the pecten, immediately posterior to the operculum, _i.e._
+according to my view, posterior to the segment originally represented by
+the VIIth nerve, is more correctly situated than an organ like the
+flabellum, which belongs to a segment anterior to the operculum.
+
+On the other hand, from the point of view of the relationship between the
+scorpions and the king-crabs, it is a possibly debatable question whether
+the pecten really belongs to a segment posterior to the operculum. The
+position of any nerve in a series depends upon its position of origin in
+the central nervous system, rather than upon the position of its peripheral
+organ. Now, Patten gives two figures of the brain of the scorpion built up
+from serial sections. In both he shows that the main portion of the
+pectinal nerve arises from a swelling, to which he gives the name _ganglion
+nodosum_. This swelling arises on each side in close connection with the
+origin of the most posterior prosomatic appendage-nerve, according to his
+drawings, and posteriorly to such origin he figures a small nerve which he
+says supplies the distal parts of the sexual organs. This nerve is the only
+nerve which can be called the opercular nerve, and apparently arises
+posteriorly to the main part of the pectinal nerve. If this is so, it would
+indicate that the pectens arose from sense-organs which were originally,
+like the flabella, pre-opercular in position, but have shifted to a
+post-opercular position.
+
+
+{377}THE ORIGIN OF THE PARACHORDALS AND AUDITORY CARTILAGINOUS CAPSULE.
+
+In addition to what I have already said, there is another reason why a
+special sense-organ such as the pecten is suggestive of the origin of the
+vertebrate auditory organ, in that such a suggestion gives a clue to the
+possible origin of the parachordals and auditory cartilaginous capsules.
+
+In the lower vertebrates the auditory organ is characterized by being
+surrounded with a cartilaginous capsule which springs from a special part
+of the axial cartilaginous skeleton on each side, known as the pair of
+parachordals. The latter, in Ammocoetes, form a pair of cartilaginous bars,
+which unite the trabecular bars with the branchial cartilaginous
+basket-work. They are recognized throughout the Vertebrata as distinct from
+the trabecular bars, thus forming a separate paired cartilaginous element
+between the trabeculæ and the branchial cartilaginous system, which of
+itself indicates a position for the auditory capsule between the prosomatic
+trabeculæ and the mesosomatic branchial cartilaginous system.
+
+The auditory capsule and parachordals when formed are made of the same kind
+of cartilage as the trabeculæ, _i.e._ of hard cartilage, and are therefore
+formed from a gelatin-containing tissue, and not from muco-cartilage.
+Judging from the origin already ascribed to the trabeculæ, viz. their
+formation from the great prosomatic entochondrite or plastron, this would
+indicate that a second entochondrite existed in the ancestor of the
+vertebrate in the region of the junction of the prosoma and mesosoma, which
+was especially connected with the sense-organ to which the auditory organ
+owes its origin. This pair of entochondrites becoming cartilaginous would
+give origin to the parachordals, and subsequently to the auditory capsules,
+their position being such that the nerve to the operculum would be
+surrounded at its origin by the growth of cartilage.
+
+On this line of argument it is very significant to find that the scorpions
+do possess a second pair of entochondrites, viz. the supra-pectinal
+entochondrites, situated between the nerve-cord and the pectens, so that if
+the ancestor of the Cephalaspid was sufficiently scorpion-like to have
+possessed a second pair of entochondrites and at the same time a pair of
+special sense-organs of the nature either of {378}the pectens or flabella,
+then the origin of the auditory apparatus would present no difficulty.
+
+It is also easy to see that the formation of the parachordals from
+entochondrites homologous with the supra-pectinal entochondrites, would
+give a reason why the VIIth or opercular nerve is involved with the VIIIth
+in the formation of the auditory capsule, especially if the special
+sense-organ which gave origin to the auditory organ was originally a
+pre-opercular sense-organ such as the flabellum, which subsequently took up
+a post-opercular position like that of the pecten.
+
+
+THE EVIDENCE OF AMMOCOETES.
+
+As to the auditory apparatus itself, we see that the elaborate organ for
+hearing--the cochlea--has been evolved in the vertebrate phylum itself. In
+the lowest vertebrates the auditory apparatus tends more and more to
+resolve itself into a simple epithelial sac, the walls of which in places
+bear auditory hairs projecting into the sac, and in part form otoliths.
+Such a simple sac forms the early stage of the auditory vesicle in
+Ammocoetes, according to Shipley; subsequently, by a series of foldings and
+growings together, the chambers of the ear of the adult Petromyzon, as
+figured and described by Retzius, are formed. Further, we see that
+throughout the Vertebrata this sac was originally open to the exterior, the
+auditory vesicle being first an open pit, which forms a vesicle by the
+approximating of its sides, the last part to close being known as the
+_recessus labyrinthicus_; in many cases, as in elasmobranchs, this part
+remains open, or communicates with the exterior by means of the _ductus
+endolymphaticus_.
+
+Judging, therefore, from the embryological evidence, it would appear that
+the auditory organ originated as a special sense-organ, formed by modified
+epithelial cells of the surface, which epithelial surface becoming
+invaginated, came to line a closed auditory vesicle under the surface. This
+special sense-organ was innervated from a large ganglionic mass of
+nerve-cells, situated close against the peripheral sense-cells, the
+axis-cylinder processes of which formed the sensory roots of the nerve.
+
+Yet another peculiarity of striking significance is seen in connection with
+the auditory organ of Ammocoetes. The opening of the cartilaginous capsule
+towards the brain is a large one (Fig. 154), and {379}admits the passage
+not only of the auditory and facial nerves, but also of a portion of the
+peculiar tissue which surrounds the brain. The large cells of this tissue,
+with their feebly staining nuclei and the pigment between them, make them
+quite unmistakable; and, as I have already stated, nowhere else in the
+whole of Ammocoetes is such a tissue found. When I first noticed these
+cells within the auditory capsule, it seemed to me almost impossible that
+my interpretation of them as the remnant of the generative and hepatic
+tissue which surrounds the brain of animals such as Limulus could be true,
+for it seemed too unlikely that a part of the generative system could ever
+have become included in the auditory capsule. Still, they are undoubtedly
+there; and, as already argued with respect to the substance round the
+brain, they must represent some pre-existing tissue which was functional in
+the ancestor of Ammocoetes. If my interpretation is right, this tissue must
+be generative and hepatic tissue, and its presence in the auditory capsule
+immediately becomes a most important piece of evidence, for it proves that
+the auditory organ must have been originally so situated that a portion of
+the generative and hepatic mass surrounding the cephalic region of the
+nervous system followed the auditory nerve to the peripheral sense-organ.
+
+[Illustration: FIG. 154.--TRANSVERSE SECTION THROUGH AUDITORY CAPSULES AND
+BRAIN OF AMMOCOETES.
+
+_Au._, auditory organ; _VIII_, auditory nerve; _gl._, ganglion cells of
+VIIIth nerve; _Au. cart._, cartilaginous auditory capsule; _gen._, cells of
+old generative tissue round brain and in auditory capsule; _bl._,
+blood-vessels]
+
+{380}Here there was a test of the truth of my theory ranking second only to
+the test of the median eyes; the strongest possible evidence of the truth
+of any theory is given when by its aid new and unexpected facts are brought
+to light. The theory said that in the group of animals from which the
+vertebrates arose, a special sense-organ of the nature of an auditory organ
+must have existed on the base of one of the appendages situated at the
+junction of the prosoma and mesosoma, and that into this basal part of the
+appendage a portion of the cephalic mass of generative and hepatic material
+must have made its way in close contiguity to the nerve of the special
+organ.
+
+The only living example which nearly approaches the ancient extinct forms
+from which, according to the theory, the vertebrates arose, is Limulus,
+and, as has already been shown, in this animal, in the very position
+postulated by the theory, a large special sense-organ--the
+flabellum--exists, which, as already stated, may well have given rise to a
+sense-organ concerned with equilibration and audition. If, further, it be
+found that a diverticulum of the generative and hepatic material does
+accompany the nerve of the flabellum in the basal part of the appendage,
+then the evidence becomes very strong that the auditory organ of
+Ammocoetes, _i.e._ of the ancient Cephalaspids, was derived from an organ
+homologous with the flabellum; that, therefore, the material round the
+brain of Ammocoetes was originally generative and hepatic material; that,
+in fact, the whole theory is true, for all the parts of it hang together so
+closely that, if one portion is accepted, all the rest must follow. As
+pointed out in my address at Liverpool, and at the meeting of the
+Philosophical Society at Cambridge, it is a most striking fact that a mass
+of the generative and hepatic tissue does accompany the flabellar nerve
+into the basal part of this appendage. Into no other appendage of Limulus
+is there the slightest sign of any intrusion of the generative and hepatic
+masses; nowhere, except in the auditory capsule, is there any sign of the
+peculiar large-celled tissue which surrounds the brain and upper part of
+the spinal cord of Ammocoetes. The actual position of the flabellum on the
+basal part of the ectognath is shown in Fig. 155, A, and in Fig. 155, B, I
+have removed the chitin, to show the generative and hepatic tissue (_gen._)
+lying beneath.
+
+The reason why, to all appearance, the generative and hepatic mass
+penetrates into the basal part of this appendage only is apparent {381}when
+we see (as Patten and Redenbaugh have pointed out) to what part of the
+appendage the flabellum in reality belongs.
+
+[Illustration: FIG. 155.--A, THE DIGGING APPENDAGE OR ECTOGNATH OF LIMULUS;
+B, THE MIDDLE PROTUBERANCE (2) OF THE ENTOCOXITE OPENED, TO SHOW THE
+GENERATIVE AND HEPATIC TISSUE (_gen._) WITHIN IT; C, ONE OF THE PROSOMATIC
+LOCOMOTOR APPENDAGES OR ENDOGNATHS OF LIMULUS, FOR COMPARISON WITH A.
+
+_fl._, flabellum; _cox._, coxopodite; _ent._, entocoxite; _m._, mandible;
+_i.m._, inner mandible or epicoxite.]
+
+Patten and Redenbaugh, in their description of the prosomatic appendages of
+Limulus, describe the segments of the limbs as (1) the dactylopodite, (2)
+the propodite, (3) the mero- and carpo-podites, (4) the ischiopodite, (5)
+the basipodite, and (6) the coxopodite (_cox._ in Fig. 155). Still more
+basal than the coxopodite is situated the entocoxite (_ent._ in Fig. 155),
+which is composed of three sclerites {382}or sensory knobs, to use Patten's
+description. The middle one of these three sclerites enlarges greatly in
+the digging appendage, and grows over the coxopodite to form the base from
+which the flabellum springs. Thus, as they have pointed out, the flabellum
+does not belong to the coxopodite of the appendage, but to the middle
+sensory knob of the entocoxite. Upon opening the prosomatic carapace, it is
+seen that the cephalic generative and hepatic masses press closely against
+the internal surface of the prosomatic carapace and also of the entocoxite,
+so that any enlargement of one of the sensory knobs of the entocoxite would
+necessarily be filled with a protrusion of the generative and hepatic
+masses. This is the reason why the generative and hepatic material
+apparently passes into the basal segment of the ectognath, and not into
+that of the endognaths; it does not really pass into the coxopodite of the
+appendage, but into an enlarged portion of the entocoxite, which can hardly
+be considered as truly belonging to the appendage. Kishinouye has stated
+that a knob arises in the embryo at the base of each of the prosomatic
+locomotor appendages, but that this knob develops only in the last or
+digging appendage (ectognath) forming the flabellum. Doubtless the median
+sclerites of the entocoxites of the endognaths represent Kishinouye's
+undeveloped knobs.
+
+I conclude, therefore, that the flabellum, together with its basal part, is
+an adjunct to the appendage rather than a part of it, and might, therefore,
+easily remain as a separate and well-developed entity, even although the
+appendage itself dwindled down to a mere tentacle.
+
+The evidence appears to me very strong that the flabellum of Limulus and
+the pecten of scorpions are the most likely organs to give a clue to the
+origin of the auditory apparatus of vertebrates. At present both the
+Eurypterids and Cephalaspids have left us in the lurch; in the former there
+is no sign of either flabellum or pecten; in the latter, no sign of any
+auditory capsule beyond Rohon's discovery of two small apertures situated
+dorsally on each side of the middle line in Tremataspis, which he considers
+to be the termination of the _ductus endolymphaticus_ on each side. In both
+cases it is probable, one might almost say certain, that any such special
+sense-organ, if present, was not situated externally, but was sunk below
+the surface as in Ammocoetes.
+
+The method by which such a sense-organ, situated externally on {383}the
+surface of the animal, comes phylogenetically to form the lining wall of an
+internally situated membranous capsule is given by the ontogeny of this
+capsule, which shows step by step how the sense-organ sinks in and forms a
+capsule, and finally is entirely removed from the surface except as regards
+the _ductus endolymphaticus_.
+
+
+SUMMARY.
+
+  The special apparatus for hearing is of a very different character from
+  that for vision or for smell, for its nerve belongs to the
+  infra-infundibular group of nerves, and not to the supra-infundibular, as
+  do those of the other two special senses. Of the five special senses the
+  nerves for touch, taste, and hearing, all belong to the
+  infra-infundibular segmental nerve-groups. The invertebrate origin, then,
+  of the vertebrate auditory nerve must be sought for in the
+  infra-oesophageal segmental group of nerves, and not in the
+  supra-oesophageal.
+
+  The organs supplied by the auditory nerve are only partly for the purpose
+  of hearing; there is always present also an apparatus--the semicircular
+  canals--concerned with equilibration and co-ordination of movements. Such
+  equilibration organs are not confined to the auditory nerve, but in the
+  water-living vertebrates are arranged segmentally along the body, forming
+  the organs of the lateral line in fishes; the auditory organ is but one
+  of these lateral line organs, which has been specially developed.
+
+  These lateral line organs have been compared to similar segmental organs
+  found in connection with the appendages in worms, especially the
+  respiratory appendages. In accordance with this suggestion we see that
+  they are all innervated from the region of the respiratory nerves--the
+  vagus, glosso-pharyngeal, and facial--nerves which originally supplied
+  the respiratory appendages of the palæostracan ancestor.
+
+  The logical conclusion is that the appendages of the Palæostraca
+  possessed special sense-organs concerned with the perception of special
+  vibrations, especially in the mesosomatic or respiratory region, and that
+  somewhere at the junction of the prosoma and mesosoma, one of these
+  sense-organs was specially developed to form the origin of the vertebrate
+  auditory apparatus.
+
+  Impressed by this reasoning I made search for some specially striking
+  sense-organ at the base of one of the appendages of Limulus, at the
+  junction of the prosoma and mesosoma, and was immediately rewarded by the
+  discovery of the extraordinary nature of the flabellum, which revealed
+  itself as an elaborate sense-organ supplied with a nerve out of all
+  proportion to its size. Up to this time no one had the slightest
+  conception that this flabellum was a special sense-organ; the discovery
+  of its nature was entirely due to the logical following out of the theory
+  of the origin of vertebrates described in this book.
+
+  The structure of this large sense-organ is comparable with that of the
+  sense-organs of the pectens of the scorpion, and of many other organs
+  found on the appendages of various members of the scorpion group, of
+  arachnids and {384}other air-breathing arthropods. Many of these organs,
+  such as the lyriform organs of arachnids, and the 'halteres' or balancers
+  of the Diptera, are usually regarded as auditory and equilibration
+  organs.
+
+  On all the mesosomatic appendages of Limulus very remarkable sense-organs
+  are found, apparently for estimating pressures, which, when the
+  appendages sank into the body to form with their basal parts the
+  branchial diaphragms of Ammocoetes, could easily be conceived as
+  remaining at the surface, and so giving rise to the lateral line organs.
+
+  Further confirmation of the view that an organ, such as the flabellum,
+  must be looked upon as the originator of the vertebrate auditory organ,
+  is afforded by the extraordinary coincidence that in Limulus a
+  diverticulum of the generative and hepatic mass accompanies the flabellar
+  nerve into the basal part of the digging appendage, while in Ammocoetes,
+  accompanying the auditory nerve into the auditory capsule, there is seen
+  a mass of cells belonging to that peculiar tissue which fills up the
+  space between the brain and the cranial walls, and has already, on other
+  grounds, been homologized with the generative and hepatic masses which
+  fill up the encephalic region of Limulus.
+
+  For all these reasons special sense-organs, such as are found in the
+  flabellum of Limulus and in the pectens of scorpions, may be looked upon
+  as giving origin to the vertebrate auditory apparatus. In such case it is
+  highly probable that the parachordals, with the auditory capsules
+  attached, arose from a second entochondrite of the same nature as the
+  plastron; a probability which is increased by the fact that the scorpion
+  does possess a second entochondrite, which, owing to its special
+  relations to the pecten, is known as the supra-pectinal entochondrite.
+
+
+
+
+{385}CHAPTER XII
+
+_THE REGION OF THE SPINAL CORD_
+
+  Difference between cranial and spinal regions.--Absence of lateral
+  root.--Meristic variation.--Segmentation of coelom.--Segmental excretory
+  organs.--Development of nephric organs; pronephric, mesonephric,
+  metanephric.--Excretory organs of Amphioxus.--Solenocytes.--Excretory
+  organs of Branchipus and of Peripatus, appendicular and
+  somatic.--Comparison of coelom of Peripatus and of
+  vertebrate.--Pronephric organs compared to coxal glands.--Origin of
+  vertebrate body-cavity (metacoele).--Segmental duct.--Summary of
+  formation of excretory organs.--Origin of somatic
+  trunk-musculature.--Atrial cavity of Amphioxus.--Pleural folds.--Ventral
+  growth of pleural folds and somatic musculature.--Pleural folds of
+  Cephalaspidæ and of Trilobita.--Significance of the ductless
+  glands.--Alteration in structure of excretory organs which have lost
+  their duct in vertebrates and in invertebrates.--Formation of lymphatic
+  glands.--Segmental coxal glands of arthropods and of vertebrates.--Origin
+  of adrenals, pituitary body, thymus, tonsils, thyroid, and other ductless
+  glands.--Summary.
+
+
+The consideration of the auditory nerve and the auditory apparatus
+terminates the comparison between the cranial nerves of the vertebrate and
+the prosomatic and mesosomatic nerves of the arthropod, and leaves us now
+free to pass on to the consideration of the vertebrate spinal nerves and
+the organs they supply. Before doing so, it is advisable to pass in review
+the conclusions already attained.
+
+Starting with the working hypothesis that the central nervous system of the
+vertebrate has arisen from the central nervous system of the arthropod, but
+has involved and enclosed the alimentary canal of the latter in the
+process, so that there has been no reversal of surfaces in the derivation
+of the one form from the other, we have been enabled to compare closely all
+the organs of the head-region in the two groups of animals, and in no
+single case have we been compelled to make any startling or improbable
+assumptions. The simple following out of this clue has led in every case in
+the most natural {386}manner to the interpretation of all the organs in the
+head-region of the vertebrate from the corresponding organs of the
+arthropod.
+
+That it is possible to bring together all the striking resemblances between
+organs in the two classes of animals, such as I have done in preceding
+chapters, has been ascribed to a perverted ingenuity on my part--a
+suggestion which is flattering to my imaginative powers, but has no
+foundation of fact. There has been absolutely no ingenuity on my part; all
+I have done is to compare organs and their nerve-supply, as they actually
+exist in the two groups of animals, on the supposition that there has been
+no turning over on to the back, no reversal of dorsal and ventral surfaces.
+The comparison is there for all to read; it is all so simple, so
+self-evident that, given the one clue, the only ingenuity required is on
+the part of those who fail to see it.
+
+The great distinction that has arisen between the two head-regions is the
+disappearance of appendages as such, never, however, of important organs on
+those appendages. If the olfactory organs of the one group were originally
+situated on antennules, the olfactory organs still remain, although the
+antennules as such have disappeared. The coxal excretory organs at the base
+of the endognaths remain and become the pituitary body. A special
+sense-organ, such as the flabellum of Limulus or the pecten of scorpion,
+remains and gives rise to the auditory organ. A special glandular organ,
+the uterus in the base of the operculum, remains, and gives rise to the
+thyroid gland. The branchiæ and sense-organs on the mesosomatic appendages
+remain, and even the very muscles to a large extent. As will be seen later,
+the excretory organs at the base of the metasomatic appendages remain. It
+is merely the appendage as such which vanishes either by dwindling away, or
+by so great an alteration as no longer to be recognizable as an appendage.
+
+This dwindling process was already in full swing before the vertebrate
+stage; it is only a continuation of a previous tendency, as is seen in the
+dwindling of the prosomatic appendages in the Merostomata and the inclusion
+of the branchiæ within the body of the scorpion. Already among the
+Palæostraca, swimming had largely taken the place of crawling. The whole
+gradual transformation from the arthropod to the vertebrate is associated
+with a transformation from a crawling to a swimming animal--with the
+concomitant loss of locomotor appendages as such, and the alteration of the
+shape of {387}the animal into the lithe fish-like form. The consideration
+of the manner in which this latter change was brought about, takes us out
+of the cranial into the spinal region.
+
+If we take Limulus as the only living type of the Palæostraca, we are
+struck with the fact that the animal consists to all intents and purposes
+of prosomatic and mesosomatic regions only; the metasoma consisting of the
+segments posterior to the mesosoma is very insignificant, so that the large
+mass of the animal consists of what has become the head-region in the
+vertebrate; the spinal region, which has become in the higher vertebrates
+by far the largest region of the body, can hardly be said to exist in such
+an animal as Limulus. As to the Eurypterids and others, similar remarks may
+be made, though not to the same extent, for in them a distinct metasoma
+does exist.
+
+In this book I have considered up to the present the cranial region as a
+system of segments, and shown how such segments are comparable, one by one,
+with the corresponding segments in the prosoma and mesosoma of the presumed
+arthropod ancestor.
+
+In the spinal region such direct comparison is not possible, as is evident
+on the face of it; for even among vertebrates themselves the spinal
+segments are not comparable one by one, so great is the variation, so
+unsettled is the number of segments in this region. This meristic
+variation, as Bateson calls it, is the great distinctive character of the
+spinal region, which distinguishes it from the cranial region with its
+fixed number of nerves, and its substantive rather than meristic variation.
+At the borderland, between the two regions, we see how the one type merges
+into the other; how difficult it is to fix the segmental position of the
+spino-occipital nerves; how much more variable in number are the segments
+supplied by the vagus nerves than those anterior to them.
+
+This meristic variation is a sign of instability, of want of fixedness in
+the type, and is evidence, as already pointed out, that the spinal region
+is newer than the cranial. This instability in the number of spinal
+segments does not necessarily imply a variability in the number of segments
+of the metasoma of the invertebrate ancestor; it may simply be an
+expression of adaptability in the vertebrate phylum itself, according to
+the requirements necessitated by the conversion of a crawling into a
+swimming animal, and the subsequent conversion of the swimming into a
+terrestrial or flying animal.
+
+{388}However many may have been the original number of segments belonging
+to the spinal region, one thing is certain--the segmental character of this
+region is remarkably clearly shown, not only by the presence of the
+segmental spinal nerves, but also by the marked segmentation of the
+mesoblastic structures. The question, therefore, that requires elucidation
+above all others is the origin of the spinal mesoblastic segments, _i.e._
+of the coelomic cavities of the trunk-region, and the structures derived
+from their walls.
+
+Proceeding on the same lines as in the case of the cranial segments, it is
+necessary in the first instance to inquire of the vertebrate itself as to
+the scope of the problem in this region. In addition to the variability in
+the number of segments so characteristic of the spinal region, the complete
+absence in each spinal segment of a lateral root affords another marked
+difference between the two regions. Here, except, of course, at the
+junction of the spinal and cranial regions, each segmental nerve arises
+from two roots only, dorsal and ventral, and these roots are separately
+sensory and motor, and not mixed in function as was the lateral root of
+each cranial segment. Now, these lateral roots were originally the nerves
+supplying the prosomatic and mesosomatic appendages with motor as well as
+sensory fibres. The absence, therefore, of lateral roots in the spinal
+region implies that in the vertebrate none of the musculature belonging to
+the metasomatic appendages has remained. Consequently, as far as muscles
+are concerned, the clue to the origin of the spinal segments must be sought
+for in the segmentation of the body-muscles.
+
+Here, in contradistinction to the cranial region, the segmentation is most
+marked, for the somatic spinal musculature of all vertebrates can be traced
+back to a simple sheet of longitudinal ventral and dorsal muscles, such as
+are seen in all fishes. This sheet is split into segments or myotomes by
+transverse connective tissue septa or myo-commata; each myotome
+corresponding to one spinal segment.
+
+In addition to the evidence of segmentation afforded by the
+body-musculature in all the higher vertebrates, similar evidence is given
+by the segmental arrangement of parts of the supporting tissue to form
+vertebræ. Such segments have received the name of sclerotomes, and each
+sclerotome corresponds to one spinal segment.
+
+Yet another marked peculiarity of this region is the segmental arrangement
+of the excretory organs. Just as our body-musculature {389}has arisen from
+the uniformly segmented simple longitudinal musculature of the lowest fish,
+so, as we pass down the vertebrate phylum, we find more and more of a
+uniform segmental arrangement in the excretory organs.
+
+The origin of all these three separate segmentations may, in accordance
+with the phraseology of the day, be included in the one term--the origin of
+the spinal mesoblastic segments--_i.e._ of the coelomic cavities of the
+trunk-region and the structures derived from their walls.
+
+
+THE ORIGIN OF THE SEGMENTAL EXCRETORY ORGANS.
+
+Of these three clues to the past history of the spinal region, the
+segmentation manifested by the presence of vertebræ is the least important,
+for in Ammocoetes there is no sign of vertebræ, and their indications only
+appear at transformation. Especially interesting is the segmentation due to
+the excretory organs, for the evidence distinctly shows that such excretory
+organs have steadily shifted more and more posteriorly during the evolution
+of the vertebrate.
+
+In Limulus the excretory organs are in the prosomatic region--the coxal
+glands; these become in the vertebrate the pituitary body.
+
+In Amphioxus the excretory organs are in the mesosomatic region,
+segmentally arranged with the gills.
+
+In vertebrates the excretory organs are in the metasomatic region posterior
+to the gills, and are segmentally arranged in this region. Their
+investigation has demonstrated the existence of three distinct stages in
+these organs: 1. A series of segmental excretory organs in segments
+immediately following the branchial segments. This is the oldest of the
+three sets, and to these organs the name of the _pronephros_ is given. 2. A
+second series which extends more posteriorly than the first, overlaps them
+to an extent which is not yet settled, and takes their place; to them is
+given the name of the _mesonephros_. 3. A third series continuous with the
+mesonephric is situated in segments still more posterior, supplants the
+mesonephros and forms the kidneys of all the higher vertebrates. This forms
+the _metanephros_.
+
+These three sets of excretory organs are not exactly alike in their origin,
+in that the pronephric tubules are formed from a different portion of the
+coelomic walls to that from which the meso- and {390}metanephric tubules
+are formed, and the former alone gives origin to a duct, which forms the
+basis for the generative and urinary ducts, and is called the _segmental
+duct_. The mesonephric tubules, called also the Wolffian body, open into
+this duct.
+
+In order to make the embryology of these excretory organs quite clear, I
+will make use of van Wijhe's phraseology and also of his illustrations. He
+terms the whole coelomic cavity the _procoelom_, which is divisible into a
+ventral unsegmented part, the body-cavity or _metacoelom_, and a dorsal
+segmented part, the _somite_. This latter part again is divided into a
+dorsal part--the _epimere_--and a part connecting the dorsal part with the
+body-cavity, to which therefore he gives the name of _mesomere_.
+
+The cavity of the epimere disappears, and its walls form the muscle and
+cutis plates of the body. The part which forms the muscles is known as the
+_myotome_, which separates off from the mesomere, leaving the latter as a
+blind sac--the _mesocoelom_--communicating by a narrow passage with the
+body cavity or _metacoelom_. At the same time, from the mesomere is formed
+the _sclerotome_, which gives rise to the skeletal tissues of the vertebræ,
+etc., so that van Wijhe's epimere and mesomere together correspond to the
+original term, protovertebra, or somite of Balfour; and when the myotome
+and sclerotome have separated off, there is still left the intermediate
+cell-mass of Balfour and Sedgwick, _i.e._ the sac-like mesocoele of van
+Wijhe, the walls of which give origin to the mesonephrotome or
+_mesonephros_. Further, according to van Wijhe, the dorsal part of the
+unsegmented metacoelom is itself segmented, but not, as in the case of the
+mesocoele, with respect to both splanchnopleuric and somatopleuric walls.
+The segmentation is manifest only on the somatopleuric side, and consists
+of a distinct series of hollow somatopleuric outgrowths, called by him
+_hypomeres_, which give rise to the _pronephros_ and the segmental duct.
+
+Van Wijhe considers that the whole metacoelom was originally segmented,
+because in the lower vertebrates the segmentation reaches further
+ventral-wards, so that in Selachia the body-cavity is almost truly
+segmental. Also in the gill-region of Amphioxus the cavities which are
+homologous with the body-cavity arise segmentally.
+
+{391}[Illustration: FIG. 156.--DIAGRAMS TO ILLUSTRATE THE DEVELOPMENT OF
+THE VERTEBRATE COELOM. (After VAN WIJHE.)
+
+_N._, central nervous system; _Nc._, notochord; _Ao._, aorta; _Mg._,
+midgut. A, _My._, myocoele; _Mes._, mesocoele; _Met._, metacoele; _Hyp._,
+hypomere (pronephric). B and C, _My._, myotome; _Mes._, mesonephros;
+_S.d._, segmental duct (pronephric); _Met._, body cavity.]
+
+{392}As is well known, Balfour and Semper were led, from their
+embryological researches, to compare the nephric organs of vertebrates with
+those of annelids, and, indeed, the nature of the vertebrate segmental
+excretory organs has always been the fact which has kept alive the belief
+in the origin of vertebrates from a segmented annelid. These segmental
+organs thus compared were the mesonephric tubules, and doubts arose,
+especially in the mind of Gegenbaur, as to the validity of such a
+comparison, because the mesonephric tubules did not open to the exterior,
+but into a duct--the segmental duct--which was an unsegmented structure
+opening into the cloaca; also because the segmental duct, which was the
+excretory duct of the pronephros, was formed first, and the mesonephric
+tubules only opened into it after it was fully formed. Further, the
+pronephros was said to arise from an outbulging of the somatopleuric
+mesoblast, which extended over a limited number of metameres, and was not
+segmental, but continuous. Gegenbaur and others therefore argued that the
+original prevertebrate excretory organ was the pronephros and its duct, not
+the mesonephros, from which they concluded that the vertebrate must have
+been derived from an unsegmented type of animal, and not from the segmented
+annelid type.
+
+Such a view, however, has no further reason for acceptance, as it was based
+on wrong premises, for Rückert has shown that the pronephros does arise as
+a series of segmental nephric tubules, and is not unsegmented. He also has
+pointed out that in Torpedo the anterior part of the pronephric duct shows
+indications of being segmented, a statement fully borne out by the
+researches of Maas on Myxine, who gives the clearest evidence that in this
+animal the anterior part of the pronephric duct is formed by the fusion of
+a series of separate ducts, each of which in all probability once opened
+out separately to the exterior.
+
+Rückert therefore concludes that Balfour and Semper were right in deriving
+the segmental organs of vertebrates from those of annelids, but that the
+annelid organs are represented in the vertebrate, not by the mesonephric
+tubules, but by the pronephric tubules and their ducts, which originally
+opened separately to the exterior. By the fusion of such tubules the
+anterior part of the segmental duct was formed, while its posterior part
+either arose by a later coenogenetic lengthening, or is the only remnant of
+a series of pronephric tubules which originally extended the whole length
+of the body, as suggested also by Maas and Boveri. Rückert therefore
+supposed that the mesonephric tubules were a secondary set of nephric
+organs, which were not necessarily directly derived from the annelid
+nephric organs.
+
+{393}At present, then, Rückert's view is the one most generally
+accepted--the original annelid nephric organs are represented by the
+pronephric tubules and the pronephric duct, not by the mesonephric tubules,
+which are a later formation. This latter statement would hold good if the
+mesonephric tubules were found entirely in segments posterior to those
+containing the pronephric tubules; such, however, is said not to be the
+case, for the two sets of organs are said to overlap in some cases; even
+when they exist in the same segments, the former are said always to be
+formed from a more dorsal part of the coelom than the pronephros, always to
+be a later formation, and never to give any indication of communicating
+with the exterior except by way of the pronephric duct.
+
+The recent observations of Brauer on the excretory organs of the
+Gymnophiona throw great doubt on the existence of mesonephric and
+pronephric tubules in the same segment. He criticizes the observations on
+which such statements are based, and concludes that, as in Hypogeophis, the
+nephrotome which is cut off after the separation of the sclero-myotome
+gives origin to the pronephros in the more anterior regions, just as it
+gives origin to the mesonephros in the more posterior regions. In fact, the
+observations of van Wijhe and others do not in reality show that two
+excretory organs may be formed in one segment, the one mesonephric from the
+remains of the mesomere and the other pronephric from the hypomere, but
+rather that in such cases there is only one organ--the pronephros--part of
+which is formed from the mesomere and part from the hypomere. Brauer goes
+further than this, and doubts the validity of any distinction between
+pronephros and mesonephros, on the ground of the former arising from a more
+ventral part of the procoelom than the latter; for, as he says, it is only
+possible to speak of one part of the somite as being more ventral than
+another part when both parts are in the same segment; so that if pronephric
+and mesonephric organs are never in the same segment, we cannot say with
+certainty that the former arises more ventrally than the latter.
+
+These observations of Brauer strongly confirm Sedgwick's original statement
+that the pronephric and mesonephric organs are homodynamous organs, in that
+they are both derived from the original serially situated nephric organs,
+the differences between them being of a subordinate nature and not
+sufficient to force us to believe that the mesonephros is an organ of quite
+different origin to the {394}pronephros. So, also, Price, from his
+investigations of the excretory organs of Bdellostoma, considers that in
+this animal both pronephros and mesonephros are derived from a common
+embryonic kidney, to which he gives the name _holonephros_.
+
+Brauer also is among those who conclude that the vertebrate excretory
+organs were derived from those of annelids; he thinks that the original
+ancestor possessed a series of similar organs over the whole pronephric and
+mesonephric regions, and that the anterior pronephric organs, which alone
+form the segmental duct, became modified for a larval existence--that their
+peculiarities were adaptive rather than ancestral. This last view seems to
+me very far-fetched, without any sufficient basis for its acceptance.
+According to the much more probable and reasonable view, the pronephros
+represents the oldest and original excretory organs, while the mesonephros
+is a later formation. Brauer's evidence seems to me to signify that the
+pronephros, mesonephros, and metanephros are all serially homologous, and
+that the pronephros bears much the same relation to the mesonephros that
+the mesonephros does to the metanephros. The great distinction of the
+pronephros is that it, and it alone, forms the segmental duct.
+
+We may sum up the conclusions at which we have now arrived as follows:--
+
+1.  The pronephric tubules and the pronephric duct are the oldest part of
+the excretory system, and are distinctly in evidence for a few segments
+only in the most anterior part of the trunk-region immediately following
+the branchial region. They differ also from the mesonephric tubules by not
+being so clearly segmental with the myotomes.
+
+2.  The mesonephric tubules belong to segments posterior to those of the
+pronephros, are strictly segmental with the myotomes, and open into the
+pronephric duct.
+
+3.  All observers are agreed that the two sets of excretory organs resemble
+each other in very many respects, as though they arose from the same series
+of primitive organs, and, according to Sedgwick and Brauer, no distinction
+of any importance does exist between the two sets of organs. Other
+observers, however, consider that the pronephric organs, in part at all
+events, arise from a part of the nephrocoele more ventral than that which
+gives origin to the mesonephric organs, and that this difference in
+position of origin, combined {395}with the formation of the segmental duct,
+does constitute a true morphological distinction between the two sets of
+organs.
+
+4. All the recent observers are in agreement that the vertebrate excretory
+organs strongly indicate a derivation from the segmental organs of
+annelids.
+
+The very strongest support has been given to this last conclusion by the
+recent discoveries of Boveri and Goodrich upon the excretory organs of
+Amphioxus. According to Boveri, the nephric tubules of Amphioxus open into
+the dorsal coelom by one or more funnels. Around each funnel are situated
+groups of peculiar cells, called by him 'Fadenzellen,' each of which sends
+a long process across the opening of the funnel. Goodrich has examined
+these 'Fadenzellen,' and found that they are typical pipe-cells, or
+solenocytes, such as he has described in the nephridial organs of various
+members of the annelid group Polychæta. Also, just as in the Polychæta, the
+ciliated nephric tubule has no internal funnel-shaped opening into the
+coelom, but terminates in these groups of solenocytes. "Each solenocyte
+consists of a cell-body and nucleus situated at the distal free extremity
+of a delicate tube; the proximal end of the tube pierces the wall of the
+nephridial canal and opens into its lumen. A single long flagellum arising
+from the cells works in the tube and projects into the canal."
+
+The exceedingly close resemblance between the organs of Amphioxus and those
+of Phyllodoce, as given in his paper, is most striking, and, as he says,
+leads to the conclusion that the excretory organs of Amphioxus are
+essentially identical with the nephridia of certain polychæte worms.
+
+It is to me most interesting to find that the very group of annelids, the
+Polychæta, which possess solenocytes so remarkably resembling those of the
+excretory organs of Amphioxus, are the highest and most developed of all
+the Annelida. I have argued throughout that the law of evolution consists
+in the origination of successive forms from the dominant group then alive,
+dominance signifying the highest type of brain-power achieved up to that
+time. The highest type among Annelida is found in the Chætopoda; from them,
+therefore, the original arthropod type must have sprung. This original
+group of Arthropoda gave rise to the two groups of Crustacea and Arachnida,
+in my opinion also to the Vertebrata, and, as already mentioned, it is
+convenient to give it a generalized {396}name, the Protostraca, from which
+subsequently the Palæostraca arose.
+
+The similarity between the excretory organs of Amphioxus and those of
+Phyllodoce suggests that the protostracan ancestor of the vertebrates arose
+from the highest group of the Chætopoda--the Polychæta. The evidence which
+I have already given points, however, strongly to the conclusion that the
+vertebrate did not arise from members of the Protostraca near to the
+polychæte stock, but rather from members in which the arthropod characters
+had already become well developed--members, therefore, which were nearer
+the Trilobita than the Polychæta. Such early arthropods would very probably
+have retained in part excretory organs of the same character as those found
+in the original polychæte stock, and thus account for the presence of
+solenocytes in the excretory organs of Amphioxus.
+
+In connection with such a possibility, I should like to draw attention to
+the observations of Claus and Spangenberg on the excretory organs of
+Branchipus--that primitive phyllopod, which is recognized as the nearest
+approach to the trilobites at present living. According to Claus, an
+excretory apparatus exists in the neighbourhood of each nerve-ganglion, and
+Spangenberg finds a perfectly similar organ in the basal segment of each
+appendage--a system, therefore, of excretory organs as segmentally arranged
+as those of Peripatus. Claus considers that although these organs formed an
+excretory system, it is not possible to compare them with the annelid
+segmental organs, because he thought the cells in question arose from
+ectoderm. Now, the striking point in the description of the excretory cells
+in these organs, as described both by Claus and Spangenberg, is that they
+closely resemble the pipe-cells or solenocytes of Goodrich; each cell
+possesses a long tube-like projection, which opens on the surface. They
+appear distinctly to belong to the category of flame-cells, and resemble
+solenocytes more than anything else. According to Goodrich, the solenocyte
+is probably an ectodermal cell, so that even if it prove to be the case, as
+Claus thought, that these pipe-cells of Branchipus are ectodermal, they
+would still claim to be derived from the segmental organs of annelids,
+especially of the Polychæta, being, to use Goodrich's nomenclature, true
+nephridial organs, as opposed to coelomostomes.
+
+These observations of Claus and Spangenberg suggest not only that the
+primitive arthropod of the trilobite type possessed segmental {397}organs
+in every segment directly derived from those of a polychæte ancestor, but
+also that such organs were partly somatic and partly appendicular in
+position. Such a suggestion is in strict accord with the observations of
+Sedgwick on the excretory organs of the most primitive arthropod known,
+viz. Peripatus, where also the excretory organs, which are true segmental
+organs, are partly somatic and partly appendicular. Further, the excretory
+organs of the Scorpion and Limulus group are again partly somatic and
+partly appendicular, receiving the name of coxal glands, because there is a
+ventral projection of the gland into the coxa of the corresponding
+appendage.
+
+Judging from all the evidence available, it is probable that when the
+arthropod stock arose from the annelids, simultaneously with the formation
+of appendages, the segmental somatic nephric organs of the latter extended
+ventrally into the appendage, and thus formed a segmental set of excretory
+organs, which were partly somatic, partly appendicular in position, and
+might therefore be called coxal glands.
+
+As already stated, all investigators of the origin of the vertebrate
+excretory organs are unanimous in considering them to be derived from
+segmental organs of the annelid type. I naturally agree with them, but, in
+accordance with my theory, would substitute the words "primitive arthropod"
+for the word "annelid," for all the evidence I have accumulated in the
+preceding chapters points directly to that conclusion. Further, the most
+primitive of the three sets of vertebrate segmental organs--the pronephros,
+mesonephros, and metanephros--is undoubtedly the pronephros; consequently
+the pronephric tubules are those which I consider to be more directly
+derived from the coxal glands of the primitive arthropod ancestor. Such a
+derivation appears to me to afford an explanation of the difficulties
+connected with the origin of the pronephros and mesonephros respectively,
+which is more satisfactory than that given by the direct derivation from
+the annelid.
+
+The only living animal which we know of as at all approaching the most
+primitive arthropod type is, as pointed out by Korschelt and Heider,
+Peripatus; and Peripatus, as is well known, possesses a true coelom and
+true coelomic excretory organs in all the segments of the body. Sedgwick
+shows that at first a true coelom, as typical as that of the annelids, is
+formed in each segment of the body, and that then this coelom (which
+represents in the vertebrate van Wijhe's pro-coelom) {398}splits into a
+dorsal and a ventral part. In the anterior segments of the body the dorsal
+part disappears (presumably its walls give origin to the mesoblast from
+which the dorsal body-muscles arise), while the ventral part remains and
+forms a nephrocoele, giving origin to the excretory organs of the adult.
+According to von Kennel, the cavity becomes divided into three spaces,
+which for a time are in communication--a lateral (I.), a median (II.), and
+a dorso-median (III.). The dorso-median portion becomes partitioned off,
+and this, as well as the greater part of the lateral portion, which lies
+principally in the foot, is used up in providing elements for the formation
+of the body- and appendage-muscles respectively and the connective tissue.
+
+In Fig. 157 I reproduce von Kennel's diagram of a section across a
+Peripatus embryo, in which I. represents the lateral appendicular part of
+the coelom, II. the ventral somatic part, and III. the dorsal part which
+separates off from the ventral and lateral parts, and, as its walls give
+origin largely to the body-muscles, may be called the myocoele. The muscles
+of the appendages are formed from the ventral part of the original
+procoelom, just as I have argued is the case with the muscles of the
+splanchnic segmentation in vertebrates.
+
+Sedgwick states that the ventral part of the coelom extends into the base
+of each appendage, and there forms the end-sac of each nephric tubule, into
+which the nephric funnel opens, thus forming a coxal gland; this end-sac or
+vesicle in the appendage is called by him the internal vesicle (_i.v._),
+because later another vesicle is formed from the ventral coelom in the body
+itself, close against the nerve-cord on each side, which he calls the
+external vesicle (_e.v._). (_Cf._ Fig. 158, taken from Sedgwick.) This
+second vesicle is, according to him, formed later in the development from
+the nephric tubule of the internal vesicle, so that it discharges its
+contents to the exterior by the same opening as the original tubule. Of
+course, as he points out, the whole system of internal and external
+vesicles and nephric tubules are all simply derivatives of the original
+ventral part of the coelom or nephrocoele.
+
+{399}[Illustration: FIG. 157.--TRANSVERSE SECTION OF PERIPATUS EMBRYO.
+(After VON KENNEL.)
+
+_Al._, alimentary canal; _N._, nerve-cord; _App._, appendage; _I_, _II_,
+_III_, the three divisions (lateral, median, and dorso-median) of the
+coelom.]
+
+[Illustration: FIG. 158.--SECTION OF PERIPATUS. (After SEDGWICK.)
+
+_Al._, alimentary canal; _N._, nerve-cord; _App._, appendage; _i.v._,
+internal, and _e.v._, external vesicles of the segmented excretory tubule
+(coxal gland).]
+
+Here, then, in Peripatus, and presumably, therefore, in members of the
+Protostraca, we see that the original segmental organs of the annelid have
+become a series of nephric organs, which extended into the base of the
+appendages, and may therefore be called coxal glands; also it is clear,
+from Sedgwick's description, that if the appendages disappeared, the
+nephric organs would still remain, not as coxal glands, but as purely
+somatic excretory glands. They would still be homologous with the annelid
+segmental organs, or with the coxal glands, but would arise _in toto_ from
+a part of the ventral coelom or nephrocoele, more dorsal than the former
+appendicular part, because the appendages and their enclosed coelom are
+always situated ventrally to the body. Again, according to Sedgwick, the
+nephric tubules are connected with two coelomic vesicles, the one in the
+appendage the internal vesicle, and the other, the so-called bladder, or
+the external vesicle, in the body itself, close against the nerve-cord.
+Sedgwick appears to consider that either of these vesicles may form the
+end-sac of a nephric tubule, for he discusses the question whether the
+single vesicle, which in each case gives origin to the nephridia of the
+first three legs, corresponds to the internal or external vesicle. He
+{400}decides, it is true, in favour of the internal vesicle, and therefore
+considers the excretory organ to be appendicular, _i.e._ a coxal gland, in
+these segments as well as in those more posterior. Still, the very
+discussion shows that in his opinion, at all events, the external vesicle
+might represent the end-sac of the tubule, in the absence of the internal
+or appendicular vesicle.
+
+Such an arrangement as Sedgwick describes in Peripatus is the very
+condition required to give rise to the pronephric and mesonephric tubules,
+as deduced by me from the consideration of the vertebrate, and harmonizes
+and clears up the controversy about the mesonephros and pronephros in the
+most satisfactory manner. Both pronephros and mesonephros are seen to be
+derivatives of the original annelid segmental organs, not directly from an
+annelid, but by way of an arthropodan ancestor; the difference between the
+two is simply that the pronephric organs were coxal glands, and indicate,
+therefore, the presence of the original metasomatic appendages, while the
+mesonephric organs were homologous organs, formed in segments of later
+origin which had lost their appendages. For this reason the pronephros is
+said to be formed, in part at least, from a portion of the coelom situated
+more ventrally than the purely somatic part which gives rise to the
+mesonephros. For this reason Sedgwick, Brauer, etc., can say that the
+mesonephros is strictly homodynamous with the pronephros; while equally
+Rückert, Semon, and van Wijhe can say it is not homodynamous, in so far
+that the two organs are not derived strictly from absolutely homologous
+parts of the coelom. For this reason Semon can speak of the mesonephros as
+a dorsal derivative of the pronephros, just as Sedgwick says that the
+external or somatic vesicle of Peripatus is a derivative of the
+appendicular nephric organ. For this reason the pronephros, or rather a
+part of it, is always derived from the somatopleuric layer, for, as is
+clear from Miss Sheldon's drawing, the part of the coelom in Peripatus
+which dips into the appendage is derived from the somatopleuric layer
+alone.
+
+Such a coelom as that of Peripatus, Fig. 157, would represent the origin of
+the vertebrate coelom, and would therefore represent the procoelom of van
+Wijhe. In strict accordance with this, we see that it separates into a
+dorsal part, the walls of which give origin to the somatic muscles, or at
+all events to the great longitudinal dorsal muscles of the animal, and a
+ventral part, which forms a nephrocoele, {401}dips into the appendage, and
+gives origin to the muscles of the appendage. In the vertebrate, after the
+somatic dorsal part or myocoele has separated off, a ventral part is left,
+which forms a nephrocoele in the trunk-region, and gives origin to the
+splanchnic striated muscles in the cranial region, _i.e._ to the muscles
+which, according to my theory, were once appendicular muscles. This ventral
+nephrocoelic part is divisible in the trunk into a segmented part, which
+forms the excretory organs proper, and an unsegmented part, the metacoele
+or true body-cavity of the vertebrate.
+
+This comparison of the procoelom of the vertebrate and arthropod signifies
+that the vertebrate metacoele was directly derived by ventral downgrowth
+from the arthropod nephrocoele, so that if, as I suppose, the vertebrate
+nervous system represents the conjoined nervous system and alimentary canal
+of the arthropod, then the vertebrate metacoele, or body-cavity, must have
+been originally confined to the region on each side of the central nervous
+system, and from this position have spread ventrally, to enclose ultimately
+the new-formed vertebrate gut. This means that the body-cavity (metacoele)
+of the vertebrate is not the same as the body-cavity of the annelid, but
+corresponds to a ventral extension of the nephrocoele, or ventral part of
+such body-cavity.
+
+Such a phylogenetic history is most probable, because it explains most
+naturally and simply the facts of the development of the vertebrate
+body-cavity; for the mesoblast always originates in the neighbourhood of
+the notochord and central nervous system, and the lumen of the body-cavity
+always appears first in that region, and then extends laterally and
+ventrally on each side until it reaches the most ventral surface of the
+embryo, thus forming a ventral mesentery, which ultimately disappears, and
+the body-cavity surrounds the gut, except for the dorsal mesentery. Thus
+Shipley, in his description of the formation of the mesoblastic plates
+which line the body-cavity in Ammocoetes, describes them as commencing in
+two bands of mesoblast situated on each side, close against the commencing
+nervous system:--
+
+"These two bands are separated dorsally by the juxtaposition of the dorsal
+wall of the mesenteron and the epiblast, and ventrally by the hypoblastic
+yolk-cells which are in contact with the epiblast over two-thirds of the
+embryo. Subsequently, but at a much later date, the mesoblast is completed
+ventrally by the downgrowth on {402}each side of these mesoblastic plates.
+The subsequent downward growth is brought about by the cells proliferating
+along the free ventral edge of the mesoblast, these cells then growing
+ventralwards, pushing their way between the yoke-cells and epiblast."
+
+The derivation of the vertebrate pronephric segmental organs from the
+metasomatic coxal glands of a primitive arthropod would mean, if the
+segmental organs of Peripatus be taken as the type, that such glands opened
+to the exterior on every segment, either at the base of the appendage or on
+the appendage itself. It is taken for granted by most observers that the
+pronephric segmental organs once opened to the exterior on each segment,
+and then, from some cause or other, ceased to do so, and the separate
+ducts, by a process of fusion, came to form a single segmental duct, which
+opened into the cloaca. Many observers have been led to the conclusion that
+the pronephric duct is epiblastic in origin, although from its position in
+the adult, it appears far removed from all epiblastic formations. However,
+at no time in the developmental history is there any clear evidence of
+actual fusion of any part of the pronephric organ with the epidermis, and
+the latest observer, Brauer, is strongly of opinion that there is never
+sufficiently close contact with the epidermis to warrant the statement that
+the epiblastic cells take part in the formation of the duct. All that can
+be said is, that the formation of the duct takes place at a time when the
+pronephric diverticulum is in close propinquity to the epidermis, before
+the ventral downgrowth of the myotome has taken place.
+
+The formation of the anterior portion of the pronephric duct is, according
+to Maas in Myxine, and Wheeler in Petromyzon, undoubtedly brought about by
+the fusion of a number of pronephric tubules, which, according to Maas, are
+clearly seen in the youngest specimens as separate segmental tubes; each of
+these tubules is supplied by a capillary network from a segmental branch of
+the aorta, as in the tubules of Amphioxus according to Boveri, and does not
+possess a glomerulus.
+
+The posterior part of the duct into which the mesonephric tubules enter
+possesses also a capillary network, which Maas considers to represent the
+original capillary network of a series of pronephric tubules, the only
+remnant of which is the duct into which the mesonephric tubules open. He
+therefore argues that the pronephric duct indicates a series of pronephric
+tubules, which originally extended {403}along the whole length of the body,
+and were supplanted by the mesonephric tubules, which also belonged to the
+same segments.
+
+I also think that the paired appendages which have left the pronephric
+tubules as signs of their past existence, existed originally, in the
+invertebrate stage, on every segment of the body. But I do not consider
+that such a statement is at all equivalent to saying that such pairs of
+tubules must have existed upon every one of the segments existing at the
+present day; for it seems to me that Rückert is much more likely to be
+right when he says that in Selachians the duct clearly does grow back, and
+is not formed throughout _in situ_; so that he gives a double explanation
+of the formation of the duct--a palingenetic anterior part formed by the
+fusion of the extremities of the original excretory tubules, to which a
+posterior coenogenetic lengthening has been added.
+
+It does not seem to me at all necessary that the immediate invertebrate
+ancestor of the vertebrate should have possessed excretory organs which
+opened out separately to the exterior on each segment; a fusion may already
+have taken place in the invertebrate stage, and so a single duct have been
+acquired for a number of organs. Such a suggestion has been made by
+Rückert, because of the fact discovered by Cunningham and E. Meyer, that
+the segmental organs of _Lanice conchilega_ are on each side connected
+together by a single strong longitudinal canal. I would, however, go
+further than this and say, that even although the nephric organs of the
+polychæte ancestor opened out on every segment, and although the primitive
+arthropodan ancestor derived from such polychæte possessed coxal glands
+which opened out either on to or at the base of each appendage, similarly
+to those of Peripatus, yet the immediate arthropodan ancestor, with its
+palæostracan affinities, may already have possessed metasomatic coxal
+glands, all of which opened into a single duct, with a single opening to
+the exterior.
+
+Judging from Limulus, such was very probably the case, for Patten and Hazen
+have shown (1) that the coxal glands of Limulus are segmental organs
+belonging to the prosomatic segments; (2) that the organs belonging to the
+cheliceral and ectognathal segments are not developed; (3) that the four
+glands belonging to the endognaths become connected together by a _stolon_,
+which communicates with a single nephric duct, opening to the exterior on
+the basal segment of the 5th prosomatic appendage (the last endognath). At
+{404}no time is there any evidence of any separate openings or any fusion
+with the ectoderm, such as might indicate separate openings of these
+prosomatic coxal segmental organs. Thus we see that in Limulus, which is
+presumably much nearer the annelid condition than the vertebrate, all
+evidence of separate nephric ducts opening to the exterior on each
+prosomatic segment has entirely disappeared, just as is the case in the
+metasomatic coxal glands (_i.e._ the pronephros) of the vertebrate. What is
+seen in the prosomatic region of Limulus, and doubtless also of the
+Eurypterids, may very probably have occurred in the metasomatic region of
+the immediate invertebrate ancestors of the vertebrate, and so account for
+the single pronephric duct belonging to a number of pronephric organs.
+
+The interpretation of these various embryological investigations may be
+summed up as follows:--
+
+1.  The ancestor of the vertebrates possessed a pair of appendages on each
+segment; into the base of each of these appendages the segmental excretory
+organ sent a diverticulum, thus forming a coxal gland.
+
+2.  Such coxal glands, even in the invertebrate stage, may have discharged
+into a common duct which opened to the exterior most posteriorly.
+
+3.  Then, from some cause, the appendages were rendered useless, and
+dwindled away, leaving only the pronephric organs to indicate their former
+presence. At the end of this stage the animal possessed vertebrate
+characteristics.
+
+4.  For the purpose of increasing mobility, of forming an efficient
+swimming instead of a crawling animal, the body-segments increased in
+number, always, as is invariably the case, by the formation of new ones
+between those already formed and the cloacal region, and so of necessity
+caused an elongation of the pronephric duct. Into this there now opened the
+ducts of the segmental organs formed by recapitulation, those, therefore,
+belonging to the body-segments--mesonephric--having nothing to do with
+appendages, for the latter had already ceased to exist functionally, and
+would not, therefore, be repeated with each meristic repetition.
+
+This, so to speak, passive lengthening of the pronephric duct in
+consequence of the lengthening of the early vertebrate body by the addition
+of metameres, each of which contained only mesonephric and no pronephric
+tubules, is, to my mind, an example of a principle {405}which has played an
+important part in the formation of the vertebrate, viz. that the meristic
+variation by which the spinal region of even the lowest of existing
+vertebrates has been formed, has largely taken place in the vertebrate
+phylum itself, and that such changes must be eliminated before we can
+picture to ourselves the pre-vertebrate condition. As an example, I may
+mention the remarkable repetition of similar segments pictured by Bashford
+Dean in Bdellostoma. Such repetition leads to passive lengthening of such
+parts as are already formed but are not meristically repeated: such are the
+notochord, the vertebrate intestine, the canal of the spinal cord, and
+possibly the lateral line nerve. The fuller discussion of this point means
+the discussion of the formation of the vertebrate alimentary canal; I will
+therefore leave it until I come to that part of my subject, and only say
+here that the evidence seems to me to point to the conclusion that at the
+time when the vertebrate was formed, the respiratory and cloacal regions
+were very close together, the whole of the metasoma being represented by
+the region of the pronephros alone.
+
+Here, as always, the evidence of Ammocoetes tends to give definiteness to
+our conceptions, for Wheeler points out that up to a length of 7 mm. the
+pronephros only is formed; there is no sign of the more posteriorly formed
+mesonephros. Now we know, as pointed out in Chapter VI., p. 228, this is
+the time of Kupffer's larval stage of Ammocoetes. This is the period during
+which the invertebrate stage is indicated in the ontogeny, so that, in
+accordance with all that has gone before, this means that the metasoma of
+the invertebrate ancestor was confined to the region of the pronephros.
+
+Again, take Shipley's account of the development of Petromyzon. He says--
+
+"The alimentary canal behind the branchial region may be divided into three
+sections. Langerhans has termed these the stomach, midgut, and hindgut, but
+as the most anterior of these is the narrowest part of the whole intestine,
+it would, perhaps, be better to call it oesophagus. This part of the
+alimentary canal lies entirely in front of the yolk, and is, with the
+anterior region, which subsequently bears the gills, raised from the rest
+of the egg when the head is folded off. It is supported by a dorsal
+mesentery, on each side of which lies the head-kidney (pronephros)."
+
+Further on he says--
+
+{406}"The hindgut is smaller than the midgut; its anterior limit is marked
+by the termination of the spiral valve, which does not extend into this
+region. The two segmental ducts open into it just where it turns ventrally
+to open to the exterior by a median ventral anus. Its lumen is from an
+early stage lined with cells which have lost their yolk, and it is in wide
+communication with the exterior from the first. This condition seems to be,
+as Scott suggests, connected with the openings of the ducts of the
+pronephros, for this gland is completed and seems capable of functioning
+long before any food could find its way through the midgut, or, indeed,
+before the stomodæum has opened."
+
+Is there no significance in this statement of Shipley? Even if it be
+possible to find some special reason why the branchial and cloacal parts of
+the gut are freed from yolk and lined with serviceable epithelium a long
+time before the midgut, why should a bit of the midgut, which Shipley calls
+the oesophagus, which is connected with the region of the pronephros and
+not of the branchiæ, differ so markedly from the rest of the midgut? Surely
+the reason is that the branchial region of the gut, the pronephric region
+of the gut, and the cloacal region of the gut, belong to a different and
+earlier phase in the phylogenetic history of the Ammocoetes than does the
+midgut between the pronephric and cloacal regions. This observation of
+Shipley fits in with and emphasizes the view that the original animal from
+which the vertebrate arose consisted of a cephalic and branchial region,
+followed by a pronephric and cloacal region; the whole intermediate part of
+the gut, which forms the midgut, with its large lumen and spiral valve, and
+belongs to the mesonephric region, being a later formation brought about by
+the necessity of increasing the length of the body.
+
+
+THE ORIGIN OF THE SOMATIC TRUNK-MUSCULATURE AND THE FORMATION OF AN ATRIAL
+CAVITY.
+
+Next comes the question, why was the pronephros not repeated in the
+meristic repetition that took place during the early vertebrate stage?
+What, in fact, caused the disappearance of the metasomatic appendages, and
+the formation of the smooth body-surface of the fish?
+
+The embryological evidence given by van Wijhe and others of the manner in
+which the original superficially situated pronephros is {407}removed from
+the surface and caused to assume the deeper position, as seen in the later
+embryo, is perfectly clear and uniform in all the vertebrate groups. The
+diagrams at the end of van Wijhe's paper, which I reproduce here,
+illustrate the process which takes place. At first the myotome (Fig. 159,
+A) is confined to the dorsal region on each side of the spinal cord and
+notochord. Then (Fig. 159, B) it separates from the rest of the somite and
+commences to extend ventrally, thus covering over the pronephros and its
+duct, until finally (Fig. 159, C) it reaches the mid-ventral line on each
+side, and the foundations of the great somatic body-muscles are finally
+laid.
+
+In order, therefore, to understand how the obliteration of the appendages
+took place, we must first find out what is the past history of the
+myotomes. Why are they confined at first to the dorsal region of the body,
+and extend afterwards to the ventral region, forcing by their growth an
+organ that was originally external in situation to become internal?
+
+In the original discussion at Cambridge, I was accused of violating the
+important principle that in phylogeny we must look at the most elementary
+of the animals whose ancestors we seek, and was told that the lowest
+vertebrate was Amphioxus, not Ammocoetes; that therefore any argument as to
+the origin of vertebrates must proceed from the consideration of the former
+and not the latter animal. My reply was then, and is still, that I was
+considering the cranial region in the first place, and that therefore it
+was necessary to take the lowest vertebrate which possessed cranial nerves
+and sense-organs of a distinctly vertebrate character, a criterion
+evidently not possessed by Amphioxus. Such argument does not apply to the
+spinal region, so that, now that I have left the cranial region and am
+considering the spinal, I entirely agree with my critics that Amphioxus is
+likely to afford valuable help, and ought to be taken into consideration as
+well as Ammocoetes. The distinction between the value of the spinal
+(including respiratory) and cranial regions of Amphioxus for drawing
+phylogenetic conclusions is recognized by Boveri, who says that, in his
+opinion, "Amphioxus shows simplicity and undifferentiation rather than
+degeneration. If truly Amphioxus is somewhat degenerated, then it is so in
+its prehensile and masticatory apparatus, its sense organs, and perhaps its
+locomotor organs, owing to its method of living."
+
+{408}[Illustration: FIG. 159.--DIAGRAMS TO ILLUSTRATE THE DEVELOPMENT OF
+THE VERTEBRATE COELOM. (After VAN WIJHE.)
+
+_N._, central nervous system; _Nc._, notochord; _Ao._, aorta; _Mg._,
+midgut. A, _My._, myocoele; _Mes._, mesocoele; _Met._, metacoele; _Hyp._,
+hypomere (pronephric). B and C, _My._, myotome; _Mes._, mesonephros;
+_S.d._, segmental duct (pronephric); _Met._, body-cavity.]
+
+{409}Hatschek describes in Amphioxus how the coelom splits into a dorsal
+segmented portion, the protovertebra, and a ventral unsegmented portion,
+the lateral plates. He describes in the dorsal part the formation of
+myotome and sclerotome, as in the Craniota. Also, he describes how the
+myotome is at first confined to the dorsal region in the neighbourhood of
+the spinal cord and notochord, and subsequently extends ventrally, until,
+just as in Ammocoetes, the body is enveloped in a sheet of somatic
+segmented muscles, the well-known myomeres.
+
+The conclusion to be drawn from this is inevitable. Any explanation of the
+origin of the somatic muscles in Ammocoetes must also be an explanation of
+the somatic muscles in Amphioxus, and conversely; so that if in this
+respect Amphioxus is the more primitive and simpler, then the condition in
+Ammocoetes must be looked upon as derived from a more primitive condition,
+similar to that found in Amphioxus. Now, it is well known that a most
+important distinction exists between Amphioxus and Ammocoetes in the
+topographical relation of the ventral portion of this muscle-sheet, for in
+the former it is separated from the gut and the body-cavity by the atrial
+space, while in the latter there is no such space. Fürbringer therefore
+concludes, as I have already mentioned, that this space has become
+obliterated in the Craniota, but that it must be taken into consideration
+in any attempt at formulating the nature of the ancestors of the
+vertebrate.
+
+Kowalewsky described this atrial space as formed by the ventral downgrowth
+of pleural folds on each side of the body, which met in the mid-ventral
+line and enclosed the branchial portion of the gut. According to this
+explanation, the whole ventral portion of the somatic musculature of the
+adult Amphioxus belongs to the extension of the pleural folds, the original
+body-musculature being confined to the dorsal region. This is expressed
+roughly on the external surface of Amphioxus by the direction of the
+connective tissue septa between the myotomes (_cf._ Fig. 162, B). These
+septa, as is well known, bend at an angle, the apex of which points towards
+the head. The part dorsal to the bend represents the part of the muscle
+belonging to the original body; the part ventral to the bend is the pleural
+part, and represents the extension into the pleural folds.
+
+Lankester and Willey have attempted to give another explanation of the
+formation of the atrial cavity; they look upon it as originating from a
+ventral groove, which becomes a canal by the meeting of two {410}outgrowths
+from the metapleure on each side. This canal then extends dorsalwards on
+each side, and so forms the atrial cavity; the metapleure still remains in
+the adult; the somatic muscles in the epipleure of the adult are the
+original body-muscles, and not extensions into an epipleuric fold, for
+there is no such fold.
+
+This explanation is a possible conception for the post-branchial portion of
+the atrium, but is impossible for the branchial region; for, as Macbride
+points out, as must necessarily be the case, the point of origin of the
+atrial wall is, in all stages of development, situated at the end of the
+gill-slit. It shifts in position with the position of the gill-slit, but
+there can be no backwards extension of the cavity. Macbride therefore
+agrees with Kowalewsky that the atrial cavity is formed by the simultaneous
+ventral extension of pleural folds, and of the branchial part of the
+original pharynx. Thus, in his summing up, he states: "In the larva
+practically the whole sides and dorsal portion of the pharynx represent
+merely the hyper-pharyngeal groove and the adjacent epithelium of the
+pharynx of the adult, the whole of the branchial epithelium of the adult
+being represented by a very narrow strip of the ventral wall of the pharynx
+of the larva. The subsequent disproportionate growth of this part of the
+pharynx of the larva, and of the adjacent portion of the atrial cavity, has
+given the impression that the atrial cavity grew upwards and displaced
+other structures, which is not the case."
+
+Further, van Wijhe states that the atrium extends beyond the atriopore
+right up to the anus, just as must have been the case if the pleural folds
+originally existed along the whole length of the body. His words are:
+"Allerdings hat sich das Atrium beim _Amphioxus lanceolatus_ eigenthümlich
+ausgebildet, indem sich dasselbe durch den ganzen Rumpf bis an den Anus,
+d.h. bis an die Wurzel des Schwanzes ausdehnt."
+
+We get, therefore, this conception of the origin of the somatic musculature
+of the vertebrate. The invertebrate ancestor possessed on each side, along
+the whole length of its body, a lateral fold or pleuron which was segmented
+with the body, and capable of movement with the body, because the dorsal
+longitudinal somatic muscles extended segmentally into each segment of the
+pleuron. By the ventral extension of these pleural folds, not only was the
+smooth body-surface of the vertebrate attained, but also the original
+appendages obliterated as such, leaving only as signs of their existence
+the {411}branchiæ, the pronephric tubules, and the sense-organs of the
+lateral line system.
+
+Such an explanation signifies that the somatic trunk-musculature of the
+vertebrate was derived from the dorsal longitudinal musculature of the body
+of the arthropod, and not from the ventral longitudinal musculature, and
+that therefore in the primitive arthropod stage the equivalent of the
+myotome of the vertebrate did not give origin to the ventral longitudinal
+muscles of the invertebrate ancestor. Now, as I have said, von Kennel
+states that in the procoelom of Peripatus a dorsal part (III. in Fig. 157)
+is cut off which gives origin to the dorsal body-musculature, while the
+ventral part which remains (I. and II. in Fig. 157) gives origin in its
+appendicular portion (I.) to the muscles of the appendage, and presumably
+in its ventral somatic portion (II.) to the ventral longitudinal muscles of
+the body. This dorsal cut-off part might be called the myotome, in the same
+sense as the corresponding part of the procoelom in the vertebrate is
+called the myotome. In both cases the muscles derived from it form only a
+part of the voluntary musculature of the animal, and in both cases the
+muscles in question are the dorsal longitudinal muscles of the body, to
+which must be added the dorso-ventral body-muscles. Now, the whole of my
+theory of the origin of vertebrates arose from the investigation of the
+structure of the cranial nerves, which led to the conception that their
+grouping is not, like the spinal, a dual grouping of motor and sensory
+elements, but a dual grouping to supply two sets of segments, characterized
+especially by the different embryological origin of their musculature. The
+one set I called the somatic segmentation, because the muscles belonging to
+it were the great longitudinal body-muscles; the other I called the
+splanchnic segmentation, because its muscles were those connected with the
+branchial and visceral arches. According to my theory, this latter
+segmentation was due to the segmentation of the appendages in the
+invertebrate ancestor; and in previous chapters, dealing as they do with
+the cranial region, attention was especially directed to the way in which
+the position of the striated splanchnic musculature could be explained by a
+transformation of the prosomatic and mesosomatic appendages. Now, I am
+dealing with the metasomatic region, in which it is true the appendages
+take a very subordinate place, but still something corresponding to the
+splanchnic segments of the cranial region might fairly be expected to
+exist, and I therefore {412}desire to emphasize what appears to me to be
+the fact, that the musculature, which in the region of the trunk would
+correspond to that derived from the ventral segmentation of the mesoblast
+in the region of the head, may have arisen not only from the musculature of
+the appendages, but also from the ventral longitudinal musculature of the
+body of the invertebrate ancestor, for it seems probable that this latter
+musculature had nothing to do with the origin of the great longitudinal
+muscles of the vertebrate body, either dorsal or ventral.
+
+The way in which I imagine the obliteration of the atrial cavity to have
+taken place is indicated in Fig. 160, B, which is a modification of a
+section across a trilobite-like animal as represented in Fig. 160, A. As is
+seen, the pleural folds on each side have nearly met the bulged-out ventral
+body-surface. A continuation of the same process would give Fig. 160, C,
+which is, to all intents and purposes, the same as Fig. 159, C, taken from
+van Wijhe, and shows how the segmental duct is left in the remains of the
+atrial cavity. The lining walls of the atrial cavity are represented very
+black, in order to indicate the presence of pigment, as indeed is seen in
+the corresponding position in Ammocoetes. In these diagrams I have
+represented the median ventral surface as a large bulged-out bag, without
+indicating any structures in it except the ventral extension of the
+procoelom to form the metacoelom. At present I will leave the space between
+the central nervous system and the ventral mesentery blank, as in the
+diagrams; in my next chapter I will discuss the possible method of
+formation within this blank space of the notochord and midgut. Boveri
+considers that the obliteration of the atrial cavity in the higher
+vertebrates is not complete, but that its presence is still visible in the
+shape of the pronephric duct. The evidence of Maas and others that the duct
+is formed by the fusion of the pronephric tubules is, it seems to me,
+conclusive against Boveri's view; but yet, as may be seen from my
+diagrammatic figures, the very place where one would expect to find the
+last remnant of the atrial cavity is exactly where the pronephric duct is
+situated. For my own part I should expect to find evidence of a former
+existence of an atrial cavity rather in the pigment round the pronephros
+and its duct than in the duct itself.
+
+{413}[Illustration: FIG. 160.--A, DIAGRAM OF SECTION THROUGH A
+TRILOBITE-LIKE ANIMAL; B, DIAGRAM TO ILLUSTRATE SUGGESTED OBLITERATION OF
+APPENDAGES AND THE FORMATION OF AN ATRIAL CAVITY BY THE VENTRAL EXTENSION
+OF THE PLEURAL FOLDS; C, DIAGRAM TO ILLUSTRATE THE COMPLETION OF THE
+VERTEBRATE TYPE BY THE MEETING OF THE PLEURAL FOLDS IN THE MID-VENTRAL LINE
+AND THE OBLITERATION OF THE ATRIAL CAVITY.
+
+_Al._, alimentary canal; _N._, nervous system; _My._, myotome; _Pl._,
+pleuron; _App._, appendage; _Neph._, nephrocoele; _Met._, metacoele;
+_S.d._, segmental duct; _At._, atrial chamber; _V.Mes._, ventral mesentery;
+_Mes._, mesonephros. The dotted line represents the splanchnopleuric
+mesoblast in all figures.]
+
+{414}The conception that Amphioxus shows us how to account for the great
+envelope of somatic muscles which wraps round the vertebrate body, in that
+the ancestor of the vertebrate possessed on each side the body a segmented
+pleuron, is exactly in accordance with the theory of the origin of
+vertebrates deduced from the study of Ammocoetes, as already set forth in
+previous chapters. For we see that one of the striking characteristics of
+such forms as Bunodes, Hemiaspis, etc., is the presence of segmented
+pleural flaps on each side of the main part of the body; and if we pass
+further back to the great group of trilobites, we find in the most manifold
+form, and in various degrees of extent, the most markedly segmented pleural
+folds. In fact, the hypothetical figure (Fig. 160, A) which I have deduced
+from the embryological evidence, might very well represent a cross-section
+of a trilobite, provided only that each appendage of the trilobite
+possessed an excretory coxal gland.
+
+The earliest fishes, then, ought to have possessed segmented pleural folds,
+which were moved by somatic muscles, and enveloped the body after the
+fashion of Ammocoetes and Amphioxus, and I cannot help thinking that
+Cephalaspis shows, in this respect also, its relation to Ammocoetes. It is
+well known that some of the fossil representatives of the Cephalaspids show
+exceedingly clearly that these animals possessed a very well-segmented
+body, and it is equally recognized that this skeleton is a calcareous, not
+a bony skeleton, and does not represent vertebræ, etc. It is generally
+called an aponeurotic skeleton, meaning thereby that what is preserved
+represents not dermal plates alone, or a vertebrate skeleton, but the
+calcified septa or aponeuroses between a number of muscle-segments or
+myomeres, precisely of the same kind as the septa between the myomeres in
+Ammocoetes. The termination of such septa on the surface would give rise to
+the appearance of dermal plates or scutes, or the septa may even have been
+attached to something of the nature of dermal plates. The same kind of
+picture would be represented if these connective tissue dissepiments of
+Ammocoetes were calcified, and the animal then fossilized. In agreement
+with this interpretation of the spinal skeleton of Cephalaspis, it may be
+noted that again and again, in parts of these dissepiments, I have found in
+old specimens of Ammocoetes nodules of cartilage formed, and at
+transformation it is in this very tissue that the spinal cartilages are
+formed.
+
+{415}[Illustration: FIG. 161.--A, FACSIMILE OF WOODWARD'S DRAWING OF A
+SPECIMEN OF _Cephalaspis Murchisoni_, AS SEEN FROM THE SIDE. THE CEPHALIC
+SHIELD IS ON THE RIGHT AND CAUDAL TO IT THE PLEURAL FRINGES ARE WELL SHOWN;
+B, ANOTHER SPECIMEN OF _Cephalaspis Murchisoni_ TAKEN FROM THE SAME BLOCK
+OF STONE, SHOWING THE DERMOSEPTAL SKELETON AND IN ONE PLACE THE PLEURAL
+FRINGES, _bc._]
+
+Now, the specimens of Cephalaspis all show, as seen in Fig. 161, that the
+skeletal septa cover the body regularly, and then along one line are bent
+away from the body to form, as it were, a fringe, or rather a free pleuron,
+which has been easily pushed at an angle to the body-skeleton in the
+process of fossilization. Patten thinks that this fringed appearance is
+evidence of a number of segmental appendages which were jointed to the
+corresponding body-segments, and in the best specimen at the South
+Kensington Natural History Museum he thinks such joints are clearly
+visible. He concludes, therefore, that the cephalaspids were arthropods,
+and not vertebrates. I have also carefully examined this specimen, and do
+not consider that what is seen resembles the joint of an arthropod
+appendage; the appearance is rather such as would be produced if the line
+of attachment of Patten's appendages to the body were the place where the
+pleural body folds became free from the body, and so with any pressure a
+{416}bending or fracture of the calcified plates would take place along
+this line. There is, undoubtedly, an appearance of finish at the
+termination of these skeletal fringes, as though they terminated in a
+definitely shaped spear-like point, just as is seen in the trilobite
+pleuræ. This, again, to my mind, is rather evidence of pleural fringes than
+of true appendages.
+
+[Illustration: FIG. 162.--A, ARRANGEMENT OF SEPTA IN AMMOCOETES (_NC._,
+position of notochord); B, ARRANGEMENT OF SEPTA IN AMPHIOXUS.]
+
+As already argued, I look upon Ammocoetes as the only living fish at all
+resembling the cephalaspids; it is therefore instructive to compare the
+arrangement of this spinal dermo-septal skeleton of Cephalaspis with that
+of the septa between the myomeres in the trunk-region of Ammocoetes and
+Amphioxus. Such a skeleton in Ammocoetes would be represented by a series
+of plates overlapping each other, arranged as in Fig. 162, A, and in
+Amphioxus as in Fig. 162, B. I have lettered the corresponding parts of the
+two structures by similar letters, _a_, _b_, _c_. Ammocoetes differs in
+configuration from Amphioxus in that it possesses an extra dorsal (_a_,
+_d_) and an extra ventral bend. Ammocoetes is a much rounder animal than
+Amphioxus, and both the dorsal and ventral bends are on the extreme ventral
+and dorsal surfaces--surfaces which can hardly be said to exist in
+Amphioxus. The part, then, of such an aponeurotic skeleton {417}in
+Ammocoetes which I imagine corresponds to _b_, _c_ in Amphioxus, and
+therefore would represent the pleural fold, is the part ventral to the bend
+at _b_. In both the animals this bend corresponds to the position of the
+notochord NC.
+
+The skeleton of Cephalaspis compares more directly with that of Ammocoetes
+than that of Amphioxus, for there is the same extra dorsal bend (Fig. 161,
+_a_, _d_) as in Ammocoetes; the lateral part of the skeleton again gives an
+angle _a_, _b_, _c_; the part from _b_ to _c_ would therefore represent the
+pleural fold. I picture to myself the sequence of events somewhat as
+follows:--
+
+First, a protostracan ancestor, which, like Peripatus, possessed appendages
+on every segment into which coelomic diverticula passed, forming a system
+of coxal glands; such glands, being derived from the segmental organs of
+the Chætopoda, discharged originally to the exterior by separate openings
+on each segment. It is, however, possible, and I think probable, that a
+fusion of these separate ducts had already taken place in the protostracan
+stage, so that there was only one external opening for the whole of these
+metasomatic coxal glands, just as there is only one external opening for
+the corresponding prosomatic coxal glands of Limulus. Then, by the ventral
+growth of pleural body-folds, such appendages became enclosed and useless,
+and the coxal glands of the post-branchial segments, with their segmental
+or pronephric duct, were all that remained as evidence of such appendages.
+This dwindling of the metasomatic appendages was accompanied by the
+getting-rid of free appendages generally, in the manner already set forth,
+with the result that a smooth fish-like body-surface was formed; then the
+necessity of increasing mobility brought about elongation by the addition
+of segments between those last formed and the cloacal region. Each of such
+new-formed segments was appendageless, so that its segmental organ was not
+a coxal gland, but entirely somatic in position, and formed, therefore, a
+mesonephric tubule, not a pronephric one. Such glands could no longer
+excrete to the exterior, owing to the enclosing shell of the pleural folds;
+but the pronephric duct was there, already formed, and so these nephric
+tubules opened into that, instead of, as in the case of the branchial
+slits, forcing their way through the pleural walls when the atrium became
+closed.
+
+
+{418}THE MEANING OF THE DUCTLESS GLANDS.
+
+If it is a right conception that the excretory organs of the protostracan
+group, which gave origin to the vertebrates as well as to the crustaceans
+and arachnids, were of the nature of coxal glands, then it follows that
+such coxal glands must have existed originally on every segment, because
+they themselves were derived from the segmental organs of the annelids; it
+is therefore worth while making an attempt to trace the fate of such
+segmental organs in the vertebrate as well as in the crustacean and
+arachnid.
+
+Such an attempt is possible, it seems to me, because there exists
+throughout the animal kingdom striking evidence that excretory organs which
+no longer excrete to the exterior do not disappear, but still perform
+excretory functions of a different character. Their cells still take up
+effete or injurious substances, and instead of excreting to the exterior,
+excrete into the blood, forming either ductless glands of special
+character, or glands of the nature of lymphatic glands.
+
+The problem presented to us is as follows:--
+
+The excretory organs of both arthropods and vertebrates arose from those of
+annelids, and were therefore originally present in every segment of the
+body. In most arthropods and vertebrates they are present only in certain
+regions; in the former case, as the coxal glands of the prosomatic or
+head-region; in the latter, as the nephric glands of the metasomatic or
+trunk-region, and, in the case of Amphioxus, of the mesosomatic or
+branchial region.
+
+In the original arthropod, judging from Peripatus, they were present, as in
+the annelid, in all the segments of the body, and formed coxal glands.
+Therefore, in the ancestors of the living Crustacea and Arachnida, coxal
+glands must have existed in all the segments of the body, and we ought to
+be able to find the vestiges of them in the mesosomatic or branchial and
+metasomatic or abdominal regions of the body.
+
+Similarly, in the vertebrates, derived, as has been shown, not from the
+annelids, but from an arthropod stock, evidence of the previous existence
+of coxal glands ought to be manifested in the prosomatic or trigeminal
+region, in the mesosomatic or branchial region, as well as in the
+metasomatic or post-branchial region.
+
+How does an excretory organ change its character when it ceases {419}to
+excrete to the exterior? What should we look for in our search after the
+lost coxal glands?
+
+The answer to these questions is most plainly given in the case of the
+pronephros, especially in Myxine, where Maas has been able to follow out
+the whole process of the conversion of nephric tubules into a tissue
+resembling that of a lymph-gland.
+
+He states, in the first place, that the pronephros possesses a capillary
+network, which extends over the pronephric duct, while the tubules of the
+mesonephros possess not only this capillary network, equivalent to the
+capillaries over the convoluted tubules in the higher vertebrates, but also
+a true glomerulus, in that the nephric segmental arteriole forms a coil
+(Knauel), and pushes in the wall of the mesonephric tubule. He describes
+the pronephros of large adult individuals as consisting of--
+
+1.  Tubules with funnels which open into the pericardial coelom.
+
+2.  A large capillary network (the glomus) at the distal end.
+
+3.  A peculiar tissue (the 'strittige Gewebe' of the Semon-Spengel
+controversy), which Spengel considers to be composed of the altered
+epithelium of pronephric tubules, while Semon looks on it as an
+amalgamation of glomeruli.
+
+Maas is entirely on the side of Spengel, and shows that this peculiar
+tissue is actually formed by modified pronephric tubules, which become more
+and more lymphatic in character.
+
+He says: "The pronephros consists of a number of nephric tubules, placed
+separately one behind the other, which were originally segmental in
+character, each one of which is supplied by a capillary network from a
+segmental branch of the aorta. The tubules begin with many mouths
+(dorso-lateral and medial-ventral) in the pericardial cavity; on their
+other blind end they have lost their original external opening, and there,
+in the cranial portion of the head-kidney, before they have joined together
+to form a collecting duct, they, together with the vascular network, are
+transformed into a peculiar adrenal-like tissue. The most posterior of the
+segmental capillary nets retain their original character, and are
+concentrated into the separate capillary mass known as the glomus."
+
+Later on he says: "Further, the separate head-kidney is more and more
+removed in structure from an excretory organ in the ordinary sense. One
+cannot, however, speak of it as an organ becoming rudimentary; this is
+proved not only by the progressive transformation {420}of its internal
+tissue into a tissue of a very definite character, but also by the cilia in
+its canals, and the steady increase in the number of its funnels. It
+appears, therefore, to be the conversion of an excretory organ into an
+organ for the transference of fluid out of the coelom into a special
+tissue, _i.e._ into its blood-sinus; in other words, into an organ which
+must be classed as belonging to the lymph-system."
+
+In exact correspondence with this transformation of a nephric tubule into a
+ductless gland of the nature of a lymphatic gland, is the formation of the
+head-kidney in the Teleostea. Thus, Weldon points out that, though the
+observations of Balfour left it highly probable that the "lymphatic" tissue
+described by him was really a result of the transformation of part of the
+embryonic kidney, he did not investigate the details of its development.
+This was afterwards done by Emery, with the following results: "In those
+Teleostea which he has studied, Professor Emery finds that at an early
+stage the kidney consists entirely of a single pronephric funnel, opening
+into the pericardium, and connected with the segmental duct, which already
+opens to the exterior. Behind this funnel, the segmental duct is surrounded
+by a blastema, derived from the intermediate cell-mass, which afterwards
+arranges itself more or less completely into a series of solid cords,
+attaching themselves to the duct. These develop a lumen, and become normal
+segmental tubules, but it is, if I may be allowed the expression, a matter
+of chance how much of the blastema becomes so transformed into kidney
+tubules, and how much is left as the 'lymphatic' tissue of Balfour, this
+'lymphatic' tissue remaining either in the pronephros only, or in both pro-
+and meso-nephros."
+
+If we turn now to the invertebrates, we see also how close a connection
+exists between lymphatic and phagocytic organs and excretory organs. The
+chief merit for this discovery is due to Kowalewsky, who, taking a hint
+from Heidenhain's work on the kidney, in which he showed how easy it was to
+find out the nature of different parts of the mammalian excretory organ by
+the injection of different substances, such as a solution of ammoniated
+carmine, or of indigo-carmine, has injected into a large number of
+different invertebrates various colouring matters, or litmus, or bacilli,
+and thus shown the existence, not only of known excretory organs, but also
+of others, lymphatic or lymphoid in nature, not hitherto suspected.
+
+In all cases he finds that a phagocytic action with respect to solid
+{421}bodies is a property of the leucocytes, and that these leucocytes
+which are found in the coelomic spaces of the Annelida, etc., are
+apparently derived from the epithelium of such spaces. Also by the
+proliferation of such epithelium in places, _e.g._ the septal glands of the
+terrestrial Oligochæta, segmental glandular masses of such tissue are
+formed which take up the colouring matter, or the bacilli. In the
+limicolous Oligochæta such septal glands are not found, but at the
+commencement of the nephridial organ, immediately following upon the
+funnel, a remarkable modification of the nephridial wall takes place to
+form a large cellular cavernous mass, the so-called filter, which in Euaxes
+is full of leucocytes; the cells are only definable by their nuclei, and
+look like and act in the same way as the free leucocytes outside this
+nephridial appendage. As G. Schneider points out, the whole arrangement is
+very like that described by Kowalewsky in the leeches Clepsine and
+Nephelis, where, also immediately succeeding the funnel of the nephridial
+organ, a large accessory organ is found, which is part of the nephridium,
+and is called the nephridial capsule. This is the organ _par excellence_
+which takes up the solid carmine-grains and bacilli, and apparently, from
+Kowalewsky's description, contains leucocytes in large numbers. We see,
+then, that in such invertebrates, just as in the vertebrate, modifications
+of the true excretory organ may give rise to phagocytic glands of the
+nature of lymphatic glands. Further, these researches of Kowalewsky suggest
+in the very strongest manner that whenever by such means new, hitherto
+unsuspected glands are discovered, such glands must belong to the excretory
+system, _i.e._ must be derived from coelomic epithelium, even when all
+evidence of any coelom has disappeared. Kowalewsky himself was evidently so
+impressed with the same feeling that he heads one of his papers "The
+Excretory Organs of the Pantopoda," although the organs in question had
+been discovered by him by this method, and appeared as ductless glands with
+no external opening.
+
+To my mind these observations of Kowalewsky are of exceeding interest, for
+it is immediately clear that if the segmental organs of the annelids, which
+must have existed on all the segments of the forefathers of the Crustacea
+and Arachnida (the Protostraca), have left any sign of their existence in
+living crustaceans and arachnids, then such indication would most likely
+take the form of lymphatic glands in the places where the excretory organs
+ought to have been.
+
+Now, as already pointed out in Peripatus, such segmental organs {422}were
+formed by the ventral part of the coelom, and dipped originally into each
+appendage. We know also that each segment of an arachnid embryo possesses a
+coelomic cavity in its ventral part which extends into the appendage on
+each side; this cavity afterwards disappears, and is said to leave no trace
+in the adult of any excretory coxal gland derived from its walls. If,
+however, it is found that in the very position where such organ ought to
+have been formed a segmentally arranged ductless gland is situated, the
+existence of which is shown by its taking up carmine, etc., then it seems
+to me that in all probability such gland is the modification of the
+original coxal gland.
+
+This is what Kowalewsky has done. Thus he states that Metschnikoff had fed
+Mysis with carmine-grains, and found tubules at the base of the thoracic
+feet coloured red with carmine. He himself used an allied species,
+_Parapodopsis cornutum_, and found here also that the carmine was taken up
+by tubules situated in the basal segments of the feet. In Nebalia, feeding
+experiments with alizarin blue and carmine stained the antennal glands, and
+showed the existence of glands at the base of the eight thoracic feet.
+These glands resemble the foot-glands of Mysis, Parapodopsis, and Palæmon,
+and lie in the space through which the blood passes from the thoracic feet,
+_i.e._ from the gills, to the heart. In Squilla also, in addition to the
+shell-glands, special glands were discovered on the branchial feet on the
+path of the blood to the heart. These glands form continuous masses of
+cells which constitute large compact glands at the base of the branchial
+feet. Single cells of the same sort are found along the whole course of the
+branchial venous canal, right up to the pericardium.
+
+These observations show that the Crustacea possess not only true excretory
+organs in the shape of coxal glands, _i.e._ antennary glands, shell-glands,
+etc., in the cephalic region, but also a series of segmental glands
+situated at the base of the appendages, especially of the respiratory
+appendages: a system, that is to say, of coxal glands which have lost their
+excretory function, through having lost their external opening, but have
+not in consequence disappeared, but still remain _in situ_, and still
+retain an important excretory function, having become lymphatic glands
+containing leucocytes. Such glands are especially found in the branchial
+appendages, and are called branchial glands by Cuénot, who describes them
+for all Decapoda.
+
+Further, it is significant that the same method reveals the {423}existence
+in Pantopoda of a double set of glands of similar character, one set in the
+basal segments of the appendage, and the other in the adjacent part of the
+body.
+
+In scorpions also, Kowalewsky has shown that the remarkable lymphatic organ
+situated along the whole length of the nerve-cord in the abdominal region
+takes up carmine grains and bacilli; an organ which in Androctonus does not
+form one continuous gland, but a number of separate, apparently irregularly
+grouped, glandular bodies.
+
+In addition to this median lymphatic gland, Kowalewsky has discovered in
+the scorpion a pair of lateral glands, to which he gives the name of
+lymphoid glands, which communicate with the thoracic body-cavity (_i.e._
+the pseudocoele), are phagocytic, and, according to him, give origin to
+leucocytes by the proliferation of their lining cells, thus, as he remarks,
+reminding us of the nephridial capsules of Clepsine. These glands are so
+closely related in position to the coxal glands on each side that he has
+often thought that the lumen of the gland communicated with that of the
+coxal gland; he, however, has persuaded himself that there is no true
+communication between the two glands. Neither of these organs appears to be
+segmental, and until we know how they are developed it is not possible to
+say whether they represent fused segmental organs or not.
+
+The evidence, then, is very strong that in the Crustacea and Arachnida the
+original segmental excretory organs do not disappear, but remain as
+ductless glands, of the nature of lymphatic glands, which supply leucocytes
+to the system.
+
+Further, the evidence shows that the nephric organs, or parts of the coelom
+in close connection with these organs, may be transformed into ductless
+glands, which do not necessarily contain free leucocytes as do
+lymph-glands, but yet are of such great importance as excretory organs that
+their removal profoundly modifies the condition of the animal. Such a gland
+is the so-called adrenal or suprarenal body, disease of which is a feature
+of Addison's disease; a gland which forms and presumably passes into the
+blood a substance of remarkable power in causing contraction of
+blood-vessels, a substance which has lately been prepared in crystalline
+form by Jokichi Takamine, and called by him "adrenalin"; a gland,
+therefore, of very distinctly peculiar properties, which cannot be regarded
+as rudimentary, but is of vital importance for the due maintenance of the
+healthy state.
+
+In the Elasmobranchs two separate glandular organs have been {424}called
+suprarenal; a segmental series of paired organs, each of which possesses a
+branch from the aorta and a sympathetic ganglion, and an unpaired series in
+close connection with the kidneys, to which Balfour gave the name of
+interrenal glands. Of these two sets of glands, Swale Vincent has shown
+that the extract of the interrenals has no marked physiological effect, in
+this respect resembling the extract of the cortical part of the mammalian
+gland, while the extract of the paired segmental organs of the Elasmobranch
+produces the same remarkable rise of blood-pressure as the extract of the
+medullary portion of the mammalian gland.
+
+The development also of these two sets of glands is asserted to be
+different. Balfour considered that the suprarenals were derived from
+sympathetic ganglion-cells, but left the origin of the interrenals
+doubtful. Weldon showed that the cortical part of the suprarenals in the
+lizard was derived from the wall of the glomerulus of a number of
+mesonephric tubules. In Pristiurus, he stated that the mesoblastic rudiment
+described by Balfour as giving origin to the interrenals is derived from a
+diverticulum of each segmental tubule, close to the narrowing of its
+funnel-shaped opening into the body-cavity. With respect to the paired
+suprarenals he was unable to speak positively, but doubted whether they
+were derived entirely from sympathetic ganglia.
+
+Weldon sums up the results of his observations by saying: "That all
+vertebrates except Amphioxus have a portion of the kidney modified for some
+unknown purpose not connected with excretion; that in Cyclostomes the
+pronephros alone is so modified, in Teleostei the pro- and part of the
+meso-nephros; while in the Elasmobranchs and the higher vertebrates the
+mesonephros alone gives rise to this organ, which has also in these forms
+acquired a secondary connection with certain of the sympathetic ganglia."
+
+Since Weldon's paper, a large amount of literature on the origin of the
+adrenals has appeared, a summary of which, up to 1891, is given by Hans
+Rabl in his paper, and a further summary by Aichel in his paper published
+in 1900. The result of the investigations up to this latter paper may be
+summed up by saying that the adrenals, using this term to include all these
+organs of whatever kind, are in all cases, partly at all events, derived
+from some part of the walls of either the mesonephric or pronephric
+excretory organs, but that in addition a separate origin from the
+sympathetic nervous system must {425}be ascribed to the medullary part of
+the organ and to the separate paired organs in the Elasmobranchs, which are
+equivalent to the medullary part in other cases.
+
+The evidence, then, of the transformation of the known vertebrate excretory
+organs--the pronephros and the mesonephros--leads to the conclusion that in
+our search for the missing coxal glands of the meso- and pro-somatic
+regions, we must look for either lymphatic glands, or ductless glands of
+distinct importance to the body. I have already considered the question in
+the prosomatic region, and have given my reasons why the pituitary gland
+must be looked upon as the descendant of the arthropod coxal gland. In this
+case also the resulting ductless gland is still of functional importance,
+for disease of it is associated with acromegaly. If, as is possible, it is
+homologous with the Ascidian hypophysial gland, then it is confirmatory
+evidence that this latter is said by Julin to be an altered nephridial
+organ.
+
+Finally, I come to the mesosomatic or branchial region; and here,
+strikingly enough, we find a perfectly segmental glandular organ of
+mysterious origin--the thymus gland--segmental with the branchiæ, not
+necessarily with the myotomes, belonging, therefore, to the appendicular
+system; and since the branchiæ represent, according to my theory, the basal
+part of the appendage, such segmental glands would be in the position of
+coxal glands. Here, then, in the thymus may be the missing mesosomatic
+coxal glands.
+
+What, then, is the thymus?
+
+The answer to this question has been given recently by Beard, who strongly
+confirms Kölliker's original view that the thymus is a gland for the
+manufacture of leucocytes, and that such leucocytes are directly derived
+from the epithelial cells of the thymus. Kölliker also further pointed out
+that the blood of the embryo is for a certain period destitute of
+leucocytes. Beard confirms this last statement, and says that up to a
+certain stage (varying from 10 to 16 mm. in length of the embryo) the
+embryos of _Raja batis_ have no leucocytes in the blood or elsewhere. Up to
+this period the thymus-placode is well formed, and the first leucocytes can
+be seen to be formed in it from its epithelial cells; then such formation
+takes place with great rapidity, and soon an enormous discharge of
+leucocytes occurs from the thymus into the tissue-spaces and blood. He
+therefore concludes that all lymphoid tissues in the body arise originally
+from the thymus gland, _i.e._ from leucocytes discharged from the thymus.
+
+{426}The segmental branchial glands, known by the name of thymus, are,
+according to this view, the original lymphatic glands of the vertebrate;
+and it is to be noted that, in fishes and in Amphibia, lymphatic glands,
+such as we know them in the higher mammals, do not exist; they are
+characteristic of the higher stages of vertebrate evolution. In the lower
+vertebrates, the only glandular masses apart from the cell-lining of the
+body-cavity itself, which give rise to leucocyte-forming tissue, are these
+segmental branchial glands, or possibly also the modified post-branchial
+segmental glands, known as the head-kidney in Teleostea, etc.
+
+The importance ascribed by Beard to the thymus in the formation of
+leucocytes in the lowest vertebrates would be considerably reduced in value
+if the branchial region of Ammocoetes possessed neither thymus glands nor
+anything equivalent to them. Such, however, is not the case. Schaffer has
+shown that in the young Ammocoetes masses of lymphatic glandular tissue are
+found segmentally arranged in the neighbourhood of each gill-slit--tissue
+which soon becomes converted into a swarming mass of leucocytes, and shows
+by its staining, etc., how different it is from a blood-space. The presence
+of this thymus leucocyte-forming tissue, as described by Schaffer, is
+confirmed by Beard, and I myself have seen the same thing in my youngest
+specimen of Ammocoetes.
+
+Further, the very methods by which Kowalewsky has brought to light the
+segmental lymph-glands of the branchial region of the Crustacea, etc., are
+the same as those by which Weiss discovered the branchial nephric glands in
+Amphioxus--excretory organs which Boveri considers to represent the
+pronephros of the Craniota. In this supposition Boveri is right, in so far
+that both pronephros and the tubules in Amphioxus belong to the same system
+of excretory organs; but I entirely agree with van Wijhe that the region in
+Amphioxus is wrong. The tubules in Amphioxus ought to be represented in the
+branchial region of the Craniota, not in the post-branchial region; van
+Wijhe therefore suggests that further researches may homologize them with
+the thymus gland in the Craniota, not with the pronephros. This suggestion
+of van Wijhe appears to me a remarkably good one, especially in view of the
+position of the thymus glands in Ammocoetes and the nephric branchial
+glands in Amphioxus. If, as I have pointed out, the atrial cavity of
+Amphioxus has been closed in Ammocoetes by the apposition of {427}the
+pleural fold with the branchial body-surface, then the remains of the
+position of the atrial chamber must exist in Ammocoetes as that
+extraordinary space between the somatic muscles and the branchial
+basket-work filled with blood-spaces and modified muco-cartilage. It is in
+this very space, close against the gill-slits, that the thymus glands of
+Ammocoetes are found, in the very place where the nephric tubules of
+Amphioxus would be found if its atrial cavity were closed completely.
+Instead, therefore, of considering with Boveri that the branchial nephric
+tubules of Amphioxus still exist in the Craniota as the pronephros, and
+that the atrial chamber has narrowed down to the pronephric duct, I would
+agree with van Wijhe that the pronephros is post-branchial, and suggest
+that by the complete closure of the atrial space in the branchial region
+the branchial nephric tubules have lost all external opening, and
+consequently, as in all other cases, have changed into lymphatic tissue and
+become the segmental thymus glands.
+
+As van Wijhe himself remarks, the time is hardly ripe for making any
+positive statement about the relationship between the thymus gland and
+branchial excretory organs. There is at present not sufficient consensus of
+opinion to enable us to speak with any certainty on the subject, yet there
+is so much suggestiveness in the various statements of different authors as
+to make it worth while to consider the question briefly.
+
+On the one hand, thymus, tonsils, parathyroids, epithelial cell-nests, and
+parathymus, are all stated to be derivatives of the epithelium lining the
+gill-slits, and Maurer would draw a distinction between the organs derived
+from the dorsal side of the gill-cleft and those derived from the ventral
+side--the former being thymus, the latter forming the epithelial
+cell-nests, _i.e._ parathyroids. The thymus in Ammocoetes, according to
+Schaffer, lies both ventral and dorsal to the gill-cleft; Maurer thinks
+that only the dorsal part corresponds to the thymus, the ventral part
+corresponding to the parathyroids, etc. Structurally, the thymus,
+parathyroids, and the epithelial cell-nests are remarkably similar, so that
+the evidence appears to point to the conclusion that, in the neighbourhood
+of the gill-slits, segmentally arranged organs of a lymphatic character are
+situated, which give origin to the thymus, parathyroids, tonsils, etc. Now,
+among these organs, _i.e._ among those ventrally situated, Maurer places
+the carotid gland, so that, if he is right, the origin of the carotid gland
+{428}might be expected to help in the elucidation of the origin of the
+thymus.
+
+The origin of the carotid gland has been investigated recently by Kohn, who
+finds that it is associated with the sympathetic nervous system in the same
+way as the suprarenals. He desires, in fact, to make a separate category
+for such nerve-glands, or paraganglia, as he calls them, and considers them
+all to be derivatives of the sympathetic nervous system, and to have
+nothing to do with excretory organs. The carotid gland is, according to
+him, the foremost of the suprarenal masses in the Elasmobranchs, viz. the
+so-called axillary heart.
+
+In my opinion, nests of sympathetic ganglion-cells necessarily mean the
+supply of efferent fibres to some organ, for all such ganglia are efferent,
+and also, if they are found in the organ, would have been brought into it
+by way of the blood-vessels supplying the organ, so that Aichel's statement
+of the origin of the suprarenals in the Elasmobranchs seems to me much more
+probable than a derivation from nerve-cells. If, then, it prove that Aichel
+is right as to the origin of the suprarenals, and Kohn is right in
+classifying the carotid gland with the suprarenals, then Maurer's
+statements would bring the parathyroids, thymus, etc., into line with the
+adrenals, and suggest that they represent the segmented glandular excretory
+organs of the branchial region, into which, just as in the interrenals of
+Elasmobranchs, or the cortical part of the adrenals of the higher
+vertebrates, there has been no invasion of sympathetic ganglion-cells.
+
+Wheeler makes a most suggestive remark in his paper on Petromyzon: he
+thinks he has obtained evidence of serial homologues of the pronephric
+tubules in the branchial region of Ammocoetes, but has not been able up to
+the present to follow them out. If what he thinks to be serial homologues
+of the pronephric tubules in the branchial region should prove to be the
+origin of the thymus glands of Schaffer, then van Wijhe's suggestion that
+the thymus represents the excretory organs of the branchial region would
+gain enormously in probability. Until some such further investigation has
+been undertaken, I can only say that it seems to me most likely that the
+thymus, etc., represent the lymphatic branchial glands of the Crustacea,
+and therefore represent the missing coxal glands of the branchial region.
+
+This, however, is not all, for the appendages of the mesosomatic region, as
+I have shown, do not all bear branchiæ; the foremost or {429}opercular
+appendage carries the thyroid gland. Again, the basal part of the appendage
+is all that is left; the thyroid gland is in position a coxal gland. It
+ought, therefore, to represent the coxal gland of this appendage, just as
+the thymus, tonsils, etc., represent the coxal glands of the rest of the
+mesosomatic appendages. In the thyroid gland we again see a ductless gland
+of immense importance to the economy, not a useless organ, but one, like
+the other modified coxal glands, whose removal involves far-reaching vital
+consequences. Such a gland, on my theory, was in the arthropod a part of
+the external genital ducts which opened on the basal joint of the
+operculum. What, then, is the opinion of morphologists as to the meaning of
+these external genital ducts?
+
+In a note to Gulland's paper on the coxal glands of Limulus, Lankester
+states that the conversion of an externally-opening tubular gland (coxal
+gland) into a ductless gland is the same kind of thing as the history of
+the development of the suprarenal from a modified portion of mesonephros,
+as given by Weldon. Further, that in other arthropods with glands of a
+tubular character opening to the exterior at the base of the appendages, we
+also have coxal nephridia, such as the shell-glands of the Entomostraca,
+green glands of Crustacea (antennary coxal gland); and further on he
+writes: "When once the notion is admitted that ducts opening at the base of
+limbs in the Arthropoda are possibly and even probably modified nephridia,
+we immediately conceive the hypothesis that the genital ducts of the
+Arthropoda are modified nephridia."
+
+So, also, Korschelt and Heider, in their general summing up on the
+Arthropoda, say: "In Peripatus, where the nephridia appear, as in the
+Annelida, in all the trunk-segments, a considerable portion of the
+primitive segments is directly utilized for the formation of the nephridia.
+In the other groups, the whole question of the rise of the organs known as
+nephridia is still undecided, but it may be mentioned as very probable that
+the salivary and anal glands of Peripatus, the antennal and shell-glands of
+the Crustacea, the coxal glands of Limulus and the Arachnida, as well as
+the efferent genital ducts, are derived from nephridia, and in any case are
+mesodermal in origin."
+
+The necessary corollary to this exceedingly probable argument is that
+glandular structures such as the uterine glands of the scorpion already
+described, which are found in connection with these terminal {430}genital
+ducts, may be classed as modified nephridial glands, and that therefore the
+thyroid gland of Ammocoetes, which, on the theory of this book, arose in
+connection with the opercular genital ducts of the palæostracan ancestor,
+represents the coxal glands of this fused pair of appendages. Such a gland,
+although its function in connection with the genital organs had long
+disappeared, still, in virtue of its original excretory function,
+persisted, and even in the higher vertebrates, after it had lost all
+semblance of its former structure and become a ductless gland of an
+apparently rudimentary nature, still, by its excretory function,
+demonstrates its vital importance even to the highest vertebrate.
+
+By this simple explanation we see how these hitherto mysterious ductless
+glands, pituitary, thymus, tonsils, thyroid, are all accounted for, are all
+members of a common stock--coxal glands--which originally, as in Peripatus,
+excreted at the base of the prosomatic and mesosomatic appendages, and are
+still retained because of the importance of their excretory function,
+although ductless owing to the modification of their original appendages.
+
+Finally, there is yet another organ in the vertebrate which follows the
+same law of the conversion of an excretory organ into a lymphatic organ
+when its connection with the exterior is obliterated, and that is the
+vertebrate body-cavity itself. According to the scheme here put forth, the
+body-cavity of the vertebrate arose by the fusion of a ventral prolongation
+of the original nephrocoele on each side; prolongations which accompanied
+the formation of the new ventral midgut, and by their fusion formed
+originally a pair of cavities along the whole length of the abdomen, being
+separated from each other by the ventral mesentery of the gut.
+Subsequently, by the ventral fusion of these two cavities, the body-cavity
+of the adult vertebrate was formed.
+
+This is simply a statement of the known method of formation of the
+body-cavity in the embryo, and its phylogenetic explanation is that the
+body-cavity of the vertebrate must be looked upon as a ventral prolongation
+of the original ancestral body-cavity. Embryology clearly teaches that the
+original body-cavity or somite was confined to the region of the notochord
+and central nervous system, and there, just as in Peripatus, was divisible
+into a dorsal part, giving origin to the myocoele, and a ventral part,
+forming the nephrocoele. From this original nephrocoele are formed the
+pronephric excretory organs, the mesonephric excretory organs, and the
+body-cavity.
+
+{431}That the vertebrate body-cavity was originally a nephrocoele is
+generally accepted, and its excretory function is shown by the fact that it
+communicates with the exterior in all the lower vertebrates, either through
+abdominal pores or by way of nephridial funnels. Bles has shown how largely
+these two methods of communicating with the exterior mutually exclude each
+other. In the higher vertebrates both channels become closed, except in the
+case of the Fallopian tubes, and thus, so to speak, the body-cavity becomes
+a ductless gland, still, however, with an excretory function, but now, as
+in all other cases, forming a part of the lymphatic rather than of the true
+excretory system.
+
+
+SUMMARY.
+
+  The consideration of the formation of the vertebrate cranial region, as
+  set forth in previous chapters, indicates that the ancestor of the
+  vertebrates was not an arachnid purely or a crustacean purely, but
+  possessed partly crustacean and partly arachnid characters. In order to
+  express this conclusion, I have used the term Protostraca, invented by
+  Korschelt and Heider, to indicate a primitive arthropod group, from which
+  both arachnids and crustaceans may be supposed to have arisen, and have
+  therefore stated that the vertebrate did not arise directly from the
+  annelids, but from the Protostraca. Such an origin signifies that the
+  origin of the excretory organs of the vertebrate must not be looked for
+  in the segmental organs of the annelid, but rather in such modified
+  annelid organs as would naturally exist in a primitive arthropod group.
+  The nature of such organs may be inferred, owing to the fortunate
+  circumstance that so primitive an arthropod as Peripatus still exists,
+  and we may conclude that the protostracan ancestor possessed in every
+  segment a pair of appendages and a pair of coelomic cavities, which
+  extended into the base of these appendages. The ventral portion of each
+  of these coelomic cavities separated off from the dorsal and formed a
+  nephrocoele, giving origin to a segmental excretory organ, which, seeing
+  that its end-vesicle was in the base of the appendage, and seeing also
+  the nature of the known arachnid and crustacean excretory organs, may
+  fitly be termed a coxal gland. This, then, is the working hypothesis to
+  explain the difficulties connected with the origin of the pronephros and
+  mesonephros--that the original segmental organs were coxal glands, and
+  therefore indicated the presence of appendages. This hypothesis leads to
+  the following conclusions:--
+
+  1. The coxal glands belonging to the post-branchial appendages of the
+  invertebrate ancestor are represented by the pronephric tubules, and
+  existed over the whole metasomatic region.
+
+  2. Such glands discharged into a common duct--the pronephric duct--which
+  opened into the cloacal region, either in the protostracan stage, when
+  the metasomatic appendages were still in existence, just as the coxal
+  glands of the prosomatic region in Limulus discharge into a common duct,
+  or else the pronephric duct was formed when the appendages were
+  obliterated.
+
+  {432}3. The metasomatic appendages disappeared owing to their enclosure
+  by pleural folds, which, meeting in the mid-ventral line, not only caused
+  the obliteration of the appendages, and gave a smooth fish-like
+  body-surface to the animal, but also caused the formation of an atrial
+  cavity.
+
+  4. Into these pleural folds the dorsal longitudinal muscles of the body
+  extended, and ultimately reached to the ventral surface, thus forming the
+  somatic muscles of the vertebrate body.
+
+  5. When the pleural folds had met in the mid-ventral line the animal had
+  become a vertebrate, and was dependent for its locomotion on the
+  movements of these somatic muscles, and not on the movements of
+  appendages. Consequently, elongation of the trunk-region took place, for
+  the purpose of increasing mobility, by the formation of new metameres.
+
+  6. Each of such metameres possessed its own segmental excretory organ,
+  formed in the same way as the previous pronephric organs, but, as there
+  were no appendages in these new-formed segments, the excretory organs
+  took on the characters of a mesonephros, not a pronephros, and opened
+  into the pronephric duct, because the direct way to the exterior was
+  blocked by the enveloping pleural folds.
+
+  7. The group of annelids from which the protostracan ancestor of the
+  vertebrates arose was the highest annelidan group, viz. the Polychæta, as
+  shown by the nature of the excretory organs in Amphioxus.
+
+  8. The coxal glands of the protostracan ancestor existed on all the
+  segments, and were, therefore, divisible into three groups, prosomatic,
+  mesosomatic, and metasomatic; these three groups of coxal glands still
+  exist in the vertebrate as ductless glands.
+
+  9. The prosomatic coxal glands form the pituitary body.
+
+  10. The mesosomatic coxal glands form the thymus, thyroid, parathyroids,
+  tonsils, etc.
+
+  11. The metasomatic coxal glands form the adrenals.
+
+  12. The procoelom of the vertebrate is the procoelom of the protostracan
+  ancestor, which splits into a dorsal part, the myocoele, and a ventral
+  part, the nephrocoele. This latter part not only forms the pronephros and
+  mesonephros, but also by a ventral extension gives origin to the walls of
+  the vertebrate body-cavity or metacoele.
+
+  13. This ventral extension of the original nephrocoele at first excreted
+  to the exterior, through abdominal pores, or through peritoneal funnels.
+  When such paths to the exterior became closed, it also became a ductless
+  gland, belonging to the lymphatic system.
+
+
+
+
+{433}CHAPTER XIII
+
+_THE NOTOCHORD AND ALIMENTARY CANAL_
+
+  Relationship between notochord and gut.--Position of unsegmented tube of
+  notochord.--Origin of notochord from a median groove.--Its function as an
+  accessory digestive tube.--Formation of notochordal tissue in
+  invertebrates from closed portions of the digestive tube.--Digestive
+  power of the skin of Ammocoetes.--Formation of new gut in Ammocoetes at
+  transformation.--Innervation of the vertebrate gut.--The three outflows
+  of efferent nerves belonging to the organic system.--The original close
+  contiguity of the respiratory chamber to the cloaca.--The elongation of
+  the gut.--Conclusion.
+
+
+In the previous chapters all the important organs of the arthropod have
+been found in the vertebrate in their appropriate place, of similar
+structure, and innervated from corresponding parts of the central nervous
+system. Such comparison is possible only as long as the ventral and dorsal
+surfaces of the vertebrate correspond with the respective surfaces of the
+arthropod, and no reversal is assumed. This method of comparative anatomy
+is the surest and most certain guide to the relationship between two
+animals, and when the facts obtained by the anatomical method are so
+strikingly confirmatory of the palæontological evidence, the combined
+evidence becomes so strong as to amount almost to a certainty that
+vertebrates did arise from arthropods in the manner mapped out in previous
+chapters, and not from a hypothetical group of animals, such as is
+postulated in the theory of their origin from forms like Balanoglossus.
+
+The latter theory derives the alimentary canal of the vertebrate from that
+of the invertebrate, and finds in the latter the commencement of the
+notochord. In the comparison which I have made the alimentary canal of the
+invertebrate ancestor has become the tube of the central nervous system of
+the vertebrate, and there is no sign of a notochord whatever. All the
+organs of the arthropod have already been allocated; where the notochord is
+situated in the {434}vertebrate there is nothing but a gap in the
+invertebrate, but the position of that gap can be settled with great
+accuracy from the previous comparison of organs in the two groups. So,
+also, the alimentary canal of the vertebrate is from the very nature of the
+case a new organ, yet, as has been shown in Chapter V., the comparison of
+the respiratory organs in the two groups gives a strong suggestion of the
+manner in which such a canal was formed.
+
+
+THE ORIGIN OF THE NOTOCHORD.
+
+The time has now come to endeavour to frame a plausible theory of the
+method of formation of the notochord and the new alimentary canal, and thus
+to complete the diagram on p. 413. The comparative method is no longer
+available, for these structures are both unrepresented as such in the
+arthropod; any suggested explanation, therefore, must be more tentative,
+and cannot give the same feeling of certainty as is the case with all the
+organs already considered. Our only chance of finding out the past history
+of the notochord lies in the embryological method, in the hope that,
+according to the 'law of recapitulation,' the ancestral history may be
+repeated in the ontogeny with sufficient clearness to enable some
+conclusion to be drawn.
+
+At the outset, one point comes out clearly--the close relationship between
+the notochord and the vertebrate gut; they are both derived from the same
+layer, both parts of the same structure. On this point all embryologists
+are agreed; it is expressed in such statements as, "the notochord, as well
+as the alimentary canal, is formed from hypoblast"; "the notochord arises
+as a thickening in the dorsal wall of the alimentary canal." The two
+structures are so closely connected together that they must be considered
+together. If we can conjecture the origin of the one, we may be sure that
+we have the clue to the origin of the other. The two together form the one
+new organ which distinguishes the vertebrate from the arthropod, the only
+thing left which requires explanation for the completion of this strange
+history.
+
+What, then, is the notochord? What are its characteristics? In the highest
+vertebrates it is conspicuous only in the embryo; with the development of
+the axial skeleton it is more and more squeezed out of existence, until in
+the adult it is no longer visible. By the 'law of recapitulation' this
+developmental history implies that, as we descend the vertebrate phylum,
+the notochord ought to be more and {435}more conspicuous, more and more
+permanent during the life of the animal. Such is, indeed, found to be the
+case, until at last, in the lowest vertebrates, such as the lamprey, and in
+forms like Amphioxus, the notochord persists throughout the life of the
+animal as a large important axial supporting rod.
+
+This rod has a number of striking characteristics which distinguish it from
+all other structures, and are the only means of guessing its probable
+origin. Its position in the body is always the same in all vertebrates and
+is very significant, for it lies just ventrally to the central nervous
+system, along nearly the whole length of the animal, not quite the whole
+length, for it invariably terminates close to the place where the
+infundibulum comes to the surface of the brain; it is, in fact, always
+confined to the infra-infundibular and spinal cord part of the central
+nervous system. Interpreting this into the language of the arthropod, it
+means that a rod was formed just ventrally to the nervous system, which
+extended the whole length of the infraoesophageal and ventral chain of
+ganglia, and terminated at the orifice of the mouth. Moreover, this rod was
+unsegmented, for the notochord is devoid of segmentation.
+
+At the anterior end the rod tapers to a point, as in Fig. 166. In its
+middle part it is very large and conspicuous, cylindrical in shape; its
+interior is filled with a peculiar vacuolated tissue, different to any
+other known vertebrate tissue, which has therefore received the name of
+notochordal tissue. Outside this is a thick sheath formed of many layers,
+of which the external one gives the staining reactions of elastin, and is
+called the external elastic layer. Between this sheath and the notochordal
+tissue a thin layer of lining cells, of normal appearance, is conspicuous
+in Ammocoetes. These cells secrete the layers of the sheath, and have
+originally, by proliferation, given rise to the notochordal tissue. In the
+notochord of Ammocoetes there is no sign of either nerves, blood-vessels,
+or muscles.
+
+The centre of the notochord presents the appearance of a slight slit, as
+though it had originated from a tube, and that is the opinion now generally
+held, for its mode of formation in the embryo is as that of a tube formed
+from an open groove, as will be explained immediately.
+
+We may, then, conceive of the notochord as originally a tube lying in the
+mid-line just ventrally to the central nervous system, and extending from
+the original mouth to the end of the body. Translate this into the language
+of the arthropod and it denotes a tube on the {436}mid-ventral surface of
+the body, which extended from mouth to anus. Such a tube might be formed
+from the mid-ventral surface as follows:--
+
+In Fig. 163, A, the lining of the ventral surface between two appendages is
+represented flat, in B is shown how the formation of a solid rod may arise
+from the bulging of that ventral surface, and in C how a groove on that
+surface may lead to the formation of a tube between the two appendages. The
+difference between a notochordal rod formed as in B from that in C would be
+shown in the sheath, for in B the sheath would be formed from the cuticle
+of the lining cells, and in C from the basement membrane. The structure of
+the sheath is in accordance with the embryological evidence that the
+notochord is formed as a tube from a groove, as in C, and not as a solid
+rod as in B, for it possesses a well-marked elastin layer, and elastin has
+never yet been found as a constituent of any cuticular secretion, but
+invariably in connection with basement-membranes.
+
+[Illustration: FIG. 163.--DIAGRAM OF TWO POSSIBLE METHODS OF THE FORMATION
+OF A NOTOCHORD.]
+
+The position, then, of the notochord and its method of formation suggests
+that the mid-ventral surface of the arthropod ancestor of the vertebrate
+formed a deep groove between the bases of all the prosomatic, mesosomatic,
+and metasomatic appendages, which was subsequently converted into a tube
+extending along the whole of the body between mouth and anus, and finally,
+by the proliferation of its lining cells and their conversion into
+notochordal tissue, became the notochordal rod of the vertebrate.
+
+As already frequently stated, Apus and Branchipus are the two living
+arthropods which most nearly resemble the extinct trilobites. The beautiful
+specimens of Triarthrus (Fig. 165) found by Beecher give an idea of the
+under surface of the trilobite such as has never been obtained before, and
+demonstrate how closely the condition of things found in Apus (Fig. 164)
+was similar to that occurring in the trilobites. In both cases the
+mid-ventral surface of the animal formed a deep groove which extended the
+whole length of the {437}animal; on each side of this groove in Apus are
+closely set the gnatho-bases of the appendages, in such a manner that the
+groove can be easily converted into a canal by the movements of these
+bases--a canal which, owing to the great number of the appendages and their
+closeness to each other, can be completely and efficiently closed.
+
+[Illustration: FIG. 164.--UNDER-SURFACE OF APUS. (After BRONN.)]
+
+[Illustration: FIG. 165.--UNDER-SURFACE OF A TRILOBITE (_Triarthrus_).
+(From BEECHER.)]
+
+All those who have seen Apus in the living state assert that this canal so
+formed is actually used by the animal for feeding purposes. By the
+movements of the gnatho-bases food is passed up from the hind end of the
+animal along the whole length of this ventral canal to the mouth, where it
+is taken in and swallowed. In this way Apus has been seen to swallow its
+own eggs.
+
+In the trilobites there is a similar deep channel formed by the mid-ventral
+surface, similar gnatho-bases, and closely set appendages, and the membrane
+of this ventral groove was extremely thin.
+
+Here, then, in the very group of animals which were the progenitors of the
+presumed palæostracan ancestor of the vertebrate--a group which is
+characterized by its extensive prevalence and its {438}enormous variety of
+form during the great trilobite era--the formation of a mid-ventral canal
+out of this deep ventral groove is seen to be not only easy to imagine, but
+most probable, provided that a necessity arose for such a conversion.
+
+For what purpose might such a tube have been formed? I would suggest that
+it might have acted as an accessory food-channel, which was of sufficient
+value at the time to give some advantage in the struggle for existence to
+those members of the group who were thus able to supplement their intake of
+food, but at the same time was so inefficient that it was quickly
+superseded by the new alimentary canal, and thus losing its temporary
+function, became solid, and was utilized to form an axial supporting rod.
+
+There is a very considerable amount of evidence in favour of the view that
+the notochord was originally a digestive tube; in fact, as far as I know,
+this conclusion is universally accepted. The evidence is based essentially
+upon its development and upon its structure. It is formed in the vertebrate
+from the same layer as the alimentary canal, _i.e._ the hypoblast, and in
+Amphioxus it commences as a groove in the dorsal wall of the future
+alimentary canal; this groove then closes to form the tube of the
+notochord, and separates from the alimentary canal. Embryologically, then,
+the notochord is looked upon as a tube formed directly from the alimentary
+canal.
+
+As regards its structure, its tissue is, as already stated, something _sui
+generis_. Notochordal tissue has no resemblance to bone or cartilage, or
+any of the usual supporting tissues. Such a tissue is not, however,
+entirely confined to the notochord of the vertebrates, but tissue closely
+resembling it has been found not only in Amphioxus and the Tunicata, but in
+certain other invertebrates, in the Enteropneusta (Balanoglossus, etc.), in
+Cephalodiscus, and in Actinotrocha. In all these latter cases, such a
+tissue is invariably found in disused portions of the alimentary canal; a
+diverticulum of the alimentary canal becomes closed, vacuolation of its
+lining cells takes place, and a tissue resembling notochordal tissue is
+formed.
+
+Owing to the notochord being invariably so striking and mysterious a
+feature of the lowest vertebrates, the term vertebrate, which is
+inappropriate in the members of the group which do not yet possess
+vertebræ, has been largely superseded by the term chordate, with the result
+of attributing an undue preponderance to this tissue in any system of
+classification. Hence, wherever any animal has been found {439}with a
+tissue resembling that of the notochord, enthusiasts have immediately
+jumped to the conclusion that a relationship must exist between it and the
+chordate animals; and, accordingly, they have classified such animals as
+follows: Amphioxus belongs to the group _Cephalochorda_ because the
+notochord projects beyond the central nervous system; the Tunicata are
+called _Urochorda_ because it is confined to the tail; the Enteropneusta,
+_Hemichorda_, because this tissue is confined to a small diverticulum of
+the gut, and, finally, _Diplochorda_ has been suggested for Actinotrocha
+and Phoronis because two separate portions of the gut are transformed in
+this way.
+
+This exaggerated importance given to any tissue resembling in structure
+that of the notochord is believed in by many of those who profess to be our
+teachers on this subject, the very men who can deliberately shut their eyes
+to the plain reading of the story of the pineal eyes, and say, "In our
+opinion this pineal organ was not an eye at all."
+
+The only legitimate inference to be drawn from the similarity of structure
+between the notochord and these degenerated gut-diverticula, is that the
+structure of the notochord may have arisen in the same way, and that
+therefore the notochord may once have functioned as a gut. With cessation
+of its function its cells became vacuolated, as in these other cases, and
+its lumen became filled with notochordal tissue. This evidence strongly
+confirms the suggestion that the notochord was once a digestive tube, but
+by no means signifies that such tissue, wherever found, indicates the
+presence of a notochord.
+
+In order to resemble a notochord, this tissue must possess not only a
+definite structure but a definite position, and this position is a
+remarkably striking and suggestive one. The notochordal tube is
+unsegmented, although the vertebrate is markedly segmented. But in all
+segmented animals the only unsegmented tube which extends the whole length
+of the body, from mouth to anus, is invariably the gut. In the vertebrate
+there are three such tubes: (1) the gut itself, (2) the central canal of
+the nervous system, and (3) the notochordal tube.
+
+The first is the present gut, the second the gut of the invertebrate
+ancestor, and the third the tube in question.
+
+These three unsegmented tubes, extending along the whole length {440}of the
+segmented animal, constitute the great peculiarity of the vertebrate group;
+it is not the unsegmented notochord alone which requires explanation, but
+the presence of three such tubes in the same animal. Any one of them might
+be the unsegmented gut of the segmented animal. The most ventral tube is
+the actual gut of the present vertebrate; the most dorsal--the neural
+canal--was, according to my view, the original gut of the invertebrate
+ancestor; the middle one--the notochordal tube--was, in all probability,
+also once a gut, formed at the time when the exigencies of the situation
+made it difficult for food to pass along the original gut.
+
+[Illustration: FIG. 166.--DIAGRAM TO SHOW THE MEETING OF THE FOUR TUBES IN
+SUCH A VERTEBRATE AS THE LAMPREY.
+
+_Nc._, neural canal with its infundibular termination; _Nch._, notochord;
+_Al._, alimentary canal with its anterior diverticulum; _Hy._, hypophysial
+or nasal tube; _Or._, oral chamber closed by septum.]
+
+Yet another circumstance in favour of this suggestion is the very striking
+position of the anterior termination of the notochord. It terminates at the
+point of convergence of three structures:--
+
+(1) The tube of the hypophysis or nasal tube.
+
+(2) The infundibulum or old mouth-termination.
+
+(3) The notochordal tube.
+
+To these may be added, according to Kupffer, in the embryonic stage, the
+anterior diverticulum of the gut (Fig. 166).
+
+This is a very significant point. Here originally, in the invertebrate
+stage, the olfactory passage opened into the old mouth and oesophagus.
+Here, finally, in the completed vertebrate the same olfactory passage opens
+into the new pharynx. In the stage between the two it may well have opened
+into an intermediate gut, the notochordal tube, its separation from which
+would leave the end of the {441}notochord blind, just as it had already
+left the end of the infundibulum blind.
+
+The whole evidence points to the derivation of the notochord from a ventral
+groove on the surface of the animal, which closed to form a tube capable of
+acting as an accessory gut at the critical period before the new gut was
+fully formed. The essentials of a gut tube are absorption and digestion of
+food; is it likely that a tube formed as I have suggested would be
+efficient for such purposes?
+
+As far as absorption is concerned, no difficulty would arise. The gut of
+the arthropod is lined with a thin layer of chitin, which is traversed,
+like all other chitinous surfaces, by fine canaliculi. Through these
+canaliculi, absorption of fluid material takes place, from the gut to the
+body. Similar canaliculi occur in the chitin covering the animal
+externally, so that, if such external surface formed a tube, and food in
+the right condition for absorption passed along it, absorption could easily
+take place through the chitinous surface. The evidence of Apus proves that
+food does pass along such a tube in the open condition, and in the
+trilobites the chitinous surface lining a similar groove was apparently
+very thin, a condition still more favourable to such an absorption process.
+
+At first sight the second essential of a gut-tube--the power of
+digestion--appears to present an insuperable difficulty to this method of
+forming an accessory gut-tube, for it necessitates the formation of a
+secretion capable of digesting proteid material by the external cells of
+the body, whereas until recently it was supposed that such a function was
+confined to cells belonging to the so-called hypoblastic layer. Experiments
+were made now years ago of turning a Hydra inside out so that its internal
+layer should become external, and _vice versâ_, and they were said to have
+been successful. Such an animal could go on living and absorbing and
+digesting food, although its epiblastic surface was now its digestive
+internal surface. More recent observations have shown that these
+experiments were fallacious. At night-time, when the observer was not
+looking, the hydra reinverted itself, so that again its original digestive
+surface was inside and it lived and prospered as before.
+
+Another piece of evidence of somewhat similar kind, which has not as yet
+been discredited, is seen in the Tunicata. In many of these, new
+individuals are formed from the parent by a process of budding, and it has
+been proved that frequently the gut of the new {442}individual thus budded
+off arises not from the gut or hypoblastic layer of the parent, but from
+the surface or epiblastic layer. Such gut so formed possesses as efficient
+digestive powers as the gut of the parent.
+
+The most remarkable evidence of all has been afforded by Miss Alcock's
+experiments. She examined the different tissues of Ammocoetes for the
+express purpose of finding out their power of digesting fibrin, with the
+result that the most active cells were those of the liver. Next in activity
+came the extract of the lining cells of the respiratory chamber and of the
+skin. The intestine itself when freed from the liver-secretion had very
+little digestive power; extracts of muscle, nervous system, and thyroid
+gland had no power whatever, but the extract of the skin-cells possessed a
+powerful digesting action.
+
+Furthermore, it is not necessary to make an extract of the skin in order to
+obtain this digestive fluid, for under the influence of chloroform the skin
+of Ammocoetes secretes copiously, and this fluid thus secreted was found to
+possess strong digestive powers. So, also, Miss Alcock has demonstrated the
+power of digesting fibrin in a similar secretion of the epithelial cells
+lining the carapace of the crayfish. In both cases a very plausible reason
+for the presence of a digestive ferment in a skin-secretion is found in the
+necessity of preventing the growth of parasites, fungoid, or otherwise,
+especially in those parts where the animal cannot keep itself clean by
+'preening.' Thus in a crayfish, in which the oesophageal commissures had
+been cut, fungus was found to grow on the ventral side, but not on the
+dorsal carapace. The animal was accustomed to keep its ventral surface
+clean by preening; owing to the paralysis it could not do so, and
+consequently the fungus grew there. In the lamprey I found that wherever
+there was a removal of the surface-epithelium, from whatever cause, that
+spot was immediately covered with a fungoid growth, although in the intact
+lamprey the skin was invariably smooth and clean.
+
+I imagine, then, that this digestive power of the skin arose as a
+protective mechanism against parasitic attacks; it is self-evident how a
+tube formed of such material must _ab initio_ act as a digestive tube.
+
+In yet another respect this skin secretion of Ammocoetes is most
+instructive. The surface of Ammocoetes is absolutely smooth, no scales
+{443}of any kind exist; this smoothness is due to the presence of a very
+well-defined cuticular layer secreted by the underlying epithelial cells.
+This cuticle is very much thicker than is usually found in vertebrates,
+and, strangely enough, has been thought to contain chitin. Whether it
+really contains chitin or not I am unable to say, but it certainly
+resembles a chitinous layer in one respect; it is perforated by innumerable
+very fine tubes or canaliculi, along which, by appropriate staining, it is
+easy to see the secretion of the underlying cell pass to the exterior (Fig.
+140). This marked digestive power of the skin of Ammocoetes, together with
+the easy passage of the secretion through the thin cuticular layer, renders
+it almost certain that a tube formed from the deep ventral groove of the
+trilobite would, from the very first, act as a digestive as well as an
+absorbent tube; in other words, the notochord as soon as formed was able to
+act as an accessory digestive tube.
+
+This suggested origin of the notochord from a groove along the mid-ventral
+surface of the body not only indicates a starting-point from a markedly
+segmented portion of the body, but also points to its formation at a stage
+previous to the formation of the operculum by the fusion of the two
+foremost mesosomatic appendages--indicates therefore its formation at a
+stage more nearly allied to the trilobite than to the sea-scorpion. The
+chance of ever finding any direct evidence of such a chordate trilobite
+stage appears to me exceedingly improbable, and I greatly fear that this
+conception of the mode of formation of the notochord can never be put to
+direct proof, but must always remain guesswork.
+
+On the other hand, evidence of a kind in favour of its origin from a
+segmented part of the body does exist, and that evidence has this special
+value, that it is found only in that most primitive animal, Amphioxus.
+
+This evidence is as follows:--
+
+At fairly regular intervals, the sheath of the notochord is interrupted on
+each side of the mid-dorsal line by a series of holes, which penetrate the
+whole thickness of the sheath. This dorsal part is pressed closely against
+the spinal cord, and through these holes fibres appear to pass from the
+spinal cord to the interior of the notochord. So greatly do these fibres
+present the appearance of ventral roots to the notochord, that Miss Platt
+looks upon them as paired motor roots to the notochord, or at all events as
+once having been such motor {444}roots. Lwoff and Rolph both describe a
+direct communication between the spinal cord and the notochord by means of
+fibres passing through these holes, without however looking upon this
+connection as a nervous one. Joseph alone asserts that no absolute
+connection exists, for the internal elastic layer of the notochord,
+according to him, is not interrupted at these holes, and forms, therefore,
+a barrier between the fibres from the spinal cord and those from the
+interior of the notochord. Still, whatever is the ultimate verdict as to
+these fibres, the suggestive fact remains of the spaces in the notochordal
+sheath and of the corresponding projecting root-like fibres from the spinal
+cord. The whole appearance gives the impression of some former connection,
+or rather series of connections, between the spinal cord and the notochord,
+such as would have occurred if nerves had once passed into the notochord.
+On the other hand, such nerves were not arranged segmentally with the
+myotomes, for, according to Joseph, in the middle of the animal ten to
+twelve such holes occur in one body-segment. In Apus the appendages are
+more numerous than the body-segments, so that it is not necessary for a
+segmental arrangement to coincide with that of the body-segments.
+
+
+THE ORIGIN OF THE ALIMENTARY CANAL.
+
+In close connection with the notochord is the alimentary canal. Any
+explanation of the one must be of assistance in explaining the other.
+
+According to the prevalent embryological teaching, the body is formed of
+three layers, epiblast, hypoblast, and mesoblast, and the gastræa theory of
+the origin of all Metazoa implies of necessity that the formation of every
+individual commences with the formation of the gut. For this reason the
+alimentary canal must in every case be regarded as the earliest formed
+organ, however late in the development it may attain its finished
+appearance. Hence the notochord is spoken of as developed from the
+mid-dorsal wall of the alimentary canal. It is possible to look at the
+question the other way round, and suppose that the organ whose development
+is finished first is older than the one still in process of making. In this
+case it would be more right to say a ventral extension of the tissue, which
+gives rise to the notochord, takes place and forms the alimentary canal. It
+is, to my mind, perfectly possible, and indeed probable, that {445}the
+formation of the vertebrate alimentary canal was a repetition of the same
+process which had already led to the formation of the notochordal tube. The
+formation of the anterior part of the alimentary canal in Ammocoetes at the
+time of transformation strongly suggests the marked similarity of the two
+processes.
+
+Of all the startling surprises which occur at transformation, this
+formation of a new anterior gut is the most startling. From the oral
+chamber of Petromyzon two tubes start: the one leads into the
+gill-chambers, is known as the bronchus, and is entirely concerned with
+respiration; the other leads without a break from the mouth to the anus,
+has no connection with respiration, and is the alimentary canal of the
+animal. Any one looking at Petromyzon would say that its alimentary canal
+was absolutely non-respiratory in character. Before transformation, this
+kind of alimentary canal commences at the end of the respiratory chamber;
+from here to the anus it is of the same character as in Petromyzon, but in
+Ammocoetes the non-respiratory anterior part simply does not exist: the
+whole anterior chamber is both respiratory and affords passage to food.
+This part of the alimentary canal of the adult is formed anew. We see,
+then, here the formation of a part of the alimentary canal taking place,
+not in an embryo full of yolk, but in a free-living, independent, grown-up
+larval form in which all yolk has long since disappeared: a condition
+absolutely unique in the vertebrate kingdom, but one which more than any
+other may be expected to give a clue to the method of formation of a
+vertebrate gut.
+
+The formation of this new gut can be easily followed at transformation, and
+was originally described by Schneider. His statement has been confirmed by
+Nestler, and its absolute truth has been demonstrated to me again and again
+by Miss Alcock, in her specimens illustrative of the transformation
+process. First, in the mid-dorsal line of the respiratory chamber a
+distinct groove is formed, the edges of which come together and form a
+solid rod. This solid rod blocks the opening of the respiratory chamber
+into the mid-gut, so that during this period of the transformation no food
+can pass out of the pharyngeal chamber. A lumen then begins to appear in
+this solid rod at the posterior end, which steadily advances mouthwards
+until it opens into the oral chamber and thus forms an open tube connecting
+the mouth with the gut.
+
+Here, then, is the foundation of a new gut on very similar lines {446}to
+that of the notochord, by the conversion of a groove into a tube. Still
+more suggestive is it to find that the tube so formed has no appearance
+whatever of segmentation; it is as unsegmented as the rest of the gut,
+although, as is seen in Fig. 62, the dorsal wall of the respiratory chamber
+from which it arose is as markedly segmented as any part of the animal.
+Here under our very eyes, in the course of a few days or weeks, an
+object-lesson in the process of the manufacture of an alimentary canal is
+carried out and completed, and the teaching of that lesson is that a
+gut-tube may be formed in the same way as the notochordal tube, by the
+conversion of a grooved surface into a canal, and that gut-tube so formed,
+like the notochord, loses all sign of segmentation, even although the
+original grooved surface was markedly segmented.
+
+The suggestion then is, that the new gut may have been formed by a
+repetition of the same process which had already given origin to the
+notochord.
+
+Such a method of formation is not, in my opinion, opposed to the evidence
+given by embryology, but in accordance with it; the discussion of this
+point will come best in the next chapter, which treats of the embryological
+evidence as a whole, and will therefore be left till then.
+
+
+THE EVIDENCE GIVEN BY THE INNERVATION OF THE VERTEBRATE ALIMENTARY CANAL.
+
+Throughout this investigation the one fixed landmark to which all other
+comparisons must be referred, is the central nervous system, and the
+innervation of every organ has given the clue to the meaning of that organ.
+So also it must be with the new alimentary canal; by its innervation we
+ought to obtain some insight into the manner of its origination. In any
+organ the nerves which are specially of value in determining its
+innervation, are of necessity the efferent or motor nerves, for the limits
+of their distribution in the organ are much more easily determined than
+those of the afferent or sensory nerves. The question therefore of primary
+importance in endeavouring to determine the nature of the origin of the
+alimentary canal from its innervation is the determination of the efferent
+supply to the musculature of its walls.
+
+Already in previous chapters a commencement has been made in {447}this
+direction; thus the musculature of the oral chamber has been derived
+directly from the musculature of the prosomatic appendages; the muscles
+which move the eyes from the prosomatic and mesosomatic dorso-ventral
+somatic muscles; the longitudinal body-muscles from the dorsal longitudinal
+somatic muscles of the arthropod; the muscles of respiration from the
+dorso-ventral muscles of the mesosomatic appendages.
+
+In all these cases we have been dealing with striated musculature and
+consequently with only the motor nerves of the muscle; but the gut
+posterior to the pharyngeal or respiratory chamber contains unstriped
+instead of striped muscle, and is innervated by two sets of nerves, those
+which cause contraction and are motor, and those which cause relaxation and
+are inhibitory. It is by no means certain that these two sets of nerves
+possess equal value from a morphological point of view. The meaning of an
+inhibitory nerve is at present difficult to understand, and in this
+instance, is rendered still more doubtful owing to the presence of
+Auerbach's plexus along the whole length of the intestine--an elaborate
+system of nerve-cells and nerve-fibres situated between the layers of
+longitudinal and circular muscles surrounding the gut-walls, which has been
+shown by the recent experiments of Magnus, to constitute a special enteric
+nervous system.
+
+One of the strangest facts known about the system of inhibitory nerves is
+their marked tendency to leave the central nervous system at a different
+level to the corresponding motor nerves, as is well known in the case of
+the heart, where the inhibitory nerve--the vagus--arises from the medulla
+oblongata, while the motor nerve--the augmentor or accelerator--leaves the
+spinal cord in the upper thoracic region. It is very difficult to obtain
+any idea of the origin of such a peculiarity; I know of only one suggestive
+fact, which concerns the innervation of the muscles which open and close
+the chela of the crayfish, lobster, etc. These muscles are antagonistic to
+each other, and both possess inhibitory as well as motor nerves. The
+central nervous system arrangements are of such a character that the
+contraction of the one muscle is accompanied by the inhibition of its
+opposer, and the nerves which inhibit the contraction of the one, leave the
+central nervous system with the nerves which cause the other to contract.
+Thus the inhibitory and motor nerves of either the abductor (opener) or
+adductor (closer) muscles of the crayfish claw do not leave the central
+nervous system together, but in separate nerves.
+
+{448}If now for some cause the one set of muscles either disappeared, or
+were so altered as no longer to present any appearance of antagonism, then
+there would be left a single set of muscles, the inhibitory and motor
+nerves of which would leave the central nervous system at different levels,
+and the older such systems might be, the greater would be the modification
+in the shape and arrangements of parts in the animal, so that the two sets
+of fibres might ultimately arise from very different levels.
+
+As mentioned in the introductory chapter, the whole of this investigation
+into the origin of vertebrates arose from my work on the system of efferent
+nerves which innervate the vascular and visceral systems. One of the main
+points of that investigation was the proof that such nerves did not leave
+the central nervous system uniformly along the whole length of it, but in
+three great outflows, cranial, thoracico-lumbar, and sacral; there being
+two marked gaps separating the three outflows, caused by the interpolation
+of the plexuses for the innervation of the anterior and posterior limbs
+respectively. All these nerves are characterized by the presence of
+ganglion-cells in their course to the periphery, they are, therefore,
+distinguished from ordinary motor nerves to striated muscle in that their
+impulses pass through a ganglion-cell before they reach the muscle.
+
+The ganglia of the large middle thoracico-lumbar outflow constitute the
+ganglia of the sympathetic system.
+
+The functions of the nerves constituting these three outflows are very
+different, as I pointed out in my original papers. Since then a large
+amount of further information has been obtained by various observers,
+especially Langley and Anderson, which enable the following statements to
+be made:--
+
+All the nerves which cause contraction of the unstriped muscles of the
+skin, whether pilomotor or not, all the nerves which cause secretion of
+sweat glands wherever situated, all the nerves which cause contraction or
+augmentation of the action of muscles belonging to the vascular system, all
+the nerves which are motor to the muscles belonging to all organs derived
+from the Wolffian and Müllerian ducts, _e.g._ the uterus, ureters, urethra,
+arise from the thoracico-lumbar outflow, never from the cranial or sacral
+outflows. It is essentially an efferent skin-system.
+
+On the other hand, the latter two sets of nerves are concerned {449}with
+the supply of motor nerves to the alimentary canal; they form essentially
+an efferent gut-system in contradistinction to the sympathetic or
+skin-system.
+
+A marked distinction exists between these cranial and sacral nerves. The
+vagus never supplies the large intestine, the sacral nerves never supply
+the small intestine. Associated with the large intestine is the bladder,
+the whole system arising from the original cloacal region; the vagus never
+supplies the bladder, its motor nerves belong to the sacral outflow. The
+motor nerves to the ureters, to the urethra, and to the trigonal portion of
+the bladder between the ureters and the urethra, do not arise from the
+sacral outflow, but from the thoracico-lumbar. These muscles belong really
+to the muscles in connection with the Müllerian and Wolffian ducts and
+skin, not to the cloacal region.
+
+The motor innervation then of the alimentary canal reveals this striking
+and suggestive state of affairs. The motor innervation of the whole of the
+small intestine arises from the cranial region, and is immediately followed
+by an innervation from the sacral region for the whole of the muscles of
+the cloaca. It thus indicates a head-region and a tail-region in close
+contiguity, the whole of the spinal cord region between these two extremes
+being apparently unrepresented. Not, however, quite unrepresented, for
+Elliott has shown recently that the ileo-colic valve at the junction of the
+small and large intestine is in reality an ileo-colic sphincter muscle, and
+that this muscle receives its motor nerves neither from the vagus nor from
+the sacral nerves, but from the thoracico-lumbar outflow or sympathetic
+system. This may mean one of two things, either that a band of fibres
+belonging to the skin-system has been added to the gut-musculature, for the
+purpose of forming a sphincter at this spot, or that the region between the
+vagus territory and the cloaca is represented by this small band of muscle.
+The second explanation seems to me the more probable of the two. Between
+the mesosomatic region represented by the vagus, and the cloacal region,
+there existed a small metasomatic region, represented by the pronephros,
+with its segmental duct, as already discussed in Chapter XII. That part of
+the new alimentary canal which belonged to this region is the short piece
+indicated by the ileo-colic sphincter, and innervated, therefore, from the
+same region as the organs derived from the segmental duct.
+
+Such innervation seems to me to suggest that originally the {450}vertebrate
+consisted, as far as its gut was concerned, of a prosomatic and mesosomatic
+(branchial) region, close behind which came the cloaca and anus. Between
+the two there was a short metasomatic region (possibly pronephric), so that
+the respiratory chamber did not open directly into the cloaca.
+
+Such an interpretation is, I think, borne out by the study of the most
+ancient forms of fish. In Bothriolepis, according to Patten, and in
+Drepanaspis, according to Traquair, the cloacal region and anus follow
+immediately upon the posterior end of the head-shield, _i.e._ immediately
+after that region which presumably contained the branchiæ. Similarly, on
+the invertebrate side, all those forms which resembled Limulus must have
+possessed a very short region between the branchial and cloacal parts of
+the body. The original cloacal part of the vertebrate gut may well have
+been the original cloaca of the arthropod, into which its intestine emptied
+itself, especially when we see the tendency of the scorpion group of
+animals to form an accessory cloacal pouch known as the stercoral pouch or
+pocket.
+
+Again, it is striking to see how, in certain of the scorpion group, _e.g._
+Thelyphonus and Phrynus, there is a caudal massing of the central
+nerve-cells as well as a cephalic massing, so that their central nervous
+system is composed of a cephalic and caudal brain. These two brains are
+connected together by commissures extending the whole length of the body,
+in which I have been unable to find any sign of ganglion-cells. What this
+caudal brain innervates I do not know; it is, I think, a matter worth
+further investigation, especially as there are many indications in the
+vertebrate that the lumbo-sacral region of the cord possesses higher
+functions than the thoracic region.
+
+The method of formation of the alimentary canal as indicated by its
+innervation is as follows:--
+
+In front an oral chamber, formed, as already pointed out, by the
+modification of the prosomatic appendages, followed by a respiratory
+chamber, the muscles and branchiæ of which were the muscles and branchiæ of
+the mesosomatic appendages. This mesosomatic, or branchial, part was in
+close contiguity to the cloaca and anus, being separated from it only by a
+short tube formed in the metasomatic or pronephric region.
+
+I imagine that this connection was originally in the form of an {451}open
+groove, as already explained for both notochord and the anterior part of
+the gut itself in Ammocoetes; an open groove formed from the mid-ventral
+surface of the body, on each side of which were the remnants of the
+pronephric appendages. By the closure of this groove ventrally, and the
+growing round of the pleural folds, as already suggested, the remains of
+the pronephric appendages are indicated by the segmental duct and the form
+of the vertebrate body is attained.
+
+Even in the branchial region the same kind of thing must, I think, have
+occurred. The grooved ventral surface became a tube, on each side of which
+were lying in regular order the in-sunk branchial appendages, the whole
+being subsequently covered by the pleural folds to form an atrial chamber.
+A tube thus formed from the grooved ventral surface would carry with it to
+the new ventral surface the longitudinal venous sinuses, and thus form, in
+the way already suggested, the heart and ventral aorta. Posterior to the
+heart in the pronephric region, the same process would give rise to the
+sub-intestinal vein.
+
+The evidence of comparative anatomy bears out most conclusively the
+suggestion that in the original vertebrate the gut was mainly a respiratory
+chamber. In man and all mammals the oral chamber opens into a small
+pharynx, followed by the oesophagus, stomach and small intestine. Of this
+whole length, a very small part is taken up by the pharynx, in which, in
+the embryo, the branchial arches are found, showing that this represents
+the original respiratory part of the gut. In the ordinary fish this
+branchial part is much more conspicuous, occupies a large proportion of the
+gut, and in the lowest fishes, such as Ammocoetes and Amphioxus, the
+branchial region extends over a large portion of the animal, while the
+intestine proper is a straight tube, the length of which is insignificant
+in comparison with its length in the higher vertebrates.
+
+Such a tube was able to act as a digestive tube, owing, as already pointed
+out, to the digestive powers of the skin-epithelium, and I imagine at first
+the respiratory chamber, seeing that it composed very nearly the whole of
+the gut, was at the same time the main digestive chamber; even in
+Ammocoetes its digestive power is superior to that of the intestine itself.
+
+Just posterior to the branchial part a diverticulum of the gut was formed
+at an early stage, as seen in Amphioxus, and provided the {452}commencement
+of the liver. This simple liver-diverticulum became the tubular liver of
+Ammocoetes, and formed, curiously enough, not a glandular organ of the same
+character as the liver of the higher vertebrates, but a hepato-pancreas,
+like the so-called liver of the arthropods, which also is a special
+diverticulum of the gut, or rather the main true gut of the animal. In both
+cases the liver is the chief agent in digestion, for in Ammocoetes the
+liver-extract is very much more powerful in the digestion of proteids than
+the extract of any other organ tried by Miss Alcock. Subsequently in the
+vertebrate the gastric and pancreatic glands arise and relieve the liver of
+the burden of proteid digestion.
+
+It is, to my mind, somewhat significant that the liver on its first
+formation in the vertebrate should have arisen as a digestive organ of the
+same character as the so-called liver in the arthropods; whether it
+originally belonged to any separate segment is in our present state of
+knowledge difficult to say.
+
+
+CONCLUSION.
+
+In conclusion, I will endeavour to illustrate crudely the way in which, on
+my theory, the notochord and vertebrate gut may have been formed, the
+agencies at work being in the main two, viz. the dwindling of appendages as
+mere organs of locomotion, and the conversion of a ventral groove into a
+tube.
+
+I imagine that, among the Protostraca, forms were found somewhat resembling
+trilobites with markedly polychætan affinities; which, like Apus, possessed
+a deep ventral groove from one end of the body to the other, and also
+pleural fringes, as in many trilobites. This might be called the Trilobite
+stage (Fig. 167, A).
+
+This groove became converted into a tube and so gave rise to the notochord,
+while the appendages were still free and the pleuræ had not met to form a
+new ventral surface. This might be called the Chordate Trilobite stage
+(Fig. 167, B).
+
+Then, passing from the protostracan to the palæostracan stage, the oral and
+respiratory chambers were formed, not communicating with each other, in the
+manner described in previous chapters, a ventral groove in the metasomatic
+region being the only connection between respiratory chamber and cloaca.
+This might be called the Chordate Palæostracan stage (Fig. 167, C).
+
+{453}[Illustration: FIG. 167.--A, DIAGRAM OF SECTION THROUGH A
+TRILOBITE-LIKE ANIMAL; B, DIAGRAM TO ILLUSTRATE THE SUGGESTED FORMATION OF
+THE NOTOCHORD FROM A VENTRAL GROOVE; C, DIAGRAM TO ILLUSTRATE THE SUGGESTED
+FORMATION OF THE POST-BRANCHIAL GUT BY THE CONTINUATION OF THE SAME PROCESS
+OF VENTRAL GROOVE-FORMATION, COMBINED WITH OBLITERATION OF APPENDAGES AND
+GROWTH OF PLEURAL FOLDS; D, DIAGRAM TO ILLUSTRATE THE COMPLETION OF THE
+VERTEBRATE TYPE BY THE MEETING OF THE PLEURAL FOLDS IN THE MID-VENTRAL LINE
+WITH THE OBLITERATION OF THE ATRIAL CAVITY AND THE CONVERSION OF THE
+VENTRAL GROOVE INTO THE CLOSED ALIMENTARY CANAL.
+
+_Al._, alimentary canal; _N._, nervous system; _My._, myotome; _Pl._,
+pleuron; _App._, appendage; _Neph._, nephrocoele; _Met._, metacoele; _Sd._,
+segmental duct; _Mes._, mesonephros; _At._, atrial chamber; _Nc._,
+notochord; _H._, heart; _F._, fat body; _Ng._, notochordal groove. (These
+diagrams are intended to complete the diagrams on p. 413, which, as stated
+there, were purposely left incomplete.)]
+
+{454}Finally, with the conversion of this groove into a tube, the opening
+of the oral into the respiratory chamber, and the formation of an atrium by
+the ventralwards growth of the pleural folds, the formation of a Vertebrate
+was completed (Fig. 167, D).
+
+In my own mind I picture to myself an animal which possessed eurypterid and
+trilobite characters combined, in which a notochordal tube had been formed
+in the way suggested, and a respiratory chamber which communicated with the
+cloaca by means of a grooved channel along the mid-ventral line of the
+metasomatic portion of the body. On each side of this channel were the
+remains of the metasomatic appendages (pronephric). The whole was enveloped
+in the pleural folds, which probably at this time did not yet meet in the
+middle line to form a new ventral surface. This respiratory chamber, owing
+to the digestive power of the epidermis, assisted in the process of
+alimentation to such an extent as to supersede the temporary notochordal
+tube, with the effect of bringing about the conversion of the metasomatic
+groove into a closed canal, and so the formation of an alimentary tube
+continuous with the respiratory chamber. The amalgamation of the pleural
+folds ventrally completed the process, and so formed an animal resembling
+the Cephalaspidæ, Ammocoetes, or Amphioxus.
+
+I have endeavoured in this chapter to make some suggestions upon the origin
+of the notochord and of the vertebrate gut in accordance with my theory of
+the origin of vertebrates. I feel, however, strongly that these suggestions
+are much more speculative than those put forward in the previous chapters,
+and of necessity cannot give the same feeling of soundness as those based
+directly upon comparative anatomy and histology. Still, the fact remains
+that the origin of the notochord is at present absolutely unknown, and that
+my speculation that it may have originated as an accessory digestive tube
+is at all events in accordance with the most widely spread opinion that it
+arises in close connection with an alimentary canal.
+
+
+
+
+{455}CHAPTER XIV
+
+_THE PRINCIPLES OF EMBRYOLOGY_
+
+  The law of recapitulation.--Vindication of this law by the theory
+  advanced in this book.--The germ-layer theory.--Its present position.--A
+  physiological not a morphological conception.--New fundamental law
+  required.--Composition of adult body.--Neuro-epithelial syncytium and
+  free-living cells.--Meaning of the blastula.--Derivation of the Metazoa
+  from the Protozoa. Importance of the central nervous system for Ontogeny
+  as well as for Phylogeny.--Derivation of free-living cells from
+  germ-cells.--Meaning of coelom.--Formation of neural canal.--Gastrula of
+  Amphioxus and of Lucifer.--Summary.
+
+
+In a discussion upon this theory of mine, which took place at Cambridge on
+November 25 and December 2, 1895, it was said that such a theory was
+absolutely and definitely put out of court, because it contravened the
+principles of embryology, was opposed, therefore, to our surest guide in
+such matters; and the law was laid down with great assurance that no claim
+for genetic relationship between two groups of animals can be allowed which
+is based upon topographical and structural coincidences revealed by the
+study of the anatomy of two adult animals, however numerous and striking
+they may be, if there are fundamental differences in the embryology of the
+members of these two groups.
+
+According to my theory the old gut of the arthropod still exists in the
+vertebrate as the tubular lining of the central nervous system, and the
+vertebrate has formed a new gut. According to the principles of embryology
+as held up to the present, in all animals above the Protozoa, the different
+structures of the body arise from three definite embryonic layers, the
+epiblast, mesoblast, and hypoblast, and in all cases the gut arises from
+the hypoblastic layer. In the vertebrate the gut also arises from the
+hypoblast, while the neural canal is epiblastic. My theory, then, makes the
+impossible assertion that what was hypoblast in the arthropod has become
+epiblast in the vertebrate, and what was epiblast in the arthropod has
+become hypoblast in the vertebrate. Such a conception is supposed to be so
+{456}absolutely impossible that it only requires to be stated to be
+dismissed as an absurdity.
+
+Against this opinion I claim boldly that my theory is not only not contrary
+to the principles of embryology, but is mainly based upon the teachings of
+embryology. I wish here not to be misunderstood. The great value of the
+study of embryology for questions of the sequence of the evolution of
+animals is to be found in what is known as the Law of Recapitulation, which
+asserts that every animal gives some indication in the stages of its
+individual development of its ancestral history. Naturally enough it cannot
+pass through all the stages of its past history with equal clearness, for
+what has taken millions of years to be evolved has to be compressed into an
+evolution lasting only a few months or weeks, or even less.
+
+When in the highest vertebrate a vestigial organ, such as the pineal gland,
+can be traced back without leaving the vertebrate kingdom to a distinct
+median eye, such as is found in the lamprey, that rudimentary organ is
+evidence of an organ which was functional in the earliest vertebrates or
+their immediate ancestors. So it is generally with well defined vestigial
+organs found in the adult animal; they always indicate an organ which was
+functional in the near ancestor.
+
+Passing from the adult to the embryo we still find the same law. Here,
+also, vestigial organs are met with, which may leave no trace in the adult,
+but indicate organs which were functional in the near ancestor. Thus, but
+for embryology, we should have no certainty that the air-breathing
+vertebrates had been derived from water-breathing fishes; the indication is
+not given by any close resemblance between the formation of the embryos in
+their earliest stages, but by the formation of vestigial gill-arches even
+in the embryos of the highest mammal.
+
+For all questions of evolution the presence of vestigial organs in the
+embryo is the important consideration, for they give an indication of near
+ancestry; the early formation of the embryo concerns a much more remote
+ancestral period, all vestigial organs of which may well have been lost and
+obscured by coenogenetic changes. Let us, then, consider the two
+things--the vestigial organs and the early formation of the
+embryo--separately, and see how far my opponents are justified in their
+statement that my theory contravenes the principles of embryology.
+
+{457}First, I will take the teachings of vestigial organs and the
+arrangement of organs found in the vertebrate embryo. Here it is impossible
+to say that my theory is contrary to the teaching of embryology, for as the
+previous chapters have shown again and again, the argument is based very
+largely upon the facts of embryology. In the first place, the comparison
+which I have chiefly made is a comparison between the larval form of a very
+low vertebrate and the arthropod group, a comparison which exists only for
+the larval form, and not for the adult. The whole theory, then, is based
+upon a developmental stage of the vertebrate, and not upon the anatomy of
+the adult.
+
+Throughout the whole history it seems to me perfectly marvellous how
+completely the law of recapitulation is vindicated by my theory of the
+origin of the vertebrate. The theory asserts that the clue to the origin of
+vertebrates is to be found in the tubular nature of the central nervous
+system of the vertebrate; in that the vertebrate central nervous system is
+in reality formed of two things: (1) a central nervous system of the
+arthropod type, and (2) an epithelial tube in the position of the
+alimentary canal of the arthropod.
+
+Is it possible for embryology to recapitulate such a phylogenetic history
+more clearly than is here the case? In order to avoid all possibility of
+our mistaking the clue, the nerve-tube in the embryo always opens into the
+anus at its posterior end, while in the larval Amphioxus it is actually
+still open to the exterior at the anterior end. The separateness of the
+tube from the nervous system at its first origin is shown especially well
+in the frog, where, as Assheton has pointed out, owing to the pigment in
+the cells of the external layer of epithelium, a pigmented tube is formed,
+on the outside of which the nervous tissue is lying, and step by step the
+gradual intermingling of the nerve-cells and the pigmented lining cells can
+be followed out.
+
+Consider the shape of the nerve-tube when first formed in the vertebrate.
+At the cephalic end a simple bulged-out tube with two simple anterior
+diverticula, which passes into a narrow straight spinal tube; from this
+large cephalic bulging a narrow diverticulum, the infundibulum, passes to
+the ventral surface of the forming brain. This tube is the embryological
+expression of the simple dilated cephalic stomach, with its ventral
+oesophagus and two anterior diverticula, which opens into the straight
+intestine of the arthropod. Nay, more, by its very shape, and the
+invariable presence of two anterior {458}diverticula, it points not only to
+an arthropod ancestry, but to a descent from a particular group of
+primitive arthropods. Then comes the formation of the cerebral vesicles,
+and the formation of the optic cup, telling us as plainly as can be how the
+invasion of nervous material over this simple cephalic stomach and its
+diverticula has altered the shape of the original tube, and more and more
+enclosed it with nervous elements.
+
+So, too, in the spinal cord region. When the tube is first formed, it is a
+large tube, the latero-ventral part of which presents two marked bulgings;
+connecting these two bulgings is the anterior commissure. These two lateral
+bulgings, with their transverse commissure, represent, with marked
+fidelity, the ventral ganglion-masses of the arthropod with their
+transverse commissure, and occupy the same position with respect to the
+spinal tube, as the ganglion-masses do with respect to the intestine in the
+arthropod. Then the further development shows how, by the subsequent growth
+of the nervous material, the calibre of the tube is diminished in size, and
+the spinal cord is formed.
+
+Again, I say, is it possible to conceive that embryology should indicate
+the nature of the origin of the vertebrate nervous system more clearly than
+it does?
+
+It is the same with all the other organs. Take, for instance, the skeletal
+tissues. The study of the vertebrate embryo asserts that the cartilaginous
+skeleton arose as simple branchial bars and a simple cranio-facial
+skeleton, and also that the parenchymatous variety of cartilage represents
+the embryonic form. Word for word, the early embryonic stage of the
+vertebrate skeleton closely resembles the stage reached in the arthropod,
+as shown by Limulus, and again records, unmistakably, the past history of
+the vertebrate.
+
+So, too, with the whole of the prosomatic region; the situation of the old
+mouth, the manner in which the nose of the cephalaspidian fishes arose from
+the palæostracan, are all shown with vivid clearness by Kupffer's
+investigations of the early stage of Ammocoetes, while at the same time the
+closure of the oral cavity by the septum shows how the oral chamber was
+originally bounded by the operculum. Nay, further, the very formation of
+this chamber embryologically was brought about by the forward growth of the
+lower lip, just as it must have been if the chilaria grew forward to form
+the metastoma.
+
+So, too, the study of the embryo teaches that the branchiæ arise as
+{459}ingrowths, that the heart arises as two longitudinal veins, just as
+the theory supposes from the facts provided by Limulus and the scorpions.
+No indication of the origin of the thyroid gland is given by the study of
+its structure in any adult vertebrate, but in the larval form of the
+lamprey there is still preserved for us a most graphic record of its past
+history.
+
+The close comparisons which it is possible to make between the eye-muscles
+of the vertebrate and the recti muscles of the scorpion group on the one
+hand, and between the pituitary and coxal glands on the other, are based
+upon, or at all events are strikingly confirmed by, the study of the
+coelomic cavities and the origin of these muscles in the two groups. In
+fact the embryological evidence of the double segmentation in the head and
+the whole nature of the cranial segments is one of the main
+foundation-stones on which the whole of my theory rests.
+
+So it is throughout. Turn to the excretory organs--it is not the kidney of
+the adult animal which leads direct to the excretory organs of the
+primitive arthropod, but the early embryonic origin of that kidney.
+
+So far from having put forward a theory which runs counter to the
+principles of embryology, I claim to have vindicated the great Law of
+Recapitulation which is the foundation-stone of embryological principles.
+My theory is largely based upon embryological facts, and its strength
+consists in the manner in which it links together into one harmonious
+whole, the facts of Embryology, Palæontology, Anatomy, and Physiology. Why,
+then, is it possible to assert that my theory disregards the principles of
+embryology, when, as we have seen, embryology is proclaiming as loudly as
+possible how the vertebrate arose? In my opinion, it is because the
+embryologists have to a large extent gone wrong in their fundamental
+principles, and have attached more weight to these faulty fundamental
+principles than to the obvious facts which, looked at thoughtfully, could
+not have failed to suggest a doubt as to the correctness of these
+'principles.'
+
+The current laws of embryology upon which such weight is laid are based on
+the homology of the germinal layers in all Metazoa, and state that in all
+cases after segmentation is finished a blastula is formed, from which there
+arises a gastrula, formed of an internal layer, the hypoblast, and an
+external layer, the epiblast; subsequently {460}between these arises a
+third layer, the mesoblast. These layers are strictly morphological
+conceptions, and are stated to be homologous in all cases, so that the
+hypoblast of one animal must be homologous to the hypoblast of another. In
+order, therefore, to compare two adult animals for the purpose of finding
+kinship between them, it is necessary to find whether parts such as the
+gut, which in both cases have the same function, arise from the same
+germinal layer in the embryo. We can, in fact, have no certainty of
+kinship, even although the two animals are built up as far as the adult
+state is concerned on a remarkably similar plan, unless we can study their
+respective embryos and find out what parts arise from the hypoblast and
+what from the epiblast. The homology of the germinal layers constitutes in
+all cases of disputed relationship the court of final appeal. A new gut,
+therefore, in any animal can only be formed from hypoblast, and any theory,
+such as that advocated in this book, which deals with the formation of a
+new gut, and does not form that gut from pre-existing hypoblast, must of
+necessity be wrong and needs no further consideration.
+
+Such is the result of current conceptions--conceptions which to be valid
+must be based upon an absolutely clear morphological definition of the
+formation of the germinal layers, a definition not based on their
+subsequent history and function, but determined solely by the uniformity of
+the manner of their origin.
+
+What, then, is a germinal layer? How can we identify it when it first
+arises? What is the morphological criterion by which hypoblast can be
+distinguished from epiblast, or mesoblast from either?
+
+This is the question put by Braem, in an admirable series of articles in
+the _Biologisches Centralblatt_, and is one that must be answered by every
+worker who bases his views of the process of evolution upon embryological
+investigation. As Braem points out, the germinal layers are definable
+either from a morphological or physiological standpoint. In the one case
+they must arise throughout on the same plan, and whatever be their fate in
+the adult, they must form at an early stage structures strictly homologous
+in all animals. In the other case the criterion is based on function, and
+the hypoblast, for instance, is that layer which is found afterwards to
+form the definitive alimentary canal. There is no longer any morphological
+homology; such layers are analogous; they may be, but are not necessarily,
+homologous. Braem gives a sketch of the history of the views held on
+{461}the germinal layers, and shows how they were originally a purely
+physiological conception, and how gradually such conception changed into a
+morphological one, with the result that what had up to that time been
+looked upon as analogous structures became strictly homologous and of
+fundamental importance in deciding the position of any animal in the whole
+animal series.
+
+This change of opinion was especially due to the lively imagination of
+Haeckel, who taught that the germinal layers of all Metazoa must be
+strictly homologous, because they were all derived from a common ancestral
+stock, represented by a hypothetical animal to which he gave the name
+Gastræa; an animal which was formed by the simple invagination of a part of
+the blastula, thus giving rise to the original hypoblast and epiblast, and
+he taught that throughout the animal kingdom the germinal layers were
+formed by such an invagination of a part of the blastula to form a simple
+gastrula. If further investigation had borne out Haeckel's idea, if
+therefore the hypoblast was in all cases formed as the invagination of a
+part of a single-layered blastula, then indeed the dogma of the homology of
+the germinal layers would be on so firm a foundation that no speculation
+which ran counter to it could be expected to receive acceptance; but that
+is just what has not taken place. The formation of the gastrula by simple
+invagination of the single-layered blastula is the exception, not the rule,
+and, as pointed out by Braem, is significantly absent in the earliest
+Metazoa; in those very places where, on the Gastræa theory, it ought to be
+most conspicuous.
+
+Braem discusses the question most ably, and shows again and again that in
+every case the true criterion upon which it is decided whether certain
+cells are hypoblastic or not is not morphological but physiological. The
+decision does not rest upon the answer to the question, Are these cells in
+reality the invaginated cells of a single-celled blastula? but to the
+question, Do these cells ultimately form the definitive alimentary canal?
+The decision is always based on the function of the cells, not on their
+morphological position. Not only in Braem's paper, but elsewhere, we see
+that in recent years the physiological criterion is becoming more and more
+accepted by morphologists. Thus Graham Kerr, in his paper on the
+development of Lepidosiren, says: "It seems to me quite impossible to
+define a layer as hypoblastic except by asking one or other of the two
+questions: (1) Does it form the lining of an archenteric cavity? and (2)
+{462}Does it become a certain part of the definitive epithelial lining of
+the gut?"
+
+The appearance of Braem's paper was followed by a criticism from the pen of
+Samassa, who agrees largely with Braem, but thinks that he presses the
+physiological argument too far. He considers that morphological laws must
+exist for the individual development as well as for the phylogenetic, and
+finishes his article with the following sentence, a sentence in which it
+appears to me he expresses what is fast becoming the prevailing view: "Mit
+dem Satz, den man mitunter lesen kann: 'es muss doch auch für die Ontogenie
+allgemeine Gesetze geben' kann leicht Missbrauch getrieben werden; diese
+allgemeinen Gesetze giebt es wohl, aber sie liegen nicht auf flacher Hand
+und bis zu ihrer Erkenntnis hat es noch gute Wege; das eine kann man aber
+wohl heute schon sagen, die Keimblätterlehre gehört zu diesen allgemeinen
+Gesetzen nicht."
+
+I conclude, then, that we ought to go back to a time previous to that of
+Haeckel and ask ourselves seriously the question, When we lay stress on the
+germinal layers and speak of this or that organ arising from this or that
+germinal layer, are we thereby adding anything to the knowledge that we
+already possess from the study of the anatomy and physiology of the adult
+body? If by hypoblast we only mean the internal surface or alimentary canal
+and its glands, etc., and by epiblast we mean the external surface or skin
+and its glands, etc., while mesoblast indicates the middle structures
+between the other two, then I fail to see what advantages we obtain by
+using Greek terms to express in the embryo what we express in English in
+the adult.
+
+The evidence given by Braem, and it could be strengthened considerably, is
+conclusive against the morphological importance of the theory of the
+germinal layers, and transfers the fundamental importance of the early
+embryonic formation, from that of a three-layered embryo to that of a
+single-layered embryo--the blastula--from which, in various ways, the adult
+animal has arisen.
+
+The derivation of both arthropod and vertebrate from such a single-layered
+animal is perfectly conceivable, even though the gut of the latter is not
+homologous with the gut of the former. We have seen that the teachings of
+embryology, as far as its later stages are concerned, afford one of the
+main supports upon which this theory rests. What, therefore, is required to
+complete the story is the way {463}in which these later stages arise from
+the blastula stage; here, as in all cases, the ontogenetic laws must be in
+harmony with the phylogenetic; of the latter the most important is the
+steady development of the central nervous system for the upward progress of
+the animal race. The study of comparative anatomy indicates the central
+nervous system, not the gut, as the keystone of the edifice. So, also, it
+must be with ontogeny; here also the central factor in the formation of the
+adult from the blastula ought to be the formation of the central nervous
+system, not that of the gut.
+
+Such, it appears to me, is the case, as may be seen from the following
+considerations.
+
+The study of the development of any animal can be treated in two ways:
+either we can trace back from the adult to the very beginning in the ovum,
+or we can trace forward from the fertilized egg to the adult. Both methods
+ought to lead to the same result; the difference is, that in the first case
+we are passing from the more known to the less known, and are expressing
+the unknown in terms of the known. In the second case we are passing from
+the less known to the more known, and are expressing the known in
+speculative terms, invented to explain the unknown. What has just been said
+with respect to the germinal layers means that, however much we may study
+the embryo and try to express the adult in terms of it, we finally come
+back to the first way of looking at the question, and, starting with the
+adult, trace the continuity of function back to the first formation of
+cells having a separate function.
+
+Let us, then, apply this throughout, and see what are the logical results
+of tracing back the various organs and tissues from the adult to the
+embryo.
+
+The adult body is built up of different kinds of tissues, which fall
+naturally, from the standpoint of physiology, into groups. Such groups are,
+in the first place--
+
+  1. All those tissues which are connected with the central nervous system,
+  including in that group the nervous system itself.
+
+  2. All those tissues which have no connection with the nervous system.
+
+In the second group the physiologist places all germinal cells, all blood-
+and lymph-corpuscles, all plasma-cells and connective tissue and its
+derivatives--in fact, all free-living cells, whether in a free state or in
+a quiescent, so to speak encysted, condition, such as is {464}found in
+connective tissue. In the first group the physiologist recognizes that the
+central nervous system is connected with all muscular tissues, whether
+striped or unstriped, somatic or splanchnic, and that such connection is of
+an intimate character. Further, all epithelial cells, either of the outer
+or inner surfaces, whether forming special sense-organs and glands, such as
+the digestive and sweat-glands, or not, are connected with the nervous
+system. Besides these structures, there is another set of organs as to
+which we cannot speak definitely at present, which must be considered
+separately, viz. all the cells, together with their derived organs, which
+line the body-spaces. Whatever may be the ultimate decision as to this
+group of cells, it must fall into one or other of the two main groups.
+
+The members of these two groups are so interwoven with one another that
+either, if taken alone, would still give the form of the body, so that, in
+a certain sense, we can speak of the body as formed of two syncytia,
+separate from each other, but interlaced, of which the one forms a
+continuous whole by means of cells connected together by a fluid medium or
+by solid threads formed in such fluid medium, while the other does not form
+a syncytium in the sense that any cell of one kind may be connected with
+any cell of another kind, but a syncytium of which all the different
+elements are connected together only through the medium of the nervous
+system.
+
+If we choose to speak of the body as made up of two syncytia in this way,
+we must at the same time recognize the fundamental difference in character
+between them. In the one case the elements are connected together only by
+what may be called non-living material; there is no direct metabolic
+activity caused by the action of one cell over a more distant cell in
+consequence of such connection, it is not a true syncytium; in the second
+case there is a living connection, the metabolism of one part is directly
+influenced by the activity of another, and the whole utility of the system
+depends upon such functional connection.
+
+The tissues composing this second syncytium may be spoken of as the
+master-tissues of the body, and we may express this conception of the
+building up of the body of the higher Metazoa by saying that it is composed
+of a syncytial host formed of the master-tissues, which contains within its
+meshes a system of free-living cells, none of which have any connection
+with the nervous system. This syncytial {465}host is in the adult composed
+of a number of double elements, a nerve-cell element, and an epithelial
+element, such as muscle-cell, gland-cell, etc., connected together by
+nerves; and if such connection is always present as we pass from the adult
+to the embryo, if there is no period when, for example, the neural element
+exists alone free from the muscle-cell, no period when the two can be seen
+to come together and join, then it follows that when the single-layered
+blastula stage is reached, muscle-cell and nerve-cell must have fused
+together to form a neuro-muscular cell. Similarly with all the other
+neuro-epithelial organs; however far apart their two components may be in
+the adult, they must come together and fuse in the embryo to form a
+neuro-epithelial element.
+
+The close connection between muscle and nerve which has always been
+recognized by physiologists, together with the origin of muscle from a
+myo-epithelial cell in Hydra and other Coelenterata, led the older
+physiologists to accept thoroughly Hensen's views of the neuro-epithelial
+origin of all tissues connected with the central nervous system. Of late
+years this conception has been largely given up owing to the statement of
+His that the nervous system arises from a number of neuroblasts, which are
+entirely separate cells, and have at first no connection with muscle-cells
+or any peripheral epithelial cells, but subsequently, by the outgrowing of
+an axial fibre, find their way to the muscle, etc., and connect with it. I
+do not think that His' statement by itself would have induced any
+physiologist to give up the conception of the intimate connection of muscle
+and nerve, if the work of Golgi, Ramón y Cajal, and others had not brought
+into prominence the neurone theory, _i.e._ that each element of the central
+nervous system is an independent element, without real connection with any
+other element and capable of influencing other cells by contact only. These
+two statements, emanating as they did from embryological and anatomical
+studies respectively, have done much to put into the background Hensen's
+conceptions of the syncytial nature of the motor, neural, and sensory
+elements, which make up the master-tissues of the body, and have led to the
+view that all the elements of the body are alike, in so far as they are
+formed of separate cells each leading an independent existence, without any
+real intimate connection with each other.
+
+The further progress of investigation is, it seems to me, bringing us back
+to the older conception, for not only has the neuroblast theory {466}proved
+very difficult for physiologists to accept, but also Graham Kerr, in his
+latest papers on the development of Lepidosiren, has shown that there is
+continuity between the nerve-cell and the muscle-cell from the very first
+separation of the two sets of elements; in fact, Hensen is right and His
+wrong in their respective interpretation of the earliest stages of the
+connection between muscle and nerve. So also, it seems to me, the intimate
+connection between the metabolism of the gland-cell, as seen in the
+submaxillary gland, and the integrity of its nervous connection implies
+that the connection between nerve-cell and gland-cell is of the same order
+as that between nerve-cell and muscle-cell. Graham Kerr also states in his
+paper that from the very commencement there is, he believes, continuity
+between nerve-cell and epithelial cell, but so far he has not obtained
+sufficiently clear evidence to enable him to speak positively on this
+point.
+
+Further, according to the researches of Anderson, the cells of the superior
+cervical ganglion in a new-born animal will continue to grow healthily as
+long as they remain connected with the periphery, even though entirely
+separated from the central nervous system by section of the cervical
+sympathetic nerve, and conversely, when separated from the periphery, will
+atrophy, even though still connected with the central nervous system. So,
+also, on the sensory side, Anderson has shown that the ganglion-cells of
+the posterior root-ganglion will grow and remain healthy after separation
+of the posterior roots in a new-born animal, but will atrophy if the
+peripheral nerve is cut, even though they are still in connection with the
+central nervous system. Further, although section of a posterior root in
+the new-born animal does not affect the development of the nerve-cells in
+the spinal ganglion, and of the nerve-fibres connecting the posterior
+root-ganglion with the periphery, it does hinder the development of that
+part of the posterior root connected with the spinal ganglion.
+
+These experiments of Anderson are of enormous importance, and force us, it
+seems to me, to the same conclusion as that to which he has already
+arrived. His words are (p. 511): "I suggest, therefore, that the section of
+peripheral nerves checked the development of motor and sensory neurones,
+not because it blocked the passage of efferent impulses in the first case
+and the reception of stimuli from the periphery in the second, but for the
+same reason in both cases, {467}viz. that the lesion disturbed the
+chemico-physical equilibrium of an anatomically continuous (neuro-muscular
+or neuro-epithelial) chain of cells, by separating the non-nervous from the
+nervous, and that the changes occurring in denervated muscle, which I shall
+describe later (and possibly those in denervated skin), are in part due to
+the reciprocal chemico-physical disturbance effected in these tissues by
+their separation from the nervous tissues; also that the section of the
+posterior roots checked the development of those portions of them still
+attached to the spinal ganglia, because the chemico-physical equilibrium in
+those processes is maintained not only by the spinal ganglion-cells, but
+also by the intra-spinal cells with which these processes are anatomically
+continuous."
+
+What is seen so strikingly in the new-born animal can be seen also in the
+adult, and in Anderson's paper references are given to the papers of Lugaro
+and others which lead to the same conclusion.
+
+These experiments seem to me distinctly to prove that the connection
+between the elements of the peripheral organ and the proximate neurone is
+more than one of contact.
+
+We can, however, go further than this, for, apart from the observations of
+Apathy, there is direct physiological evidence that the vitality of other
+neurones besides the terminal neurones is dependent upon their connection
+with the peripheral organ, even though their only connection with the
+periphery is by way of the terminal neurone. Thus, as is seen from
+Anderson's experiments, section of the cervical sympathetic nerve in a very
+young animal causes atrophy of many of the cells in the corresponding
+intermedio-lateral tract, cells which I supposed gave origin to all the
+vaso-constrictor, pilomotor, and sweat-gland nerves. A still more striking
+experiment given by Anderson is the effect of the removal of the periphery
+upon the medullation of those efferent fibres which arise from these same
+spinal cells, for, as he has shown, section of the nerves from the superior
+cervical ganglion to the periphery in a very young animal delays the
+medullation in the fibres of the cervical sympathetic--that is, in
+preganglionic fibres which are not directly connected with the periphery
+but with the terminal neurones in the superior cervical ganglion. So also
+on the afferent side a sufficiently extensive removal of sensory field will
+cause atrophy of the cells of Clarke's column, so that, just as in the case
+of the primary neurones, {468}the secondary neurones show by their
+degenerative changes the importance of their connection with the peripheral
+organs.
+
+In this way I can conceive the formation of a series of both efferent and
+afferent relays in the nervous system by proliferation of the original
+neural moiety of the neuro-epithelial elements, every one of which is
+dependent upon its connection with the peripheral epithelial elements for
+its due vitality, the whole system being a scheme for co-ordination of a
+larger and larger number of peripheral elements. Thus the cells of the
+vasomotor centre are in connection with the whole system of segmental
+vaso-constrictor centres in the lateral horns of the thoracic region of the
+cord, so that to cause atrophy of these cells a very extensive removal of
+the vascular system would be required. Each of the segmental centres in the
+cord supplies a number of sympathetic segments, the connection with all of
+which would have to be cut in order to ensure complete removal of the
+connection of each of its cells with the periphery, and finally each of the
+cells in the sympathetic ganglia supplies a number of peripheral elements,
+all of which must be removed to ensure complete severance.
+
+Thus, if we take any arbitrary number, such as 4, to represent the number
+of peripheral organ-elements with which each terminal neurone is connected,
+and suppose that each neurone has proliferated into sets of 4, then a cell
+of the third order, such as a cell of the vasomotor centre, would require
+the removal of 64 peripheral elements to cause its complete separation from
+the periphery, one of the second order (a cell of the thoracic lateral
+horn) 16 elements, one of the first order (a cell of a sympathetic
+ganglion) 4 elements.
+
+Such intimate inter-relationship between the neurones, both afferent and
+efferent, and their corresponding peripheral organs does not imply that all
+nerve-cells are necessarily as closely dependent upon some connection with
+the periphery, for just as the proliferation of epithelial or muscle-cells
+forms an epithelial or muscular sheet, the elements of which are so
+loosely, if at all, connected together that their metabolism is in no way
+dependent upon such connection, so also a similar proliferation of the
+neural elements may form connections between nerve-cell and nerve-cell of a
+similarly loose nature.
+
+It is this kind of proliferation which, in my opinion, would bind together
+the separate relays of efferent and afferent neurones, and {469}so give
+origin to reflex actions at different levels. Such neurones would not be in
+the direct chain of either the afferent or efferent neurones, and so not
+directly connected with the periphery, and could therefore be removed
+without affecting the vitality of either the efferent or afferent chain of
+neurones. In other words, the vitality of the cells on the efferent side
+ought not to be dependent on the integrity of the reflex arc. With regard
+to the development of the anterior roots, Anderson has shown that this is
+the case, for section of all the posterior roots conveying afferent
+impulses from the lower limb in a new-born animal does not hinder the
+normal development of the anterior roots supplying that limb. Also Mott,
+who originally thought that section of all the posterior roots to a limb
+caused atrophy of the corresponding anterior roots, has now come to the
+same conclusion as other observers, and can find no degeneration on the
+efferent side due to removal of afferent impulses.
+
+Again, the process of regeneration after section of a nerve is not in
+favour of the neuroblast theory. There is no evidence that the cut end of a
+nerve can grow down and attach itself to a muscular or epithelial element
+without the assistance of a nerve tube down which to grow. When the cut
+nerves connected with the periphery degenerate, that applies only to the
+axis-cylinder and the medullary sheath, not to the neurilemma; the
+connective tissue elements remain alive and form a tube into which the
+growing axon finds its way, and so is conducted to the end-plate or
+end-organ of the peripheral structure.
+
+Possibly, as suggested by Mott and Halliburton, the products of
+degeneration of the axis-cylinder and medullary sheath stimulate these
+connective tissue sheath-cells into active proliferation, and so bring
+about the great multiplication of cells arranged as cell-chains, which are
+so often erroneously spoken of as forming the young nerves. These
+sheath-cells are then supposed to re-form and secrete a pabulum which is
+important for the process of regeneration of the down-growing axis-cylinder
+and medullary sheath. Without such pabulum regeneration does not take
+place, as is seen in the central nervous system, where the sheath of
+Schwann is absent.
+
+Again, it is becoming more and more doubtful whether the peripheral
+terminations of nerves are ever really free. As far as efferent nerves are
+concerned the nervous element may entirely {470}predominate over the
+muscular or glandular, as in the formation of the electric organs of the
+Torpedo and Malapterurus, but still the final effect is produced by the
+alteration of the muscle or gland-cell. On the afferent side especially
+free nerve-terminations are largely recognized, or, as in Barker's book,
+nerves are spoken of as arising in connective tissue. Thus the numerous
+kinds of special sense-organs, such as Pacinian bodies, tendon-organs,
+genital corpuscles, etc., are all referred to by Barker under the heading
+of "sensory nerve beginnings in mesoblastic tissues." Yet the type of these
+organs has been known for a long time in the shape of Grandry's corpuscles
+or the tactile corpuscles in the duck's bill, where it has been proved that
+the nerve terminates in special large tactile cells derived from the
+surface-epithelium.
+
+So also with all the others, further investigation tends to put them all in
+the same category, all special sensory organs originating from a localized
+patch of surface-epithelium. Thus Anderson has shown me in his specimens
+how the young Pacinian body is composed of rows of epithelial cells, into
+each of which a twig from the nerve passes. He has also shown me how, in
+the case of the tendon-organ, each nerve-fibre passes towards the
+attachment of the tendon and then bends back to supply the tendon-organ,
+thus indicating, as he suggests, how the nest of epithelial cells has
+wandered inwards from the surface to form the tendon-organ. Again,
+Meissner's corpuscles and Herbst's corpuscles are evidently referable to
+the same class as those of Grandry and Pacini.
+
+Yet another instance of the same kind is to be found in the chromatophores
+of the frog and other animals which are under the influence of the central
+nervous system and yet have been supposed by various observers to be
+pigmented connective tissue cells. The most recent work of Leo Loeb and
+others has conclusively shown that such cells are invariably derived from
+the surface-epithelium.
+
+Finally, in fishes we find the special sense-organs of the lateral line and
+other accessory sensory organs, all of which are indisputably formed from
+modified surface epithelial cells.
+
+The whole of this evidence seems to me directly against Barker's
+classification of sensory nerve-beginnings in mesoblastic tissues; in none
+of these cases are we really dealing with free nervous tissue alone, the
+starting point is always a neuro-epithelial couple.
+
+We may then, I would suggest, look upon the adult as formed of {471}a
+neural syncytium, which we may call the host, which carries with it in its
+meshes a number of free cells not connected with the nervous system. If,
+then, we confine our attention to the host and trace back this neural
+syncytium to its beginnings in the embryo, we see that, from the very
+nature of the neuro-epithelial couple, each epithelial moiety must approach
+nearer and nearer to its neural moiety, until at last it merges with it;
+the original neuro-epithelial cell results, and we must obtain, as far as
+the host is concerned, a single-layered blastula as the origin of all
+Metazoa. It follows, further, that there must always be continuity of
+growth in the formation of the host, _i.e._ in the formation of the
+neuro-epithelial syncytium; that therefore cells which have been previously
+free cannot settle down and take part in its formation, as, for instance,
+in the case of the formation of any part of the gut-epithelium or of
+muscle-cells from free-living cells.
+
+Further, since the neural moiety is the one element common to all the
+different factors which constitute the host, it follows that the
+convergence of each epithelial moiety to the neural moiety, as we pass from
+the adult to the embryo, is a convergence of all outlying parts to the
+neural moiety, _i.e._ to the central nervous system, if there is a
+concentrated nervous system. Conversely, in the commencing embryo the place
+from which the spreading out of cells takes place, _i.e._ from which growth
+proceeds, must be the position of the central nervous system, if the
+nervous system is concentrated. If the nervous system is diffuse, and forms
+a general sub-epithelial layer, then the growth of the embryo would take
+place over the whole surface of the blastula.
+
+Turning now to the consideration of the second group of tissues, those that
+are not connected with the central nervous system, we find that they
+include among them such special cells as the germinal cells, free cells of
+markedly phagocytic nature, and cells which were originally free and
+phagocytic, but have settled down to form a supporting framework of
+connective tissue, and are known as plasma-cells. In the embryo we find
+also in many cases free cells in the yolk, forming more or less of a layer,
+which function as phagocytes and prepare the pabulum for the fixed cells of
+the growing embryo; these cells are known by the name of vitellophags, and
+in meroblastic vertebrate eggs form somewhat of a layer known by the name
+of periblast. Such cells must be included in the second group, and,
+{472}indeed, have been said again and again to give origin to the
+free-living blood-corpuscles of the adult. In other cases they are said to
+disintegrate after their work is done.
+
+In the adult the free-living lymphocytes and hæmocytes reproduce themselves
+from already existing free-living cells, but as we pass back to the embryo
+there comes a time, comparatively late in the history of the embryo, when
+such free-living cells are not found in the fluids of the body, and they
+are said to arise from the proliferation and setting free of cells which
+form a lining epithelium. Such formation of leucocytes has been especially
+described in connection with the lining epithelium of the coelomic
+cavities, as stated in Chapter XII., so that anatomists look upon the
+origin of these free cells as being largely from the coelomic epithelium,
+or mesothelium, as Minot calls it.
+
+Then, again, the free cells which form the germinal cells can be traced
+back to a germinal epithelium, which also is part of the coelom. Thus the
+suggestion arises that in the embryo a cellular lining is formed to a
+coelomic cavity (mesothelium) composed of cells which have no communication
+with the nervous system, and are capable of a separate existence as free
+individuals, either in the form of germinal cells or of lymphocytes,
+hæmocytes, and plasma-cells, so that these latter free cells may be
+considered as living an independent existence in the body, and ministering
+to it in the same sense as the germ-cells live an independent existence in
+the body. Again, the function of this mesothelium apart from the germ-cell
+is essentially excretory and phagocytic. It is the cells of the excretory
+organs as well as the lymphocytes which pick up carmine-grains when
+injected. It is the cells of the modified excretory organs, as mentioned in
+Chapter XII., which, according to Kowalewsky and others, give origin to the
+free leucocytes.
+
+We see, then, that the conception of a syncytial neuro-epithelial host
+holding in its meshes a number of free cells leads directly to the
+questions: What is the coelom? To which category does its lining membrane
+belong? and further, also, What is the origin of these free cells?
+
+The Metazoa have been divided into two great groups--those which possess a
+coelom (the Coelomata; Lankester's Coelomocoela) and those which do not
+(Coelenterata; Lankester's Enterocoela). As an example of the latter we may
+take Hydra, because it is a very {473}primitive form, and because its
+development has been carefully worked out recently by Brauer.
+
+In Hydra we find a dermal layer of cells and an inner layer of cells
+separated by a gelatinous mass known as mesogloea; in this mass between the
+dermal and inner layers scattered cells are found, the interstitial cells.
+Now, according to Brauer the position of the germ in Hydra is the
+interstitial cell-layer. One cell of the ovarium becomes the egg-cell, the
+others have their substance changed into yolk-grains, forming the so-called
+pseudo-cells, and as such afford pabulum to the growing egg-cell. Thus we
+see that in between the dermal and gastral layer of cells a third layer of
+cells is found, composed of free living germ-cells, some of which, by the
+formation of yolk-granules, become degraded into pabulum for their more
+favoured kinsfolk. These interstitial cells are said to arise from the
+dermal layer, or ectoderm, but clearly, as in other cases, germ-cells
+constitute a class by themselves and cannot be spoken of as originating
+from ectoderm-cells or from hypoderm-cells.
+
+So also in Porifera, Minchin states: "In addition to the collared cells of
+the gastral layer, and the various cell-elements of the dermal layer, the
+body-wall contains numerous wandering cells or amoebocytes, which occur
+everywhere among the cells and tissues. Though lodged principally in the
+dermal layer, they are not to be regarded as belonging to it, but as
+constituting a distinct class of cells by themselves. They are concerned
+probably with the functions of nutrition and excretion, and from them arise
+the genital products." Further (p. 31): "At certain seasons some of these
+cells become germ-cells; hence the wandering cells and the reproductive
+cells may be included together under the general term archæocytes." Also
+(p. 51): "The mesogloea is the first portion to appear as a structureless
+layer between the dermal and gastral epithelia, and is probably a secretion
+of the former."
+
+He also points out that in these, the very lowest of the Metazoa, the
+separate origin of these archæocytes can be traced back to a very early
+period of embryonic life. Thus in _Clathrina blanca_ the ovum undergoes a
+regular and total cleavage, resulting in the formation of a hollow ciliated
+blastula of oval form. At one point, the future posterior pole of the
+larva, are a pair of very large granular cells with vesicular nuclei, which
+represent undifferentiated blastomeres and are destined to give rise to the
+archæocytes, and, therefore, also to the {474}sexual cells of the adult.
+Thus, as he says, from the very earliest period a distinction is made
+between the "tissue-forming" cells (my syncytial host) and the archæocytes.
+
+We see, then, that the origin of all these free-living cells can be traced
+back to the very earliest of the Metazoa. Here between the dermal and
+gastral layers a gelatinous material, the mesogloea is secreted by these
+layers. This material is non-living, non-cellular. In it live free cells
+which may either be germ-cells, amoebocytes, or 'collencytes' (connective
+tissue cells). If this mesogloea were a fluid secretion, then we should
+have a tissue of the nature of blood or lymph; if it were solid, then we
+should have the foundation of connective tissue, cartilage, and bone.
+
+From this primitive tissue it is easy to see how the special elements of
+the vascular, lymphatic, and skeletal tissues gradually arose, the matrix
+being provided by the cells of the syncytial host and the cellular elements
+by the archæocytes. In fact, we have no right to speak of these lowest
+members of the Metazoa as not being triploblastic, as possessing nothing
+corresponding to mesoblast, for in these free cells in the mesogloea we
+have the origin of the mesenchyme of the higher groups. Thus Lankester,
+talking of mesenchyme, says: "I think we are bound to bring into
+consideration here the existence in many Coelentera of a tissue resembling
+the mesenchyme of Coelomocoela. In Scyphomedusæ, in Ctenophora, and in
+Anthozoa, branched fixed and wandering cells are found in the mesogloea
+which seem to be the same thing as a good deal of what is distinguished as
+mesenchyme in Coelomocoela. These appear to be derived from both the
+primitive layers; some produce spicules, others fibrous substance, others
+again seem to be amoebocytes with various functions. It appears to be
+probable that, though it may be necessary to distinguish other elements in
+it, the mesenchyme of Coelomocoela is largely constituted by cells, which
+are the mother-cells of the skeletotrophic group of tissues, and are
+destined to form connective tissues, blood-vessels, and blood."
+
+Thus we see that the earliest Metazoa were composed of a dermal and gastral
+epithelium, with a sub-epithelial nervous system connecting the parts
+together, which formed, as it were, a host, carrying around free living
+cells of varying function, all of which may be looked on as derived from
+archæocytes, _i.e._ germ-cells. From these the coelomatous animals arose,
+and here also we find, according to {475}present-day opinion, that the
+coelom arose in the first place in the very closest connection with the
+germ-cells or gonads. Thus Lankester, in his review of the history of the
+coelom, states:--
+
+"The numerous embryological and anatomical researches of the past twenty
+years seem to me to definitely establish the conclusion that the coelom is
+primarily the cavity, from the walls of which the gonad cells (ova or
+spermata) develop, or which forms around those cells. We may suppose the
+first coelom to have originated by a closing or shutting off of that
+portion of the general archenteron of Enterocoela (Coelentera), in which
+the gonads developed as in Aurelia or as in Ctenophora. Or we may suppose
+that groups of gonad mother cells, having proliferated from the endoderm,
+took up a position between it and the ectoderm, and there acquired a
+vesicular arrangement, the cells surrounding the cavity in which liquid
+accumulated.
+
+"The coelom is thus essentially and primarily (as first clearly formulated
+by Hatschek) the perigonadial cavity or gonocoel, and the lining cells of
+gonadial chambers are coelomic epithelium. In some few groups of
+Coelomocoela the coeloms have remained small and limited to the character
+of gonocoels. This seems to be the case in the Nemertina, the Planarians,
+and other Platyhelmia. In some Planarians they are limited in number, and
+of individually large size; in others they are numerous."
+
+When Lankester says that "the lining cells of gonadial chambers are
+coelomic epithelium," that is equivalent to saying that the lining cells of
+the coelom form an epithelium which was originally gonadial, provided that,
+as seems to me most probable, his second suggestion, of the coelom being
+formed from gonadial mother-cells which have taken up an intermediate
+position between endoderm and ectoderm and there acquired a vesicular
+arrangement, is the true one. It does not seem to me possible to conceive
+of the gonads arising from cells of the epiblast or of the hypoblast, in
+the sense that such cells are differentiated cells belonging to a layer
+with a definite meaning. When we consider that the gonad gives origin to
+the whole of a new individual, that in the protozoan ancestors of the
+Metazoa their ultimate aim and object was the formation of gonads, it seems
+a wrong conception to speak of the gonads as formed from cells belonging
+either to the gut-wall or to the external epithelium. The gonads must stand
+in a category by themselves; they represent a whole, {476}while the other
+cells represent only a part; they cannot therefore be derived from the
+latter. They may, and indeed do, give rise to cells of a subordinate
+character, but they cannot rightly be spoken of as derived from such cells.
+The very fact mentioned by Lankester, that in the lowest coelomatous
+Metazoa, the Platyhelminthes, the coeloms are limited to the character of
+simple gonocoels, strongly points to the conclusion that all the coelomic
+cells were originally of the nature of gonadial cells, and therefore
+free-living and independent of the rest of the cells of the body. Whether
+the germ-cells appear, as in Hydra, to be derived from the ectoblast, or,
+as is usually stated, from the endoblast, in neither case ought they to be
+classed with the internal or external epithelium; they are germ-cells, and
+the epithelium which they form is neither epiblastic nor hypoblastic, but
+germinal, forming originally a simple gonocoele, afterwards, in the higher
+forms, the coelom with its cells of various function. Thus, to quote again
+from Lankester, "The coelomic fluid and the coelomic epithelium, as well as
+the floating corpuscles derived from that epithelium, acquire special
+properties and importance over and above the original functions subservient
+to the maturation of the gonadial cells ... the most important developments
+of the coelom are in connection with the establishment of an exit for the
+generative products through the body-wall to the outer world, and further
+in the specialization of parts of its lining epithelium for renal excretory
+functions."
+
+Such exits led very early to the formation of coelomoducts, which are true
+outgrowths of the coelom itself (p. 14): "The coelomoducts and the
+gonocoels of which they are a part, frequently acquire a renal excretory
+function, and may retain both the function of genital conduits and of renal
+organs, or may, where several pairs are present (metamerized or segmented
+animals), subserve the one function in some segments of the body, and the
+other function in other segments."
+
+The origin of the coelom and its derivatives from a germinal membrane, as
+suggested by Lankester, appears to me most probable, and, if true, it
+carries with it conclusions of far-reaching importance, for it necessitates
+that all the cells which line true coelomic cavities, and their
+derivatives, belong to the category of free-living cells, and are not
+connected with the nervous system. The cells in question are essentially
+those which line serous cavities and those which form excretory glands such
+as the kidneys. In the latter organ we ought especially to be able to
+obtain a clear answer to this question, for is {477}it not a gland which
+secretes into a duct and might therefore be expected to be innervated in
+the same way as other secretory glands? Although there is a strong _primâ
+facie_ presumption in favour of the existence of renal secretory nerves,
+yet according to the universal opinion of physiologists no evidence in
+favour of such nerves has hitherto been found; all the phenomena of
+excretion of urine consequent on nerve stimulation are explicable by the
+action of nerves on the renal vessels, not on the renal cells. Not only is
+the physiological evidence negative up to the present time, but also, I
+think, the histological. On the one hand, Retzius has failed to find
+nerve-connections with kidney-cells; on the other, Berkley has obtained
+such evidence with the Golgi method, but failed entirely with methylene
+blue. I do not myself think that the evidence of the Golgi method alone is
+sufficient without corroboration by other methods, and, in any case,
+Berkley's evidence does not show the nerve-fibres terminating in the
+kidney-cells, in the same way as can be shown by modern methods to exist in
+the case of epithelial cells of the surface, etc. Quite recently another
+paper on this subject has appeared by Smirnow, who appears to have obtained
+better results than those given by Berkley.
+
+Apart from these physiological and histological considerations, this
+question is also dependent upon the nature of the development of the
+excretory organs, for, according to Lankester, all excretory organs may be
+divided into the two classes of nephridial organs and coelomostomes, of
+which the former are largely derived from epiblast. We should, therefore,
+expect to find secretory nerves to nephridial organs, though possibly not
+to coelomostomes. The kidneys of the Mammalia are supposed to be true
+coelomostomes, although, according to Goodrich's researches, the excretory
+organs in Amphioxus are solenocytes, _i.e._ true nephridia.
+
+As to the lining epithelium of the peritoneal, pleural, and pericardial
+cavities--_i.e._ the mesothelium--there is no definite evidence that these
+cells are provided with nerves. Such surfaces are remarkably insensitive in
+the healthy condition, and the pain in such cavities is essentially a
+pressure phenomenon and referable to special sense-organs, such as Pacinian
+bodies, etc., rather than to the mesothelium itself.
+
+These sense-organs are identical in structure with those in the skin, and,
+as Anderson has shown, the nerves of these organs {478}medullate at the
+same time as those in the skin, and both obtain their medullary sheaths
+earlier than any other nerves, whether afferent or efferent. However
+difficult it may be to explain this fact, only one conclusion seems to me
+possible--these Pacinian bodies, like the skin Pacinians, originate from a
+nest of surface epithelial cells, a conclusion which is extremely probable
+on my theory of the origin of vertebrates, but not, as far as I can see, on
+any other.
+
+At the present moment the weight of evidence is, to my mind, in favour of
+the lining endothelium of the coelomic cavities being composed of free
+cells, unconnected with the nervous system rather than the reverse, but I
+must confess that the question is undecided. If it be true that the
+coelomic lining is partly enterocoelic and partly gonocoelic, as Lankester
+teaches, then it would be natural that its cells should be in connection
+with the nervous system, to some extent at all events. This view is,
+however, based on very slender foundations. If the mesothelium is composed
+of cells capable of becoming free, it cannot give rise to the skeletal
+muscles, and it cannot therefore be right to speak of the skeletal muscles
+as derived from the lining cells of a part of the primary coelom. The
+phylogenetic history of the musculature of the different animals points
+strongly to its intimate connection with and derivation from surface
+epithelial cells rather than from coelomic mesothelial cells. Thus in the
+coelenterates, as seen in Hydra, the muscular layer arises directly from a
+modification of the surface epithelial cells; and right up to the annelids,
+even to the highest form in the Polychæta, we still see it stated that the
+musculature, both circular and longitudinal, arises from the ectoderm. In
+the Oligochæta and Hirudinea, according to Bergh, there are five rows of
+teloblasts on each side, of which four are ectodermic and give rise to the
+nerve-ganglia and the circular muscles, while one is mesoblastic and forms
+the nephridial organs and the longitudinal muscles. (The latter statement
+is, according to Bergh, well known, and is not particularly shown by him.
+These longitudinal muscle-bands always lie close against the nervous system
+at their first formation, and may well have been derived in connection with
+it.)
+
+It is apparently only in the Vertebrata that the lining cells of the
+coelomic cavity are definitely stated to give origin to the
+body-musculature, and taking into account on the one hand the evidence of
+Graham Kerr as to the intimate connection between nerve-cell and
+{479}muscle-cell from the very beginning, and on the other the manner in
+which all the skeletal muscles of the adult are lined with a lymphatic
+endothelium, I am strongly inclined to believe that at the closing up of
+the myocoele, when the myomere separates from the mesomere, the lining
+cells remain scattered in among the forming muscle-cells and form the
+ultimate lymphatic tissue of the muscles. If this is really so, then the
+evidence in favour of the mesothelium being composed of free cells not
+connected with the nervous system would be much strengthened, for, on the
+one hand, an intimate relation exists between the connective tissue cells
+and the endothelium of the roots of the lymphatic vessels, a relation
+which, according to Virchow, has rendered it impossible to draw any sharp
+line of distinction between the two; and, on the other, the lymphatic
+endothelium merges into the lining cells of the great serous cavities of
+the body.
+
+It is impossible to conceive of an animal possessing a nervous system which
+is not in connection with sensory and muscular tissues; an isolated
+nerve-cell is a meaningless possession; but it is equally natural to
+conceive of a germ-cell being isolated, capable of living an independent
+existence. Such a difference between the two kinds of tissues must have
+existed from the very commencement of the Metazoa, so that we must, it
+seems to me, imagine that in the formation of the Metazoa from the Protozoa
+the whole of the body of the latter did not break up into a mass of
+separate gonads, each capable of becoming a free-living protozoan similar
+to its parent, but that a portion proliferated into a multinucleated
+syncytium while the remainder formed the free-living gonads. This
+multinucleated syncytium, or host, as it might be called, would still
+continue to exist for the purpose of carrying further afield the immortal
+gonads, which need no longer be all shed at one time.
+
+In such an animal as _Volvox globator_ we have an indication of the very
+kind of animal postulated as connecting the single-celled Protozoa and the
+multi-cellular Metazoa, for it consists of a many-celled case which forms a
+hollow sphere, each of the cells being provided with flagella for the
+purpose of locomotion of the sphere, except a certain number which are not
+flagellated; the latter leave the case to swim freely in the fluid
+contained within the sphere, and forming spermaries and ovaries, conjugate,
+maturate, and then are set free by the rupture of the encircling locomotor
+host.
+
+{480}This conception of the predecessors of the Metazoa being composed of a
+mortal host, holding within itself the immortal sexual products, leads
+naturally to the idea of the separate development of the host from that of
+the germ-cells _ab initio_, so that the study of the development of the
+Metazoa means the study of two separate constituents of the metazoan
+individual--on the one hand, the elaboration of the elements forming the
+syncytial host, on the other, of those derived from the free-living
+independent germ-cells. The elaboration of the host means the
+differentiation of the protoplasm into epithelial, muscular, and nervous
+elements, by means of which the gonads were carried further afield and
+their nourishment as well as that of the host ensured.
+
+The _rôle_ of the nervous system as the middleman between internal and
+external muscular and epithelial surfaces was, I imagine, initiated from
+the very earliest time. The further evolution of the host consisted in a
+greater and greater differentiation and elaboration of this
+neuro-epithelial syncytium, with the result of a steadily increasing
+concentration and departmental centralization of the main factor of the
+syncytium; in other words, it led to the origin and elaboration of a
+central nervous system. In the interstices of this syncytium the gonads
+were placed, and at first, doubtless, the life of the host ended when all
+the germ-cells had been set free. 'Reproduce and die' was, I imagine, the
+law of the Metazoa at its earliest origin, and throughout the ages, during
+all the changes of evolution, the reminiscence of such law still manifests
+itself even up to the highest forms as yet reached. With the
+differentiation of the syncytial host there came also differentiation of
+the free-living gonads, so that only some of them attained to the
+perfection of independent existence, capable of continuing the species;
+while others became subordinate to the first and provided them with
+pabulum, manufacturing within themselves yolk-spherules, and thus in the
+shape of yolk-cells ministered to the developing egg-cell. Thus arose a
+germinal epithelium of which only a few of the elements passed out of the
+host as perfect individuals, the remainder being utilized for the nutrition
+of these few. Such yolk-cells of the germinal epithelium would still,
+however, retain their character as free cells totally independent of the
+syncytial host, and, situated as they were between the internal and
+external epithelium, capable of amoeboid movement, would naturally have
+their phagocytic action {481}utilized either as yolk-cells for the
+providing of pabulum to the egg-cell, or as excretory cells for the removal
+and rendering harmless of deleterious products of all kinds. Thus the free
+cells of the body would become differentiated into the three classes of
+germ-cells, yolk-cells, and excretory cells.
+
+Further, the mass of gonads, which originally occupied so large a space
+within the interior of the host, necessarily, as the tissues of the host
+differentiated more and more, took up less and less space in proportion to
+the whole bulk of the host and formed a germinal mass of cells between the
+outer and inner epithelial layers. This germinal mass formed an epithelium,
+some of the members of which acted as scavengers for the inner and outer
+layers of the host, with the result that fluid accumulated between the two
+parts of the germinal epithelium in connection respectively with the
+external and internal epithelial surfaces of the host, and thus led to the
+formation of a gonocoele, which, by obtaining an external opening, a
+coelomostome, gave origin to the coelom.
+
+Again, with the longer life of the host, the setting free of the gonads no
+longer necessitating the destruction of the host, and also the gonads
+themselves requiring a longer and longer time to be fed up to maturity, the
+bulk and complexity of the whole organism increased and special supporting
+structures became a necessity. The host itself could and did provide these
+to a certain extent by secretions from its epithelial elements, but the
+intermediate supports were provided by the system of phagocytic cells
+utilizing the fluids of the body, at first in the shape of plasma-cells
+able to move from place to place, then settling down to form a connective
+tissue framework, and, later on, cartilage and bone.
+
+So also were gradually evolved the whole of the endothelial structures; the
+lymph-cells, blood-cells, etc., all having their origin from the free cells
+of the body, which themselves originated in the extension of a germinal
+epithelium. Just as in a bee-hive the egg-cells may form the fully
+developed sexual animal, whether drone or queen bee, or the asexual host of
+workers, so in the body of the Metazoa the free cells may form either male
+or female germ-cells spermatozoa, or ova, or a host of workers, scavengers,
+repairers, food-providers, all useful to the community, all showing their
+common origin by their absolute independence of the nervous system.
+
+Two points of great importance follow from this method of looking {482}at
+the problem. First, the evolution of the animal kingdom means essentially
+the evolution of the host, for that is what forms the individual; secondly,
+as the host is composed of a syncytium, the common factor of whose elements
+is the neural moiety, it follows that the tissue of central importance for
+the evolution of the host must be, as indeed it is, the nervous system.
+Further, seeing that the growth of the individual means the orderly
+spreading out of the epithelial moiety away from the neural moiety, it
+follows that the germ-band or germ-area from which growth starts must be in
+the position of the nervous system. If then, the nervous system in the
+animal is a concentrated one, then the growth will emanate from the
+position of such nervous system. If, on the other hand, the nervous system
+is diffused, then the growth will also be diffused.
+
+In this book I have throughout argued that the ancestors of vertebrates
+belonged to a great group of animals which gave origin also to Limulus and
+scorpion-like animals; it is therefore instructive to see what is the
+nature of the development of such animals. For this purpose I will take the
+development of the scorpion, as given by Brauer, for he has worked out its
+development with great thoroughness and care. His papers show that the
+segmentation is discoidal, and results in an oval blastodermic area lying
+on a large mass of yolk. Very early there separates out in this area
+genital cells and yolk-cells, which latter move freely into the yolk and
+prepare it into a fluid pabulum for the nutrition of the cells of the
+embryonic shield or germ-band. These free yolk-cells do not take part in
+the formation of the germinal layers, nor does the endoderm when formed
+give origin to free yolk-cells.
+
+The cells of the germ-band form a small compact area, in which by continual
+mitosis the cells become more than one-layered, and soon it is found that
+those cells which lie close against the fluid pabulum form a continuous
+layer and absorb the nutritious material for themselves and the rest of the
+embryo. While this area is thus increasing in thickness by continuous
+development, the group of genital cells remains always apart, increasing in
+number, but being always in a state of isolation from the cells of the rest
+of the growing area. Thus from the very first Brauer's observations on the
+development of the scorpion point to the formation of a syncytial host
+containing separate genital cells. The continuous layer of cells against
+the fluid pabulum, which is already functioning as a gut, and may
+{483}therefore be called hypoblast, spreads continuously over the yolk, as
+also does the surface epithelial layer, or epiblast. Such spreading is
+always a continuous one for both surfaces, so that the yolk is gradually
+enclosed by a continuous orderly growth from the germ-band, and not by the
+settling down of free cells in the yolk here and there to form the
+gut-lining. This steady orderly development proceeds owing to the
+nourishment afforded by the activity of the free cells or vitellophags and
+the absorbing power of the hypoblast, a steady growth round the yolk which
+results in the formation of the gut-tube, the outer covering and all the
+muscular and excretory organs. Where, then, is this starting-point, this
+germ-band from which the whole embryo grows? It forms the mid ventral area
+of the adult animal, it corresponds exactly to the position of the central
+nervous system. The whole phenomenon of embryonic growth in the scorpion is
+exactly what must take place on the argument deduced from the study of the
+adult that the animal arises as a neuro-epithelial syncytium, and we see
+that that layer of cells which is situated next to the food-material forms
+the alimentary tube. It is not a question whether such layer is ventral or
+dorsal to the neural cells, but whether it is contiguous to or removed from
+the food-material.
+
+Take, again, a meroblastic vertebrate egg as of the bird. Again we find
+free cells passing into the yolk to act as vitellophags, the so-called
+periblast cells; again we see that the embryo starts from a germ-band or
+embryonic shield, and spreads from there continuously and steadily; again
+we see that the layer of cells which lies against the yolk absorbs the
+fluid pabulum for the growing cells; again we see that the area from which
+the whole process of growth starts is that of the central nervous system,
+and again we see that those cells which are contiguous to the food form the
+commencing gut, and are therefore called hypoblast, though in this case
+they are ventral not dorsal to the neural layer.
+
+The comparison of these two processes shows that there is one common
+factor, one thing comparable in the two, one thing that is homologous and
+is the essential in the formation of that part of the animal which I have
+called the host, and that is the central nervous system. Whether the
+epithelial layer which lies ventrally to it or the one that is dorsal forms
+the gut depends upon the position of the food-mass. Where the food is,
+there will be the absorbing layer. {484}Where the food is not, there will
+be no gut formation, whatever may have been the previous history of that
+layer. If, then, we suppose, as I do, that the vertebrate arose from a
+scorpion-like animal without any reversal of dorsal and ventral surfaces,
+and that the central nervous system remained the same in the two animals,
+then the comparison of the development of the two embryos shows that the
+one would be derived from the other if the yolk-mass shifted from the
+dorsal to the ventral side of the nervous system. This would leave the
+dorsal epithelial layer of the original syncytium free from pabulum; it
+would no longer form the definite gut, _but it would still tend to form
+itself in the same manner as before, would still grow from a ventrally
+situated germ-band dorsalwards to form a tube, would recapitulate its past
+history, and show how the alimentary canal of the arthropod became the
+neural canal of the vertebrate_. Although this alimentary canal is formed
+in the same way as before, it is no longer recognized as homologous with
+the scorpion's alimentary canal, but because it no longer absorbs pabulum,
+and does not therefore form the definite gut, it is called an epiblastic
+tube, and, in the words of Ray Lankester, has no developmental importance.
+
+All the arthropods are built up on the same type, and in all the
+development may in its broad outlines be referred to the type just
+mentioned. So also with the vertebrate group; in both cases the position of
+the central nervous system determines the starting area of embryonic
+growth. In both cases the absorbing layer shows the position of the
+definite gut. A concentrated nervous system of this type is common to all
+the segmented animals from the annelids to the vertebrates, and in all
+cases the germ-band which indicates the first formation of the embryo is in
+the position of this nervous system.
+
+As far as the embryo is concerned, there is no great difficulty in the
+conception that the yolk-mass may have shifted from one side to the other
+in passing from the arthropod to the vertebrate, for in the arthropod the
+embryo at first is surrounded by yolk and then passes to the periphery of
+the egg. If it is permissible to speak of a dorsal and ventral surface to
+an egg, and we may imagine the egg held with such dorsal surface uppermost,
+then the yolk would be situated ventrally to the embryo, as in the
+vertebrate, if the protoplasmic cells of the embryo rose from their central
+position to the surface through the yolk, while if they sank through the
+yolk, the yolk would be situated dorsally to the embryo, as in the
+arthropod.
+
+{485}In cases where there is no yolk, or very little, as in Lucifer and
+Amphioxus respectively, the embryo is compelled to feed itself at a very
+early age; such embryos form a free-swimming pelagic ciliated blastula, the
+invagination of which, for the purpose of collecting food material out of
+the open sea, is the simplest method of obtaining nutriment. Here, as in
+other cases, it is the physiological necessity which determines the method
+of formation of the gut, and such similarity of appearance as exists
+between the gastrula of Lucifer and that of Amphioxus, by no means implies
+that the gut of the adult Lucifer is homologous with the gut of Amphioxus.
+
+I have compared two meroblastic eggs of the two classes respectively,
+because the scorpion's egg is meroblastic. I imagine that no real
+difficulty arises with respect to holoblastic eggs, for the experiments of
+O. Hertwig and Samassa show that by centrifugalizing, stimulating, and
+breaking down of large spheres the holoblastic amphibian egg may be
+converted into a meroblastic one, and then development will proceed
+regularly, _i.e._ in this case also the growth proceeds from the animal
+pole; the large cells of the vegetal pole, like the yolk-cells of the
+meroblastic egg, manufacture pabulum for the growing syncytial host.
+
+
+SUMMARY.
+
+  Any attempt to discover how vertebrates arose from invertebrates must be
+  based upon the study of Comparative Anatomy, of Palæontology, and of
+  Embryology. The arguments and evidence put forward in the preceding
+  chapters show most clearly how the theory of the origin of vertebrates
+  from palæostracans is supported by the geological evidence, by the
+  anatomical evidence, and by the embryological evidence. Of the three the
+  latter is the strongest and most conclusive, if it be taken to include
+  the evidence given by the larval stage of the lamprey.
+
+  The stronghold of embryology for questions of this sort is the Law of
+  Recapitulation, which asserts that the history of the race is
+  recapitulated to a greater or less extent in the development of the
+  individual. In the previous chapters such recapitulation has been shown
+  for all the organs of the vertebrate body. In this respect, then,
+  embryology has proved of the greatest value in confirming the evidence of
+  relationship between the palæostracan and the vertebrate, given by
+  anatomical and geological study.
+
+  There is, however, another side to embryology, which claims that the
+  tissues of all the Metazoa are built up on the same plan; that in all
+  cases in the very early stage of the embryo three layers are formed, the
+  epiblast, mesoblast, and hypoblast; that in all animals above the
+  Protozoa these three layers are {486}homologous, the epiblast in all
+  cases forming the external or skin-layer, the hypoblast the internal or
+  gut-layer.
+
+  Such a theory, therefore, as is advocated in this book, which turns the
+  gut of the arthropod into the neural canal of the vertebrate, and makes a
+  new gut for the vertebrate from the external surface must be wrong, as it
+  flatly contradicts the fundamental germ-layer theory.
+
+  Of recent years grave doubts have been thrown upon the validity of this
+  theory, doubts which have increased in force year by year as more and
+  more facts have been discovered which are not in agreement with the
+  theory. So much is it now discredited that any criticism against my
+  theory, which is based upon it, weighs nothing in the balance against the
+  positive evidence of recapitulation already stated. If the germ-layer
+  theory is no longer credited, upon what fundamental laws is embryology
+  based?
+
+  In this chapter I have ventured to suggest a reply to this question,
+  based on the uniformity of the laws of growth throughout the existence of
+  the individual.
+
+  In the adult animal the body is composed of two kinds of tissues, those
+  which are connected with or at all events are under the control of the
+  nervous system, and those which are capable of leading a free life
+  independent of the nervous system. These two kinds of tissues can be
+  traced back from the adult to the embryo, and it is the task of
+  embryology to find out how these two kinds of tissue originate.
+
+  The following out of this line of thought leads to the conception that,
+  throughout the Metazoa, the body is composed of a host which consists of
+  the master-tissues of the body, and takes the form of a neuro-epithelial
+  syncytium, within the meshes of which free living independent organisms
+  or cells live, so to speak, a symbiotic existence.
+
+  The evidence points to the origin of all these free cells from
+  germ-cells, and thus leads to the conception that the blastula stage of
+  every embryo represents two kinds of cells, the one which will form the
+  mortal host being the locomotor neuro-epithelial cell, the other the
+  independent immortal symbiotic germ-cell. Such conception leads directly
+  to the conclusion that the blastula stage of every member of the Metazoa
+  is the embryonic representation of a Protozoan ancestor of the Metazoa;
+  an ancestor, whose nature may be illustrated by such a living form as
+  _Volvox globator_, which, like a blastula, is composed of a layer of
+  cells forming a hollow sphere. These cells partly bear cilia, and so form
+  a locomotor host, partly are of a different character, and form male and
+  female germ-cells. The latter leave the surface of the sphere, pass as
+  free individuals into its fluid contents, form spermaries and ovaries,
+  and then by the rupture of the mortal locomotor host pass out into the
+  external medium, as free swimming young Volvox.
+
+  It is of interest to note that such members of the Protozoa are among the
+  most highly developed of the members of this great group.
+
+  From such a beginning arose in orderly evolution, on the one hand, all
+  the neuro-muscular and neuro-epithelial structures of the body--the
+  so-called master-tissues; on the other, the germ-cells, the
+  blood-corpuscles, lymph-corpuscles plasma and excretory cells, connective
+  tissue cells, cartilage and bone-cells, etc., all of them independent of
+  the central nervous system, all traceable to a modification of the
+  original germ-cells.
+
+  {487}Such a view of the processes of embryology brings embryology into
+  harmony with comparative anatomy and phylogeny, for it makes the central
+  nervous system and not the alimentary canal the most important factor in
+  the development of the host.
+
+  The growth of the individual, whether arthropod or vertebrate, spreads
+  from the position of the central nervous system, regardless of whether
+  that position is a ventral or dorsal one with respect to the yolk-mass.
+  Where the pabulum is, there is the definite gut, the lining walls of
+  which are called in the embryo, hypoblast; but when the pabulum is no
+  longer there, although a tube is formed in the same manner as the
+  alimentary canal of the arthropod, it is now called an epiblastic tube,
+  and is known as the neural tube of the vertebrate.
+
+  This is the great fallacy of the germ-layer theory, a fallacy which
+  consists of an argument in a vicious circle: thus the alimentary canal is
+  homologous in all of the Metazoa, because it is formed of hypoblast, but
+  there is no definition of hypoblast, except that it is always that layer
+  which forms the definitive alimentary canal.
+
+  When, after the process of segmentation has been completed, a free
+  swimming blastula results, unprovided with any store of pabulum in the
+  shape of yolk, then the same physiological necessity causes such a form
+  to obtain its nutriment from the surrounding medium. The simplest way to
+  do this is by a process of invagination, in consequence of which food
+  particles are swept into the invaginated part and then absorbed. For this
+  reason in such cases true gastrulæ are formed, as in the case of
+  Amphioxus among the vertebrates, and Lucifer among the crustaceans; such
+  a formation does not in the least imply that the gut of the arthropod is
+  homologous with that of the vertebrate. The resemblance between the two
+  is not a morphological one, but due to the same physiological necessity.
+  They are analogous formations, not homologous.
+
+  The muscular tissues are found to be formed in close connection with the
+  nervous tissues, and in very many cases are described as formed from
+  epiblast, so that there are strong reasons for placing them in a special
+  category of the so-called mesoblastic tissues. If they be separated out,
+  then it seems to me, the rest of the mesoblast would consist of the
+  free-living cells of the body, which are not connected with the central
+  nervous system. In watching, then, the formation of mesoblast, defined in
+  this way, we are watching the separation out from the master-tissues of
+  the body of the independent skeletal and excretory cells.
+
+
+
+
+{488}CHAPTER XV
+
+_FINAL REMARKS_
+
+  Problems requiring investigation--
+
+  Giant nerve-cells and giant-fibres; their comparison in fishes and in
+  arthropods; blood- and lymph-corpuscles; nature of the skin; origin of
+  system of unstriped muscles; origin of the sympathetic nervous system;
+  biological test of relationship.
+
+  Criticism of Balanoglossus theory.--Theory of parallel
+  development.--Importance of the theory advocated in this book for all
+  problems of Evolution.
+
+
+The discussion in the last chapter on the "Principles of Embryology"
+completes the evidence which I am able to offer up to the present time in
+favour of my theory of the "Origin of Vertebrates." There are various
+questions which I have left untouched, but still are well worth discussion,
+and may be mentioned here. The first of these is the significance of the
+giant nerve-cells and giant nerve-fibres so characteristic of the
+brain-region of the lower vertebrates. In most fishes two very large cells
+are most conspicuous objects in any transverse section of the _medulla
+oblongata_ at the level of entrance of the auditory nerves. Each of these
+cells gives off a number of processes, some of which pass in the direction
+of the auditory nerves and one very large axis-cylinder process which forms
+a giant-fibre, known by the name of a Mauthnerian fibre. Each Mauthnerian
+fibre crosses the middle line soon after its origin from the giant-cell,
+and passes down the spinal cord on the opposite side right to the tail.
+Here, near the end of the spinal cord, it breaks up into smaller fibres,
+which are believed by Fritsch and others to pass out directly into the
+ventral roots to supply the muscles of the tail. Thus Bela Haller says:
+"The Mauthnerian fibres are known to give origin to certain fibres which
+supply the ventral roots of the last three spinal nerves, so that their
+terminal branches serve, in all probability, for the innervation of the
+muscles of the tail-fin." They do not occur in the eel, according to
+Haller, or in Silurus, according to Kölliker. {489}Their absence in those
+fishes, in which a well-developed tail-fin is also absent, increases the
+probability of the truth of Fritsch's original conclusion that these
+giant-fibres are associated axis-cylinders for certain definite
+co-ordinated movements of the fish, especially for the lateral movement of
+the tail.
+
+In Ammocoetes, instead of two Mauthnerian fibres, a number of giant-fibres
+are found. They are called Müllerian fibres, and arise from giant-cells
+which are divisible into two groups. The first group consists of three
+pairs situated headwards of the level of exit of the trigeminal nerves. Two
+of these lie in front of the level of exit of the oculomotor nerves, and
+one pair is situated at the same level as the origin of the oculomotor
+nerves. The second group consists of a number of cells on each side at the
+level of the entrance of the fibres of the auditory nerves.
+
+The Müllerian fibres largely decussate, as described by Ahlborn, and then
+become the most anterior portion of the white matter of the spinal cord,
+forming a group of about eight fibres on each side (Fig. 73). A few fibres
+are also found laterally, and slightly dorsally, to the grey matter. These
+giant-fibres pass down the spinal cord right to the anal region; their
+ultimate destination is unknown. Mayer considers that in the first part of
+their course they correspond to those tracts of fibres known as the
+"posterior longitudinal bundles" in other vertebrates. I imagine,
+therefore, that the spinal part of their course represents the two
+antero-lateral descending tracts. The second group of giant-cells, which
+appears to have some connection with the auditory nerves, may represent
+"Deiter's nucleus." The whole system is probably the central nervous part
+of a co-ordination mechanism, which arises entirely in the pro-otic or
+prosomatic region of the brain--the great co-ordinating and equilibrating
+region _par excellence_.
+
+If we turn now to the arthropod it is a striking coincidence that in the
+crayfish and in the lobster the work of Retzius, of Celesia, of Allen, and
+of many others demonstrates the existence of an equilibration-mechanism for
+the swimming movements of the tail-muscles, which is carried out by means
+of giant-fibres. These giant-fibres are the axis-cylinder processes of
+giant-cells, situated exclusively in the brain-region, and they run through
+the whole ventral ganglionic chain in order to supply the muscles of the
+tail. In the ventral nerve-cord of the crayfish, according to Retzius, two
+specially large {490}giant-fibres exist, each of which breaks up, at the
+last abdominal ganglion, into smaller fibres, which pass directly out with
+the nerves to the tail-fin. Allen has shown that, in addition to these two
+specially large giant-fibres, there are a number of others, some of which,
+similarly to the Müllerian fibres of Ammocoetes, cross the middle line,
+while some do not. Each of these arises from a large nerve-cell and passes
+to one or other of the last pair of abdominal ganglia. The latter fibres,
+he says, send off collaterals, while the two specially large giant-fibres
+do not. The cells which give origin to all these large, long fibres are
+situated in or in front of the prosomatic region of the brain, similarly to
+the giant-cells, which give rise to the corresponding Müllerian fibres of
+Ammocoetes. I do not know how far this system is represented in Limulus or
+Scorpio.
+
+It is, to my mind, improbable that the Mauthnerian fibres pass out directly
+as motor fibres to the muscles of the tail-fin; it is more likely that they
+are conducting paths between the equilibration-mechanism in connection with
+the VIIIth nerve and the spinal centres for the movements of the tail.
+Similarly, with respect to the arthropod, it is difficult to believe that
+the motor fibres for the tail-muscles arise in the brain-region. In either
+case, the striking coincidence remains that the movements of the tail-end
+of the body are regulated by means of giant-fibres which arise from
+giant-cells in the head-region of the body in both the Arthropoda and the
+lowest members of the Vertebrata.
+
+The meaning of this system of giant-cells and giant-fibres in both classes
+of animals is well worthy of further investigation.
+
+Another important piece of comparative work which ought to help in the
+elucidation of this problem is the comparison of the blood- and
+lymph-corpuscles of the vertebrate with those of the invertebrate groups.
+As yet, I have not myself made any observations in this direction, and feel
+that it is inadvisable to discuss the results of others until I know more
+about the facts from personal observation.
+
+The large and important question of the manner of formation of the
+vertebrate skin has only been considered to a slight extent. A much more
+thorough investigation requires to be made into the nature of the skin of
+the oldest fishes in comparison with the skin of Ammocoetes on the one
+side, and of Limulus and the Palæostraca on the other.
+
+The muscular system requires further investigation, not so much {491}the
+different systems of the striated voluntary musculature--for these have
+been for the most part compared in the two groups of animals in previous
+chapters--as the involuntary unstriped musculature, about which no word has
+been said. The origin of the different systems of unstriped muscles in the
+vertebrate is bound up with the origin of the sympathetic system and its
+relation to the cranial and sacral visceral systems. The reason why I have
+not included in this book the consideration of the sympathetic nervous
+system is on account of the difficulty in finding any such system in
+Ammocoetes. Also, so far as I know, the distribution of unstriped muscle in
+Ammocoetes has not been worked out.
+
+One clue has arisen quite recently which is of great importance, and must
+be worked out in the future, viz. the extraordinary connection which exists
+between the action of the sympathetic nervous system and the action of
+adrenalin. This substance, which is obtained from the medullary part of the
+adrenal or suprarenal glands, when injected into an animal produces the
+same effects as stimulation of the nerves, which belong to the
+lumbo-thoracic outflow of visceral nerves, _i.e._ the system known as the
+sympathetic nervous system, which is distinct from both the cranial and
+sacral outflows of visceral nerves. The similarity of its action to
+stimulation of nerves is entirely confined to the nerves of this
+sympathetic system, and never resembles that of either the cranial or
+sacral visceral nerves.
+
+Another most striking fact which confirms the great importance of this
+connection between the adrenals and the sympathetic nervous system from the
+point of view of the evolution of the latter system is that the extract of
+the adrenals always produces the same effect as that of stimulation of the
+nerves of the sympathetic system, whatever may be the animal from which the
+extract is obtained. Thus adrenalin obtained from the elasmobranch fishes
+will produce in the highest mammal all the effects known to occur upon
+stimulation of the nerves of its sympathetic system.
+
+Further, the cells, which are always associated with the presence of this
+peculiar substance--adrenalin--stain in a characteristic manner in the
+presence of chromic salts. In Ammocoetes patches of cells which stain in
+this manner have been described in connection with blood-vessels in certain
+parts, so that, although I know of no definite evidence of the existence of
+cell-groups in Ammocoetes corresponding to the ganglia of the sympathetic
+system in other vertebrates, it is {492}possible that further investigation
+into the nature and connection of these "chromaffine" cells may afford a
+clue to the origin of the sympathetic nervous system. At present it is
+premature to discuss the question further.
+
+Finally, another test as to the kinship of two animals of different species
+must be considered more fully than I have been able to do up to the present
+time. This test is of a totally different nature to any put forth in
+previous pages. It is known as the "biological test" of relationship, and
+is the outcome of pathological rather than of physiological or anatomical
+research. It is possible that this test may prove the most valuable of all.
+At present we do not know sufficiently its limitations and its sources of
+error, especially in the case of cold-blooded animals, to be able to look
+upon it as decisive in a problem of the kind considered in this book.
+
+The nature of this test is as follows: It has been found that the serum of
+the blood of another animal, when injected in sufficient quantity into a
+rabbit, will cause such a change in the serum of that rabbit's blood that
+when it is added to the serum of the other animal a copious precipitate is
+formed, although the serum of normal rabbit's blood when mixed with that of
+another animal will cause no precipitate whatever. This extraordinary
+production of a precipitate in the one case and not in the other indicates
+the production of some new substance in the rabbit's serum in consequence
+of the introduction of the foreign serum into the rabbit, which brings
+about a precipitate when the rabbit's serum containing it is mixed with the
+serum originally injected. The barbarous name "antibody" has been used to
+express this supposed substance in accordance with the meaning of such a
+word as "antitoxin," which has been a long time in use in connection with
+preventive remedies against pathogenic bacteria. Now, it is found that the
+rabbit's serum containing a particular "antibody" will cause a precipitate
+only when added to the serum of the blood of the animal from which the
+"antibody" was produced or to the serum of the blood of a nearly related
+animal.
+
+Further, if that animal is closely related a precipitate will be formed
+nearly as copious as with the original serum, if more distantly related a
+cloudiness will occur rather than a precipitate, and if the relationship is
+still more distant the mixture of the two sera will remain absolutely
+clear. Thus this test demonstrates the close relationship of man to the
+anthropoid apes and his more distant {493}relationship to monkeys in
+general. By this method very evident blood-relationships have been
+demonstrated, especially between members of the Mammalia.
+
+I therefore started upon an investigation into the possibility of proving
+relationship in this way between Limulus and Ammocoetes, with the kind
+assistance of Mr. Graham Smith. I must confess I was not sanguine of
+success, as I thought the distance between Limulus and Ammocoetes was too
+great. Dr. Lee, of New York, kindly provided me with most excellent serum
+of Limulus, and the first experiments showed that the anti-serum of Limulus
+gave a most powerful precipitate with its own serum. Graham Smith then
+tried this anti-serum of Limulus with the serum of Ammocoetes, and to his
+surprise, and mine, he obtained a distinct cloudiness, indicative of a
+relationship between the two animals. This, however, is not considered
+sufficient, the reverse experiment must also succeed. I therefore, with
+Graham Smith, obtained a considerable amount of blood from the adult
+lampreys at Brandon, and produced an anti-serum of Petromyzon, which gave
+some precipitate with its own serum, but not a very powerful one. This
+anti-serum tried with Limulus gave no result whatever, but at the same time
+it gave no result with serum from Ammocoetes, so that the experiment not
+only showed that Petromyzon was not related to Limulus, but also was not
+related to its own larval form, which is absurd.
+
+Considerable difficulties were encountered in preparing the Petromyzon
+anti-serum owing to the extreme toxic character of the lamprey's serum to
+the rabbit; in this respect it resembled that of the eel. It is possible
+that the failure of the lamprey's anti-serum was due to the necessity of
+heating the serum sufficiently to do away with its toxicity before
+injecting it into the rabbit. At this point the experiments have been at
+present left. It will require a long and careful investigation before it is
+possible to speak decisively one way or the other. At present the
+experiment is positive to a certain extent, and also negative; but the
+latter proves too much, for it proves that the larva is not related to the
+adult.
+
+Some day I hope this "biological test" will be of use for determining the
+relationships of the Tunicata, the Enteropneusta, Amphioxus, etc., as well
+as of Limulus and Ammocoetes.
+
+The origin of Vertebrates from a Palæostracan stock, as put forward in this
+book, gives no indication of the systematic position {494}of the Tunicata
+or Enteropneusta. Neither the Tunicata nor Amphioxus can by any possibility
+be on the direct line of ascent from the invertebrate to the vertebrate.
+They must both be looked upon as persistent failures, relics of the time
+when the great change to the vertebrate took place. The Enteropneusta are
+on a different footing; in their case any evidence of affinity with
+vertebrates is very much more doubtful.
+
+The observer Spengel, who has made the most exhaustive study of these
+strange forms, rejects _in toto_ any connection with vertebrates, and
+considers them rather as aberrant annelids. The so-called evidence of the
+tubular central nervous system is worth nothing. There is not the slightest
+sign of any tubular nervous system in the least resembling that of the
+vertebrate. It is simply that in one place of the collar-region the piece
+of skin containing the dorsal nerve of the animal, owing to the formation
+of the collar, is folded, and thus forms just at this region a short tube.
+My theory explains in a natural manner every portion of the elaborate and
+complicated tube of the vertebrate central nervous system. In the
+Balanoglossus theory the evolution of the vertebrate tube in all its
+details from this collar-fold is simple guesswork, without any reasonable
+standpoint. Similarly, the small closed diverticulum of the gut in
+Balanoglossus, which is dignified with the name of "notochord," has no
+right to the name. As I have already said, it may help to understand why
+the notochord has such a peculiar structure, but it gives no help to
+understanding the peculiar position of the notochord. The only really
+striking resemblance is between the gill-slits of Amphioxus and of the
+Enteropneusta. In this comparison there is a very great difficulty, very
+similar to that of the original attempts to derive vertebrates from
+annelids--the gill-slits open ventrally in the one animal and dorsally in
+the other. In both animals an atrial cavity exists which is formed by
+pleural folds, and in these pleural folds the gonads are situated so that
+the similarity of the two branchial chambers seems at first sight very
+complete. In the Enteropneusta, however, there are certain
+forms--Ptychodera--in which these pleural folds have not met in the
+mid-line in this branchial region, and in these it is plainly visible that
+these folds, with their gonads, spring from the ventral mid-line and arch
+over the dorsal region of the body. Equally clearly Amphioxus shows that
+its pleural folds, with the gonads, spring from the dorsal side of the
+animal, {495}and grow ventralwards until they fuse in the ventral mid-line
+(_cf._ Fig. 168).
+
+As far, then, as this one single striking similarity between Amphioxus and
+the Enteropneusta is concerned it necessitates the reversal of dorsal and
+ventral surfaces to bring the two branchial chambers into harmony.
+
+[Illustration: FIG. 168.--DIAGRAM ILLUSTRATING THE POSITION OF THE PLEURAL
+FOLDS AND GONADS IN PTYCHODERA (A) AND AMPHIOXUS (B) RESPECTIVELY.
+
+_Al._, alimentary canal; _D.A._, dorsal vessel; _V.A._, ventral vessel;
+_g._, gonads; _NC._, notochord; _C.N.S._, central nervous system.]
+
+In a mud-dwelling animal, like Balanoglossus, which possesses no
+appendages, no special sense-organs, it seems likely enough that ventral
+and dorsal may be terms of no particular meaning, and consequently what is
+called ventral in Balanoglossus may correspond to what is dorsal in
+Amphioxus; in this way the branchial regions of the two animals may be
+closely compared. Such comparison, however, immediately upsets the whole
+argument of the vertebrate nature of Balanoglossus based on the relative
+position of the central nervous system and gut, for now that part of its
+nervous system which is looked upon as the central nervous system in
+Balanoglossus is ventral to the gut, just as in a worm-like animal, and not
+dorsal to it as in a vertebrate.
+
+There is absolutely no possibility whatever of making such a detailed
+comparison between Balanoglossus and any vertebrate, as I have done between
+a particular kind of arthropod and Ammocoetes. In the latter case not only
+the topographical anatomy of the organs in the two animals is the same, but
+the comparison is valid even to microscopical structure. In the former case
+the origin of almost all {496}the vertebrate organs is absolutely
+hypothetical, no clue is given in Balanoglossus, not even to the segmented
+nature of the vertebrate. The same holds good with the evidence from
+Embryology and from Palæontology. I have pointed out how strongly the
+evidence in both cases confirms that of Comparative Anatomy. In neither
+case is the strength of the evidence for Balanoglossus in the slightest
+degree comparable. In Embryology an attempt has been made to compare the
+origin of the coelom in Amphioxus and in Balanoglossus. In Palæontology
+there is nothing, only an assumption that in the Cambrian and Lower
+Silurian times a whole series of animals were evolved between Balanoglossus
+and the earliest armoured fishes, which have left no trace, although they
+were able to hold their own against the dominant Palæostracan race. The
+strangeness of this conception is that, when they do appear, they are fully
+armoured, as in Pteraspis and Cephalaspis, and it is extremely hard luck
+for the believers in the Balanoglossus theory that no intermediate less
+armoured forms have been found, especially in consideration of the fact
+that the theory of the origin from the Palæostracan does not require such
+intermediate forms, but finds that those already discovered exactly fulfil
+its requirements.
+
+One difficulty in the way of accepting the theory which I have advocated is
+perhaps the existence of the Tunicata. I cannot see that they show any
+affinities to the Arthropoda, and yet they are looked upon as allied to the
+Vertebrata. I can only conclude that both they and Amphioxus arose late,
+after the vertebrate stock had become well established, so that in their
+degenerated condition they give indications of their vertebrate ancestry
+and not of their more remote arthropod ancestry.
+
+In conclusion, the way in which vertebrates arose on the earth as suggested
+in this book carries with it many important far-reaching conclusions with
+respect to the whole problem of Evolution.
+
+When the study of Embryology began, great hopes were entertained that by
+its means it would be possible to discover the pedigree of every group of
+animals, and for this end all the stages of development in all groups of
+animals were sought for and, as far as possible, studied. It was soon
+found, however, that the interpretation of what was seen was so difficult,
+as to give rise to all manner of views, depending upon the idiosyncrasy of
+the observer. At his will he decided whether any appearance was
+coenogenetic or palingenetic, {497}with the result that, in the minds of
+many, embryology has failed to afford the desired clue.
+
+At the same time, the geological record was looked upon as too imperfect to
+afford any real help; it was said, and is said, that the Cambrian and
+pre-Cambrian periods were so immense, and the animals discovered in the
+lower Silurian so highly organized, as to compel us to ascribe the
+origination of all the present-day groups to this immense early period, the
+animals of which have left no trace of their existence as fossils.
+
+In consequence of, or at all events following upon, the supposed failure of
+embryology and of geology to solve the problem of the sequence of evolution
+of animal life, a new theory has arisen, which goes very near to the denial
+of evolution altogether. This is the theory of parallel development. It
+discards the old picture of a genealogical tree with main branches arising
+at different heights, these again branching and branching into smaller and
+smaller twigs, and substitutes instead the picture of the ribs of a fan,
+every rib running independently of every other, each group represented by a
+rib reaching its highest development on the circumference of the fan and
+coming nearer and nearer to a common point at the handle of the fan. This
+point of convergence, where all the groups ultimately meet, is so far back
+as to reach to the lowest living organisms.
+
+This, in my opinion, unscientific and inconceivable suggestion has arisen
+largely in consequence of a conception which has become firmly fixed in the
+minds of very many writers on this subject--the conception that in the
+evolution of every group, the higher members of the group are the most
+specialized in the peculiarities of that group, and it is impossible to
+obtain a new group with different peculiarities from such specialized
+members. If, then, a higher group is to arise from a lower, it must arise
+from the generalized members of that lower group, in other words, from the
+lowest members or those nearly akin to the next lower group.
+
+Similarly, the highest members of this latter group are too specialized,
+and again we must go to the more generalized members of the group. In this
+way each separate specialized group is put on one side, and so the
+conception of parallel development comes into being.
+
+The evidence given in this book dealing with the origin of vertebrates
+strikes at the foundations of this belief, for it presents an {498}image of
+the sequence of evolution of animal forms in orderly upward progress,
+caused by the struggle for existence among the members of the race dominant
+at the time, which brought about the origin of the next higher group not
+from the lowest members of the dominant group, but from some one of the
+higher members of that group.
+
+The great factor in evolution has been throughout the growth of the central
+nervous system; from that group of animals which possessed the highest
+nervous system evolved up to that time the next higher group must have
+arisen.
+
+In this way we can trace without a break, always following out the same
+law, the evolution of man from the mammal, the mammal from the reptile, the
+reptile from the amphibian, the amphibian from the fish, the fish from the
+arthropod, the arthropod from the annelid, and we may be hopeful that the
+same law will enable us to arrange in orderly sequence all the groups in
+the animal kingdom.
+
+This very same law of the paramount importance of the development of the
+central nervous system for all upward progress will, I firmly believe, lead
+to the establishment of a new and more fruitful embryology, the leading
+feature of which will be, as suggested in the last chapter, not the attempt
+to derive from the blastula three germ-layers common to all animals, but
+rather two sets of organs--those which are governed by the nervous system
+and those which are not--and thus by means of the development of the
+central nervous system obtain from embryology surer indications of
+relationship than are given at present.
+
+The great law of recapitulation, which asserts that the past history of the
+race is indicated more or less in the development of each individual, a law
+which of late years has fallen somewhat into disrepute, owing especially to
+the difficulty of interpreting the embryological history of the vertebrate,
+is triumphantly vindicated by the theory put forward in this book. Each
+separate vertebrate organ, one after the other, as shown in the last
+chapter, indicates in its development the manner in which it arose from the
+corresponding organ of the arthropod. There is no failure in the evidence
+of embryology, the failure is in the interpretation thereof.
+
+So, too, my theory vindicates the geological method. There is no failure
+here; on the contrary, the record of the rocks proclaims with startling
+clearness not only the sequence of evolution in the {499}vertebrate kingdom
+itself, but the origin of the vertebrate from the most highly-developed
+invertebrate race.
+
+The study of the comparative anatomy of organs down to the finest details
+has always been a most important aid in finding out relationship between
+animals or groups of animals. My theory endorses this view to the
+uttermost, and especially indicates the study of the central nervous system
+and its outgoing nerves as that comparative study which is most likely to
+afford valuable results.
+
+As for the individual, so for the nation; as for the nation, so for the
+race; the law of evolution teaches that in all cases brain-power wins.
+Throughout, from the dawn of animal life up to the present day, the
+evidence given in this book suggests that the same law has always held. In
+all cases, upward progress is associated with a development of the central
+nervous system.
+
+The law for the whole animal kingdom is the same as for the individual.
+"Success in this world depends upon brains."
+
+
+
+
+{501}BIBLIOGRAPHY AND INDEX OF AUTHORS
+
+  --------------+------------------------------------------+---------------
+  Author's name.|            Title of Paper.               |    Pages of
+                |                                          |   reference.
+  --------------+------------------------------------------+---------------
+  AHLBORN       |"Untersuchungen über das Gehirn der       | 210, 489
+                | Petromyzonten"                           |
+                |  _Zeitsch. f. wiss. Zool._ Vol. 39. 1883 |
+                |                                          |
+                |"Ueber die Segmentation des               | 260
+                | Wirbelthierkörpers"                      |
+                |  _Zeitsch. f. wiss. Zool._ Vol. 40. 1884 |
+                |                                          |
+  AICHEL        |"Vergleichende Entwicklungsgeschichte     | 424, 428
+                | und Stammesgeschichte der Nebennieren"   |
+                |     _Arch. f. Mikr. Anat._ Vol. 56. 1900 |
+                |                                          |
+  ALCOCK        |                                          | 135, 287, 288,
+                |                                          | 289, 304, 307,
+                |                                          | 347, 445
+                |                                          |
+                |"The Peripheral Distribution of the       | 164, 171, 177,
+                | Cranial Nerves of Ammocoetes"            | 188, 202, 297,
+                |           _Journ. of Anat. and Physiol._ | 300, 310, 311,
+                |                            Vol. 33. 1898 | 316
+                |                                          |
+                |"On Proteid Digestion in Ammocoetes"      | 58, 213, 442,
+                |           _Journ. of Anat. and Physiol._ |
+                |                            Vol. 33. 1898 | 452
+                |                                          |
+  ALLEN         |"Studies on the Nervous System of         | 489
+                | Crustacea"                               |
+                |         _Q. J. Micr. Sci._ Vol. 36. 1894 |
+                |                                          |
+  ANDERSON,     |                                          | 448, 470
+   H. K.        |                                          |
+                |"The Nature of the Lesions which hinder   | 466, 467, 469
+                | the Development of Nerve-cells and their |
+                | Processes                                |
+                |       _Journ. of Physiol._ Vol. 28. 1902 |
+                |                                          |
+                |"On the Myelination of Nerve-fibres"      | 467, 477
+                |         _Report of the Brit. Assn._ 1898 |
+                |                                          |
+  APATHY        |"Das leitende Element des Nervensystems   | 467
+                | und seine topographischen Beziehung zu   |
+                | den Zellen"                              |
+                |    _Mitth. a. d. Zool. Stat. zu Neapel._ |
+                |                            Vol. 12. 1896 |
+                |                                          |
+  ASSHETON      |"On the Phenomenon of the Fusion of the   | 42
+                | Epiblastic Layers in the Rabbit and in   |
+                | the Frog"                                |
+                |         _Q. J. Micr. Sci._ Vol. 37. 1894 |
+                |                                          |
+                |"An Experimental Examination into the     | 154
+                | Growth of the Blastoderm of the Chick"   |
+                |       _Proc. of Roy. Soc._ Vol. 60. 1896 |
+                |                                          |
+  ASSHETON      |"On the Growth in Length of the Frog      | 154
+                | Embryo"                                  |
+                |         _Q. J. Micr. Sci._ Vol. 37. 1894 |
+                |                                          |
+                |"A Re-investigation into the Early Stages | 154
+                | of the Development of the Rabbit"        |
+                |         _Q. J. Micr. Sci._ Vol. 37. 1894 |
+                |                                          |
+                |"The Primitive Streak of the Rabbit: the  | 154
+                | Causes which may determine its Shape,    |
+                | and the part of the Embryo formed by its |
+                | Activity"                                |
+                |         _Q. J. Micr. Sci._ Vol. 37. 1894 |
+                |                                          |
+  BALFOUR       |'Comparative Embryology.' Vol. 2          | 73, 74, 94,
+                |            London. 1881. Macmillan & Co. | 103, 104, 120,
+                |                                          | 181, 259, 424
+                |                                          |
+                |"On the Origin and History of the         | 390, 392
+                | Urino-genital Organs of Vertebrates"     |
+                |           _Journ. of Anat. and Physiol._ |
+                |                            Vol. 10. 1876 |
+                |                                          |
+                |"On the Nature of the Organ in Adult      | 420
+                | Teleosteans and Ganoids, which is usually|
+                | regarded as the Head-kidney or           |
+                | Pronephros"                              |
+                |         _Q. J. Micr. Sci._ Vol. 22. 1882 |
+                |                                          |
+  BARKER        |'The Nervous System'                      | 470
+                |                             London. 1901 |
+                |                                          |
+  BATESON       |"The Ancestry of the Chordata"            | 11
+                |         _Q. J. Micr. Sci._ Vol. 26. 1886 |
+                |                                          |
+                |'Materials for the Study of Variation'    | 387
+                |                             London. 1894 |
+                |                                          |
+  BEARD         |"The System of Branchial Sense Organs and | 262, 281, 283
+                | their Associated Ganglia in Ichthyopsida"|
+                |         _Q. J. Micr. Sci._ Vol. 26. 1885 |
+                |                                          |
+                |"The Development of the Peripheral        | 262, 281, 283
+                | Nervous System in Vertebrates"           |
+                |         _Q. J. Micr. Sci._ Vol. 29. 1888 |
+                |                                          |
+                |"The Old Mouth and the New"               | 318
+                |                   _Anat. Anzeiger._ 1888 |
+                |                                          |
+                |"The Source of Leucocytes and the True    | 425, 426
+                | Function of the Thymus"                  |
+                |          _Anat. Anzeiger._ Vol. 18. 1900 |
+                |                                          |
+                |"The Parietal Eye of the Cyclostome       | 84
+                | Fishes"                                  |
+                |         _Q. J. Micr. Sci._ Vol. 29. 1882 |
+                |                                          |
+  BECK AND      |"On the Muscular and Endo-skeletal        | 171, 222, 224,
+   LANKESTER    | Tissues of Scorpio"                      | 247, 268-277
+                |        _Trans. Zool. Soc._ Vol. 11. 1885 |
+                |                                          |
+  BEECHER       |"Natural Classification of the Trilobites"| 283, 351, 436,
+                |                   _Amer. Journ. of Sci._ | 437
+                |                     Ser. 4. Vol. 3. 1897 |
+                |                                          |
+  BELL, C.      |'The Nervous System of the Human Body'    | 155, 156, 183
+                |                             London. 1830 |
+                |                                          |
+  BELLONCI      |"Système Nerveux et Organes des sens du   | 62, 90, 92,
+                | _Sphæroma serratum_"                     | 101
+                |    _Archiv. Ital. de Biol._ Vol. 1. 1882 |
+                |                                          |
+                |"Sur la structure et les rapports des     | 221, 225
+                | lobes olfactives dans les Arthropods     |
+                | superieurs et les Vertébrés"             |
+                |    _Archiv. Ital. de Biol._ Vol. 3. 1883 |
+                |                                          |
+  BENHAM AND    |"On the Muscular and Endo-skeletal        | 143, 171, 176,
+   LANKESTER    | Systems of Limulus"                      | 177, 247
+                |        _Trans. Zool. Soc._ Vol. 11. 1885 |
+                |                                          |
+  BERGER        |"Untersuchungen über den Bau des Gehirns  | 88-92, 97,
+                | und der Retina der Arthropoden"          | 100, 101
+                |      _Arbeit. a. d. Zool. Instit. Wien._ |
+                |                             Vol. 1. 1878 |
+                |                                          |
+  BERGH         |"Neue Beiträge zur Embryologie der        | 478
+                | Anneliden"                               |
+                |  _Zeitsch. f. wiss. Zool._ Vol. 50. 1890 |
+                |                                          |
+  BERKLEY       |"The Intrinsic Nerves of the Kidney"      | 477
+                | _Bulletin of the Johns Hopkins Hospital._|
+                |                                   Vol. 4 |
+                |                                          |
+  BERNARD       |'The Apodidæ: a Morphological Study'      | 284
+                |                    _Nature Series._ 1892 |
+                |                                          |
+  BERTKAU       |"Beiträge zur Kenntniss der Sinnesorgane  | 369
+                | der Spinnen. 1. Die Augen der Spinnen"   |
+                |   _Archiv. f. mikr. Anat._ Vol. 27. 1886 |
+                |                                          |
+  BIEDERMANN    |'Electro-physiology'                      | 20
+                |  Translated by F. A. Welby. London. 1896 |
+                |                                          |
+  BLANCHARD     | Quoted by Huxley                         | 225
+                |                                          |
+                |'L'Organisation du Règne Animal.          | 109, 177, 190,
+                | Arachnides'                              | 206, 313, 315
+                |                              Paris. 1852 |
+                |                                          |
+  BLES          |"The Correlated Distribution of Abdominal | 431
+                | Pores and Nephrostomes in Fishes"        |
+                |           _Journ. of Anat. and Physiol._ |
+                |                            Vol. 32. 1898 |
+                |                                          |
+  BOBRETSKY     |'Development of Astacus and Palæmon'      | 74
+                |                               Kiew. 1873 |
+                |                                          |
+  BOURNE AND    | _See_ Lankester and Bourne.              |
+   LANKESTER    |                                          |
+                |                                          |
+  BOVERI        |"Die Nieren Canälchen des Amphioxus"      | 392, 395, 402,
+                |           _Zool. Jahrbuch._ Vol. 5. 1892 | 407, 412, 426,
+                |                                          | 427
+                |                                          |
+  BRAEM         |"Was ist ein Keimblatt"                   | 460, 461, 462
+                |      _Biol. Centralblatt._ Vol. 15. 1895 |
+                |                                          |
+  BRAUER        |"Beiträge zur Kenntniss der               |
+                | Entwicklungsgeschichte des Skorpions"    | 62, 167, 222,
+                |                   _Zeit. f. wiss. Zool._ | 237, 281, 482
+                |                    Part I. Vol. 57. 1894 |
+                |                   Part II. Vol. 59. 1895 |
+                |                                          |
+                |"Beiträge zur Kenntniss der Entwicklung   | 393, 394, 400,
+                | und Anatomie der Gymnophionen." III.     | 402
+                | "Die Entwicklung der Excretionsorgane"   |
+                |          _Zool. Jahrbuch._ Vol. 16. 1902 |
+                |                                          |
+                |"Ueber die Entwicklung von Hydra"         | 473
+                |     _Zeit. f. wiss. Zool._ Vol. 52. 1891 |
+                |                                          |
+  BÜTSCHLI      |"Notiz zur Morphologie des Auges der      | 114
+                | Muscheln"                                |
+                |         _Festschrift des Natur-hist-med. |
+                |             Vereins zu Heidelberg._ 1886 |
+                |                                          |
+  BUJOR         |"Contribution a l'étude de la métamorphose| 135, 304
+                | de _l'Ammocoetes branchialis_ en         |
+                | _Petromyzon Planeri_"                    |
+                | _Revue Biologique du Nord de la France._ |
+                |                             Vol. 3. 1891 |
+                |                                          |
+  CARLSON       |                                          | 177, 315, 316
+                |                                          |
+  CELESIA       |'Differenziamento della proprietà         | 489
+                | inibitoria e dei funzioni coordinatrici  |
+                | nella catena gangliare dei crustacei     |
+                | decapodi'                                |
+                |                              Genoa. 1897 |
+                |                                          |
+  CLAUS         |"Untersuchungen über den Organismus und   | 90-92, 97,
+                | Entwicklung von Branchipus und Artemia"  | 100, 396
+                |      _Arbeit a.d. Zool. Institut. Wien._ |
+                |                             Vol. 6. 1886 |
+                |                                          |
+  COPE          |"On the Phylogeny of the Vertebrata"      | 343
+                | _Proc. Amer. Philos. Soc._ Vol. 30. 1892 |
+                |                                          |
+  CRONEBERG     |"Ueber die Mundtheile der Arachniden"     | 221-224, 241
+                |       _Archiv. f. Naturgeschichte._ 1880 |
+                |                                          |
+  CUÉNOT        |"Études sur le sang et les glandes        | 422
+                | lymphatiques dans la série animale;      |
+                | 2nd partie; invertébrés"                 |
+                |             _Arch. d. Zool. exper. gen._ |
+                |                    2nd Ser. Vol. 9. 1891 |
+                |                                          |
+  CUNNINGHAM,   |"The Significance of Kupffer's Vesicle,   | 318
+   J. T.        | with Remarks on other Questions of       |
+                | Vertebrate Morphology"                   |
+                |         _Q. J. Micr. Sci._ Vol. 25. 1885 |
+                |                                          |
+                |"The Nephridia of _Lanice conchilega_"    | 403
+                |                  _Nature._ Vol. 36. 1887 |
+                |                                          |
+  DANA          |"On Cephalization"                        | 53
+                |                _Mag. of Nat. Hist._ 1863 |
+                |                                          |
+  DEAN-BASHFORD |'Fishes, Living and Fossil'               | 344
+                |                           New York. 1895 |
+                |                                          |
+                |"On the Embryology of _Bdellostoma        | 405
+                | Stouti_"                                 |
+                |  _Festschr. z. siebenzigsten Geburtstag. |
+                |           von C. v. Kupffer._ Jena. 1899 |
+                |                                          |
+  DENDY         |"On the Parietal Sense-organs and         | 80, 82
+                | Associated Structures in the New Zealand |
+                | Lamprey (_Geotria australis_)"           |
+                |         _Q. J. Micr. Sci._ Vol. 51. 1907 |
+                |                                          |
+  DIETL         |"Die Organisation des Arthropoden Gehirns"| 101
+                |  _Zeitsch. f. wiss. Zool._ Vol. 27. 1876 |
+                |                                          |
+  DOHRN         |'Der Ursprung der Wirbelthiere und das    | 14, 60, 185,
+                | Princip des Functionswechsels'           | 186, 317, 318
+                |                            Leipzig. 1875 |
+                |                                          |
+                | Studien zur Urgeschichte des Wirbelthiere| 188, 195-198,
+                | Körpers. VIII. "Die Thyroidea bei        | 199, 212, 213
+                | Petromyzon, Amphioxus, und Tunicaten"    |
+                |          _Mitth. Zool. Stat. z. Neapel._ |
+                |                             Vol. 6. 1886 |
+                |                                          |
+                |"Neue Grundlagen zur Beurtheilung der     | 262, 263, 279
+                | Metamerie des Kopfes"                    |
+                |          _Mitth. Zool. Stat. z. Neapel._ |
+                |                             Vol. 9. 1890 |
+                |                                          |
+                | Studien zur Urgeschichte des Wirbelthiere| 167, 314, 337
+                | Gefässe Körpers. XIII. "Ueber Nerven     |
+                | und bei Ammocoetes und _Petromyzon       |
+                | Planeri_"                                |
+                |          _Mitth. Zool. Stat. z. Neapel._ |
+                |                             Vol. 8. 1888 |
+                |                                          |
+  DREVERMANN    |"Ueber _Pteraspis dunensis_"              | 29, 30
+                |             _Zeitschr. d. Deutsch. Geol. |
+                |             Gesellschaft._ Vol. 56. 1904 |
+                |                                          |
+  EDGEWORTH     |"The Development of the Head-muscles in   | 266
+                | _Gallus domesticus_, and the Morphology  |
+                | of the Head-muscles in the Sauropsida"   |
+                |         _Q. J. Micr. Sci._ Vol. 51. 1907 |
+                |                                          |
+  EDINGER       |'Anatomy of Central Nervous System in Man | 17, 264
+                | and in Vertebrates'                      |
+                |                 Translated by Hall. 1899 |
+                |                                          |
+  v. EICHWALD   |"Die Thier- und Pflanzenreste des alten   | 327
+                | rothen Sandsteins und Bergkalks im       |
+                | Nowgorodschen Gouvernement"              |
+                |            _Bull. Sci. de l'Acad. Impér. |
+                |                d. St. Petersbourg._ 1840 |
+                |                                          |
+  EISIG         |"Die Seiten-organe und becherförmigen     | 357
+                | Organe der Capitelliden"                 |
+                |    _Mitth. a. d. Zool. Stat. z. Neapel._ |
+                |                             Vol. 1. 1879 |
+                |                                          |
+                |"Capitelliden"                            | 357
+                |    _Faun. u. Flor. d. Golfes v. Neapel._ |
+                |                            Vol. 16. 1887 |
+                |                                          |
+  ELLIOTT       |"On the Innervation of the Ileo-colic     | 449
+                | Sphincter"                               |
+                |       _Journ. of Physiol._ Vol. 31. 1904 |
+                |                                          |
+  EMERY         | Quoted by Weldon                         | 420
+                |                                          |
+  FOSTER, M.    | Text-book of Physiology                  | 108
+                |                                          |
+  FREUND        |"Die Beziehungen der Schilddrüse zu den   | 215
+                | weiblichen Geschlechtsorganen"           |
+                |         _Deutsch. Zeitsch. f. Chirugie._ |
+                |                            Vol. 18. 1883 |
+                |                                          |
+  FRITSCH, G.   |'Untersuchungen über den feineren Bau des | 488, 489
+                | Fischgehirns'                            |
+                |                             Berlin. 1878 |
+                |                                          |
+  FRORIEP       |"Ueber Anlagen von Sinnesorganen am       | 261, 262, 281,
+                | Facialis, Glossopharyngeus und Vagus,    | 283
+                | über die genetische Stellung des Vagus   |
+                | zum Hypoglossus, und über die Herkunft   |
+                | der Zungenmusculatur"                    |
+                |              _Arch. f. Anat. u. Physiol; |
+                |                     Anat. Abtheil._ 1885 |
+                |                                          |
+  FÜRBRINGER,   |'Ueber die Spino-occipetalen Nerven der   |
+   M.           | Selachier und Holocephalen'              | 276-278, 409
+                |    Fest-schrift für Carl Gegenbaur. 1897 |
+                |                                          |
+  GAUBERT       |'Recherches sur les organes des sens et   | 364, 368-375
+                | sur les systèmes tegumentaire,           |
+                | glandulaire et musculaire des appendices |
+                | des arachnides'                          |
+                |                              Paris. 1892 |
+                |                                          |
+  GEGENBAUR     |"Anatomische Untersuchung eines Limulus"  | 20, 358-360
+                |     _Abhandl. d. Naturforsch. Gesellsch. |
+                |                  z. Halle._ Vol. 4. 1858 |
+                |                                          |
+                |"Ueber die Skeletgewebe der Cyclostomen"  | 181
+                |         _Jen. Zeitschrift._ Vol. 5. 1870 |
+                |                                          |
+                | Untersuchungen zur vergleichende Anatomie| 151, 259, 261
+                | der Wirbelthiere III. Heft. 'Das         |
+                | Kopfskeletder Selachiern'                |
+                |                            Leipzig. 1872 |
+                |                                          |
+                |'Grundriss der vergleichenden Anatomie'   | 392
+                |                            Leipzig. 1878 |
+                |                                          |
+  v. GEHUCHTEN  |"De l'origine du pathétique et de la      | 264
+                | racine supérieure du trijumeau"          |
+                |          _Acad. d. Sci. Belg. Bulletin._ |
+                |                   3rd Ser. Vol. 29. 1895 |
+                |                                          |
+  GOETHE        |                                          | 258
+                |                                          |
+  GÖTTE         |'Entwicklungsgeschichte der Unke'         | 101, 102, 114
+                |                            Leipzig. 1875 |
+                |                                          |
+  GOLGI         |                                          | 72, 465, 477
+                |                                          |
+  GOODRICH      |"On the Structure of the Excretory Organs |
+                | of Amphioxus"                            |
+                |         _Q. J. Micr. Sci._ Vol. 45. 1902 | 395, 396, 477
+                |                                          |
+                |"On the Nephridia of the Polychæta."      | 395
+                | Parts I., II., III.                      |
+                |      _Q. J. Micr. Sci._ Vols. 40, 41, 43 |
+                |                                          |
+                |"On the Excretory Organs of Amphioxus"    | 477
+                |          _Proc. Roy. Soc._ Vol. 69. 1902 |
+                |                                          |
+  GRABER        |"Die Chordo-tonalen Sinnesorgane und das  | 364, 369-371
+                | Gehör der Insecten"                      |
+                |                 _Archiv. f. Mikr. Anat._ |
+                |                    Vols. 20 and 21. 1882 |
+                |                                          |
+  GRENACHER     |'Untersuchungen über das Sehorgan der     | 76, 100
+                | Arthropoden'                             |
+                |                          Göttingen. 1879 |
+                |                                          |
+  GUDDEN        | Quoted in Obersteiner                    | 264
+                |                                          |
+  HAECKEL       |                                          | 461, 462
+                |                                          |
+  HALLER, BELA  |"Untersuchungen über die Hypophyse und    | 320, 321
+                | die Infundibulärorgane"                  |
+                |         _Morph. Jahrbuch._ Vol. 25. 1898 |
+                |                                          |
+                |"Untersuchungen über das Rückenmark der   | 488
+                | Teleostier"                              |
+                |         _Morph. Jahrbuch._ Vol. 23. 1895 |
+                |                                          |
+  HARDY         |"On the Histological Features and         | 110, 159
+                | Physiological Properties of the          |
+                | Post-oesophageal Nerve-cord of the       |
+                | Crustacea"                               |
+                |        _Phil. Trans. Roy. Soc._ 1894. B. |
+                |                                          |
+  HARDY AND     |"On the Structure and Functions of the    | 112, 206
+   MACDOUGALL   | Alimentary Canal of Daphnia"             |
+                |    _Proc. Camb. Phil. Soc._ Vol. 8. 1893 |
+                |                                          |
+  HATSCHEK      |"Die Metamerie des Amphioxus und des      | 289, 300, 337
+                | Ammocoetes"                              |
+                |       _Anat. Anzeig._, 7 Jahrgang, 1892. |
+                |    _Verhandl. d. Anat. Gesell. in Wien_, |
+                |                                   p. 136 |
+                |                                          |
+                |"Studien über Entwicklung des Amphioxus"  |  407
+                |        _Arbeit. d. Zool. Inst. z. Wien._ |
+                |                             Vol. 4. 1881 |
+                |                                          |
+                | Quoted by Lankester                      | 475
+                |                                          |
+  HAZEN         | _See_ Patten and Hazen.                  |
+                |                                          |
+  HEIDENHAIN    |                                          | 258, 259
+                |                                          |
+  HEIDER        | _See_ Korschelt and Heider.              |
+                |                                          |
+  HENSEN        |"Zur Entwicklung des Nervensystem"        | 465, 466
+                |         _Virchows Archiv._ Vol. 30. 1864 |
+                |                                          |
+  HENSEN AND    | _Archiv. f. Opthalmol._ Vol. 24. 1878    | 265, 266
+   VÖLCKERS     |                                          |
+                |                                          |
+  HERTWIG, O.,  | Quoted in Zeigler's 'Lehrbuch der        |  485
+   AND SAMASSA  | vergleichenden Entwicklungsgeschichte    |
+                | der niederen Wirbelthiere.' 1902         |
+                |                                          |
+  HIS           |"Die Neuroblasten und deren Entstehung    | 465, 466
+                | im embryonalen Mark"                     |
+                |            _Archiv. f. Anat. u. Physiol. |
+                |                        Anat. Abth._ 1889 |
+                |                                          |
+  HOFFMANN      |"Ueber die Metamerie des Nachhirns und    | 276
+                | Hinterhirns, und ihre Beziehung zu den   |
+                | segmentalen Kopfnerven bei Reptilien     |
+                | embryonen"                               |
+                |          _Zool. Anzeiger._ Vol. 12. 1889 |
+                |                                          |
+  HOLM          |"Ueber die Organisation des _Eurypterus   | 192, 240, 241,
+                | Fischeri_"                               | 306
+                |         _Mem. d. l'Acad. Imp. d. Sci. d. |
+                |           St. Petersbourg._ Vol. 8. 1898 |
+                |                                          |
+  HOYER         |"Ueber den Nachweis des Mucins in Geweben | 131
+                | Mittelst der Färbe-Methode"              |
+                |   _Archiv. f. Mikr. Anat._ Vol. 36. 1890 |
+                |                                          |
+  HUXLEY        |"Hunterian Lectures." 1869                | 124, 258, 259
+                |                                          |
+                |"On the Structure of the Mouth and Pharynx| 222, 225, 271
+                | of the Scorpion"                         |
+                |          _Q. J. Micr. Sci._ Vol. 8. 1860 |
+                |                                          |
+                |"On the Anatomy and Affinities of the     | 238
+                | Genus Pterygotus"                        |
+                |              _Mem. of the Geol. Survey._ |
+                |                        Monograph I. 1859 |
+                |                                          |
+                |"On Cephalaspis and Pteraspis"            | 327
+                |      _Q. J. of Geol. Soc._ Vol. 14. 1858 |
+                |                                          |
+  JAEKEL        |"Ueber Tremataspis und Patten's Ableitung | 329, 339, 340,
+                | der Wirbelthiere von Arthropoden"        | 351
+                |         _Protocoll der Deutschen Geolog. |
+                |    Gesellschaft_, p. 84; in _Zeitsch. d. |
+                |       Deutschen Geologischen Gesellsch._ |
+                |                            Vol. 55. 1903 |
+                |                                          |
+                |"Ueber die Organisation und systematische | 345
+                | Stellung der Asterolepiden"              |
+                |                           _Ibid._, p. 41 |
+                |                                          |
+  JOHNSON       |"Contributions to the Comparative Anatomy | 70
+                | of the Mammalian Eye, chiefly based on   |
+                | Opthalmoscopic Examination"              |
+                |              _Phil. Trans. Roy. Soc. B._ |
+                |                           Vol. 194. 1901 |
+                |                                          |
+  JOSEPH        |"Ueber das Achsenskelett des Amphioxus"   | 444
+                |  _Zeitsch. f. wiss. Zool._ Vol. 59. 1895 |
+                |                                          |
+  JULIN AND     | Recherches sur l'Organisation des        | 425
+   VAN BENEDEN  | Ascidies simples. "Sur l'hypophyse," etc.|
+                |     _Archives de Biologie._ Vol. 2. 1881 |
+                |                                          |
+  KAENSCHE      |"Beiträge zur Kenntniss der Metamorphose  | 135, 304
+                | des _Ammocoetes branchialis_ in          |
+                | _Petromyzon_"                            |
+                |     _Schneider's Beiträge._ Vol. 2. 1890 |
+                |                                          |
+  v. KENNEL     |"Entwickelungsgeschichte von _Peripatus   | 398, 399, 411
+                | Edwardsii_ und _Peripatus torquatus_."   |
+                | II. Theil                                |
+                |       _Arbeit. a. d. Zool. Zoot. Instit. |
+                |                  Würzburg._ Vol. 8. 1888 |
+                |                                          |
+  KERR          |"On some Points in the Early Development  | 461, 466, 478
+                | of Motor Nerve-trunks and Myotomes in    |
+                | _Lepidosiren paradoxa_"                  |
+                |   _Trans. Roy. Soc. Edin._ Vol. 41. 1904 |
+                |                                          |
+  KILLIAN       |"Zur Metamerie des Selachierkopfes"       | 262
+                |              _Verhandl. d. Anat. Gesell. |
+                |              Versamml. in München._ 1891 |
+                |                                          |
+  KISHINOUYE    |"On the Development of _Limulus           | 167, 238, 252,
+                | longispina_"                             | 253, 273, 320,
+                |        _Journ. of Coll. of Sci., Tokio._ | 382
+                |                             Vol. 5. 1891 |
+                |                                          |
+  KLEINENBERG   | Quoted by Beard                          | 318
+                |                                          |
+  v. KÖLLIKER   |"Die obere Trigeminus-Wurzel"             | 280
+                |     _Arch. f. Mikr. Anat._ Vol. 53. 1899 |
+                |                                          |
+  v. KÖLLIKER   |                                          |
+   AND          | Handbuch der Gewebe-Lehre. 6th Auflage.  | 264, 425, 488
+   TERTERJANZ   | 1893                                     |
+                |                                          |
+  KOHL          |"Rudimentäre Wirbelthieraugen"            | 94, 96, 99,
+                |      _Bibliotheca Zoologica. Leukart und | 101
+                |                 Chun._ Vol. 4 and Vol. 5 |
+                |                                          |
+  KOHN          |"Ueber den Bau und die Entwicklung der    | 428
+                | sogenannten Carotis-drüse"               |
+                |   _Archiv. f. Mikr. Anat._ Vol. 56. 1900 |
+                |                                          |
+  KORSCHELT AND |'Text-book of the Embryology of the       | 27, 73, 88,
+   HEIDER       | Invertebrates.' Translated by M. Bernard.| 114-116, 397,
+                | 1900. Part III. and Part IV.             | 429, 431
+                |                                          |
+  KOWALEWSKY    |"Ein Beitrag zur Kenntniss der            | 421
+                | Excretionsorgane der Pantopoden"         |
+                |  _Mem. d. l'Acad. d. Imp. d. Sci. d. St. |
+                |    Petersbourg._ Ser. VII. Vol. 38. 1890 |
+                |                                          |
+                |"Une nouvelle glande lymphatique chez le  | 423
+                | scorpion d'Europe"                       |
+                |          _Ibid._ Ser. VIII. Vol. 5. 1897 |
+                |                                          |
+                |"Étude Biologique sur les Clepsines"      | 421
+                |          _Ibid._ Ser. VIII. Vol. 5. 1897 |
+                |                                          |
+                |"Ein Beitrag zur Kenntniss der            | 420, 422, 472
+                | Excretionsorgane"                        |
+                |        _Biologisches Centralblatt._ 1889 |
+                |                                          |
+                |"Weitere Studien über die                 | 409, 410
+                | Entwicklungsgeschichte des               |
+                | _Amphioxus lanceolatus_"                 |
+                |   _Archiv. f. Mikr. Anat._ Vol. 13. 1877 |
+                |                                          |
+  KRIEGER       |"Ueber das Centralnervensystem des        | 101
+                | Flusskrebses                             |
+                |  _Zeitsch. f. wiss. Zool._ Vol. 33. 1880 |
+                |                                          |
+  v. KUPFFER    |'Studien zur vergleichenden Entwicklungs- |
+                | geschichte des Kopfes der Kranioten.'    |
+                |   Heft. 1. 'Die Entwicklung des Kopfes   | 318, 319, 320,
+                |   von _Acipenser_'                       | 440
+                |                            München. 1893 |
+                |                                          |
+                |   Heft. 2. 'Die Entwicklung des Kopfes   | 300, 440
+                |   von _Ammocoetes Planeri_'              |
+                |                            München. 1894 |
+                |                                          |
+                |   Heft. 3. 'Die Entwicklung der          | 228, 263, 282,
+                |   Kopfnerven von _Ammocoetes Planeri_.'  | 283, 405, 458
+                |    Dritter Abschnitt. 'Die Metamorphose  |
+                |    des larvalen Nervensystems des Kopfes'|
+                |                            München. 1895 |
+                |                                          |
+  LANG          |'Text-book of Comparative Anatomy.'       | 357
+                | Translated by H. M. and M. Bernard       |
+                |                                          |
+  LANGERHANS    |"Untersuchungen über _Petromyzon          | 94-101, 301,
+                | Planeri_"                                |
+                | _Bericht v. d. Verhandl. d. Naturforsch. | 405
+                |            Gesellsch. z. Freiburg._ 1873 |
+                |                                          |
+  LANGLEY       | Schäfer's 'Text-book of Physiology.'     | 2, 3, 448
+                | Vol. 2. 1900                             |
+                |                                          |
+  LANKESTER     | Article "Vertebrata" in the              | 484
+                | 'Encyclopædia Britannica'                |
+                |                                          |
+                |"On the Skeleto-trophic Tissues and Coxal | 137, 139, 253,
+                | Glands of Limulus, Scorpio, and Mygale   | 320, 321
+                |         _Q. J. Micr. Sci._ Vol. 24. 1884 |
+                |                                          |
+                |"Limulus an Arachnid"                     | 62, 238, 241,
+                |         _Q. J. Micr. Sci._ Vol. 21. 1881 | 306, 361, 366
+                |                                          |
+                |'Extinct Animals'                         | 22, 150, 345
+                |             London. Constable & Co. 1906 |
+                |                                          |
+                | A treatise on Zoology. Edited by E. Ray  |
+                | Lankester.                               |
+                |   Part II. 'The Entero-coela and the     | 472-478
+                |   Coelomocoela'                          |
+                |                                          |
+  LANKESTER AND |"A Monograph of the Fishes of the Old Red | 29, 275, 327,
+   POWRIE       | Sandstone of Britain."                   | 339, 345
+                |   Part I. "The Cephalaspidæ"             |
+                |            _Palæontographical Soc._ 1868 |
+                |                                          |
+  LANKESTER,    |"On the Muscular and Endo-skeletal Systems| 177, 222, 224,
+   BENHAM, AND  | of Limulus and Scorpio, with some Notes  | 313
+   BECK         | on the Anatomy and Generic Characters of |
+                | Scorpions"                               |
+                |        _Trans. Zool. Soc._ Vol. 11. 1885 |
+                |                                          |
+  LANKESTER AND |"The Minute Structure of the Lateral and  | 74, 81-83
+   BOURNE       | Central Eyes of Scorpio and Limulus"     |
+                |               _Q. J. Micr. Sci._ Vol. 23 |
+                |                                          |
+  LANKESTER AND |"The Development of the Atrial Chamber of |
+   WILLEY       | Amphioxus"                               | 409
+                |         _Q. J. Micr. Sci._ Vol. 31. 1890 |
+                |                                          |
+  LANKESTER AND |"Evidence in Favour of the View that the  | 429
+   GULLAND      | Coxal Gland of Limulus and of other      |
+                | Arachnids is a Modified Nephridium"      |
+                |         _Q. J. Micr. Sci._ Vol. 25. 1885 |
+                |                                          |
+  LATREILLE     |                                          | 221
+                |                                          |
+  LAURIE        |"The Anatomy and Relations of the         | 237
+                | Eurypteridæ"                             |
+                |   _Trans. Roy. Soc. Edin._ Vol. 37. 1893 |
+                |                                          |
+                |"On a Silurian Scorpion and some          | 238, 239
+                | Additional Eurypterid Remains from the   |
+                | Pentland Hills                           |
+                |                    _Ibid._ Vol. 34. 1899 |
+                |                                          |
+  LEYDIG        |                                          | 91
+                |                                          |
+  LOCY          |"Contributions to the Structure and       | 179, 262
+                | Development of the Vertebrate Head"      |
+                |            _Journ. Morph._ Vol. 11. 1895 |
+                |                                          |
+  LOEB, LEO, AND|"On Regeneration in the Pigmented Skin of | 470
+   R. M. STRONG | the Frog, and on the Character of the    |
+                | Chromatophores"                          |
+                |      _Amer. Jour. of Anat._ Vol. 3. 1904 |
+                |                                          |
+  LOWNE         |'The Anatomy, Physiology, Morphology, and | 369, 370, 375
+                | Development of the Blow-fly'             |
+                |                             London. 1895 |
+                |                                          |
+  LUGARO        | Quoted by Anderson                       | 467
+                |                                          |
+  LWOFF         |"Ueber den Zusammenhang von Markrohr und  | 444
+                | Chorda beim Amphioxus und ähnliche       |
+                | Verbältnisse bei Anneliden"              |
+                |  _Zeitsch. f. wiss. Zool._ Vol. 56. 1893 |
+                |                                          |
+  MAAS          |"Ueber Entwicklungstadien der Vorniere und| 392, 402, 412,
+                | Urniere bei Myxine"                      | 419
+                |          _Zool. Jahrbuch._ Vol. 10. 1897 |
+                |                                          |
+  MACBRIDE      |"Further Remarks on the Development of    | 410
+                | Amphioxus"                               |
+                |         _Q. J. Micr. Sci._ Vol. 43. 1900 |
+                |                                          |
+  McDOUGALL     | _See_ Hardy and McDougall.               |
+                |                                          |
+  MACLEOD       |"Recherches sur la structure et la        | 169, 174
+                | signification de l'appareil respiratoire |
+                | des Arachnides"                          |
+                |          _Archiv. de Biol._ Vol. 5. 1881 |
+                |                                          |
+  MAGNUS        |"Versuche am überlebenden Dünndarm von    | 447
+                | Säugethieren"                            |
+                |        _Archiv. f. d. Ges. Physiologie._ |
+                |                     Vols. 102, 103. 1904 |
+                |                                          |
+  MARK          |                                          | 115
+                |                                          |
+  MARSHALL      |"On the Head-cavities and Associated      | 185, 186
+                | Nerves of Elasmobranchs"                 |
+                |         _Q. J. Micr. Sci._ Vol. 21. 1881 |
+                |                                          |
+                |"The Segmental Value of the Cranial       | 260
+                | Nerves"                                  |
+                |           _Journ. of Anat. and Physiol._ |
+                |                            Vol. 16. 1882 |
+                |                                          |
+  MASTERMAN     |"On the Diplochorda"                      | 16
+                |         _Q. J. Micr. Sci._ Vol. 43. 1900 |
+                |                                          |
+  MAURER        |"Die Schilddrüse, Thymus und andere       | 427, 428
+                | Schlundspaltenderivate bei den Eidechse" |
+                |         _Morph. Jahrbuch._ Vol. 27. 1899 |
+                |                                          |
+  MAYER, F.     |"Das Centralnervensystem von Ammocoetes"  | 489
+                |            _Anat. Anzeig._ Vol. 13. 1897 |
+                |                                          |
+  MAYER, P.     |"Ueber die Entwicklung des Herzens und der| 179
+                | grossen Gefässstämme bei den             |
+                | Selachiern"                              |
+                |    _Mitth. a. d. Zool. Stat. z. Neapel._ |
+                |                             Vol. 7. 1887 |
+                |                                          |
+  METSCHNIKOW   | Quoted by Kowalewsky                     | 422
+                |                                          |
+  MEYER         |"Studien über den Körperbau der           | 403
+                | Anneliden"                               |
+                |    _Mitth. a. d. Zool. Stat. z. Neapel._ |
+                |                             Vol. 7. 1887 |
+                |                                          |
+  MILNE-EDWARDS |"Anatomie des Limules"                    | 157, 159, 176,
+                |       _Annales des Sciences Naturelles._ | 177, 313
+                |                    Ser. 5. Vol. 17. 1872 |
+                |                                          |
+  MINCHIN       | A treatise on Zoology. Edited by Ray     | 473
+                | Lankester. Part II. "The Porifera and    |
+                | Coelenterata"                            |
+                |                                          |
+  MITSUKURI     |"On the Fate of the Blastopore, the       | 179
+                | Relations of the Primitive Streak, and   |
+                | the Formation of the Posterior End of the|
+                | Embryo in Chelonia," etc.                |
+                |               _Journ. Coll. Sci._ Tokyo. |
+                |                            Vol. 10. 1896 |
+                |                                          |
+  MOTT          |"Croonian Lectures of the Roy. Coll. of   | 469
+                | Physicians," 1900                        |
+                |                                          |
+  MOTT AND      |"On the Chemistry of Nerve-degeneration"  | 469
+   HALLIBURTON  |              _Phil. Trans. Roy. Soc. B._ |
+                |                           Vol. 194. 1901 |
+                |                                          |
+  MÜLLER, J.    |                                          | 1
+                |                                          |
+                |"Vergleichende Anatomie der Myxinoiden"   | 126
+                |        _Abhandl. d. Kgl. Akad. d. Wiss._ |
+                |                             Berlin. 1834 |
+                |                                          |
+  MÜLLER, W.    |"Ueber die Stammes Entwickelung des       | 96-100, 105,
+                | Sehorgans der Wirbelthiere"              | 108
+                |        Festgabe C. Ludwig. Leipzig. 1874 |
+                |                                          |
+  NEAL          |"The Segmentation of the Nervous System   | 179, 266, 300
+                | in _Squalus acanthias_"                  |
+                |              _Bull. of Mus. Comp. Zool._ |
+                |                   Harvard. Vol. 31. 1898 |
+                |                                          |
+  NESTLER       |"Beiträge zur Anatomie und                | 168, 171, 175,
+                | Entwicklungsgeschichte von _Petromyzon   | 445
+                | Planeri_"                                |
+                |   _Archiv. f. Naturgesch. Jahrgang_, 56. |
+                |                             Vol. I. 1890 |
+                |                                          |
+  NIESKOWSKI    |"Der _Eurypterus Remipes_ aus den ober-   | 26, 239, 240
+                | silurischen Schichten der Insel Oesel"   |
+                |     _Arch. f. d. Naturkunde Liv-Ehst-und |
+                |         Kurlands._ 1st Ser. Vol. 3. 1858 |
+                |                                          |
+  NUSBAUM, J.   |"Einige neue Thatsachen zur               | 320
+                | Entwicklungsgeschichte des _Hypophysis   |
+                | Cerebri_ bei Säugethieren"               |
+                |          _Anat. Anzeiger._ Vol. 12. 1896 |
+                |                                          |
+  OBERSTEINER   |'Central Nervous System.' Translated by   | 264, 280
+                | Hill. 1896                               |
+                |                                          |
+  OKEN          |                                          | 258
+                |                                          |
+  OWEN          |"Essays on the Conario-Hypophysial Tract, | 14
+                | and the Aspects of the Body in Vertebrate|
+                | and Invertebrate Animals"                |
+                |                                          |
+                |"On the Anatomy of the American King-crab | 211
+                |(_Limulus polyphemus_)"                   |
+                |        _Trans. Linn. Soc._ Vol. 28. 1873 |
+                |                                          |
+  PANDER        |'Monographie der fossilen Fische des      | 327
+                | Silurischen Systems des russisch-        |
+                | baltischen Gouvernements'                |
+                |                    St. Petersbourg. 1856 |
+                |                                          |
+  PARKER, G. H. |"The Retina and Optic Ganglia in Decapods,| 91, 93, 97
+                | especially in Astacus"                   |
+                |    _Mitth. a. d. Zool. Stat. z. Neapel._ |
+                |                            Vol. 12. 1895 |
+                |                                          |
+                |"The Compound Eyes in Crustaceans"        | 99, 100, 114
+                |   _Bull. of Harvard Mus. of Comp. Zool._ |
+                |                            Vol. 20. 1890 |
+                |                                          |
+                |"The Function of the Lateral-line Organs  | 357
+                | in Fishes"                               |
+                |         _Bull. of the Fisheries Bureau._ |
+                |                Washington. Vol. 24. 1904 |
+                |                                          |
+                |"Studies on the Eyes of Arthropods"       | 73, 79, 83-85,
+                |                  _Journ. of Morphology._ | 114
+                |             Vols. 1 and 2. 1887 and 1889 |
+                |                                          |
+  PARKER, W. K. |"On the Skeleton of the Marsipobranch     | 120, 125, 126,
+                | Fishes"                                  | 131
+                |            _Phil. Trans. Roy. Soc._ 1883 |
+                |                                          |
+  PATTEN        |"On the Origin of Vertebrates from        | 352, 353
+                | Arachnids"                               |
+                |         _Q. J. Micr. Sci._ Vol. 31. 1890 |
+                |                                          |
+                |"On the Morphology and Physiology of the  | 358-367, 371
+                | Brain and Sense-organs of Limulus"       |
+                |         _Q. J. Micr. Sci._ Vol. 35. 1893 |
+                |                                          |
+                |"New Facts concerning Bothriolepis"       | 32, 351, 450
+                |      _Biological Bulletin._ Vol. 7. 1904 |
+                |                                          |
+                |"On the Structure and Classification of   | 329
+                | the Tremataspidæ"                        |
+                |         _Mem. d. l'Acad. Imp. d. Sci. de |
+                |          St. Petersbourg._ Vol. 13. 1903 |
+                |                                          |
+                |"On the Structure of the Pteraspidæ and   |
+                | Cephalaspidæ"                            | 415
+                | _The American Naturalist._ Vol. 37. 1903 |
+                |                                          |
+                |"On the Appendages of Tremataspis"        | 351
+                | _The American Naturalist._ Vol. 37. 1903 |
+                |                                          |
+                |"On Structures Resembling Dermal Bones in | 346
+                | Limulus"                                 |
+                |             _Anat. Anzeig._ Vol. 9. 1894 |
+                |                                          |
+  PATTEN AND    |"The Development of the Coxal Gland, etc.,| 408
+   HAZEN        | of _Limulus Polyphemus_"                 |
+                |       _Journ. of Morphol._ Vol. 16. 1900 |
+                |                                          |
+  PATTEN AND    | Studies on Limulus. II. "The Nervous     | 314, 315, 381,
+   REDENBAUGH   | System of _Limulus Polyphemus_"          | 382
+                |       _Journ. of Morphol._ Vol. 16. 1900 |
+                |                                          |
+  PERLIA        | Quoted by Edinger                        | 264
+                |                                          |
+  PICK          |   "         "                            | 265
+                |                                          |
+  PLATT         |"A Contribution to the Morphology of the  | 253, 265-267,
+                | Vertebrate Head, based on a Study of     | 273, 274, 279,
+                | _Acanthias vulgaris_"                    | 284
+                |           _Journ. Morphol._ Vol. 5. 1891 |
+                |                                          |
+                |"Fibres connecting the Central Nervous    | 443
+                | System and Chorda in Amphioxus"          |
+                |                     _Anat. Anzeig._ 1892 |
+                |                                          |
+  PRICE         |"Development of the Excretory Organs of   | 394
+                | _Bdellostoma Stouti_"                    |
+                |          _Zool. Jahrbuch._ Vol. 10. 1897 |
+                |                                          |
+  RABL          |"Ueber die Metamerie des Wirbel-          | 258, 262
+                | thierkopfes"                             |
+                | _Verhandl. der Anat. Gesellsch. Versamml.|
+                |             in Wien. Anat. Anzeig._ 1892 |
+                |                                          |
+                |"Die Entwicklung und Structur der         | 424
+                | Nebennieren bei den Vögeln"              |
+                |     _Arch. f. mikr. Anat._ Vol. 38. 1891 |
+                |                                          |
+  RAMÓN Y. CAJAL|                                          | 72, 465
+                |                                          |
+  RATHKE        |"Anatomie des Querders"                   | 161, 169, 304
+                |    _Naturforsch. Gesellsch. zu Dantzig._ |
+                |                             Vol. 2. 1827 |
+                |                                          |
+  REDENBAUGH    | _See_ Patten and Redenbaugh.             |
+                |                                          |
+  REICHENBACH   |"Entwicklungs-geschichte des Flusskrebses"| 98-100, 114
+                |           _Abhandl. d. Senckenbergischen |
+                |  Naturforsch. Gesellsch._ Vol. 14. 1886. |
+                |                                          |
+  RETZIUS       |'Biologische Untersuchungen.' Vol. 1.     | 20, 489
+                | 1890. "Zur Kenntniss des Nervensystem der|
+                | Crustaceen"                              |
+                |                                          |
+  ROHON         | Die Obersilurischen Fische von Oesel. 1st| 32, 275, 276
+                | Theil. "Thyestidæ und Tremataspidæ"      |
+                |     _Mem. d. l'Acad. Imp. d. Sci. d. St. |
+                |     Petersbourg._ 7th Ser. Vol. 38. 1892 |
+                |                                          |
+                |"Weitere Mittheilungen über die Gattung   | 327-330,
+                | _Thyestes_"                              | 339-341, 382
+                |   _Bull. d. l'Acad. d. St. Petersbourg._ |
+                |                    5th Ser. Vol. 4. 1896 |
+                |                                          |
+  ROLPH         |"Untersuchungen über den Bau des          | 444
+                | _Amphioxus lanceolatus_"                 |
+                |        _Morphol. Jahrbuch._ Vol. 2. 1887 |
+                |                                          |
+  RÜCKERT, J.   |"Entwicklung der Excretionsorgane"        | 392, 393, 400
+                |        _Merkel und Bonnet; Anat. Hefte._ |
+                |                            Vol. 1. 1891. |
+                |                                          |
+                |"Ueber die Entstehung der Excretionsorgane| 403
+                | bei Selachiern"                          |
+                |              _Archiv. f. Anatomie._ 1888 |
+                |                                          |
+  ST. HILAIRE   |"Sur la Vertèbre"                         | 11
+                |          _La Revue Encyclopédique._ 1822 |
+                |                                          |
+  SAMASSA       |"Bemerkungen über die Methode der         | 462
+                | Vergleichenden Entwicklungsgeschichte"   |
+                |      _Biol. Centralblatt._ Vol. 18. 1898 |
+                |                                          |
+  SCHAFFER      |"Ueber das Knorpelige Skelett von         | 126-135
+                | Ammocoetes"                              |
+                |  _Zeitsch. f. wiss. Zool._ Vol. 61. 1896 |
+                |                                          |
+                |"Ueber die Thymusanlage bei _Petromyzon   | 426-428
+                | Planeri_"                                |
+                |        _Sitzungsber. d. K. Akad d. Wiss. |
+                |                 in Wien._ Vol. 103. 1894 |
+                |                                          |
+  SCHIMKÉWITSCH |"Sur la structure et sur la signification | 143-145, 342
+                | de l'Endosternite des Arachnides"        |
+                |                     _Zool. Anzeig._ 1893 |
+                |                                          |
+                |"Anatomie de l'Epeire"                    | 369
+                |        _Ann. d. Sci. Nat._ Vol. 17. 1884 |
+                |                                          |
+  SCHMIDT       |"Die Crustaceen-fauna der Eurypteren-     | 190, 191, 236,
+                | schichten von Rootziküll auf Oesel"      | 240, 329, 341
+                |            _Mem. d'Acad. Imp. d. Sci. d. |
+                |          St. Petersbourg._ Vol. 31. 1883 |
+                |                                          |
+  SCHMIEDEBERG  |"Ueber die chemische Zusammensetzung des  | 147
+                | Knorpels"                                |
+                |   _Arch. f. exper. Pathol. und Pharmak._ |
+                |                            Vol. 28. 1891 |
+                |                                          |
+  SCHNEIDER, A. |'Beiträge zur Anatomie und                | 128, 130, 172,
+                |Entwicklungsgeschichte der Wirbelthiere'  | 195, 197, 213,
+                |                             Berlin. 1879 | 310, 445
+                |                                          |
+  SCHNEIDER, G. |"Ueber phagocytäre Organe und             | 421
+                | Chloragogenzellen der Oligochæta"        |
+                |  _Zeitsch. f. wiss. Zool._ Vol. 61. 1896 |
+                |                                          |
+  SCOTT         |"Notes on the Development of Petromyzon"  | 42, 78, 111,
+                |        _Journ. of Morphol._ Vol. 1. 1887 | 112, 406
+                |                                          |
+  SEDGWICK      |"A Monograph of the Development of        | 397-400
+                | _Peripatus capensis_"                    |
+                |          _Studies from the Morphological |
+                |     Laboratory, Cambridge._ Vol. 4. 1888 |
+                |                                          |
+                |"Development of the Kidney in its Relation| 390
+                | to the Wolffian Body in the Chick"       |
+                |         _Q. J. Micr. Sci._ Vol. 20. 1880 |
+                |                                          |
+                |"Early Development of the Wolffian Duct   | 393, 394, 400
+                | and Anterior Wolffian Tubules in the     |
+                | Chick; with some Remarks on the          |
+                | Vertebrate Excretory System"             |
+                |         _Q. J. Micr. Sci._ Vol. 21. 1881 |
+                |                                          |
+  SEMON         |"Das Excretionssystem der Myxinoiden"     | 400, 419
+                | _Festschrift f. Gegenbaur._ Leipzig. 1897|
+                |                                          |
+  SEMPER        |"Die Stammesverwandschaft der Wirbelthiere| 390, 392
+                | und Wirbellosen"                         |
+                |         _Arbeit. a. d. Zool. Zoot. Inst. |
+                |                  Würzburg._ Vol. 2. 1875 |
+                |                                          |
+                |"Das Urinogenitalsystem der Plagiostomen  | 390, 392
+                | und seine Bedeutung für die übrigen      |
+                | Wirbelthiere"                            |
+                |                     _Ibid._ Vol. 2. 1875 |
+                |                                          |
+  SHELDON       |"On the Development of _Peripatus         | 400
+                | Nova-Zealandiæ_"                         |
+                |          _Studies from the Morphological |
+                |     Laboratory, Cambridge._ Vol. 4. 1889 |
+                |                                          |
+  SHERRINGTON   |"On the Anatomical Constitution of the    | 267
+                | Nerves of Muscles"                       |
+                |      _Journ. of Physiol._ Vol. 17. 1894. |
+                |         _Proc. of Physiol. Soc._ June 23 |
+                |                                          |
+  SHIPLEY       |                                          | 334
+                |                                          |
+                |"On some points in the Development of     | 167, 305, 378,
+                | _Petromyzon fluviatilis_"                | 401, 405, 406
+                |         _Q. J. Micr. Sci._ Vol. 27. 1887 |
+                |                                          |
+  v. SMIRNOW    |"Ueber die Nervenendigungen in den Nieren | 477
+                | der Säugethiere"                         |
+                |          _Anat. Anzeiger._ Vol. 19. 1901 |
+                |                                          |
+  SMITH, ELLIOT |                                          | 17
+                |                                          |
+  SPANGENBERG   |"Zur Kenntniss von _Branchipus stagnalis_"| 396
+                |  _Zeitsch. f. wiss. Zool._ Vol. 25. 1875 |
+                |                                          |
+  SPENGEL       |'Die Enteropneusten'                      | 494
+                |                             Berlin. 1893 |
+                |                                          |
+  STARR         | Quoted by Edinger                        | 265, 266
+                |                                          |
+  STUDNIÇKA     |"Sur les organes pariétaux de _Petromyzon | 80, 81, 86
+                | Planeri_"                                |
+                |           _Sitzungsber. d. K. Gesell. d. |
+                |                     Wiss. in Prag._ 1893 |
+                |                                          |
+                |"Ueber den feineren Bau der               | 81, 86
+                | Parietalorgane von _Petromyzon marinus_" |
+                |   _Sitzungsber. d. K. böhmischen Gesell. |
+                |                     d. Wiss. Prag._ 1899 |
+                |                                          |
+  TAKAMINE      |"The Isolation of the Active Principle    | 423
+                | of the Supra-renal Gland"                |
+                |            _Journ. of Physiol._ Vol. 27. |
+                |  _Proc. of Physiol. Soc._, Dec. 14, 1901 |
+                |                                          |
+  TARNANI       |"On the Anatomy of the Thelyphonides"     | 190, 206-208
+                |          _Revue des Sciences Naturelles, |
+                |                   St. Petersbourg._ 1890 |
+                |                                          |
+                |"Die genitalen Organe der Thelyphonus"    | 190, 206-208
+                |       _Biol. Centralblatt._ Vol. 9. 1889 |
+                |                                          |
+  TRAQUAIR      |"Report on Fossil Fishes collected by the | 343-345, 350
+                | Geological Survey of Scotland in the     |
+                | Silurian Rocks of the South of Scotland" |
+                |  _Trans. Roy. Soc., Edin._ Vol. 39. 1899 |
+                |                                          |
+  VIALLANES     |"Contribution à l'histologie du système   | 100
+                | nerveux des Invertébrés; la lame         |
+                | ganglionnaire de la Langouste"           |
+                |                 _Ann. Sci. Nat._ Vol. 13 |
+                |                                          |
+  VINCENT,      |"The Carotid Gland of Mammalia and its    | 424
+   SWALE        | Relation to the Supra-renal Capsule, with|
+                | some Remarks upon Internal Secretion and |
+                | the Phylogeny of the latter Organ"       |
+                |          _Anat. Anzeiger._ Vol. 18. 1900 |
+                |                                          |
+                |"Contributions to the Comparative Anatomy | 424
+                | and Histology of the Supra-renal         |
+                | Capsules"                                |
+                |        _Trans. Zool. Soc._ Vol. 14. 1897 |
+                |                                          |
+  VIRCHOW       |"Transformation and Descent"              | 479
+                |            _Journ. of Path. and Bacter._ |
+                |                             Vol. 1. 1893 |
+                |                                          |
+  VOGT          |                                          | 258
+                |                                          |
+  VÖLCKERS      | _See_ Hensen and Völckers.               |
+                |                                          |
+  WAGNER        | Quoted by Gaubert                        |
+                |                                          |
+  WEISS         |"Excretory Tubules in _Amphioxus          | 426
+                | Lanceolatus_"                            |
+                |         _Q. J. Micr. Sci._ Vol. 31. 1890 |
+                |                                          |
+  WELDON        |"On the Supra-renal Bodies of Vertebrates"| 420, 424, 429
+                |         _Q. J. Micr. Sci._ Vol. 25. 1885 |
+                |                                          |
+                |"Note on the Origin of the Supra-renal    | 424
+                | Bodies in Vertebrates"                   |
+                |          _Proc. Roy. Soc._ Vol. 37. 1884 |
+                |                                          |
+  WHEELER       |"Development of the Urino-genital         | 402, 405
+                | Organs of the Lamprey"                   |
+                |          _Zool. Jahrbuch._ Vol. 13. 1899 |
+                |                                          |
+  v. WIJHE      |"Ueber die Mesodermsegmente des Rumpfes   | 155-157, 172,
+                | und die Entwicklung des Excretionsystems | 173, 188, 234,
+                | bei Selachiern"                          | 258, 260, 262,
+                |   _Archiv. f. Mikr. Anat._ Vol. 33. 1889 | 263, 266, 273,
+                |                                          | 280, 308, 390-
+                |                                          | 393, 397, 400,
+                |                                          | 406-408, 412
+                |                                          |
+                |"Beiträge zur Anatomie der Kopfregion des | 410, 426-428
+                | _Amphioxus lanceolatus_"                 |
+                | _Petrus Camper. Deel._ 1; _Aflevering._ 2|
+                |                                          |
+  WILLEY        | _See_ Lankester and Willey.              |
+                |                                          |
+  WOLFF         |"Die Cuticula der Wirbelthierepidermis"   | 302
+                |    _Jen. Zeitsch. f. Naturwissenschaft._ |
+                |                            Vol. 23. 1889 |
+                |                                          |
+  WOODWARD, H.  |"A Monograph of the British Fossil        | 235-240, 249,
+                | Crustacea, belonging to the order        | 251, 275
+                | Merostomata"                             |
+                |        _Palæontographical Society._ 1878 |
+                |                                          |
+  WOODWARD,     |                                          | 339
+   SMITH        |                                          |
+                |'Catalogue of Fossil Fishes in the British| 29, 326, 327,
+                | Museum.' Part II.                        | 344, 349, 351
+                |                             London. 1891 |
+                |                                          |
+  v. ZITTEL     | Handbuch der Palæontologie               | 190
+
+
+
+
+GENERAL INDEX
+
+[_The numbers in dark type refer to illustrations_]
+
+
+  Acilius larva, eye of, 78, 83
+  Acromegaly, 425
+  Actinotrocha, 438
+  Addison's disease, 423
+  Adelopthalmus, 249
+  Adrenalin, 423, 491
+  Adrenals, 423, 491
+  Agnathostomatous fishes, 29, 343
+  Alimentary canal, 433
+      "       "     Ammocoetes, 168, 405, 445
+      "       "     invertebrate, compared to tube of central nervous
+                      system of vertebrate, 43, 433
+      "       "     innervation of, 447
+      "       "     origin of, 444
+      "       "     position of vertebrate and invertebrate, 10
+      "       "     possibility of formation of new, 58
+      "       "     relationship between notochord and, 434
+  Ammocoetes, 32, 245
+      "       an ancestral type, 35, 309
+      "       alimentary canal, 168, 405, 445
+      "       auditory organ, 378, 379
+      "       brain, 39, 40, 41, 45, 46, 48, 54, 61
+      "       branchial appendages, 161, 162, 163, 164
+      "          "      basket-work, 126, 128, 296, 331, 335
+      "          "      chamber, 161, 168, 162, 163
+      "          "      circulation in Limulus and, 174
+      "          "      diaphragms, 161, 167
+      "          "      lamellæ, 175
+      "          "      muscles, 171
+      "          "      nerves, 164
+      "          "      segments, 178, 312
+      "       cartilage, hard, 133, 133, 293, 294, 377
+      "           "      muco, 130, 131, 291, 293, 294, 296, 330, 331, 333,
+                          334, 335, 338
+      "           "      soft, 129, 130, 293, 294, 296, 335
+      "       degeneracy, evidence of, 59, 94, 343
+      "       development, 228, 458
+      "       digestion, 58, 442
+      "       epithelial cells of gills, 214
+      "       epithelial cells of skin, 347
+      "           "      pits, 173, 200
+      "       eye, 93
+      "        "   muscles, 267
+      "        "   median or pineal, 63, 75, 76, 77, 78, 80, 85, 86
+      "        "     "         "    left, 78, 79
+      "       fat-column, 181, 182
+      "        "  in degenerated muco-cartilage, 333, 334
+      "       ganglia in embryo, 229, 283
+      "       gland-tissue round the brain, 209, 210, 379
+      "       head-region, 128, 162, 163, 193, 293, 294, 296, 298, 335
+      "       head-shield, 329, 331, 338
+      "       liver, 442, 452
+      "       lymphatic glandular tissue, 426
+      "       Müllerian fibres, 489
+      "       muscles, eye, 173, 267
+      "         "      lip, lower, 297
+      "         "        "   upper, 305
+      "         "       respiratory, 171
+      "         "       somatic, 332, 336, 409
+      "         "       tubular, 173, 298, 309
+      "       nerves, cranial, 141
+      "         "     facial, 186, 311
+      "         "     glossopharyngeal, 186
+      "         "     optic, 105
+      "         "     trigeminal, 282, 288, 288
+      "         "     vagus, 153, 173, 186
+      "       nerve-fibres, medullation of, 20
+      "       notochord, 182, 435
+      "       olfactory tube, 219, 225, 227, 317
+      "       oral chamber, 317, 243, 287, 458
+      "       parasitism, 60, 286
+      "       pituitary, 321
+      "       prosomatic region, 243
+      "       pronephric duct, 402, 405
+      "       relationship to Ostracodermata, 326, 338, 344, 414, 416
+      "       retina, 93, 111
+      "       skin, 58, 346, 348, 442
+      "       skeleton, 125, 126, 132, 291, 296, 335
+      "       segments, comparison with segments of Eurypterus, 323
+      "           "     facial, 201
+      "           "     hyoid, 186, 201
+      "           "     prosomatic, 286
+      "       septa between myomeres, 416
+      "       tentacles of upper lip, 303
+      "       test, biological, to show relationship with Limulus, 493
+      "       thyroid, 192, 194, 196, 205, 213, 430
+      "       transformation, 18, 59, 125, 168, 193, 199, 200, 220, 227,
+                228, 287, 291, 304, 307, 309, 331, 336, 347, 349, 389, 445
+      "       velum, 228, 289, 298, 302
+  Amoebocytes, 473
+  Amphibia, 23, 345
+  Amphioxus, 33, 407
+       "     atrial cavity, 409
+       "     branchial nephric glands, 426
+       "     endostyle, 198, 212
+       "     excretory organs, 389, 395, 477
+       "     neuropore, 220, 457
+       "     notochord, 435, 436, 443
+       "     pleural folds, 495
+       "     septa between myomeres, 416
+       "     somatic muscles, 409
+       "     yolk, 485
+  Androctonus, 53, 54, 372, 423
+  Annelids, lateral sense-organs, 357, 367
+     "      nephric organs, 390
+     "      rigin of Arthropods from, 395
+     "      parapodal ganglia, 283
+     "      phagocytic glands, 421
+  Anthozoa, 474
+  Antiarcha, 29, 326, 343
+  Antibody, 492
+  Antitoxin, 492
+  Anus, 43, 457
+  Aponeuroses, 327, 342, 414
+  Apparatus, auditory, 355
+      "      dioptric, 83
+      "      respiratory, 148
+      "      suctorial, of Petromyzon, 287
+  Appendages, branchial, of Ammocoetes, 161, 162, 163, 164
+      "          "          Limulus, 164
+      "          "          internal, 149
+      "       derivation of suctorial apparatus of Petromyzon from, 290
+      "       disappearance of, in transformation of Arthropod into
+                Vertebrate, 386, 413
+      "       evidence of, in prosomatic region of ancient fishes, 342
+      "       muscles, in Limulus and Scorpion, 247
+      "       prosomatic, of Gigantostraca, 234
+      "       Trilobites, 351
+  Apus, 28, 137, 436, 437
+  Arachnids, eyes, 75, 87
+      "      diverticula of stomach, 109
+      "      lyriform organs, 364, 368
+      "      segmental excretory organs, 423
+  Archæocytes, 473
+  Artemia, _v._ Branchipus
+  Arthropleura, 249
+  Arthropoda, arrangement of organs, 10
+       "      evolution, 11
+       "      excretory organs, 396, 418
+       "      eyes, 75, 89
+       "      giant-fibres, 489
+       "      musculature, 411
+       "      olfactory organs, 220
+       "      resemblance to ancient fishes, 29
+  Astacus, brain, 54
+     "     digestive ferment in cells lining the carapace, 442
+     "     optic chiasma, 101
+     "     optic stalk, 91
+     "     etina, 98
+  Asterolepis, 326, 342
+  Atrium, 410
+  Auchenaspis (Thyestes), 30, 31, 75, 275, 326, 327, 328, 338
+  Auditory apparatus, 355
+  Auerbach, plexus of, 447
+  Aurelia, 475
+  Autonomic nerves, 3
+
+  Balanoglossus, 12, 12, 433, 438, 494
+  Bdellostoma, 394, 405
+  Belinurus, 24, 249, 351
+  Bird, rhomboidal sinus, 46
+  Bladder, 449
+     "     swim, 148
+  Blastula, 459, 471, 473
+  Blood, 463, 472, 474
+    "    circulation, in Ammocoetes and Limulus, 174
+    "    secretion of ductless glands into, 418
+  Bothriolepis, 29, 32, 239, 326, 351, 450
+  Bone, 344, 474, 481
+  Brain, Ammocoetes and Arthropod, 54, 61
+    "    and brain-case of Ammocoetes, 40, 41, 46, 209
+    "    caudal, of Thelyphonus, 450
+    "    epithelial lining of, 38
+    "    roof, 39
+    "    Sphæroma serratum, 62, 90
+    "    Thelyphonus, 56
+    "    ventricles, 4
+    "    vesicles, 48
+  Branchial basket-work of Ammocoetes, 126, 128, 296, 331, 335
+  Branchipus, 28
+      "       brain, 51, 54
+      "       eyes, lateral, 88
+      "        "       "     retina of, 91, 97
+      "        "    median, 75
+      "       excretory organs, 396
+      "       (Artemia) diverticula of gut and retinal ganglion, 110, 111,
+                113
+      "       nerves of appendages, 157
+      "       segmentation, 159
+      "       resemblance to Trilobite, 436
+  Bunodes, 24, 30, 249, 341, 351, 414
+  Bundle of Meynert, 48, 77
+  Bundles, posterior longitudinal, 489
+  Buthus, muscles, 270
+
+  Calcification in aponeuroses of Cephalaspis, 414
+         "      cartilage, 140,330
+         "      successive layers of the skin, 348
+  Camerostome, 221, 222, 223, 224, 241, 271
+  Canal, alimentary, formation of vertebrate, 58, 433, 446
+    "         "      innervation, 447
+    "         "      relationships between notochord and, 434
+    "         "      origin, 444
+    "    Haversian, 329
+    "    central, of spinal cord, 405, 439, 455
+    "    spinal, 182
+  Capsule, auditory, 377, 379
+  Cartilage Ammocoetes, muco, 127, 130, 131, 200, 291, 303, 330, 333, 334,
+                                344
+      "         "       hard, 133, 133, 377
+      "         "       soft, 126, 129, 130
+      "         "       spinal cartilages, 414
+      "     Hypoctonus, 133, 142
+      "     Limulus, hard, 142
+      "        "     muco, 139
+      "        "     soft, 20, 130, 137
+      "     origin, 474, 481
+      "     staining reactions, 131, 133, 139, 330, 336
+  Cavity, atrial, 409, 413
+     "    coelomic, 167, 251, 266, 320, 389, 391, 408, 422, 430, 472
+  Cells, free-living, 463
+  Centre, vaso-motor, 468
+  Cephalaspis, diverticula of gut, 109
+      "        eyes, lateral, 75, 275
+      "         "    median, 75
+      "        head-shield, 327, 328, 330, 338
+      "        muscles on head-shield, 269
+      "        resemblance to Ammocoetes, 145, 291, 326, 329, 338, 348, 414
+      "             "         Arthropod, 29
+      "        segmentation, 339
+  Ceratodus, 148
+  Cephalization, 51
+  Cephalodiscus, 438
+  Cephalopod, 23
+  Cerebellum, 47, 50
+  Chætopoda, 395
+  Chamber, oral, of Ammocoetes, 243, 287, 458
+  Cheliceræ, 235
+  Chiasma, optic, 101
+  Chilaria, 235, 238, 291, 301, 458
+  Chitin, 85, 119, 139, 205, 206, 302, 329, 346, 359, 440, 443
+  Cilia, 206
+  Circulation, branchial, 174
+  Cirri, 357
+  Clarke's column, 467
+  Clepsine, nephridial glands, 423
+  Cochlea, 378
+  Coelenterata, 465, 472
+  Coelolepidæ, 344
+  Coelom, 167, 251, 400, 472, 481
+  Coelomata, 472
+  Coelomocoela, 472, 475
+  Coelomostomes, 477, 481
+  Colleneytes, 474
+  Commissure, anterior, 49
+      "       oesophageal, 14
+      "       posterior, 48, 280
+  Comparison of brains of Ammocoetes and Arthropod, 61
+      "           "       invertebrate from Branchipus to Ammocoetes, 54
+      "           "       vertebrate, 40
+      "         branchial circulation in Ammocoetes and Limulus, 174
+      "             "     lamellæ of Scorpion and Ammocoetes, 175
+      "             "     segments of Ammocoetes and Petromyzon, 169
+      "         Cephalaspidian and Palæostracan fish, 31
+      "         Coelom of Peripatus and Vertebrate, 400
+      "         dermal covering of Pteraspis with chitin of Limulus or
+                  dentine of fish scales, 346
+      "         entosternite or plastron of Limulus with trabeculæ of
+                  Ammocoetes, 145
+      "         excretory organs of vertebrates and invertebrates, 389
+      "         gut of Arthropod and tube of central nervous system of
+                  Vertebrate, 43, 244, 433, 440, 455, 457
+      "         head-shield of Cephalaspis and Ammocoetes, 291, 329, 338
+      "         hypophysial tube with olfactory tube of Arthropod ancestor,
+                                   229
+      "              "       "   with position of palæostoma, 317
+      "         mesosomatic region of Ammocoetes and Eurypterus, 192
+      "         muscles, branchial, of Ammocoetes and appendage muscles of
+                           Scorpion, 171, 447
+      "           "      eye, of Vertebrate with dorso-ventral muscles of
+                           Scorpion, 267, 272, 459
+      "           "      of oral chamber of Ammocoetes and prosomatic
+                           musculature of Limulus, 247, 447
+      "           "      longitudinal body-muscles of Vertebrate and dorsal
+                           longitudinal muscles of Arthropod, 411, 447
+      "         nerves, appendage of Limulus and Branchipus to lateral root
+                          system of Vertebrate, 157
+      "           "     cranial and spinal segmental, 152
+      "         nervous systems of Vertebrate and Arthropod, 36
+      "         pineal gland of vertebrates and median eyes of Arthropod,
+                  63, 456
+      "         pituitary body and coxal glands, 246, 319, 321
+      "         prosoma and mesosoma of Limulus and Ammocoetes, 140, 141
+      "         prosomatic region of Ammocoetes and Eurypterus, 244, 333
+      "         retina in Ammocoetes and Musca, 97
+      "           "    compound in Arthropod and Vertebrate, 87
+      "         skeleton of Limulus and Ammocoetes, 126, 136
+      "         sense-organs of Arthropod appendages with auditory organs
+                  of Vertebrate, 375
+      "         thyroid with endostyle, 198
+      "            "     "   uterus of Scorpion, 205
+  Corneagen, 69
+  Corpora quadrigemina, 47
+  Corpuscles, Pacinian, Herbst, Grandry, etc., 470
+  Coxal glands, 242, 246, 319, 321, 389, 398, 403, 429
+  Cranium, 121, 145, 339
+  Crayfish, 442, 489
+  Crest, neural, 281
+  Cromatophores of frog, 470
+  Crura cerebri, 14
+  Crustacea, first appearance, 27
+       "     eyes, 76, 87
+       "     retina, 100
+       "     segmental glands, 422
+  Ctenophora, 474
+  Cyathaspis, 29, 326, 340, 343
+  Cyclostomata, 165, 229, 343, 353, 424
+  Cysts, 50
+
+  Daphnia, 112
+  Degeneration, 17, 19, 59, 74, 78, 94, 107, 212, 309, 333, 336, 343
+  Deiters' nucleus, 489
+  Dendrites, 72
+  Development, parallel, 497
+       "       of two types of eye, 73
+       "       vertebrate retina, 101
+  Diaphragms, 161, 167
+  Didymaspis, 327, 338
+  Digestion, 441
+  Dinosaurs, 17
+  Dipnoans, 23, 45, 148
+  Diptera, 89, 369
+  Diverticula, optic, 102
+  Dogfish, skull, 121, 123
+  Drepanaspis, 344, 345, 450
+  Drepanopterus Bembycoides, 238
+
+  Ectognath, 238, 242, 271, 304, 342, 381
+  Eel, 488
+  Elasmobranchs, 23, 343, 423
+  Elastin, 435
+  Embryo, head of dogfish, 121, 123
+    "     skull of pig, 121
+  Embryology, principles of, 455
+  Encepalomeres, 262
+  Endognath, 238, 271, 304, 381
+  Endostoma, 241, 306
+  Endostyle, 198, 212
+  Entapophysis of Limulus, 139
+  Enterocoela, 472
+  Enteropneusta, 438, 494
+  Entochondrites, 377
+  Entosclerite, 222, 271
+  Entosternite, 143
+  Epiblast, 444, 445, 459
+  Epithelium cells of Ammocoetes, 347
+       "     of central nervous system of vertebrates, 38, 457
+       "        coelomic spaces in annelids, 421
+       "        optic diverticula, 103
+       "        peritoneal, pleural, and pericardial cavities, 477
+       "        velum of Ammocoetes, 301, 302
+  Equilibration, 358
+  Eukeraspis, 326
+  Eurypterus, 26, 150, 191, 237
+      "       appendages, 150, 236, 237
+      "       classification, 249
+      "       comparison with Ammocoetes, 170, 323
+      "       diagram of sagittal median section, 240, 245
+      "       endostoma, 241, 306
+      "       eyes, 275
+      "       mesosomatic segments, 192
+      "       muscles of carapace, 269
+      "       operculum, 150, 190, 212
+  Evidence of alimentary canal, innervation, 446
+       "      auditory apparatus and lateral line organs, 355
+       "      coelomic cavities in Limulus, 251
+       "      degeneracy in Ammocoetes, 59, 94, 343
+       "      embryology, cartilage, 20, 129
+       "           "      eye-muscles, 263
+       "           "      excretory organs, 390
+       "           "      heart, 179, 451
+       "           "      nervous system, central, cerebral vesicles, 48,
+                                                     458
+       "           "         "      "       "      epithelial tube, 37, 42,
+                                                     102, 244, 433, 455
+       "           "         "      "       "      neurenteric canal, 37
+       "           "         "      "       "      neuropore, 220, 457
+       "           "         "      "       "      optic diverticula, 102
+       "           "         "      "       "      spinal cord, 46
+       "           "      oral chamber, 228, 242, 243, 290
+       "           "      olfactory organ, 220, 227
+       "           "      palæostoma or old mouth, 317
+       "           "      pineal or median eyes, 15, 63, 74, 456
+       "           "      pituitary body and coxal glands, 246, 319
+       "           "      thyroid, 192, 194
+       "           "      segmentation, double, of head, 157, 234, 258
+       "           "      skeleton, cranial, 120, 153
+       "      nervous system, central, 8
+       "      notochord, origin from segmented region, 443
+       "      olfactory apparatus, 218
+       "      organs of vision, 68
+       "      palæontology, 20, 497
+       "      pineal or median eyes, 74
+       "      prosomatic musculature, 247
+       "      respiratory apparatus, 148
+       "      segmentation in head-shield, 339
+       "      skeleton, 119
+  Evolution, 8, 15, 20, 149, 482, 497
+      "      of brain in brain-case, 210
+      "         cranium of Vertebrate, 342
+      "         excretory organs, 389
+      "         eye of Vertebrate, 114
+      "         nervous system, central, 34
+      "         tissues, 19
+      "         Vertebrate from Balanoglossus and Amphioxus, 33
+  Eyes, 68
+    "   lateral, 87, 105, 108
+    "   median or pineal, 74, 77, 78, 79
+
+  Fat-cells in muco-cartilage, 332
+  Fat-column of Ammocoetes, 181, 182
+  Fibres, Mauthnerian, 488
+     "    Müllerian, of Ammocoetes central nervous system, 489
+     "        "         retina, 96, 107
+  Fishes, classification, 218
+     "    ancient, classification, 326, 343
+     "       "     cloacal region, 450
+     "       "     dominance, 23
+     "       "     eyes, 75
+     "       "     head-shields. _See_ Head-shields
+     "       "     pleural folds, 414
+  Fissure, posterior, 43
+  Fittest, survival of, 16, 34
+  Flabellum, 359, 360, 362, 363, 366
+  Folds, pleural, 410, 414
+  Function of auditory organ, double, 358
+      "       lateral line sense-organs, 357
+      "       nerves, 448
+      "       thyroid, 212, 215
+  Fusion of ganglia, 52
+
+  Galeodes, 230
+      "     brain, and camerostome, 222, 223
+      "     primordial cranium, 341
+      "     racquet-organs, 369, 375
+  Ganglia, infraoesophageal, 4, 12, 14, 51, 221
+      "    supraoesophageal, 4, 12, 14, 49, 52, 221, 225
+      "    origin of, of cranial and spinal nerves, 281
+  Ganglion, epibranchial, 164, 282
+      "     habenulæ, 48, 78
+      "     optic of retina, 72, 89, 97
+      "     of posterior root, 466
+      "     cells of sympathetic system, 424, 428, 448
+  Ganoids, 23, 345
+  Gastrula theory, 165, 459
+  Genital corpuscles, 470
+  Geological record, 20
+      "      strata, 22
+  Geotria australis, 80
+  Germ-band, 482
+  Germ-cells, 471
+  Giant-fibres, 489
+  Gigantostraca, 25, 234
+  Gills, 148, 161, 185, 214, 494
+  Glabellum, 339
+  Glands, carotid, 427
+     "    coxal, 242, 246, 319, 321, 425, 429
+     "    ductless, 418
+     "    generative, of Limulus, 209
+     "    internal secretion of, 214
+     "    lymphatic, 418
+     "    pineal, 15, 63, 75, 456
+     "    pituitary, 244, 246, 319, 425
+     "    segmental, of Crustacea, 422
+     "    submaxillary, 466
+     "    sweat, 448
+     "    thymus, 425
+     "    thyroid, of Ammocoetes, 193, 194, 196, 201, 205, 429
+     "    tissue round brain of Ammocoetes, 209, 379
+     "    uterine, of Scorpion, 202, 203, 204, 205
+  Gnathostomata, 60, 343
+  Goblet, 359, 360, 373
+  Goitre, 215
+  Gonad, 475, 479
+  Gonocoele, 475, 481
+  Grooves, ciliated, 188, 197, 212
+     "     hyper-pharyngeal of Amphioxus, 410
+     "     ventral, of apus and trilobites, 436
+  Gymnophiona, 393
+
+  Hæmocytes, 472
+  Head of embryo dogfish, 121, 123
+  Head-shield, dorsal, of Ammocoetes, 330, 331, 338
+        "         "       Auchenaspis, 29, 31, 338
+        "         "       Cephalaspis, 327, 328, 330, 338, 348
+        "         "       Cyathaspis, 340
+        "         "       Didymaspis, 338
+        "         "       evidence of segmentation, 339
+        "         "       Keraspis, 328
+        "         "       Ostreostraci, 327, 348
+        "         "       Palæostracan, 348
+        "         "       Pteraspis, 29
+        "         "       Thyestes, 29, 31, 327, 332, 338, 340, 341, 348
+        "      ventral, Scaphaspis, 349
+  Heart, nerves, 2, 447
+     "   origin of vertebrate, 179, 451, 459
+     "   relative position in vertebrate and invertebrate, 175
+     "   veins forming vertebrate, 180
+  Hemiaspis, 24, 25, 249, 250, 351, 414
+  Hemispheres, cerebral, 47
+  Hepatopancreas of Ammocoetes, 452
+         "          Limulus, 211
+  Heterostraci, 29, 275, 326, 343
+  Hirudinea, 478
+  Histolysis in transformation of the lamprey, 59
+  Homology of branchial region of vertebrate and invertebrate, 149
+       "      ductless glands and nephridial organs, 418
+       "      external genital ducts of arthropods and nephridia of
+                annelids, 429
+       "      germinal layers in all Metazoa, 459
+       "      pituitary body of Ammocoetes and coxal glands of Limulus, 319
+       "      tubular muscles of Ammocoetes and veno-pericardial muscles of
+                Limulus, 309
+       "      ventral aorta of vertebrate and longitudinal venous sinuses
+                of Limulus, 178
+  Hydra, 441, 465, 472, 476
+  Hydrophilus larva, eye, 84
+  Hyoid segment in Ammocoetes, 186, 267
+  Hypoblast, 434, 438, 444, 445, 459
+  Hypoctonus, cartilage cells in entosternite, 133
+      "       operculum, 189, 207
+  Hypogastric plexus, 3
+  Hypogeophis, 393
+  Hypophysis, 229, 244, 317, 318, 340
+
+  Infundibulum, position, 122,132
+       "        tube, the ancestral oesophagus, 4, 37, 244, 318
+       "          "   relation to neural canal, 14, 36, 318, 440, 457
+       "          "         "     notochord, 318, 435,440
+       "          "         "     olfactory tube, 220, 228, 318, 340
+  Insects, chordotonal organs, 364, 370
+  Invertebrate, heart, 175, 179
+        "       excretory organs, 418
+        "       nervous system, 13, 54
+        "       segmental nerves, 152
+
+  Keraspis, 75, 328, 338
+  Kidney, 420, 459, 476
+     "    nerves, 477
+  King-crab, _v._ Limulus
+
+  Labyrinthodont, 21, 28
+  Lamina terminalis, 49
+  Lamprey, _v._ Ammocoetes and Petromyzon
+  Larva, _v._ Transformation of the Lamprey
+  Lateral line system, 261, 355, 411, 470
+  Law of Progress, 19
+    "    Recapitulation, 434, 456, 498
+  Layer, germinal, 459
+     "   laminated, 347, 348
+  Leech, 421
+  Lens, formation, 83, 115
+  Lepidosiren, 148, 461, 466
+  Limulus or king-crab, 25, 140, 236, 240
+      "   appendages, branchial, 138, 164, 175
+      "   appendages, prosomatic, 381
+      "   brain, 54
+      "   circulation, 174, 176
+      "   classification, 26, 249
+      "   coelomic cavities, 252, 328
+      "   coxal glands, 321, 389, 397, 403, 429
+      "   eyes, median, 62, 74, 81
+      "   entosternite or plastron, 142, 143
+      "   flabellum, 360, 362, 363, 380, 381
+      "   generative organs and ducts, 189, 202, 208, 209, 380
+      "   heart, 180
+      "   musculature, branchial, 170
+      "        "       prosomatic, 247
+      "        "       veno-pericardial, 177, 297, 309, 313
+      "   nerves, appendage, 140, 157
+      "      "    cardiac, 314
+      "      "    segmental, tripartite division of, 157, 235, 267, 355
+      "   segments, branchial, 152
+      "       "     first mesosomatic, 188
+      "       "     prosomatic, 233
+      "   operculum, 189, 202, 235, 295
+      "   sense-organs, poriferous, of appendages, 359
+  Lip, lower, of Ammocoetes, 246, 289, 297, 458
+    "  upper,      "         228, 243, 303, 336
+  Liver, Ammocoetes, 452
+     "   Limulus, 209, 211
+  Lizard, pineal eye, 80
+     "    suprarenals, 424
+     "    tail, 50
+  Lobes, optic, 101
+  Lobster, 489
+  Lungs, 148
+  Lung-books of scorpions, 150
+  Lymph, 474
+  Lymph-corpuscles, 463, 490
+  Lymphocytes, 472
+
+  Malapterurus, 470
+  Mammal, dominance of, 21
+  Man, dominance of, 17
+  Marsipobranchs, 23, 35
+  Medullation of nerve-fibres, 20, 267, 467, 477
+  Membranes, basement, 436
+  Meroblastic egg, 485
+  Merostomata, 25, 249, 321
+  Mesencepalon, 48
+  Mesoblast, 444, 455, 459
+  Mesogloea, 474
+  Mesonephros, 389, 400, 424, 429
+  Mesosoma, 52
+  Mesothelium, 472, 477
+  Metanephros, 389
+  Metasoma, 52, 387, 411
+  Metastoma, 239, 246, 272, 289, 342, 458
+  Metazoa, 444, 459, 471, 472
+  Meynert's bundle, 48, 77
+  Mollusca, dominance of, 23
+  Mouth, old, or palæostoma, 14, 317, 322, 440, 458
+    "    vertebrate, 317
+  Muco-cartilage, _v._ Cartilage
+  Muscles, antagonistic, 447
+     "     branchial, 170
+     "     connection of, with central nervous system, 464
+     "     eye, and their nerves, 263
+     "     prosomatic, 243, 247
+     "     phylogeny of origin of skeletal, 478
+     "     rudimentary, in Ammocoetes, 289
+     "     somatic trunk, origin of, 406
+     "     striated, 20, 155
+     "     tubular, of Ammocoetes, 309
+     "     unstriped, 20, 447, 491
+     "     visceral and parietal, 155, 172
+     "     veno-pericardial of Limulus and Scorpion, 177, 297, 309
+  Muscle-spindles, 267
+  Mygalidæ, stomach, 109
+      "     segmentation, 249, 306
+  Myomeres, 262, 337, 414, 479
+  Myotomes, 332, 337, 338, 391, 407, 408
+  Mysis, eyes, 100
+    "    ductless glands, 422
+  Myxine, 220, 392, 402, 419
+
+  Nebalia, 144, 422
+  Nemertina, 475
+  Nephridia, 395, 421, 429
+  Nephrocoele, 430
+  Nephrotome, 393
+  Nerves, abducens, 155, 263, 266
+     "    auditory, 356, 376
+     "    autonomic, 3
+     "    facial, 155, 156, 186, 188, 192, 311, 356, 378
+     "      "     ramus branchialis profundus, 311
+     "    to flabellum, in Limulus, 361, 375
+     "    glossopharyngeal, 155, 156, 186, 356
+     "    hypoglossal, 156
+     "    inhibitory, 447
+     "    inedullation of, 20, 267, 467, 477
+     "    occulomotor, 155, 234, 263, 274
+     "    olfactory, 229
+     "    optic, 101, 104
+     "      "    of pineal eye, 79
+     "    origin of ganglia of cranial and spinal, 281
+     "    to pecten of Scorpion, 375, 376
+     "    preganglionic, 2
+     "    of prosoma in Limulus, 235, 355
+     "    regeneration of, 469
+     "    roots, of Limulus, 157
+     "    sacral, 448
+     "    segmental, 152, 156
+     "    segmental nature of cranial, 259, 411
+     "    spinal, absence of lateral roots in, 388
+     "    spinal accessory, 154
+     "    trigeminal, 151, 155, 156, 234, 243, 257, 279
+     "         "      motor nucleus of, 280
+     "         "      of Ammocoetes, 288
+     "    tripartite arrangement of cranial nerves, 154, 157, 235, 267, 355
+     "    trochlear, 48, 155, 234, 263, 276
+     "    vagus, 151, 154, 156, 173, 186, 356, 447, 449
+  Nervous system, central, comparison of Vertebrate and Arthropod, 36, 457
+          "         "      connection of, with muscular and epithelial
+                                          tissues, 464
+          "         "           "         with retina, 71
+          "         "      disease of, 50
+          "         "      evidence of, 8
+          "         "      evolution of, 34
+          "         "      importance of, 16, 463, 482, 498
+          "         "      invertebrate, 10, 13, 54
+          "         "      origin of, 480
+          "         "      relation of germ-band to, 483
+          "         "      segmentation of vertebrate, 51
+          "         "      tube of, 36-51, 102, 211, 433, 455, 457
+          "         "      vertebrate, 10, 13, 40, 41, 152
+          "       enteric, 447
+          "       sympathetic, 2, 424, 428, 448, 491
+  Neurenteric canal, 37
+  Neuroblast, 465
+  Neuromeres, 55, 247, 262, 312, 316
+  Neurones, 72, 92, 465
+  Neuropil, 71, 91
+  Neuropore, 220, 457
+  Nose, 219
+    "   of Osteostraci, 329, 352, 458
+  Notochord, 120, 122, 180, 181, 220, 244, 295, 318, 405, 417, 433, 436,
+      494
+
+  Ocelli, 70
+  Oesophagus of Ammocoetes, 405
+       "        Arthropod, compared to tube of infundibulum, 4, 244, 440
+  Olfactory apparatus, evidence of the, 218
+      "     organs of the Scorpion group, 220
+      "     tube of Ammocoetes, 219, 225, 244, 317
+  Oligochæta, 421, 478
+  Operculum of Eurypterus, 191, 212, 291
+       "       Limulus, 189, 202, 235, 295
+       "       Phrynus, 191
+       "       Scorpion, 189, 206, 212, 372
+       "       Thelyphonus, 189, 190, 206
+  Organs, arrangement of, 10
+     "    auditory, of arachnids and Insects, 368
+     "    branchial, innervation of vertebrate, 151
+     "        "      sense-organs of embryo vertebrate, 261, 281
+     "    chordotonal, of insects, 364, 369, 370
+     "    electric, 470
+     "    generative, of Limulus, 208, 209
+     "         "      connection between Thyroid gland and, 215
+     "    genital, of sea-scorpions, 206
+     "    lateral line, 355, 411
+     "    lyriform, of arachnids, 364, 369
+     "    olfactory, of Scorpion group, 220
+     "    phagocytic, 420
+     "    racquet, of Galeodes, 369, 375
+     "    segmental excretory, 389, 391, 408, 418, 459, 477
+     "    sense, of appendages of Limulus, 358
+     "    vestigial, 456
+     "    of vision, evidence of, 68
+     "    vital, 57
+  Origin of alimentary canal, 444
+     "      arthropods from annelids, 395
+     "      atrial cavity, 409
+     "      auditory capsules and parachordals, 377
+     "      coelom, 475, 481
+     "      ductless glands, 428
+     "      free cells, 472
+     "      heart of vertebrate, 179
+     "      lateral line organs, 356
+     "      muscles, 478
+     "      musculature, branchial, 170
+     "            "      somatic trunk, 406
+     "      nervous system, central, 480
+     "      notochord, 434
+     "      segmental excretory organs, 389
+     "      skeleton of vertebrates, 119
+     "      vertebrates, 9, 36, 351, 433, 493
+  Ostracodermata, 326, 343
+  Osteostraci, 29, 75, 275, 326, 343
+  Otoliths, 378
+  Ovum, 473
+
+  Pacinian bodies, 470, 477
+  Palæmon, 20, 422
+  Palæontology, evidence of, 20, 497
+  Palæostoma, 317
+  Palæostraca, 27, 396
+       "       median eyes, 74
+       "       mesosomatic appendages, 188
+       "       olfactory organs, 221
+       "       segments, compared to Ammocoetes, 308
+  Pantopoda, glands, 423
+  Parachordals, 121, 132, 377
+  Parapodia, 357
+  Parapodopsis, foot glands, 422
+  Parathymus, 427
+  Parathyroids, 427
+  Parietal organ, 76
+  Pecten of scorpion, 114, 359, 366, 371, 372, 373, 374
+  Pedipalpi, 190
+  Periblast, 471
+  Peripatus, 396, 399, 400, 411, 421, 429
+  Petromyzon, alimentary canal, 405, 445
+       "      auditory organ, 378
+       "      branchial segments, 169
+       "      life-history, 59
+       "      olfactory tube, 219, 226
+       "      pronephric duct, 402
+       "      retina and optic nerve, 95
+       "      skeleton, 125
+       "      suctorial apparatus, 287, 304
+       "      transformation, _v._ Transformation of the Lamprey
+  Phagocytes, 420, 471
+  Pharynx of Amphioxus, 410
+      "      Vertebrate, 440
+  Phoronis, 439
+  Phrynus, brain, 53
+      "    caudal brain, 450
+      "    carapace and carapace removed, 250
+      "    coecal diverticula, 109
+      "    evidence of segmentation of carapace, 249, 250, 341
+      "    operculum, 191
+      "    prosomatic appendages, 306
+      "    crossing of dorso-ventral muscles, 271, 277
+      "    stridulating apparatus, 368
+  Phyllodoce, 395
+  Phyllopoda, 321
+  Pigment, in Ammocoetes, in position of atrial cavity, 412
+      "       epithelial lining of central nervous system, 43, 457
+      "       choroid of vertebrate eye, 104, 107
+      "       between glandular cells round brain of Ammocoetes, 211, 379
+      "       tapetal layer of retina, 70
+      "       white, of right pineal eye of Lamprey, 76, 80
+  Pineal body, 14, 15
+     "   eyes, 74, 233, 244
+     "     "   of Ammocoetes, 80, 78, 85
+     "   gland, 63, 75, 456
+  Pits, epithelial, of diaphragms in Ammocoetes, 164
+    "        "         skin in Ammocoetes, 173, 200
+  Pituitary body, 244, 246, 319, 321, 425, 430
+  Plasma-cells, 471
+  Plakodes, 283
+  Planarians, 475
+  Plastron, formation of cranial walls from the, 86, 322, 341
+      "     of Limulus, 136, 142, 143
+      "     Palæostracan, compared to trabeculæ of Ammocoetes, 145, 377
+      "     muscles attached to the, 270
+      "     of Thelyphonus, 143
+  Platyhelmia, 475
+  Pleuron, 410, 415
+  Plexus, of Auerbach, 447
+      "      choroid, 38, 45, 49, 103
+      "      hypogastric, 3
+  Polychæta, 357, 395
+  Pores, abdominal, 430
+  Porifera, 473
+  Pouch, formation of gill, 165, 166
+  Prestwichia, 24, 25, 249, 351
+  Principle of concentration and cephalization, 51
+       "       embryology, 455
+  Pristiurus, 424
+  Progress, law of, 19
+       "    result of, 56
+  Pronephros, 389, 397, 419, 424, 449
+  Prosencephalon, 48
+  Prosoma, 52
+  Protopterus, 148
+  Protostraca, 27, 396, 417
+       "       dominance of, 28
+  Protozoa, 166, 479
+  Pseudoniscus, 25, 249
+  Pteraspis, 29, 30, 275, 326, 343, 344, 350
+  Pterichthys, 29, 31, 239, 326, 351
+  Pterygoid, pedicle of, 295
+  Pterygotus, 25, 27, 56, 170, 191, 221, 235, 238, 249, 276
+  Ptychodera, 494, 495
+
+  Ramus branchialis profundus of facial nerve, 311
+    "   communicans, 2, 3
+  Raphe, 46
+  Recapitulation, law of, 434, 456, 498
+  Regeneration of nerves, 469
+  Reptiles, dominance of, 21
+  Retina, compound, 71
+     "    development of, 101
+     "    inversion of, in Vertebrates, 114
+     "    inverted, 70
+     "    layers of compound, 73
+     "       "      in Crustacean eye, 100
+     "    of lateral eye of Ammocoetes, 93, 95, 111
+     "    Musca, 89
+     "    Pecten and Spondylus, 114
+     "    upright compound, 72
+     "        "   simple, 69
+  Rhabdites, 69, 81
+
+  Saccus vasculosus, 244, 322
+  Scales, 345
+  Scaphaspis, 349
+  Schwann, sheath of, 469
+  Sclerotomes, 388
+  Scorpion, brain, 54
+      "     branchial lamellæ, 175
+      "     development, 482
+      "     entochondrites, 377
+      "     excretory organs, 397
+      "     eyes, 75
+      "     lung-books, 150, 170
+      "     lymphatic glands, 423
+      "     muscles, oblique, 278
+      "        "     recti, 271
+      "        "     respiration, 171
+      "        "     veno-pericardial, 177
+      "     muscular system, 247, 268, 269
+      "     nerves to Cheliceræ, 237
+      "     olfactory organs, 220
+      "     operculum of male, 189, 206, 212
+      "     pecten, 359, 366, 371, 373, 374, 377
+      "     under surface, 372
+      "     uterus, 189, 202, 203, 204, 205, 212
+  Sea-scorpions, 25, 26, 27, 56, 150, 170, 191, 208, 221, 232, 235, 241,
+      349, 359
+  Segmentation, branchiomeric, 124
+        "       body-muscles in vertebrate, 388
+        "       eye-muscles, 248
+        "       of head, double, 155, 157, 173, 234, 258, 411, 459
+        "       of head-shield, 339
+        "       history of cranial, 258
+  Segments, branchial of Ammocoetes, 161, 178, 186
+      "     hyoid, in Ammocoetes, double, 186, 201, 267, 300
+      "     innervation of branchial, 151
+      "     first mesosomatic, in Limulus and its allies, 188
+      "     mesosomatic, of Eurypterus, 192
+      "     prosomatic of Limulus and its allies, 233, 249
+      "          "        Ammocoetes, 286
+      "     of spinal region of Vertebrates, 388
+      "     of trigeminal nerve-group, 257, 279
+      "     tubular muscles of hyoid, 299
+  Sense-organs of Amphioxus, 34
+         "        branchial, of Limulus, 359, 360
+         "        lateral, of Annelids, 357, 367
+         "        lateral-line system, 356, 411, 470
+  Serum, 492
+  Significance of the optic diverticula, 102
+  Silurus, 488
+  Sinus, longitudinal venous, of Limulus, 176, 312, 451
+     "   rhomboidal of bird, 46
+  Skeleton, Ammocoetes, 126, 296, 335
+      "        "        branchial, 126, 126
+      "        "        basi-cranial, 132
+      "        "        muco-cartilaginous, 291, 296, 330, 331
+      "     aponeurotic, 414
+      "     Cephalaspis, 414, 415
+      "     evidence of the, 119
+      "     Limulus, cartilaginous, 126, 136
+      "        "     mesosomatic, 137
+      "        "     prosomatic, 142
+      "     Petromyzon, 125
+      "     Vertebrate, commencement of bony, 120, 121
+  Skin, digestive power of cells of, in Ammocoetes, 58, 442
+    "   of Ammocoetes, 346
+    "   nerves of, 448
+  Skull of dogfish, 123
+     "     pig-embryo, 121
+  Slimonia, 27, 56, 170, 235, 238, 249, 276, 303
+  Solenocytes, 395, 477
+  Solpugidæ, 109
+  Sphæroma serratum, brain, 62, 90, 101, 225
+  Spiders, eyes, 75
+     "     stomach, 109
+  Spina bifida, 50
+  Spinal cord, difference between brain and, 45
+     "     "   region of, 385
+     "     "   termination in bird-embryo, 51
+  Spondylus, retina of, 114
+  Squilla, eyes, 100
+     "     glands, 422
+  Stomach, cephalic, 4, 43, 102, 244
+  Stylonurus Lagani, 27, 235, 239, 249
+  Substantia gelatinosa Rolandi, 44
+  Suprarenal body, 423
+  Surfaces, dorsal and ventral, 11
+      "     reversal of, 15, 29, 36, 87, 175, 352, 433, 484
+  Synapse, 72
+  Syncytium, 464, 471, 479
+
+  Tail of lizards, 50
+  Tapetum, 69
+  Teleosteans, 23, 345, 420, 424
+  Tendon-organs, 470
+  Tentacles of Ammocoetes, 246, 289, 303
+  Tergo-coxal muscles, 247
+  Test, biological, of relationship of animals, 492
+  Thalainencephalon, 48
+  Thelodus, 344
+  Thelyphonus, 231
+       "       brain, 53, 54, 56, 224
+       "         "    caudal, 450
+       "       coecal diverticula, 109
+       "       entosternite, 143
+       "       genital organs, 206
+       "       lyriform organs, 368
+       "       olfactory passage, 226, 306
+       "       operculum, 189, 190, 206, 207
+  Theory, gastræa, 444, 461
+  Theories of the origin of vertebrates, 9, 411, 433, 457
+  Thionin reaction, 131, 139, 213, 330, 336
+  Throat, formation of, 179
+  Thyestes, 30, 31, 275, 326, 328, 329, 339, 340, 341
+  Thymus, 425, 430
+  Thyroid gland of Ammocoetes, 61, 127, 192, 194, 196, 429, 459
+      "     "   evidence of the, 185
+      "     "   function of, in Ammocoetes, 213
+  Tissues, connective, 471, 474, 481
+      "    evolution of, 19
+      "    notochordal, 435
+      "    two groups of, 463
+  Tongue of Ammocoetes, 246, 303
+  Tonsils, 427, 430
+  Torpedo, 262, 392, 470
+  Trabeculæ, 121, 132, 133, 145, 277, 295, 377
+  Transformation of the Lamprey, 18, 35, 59, 61, 125, 168, 193, 199, 200,
+      220, 227, 228, 287, 291, 304, 307, 309, 331, 336, 347, 349, 389, 445
+  Tremataspis, 32, 75, 275, 326, 351, 352
+  Trilobites, 24, 25, 26, 437
+       "      appendages, 351, 437
+       "      diagram of section through a trilobite-like animal, 413
+       "      dominance of, 26
+       "      excretory organs, 396
+       "      eyes, 74, 88
+       "      glabellum, 339
+       "      relations of, 249, 283
+       "      respiratory apparatus, 170
+       "      ventral surface, 437
+  Tube of central nervous system, 37, 38, 42, 102, 211, 433, 455, 457
+    "  from IVth ventricle to surface of brain in Ammocoetes, 209
+    "  Fallopian, 431
+    "  hypophysial, 229, 244, 317, 440
+    "  meeting of four tubes in vertebrate, 318, 440
+    "  notochord originally a, 436, 440
+    "  olfactory, of Ammocoetes, 219, 225, 317, 440
+    "  unsegmented, in segmented animal, 439
+  Tunicata, 16
+      "     budding of, 441
+      "     degeneration, 12, 17, 19, 60
+      "     endostyle, 198, 212
+      "     hypophysis, 425
+      "     notochord, 438
+      "     position of, 494
+
+  Unit, appendage, in non-branchial segments, 185
+    "   branchial, 161, 165, 168, 185
+  Ureters, nerves of, 448
+  Uterus of Scorpion group, 189, 202, 203, 204, 205, 214
+     "   vertebrate, nerves of, 448
+
+  Valve, ileo-colic, 449
+     "   of Vieussens, 48
+  Variation in dominant races, 21, 88
+      "     meristic, in spinal nerves, 154, 387
+  Veins, forming vertebrate heart, 180
+  Velum, 228, 289, 298, 302
+  Vertebrates, alimentary canal, innervation of, 446
+       "       atrial cavity, 410
+       "       auditory apparatus and lateral-line system, 356
+       "       body-cavity, 401, 430
+       "       brains, 40
+       "       branchial organs, 151
+       "       coelomic cavities in head region, 251, 266
+       "       cranium, evolution of, 342
+       "       egg of, 483
+       "       evolution of, 11
+       "       excretory organs, 389, 391, 408
+       "       glands, ductless, 418
+       "          "    internal secretion of, 215
+       "       heart, 175, 179, 180
+       "       muscles, evidence of segmentation of eye, 248
+       "          "     oblique, 278
+       "          "     origin of somatic trunk, 406
+       "       nervous system, central, 13
+       "       nerves, segmental, 152
+       "       notochord and gut, 434
+       "       organs of, 10
+       "       origin of, 9, 411, 433, 457
+       "       segments, prosomatic, 257
+       "       skeleton, commencement of bony, 120, 458
+       "       spinal cord and medulla oblongata, 44
+       "       spinal region, 385
+       "       thyroid, connection between generative organs and, 215
+       "       tubes, meeting of four, 318, 440
+  Vesicles, cerebral, formation of, 48, 458
+  Vitellophags, 471, 483
+  Volvox, 479
+
+  Wolffian body, 390
+
+  Xiphosura, 24, 26, 249
+
+  Yolk, 482
+
+
+THE END
+
+
+
+Notes.
+
+[1] N.B.--In addition to the nerves mentioned, C. Bell included, in his
+    respiratory system of nerves, the fourth nerve or trochlearis, the
+    phrenic and the external respiratory of Bell.
+
+[2] "The Origin of Vertebrates, deduced from the Study of Ammocoetes." Part
+    X., "The Origin of the Auditory Organ: the Meaning of the VIIIth
+    Cranial Nerve." _Journ. Anat. and Physiol._, vol. 36, 1902.
+
+       *       *       *       *       *
+
+
+
+Corrections made to printed text
+
+Fig. 6: 'Dalmanites' corrected from 'Dalmatites' (which is an ammonite).
+
+Fig. 15 caption: 'Pterichthys' corrected from 'Ptericthys'. So also on P.
+239 and twice on P. 324.
+
+P. 60: 'gnathostomatous condition' corrected from 'gnathostomotous ...'.
+
+P. 409: 'well known' corrected from 'well know'.
+
+P. 420: 'meso-nephros' corrected from 'neso-nephros'.
+
+P. 432: 'had become a vertebrate' corrected from 'had became ...'.
+
+Fig. 167 caption: 'nephrocoele' corrected from 'nephrocele'.
+
+P. 474: 'Scyphomedusæ' corrected from 'Scyphomedusoe'.
+
+P. 497: 'idiosyncrasy' corrected from 'idiosyncracy'.
+
+Bibliography, Dietl: 'Gehirns' corrected from 'Gehirus'.
+
+Bibliography, Goodrich: 'Polychæta' corrected from 'Polychoeta'.
+
+Bibliography, Graber: 'Chordo-tonalen' corrected from 'Chordo-tonalem'.
+
+Bibliography, Vincent: 'Phylogeny' corrected from 'Phyogeny'.
+
+Index, Homology: 'Metazoa' corrected from 'Metozoa'.
+
+
+
+
+
+End of the Project Gutenberg EBook of The Origin of Vertebrates, by 
+Walter Holbrook Gaskell
+
+*** END OF THE PROJECT GUTENBERG EBOOK 44000 ***
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+    Origin of Vertebrates.
+  </title>
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+<body>
+<div>*** START OF THE PROJECT GUTENBERG EBOOK 44000 ***</div>
+
+  <table class="sp5 transnote nothand" title="Transcriber's note" summary="Transcriber's note">
+    <tr>
+      <td class="w25">Transcriber's note:</td>
+      <td>A few typographical errors have been corrected. They appear in the text <span
+      class="correction" title="explanation will pop up">like this</span>, and the explanation will
+      appear when the mouse pointer is moved over the marked passage.</td>
+    </tr>
+  </table>
+
+  <p class="ac" style="margin-bottom:1.8ex;"><span class="larger">THE</span></p>
+
+  <p class="ac" style="margin-bottom:6.1ex;"><span class="xxx-larger">ORIGIN OF
+  VERTEBRATES</span></p>
+
+  <p class="ac" style="margin-bottom:1.7ex;"><span class="smaller">BY</span></p>
+
+  <p class="ac" style="margin-bottom:1.3ex;"><span class="larger">WALTER HOLBROOK GASKELL</span></p>
+
+  <p class="ac" style="margin-bottom:5.8ex;"><span class="xx-smaller">M.A., M.D. (CANTAB.), LL.D.
+  (EDIN. AND McGILL UNIV.); F.R.S.; FELLOW OF TRINITY<br/>
+  HALL AND UNIVERSITY LECTURER IN PHYSIOLOGY, CAMBRIDGE; HONORARY FELLOW<br/>
+  OF THE ROYAL MEDICAL AND CHIRURGICAL SOCIETY; CORRESPONDING MEMBER<br/>
+  OF THE IMPERIAL MILITARY ACADEMY OF MEDICINE, ST. PETERSBURG, ETC.</span></p>
+
+  <p class="ac" style="margin-bottom:0.6ex;"><span class="larger"><span class="gsp">LONGMANS, GREEN,
+  AND CO.</span></span></p>
+
+  <p class="ac" style="margin-bottom:0.6ex;"><span class="smaller">39 PATERNOSTER ROW,
+  LONDON</span></p>
+
+  <p class="ac" style="margin-bottom:0.9ex;"><span class="smaller">NEW YORK, BOMBAY, AND
+  CALCUTTA</span></p>
+
+  <p class="ac" style="margin-bottom:0.7ex;"><span class="smaller">1908</span></p>
+
+  <p class="sp5 ac" style="margin-bottom:9.6ex;"><span class="smaller"><i>All rights
+  reserved</i></span></p>
+
+  <p class="sp3 ac" style="margin-bottom:4.3ex;"><span class="x-larger">CONTENTS</span></p>
+
+  <table class="sp5 mc w65" title="Contents" summary="Contents">
+    <tr>
+      <td colspan="2" class="ar smaller">PAGE</td>
+    </tr>
+    <tr>
+      <td class="pl0"><span class="sc">Introduction</span></td>
+      <td class="ar"><a href="#page1">1</a></td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac pt1">CHAPTER I</td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac"><span class="sc">The Evidence of the Central Nervous
+      System</span></td>
+    </tr>
+    <tr>
+      <td class="it1">Theories of the origin of vertebrates&mdash;Importance of the central nervous
+      system&mdash;Evolution of tissues&mdash;Evidence of Palæontology&mdash;Reasons for choosing
+      Ammoc&#x0153;tes rather than Amphioxus for the investigation of this problem&mdash;Importance
+      of larval forms&mdash;Comparison of the vertebrate and arthropod central nervous
+      systems&mdash;Antagonism between cephalization and alimentation&mdash;Life-history of lamprey,
+      not a degenerate animal&mdash;Brain of Ammoc&#x0153;tes compared with brain of
+      arthropod&mdash;Summary</td>
+      <td class="ar vbm"><a href="#page8">8</a></td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac pt1">CHAPTER II</td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac"><span class="sc">The Evidence of the Organs of Vision</span></td>
+    </tr>
+    <tr>
+      <td class="it1">Different kinds of eye&mdash;Simple and compound retinas&mdash;Upright and
+      inverted retinas&mdash;Median eyes&mdash;Median or pineal eyes of Ammoc&#x0153;tes and their
+      optic ganglia&mdash;Comparison with other median eyes&mdash;Lateral eyes of vertebrates
+      compared with lateral eyes of crustaceans&mdash;Peculiarities of the lateral eye of the
+      lamprey&mdash;Meaning of the optic diverticula&mdash;Evolution of vertebrate
+      eyes&mdash;Summary</td>
+      <td class="ar vbm"><a href="#page68">68</a></td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac pt1">CHAPTER III</td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac"><span class="sc">The Evidence of the Skeleton</span></td>
+    </tr>
+    <tr>
+      <td class="it1">The bony and cartilaginous skeleton considered, not the notochord&mdash;Nature
+      of the earliest cartilaginous skeleton&mdash;The mesosomatic skeleton of Ammoc&#x0153;tes; its
+      topographical arrangement, its structure, its origin in muco-cartilage&mdash;The prosomatic
+      skeleton of Ammoc&#x0153;tes; the trabeculæ and parachordals, their structure, their origin in
+      white fibrous tissue&mdash;The mesosomatic skeleton of Limulus compared with that of
+      Ammoc&#x0153;tes; similarity of position, of structure, of origin in muco-cartilage&mdash;The
+      prosomatic skeleton of Limulus; the entosternite, or plastron, compared with the trabeculæ of
+      Ammoc&#x0153;tes; similarity of position, of structure, of origin in fibrous
+      tissue&mdash;Summary</td>
+      <td class="ar vbm"><a href="#page119">119</a></td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac pt1">
+        <p class="sp0">CHAPTER IV</p>
+      </td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac"><span class="sc">The Evidence of the Respiratory
+      Apparatus</span></td>
+    </tr>
+    <tr>
+      <td class="it1">Branchiæ considered as internal branchial appendages&mdash;Innervation of
+      branchial segments&mdash;Cranial region older than spinal&mdash;Three-root system of cranial
+      nerves: dorsal, lateral, ventral&mdash;Explanation of van Wijhe's segments&mdash;Lateral mixed
+      root is appendage-nerve of invertebrate&mdash;The branchial chamber of
+      Ammoc&#x0153;tes&mdash;The branchial unit, not a pouch but an appendage&mdash;The origin of
+      the branchial musculature&mdash;The branchial circulation&mdash;The branchial heart of the
+      vertebrate&mdash;Not homologous with the systemic heart of the arthropod&mdash;Its formation
+      from two longitudinal venous sinuses&mdash;Summary</td>
+      <td class="ar vbm"><a href="#page148">148</a></td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac pt1">CHAPTER V</td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac"><span class="sc">The Evidence of the Thyroid Gland</span></td>
+    </tr>
+    <tr>
+      <td class="it1">The value of the appendage-unit in non-branchial segments&mdash;The double
+      nature of the hyoid segment&mdash;Its branchial part&mdash;Its thyroid part&mdash;The double
+      nature of the opercular appendage&mdash;Its branchial part&mdash;Its genital part&mdash;Unique
+      character of the thyroid gland of Ammoc&#x0153;tes&mdash;Its structure&mdash;Its
+      openings&mdash;The nature of the thyroid segment&mdash;The uterus of the scorpion&mdash;Its
+      glands&mdash;Comparison with the thyroid gland of Ammoc&#x0153;tes&mdash;Cephalic generative
+      glands of Limulus&mdash;Interpretation of glandular tissue filling up the brain-case of
+      Ammoc&#x0153;tes&mdash;Function of thyroid gland&mdash;Relation of thyroid gland to sexual
+      functions&mdash;Summary</td>
+      <td class="ar vbm"><a href="#page185">185</a></td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac pt1">CHAPTER VI</td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac"><span class="sc">The Evidence of the Olfactory
+      Apparatus</span></td>
+    </tr>
+    <tr>
+      <td class="it1">Fishes divided into Amphirhinæ and Monorhinæ&mdash;Nasal tube of the
+      lamprey&mdash;Its termination at the infundibulum&mdash;The olfactory organs of the scorpion
+      group&mdash;The camerostome&mdash;Its formation as a tube&mdash;Its derivation from a pair of
+      antennæ&mdash;Its termination at the true mouth&mdash;Comparison with the olfactory tube of
+      Ammoc&#x0153;tes&mdash;Origin of the nasal tube of Ammoc&#x0153;tes from the tube of the
+      hypophysis&mdash;Direct comparison of the hypophysial tube with the olfactory tube of the
+      scorpion group&mdash;Summary</td>
+      <td class="ar vbm"><a href="#page218">218</a></td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac pt1">CHAPTER VII</td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac"><span class="sc">The Prosomatic Segments of Limulus and its
+      Allies</span></td>
+    </tr>
+    <tr>
+      <td class="it1">Comparison of the trigeminal with the prosomatic region&mdash;The prosomatic
+      appendages of the Gigantostraca&mdash;Their number and nature&mdash;Endognaths and
+      ectognath&mdash;The metastoma&mdash;The coxal glands&mdash;Prosomatic region of Eurypterus
+      compared with that of Ammoc&#x0153;tes&mdash;Prosomatic segmentation shown by marks on
+      carapace&mdash;Evidence of c&#x0153;lomic cavities in Limulus&mdash;Summary</td>
+      <td class="ar vbm"><a href="#page233">233</a></td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac pt1">
+        <p class="sp0">CHAPTER VIII</p>
+      </td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac"><span class="sc">The Segments belonging to the Trigeminal
+      Nerve-Group</span></td>
+    </tr>
+    <tr>
+      <td class="it1">The prosomatic segments of the vertebrate&mdash;Number of segments belonging
+      to the trigeminal nerve-group&mdash;History of cranial segments&mdash;Eye-muscles and their
+      nerves&mdash;Comparison with the dorso-ventral somatic muscles of the
+      scorpion&mdash;Explanation of the oculomotor nerve and its group of muscles&mdash;Explanation
+      of the trochlear nerve and its dorsal crossing&mdash;Explanation of the abducens
+      nerve&mdash;Number of segments supplied by the trigeminal nerves&mdash;Evidence of their motor
+      nuclei&mdash;Evidence of their sensory ganglia&mdash;Summary</td>
+      <td class="ar vbm"><a href="#page257">257</a></td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac pt1">CHAPTER IX</td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac"><span class="sc">The Prosomatic Segments of
+      Ammoc&#x0153;tes</span></td>
+    </tr>
+    <tr>
+      <td class="it1">The prosomatic region in Ammoc&#x0153;tes&mdash;The suctorial apparatus of the
+      adult Petromyzon&mdash;Its origin in Ammoc&#x0153;tes&mdash;Its derivation from
+      appendages&mdash;The segment of the lower lip or the metastomal segment&mdash;The tentacular
+      segments&mdash;The tubular muscles&mdash;Their segmental arrangement&mdash;Their peculiar
+      innervation&mdash;Their correspondence with the system of veno-pericardial muscles in
+      Limulus&mdash;The old mouth or palæostoma&mdash;The pituitary gland&mdash;Its comparison with
+      the coxal gland of Limulus&mdash;Summary</td>
+      <td class="ar vbm"><a href="#page286">286</a></td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac pt1">CHAPTER X</td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac"><span class="sc">The Relationship of Ammoc&#x0153;tes to the most
+      Ancient Fishes&mdash;the Ostracodermata</span></td>
+    </tr>
+    <tr>
+      <td class="it1">Cephalaspis&mdash;Ammoc&#x0153;tes only living representative of these ancient
+      fishes&mdash;Formation of cranium&mdash;Closure of old mouth&mdash;Rohon's primordial
+      cranium&mdash;Primordial cranium of Phrynus and Galeodes&mdash;Summary</td>
+      <td class="ar vbm"><a href="#page326">326</a></td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac pt1">CHAPTER XI</td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac"><span class="sc">The Evidence of the Auditory Apparatus and the
+      Organs of the Lateral Line</span></td>
+    </tr>
+    <tr>
+      <td class="it1">Lateral line organs&mdash;Function of this group of organs&mdash;Poriferous
+      sense-organs on the appendages in Limulus&mdash;Branchial sense-organs&mdash;Prosomatic
+      sense-organs&mdash;Flabellum&mdash;Its structure and position&mdash;Sense-organs of
+      mandibles&mdash;Auditory organs of insects and arachnids&mdash;Poriferous chordotonal
+      organs&mdash;Balancers of Diptera&mdash;Resemblance to organs of
+      flabellum&mdash;Racquet-organs of Galeodes&mdash;Pectens of scorpions&mdash;Large size of
+      nerve to all these special sense-organs&mdash;Origin of parachordals and auditory
+      capsule&mdash;Reason why VIIth nerve passes in and out of capsule&mdash;Evidence of
+      Ammoc&#x0153;tes&mdash;Intrusion of glandular mass round brain into auditory
+      capsule&mdash;Intrusion of generative and hepatic mass round brain into base of
+      flabellum&mdash;Summary</td>
+      <td class="ar vbm"><a href="#page355">355</a></td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac pt1">
+        <p class="sp0">CHAPTER XII</p>
+      </td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac"><span class="sc">The Region of the Spinal Cord</span></td>
+    </tr>
+    <tr>
+      <td class="it1">Difference between cranial and spinal regions&mdash;Absence of lateral
+      root&mdash;Meristic variation&mdash;Segmentation of c&#x0153;lom&mdash;Segmental excretory
+      organs&mdash;Development of nephric organs; pronephric, mesonephric,
+      metanephric&mdash;Excretory organs of Amphioxus&mdash;Solenocytes&mdash;Excretory organs of
+      Branchipus and Peripatus, appendicular and somatic&mdash;Comparison of c&#x0153;lom of
+      Peripatus and of vertebrate&mdash;Pronephric organs compared to coxal glands&mdash;Origin of
+      vertebrate body-cavity (metac&#x0153;le)&mdash;Segmental duct&mdash;Summary of formation of
+      excretory organs&mdash;Origin of somatic trunk-musculature&mdash;Atrial cavity of
+      Amphioxus&mdash;Pleural folds&mdash;Ventral growth of pleural folds and somatic
+      musculature&mdash;Pleural folds of Cephalaspidæ and of Trilobita&mdash;Meaning of the ductless
+      glands&mdash;Alteration in structure of excretory organs which have lost their duct in
+      vertebrates and in invertebrates&mdash;Formation of lymphatic glands&mdash;Segmental coxal
+      glands of arthropods and of vertebrates&mdash;Origin of adrenals, pituitary body, thymus,
+      tonsils, thyroid, and other ductless glands&mdash;Summary</td>
+      <td class="ar vbm"><a href="#page385">385</a></td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac pt1">CHAPTER XIII</td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac"><span class="sc">The Notochord and Alimentary Canal</span></td>
+    </tr>
+    <tr>
+      <td class="it1">Relationship between notochord and gut&mdash;Position of unsegmented tube of
+      notochord&mdash;Origin of notochord from a median groove&mdash;Its function as an accessory
+      digestive tube&mdash;Formation of notochordal tissue in invertebrates from closed portions of
+      the digestive tube&mdash;Digestive power of the skin of Ammoc&#x0153;tes&mdash;Formation of
+      new gut in Ammoc&#x0153;tes at transformation&mdash;Innervation of the vertebrate
+      gut&mdash;The three outflows of efferent nerves belonging to the organic system&mdash;The
+      original close contiguity of the respiratory chamber to the cloaca&mdash;The elongation of the
+      gut&mdash;Conclusion</td>
+      <td class="ar vbm"><a href="#page433">433</a></td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac pt1">CHAPTER XIV</td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac"><span class="sc">The Principles of Embryology</span></td>
+    </tr>
+    <tr>
+      <td class="it1">The law of recapitulation&mdash;Vindication of this law by the theory advanced
+      in this book&mdash;The germ-layer theory&mdash;Its present position&mdash;A physiological not
+      a morphological conception&mdash;New fundamental law required&mdash;Composition of adult
+      body&mdash;Neuro-epithelial syncytium and free-living cells&mdash;Meaning of the
+      blastula&mdash;Derivation of the Metazoa from the Protozoa&mdash;Importance of the central
+      nervous system for Ontogeny as well as for Phylogeny&mdash;Derivation of free-living cells
+      from germ-cells&mdash;Meaning of c&#x0153;lom&mdash;Formation of neural canal&mdash;Gastrula
+      of Amphioxus and of Lucifer&mdash;Summary</td>
+      <td class="ar vbm"><a href="#page455">455</a></td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac pt1">
+        <p class="sp0">CHAPTER XV</p>
+      </td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac"><span class="sc">Final Remarks</span></td>
+    </tr>
+    <tr>
+      <td class="it1">
+        <p>Problems requiring investigation&mdash;</p>
+        <p>Giant nerve-cells and giant nerve-fibres; their comparison in fishes and arthropods;
+        blood- and lymph-corpuscles; nature of the skin; origin of system of unstriped muscles;
+        origin of the sympathetic nervous system; biological test of relationship.</p>
+        <p class="sp0">Criticisms of Balanoglossus theory&mdash;Theory of parallel
+        development&mdash;Importance of the theory advocated in this book for all problems of
+        Evolution</p>
+      </td>
+      <td class="ar vbm"><a href="#page488">488</a></td>
+    </tr>
+    <tr>
+      <td class="pl0 pt1"><span class="sc">Bibliography and Index of Authors</span></td>
+      <td class="ar pt1"><a href="#page501">501</a></td>
+    </tr>
+    <tr>
+      <td class="pl0 pt1"><span class="sc">General Index</span></td>
+      <td class="ar pt1"><a href="#page517">517</a></td>
+    </tr>
+  </table>
+
+  <div class="poem sp5">
+    <p>"<i><span class="sc">Go on and prosper; there is nothing so</span></i></p>
+    <p><i><span class="sc">useful in science as one of those earthquake</span></i></p>
+    <p><i><span class="sc">hypotheses, which oblige one to face</span></i></p>
+    <p><i><span class="sc">the possibility that the solidest-looking</span></i></p>
+    <p><i><span class="sc">structures may collapse.</span></i>"</p>
+    <p style="margin-left:7.00em" class="stanza"><span class="sc">Letter from Prof. Huxley
+    to</span></p>
+    <p style="margin-left:7.00em"><span class="sc">the Author. June 2, 1889.</span></p>
+  </div>
+
+  <div><span class="pagenum" id="page1">{1}</span></div>
+
+  <p class="ac">THE</p>
+
+  <p class="sp5 ac" style="margin-bottom:5.3ex;"><span class="x-larger">ORIGIN OF
+  VERTEBRATES</span></p>
+
+  <p class="sp3 ac"><i>INTRODUCTION</i></p>
+
+  <p>In former days it was possible for a man like Johannes Müller to be a leader both in physiology
+  and in comparative anatomy. Nowadays all scientific knowledge has increased so largely that
+  specialization is inevitable, and every investigator is confined more and more not only to one
+  department of science, but as a rule to one small portion of that department. In the case of such
+  cognate sciences as physiology and comparative anatomy this limiting of the scope of view is
+  especially deleterious, for zoology without physiology is dead, and physiology in many of its
+  departments without comparative anatomy can advance but little. Then, again, the too exclusive
+  study of one subject always tends to force the mind into a special groove&mdash;into a line of
+  thought so deeply tinged with the prevalent teaching of the subject, that any suggestions which
+  arise contrary to such teaching are apt to be dismissed at once as heretical and not worthy of
+  further thought; whereas the same suggestion arising in the mind of one outside this particular
+  line of thought may give rise to new and valuable scientific discoveries.</p>
+
+  <p>Nothing but good can, in my opinion, result from the incursion of the non-specialist into the
+  realm of the specialist, provided that the former is in earnest. Over and over again the chemist
+  has given valuable help to the physicist, and the physicist to the chemist, so closely allied are
+  the two subjects; so also is it with physiology and anatomy, the two subjects are so
+  interdependent that a worker in the one may give valuable aid towards the solution of some large
+  problem which is the special territory of the other.</p>
+
+  <p>It has been a matter of surprise to many how it came about that <span class="pagenum"
+  id="page2">{2}</span>I, a worker in the physiological laboratory at Cambridge ever since Foster
+  introduced experimental physiology into English-speaking nations, should have devoted so much time
+  to the promulgation of a theory of the origin of vertebrates&mdash;a subject remote from
+  physiology, and one of the larger questions appertaining to comparative anatomy. By what process
+  of thought was I led to take up the consideration of a subject apparently so remote from all my
+  previous work, and so foreign to the atmosphere of a physiological laboratory?</p>
+
+  <p>It may perhaps be instructive to my readers to see how one investigation leads to another,
+  until at last, <i>nolens volens</i>, the worker finds himself in front of a possible solution to a
+  problem far removed from his original investigation, which by the very magnitude and importance of
+  it forces him to devote his whole energy and time to seeing whether his theory is good.</p>
+
+  <p>In the years 1880-1884 I was engaged in the investigation of the action of the heart, and the
+  nature of the nerves which regulate that action. In the course of that investigation I was struck
+  by the ease with which it was possible to distinguish between the fibres of the vagus and
+  accelerator nerves on their way to the heart, owing to the medullation of the former and the
+  non-medullation of the latter. This led me to an investigation of the accelerator fibres, to find
+  out how far they are non-medullated, and so to the discovery that the <i>rami communicantes</i>
+  connecting together the central nervous system and the sympathetic are in reality single, not
+  double, as had hitherto been thought; for the grey <i>ramus communicans</i> is in reality a
+  peripheral nerve which supplies the blood-vessels of the spinal cord and its membranes, and is of
+  the same nature as the grey accelerators to the heart.</p>
+
+  <p>This led to the conclusion that there is no give and take between two independent nervous
+  systems, the cerebro-spinal and the sympathetic, as had been taught formerly, but only one nervous
+  system, the cerebro-spinal, which sends special medullated nerve-fibres, characterized by their
+  smallness, to the cells of the sympathetic system, from which fibres pass to the periphery,
+  usually non-medullated. These fine medullated nerves form the system of white <i>rami
+  communicantes</i>, and have since been called by Langley the preganglionic nerves. Further
+  investigation showed that such white rami are not universally distributed, but are confined to the
+  thoracico-lumbar region, where their distribution is easily seen in <span class="pagenum"
+  id="page3">{3}</span>the ventral roots, for the cells of the sympathetic system are entirely
+  efferent in nature, not afferent; therefore, the fibres entering into them from the central
+  nervous system leave the spinal cord by ventral, not dorsal roots.</p>
+
+  <p>Following out this clue, I then found that in addition to this thoracico-lumbar outflow of
+  efferent ganglionated visceral nerves, there are similar outflows in the cranial and sacral
+  regions, belonging in the former case especially to the vagus system of nerves, and in the latter
+  to the system of nerves which pass from the sacral region of the cord to the ganglion-cells of the
+  hypogastric plexus, and from them supply the bladder, rectum, etc. To this system of nerves,
+  formerly called the <i>nervi erigentes</i>, I gave the name pelvic splanchnics, in order to show
+  their uniformity with the abdominal splanchnics. These investigations led to the conclusion that
+  the organic system of nerves, characterized by the possession of efferent nerve-cells situated
+  peripherally, arises from the central nervous system by three distinct outflows&mdash;cranial,
+  thoracico-lumbar, and sacral, respectively. To this system Langley has lately given the name
+  'autonomic.' These three outflows are separated by two gaps just where the plexuses for the
+  anterior and posterior extremities come in.</p>
+
+  <p>This peculiar arrangement of the white <i>rami communicantes</i> set me thinking, for the gaps
+  corresponded to an increase of somatic musculature to form the muscles of the fore and hind limbs,
+  so that if, as seemed probable, the white <i>rami communicantes</i> arise segmentally from the
+  spinal cord, then a marked distinction must exist in structure between the spinal cord in the
+  thoracic region, where the visceral efferent nerves are large in amount and the body musculature
+  scanty, and in the cervical or lumbar swellings, where the somatic musculature abounds, and the
+  white <i>rami communicantes</i> scarcely exist.</p>
+
+  <p>I therefore directed my attention in the next place to the structure of the central nervous
+  system in the endeavour to associate the topographical arrangement of cell-groups in this system
+  with the outflow of the different kinds of nerve-fibres to the peripheral organs.</p>
+
+  <p>This investigation forcibly impressed upon my mind the uniformity in the arrangement of the
+  central nervous system as far as the centres of origin of all the segmental nerves are concerned,
+  <span class="pagenum" id="page4">{4}</span>both cranial and spinal, and also the original
+  segmental character of this part of the nervous system.</p>
+
+  <p>I could not, therefore, help being struck by the force of the comparison between the central
+  nervous systems of Vertebrata and Appendiculata as put forward again and again by the past
+  generation of comparative anatomists, and wondered why it had been discredited. There in the
+  infundibulum was the old &#x0153;sophagus, there in the cranial segmental nerves the
+  infra&#x0153;sophageal ganglia, there in the cerebral hemispheres and optic and olfactory nerves
+  the supra&#x0153;sophageal ganglia, there in the spinal cord the ventral chain of ganglia. But if
+  the infundibulum was the old &#x0153;sophagus, what then? The old &#x0153;sophagus was continuous
+  with and led into the cephalic stomach. What about the infundibulum? It was continuous with and
+  led into the ventricles of the brain, and the whole thing became clear. The ventricles of the
+  brain were the old cephalic stomach, and the canal of the spinal cord the long straight intestine
+  which led originally to the anus, and still in the vertebrate embryo opens out into the anus. Not
+  having been educated in a morphological laboratory and taught that the one organ which is
+  homologous throughout the animal kingdom is the gut, and that therefore the gut of the
+  invertebrate ancestor must continue on as the gut of the vertebrate, the conception that the
+  central nervous system has grown round and enclosed the original ancestral gut, and that the
+  vertebrate has formed a new gut did not seem to me so impossible as to prevent my taking it as a
+  working hypothesis, and seeing to what it would lead.</p>
+
+  <p>This theory that the so-called central nervous system of the vertebrate is in reality composed
+  of two separate parts, of which the one, the segmented part, corresponds to the central nervous
+  system of the highest invertebrates, while the other, the unsegmented tube, was originally the
+  alimentary canal of that same invertebrate, came into my mind in the year 1887. The following
+  year, on June 23, 1888, I read a paper on the subject before the Anatomical Society at Cambridge,
+  which was published in the <i>Journal of Anatomy and Physiology</i>, vol. 23, and more fully in
+  the <i>Journal of Physiology</i>, vol. 10. Since that time I have been engaged in testing the
+  theory in every possible way, and have published the results of my investigations in a series of
+  papers in different journals, a list of which I append at the end of this introductory
+  chapter.</p>
+
+  <div><span class="pagenum" id="page5">{5}</span></div>
+
+  <p>It is now twenty years since the theory first came into my mind, and the work of those twenty
+  years has convinced me more and more of its truth, and yet during the whole time it has been
+  ignored by the morphological world as a whole rather than criticized. Whatever may have been the
+  causes for such absence of criticism, it is clear that the serial character of its publication is
+  a hindrance to criticism of the theory as a whole, and I hope, therefore, that the publication of
+  the whole of the twenty years' work in book-form will induce those who differ from my conclusions
+  to come forward and show me where I am wrong, and why my theory is untenable. Any one who has been
+  thinking over any one problem for so long a time becomes obsessed with the infallibility of his
+  own views, and is not capable of criticizing his own work as thoroughly as others would do. I have
+  been told that it is impossible for one man to consider so vast a subject with that thoroughness
+  which is necessary, before any theory can be accepted as the true solution of the problem. I
+  acknowledge the vastness of the task, and feel keenly enough my own shortcomings. For all that, I
+  do feel that it can only be of advantage to scientific progress and a help to the solution of this
+  great problem, to bring together in one book all the facts which I have been able to collect,
+  which appeal to me as having an important bearing on this solution.</p>
+
+  <p>In this work I have been helped throughout by Miss R. Alcock. It is not too much to say that
+  without the assistance she has given me, many an important link in the chain of evidence would
+  have been missing. With extraordinary patience she has followed, section by section, the smallest
+  nerves to their destination, and has largely helped to free the transformation process in the
+  lamprey from the mystery which has hitherto enveloped it. She has drawn for me very many of the
+  illustrations scattered through the pages in this book, and I feel that her aid has been so
+  valuable and so continuous, lasting as it does over the whole period of the work, that her name
+  ought fittingly to be associated with mine, if perchance the theory of the Origin of Vertebrates,
+  advocated in the pages of this book, gains acceptance.</p>
+
+  <p class="sp5">I am also indebted to Mr. J. Stanley Gardiner and to Dr. A. Sheridan Lea for
+  valuable assistance in preparing this book for the press. I desire to express my grateful thanks
+  to the former for valuable criticism of the scientific evidence which I have brought <span
+  class="pagenum" id="page6">{6}</span>forward in this book, and to the latter for his great
+  kindness in undertaking the laborious task of collecting the proofs.</p>
+
+  <p class="ac">LIST OF PREVIOUS PUBLICATIONS BY THE AUTHOR, CONCERNING THE ORIGIN OF
+  VERTEBRATES.</p>
+
+  <table class="sp3 mc w50" title="Previous Publications by the Author"
+  summary="Previous Publications by the Author">
+    <tr>
+      <td>1888.</td>
+      <td class="it1p05">"Spinal and Cranial Nerves." <i>Proceedings of the Anatomical Society</i>,
+      June, 1888. <i>Journal of Anatomy and Physiology</i>, vol. xxiii.</td>
+    </tr>
+    <tr>
+      <td class="pt1">1889.</td>
+      <td class="it1p05 pt1">"On the Relation between the Structure, Function, Distribution, and
+      Origin of the Cranial Nerves; together with a Theory of the Origin of the Nervous System of
+      Vertebrata." <i>Journal of Physiology</i>, vol. x., p. 153.</td>
+    </tr>
+    <tr>
+      <td class="pt1">1889.</td>
+      <td class="it1p05 pt1">"On the Origin of the Central Nervous System of Vertebrates."
+      <i>Brain</i>, vol. xii., p. 1.</td>
+    </tr>
+    <tr>
+      <td class="pt1">1890.</td>
+      <td class="it1p05 pt1">"On the Origin of Vertebrates from a Crustacean-like Ancestor."
+      <i>Quarterly Journal of Microscopical Science</i>, vol. xxxi., p. 379.</td>
+    </tr>
+    <tr>
+      <td class="pt1">1895.</td>
+      <td class="it1p05 pt1">"The Origin of Vertebrates." <i>Proceedings of the Cambridge
+      Philosophical Society</i>, vol. ix., p. 19.</td>
+    </tr>
+    <tr>
+      <td class="pt1">1896.</td>
+      <td class="it1p05 pt1">Presidential Address to Section I. at the meeting of the British
+      Association for the Advancement of Science in Liverpool. <i>Report of the British
+      Association</i>, 1896, p. 942.</td>
+    </tr>
+    <tr>
+      <td class="pt1">1899.</td>
+      <td class="it1p05 pt1">"On the Meaning of the Cranial Nerves." Presidential Address to the
+      Neurological Society for the year 1899. <i>Brain</i>, vol. xxii., p. 329.</td>
+    </tr>
+  </table>
+
+  <p class="sp3">A series of papers on "The Origin of Vertebrates, deduced from the study of
+  Ammoc&#x0153;tes," in the <i>Journal of Anatomy and Physiology</i>, as follows<span
+  class="wnw">:&mdash;</span></p>
+
+  <table class="sp5 mc w50" title="Previous Publications by the Author,
+  continued" summary="Previous Publications by the Author,
+  continued">
+    <tr>
+      <td class="">1898.</td>
+      <td class="ac prhs">Part</td>
+      <td class="plhs ar">I.</td>
+      <td class="it1p05">"The Origin of the Brain," vol. xxxii., p. 513.</td>
+    </tr>
+    <tr>
+      <td></td>
+      <td class="ac prhs">"</td>
+      <td class="plhs ar">II.</td>
+      <td class="it1p05">"The Origin of the Vertebrate Cranio-facial Skeleton," vol. xxxii., p.
+      553.</td>
+    </tr>
+    <tr>
+      <td></td>
+      <td class="ac prhs">"</td>
+      <td class="plhs ar">III.</td>
+      <td class="it1p05">"The Origin of the Branchial Segmentation," vol. xxxiii., p. 154.</td>
+    </tr>
+    <tr>
+      <td class="">1899.</td>
+      <td class="ac prhs">"</td>
+      <td class="plhs ar">IV.</td>
+      <td class="it1p05">"The Thyroid, or Opercular Segment: the Meaning of the Facial Nerve," vol.
+      xxxiii., p. 638.</td>
+    </tr>
+    <tr>
+      <td class="">1900.</td>
+      <td class="ac prhs">"</td>
+      <td class="plhs ar">V.</td>
+      <td class="it1p05">"The Origin of the Pro-otic Segmentation: the Meaning of the Trigeminal and
+      Eye-muscle Nerves," vol. xxxiv., p. 465.</td>
+    </tr>
+    <tr>
+      <td class="">1900.</td>
+      <td class="ac prhs">"</td>
+      <td class="plhs ar">VI.</td>
+      <td class="it1p05">"The Old Mouth and the Olfactory Organ: the Meaning of the First Nerve,"
+      vol. xxxiv., p. 514.</td>
+    </tr>
+    <tr>
+      <td class="">1900.</td>
+      <td class="ac prhs">"</td>
+      <td class="plhs ar">VII.</td>
+      <td class="it1p05">"The Evidence of Prosomatic Appendages in Ammoc&#x0153;tes, as given by the
+      Course and Distribution of the Trigeminal Nerve," vol. xxxiv., p. 537.</td>
+    </tr>
+    <tr>
+      <td class="">1900.</td>
+      <td class="ac prhs">"</td>
+      <td class="plhs ar">VIII.</td>
+      <td class="it1p05">"The Palæontological Evidence: Ammoc&#x0153;tes a Cephalaspid," vol.
+      xxxiv., p. 562.</td>
+    </tr>
+    <tr>
+      <td class="">1901.</td>
+      <td class="ac prhs">"</td>
+      <td class="plhs ar">IX.</td>
+      <td class="it1p05">"The Origin of the Optic Apparatus: the Meaning of the Optic Nerves," vol.
+      xxxv., p. 224.</td>
+    </tr>
+    <tr>
+      <td class=""><span class="pagenum" id="page7">{7}</span>
+        <p class="sp0">1902.</p>
+      </td>
+      <td class="ac prhs">"</td>
+      <td class="plhs ar">X.</td>
+      <td class="it1p05">"The Origin of the Auditory Organ: the Meaning of the VIIIth Cranial
+      Nerve," vol. xxxvi., p. 164.</td>
+    </tr>
+    <tr>
+      <td class="">1903.</td>
+      <td class="ac prhs">"</td>
+      <td class="plhs ar">XI.</td>
+      <td class="it1p05">"The Origin of the Vertebrate Body-cavity and Excretory Organs: the Meaning
+      of the Somites of the Trunk and of the Ductless Glands," vol. xxxvii., p. 168.</td>
+    </tr>
+    <tr>
+      <td class="">1905.</td>
+      <td class="ac prhs">"</td>
+      <td class="plhs ar">XII.</td>
+      <td class="it1p05">"The Principles of Embryology," vol. xxxix., p. 371.</td>
+    </tr>
+    <tr>
+      <td class="">1906.</td>
+      <td class="ac prhs">"</td>
+      <td class="plhs ar">XIII.</td>
+      <td class="it1p05">"The Origin of the Notochord and Alimentary Canal," vol. xl., p. 305.</td>
+    </tr>
+  </table>
+
+  <div><span class="pagenum" id="page8">{8}</span></div>
+
+  <p class="ac">CHAPTER I</p>
+
+  <p class="ac"><i>THE EVIDENCE OF THE CENTRAL NERVOUS SYSTEM</i></p>
+
+  <div class="bq1 it1 smaller sp3">
+    <p class="sp0">Theories of the origin of vertebrates.&mdash;Importance of the central nervous
+    system.&mdash;Evolution of tissues.&mdash;Evidence of Palæontology.&mdash;Reasons for choosing
+    Ammoc&#x0153;tes rather than Amphioxus.&mdash;Importance of larval forms.&mdash;Comparison of
+    the vertebrate and arthropod central nervous systems.&mdash;Antagonism between cephalization and
+    alimentation.&mdash;Life-history of lamprey: not a degenerate animal.&mdash;Brain of
+    Ammoc&#x0153;tes compared with brain of arthropod.&mdash;Summary.</p>
+  </div>
+
+  <p>At the present time it is no longer a debatable question whether or no Evolution has taken
+  place. Since the time of Darwin the accumulation of facts in its support has been so overwhelming
+  that all zoologists look upon this question as settled, and desire now to find out the manner in
+  which such evolution has taken place. Here two problems offer themselves for investigation, which
+  can be and are treated separately&mdash;the one dealing with the question of those laws of
+  heredity and variation which have brought about in the past and are still causing in the present
+  the evolution of living beings, <i>i.e.</i> the causes of evolution; the other concerned with the
+  relationship of animals, or groups of animals, rather than with the causes which have brought
+  about such relationship, <i>i.e.</i> the sequence of evolution.</p>
+
+  <p>It is the latter problem with which this book deals, and, indeed, not with the whole question
+  at all, but only with that part of it which concerns the origin of vertebrates.</p>
+
+  <p>This problem of the sequence of evolution is of a twofold character: first, the finding out of
+  the steps by which the higher forms in any one group of animals have been evolved from the lower;
+  and secondly, the evolution of the group itself from a lower group.</p>
+
+  <p>In any classification of the animal kingdom, it is clear that large groups of animals exist
+  which have so many common characteristics as to necessitate their being placed in one larger group
+  or kingdom; <span class="pagenum" id="page9">{9}</span>thus zoologists are able to speak
+  definitely of the Vertebrata, Arthropoda, Annelida, Echinodermata, Porifera, C&#x0153;lenterata,
+  Mollusca, etc. In each of these groups affinities can be traced between the members, so that it is
+  possible to speak of the progress from lower to higher members of the group, and it is
+  conceivable, given time to work out the details, that the natural relationships between the
+  members of the whole group will ultimately be discovered.</p>
+
+  <p>Thus no one can doubt that a sequence of the kind has taken place in the Vertebrata as we trace
+  the progress from the lowest fishes to man, and already the discoveries of palæontology and
+  anatomy give us a distinct clue to the sequence from fish to amphibian, from amphibian to reptile,
+  from reptile to mammal on the one hand, and to bird on the other. That the different members of
+  the vertebrate group are related to each other in orderly sequence is no longer a matter of doubt;
+  the connected problems are matters of detail, the solution of which is certain sooner or later.
+  The same may be said of the members of any of the other great natural groups, such as the
+  Arthropoda, the Annelida, the Echinodermata, etc.</p>
+
+  <p>It is different, however, when an attempt is made to connect two of the main divisions
+  themselves. It is true enough that there is every reason to believe that the arthropod group has
+  been evolved from the segmented annelid, and so the whole of the segmented invertebrates may be
+  looked on as forming one big division, the Appendiculata, all the members of which will some day
+  be arranged in orderly sequence, but the same feeling of certainty does not exist in other
+  cases.</p>
+
+  <p class="sp3">In the very case of the origin of the Appendiculata we are confronted with one of
+  the large problems of evolution&mdash;the origin of segmented from non-segmented animals&mdash;the
+  solution of which is not yet known.</p>
+
+  <p class="ac"><span class="sc">Theories of the Origin of Vertebrates.</span></p>
+
+  <p>The other large problem, perhaps the most important of all, is the question of the relationship
+  of the great kingdom of the Vertebrata: from what invertebrate group did the vertebrate arise?</p>
+
+  <p>The great difficulty which presents itself in attempting a solution of this question is not so
+  much, as used to be thought, the difficulty of deriving a group of animals possessing an internal
+  bony and <span class="pagenum" id="page10">{10}</span>cartilaginous skeleton from a group
+  possessing an external skeleton of a calcareous or chitinous nature, but rather the difficulty
+  caused by the fundamental difference of arrangement of the important internal organs, especially
+  the relative positions of the central nervous system and the digestive tube.</p>
+
+  <div class="ac w50 fcenter sp2">
+    <a href="images/fig001.png" id="fig1"><img style="width:100%" src="images/fig001.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 1.</span>&mdash;<span class="sc">Arrangement of Organs in the
+      Vertebrate (A) and Arthropod (B).</span></p>
+      <p class="sp0"><i>Al</i>, gut; <i>H</i>, heart; <i>C.N.S.</i>, central nervous system; V,
+      ventral side; D, dorsal side.</p>
+    </div>
+  </div>
+
+  <p>Now, if we take a broad and comprehensive view of the invertebrate kingdom, without arguing out
+  each separate case, we find that it bears strongly the stamp of a general plan of evolution
+  derived from a c&#x0153;lenterate animal, whose central nervous system formed a ring surrounding
+  the mouth. Then when the radial symmetry was given up, and an elongated, bilateral, segmented form
+  evolved, the central nervous system also became elongated and segmented, but, owing to its
+  derivation from an oral ring, it still surrounded the mouth-tube, or &#x0153;sophagus, and thus in
+  its highest forms is divided into supra-&#x0153;sophageal and infra-&#x0153;sophageal nervous
+  masses. These latter <span class="pagenum" id="page11">{11}</span>nervous masses are of necessity
+  ventral to the digestive tube, because the mouth of the c&#x0153;lenterate is on the ventral side.
+  The striking characteristic, then, of the invertebrate kingdom is the situation of a large portion
+  of the central nervous system ventrally to the alimentary canal and the piercing of the nervous
+  system by a tube&mdash;the &#x0153;sophagus&mdash;leading from the mouth to the alimentary canal.
+  The equally striking characteristic of the vertebrate is the dorsal position of the central
+  nervous system and the ventral position of the alimentary canal combined with the absence of any
+  piercing of the central nervous system by the &#x0153;sophagus.</p>
+
+  <p>So fundamentally different is the arrangement of the important organs in the two groups that it
+  might well give rise to a feeling of despair of ever hoping to solve the problem of the Origin of
+  Vertebrates; and, to my mind, this is the prevalent feeling among morphologists at the present
+  time. Two attempts at solution have been made. The one is associated with the name of Geoffrey St.
+  Hilaire, and is based on the supposition that the vertebrate has arisen from the invertebrate by
+  turning over on its back, swimming in this position, and so gradually converting an originally
+  dorsal surface into a ventral one, and <i>vice versâ</i>; at the same time, a new mouth is
+  supposed to have been formed on the new ventral side, which opened directly into the alimentary
+  canal, while the old mouth, which had now become dorsal, was obliterated.</p>
+
+  <p>The other attempt at solution is of much more recent date, and is especially associated with
+  the name of Bateson. It supposes that bilaterally symmetrical, elongated, segmented animals were
+  formed from the very first in two distinct ways. In the one case the digestive tube pierced the
+  central nervous system, and was situated dorsally to its main mass. In the other case the
+  segmented central nervous system was situated from the first dorsally to the alimentary canal, and
+  was not pierced by it. In the first case the highest result of evolution led to the Arthropoda; in
+  the second case to the Vertebrata.</p>
+
+  <p>Neither of these views is based on evidence so strong as to cause universal acceptance. The
+  great difficulty in the way of accepting the second alternative is the complete absence of any
+  evidence, either among animals living on the earth at the present day or among those known to have
+  existed in the past, of any such chain of intermediate animal forms as must, on this hypothesis,
+  have existed in order to link together the lower forms of life with the vertebrates.</p>
+
+  <div><span class="pagenum" id="page12">{12}</span></div>
+
+  <div class="ac w30 fcenter sp2">
+    <a href="images/fig002.png" id="fig2"><img style="width:100%" src="images/fig002.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p class="sp0"><span class="sc">Fig. 2.</span>&mdash;<span class="sc">Larval
+      Balanoglossus</span> (from the Royal Natural History).</p>
+    </div>
+  </div>
+
+  <p>It has been supposed that the Tunicata and the Enteropneusta (<i>Balanoglossus</i>) (Fig. <a
+  href="#fig2">2</a>) are members of this missing chain, and that in Amphioxus the vertebrate
+  approaches in organization to these low invertebrate forms. The tunicates, indeed, are looked upon
+  as degenerate members of an early vertebrate stock, which may give help in picturing the nature of
+  the vertebrate ancestor but are not themselves in the direct line of descent. Balanoglossus is
+  supposed to have arisen from the Echinodermata, or at all events to have affinities with them, so
+  that to fill up the enormous gap between the Echinodermata and the Vertebrata on this theory there
+  is absolutely nothing living on the earth except Balanoglossus, Rhabdopleura, and Cephalodiscus.
+  The characteristics of the vertebrate upon which this second theory is based are the notochord,
+  the respiratory character of the anterior part of the alimentary canal, and the tubular nature of
+  the central nervous system; it is claimed that in Balanoglossus the beginnings of a notochord and
+  a tubular central nervous system are to be found, while the respiratory portion of the gut is
+  closely comparable to that of Amphioxus.</p>
+
+  <p>The strength of the first theory is essentially based on the comparison of the vertebrate
+  central nervous system with that of the segmented invertebrate, annelid or arthropod. In the
+  latter the central nervous system is composed of&mdash;</p>
+
+  <p>1.  The supra-&#x0153;sophageal ganglia, which give origin to the nerves of the eyes and
+  antennules, <i>i.e.</i> to the optic and olfactory nerves, for the first pair of antennæ are
+  olfactory in function. These are connected with the infra-&#x0153;sophageal ganglia by the
+  &#x0153;sophageal commissures which encircle the &#x0153;sophagus.</p>
+
+  <p>2.  The infra-&#x0153;sophageal ganglia and the two chains of ventral ganglia, which are
+  segmentally-arranged sets of ganglia. Of these, <span class="pagenum" id="page13">{13}</span>each
+  pair gives rise to the nerves of its own segment, and these nerves are not nerves of special sense
+  as are the supra-&#x0153;sophageal nerves, but motor and sensory to the segment; nerves by the
+  agency of which food is taken in and masticated, respiration is effected, and the animal moves
+  from place to place.</p>
+
+  <p>In the vertebrate the central nervous system consists of&mdash;</p>
+
+  <p>1. The brain proper, from which arise only the olfactory and optic nerves.</p>
+
+  <div class="ac w50 fcenter sp2">
+    <a href="images/fig003.png" id="fig3"><img style="width:100%" src="images/fig003.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 3.</span>&mdash;<span class="sc">Vertebrate Central
+      Nervous System compared with the Central Nervous System and Alimentary Canal of the
+      Arthropod.</span></p>
+      <p>A.  Vertebrate central nervous system. <i>S. Inf. Br.</i>, supra-infundibular brain; <i>I.
+      Inf. Br.</i>, infra-infundibular brain and cranial segmental nerves; <i>C.Q.</i>, corpora
+      quadrigemina; <i>Cb.</i>, cerebellum; <i>C.C.</i>, crura cerebri; <i>C.S.</i>, corpus
+      striatum; <i>Pn.</i>, pineal gland.</p>
+      <p class="sp0">B.  Invertebrate central nervous system. <i>S. &#x0152;s. G.</i>,
+      supra-&#x0153;sophageal ganglia; <i>I. &#x0152;s. G.</i>, infra-&#x0153;sophageal ganglia;
+      <i>&#x0152;s. Com.</i>, &#x0153;sophageal commissures.</p>
+    </div>
+  </div>
+
+  <p>2. The region of the mid-brain, medulla oblongata, and spinal cord; from these arises a series
+  of nerves segmentally arranged, which, as in the invertebrate, gives origin to the nerves
+  governing mastication, respiration, and locomotion.</p>
+
+  <p>Further, the vertebrate central nervous system possesses the peculiarity, found nowhere else,
+  of being tubular, and the tube is of a striking character. In the spinal region it is a small,
+  simple canal of uniform calibre, which at the front end dilates to form the ventricles of the
+  region of the brain. From that part of this dilated <span class="pagenum"
+  id="page14">{14}</span>portion, known as the third ventricle, a narrow tube passes to the ventral
+  surface of the brain. This tube is called the <i>infundibulum</i>, and, extraordinary to relate,
+  lies just anteriorly to the exits of the third cranial or oculomotor nerves; in other words, it
+  marks the termination of the series of spinal and cranial segmental nerves. Further, on each side
+  of this infundibular tube are lying the two thick masses of the <i>crura cerebri</i>, the strands
+  of fibres which connect the higher brain-region proper with the lower region of the medulla
+  oblongata and spinal cord. Not only, then, are the nerve-masses in the two systems exactly
+  comparable, but in the very place where the &#x0153;sophageal tube is found in the invertebrate,
+  the infundibular tube exists in the vertebrate, so that if the words infundibular and
+  &#x0153;sophageal are taken to be interchangable, then in every respect the two central nervous
+  systems are comparable. The brain proper of the vertebrate, with its olfactory and optic nerves,
+  becomes the direct descendant of the supra-&#x0153;sophageal ganglia; the crura cerebri become the
+  &#x0153;sophageal commissures, and the cranial and spinal segmental nerves are respectively the
+  nerves belonging to the infra-&#x0153;sophageal and ventral chain of ganglia.</p>
+
+  <p>This overwhelmingly strong evidence has always pointed directly to the origin of the vertebrate
+  from some form among the segmented group of invertebrates, annelid or arthropod, in which the
+  original &#x0153;sophagus had become converted into the infundibulum, and a new mouth formed. So
+  far, the position of this school of anatomists was extremely sound, for it is impossible to
+  dispute the facts on which it is based. Still, however, the fact remained that the gut of the
+  vertebrate lies ventrally to the nervous system, while that of the invertebrate lies dorsally;
+  consequently, since the infundibulum was in the position of the invertebrate &#x0153;sophagus, it
+  must originally have entered into the gut, and since the vertebrate gut was lying ventrally to it,
+  it could only have opened into that gut in the invertebrate stage by the shifting of dorsal and
+  ventral surfaces. From this argument it followed that the remains of the original mouth into which
+  the infundibulum, <i>i.e.</i> &#x0153;sophagus, opened were to be sought for on the dorsal side of
+  the vertebrate brain. Here in all vertebrates there are two spots where the roof of the brain is
+  very thin, the one in the region of the pineal body, and the other constituting the roof of the
+  fourth ventricle. Both of these places have had their advocates as the position of the old mouth,
+  the former being upheld by Owen, the latter by Dohrn.</p>
+
+  <div><span class="pagenum" id="page15">{15}</span></div>
+
+  <p>The discovery that the pineal body was originally an eye, or, rather, a pair of eyes, has
+  perhaps more than anything else proved the impossibility of accepting this reversal of surfaces as
+  an explanation of the genesis of the vertebrate from the annelid group. For whereas a pair of eyes
+  close to the mid-dorsal line is not only likely enough, but is actually found to exist among large
+  numbers of arthropods, both living and extinct, a pair of eyes situated close to the mid-ventral
+  line near the mouth is not only unheard of in nature, but so improbable as to render impossible
+  the theory which necessitates such a position.</p>
+
+  <p>Yet this very discovery gives the strongest possible additional support to the close identity
+  in the plan of the central nervous system of vertebrate and appendiculate.</p>
+
+  <p>A truly paradoxical situation! The very discovery which may almost be said to prove the truth
+  of the hypothesis, is the very one which has done most to discredit it, because in the minds of
+  its authors the only possible solution of the transition from the one group to the other was by
+  means of the reversal of surfaces.</p>
+
+  <p>Still, as already said, even if the theory advanced to explain the facts be discredited, the
+  facts remain the same; and still to this day an explanation is required as to why such
+  extraordinary resemblances should exist between the two nervous systems, unless there is a genetic
+  connection between the two groups of animals. An explanation may still be found, and must be
+  diligently sought for, which shall take into account the strong evidence of this relationship
+  between the two groups, and yet not necessitate any reversal of surfaces. It is the object of this
+  book to consider the possibility of such an explanation.</p>
+
+  <p>What are the lines of investigation most likely to meet with success? Is it possible to lay
+  down any laws of evolution? It is instructive to consider the nature of the investigations which
+  have led to the two theories just mentioned, for the fundamental starting-point is remarkably
+  different in the two cases. The one theory is based upon the study of the vertebrate itself, and
+  especially of its central nervous system, and its supporters and upholders have been and are
+  essentially anatomists, whose chief study is that of vertebrate and human anatomy. The other
+  theory is based upon the study of the invertebrate, and consists especially of an attempt to find
+  in the invertebrate some structure resembling a notochord, such <span class="pagenum"
+  id="page16">{16}</span>organ being considered by them as the great characteristic of the
+  vertebrate; indeed, so much is this the case, that a large number of zoologists speak now of
+  Chordata rather than of Vertebrata, and in order to emphasize their position follow Bateson, and
+  speak of the Tunicata as Uro-chordata, of Amphioxus as Cephalo-chordata, of the Enteropneusta as
+  Hemi-chordata, and even of Actinotrocha (to use Masterman's term), as Diplo-chordata.</p>
+
+  <p>The upholders of this theory lay no stress on the nature of the central nervous system in
+  vertebrates, they are essentially zoologists who have made a special study of the invertebrate
+  rather than of the vertebrate.</p>
+
+  <p>Of these two methods of investigating the problem, it must be conceded that the former is more
+  likely to give reliable results. By putting the vertebrate to the question in every possible way,
+  by studying its anatomy and physiology, both gross and minute, by inquiring into its past history,
+  we can reasonably hope to get a clue to its origin, but by no amount of investigation can we tell
+  with any certainty what will be its future fate; we can only guess and prophesy in an uncertain
+  and hesitating manner. So it must be with any theory of the origin of vertebrates, based on the
+  study of one or other invertebrate group. Such theory must partake rather of the nature of
+  prophecy than of deduction, and can only be placed on a firm basis when it so happens that the
+  investigation of the vertebrate points irresistibly to its origin from the same group; in fact,
+  "never prophesy unless you know."</p>
+
+  <p class="sp3">The first principle, then, I would lay down is this: In order to find out the
+  origin of vertebrates, inquire, in the first place, of the vertebrate itself.</p>
+
+  <p class="ac"><span class="sc">Importance of the Central Nervous System.</span></p>
+
+  <p>Does the history of evolution pick out any particular organ or group of organs as more
+  necessary than another for upward progress? If so, it is upon that organ or group of organs that
+  special stress must be laid.</p>
+
+  <p>Since Darwin wrote the "Origin of Species," and laid down that the law of the 'survival of the
+  fittest' is the factor upon which evolution depends, it has gradually dawned upon the scientific
+  mind that 'the fittest' may be produced in two diametrically opposite ways: <span class="pagenum"
+  id="page17">{17}</span>either by progress upwards to a superior form, or by degeneration to a
+  lower type of animal. The principle of degeneration as a factor in the formation of groups of
+  animals, which are thereby enabled to survive, is nowadays universally admitted. The most striking
+  example is to be found in the widely distributed group of Tunicata, which live, in numbers of
+  instances, a sedentary life upon the rocks, have the appearance of very low forms of animal life,
+  propagate by budding, have lost all the characteristics of higher forms, and yet are considered to
+  be derived from an original vertebrate stock. Such degenerate forms remain degenerate, and are
+  never known to regenerate and again to reach the higher stage of evolution from which they arose.
+  Such forms are of considerable interest, but cannot help, except negatively, to decide what factor
+  is especially important for upward progress.</p>
+
+  <p>At the head of the animal race at the present day stands man, and in mankind itself some races
+  are recognized as higher than others. Such recognition is given essentially on account of their
+  greater brain-power, and without doubt the great characteristic which puts man at the head is the
+  development of his central nervous system, especially of the region of the brain. Not only is this
+  point most manifest in distinguishing man from the lower animals, but it applies to the latter as
+  well. By the amount of convolution of the brain, the amount of grey matter in the cerebral
+  hemispheres, the enlargement and increasing complexity of the higher parts of the central nervous
+  system, the anthropoid apes are differentiated from the lower forms, and the higher mammals from
+  the lower. In the recent work of Elliot Smith, and of Edinger, most conclusive proof is given that
+  the upward progress in the vertebrate phylum is correlated with the increase of brain-power, and
+  the latter writer shows how steady and remarkable is the increase in substance and in complexity
+  of the brain-region as we pass from the fishes, through the amphibians and reptiles, to the birds
+  and mammals.</p>
+
+  <p>The study of the forms which lived on the earth in past ages confirms and emphasizes this
+  conclusion, for it is most striking to see how small is the cranium among the gigantic Dinosaurs;
+  how in the great reptilian age the denizens of the earth were far inferior in brain-power to the
+  lords of creation in after-times.</p>
+
+  <p>What applies to the vertebrate phylum applies also to the invertebrate groups. Here also an
+  upward progress is recognized as we <span class="pagenum" id="page18">{18}</span>pass from the
+  sponges to the arthropods&mdash;a progress which is manifested, first by the concentration of
+  nervous material to form a central nervous system, and then by the increase in substance and
+  complexity of that nervous system to form a higher and a higher type, until the culmination is
+  reached in the nervous system of the scorpions and spiders. No upward progress is possible with
+  degeneration of the central nervous system, and in all those cases where a group owes its
+  existence to degeneration, the central nervous system takes part in the degeneration.</p>
+
+  <p>This law of the paramount importance of the growth of the central nervous system for all upward
+  progress in the evolution of animals receives confirmation from the study of the development of
+  individuals, especially in those cases where a large portion of the life of the animal is spent in
+  a larval condition, and then, by a process of transformation, the larva changes into the adult
+  form. Such cases are well known among Arthropoda, the familiar instance being the change from the
+  larval caterpillar to the adult imago. Among Vertebrata, the change from the tadpole to the frog,
+  from the larval form of the lamprey (<i>Ammoc&#x0153;tes</i>) to the adult form
+  (<i>Petromyzon</i>), are well-known instances. In all such cases the larva shows signs of having
+  attained a certain stage in evolution, and then a remarkable transformation takes place, with the
+  result that an adult animal emerges, whose organization reaches a higher stage of evolution than
+  that of the larva.</p>
+
+  <p>This transformation process is characterized by a very great destruction of the larval tissues
+  and a subsequent formation of new adult tissues. Most extensive is the destruction in the
+  caterpillar and in the larval lamprey. But one organ never shares in this process of histolysis,
+  and that is the central nervous system; amidst the ruins of the larva it remains, leading and
+  directing the process of re-formation. In the Arthropoda, the larval alimentary canal may be
+  entirely destroyed and eaten up by phagocytes, but the central nervous system not only remains
+  intact but increases in size, and by the concentration and cephalization of its
+  infra-&#x0153;sophageal ganglia forms in the adult a central nervous system of a higher type than
+  that of the larva.</p>
+
+  <p>So, too, in the transformation of the lamprey, there is not the slightest trace of any
+  destruction in the central nervous system, but simply a development and increase in nervous
+  material, which <span class="pagenum" id="page19">{19}</span>results in the formation of a brain
+  region more like that of the higher vertebrates than exists in Ammoc&#x0153;tes.</p>
+
+  <p>In these cases the development is upward&mdash;the adult form is of a higher type than that of
+  the larva. It is, however, possible for the reverse to occur, so that the individual development
+  leads to degeneration, not to a higher type. Instances are seen in the Tunicata, and in various
+  parasitic arthropod forms, such as Lernæa, etc. In these cases, the transformation from the larval
+  to the adult form leads to degradation, and in this degradation the central nervous system is
+  always involved.</p>
+
+  <p>It is perhaps a truism to state that upward progress is necessarily accompanied by increased
+  development of the central nervous system; but it is necessary to lay special stress upon the
+  importance of the central nervous system in all problems of evolution, because there is, in my
+  opinion, a tendency at the present time to ignore this factor to too great an extent.</p>
+
+  <p>The law of progress is this&mdash;The race is not to the swift, nor to the strong, but to the
+  wise.</p>
+
+  <p>This law carries with it the necessary corollary that the immediate ancestor of the vertebrate
+  must have had a central nervous system nearly approaching that of the lowest undegenerated
+  vertebrate. Among all the animals living on the earth at the present time, the highest
+  invertebrate group, the Arthropoda, possesses a central nervous system most closely resembling
+  that of the vertebrate.</p>
+
+  <p class="sp3">The law, then, of the paramount importance of a steady development of the central
+  nervous system for the upward progress of the animal kingdom, points directly to the arthropod as
+  the most probable ancestor of the vertebrate.</p>
+
+  <p class="ac"><span class="sc">Evolution of Tissues.</span></p>
+
+  <p>In the whole scheme of evolution we can recognize, not only an upward progress in the
+  organization of the animal as a whole, but also a distinct advance in the structure of the tissues
+  composing an individual, which accompanies that upward progress. Thus it is possible to speak of
+  an evolution of the supporting tissues from the simplest form of connective tissue up to cartilage
+  and thence to bone; of the contractile tissues, from the simplest contractile protoplasm <span
+  class="pagenum" id="page20">{20}</span>to unstriped muscle, and thence to the highest forms of
+  striated muscle; of the nervous connecting strands, from undifferentiated to fine strands, then to
+  thicker, more separated ones, resembling non-medullated fibres, and finally to well-differentiated
+  separate fibres, each enclosed in a medullated sheath.</p>
+
+  <p>In the connective tissue group, bone is confined to the vertebrates, cartilage is found among
+  invertebrates, and the closest resemblance to vertebrate embryonic or parenchymatous cartilage is
+  found in the cartilage of Limulus. Also, as Gegenbaur has pointed out, Limulus, more than any
+  other invertebrate, possesses a fibrous connective tissue resembling that of vertebrates.</p>
+
+  <p>In the muscular group, Biedermann, who has made a special study of the physiology of striated
+  muscle, says that among invertebrates the striated muscle of the arthropod group resembles most
+  closely that of the vertebrate.</p>
+
+  <p class="sp3">In the nervous group the resemblance between the nerve-fibres of Limulus and
+  Ammoc&#x0153;tes, both of which are devoid of any marked medullary sheath, is very apparent, and
+  Retzius points out that the only evidence of medullation, so characteristic of the vertebrates, is
+  found in a species of prawn (<i>Palæmon</i>). In all these cases the nearest resemblance to the
+  vertebrate tissues is to be found in the arthropod.</p>
+
+  <p class="ac"><span class="sc">The Evidence of Palæontology.</span></p>
+
+  <p>Perhaps the most important of all the clues likely to help in the solution of the origin of
+  vertebrates is that afforded by Geology, for although the geological record is admittedly so
+  imperfect that we can never hope by its means alone to link together the animals at present in
+  existence, yet it does undoubtedly point to a sequence in the evolution of animal forms, and gives
+  valuable information as to the nature of such sequence. In different groups of animals there are
+  times when the group can be spoken of as having attained its most flourishing period. During these
+  geological epochs the distribution of the group was universal, the numbers were very great, the
+  number of species was at the maximum, and some of them had attained a maximal size. Such races
+  were at that time dominant, and the struggle for existence was essentially among members of the
+  same group. At the present time the dominant race is man, and the <span class="pagenum"
+  id="page21">{21}</span>struggle for existence is essentially between the members of that race, and
+  not between them and any inferior race.</p>
+
+  <p>The effect of such conditions is, as Darwin has pointed out, to cause great variation in that
+  group; in consequence of that variation and that dominance the evolution of the next higher group
+  is brought about from some member of the dominant group. Thus the present age is the outcome of
+  the Tertiary period, a time when giant mammals roamed the earth and left as their successors the
+  mammals of the present day; a time of dominance of quadruped mammals; a time of which the period
+  of maximum development is long past, and we now see how the dominance of the biped mammal, man, is
+  accompanied by the rapid diminution and approaching extermination of the larger mammals. No
+  question can possibly arise as to the immediate ancestor of the biped mammal; he undoubtedly arose
+  from one of the dominant quadrupedal mammals.</p>
+
+  <p>Passing along to the next evidence of the rocks, we find an age of reptiles in the Mesozoic
+  period. Here, again, the number and variety is most striking; here, again, the size is enormous in
+  comparison with that of the present-day members of the group. This was the dominant race at the
+  time when the birds and mammals first appeared on the earth, and anatomists recognize in these
+  extinct reptilian forms two types; the one bird-like, the other more mammalian in character. From
+  some members of the former group birds are supposed to have been evolved, and mammals from members
+  of the other group. There is no question of their origin directly from lower fish-like forms; the
+  time of their appearance on the earth, their structure, all point irresistibly to the same
+  conclusion as we have arrived at from the consideration of the origin of the biped from the
+  quadruped mammal, viz. that birds and mammals arose, in consequence of the struggle for existence,
+  from some members of the reptilian race which at that time was the dominant one on earth.</p>
+
+  <p>Passing down the geological record, we find that when the reptiles first appear in the
+  Carboniferous age there is abundant evidence of the existence of numbers of amphibian forms. At
+  this time the giant Labyrinthodonts flourished. Here among the swamps and marshes of the
+  coal-period the prevalent vertebrate was amphibian in structure. Their variety and number were
+  very great, and at that period they attained their greatest size. Here, again, from the geological
+  record we draw the same conclusion as before, that the reptiles arose from the race which was then
+  predominant on the earth&mdash;the Amphibia.</p>
+
+  <div><span class="pagenum" id="page22">{22}</span></div>
+
+  <div class="ac w45 fcenter sp2">
+    <a href="images/fig004.png" id="fig4"><img style="width:100%" src="images/fig004.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p class="sp0"><span class="sc">Fig. 4.&mdash;Plan of Geological Strata.</span> (From <span
+      class="sc">Lankester</span>.)</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page23">{23}</span></div>
+
+  <p>Again, another point of great interest is seen here, and that is that these Labyrinthodonts, as
+  Huxley has pointed out, possess characters which bring them more closely than the amphibians of
+  the present day into connection with the fishes; and further, the fish-like characters they
+  possessed are those of the Ganoids, the Marsipobranchs, the Dipnoans, and the Elasmobranchs,
+  rather than of the Teleosteans.</p>
+
+  <p>Now, it is a striking fact that the ancient fishes at the time when the amphibians appeared had
+  not reached the teleostean stage. The ganoids and elasmobranchs swarmed in the waters of the
+  Devonian and Carboniferous times. Dipnoans and marsipobranchs were there, too, in all probability,
+  but teleosteans do not appear until the Mesozoic period. The very kinds of fish, then, which
+  swarmed in the seas at that time, and were the predominant race before the Carboniferous epoch,
+  are those to which the amphibians at their first appearance show the closest affinity. Here,
+  again, the same law appears; from the predominant race at the time, the next higher race arose,
+  and arose by a most striking modification, which was the consequence of altering the medium in
+  which it lived. By coming out of the water and living on the land, or, rather, being able to live
+  partly on land and partly in the water, by the acquisition of air-breathing respiratory organs or
+  lungs in addition to, and instead of, water-breathing organs or gills, the amphibian not only
+  arose from the fish, but made an entirely new departure in the sequence of progressive forms.</p>
+
+  <p>This was a most momentous step in the history of evolution&mdash;one fraught with mighty
+  consequences and full of most important suggestions.</p>
+
+  <p>From this time onwards the struggle for existence by which upward progress ensued took place on
+  the land, not in the sea, and, as has been pointed out, led to the evolution of reptiles from
+  amphibians, birds and quadrupedal mammals from reptiles, and man from quadrupeds. In the sea the
+  fishes were left to multiply and struggle among themselves, their only opponents being the giant
+  cephalopods, which themselves had been evolved from a continual succession of the Mollusca. For
+  this reason the struggle for existence between the fishes and the higher race evolved from them
+  did not <span class="pagenum" id="page24">{24}</span>take place until some members of that higher
+  race took again to the water, and so competed with the fish-tribe in their own element.</p>
+
+  <p>Another most important conclusion to be derived from the uprising of the Amphibia is that at
+  that time there was no race of animals living on the land which had a chance against them. No race
+  of land-living animals had been evolved whose organization enabled them to compete with and
+  overcome these intruders from the sea in the struggle for existence. For this reason that the
+  whole land was their own, and no serious competition could arise from their congeners, the fish,
+  they took possession of it, and increased mightily in size; losing more and more the habit of
+  going into the water, becoming more and more truly terrestrial animals. Henceforth, then, in
+  trying to find out the sequence of evolution, we must leave the land and examine the nature of the
+  animals living in the sea; the air-breathing animals which lived on the land in the Upper Silurian
+  and Devonian times cannot have reached a stage of organization comparable with that of the fishes,
+  seeing how easily the amphibians became dominant.</p>
+
+  <p>We arrive, then, at the conclusion that the ancestors of the fishes must have lived in the sea,
+  and applying still the same principles that have held good up to this time, the ancestors of the
+  fishes must have arisen from some member of the race predominant at the time when they first
+  appeared, and also the earliest fishes must have much more closely resembled the ancestral form
+  than those found in later times or at the present day.</p>
+
+  <p>What, then, is the record of the rocks at the time of the first appearance of fish-like forms?
+  What kind of fishes were they, and what was the predominant race at the time?</p>
+
+  <p>We have now reached the Upper Silurian and Lower Devonian times, and most instructive and
+  suggestive is the revelation of the rocks. Here, when the first vertebrates appeared, the sea was
+  peopled with corals, brachiopods, early forms of cephalopods, and other invertebrates; but, above
+  all, with the great tribe of trilobites (Fig. <a href="#fig6">6</a>) and their successors. From
+  the trilobites arose, as evidenced by their larval form, the king-crab group, called the Xiphosura
+  (Fig. <a href="#fig5">5</a>). Closely connected with them, and forming intermediate stages between
+  trilobites and king-crabs, numerous forms have been discovered, known as Belinurus, Prestwichia,
+  Hemiaspis, Bunodes, etc. (Fig. <a href="#fig5">5</a> and Fig. <a href="#fig12">12</a>). From them
+  also arose the most striking group <span class="pagenum" id="page25">{25}</span>of animals which
+  existed at this period&mdash;the giant sea-scorpions, or Gigantostraca. This group was closely
+  associated with the king-crabs, and the two groups together are classified under the title
+  Merostomata.</p>
+
+  <div class="ac w45 fcenter sp3">
+    <a href="images/fig005.png" id="fig5"><img style="width:100%" src="images/fig005.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="sp0"><span class="sc">Fig. 5</span> (from <span class="sc">H.
+      Woodward</span>).&mdash;1. <i>Limulus polyphemus</i> (dorsal aspect). 2. <i>Limulus,</i>
+      young, in trilobite stage. 3. <i>Prestwichia rotundata.</i> 4. <i>Prestwichia Birtwelli.</i>
+      5. <i>Hemiaspis limuloides.</i> 6. <i>Pseudoniscus aculeatus.</i></p>
+    </div>
+  </div>
+
+  <p>The appearance of these sea-scorpions is given in Figs. 7 and 8, representing Stylonurus,
+  Slimonia, Pterygotus, Eurypterus. They must have been in those days the tyrants of the deep, for
+  specimens of Pterygotus have been found over six feet in length.</p>
+
+  <p>At this time, then, by every criterion hitherto used, by the multitude of species, by the size
+  of individual species, which at this period reached the maximum, by their subsequent decay and
+  final extinction, we must conclude that these forms were in their zenith, that the predominant
+  race at this time was to be found in this group of arthropods. Just previously, the sea swarmed
+  with trilobites, and right into the period when the Gigantostraca flourished, the trilobites <span
+  class="pagenum" id="page26">{26}</span>are still found of countless forms, of great difference in
+  size. The whole period may be spoken of as the great trilobite age, just as the Tertiary times
+  form the mammalian age, the Mesozoic times the reptilian age, etc. From the trilobites the
+  Gigantostraca and Xiphosura arose, as evidenced by the embryology of Limulus, and, therefore, in
+  the term trilobite age would be included the whole of those peculiar forms which are classified by
+  the names Trilobita, Gigantostraca, Xiphosura, etc. Of all these the only member alive at the
+  present time is Limulus, or the King-Crab.</p>
+
+  <table class="mc tlf sp2 w45" summary="Group of illustrations" title="Group of illustrations">
+    <tr>
+      <td class="vbm ac" style="width:44%;"><a href="images/fig006.png" id="fig6"><img
+      style="width:100%" src="images/fig006.png" alt="" title=""/></a></td>
+      <td class="vbm ac" style="width:44%;"><a href="images/fig007.png" id="fig7"><img
+      style="width:100%" src="images/fig007.png" alt="" title=""/></a></td>
+    </tr>
+    <tr>
+      <td>
+        <div class="smaller aj it1">
+          <p class="sp0"><span class="sc">Fig. 6.&mdash;A Trilobite</span> (<i><span
+          class="correction" title="Original reads 'Dalmatites' (which is an
+          ammonite)">Dalmanites</span></i>) (after <span class="sc">Pictet</span>). Dorsal view.</p>
+        </div>
+      </td>
+      <td>
+        <div class="smaller aj it1">
+          <p class="sp0"><span class="sc">Fig. 7.</span>&mdash;<i>Eurypterus remipes</i> (after
+          <span class="sc">Nieskowski</span>). Dorsal view.</p>
+        </div>
+      </td>
+    </tr>
+  </table>
+
+  <p>As, however, the term 'trilobite' does not include the members of the king-crab or sea-scorpion
+  groups, it is advisable to use some other term to represent the whole group. They cannot be called
+  crustaceans or arachnids, for in all probability they gave origin to both; the nearest approach to
+  the Trilobite stage of development at the present time is to be found perhaps in Branchipus (Fig.
+  <a href="#fig10">10</a>) and Apus (Fig. <a href="#fig9">9</a>), just as the nearest approach to
+  the Eurypterid <span class="pagenum" id="page27">{27}</span>form is Limulus. Crustaceans such as
+  crabs and lobsters are of much later origin, and do not occur in any quantity until the late
+  Mesozoic period. The earliest found, a kind of prawn, occurs in the Carboniferous age.</p>
+
+  <div class="ac w50 fcenter sp2">
+    <a href="images/fig008.png" id="fig8"><img style="width:100%" src="images/fig008.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="sp0"><span class="sc">Fig. 8.</span>&mdash;A, <i>Pterygotus Osiliensis</i> (from
+      <span class="sc">Schmidt</span>). B, <i>Stylonurus Logani</i> (from <span
+      class="sc">Woodward</span>). C, <i>Slimonia acuminata</i> (from <span
+      class="sc">Woodward</span>).</p>
+    </div>
+  </div>
+
+  <p>Korschelt and Heider have accordingly suggested the name <i>Palæostraca</i> for this whole
+  group, and <i>Protostraca</i> for the still earlier <span class="pagenum"
+  id="page28">{28}</span>arthropod-like animals which gave origin to the trilobites themselves. This
+  name I shall adopt, and speak, therefore, of the <i>Palæostraca</i> as the dominant race at the
+  time when vertebrates first appeared.</p>
+
+  <p>If, then, there is no break in the law of evolution here, the race which was predominant at the
+  time when the vertebrate first appeared must have been that from which the first fishes arose, and
+  these fishes must have resembled, not the crustacean proper, or the arachnid proper, but a member
+  of the palæostracan group. Moreover, just as the Labyrinthodonts show special affinities to the
+  fishes which were then living, so we should expect that the forms of the earliest fish would
+  resemble the arthropodan type dominant at the time more closely than the fish of a later era.</p>
+
+  <p>At first sight it seems too great an absurdity even to imagine the possibility of any genetic
+  connection between a fish and an arthropod, for to the mind's eye there arises immediately the
+  picture of a salmon or a shark and a lobster or a spider. So different in appearance are the two
+  groups of animals, so different their methods of locomotion, that it is apparently only an inmate
+  of a lunatic asylum who could possibly suggest such a connection. Much more likely is it that a
+  fish-like form should have been developed out of a smooth, wriggling, worm-like animal, and it is
+  therefore to the annelids that the upholders of the theory of the reversal of surfaces look for
+  the ancestor of the vertebrate.</p>
+
+  <table class="mc tlf sp2 w85" summary="Group of illustrations" title="Group of illustrations">
+    <tr>
+      <td class="vbm ac" style="width:47%;"><a href="images/fig009.png" id="fig9"><img
+      style="width:47%" src="images/fig009.png" alt="" title=""/></a></td>
+      <td class="vbm ac" style="width:43%;"><a href="images/fig010.png" id="fig10"><img
+      style="width:100%" src="images/fig010.png" alt="" title=""/></a></td>
+    </tr>
+    <tr>
+      <td>
+        <div class="smaller ac">
+          <p class="sp0"><span class="sc">Fig. 9.</span>&mdash;<i>Apus</i> (from the Royal Natural
+          History). Dorsal view.</p>
+        </div>
+      </td>
+      <td>
+        <div class="smaller ac">
+          <p class="sp0"><span class="sc">Fig. 10.</span>&mdash;<i>Branchipus stagnalis.</i> (From
+          <span class="sc">Claus</span>.)</p>
+        </div>
+      </td>
+    </tr>
+  </table>
+
+  <div><span class="pagenum" id="page29">{29}</span></div>
+
+  <p>We must endeavour to dismiss from our imagination such forms as the salmon and shark as
+  representatives of the fish-tribe, and the lobster and spider of the arthropods, and try to
+  picture the kind of animals living in the seas in the early Devonian and Upper Silurian times, and
+  then we find, to our surprise, that instead of the contrast between fishes and arthropods being so
+  striking as to make any comparison between the two seem an absurdity, the difficulty in the last
+  century, and even now, is to decide in many cases whether a fossil is an arthropod or a fish.</p>
+
+  <p>I have shown what kind of animal the palæostracan was like. What information is there of the
+  nature of the earliest vertebrate?</p>
+
+  <p>The most ancient fishes hitherto discovered have been classified by Lankester and Smith
+  Woodward into the three orders, Heterostraci, Osteostraci, and Antiarcha. Of these the
+  Heterostraci contain the genera Pteraspis and Cyathaspis, and are the very earliest vertebrates
+  yet discovered, being found in the Lower Silurian. The Osteostraci are divided into the
+  Cephalaspidæ, Tremataspidæ, etc., and are found in the Upper Silurian and Devonian beds. The
+  Antiarcha, comprising Pterichthys and Bothriolepis, belong to the Devonian and are not found in
+  Silurian deposits. This, then, is the order of their appearance&mdash;Pteraspis, Cephalaspis, and
+  Pterichthys.</p>
+
+  <p>In none of these families is there any resemblance to an ordinary fish. In no case is there any
+  sign of vertebræ or of jaws. They, like the lampreys, were all agnathostomatous. Strange indeed is
+  their appearance, and it is no wonder that there should have been a difficulty in deciding whether
+  they were fish or arthropod. Their great characteristic is their buckler-plated cephalic shield,
+  especially conspicuous on the dorsal side of the head. Figs. 11, 14, 15, 16, give the dorsal
+  shields of Pteraspis, Auchenaspis, Pterichthys, and Bothriolepis.</p>
+
+  <p>In 1904, Drevermann discovered a mass of <i>Pteraspis Dunensis</i> embedded in a single stone,
+  showing the same kind of head-shield as <i>P. rostrata</i>, but the rostrum was longer and the
+  spine at the extremity of the head-shield much longer and more conspicuous. The whole shape of the
+  animal as seen in this photograph recalls the shape of a Hemiaspid rather than of a fish. It is,
+  then, natural enough for the earlier observers to have looked upon such a fossil as related to an
+  arthropod rather than a fish.</p>
+
+  <div><span class="pagenum" id="page30">{30}</span></div>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig011.jpg" id="fig11"><img style="width:100%" src="images/fig011.jpg" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p class="sp0"><span class="sc">Fig. 11.</span>&mdash;<i>Pteraspis dunensis</i> (from <span
+      class="sc">Drevermann</span>). Dorsal view of body and spine on the right side. Head-end,
+      showing long rostrum on the left side.</p>
+    </div>
+  </div>
+
+  <table class="mc tlf sp2 w35" summary="Group of illustrations" title="Group of illustrations">
+    <tr>
+      <td class="vbm ac" style="width:42%;"><a href="images/fig012.jpg" id="fig12"><img
+      style="width:100%" src="images/fig012.jpg" alt="" title=""/></a></td>
+      <td class="vbm ac" style="width:42%;"><a href="images/fig013.jpg" id="fig13"><img
+      style="width:100%" src="images/fig013.jpg" alt="" title=""/></a></td>
+    </tr>
+    <tr>
+      <td>
+        <div class="smaller ac">
+          <p class="sp0"><span class="sc">Fig. 12.</span>&mdash;<i>Bunodes lunula.</i> (From <span
+          class="sc">Schmidt</span>.)</p>
+        </div>
+      </td>
+      <td>
+        <div class="smaller ac">
+          <p class="sp0"><span class="sc">Fig. 13.</span>&mdash;<i>Auchenaspis (Thyestes)
+          verrucosus</i>, natural size. (From <span class="sc">Woodward</span>.)</p>
+        </div>
+      </td>
+    </tr>
+  </table>
+
+  <div><span class="pagenum" id="page31">{31}</span></div>
+
+  <p>In Figs. 12 and 13 I have placed side by side two Silurian fossils which are found in the same
+  geological horizon. They are both life size and possess a general similarity of appearance, yet
+  the one is a Cephalaspidian fish known by the name of <i>Auchenaspis</i> or <i>Thyestes
+  verrucosa</i>, the other a Palæostracan called <i>Bunodes lunula</i>.</p>
+
+  <table class="mc tlf sp2 w50" summary="Group of illustrations" title="Group of illustrations">
+    <tr class="vmi">
+      <td class="vbm ac" style="width:42%;"><a href="images/fig014.jpg" id="fig14"><img
+      style="width:100%" src="images/fig014.jpg" alt="" title=""/></a>
+        <div class="smaller aj it1">
+          <p><span class="sc">Fig. 14.&mdash;Dorsal Head-shield of</span> <i>Thyestes (Auchenaspis)
+          verrucosus</i>. (From <span class="sc">Rohon</span>.)</p>
+          <p class="sp0"><i>Fro.</i>, narial opening; <i>l.e.</i>, lateral eyes; <i>gl.</i>,
+          glabellum or plate over brain; <i>Occ.</i>, occipital region.</p>
+        </div>
+      </td>
+      <td class="vbm ac" style="width:46%;"><a href="images/fig015.png" id="fig15"><img
+      style="width:100%" src="images/fig015.png" alt="" title=""/></a>
+        <div class="smaller aj it1">
+          <p class="sp0 ac"><span class="sc">Fig. 15.</span>&mdash;<i><span class="correction"
+          title="Original reads 'Ptericthys'.">Pterichthys</span>.</i></p>
+        </div>
+      </td>
+    </tr>
+  </table>
+
+  <p>In a later chapter I propose to discuss the peculiarities and the nature of the head-shields of
+  these earliest fishes, in connection with the question of the affinities of the animals which bore
+  them. At this point of my argument I want simply to draw attention to the undoubted fact of the
+  striking similarity in appearance between the <span class="pagenum"
+  id="page32">{32}</span>earliest fishes and members of the Palæostraca, the dominant race of
+  arthropods which swarmed in the sea at the time: a similarity which could never have been
+  suspected by any amount of investigation among living forms, but is immediately revealed when the
+  ages themselves are questioned.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig016.png" id="fig16"><img style="width:100%" src="images/fig016.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 16.</span>&mdash;<i>Bothriolepis.</i> (After <span
+      class="sc">Patten</span>.)</p>
+      <p class="sp0"><i>An.</i>, position of anus.</p>
+    </div>
+  </div>
+
+  <p>I have not reproduced any of the attempted restorations of these old forms, as usually given in
+  the text-books, because all such restorations possess a large element of fancy, due to the
+  personal bias of the observer. I have put in Rohon's idea of the general shape of Tremataspis
+  (Fig. <a href="#fig17">17</a>) in order to draw attention to the lamprey-like appearance of the
+  fish according to his researches (<i>cf.</i> Fig. <a href="#fig18">18</a>).</p>
+
+  <div class="ac w50 fcenter sp2">
+    <a href="images/fig017.jpg" id="fig17"><img style="width:100%" src="images/fig017.jpg" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p class="sp0"><span class="sc">Fig. 17.</span>&mdash;<span class="sc">Restoration of</span>
+      <i>Tremataspis</i>. (After <span class="sc">Rohon</span>, slightly modified.)</p>
+    </div>
+  </div>
+
+  <div class="ac w50 fcenter sp2">
+    <a href="images/fig018.png" id="fig18"><img style="width:100%" src="images/fig018.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p class="sp0"><span class="sc">Fig. 18.</span>&mdash;<i>Ammoc&#x0153;tes.</i></p>
+    </div>
+  </div>
+
+  <p class="sp3">The argument, then, from geology, like that from comparative anatomy and from the
+  consideration of the importance of the central nervous system in the upward development of the
+  animal race, not only points directly to the arthropod group as the ancestor of the <span
+  class="pagenum" id="page33">{33}</span>vertebrate, but also to a distinct ancient type of
+  arthropod, the Palæostracan, the only living example of which is the King-Crab or Limulus; while
+  the nearest approach to the trilobite group among living arthropods are Branchipus and Apus. It
+  follows, therefore, that for the following up of this clue, Limulus especially must be taken into
+  consideration, while Branchipus and Apus are always to be kept in mind.</p>
+
+  <p class="ac"><span class="sc">Ammoc&#x0153;tes rather than Amphioxus is the Best Subject for
+  Investigation.</span></p>
+
+  <p>It is not, however, Limulus that must be investigated in the first instance, but the vertebrate
+  itself; for it can never be insisted on too often that in the vertebrate itself its past history
+  will be found, but that Limulus cannot reveal the future of its race. What vertebrate must be
+  chosen for investigation? Reasons have been given why our attention should be fixed upon the
+  king-crab rather than on the lobster on the invertebrate side; what is the most likely animal on
+  the vertebrate side?</p>
+
+  <p>From the evidence already given it is manifest that the earliest mammal belonged to the lowest
+  group of mammals; that the birds on their first appearance presented reptilian characteristics,
+  that the earliest reptiles belonged to a low type of reptile, that the amphibians at their first
+  appearance were nearer in type to the fishes than were the later forms. As each of these groups
+  advances in number and power, specialization takes place in it, and the latest developed members
+  become further and further removed in type from the earliest. So also it must have been with the
+  origin of fishes: here too, in the quest for information as to the structure and nature of the
+  first-formed fishes, we must look to the lowest rather than to the highest living members of the
+  group.</p>
+
+  <p>The lowest fish-like animal at present living is Amphioxus, and on this ground it is argued
+  that the original vertebrate must have approached in organization to that of Amphioxus; it is upon
+  the comparison between the structure of Amphioxus and that of Balanoglossus, that the theory of
+  the origin of vertebrates from forms like the latter animal is based. For my own part, I think
+  that in the first instance, at all events, Amphioxus should be put on one side, although of course
+  its structure must always be kept in mind, for the following reasons<span
+  class="wnw">:&mdash;</span></p>
+
+  <div><span class="pagenum" id="page34">{34}</span></div>
+
+  <p>Amphioxus, like the tunicates, does not possess the characteristics of other vertebrates. In
+  all vertebrates above these forms the great characteristic is a well-defined brain-region from
+  which arise nerves to organs of special sense, the eyes and nose. In Amphioxus no eyes exist, for
+  the pigmented spot at the anterior extremity of the brain-region is no eye but only a mass of
+  pigment, and the so-called olfactory pit is a very rudimentary and inferior organ of smell. In
+  connection with the nearly complete absence of these two most important sense-organs, the most
+  important part of the central nervous system, the region corresponding to the cerebral
+  hemispheres, is also nearly completely absent.</p>
+
+  <p>Now, the history of the evolution of the central nervous system in the animal race points
+  directly to its formation as a concentrated mass of nervous material at the anterior extremity of
+  the body, in consequence of the formation of special olfactory and visual organs at that
+  extremity. As already stated, the concentration of nervous material around the mouth as an oral
+  ring was its beginning. In connection with this there arose special sense-organs for the guidance
+  of the animal to its food which took the form of olfactory and optic organs. With the shifting
+  from the radial to the elongated form these sense-organs remained at the anterior or mouth-end of
+  the animal, and owing to their immense importance in the struggle for existence, that part of the
+  central nervous system with which they were connected developed more than any other part, became
+  the leader to which the rest of the nervous system was subservient, and from that time onwards the
+  development of the brain-region was inevitably associated with the upward progress of animal
+  life.</p>
+
+  <p>To those who believe in Evolution and the Darwinian theory of the survival of the fittest, it
+  is simply inconceivable that a soft-bodied animal living in the mud, blind, with a rudimentary
+  brain and rudimentary olfactory organs, such as is postulated when we think of Balanoglossus and
+  Amphioxus, should hold its own and come victorious out of the struggle for existence at a time
+  when the sea was peopled with powerful predaceous scorpion- and crab-like armour-plated animals
+  possessing a well-developed brain, good eyes and olfactory organs, and powerful means of
+  locomotion. Wherever in the scale of animal development Amphioxus may ultimately be placed, it
+  cannot be looked upon as the type of the earliest formed fishes such as appeared in Silurian
+  times.</p>
+
+  <div><span class="pagenum" id="page35">{35}</span></div>
+
+  <p>The next lowest group of living fishes is the Marsipobranchii which include the lampreys and
+  hag-fishes. To these naturally we must turn for a clue as to the organization of the earliest
+  fish, for here we find all the characteristics of the vertebrates represented: a well-formed
+  brain-region, well-developed eyes and nose, cranial nerves directly comparable with those of other
+  vertebrates, and even the commencement of vertebræ.</p>
+
+  <p>Among these forms the lamprey is by far the best for investigation, not only because it is
+  easily obtainable in large quantities, but especially because it passes a large portion of its
+  existence in a larval condition, from which it emerges into the adult state by a wonderful process
+  of transformation, comparable in extent with the transformation of the larval caterpillar into the
+  adult imago. So long does the lamprey live in this free larval condition, and so different is it
+  in the adult stage, that the older anatomists considered that the two states were really different
+  species, and gave the name of <i>Ammoc&#x0153;tes branchialis</i> to the larval stage, while the
+  adult form was called <i>Petromyzon planeri</i>, or <i>Petromyzon fluviatilis</i>.</p>
+
+  <p>This long-continued free-living existence in the larval or Ammoc&#x0153;tes stage makes the
+  lamprey, more than any other type of lowly organized fish, invaluable for the present
+  investigation, for throughout the animal kingdom it is recognized that the larval form approaches
+  nearer to the ancestral type than the adult form, whether the latter is progressive or degenerate.
+  Not only are the tissues formed during the stages which are passed through in a free-living larval
+  form, serviceable tissues comparable to those of adult life, but also these stages proceed at so
+  much slower a rate than do those in the embryo <i>in utero</i> or in the egg, as to make the
+  larval form much more suitable than the embryo for the investigation of ancestral problems. It is
+  true enough that the free life of the larva may bring about special adaptations which are not of
+  an ancestral character, as may also occur during the life of the adult; but the evidence is very
+  strong that although some of the peculiarities of the larva may be due to such c&#x0153;nogenetic
+  factors, yet on the whole many of them are due to ancestral characters, which disappear when
+  transformation takes place, and are not found in the adult.</p>
+
+  <p>Thus if it be supposed that the amphibian arose from the fish, the tadpole presents more
+  resemblance to the fish than the frog. If <span class="pagenum" id="page36">{36}</span>it be
+  supposed that the arthropod arose from the segmented worm, the caterpillar bears out the
+  suggestion better than the adult imago. If it be supposed that the tunicate arose from a stock
+  allied to the vertebrate, it is because of the peculiarities of the larva that such a supposition
+  is entertained. So, too, if it be supposed that the fish arose from a member of the arthropod
+  group, the larval form of the fish is most likely to give decisive information on the point.</p>
+
+  <p class="sp3">For all these reasons the lowest form of fish to be investigated, in the hopes of
+  finding out the nature of the earliest formed fish, is not Amphioxus, but Ammoc&#x0153;tes, the
+  larval form of the lamprey&mdash;a form which, as I hope to satisfy my reader after perusal of
+  subsequent pages, more nearly resembles the ancient Cephalaspidian fishes than any other living
+  vertebrate.</p>
+
+  <p class="ac"><span class="sc">Comparison of Central Nervous Systems of Vertebrate and Arthropod
+  without Reversal of Surfaces.</span></p>
+
+  <p>So far different lines of investigation all point to the origin of the vertebrate from
+  arthropods, the group of arthropods in question being now extinct, the nearest living
+  representative being Limulus; also to the fact that of the two theories of the origin of
+  vertebrates, that one which is based on the resemblance between the central nervous systems of the
+  Vertebrata and the Appendiculata (Arthropoda and Annelida) is more in accordance with this
+  evidence than the other, which is based mainly on the supposed possession of a notochord among
+  certain animals.</p>
+
+  <p>How is it, then, that this theory has been discredited and lost ground? Simply, I imagine,
+  because it was thought to necessitate the turning over of the animal. Let us, then, again look at
+  the nervous system of the vertebrate, and see whether there is any such necessity.</p>
+
+  <p>As previously mentioned, the comparison of the two central nervous systems showed such close
+  resemblances as to force those anatomists who supported this theory to the conclusion that the
+  infundibular tube was in the position of the original &#x0153;sophagus; they therefore looked for
+  the remains of a mouth opening in the dorsal roof of the brain, but did not attempt to explain the
+  extraordinary fact that the infundibular tube is only a ventral offshoot from the tube of the
+  central nervous system. Yet this latter tube <span class="pagenum" id="page37">{37}</span>is one,
+  if not the most striking, of the peculiarities which distinguish the vertebrate; a tubular central
+  nervous system such as that of the vertebrate is totally unlike any other nervous system, and the
+  very fact that the two nervous systems of the vertebrate and arthropod are so similar in their
+  nervous arrangements, makes it still more extraordinary that the nervous system should be grouped
+  round a tube in the one case and not in the other.</p>
+
+  <p>Now, in the arthropod the &#x0153;sophagus leads directly into the stomach, which is situated
+  in the head-region, and from this a straight intestine passes directly along the length of the
+  body to the anus, where it terminates. The relations of mouth, &#x0153;sophagus, alimentary canal,
+  and nervous system in these animals are represented in the diagram (Fig. <a
+  href="#fig3">3</a>).</p>
+
+  <p>Any tube, therefore, such as that of the infundibulum, which would represent the
+  &#x0153;sophagus of such an animal, must have opened into the mouth on the ventral side, and into
+  the stomach on the dorsal side, and the lining epithelium of such an &#x0153;sophagus must have
+  been continuous with that of the stomach, and so of the whole intestinal tract.</p>
+
+  <p>Supposing, then, the animal is not turned over, but that the dorsal side still remains dorsal
+  and ventral ventral, then the original mouth-opening of the &#x0153;sophagus must be looked for on
+  the ventral surface of the vertebrate brain in the region of the pituitary body or hypophysis, and
+  on the dorsal side the tube representing the &#x0153;sophagus must be continuous with a large
+  cephalically dilated tube, which ought to pass into a small canal, to run along the length of the
+  body and terminate in the anus.</p>
+
+  <p>This is exactly what is found in the vertebrate, for the infundibular tube passes into the
+  third ventricle of the brain, which forms, with the other ventricles of the brain, the large
+  dilated cephalic portion of the so-called nerve tube, and at the junction of the medulla oblongata
+  and spinal cord, this dilated anterior part passes into the small, straight, central canal of the
+  spinal cord, which in the embryo terminates in the anus by way of the neurenteric canal. If the
+  animal is regarded as not having been turned over, then the conclusion that the infundibulum was
+  the original &#x0153;sophagus leads immediately to the further conclusion that the ventricles of
+  the vertebrate brain represent the original cephalic stomach, and the central canal of the spinal
+  cord the straight intestine of the arthropod ancestor.</p>
+
+  <div><span class="pagenum" id="page38">{38}</span></div>
+
+  <p>For the first time a logical, straightforward explanation is thus given of the peculiarities of
+  the tube of the central nervous system, with its extraordinary termination in the anus in the
+  embryo, its smallness in the spinal cord, its largeness in the brain region, and its offshoot to
+  the ventral side of the brain as the infundibular channel. It is so clear that, if the
+  infundibular tube be looked on as the old &#x0153;sophagus, then its lining epithelium is the
+  lining of that &#x0153;sophagus; and the fact that this lining epithelium is continuous with that
+  of the third ventricle, and so with the lining of the whole nerve-tube, must be taken into account
+  and not entirely ignored as has hitherto been the case. If, then, we look at the central nervous
+  system of the vertebrate in the light of the central nervous system of the arthropod without
+  turning the animal over, we are led immediately to the conclusion that what has hitherto been
+  called the vertebrate nervous system is in reality composed of two parts, viz. a nervous part
+  comparable in all respects with that of the arthropod ancestor, which has grown over and included
+  into itself, to a greater or less extent, a tubular part comparable in all respects with the
+  alimentary canal of the aforesaid ancestor. If this conclusion is correct, it is entirely wrong to
+  speak of the vertebrate central nervous system as being tubular, for the tube does not belong to
+  the nervous system, but was originally a simple epithelial tube, such as characterizes the
+  &#x0153;sophagus, cephalic stomach, and straight intestine of the arthropod.</p>
+
+  <p>Here, then, is the crux of the position&mdash;either the so-called nervous tube of the
+  vertebrate is composed of two separate factors, consisting of a true non-tubular nervous system
+  and a non-nervous epithelial tube, these two elements having become closely connected together; or
+  it is composed of one factor, an epithelial tube which constitutes the nervous system, its
+  elements being all nervous elements.</p>
+
+  <p>If this latter hypothesis be accepted, then it is necessary to explain why parts of that tube,
+  such as the roof of the fourth ventricle, the choroid plexuses of the various ventricles, which
+  are parts of the original roof inserted into the ventricles, are not composed of nervous material,
+  but form simple single-layered epithelial sheets, which by no possibility can be included among
+  functional nervous structures. The upholders of this hypothesis can only explain the nature of
+  these thin epithelial parts of the nervous tube in one of two ways; either the tube was originally
+  formed of nervous <span class="pagenum" id="page39">{39}</span>material throughout, and for some
+  reason parts of it have lost their nervous function and thinned down; or else these thin
+  epithelial parts are on their way to become nervous material, are still in an embryonic condition,
+  and are of the nature of epiblast-epithelium, from which the central nervous system originally
+  arose.</p>
+
+  <p>The first explanation is said to be supported by embryology, for at first the nerve-tube is
+  formed in a uniform manner, and then later, parts of the roof appear to thin out and so form the
+  thin epithelial parts. If this were the right explanation, then it ought to be found that in the
+  lowest vertebrates there is greater evidence of a uniformly nervous tube than in the higher
+  members of the group: while conversely, if, on the contrary, as we descend the vertebrate phylum,
+  it is found that more and more of the tube presents the appearance of a single layer of
+  epithelium, and the nervous material is limited more and more to certain parts of that tube, then
+  the evidence is strong that the tubular character of the central nervous system is not due to an
+  original nervous tube, but to a non-nervous epithelial tube with which the original nervous system
+  has become closely connected.</p>
+
+  <p>The comparison of the brain region of the different groups of vertebrates (Fig. <a
+  href="#fig19">19</a>) is most instructive, for it demonstrates in the most conclusive manner how
+  the roof of the nervous tube in that region loses more and more its nervous character, and takes
+  on the appearance of a simple epithelial tube, as we descend lower and lower; until at last, in
+  the brain of Ammoc&#x0153;tes, as represented in the figures, the whole of the brain-roof, from
+  the region of the pineal eye to the commencement of the spinal cord, is composed of fold upon fold
+  of a thin epithelial membrane forming an epithelial bag, which is constricted in only one place,
+  where the fourth cranial nerve crosses over it.</p>
+
+  <p>Further, the brain of Ammoc&#x0153;tes (Fig. <a href="#fig20">20</a>) shows clearly not only
+  that it is composed of two parts, an epithelial tube and a nervous system, but also that the
+  nerve-masses are arranged in the same relative position with respect to this tube as are the
+  nerve-masses in the invertebrate with respect to the cephalic stomach and &#x0153;sophagus. This
+  evidence is so striking, so conclusive, that it is impossible to resist the conclusion that the
+  tube did not originate as part of the central nervous system, but was originally independent of
+  the central nervous system, and has been invaded by it.</p>
+
+  <div><span class="pagenum" id="page40">{40}</span></div>
+
+  <div class="ac w45 fcenter sp2">
+    <a href="images/fig019.png" id="fig19"><img style="width:100%" src="images/fig019.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 19.&mdash;Comparison of Vertebrate Brains.</span></p>
+      <p class="sp0"><i>CB.</i>, cerebellum; <i>PT.</i>, pituitary body; <i>PN.</i>, pineal body;
+      <i>C. STR.</i>, corpus striatum; <i>G.H.R.</i>, right ganglion habenulæ. <i>I.</i>, olfactory;
+      <i>II.</i>, optic nerves.</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page41">{41}</span></div>
+
+  <div class="ac w50 fcenter sp2">
+    <a href="images/fig020.png" id="fig20"><img style="width:100%" src="images/fig020.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 20.&mdash;Brain of Ammoc&#x0153;tes.</span></p>
+      <p class="sp0">A, dorsal view; B, lateral view; C, ventral view. <i>C.E.R.</i>, cerebral
+      hemispheres; <i>G.H.R.</i>, right ganglion habenulæ; <i>PN.</i>, right pineal eye;
+      <i>CH<sub>2</sub></i>, <i>CH<sub>3</sub></i>, choroid plexuses; <i>I.-XII.</i> cranial nerves;
+      <i>C.P.</i>, <i>Conus post-commissuralis</i>.</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page42">{42}</span></div>
+
+  <p>The second explanation is hardly worth serious consideration, for it supposes that the nervous
+  system, for no possible reason, was laid down in its most important parts&mdash;the
+  brain-region&mdash;as an epithelial tube with latent potential nervous functions; that even up to
+  the highest vertebrate yet evolved these nervous functions are still in abeyance over the whole of
+  the choroid plexuses and the roof of the fourth ventricle. Further, it supposes that this
+  prophetic epithelial tube originally developed into true nervous material only in certain parts,
+  and that these parts, curiously enough, formed a nervous system absolutely comparable to that of
+  the arthropod, while the dormant prophetic epithelial part was formed so as just to mimic, in
+  relation to the nervous part, the alimentary canal of that same arthropod.</p>
+
+  <p>The mere facts of the case are sufficient to show the glaring absurdity of such an explanation.
+  This is not the way Nature works; it is not consistent with natural selection to suppose that in a
+  low form nervous material can be laid down as non-nervous epithelial material in order to provide
+  in some future ages for the great increase in the nervous system.</p>
+
+  <p>Every method of investigation points to the same conclusion, whether the method is
+  embryological, anatomical, or pathological.</p>
+
+  <p>First, take the embryological evidence. On the ground that the individual development
+  reproduces to a certain extent the phylogenetic development, the peculiarities of the formation of
+  the central nervous system in the vertebrate embryo ought to receive an appropriate explanation in
+  any theory of phylogenetic development. Hitherto such explanation has been totally lacking; any
+  suggestion of the manner in which a tubular nervous system may have been formed takes no account
+  whatever of the differences between different parts of the tube; its dilated cephalic end with its
+  infundibular projection ventrally, its small straight spinal part, and its termination in the
+  anus. My theory, on the other hand, is in perfect harmony with the embryological history, and
+  explains it point by point.</p>
+
+  <p>From the very first origin of the central nervous system there is evidence of two
+  structures&mdash;the one nervous, and the other an epithelial surface-layer which ultimately forms
+  a tube; this was first described by Scott in Petromyzon, and later by Assheton in the frog. In the
+  latter case the external epithelial layer is pigmented, while the underlying nervous layer
+  contains no pigment; a marked <span class="pagenum" id="page43">{43}</span>and conspicuous
+  demarcation exists, therefore, between the two layers from the very beginning, and it is easy to
+  trace the subsequent fate of the two layers owing to this difference of pigmentation. The
+  pigmented cells form the lining cells of the central canal, and becoming elongated, stretch out
+  between the cells of the nervous layer; while the latter, on their side, invade and press between
+  the pigmented cells. In this case, owing to the pigmentation of the epithelial layer, embryology
+  points out in the clearest possible manner how the central nervous system of the vertebrate is
+  composed of two structures&mdash;an epithelial non-nervous tube, on the outside of which the
+  central nervous system was originally grouped; how, as development proceeds, the elements of these
+  two structures invade each other, until at last they become so involved together as to give rise
+  to the conception that we are dealing with one single nerve tube. It is impossible for embryology
+  to give a clearer clue to the past history than it does in this case, for it actually shows, step
+  by step, how the amalgamation between the central nervous system and the old alimentary canal took
+  place.</p>
+
+  <p>Further, consider the shape of the tube when it is first formed, how extraordinary and
+  significant that is. It consists of a simple dilated anterior end leading into a straight tube,
+  the lumen of which is much larger than that of the ultimate spinal canal, and terminates by way of
+  the neurenteric canal in the anus.</p>
+
+  <p>Why should the tube take this peculiar shape at its first formation? No explanation is given or
+  suggested in any text-book of embryology, and yet it is so natural, so simple: it is simply the
+  shape of the invertebrate alimentary canal with its cephalic stomach and straight intestine ending
+  in the anus. Again embryology indicates most unmistakably the past history of the race. How are
+  the nervous elements grouped round this tube when it is first formed? Here embryology shows that a
+  striking difference exists between the part of the tube which forms the spinal cord and the
+  dilated cephalic part. Fig. <a href="#fig21">21</a>, A (2), represents the relation between the
+  nervous masses and the epithelial tube in the first instance. At this stage the nervous material
+  in the spinal cord lies laterally and ventrally to this tube, and at a very early stage the white
+  anterior commissure is formed, joining together these two lateral masses; as yet there is no sign
+  of any posterior fissure, the tube with its open lumen extends right to the dorsal surface.</p>
+
+  <div><span class="pagenum" id="page44">{44}</span></div>
+
+  <p>The interpretation of this stage is that in the invertebrate ancestor the nerve-masses were
+  situated laterally and ventrally to the epithelial tube, and were connected together by
+  commissures on the ventral side of the tube (Fig. <a href="#fig21">21</a>, A (1)); in other words,
+  the chain of ventral ganglia and their transverse commissures lying just ventrally to the
+  intestine, which are so characteristic of the arthropod nervous system, is represented at this
+  stage.</p>
+
+  <div class="ac w45 fcenter sp2">
+    <a href="images/fig021.png" id="fig21"><img style="width:100%" src="images/fig021.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="sp0"><span class="sc">Fig. 21.&mdash;A, Method of Formation of the Vertebrate Spinal
+      Cord from the Ventral Chain of Ganglia and the Intestine of an Arthropod, represented in 1; B,
+      Method of Formation of the Vertebrate Medulla Oblongata from the Infra-&#x0153;sophageal
+      Ganglia and the Cephalic Stomach of an Arthropod.</span></p>
+    </div>
+  </div>
+
+  <p>Subsequently, by the growth dorsalwards of nervous material to form the posterior columns, the
+  original epithelial tube is compressed dorsally and laterally to such an extent that those parts
+  lose all signs of lumen, the one becoming the posterior fissure and the others the <i>substantia
+  gelatinosa Rolandi</i> on each side. The original tube is thus reduced to a small canal formed by
+  its ventral portion only (Fig. <a href="#fig21">21</a>, A (3)). In this way the spinal cord is
+  formed, and the walls of the original epithelial tube are finally visible only as the lining of
+  the central canal (Fig. <a href="#fig21">21</a>, A (4)).</p>
+
+  <p>When we pass to the brain-region, to the anterior dilated portion of the tube, embryology tells
+  a different story. Here, as in the spinal cord, the nervous masses are grouped at first laterally
+  and ventrally to the epithelial tube, as is seen in Fig. <a href="#fig21">21</a>, B (2), but owing
+  to the large size of its lumen here, the nervous material is not able to enclose it completely, as
+  in the case of the spinal cord; <span class="pagenum" id="page45">{45}</span>consequently there is
+  no posterior fissure formed; but, on the contrary, the dorsal roof, not enclosed by the
+  nerve-masses, remains epithelial, and so forms the membranous roof of the fourth ventricle and of
+  the other ventricles of the brain (Fig. <a href="#fig21">21</a>, B (3)). In the higher animals,
+  owing to the development of the cerebrum and cerebellum, this membranous roof becomes pushed into
+  the larger brain cavity, and thus forms the choroid plexuses of the third and lateral ventricles.
+  In the lower vertebrates, as in Ammoc&#x0153;tes and the Dipnoi, it still remains as a dorsal
+  epithelial roof and forms a most striking characteristic of such brains.</p>
+
+  <p>In this part of the nervous system, then, the nervous material is all grouped in its original
+  position on the ventral side of the tube; and yet it is the same nervous material as that of the
+  spinal cord, all the elements are there, giving origin here to the segmental cranial nerves just
+  as lower down they give rise to the segmental spinal nerves, connecting together the separate
+  segments each with the other and all with the higher brain-centres&mdash;the supra-infundibular
+  centres&mdash;just as they do in the spinal region.</p>
+
+  <p>Why should there be this striking difference between the formation of the infra-infundibular
+  region of the brain and that of the spinal cord? Do the advocates of the origin of vertebrates
+  from Balanoglossus give the slightest reason for it? They claim that their view also provides a
+  tubular nervous system for the vertebrate, but give not the slightest sign or indication as to why
+  the nervous material should be grouped entirely on the ventral side of an epithelial tube in the
+  infra-infundibular region and yet surround it in the spinal cord region. And the explanation is so
+  natural, so simple: embryology does its very best to tell us the past history of the race, if only
+  we look at it the right way.</p>
+
+  <p>The infra-infundibular nervous mass is naturally confined to the ventral side of the epithelial
+  tube, because it represents the infra-&#x0153;sophageal ganglia, situated as they are on the
+  ventral side of the cephalic stomach, and, owing to the size of the stomach, they could not
+  enclose it by dorsal growth, as they do in the case of the formation of the spinal cord (Fig. <a
+  href="#fig21">21</a>, B (1)). Still these nervous masses have grown dorsalwards, have commenced to
+  involve the walls of the cephalic stomach even in the lowest vertebrate, as is seen in
+  Ammoc&#x0153;tes, in which animal a ventral portion of the epithelial bag has been evidently
+  compressed and its lumen finally obliterated <span class="pagenum" id="page46">{46}</span>by the
+  growth of the nerve-masses on each side of it. Throughout the whole vertebrate kingdom this
+  obliterated portion still leaves its mark as the <i>raphé</i> or seam, which is so characteristic
+  of the infra-infundibular portion of the brain.</p>
+
+  <div class="ac w25 fcenter sp2">
+    <a href="images/fig022.png" id="fig22"><img style="width:100%" src="images/fig022.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 22.&mdash;Horizontal Section through the Brain of
+      Ammoc&#x0153;tes.</span></p>
+      <p class="sp0"><i>Cr.</i>, membranous cranium; <i>I</i>, olfactory nerves; <i>l.v.</i>,
+      lateral ventricles; <i>gl.</i>, glandular tissue which fills up the cranial cavity.</p>
+    </div>
+  </div>
+
+  <p>Here, again, it is seen how simple is the explanation of a peculiarity which has always puzzled
+  anatomists&mdash;why should there be this seam in the infra-infundibular portion of the brain and
+  not in the supra-infundibular or in the spinal cord? The corresponding compression in the upper
+  brain-region forms the lateral ventricles, as is seen in the accompanying figure of the brain of
+  Ammoc&#x0153;tes (Fig. <a href="#fig22">22</a>).</p>
+
+  <div class="ac w25 fcenter sp2">
+    <a href="images/fig023.png" id="fig23"><img style="width:100%" src="images/fig023.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p class="sp0"><span class="sc">Fig. 23.&mdash;Section through Rhomboidal Sinus of
+      Bird.</span></p>
+    </div>
+  </div>
+
+  <p>In yet another instance it is seen how markedly the nervous masses are arranged in the same
+  position with respect to the central tube as are the nerve ganglia with respect to the intestinal
+  tube in the case of the invertebrate. Thus in birds a portion of the spinal cord in the
+  lumbo-sacral region presents a very different appearance from the rest of the cord; it is known as
+  the rhomboidal sinus, and a section of the cord of an adult pigeon across this region is given in
+  Fig. <a href="#fig23">23</a>. As is seen, the nervous portions are entirely confined to two masses
+  connected together by the white anterior commissures which are situated laterally and ventrally to
+  a median gelatinous mass; the small central canal is visible and <span class="pagenum"
+  id="page47">{47}</span>the whole dorsal area of the cord is taken up by a peculiar non-nervous
+  wedge-shaped mass of tissue. At its first formation this portion of the cord is formed exactly in
+  the same manner as the rest of the cord; instead, however, of the nervous material invading the
+  dorsal part of the tube to form the posterior fissure, it has been from some cause unable to do
+  so, the walls of the original non-nervous tube have become thickened dorsally, been transformed
+  into this peculiar tissue, and so caused the peculiar appearance of the cord here. The nervous
+  parts have not suffered in their development; the mechanism for walking in the bird is as well
+  developed as in any other animal; their position only is different, for they still retain the
+  original ventro-lateral position, but the non-nervous tube, the remains of the old intestine, has
+  undergone a peculiar gelatinous degeneration just where it has remained free from invasion by the
+  nervous tissue.</p>
+
+  <p>Throughout the whole of that part of the nervous system which gives origin to the cranial and
+  spinal segmental nerves, the evidence is absolutely uniform that the nervous material was
+  originally arranged bilaterally and ventrally on each side of the central tube, exactly in the
+  same way as the nerve-masses of the infra-&#x0153;sophageal and ventral chain of ganglia are
+  arranged with respect to the cephalic stomach and straight intestine of the arthropod. But, in
+  addition, we find in the vertebrate nervous masses, the cerebral hemispheres, the corpora
+  quadrigemina and the cerebellum situated on the dorsal side of the central tube in the
+  brain-region; this nervous material is, however, of a different character to that which gives
+  origin to the spinal and cranial segmental nerves. How is the presence of these dorsal masses to
+  be explained on the supposition that the dilated anterior part of the nerve-tube was originally
+  the cephalic stomach of the arthropod ancestor? The cerebral hemispheres are simple enough, for
+  they represent the supra-&#x0153;sophageal ganglia, which of necessity, as they increased in size,
+  would grow round the anterior end of the cephalic stomach and become more and more dorsal in
+  position.</p>
+
+  <p>The difficulty lies rather in the position of the cerebellum and corpora quadrigemina, and the
+  solution is as simple as it is conclusive.</p>
+
+  <p>Let us again turn to embryology and see what help it gives. In all vertebrates the dilated
+  anterior portion of the nerve-tube does not, <span class="pagenum" id="page48">{48}</span>as it
+  grows, increase in size uniformly, but a constriction appears on its dorsal surface at one
+  particular place, so as to divide it into an anterior and posterior vesicle; then the latter
+  becomes divided into two portions by a second constriction. In this way three cerebral vesicles
+  are formed; these three primary cerebral vesicles indicate the region of the fore-brain,
+  mid-brain, and hind-brain respectively. Subsequently the first cerebral vesicle becomes divided
+  into two to form the prosencephalon and thalamencephalon, while the third cerebral vesicle is also
+  divided into two to form the region of the cerebellum and medulla oblongata.</p>
+
+  <p>These constrictions are in the position of commissural bands of nervous matter; of these the
+  limiting nervous strands between the thalamencephalon and mesencephalon and between the
+  mesencephalon and the hind-brain are of primary importance. The first of these commissural bands
+  is in the position of the posterior commissure connecting the two optic thalami. In close
+  connection with this are found, on the mid-dorsal region, the two pineal eyes with their optic
+  ganglia, the so-called <i>ganglia habenulæ</i>. From these ganglia a peculiar tract of fibre,
+  known as Meynert's bundle, passes on each side to the ventral infra-infundibular portion of the
+  brain. In other words, the first constriction of the dilated tube is due to the presence and
+  growth of nervous material in connection with the median pineal eyes. Here in precisely the same
+  spot, as will be fully explained in the next chapter, there existed in the arthropod ancestor a
+  pair of median eyes situated dorsally to the cephalic stomach, the pre-existence of which explains
+  the reason for the first constriction.</p>
+
+  <p>The second primary constriction separating the mid-brain from the hind-brain is still more
+  interesting, for it is coincident with the position of the trochlear or fourth cranial nerve. In
+  all vertebrates without exception this nerve takes an extraordinary course; all other nerves,
+  whether cranial or spinal, pass ventralwards to reach their destination. This nerve passes
+  dorsalwards, crosses its fellow mid-dorsally in the valve of Vieussens, where the roof of the
+  brain is thin, and then passes out to supply the superior oblique muscle of the eye of the
+  opposite side. The two nerves form an arch constricting the dilated tube at this place. In the
+  lowest vertebrate (Ammoc&#x0153;tes) the constriction formed by this nerve-pair is evident not
+  only in the embryonic condition as in other vertebrates, but during the whole larval stage. As
+  Fig. <a href="#fig20">20</a>, A and B, shows, the whole of the dorsal <span class="pagenum"
+  id="page49">{49}</span>region of the brain up to the region of the pineal eye and <i>ganglion
+  habenulæ</i> is one large membranous bag, except for the single constriction where the fourth
+  nerve on each side crosses over. The explanation of this peculiarity is given in Chapter VII., and
+  follows simply from the facts of the arrangement of that musculature in the scorpion-group which
+  gave rise to the eye-muscles of the vertebrate.</p>
+
+  <p>In Ammoc&#x0153;tes both cerebellum and posterior corpora quadrigemina can hardly be said to
+  exist, but upon transformation a growth of nervous material takes place in this region, and it is
+  seen that this commencing cerebellum and the corpora quadrigemina arise from tissue that is
+  present in Ammoc&#x0153;tes along the course of the fourth nerve.</p>
+
+  <p>Here, then, again Embryology does its best to tell us how the vertebrate arose. The formation
+  of the two primary constrictions in the dilated anterior vesicle whereby the brain is divided into
+  fore-brain, mid-brain, and hind-brain is simply the representation ontogenetically of the two
+  nerve-tracts which crossed over the cephalic stomach in the prevertebrate stage, in consequence of
+  the mid-dorsal position of the pineal eyes and of the insertion of the original superior oblique
+  muscles.</p>
+
+  <p>The subsequent constriction by which the prosencephalon is separated from the thalamencephalon
+  is in the position of the anterior commissure, that commissure which connects the two
+  supra-infundibular nerve-masses, and is one of the first-formed commissures in every vertebrate.
+  This naturally is simply the commissure between the two supra-&#x0153;sophageal ganglia; anterior
+  to it, in the middle line, equally naturally, the anterior end of the old stomach wall still
+  exists as the <i>lamina terminalis</i>.</p>
+
+  <p>The other division in the hind-brain region, which separates the region of the cerebellum from
+  the medulla oblongata, is due to the growth of the cerebellum, and indicates its posterior limit.
+  In such an animal as the lamprey, where the cerebellum is only commencing, this constriction does
+  not occur in the embryo.</p>
+
+  <p>From such simple beginnings as are seen in Ammoc&#x0153;tes, the higher forms of brain have
+  been evolved, to culminate in that of man, in which the massive cerebrum and cerebellum conceals
+  all sign of the dorsal membranous roof, those parts of the simple epithelial tube which still
+  remain being tucked away into the cavities to form the various choroid plexuses.</p>
+
+  <div><span class="pagenum" id="page50">{50}</span></div>
+
+  <p>In the whole evolution from the brain of Ammoc&#x0153;tes to that of man, the same process is
+  plainly visible, viz. growth and extension of nervous material over the epithelial tube; extension
+  dorsally and posteriorly of the supra-infundibular nervous masses (as seen in Fig. <a
+  href="#fig19">19</a>), combined with a dorsal growth of parts of the infra-infundibular nervous
+  masses to form the cerebellum and posterior corpora quadrigemina.</p>
+
+  <p>Especially instructive is the formation of the cerebellum. It consists at first of a small mass
+  of nervous tissue accompanying the fourth nerve, then by the growth of that mass surrounding and
+  constricting a fold of the membranous roof, the <i>worm</i> of the cerebellum is formed, as in the
+  dog-fish. This very constriction causes the membrane to be thrown into a lateral fold on each
+  side, as seen in Fig. <a href="#fig24">24</a>, and in the dog-fish the nervous material on each
+  side, known as the fimbriæ, is already commencing to grow from the ventral mass of the medulla
+  oblongata to surround these lateral membranous folds. These <i>fimbriæ</i> develop more and more
+  in higher forms, and thus form the cerebellar hemispheres.</p>
+
+  <p>Not only does comparative anatomy confirm the teachings of embryology, but also pathology gives
+  its quota in the same direction.</p>
+
+  <div class="ac w30 fcenter sp2">
+    <a href="images/fig024.png" id="fig24"><img style="width:43%" src="images/fig024.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 24.&mdash;Cerebellum of Dog-fish.</span></p>
+      <p class="sp0"><i>v</i>, worm of cerebellum; <i>IV.</i>, membranous roof of fourth ventricle
+      continuous with the membranous folds on each side. Through these the fimbriæ (<i>fb.</i>) can
+      be dimly seen.</p>
+    </div>
+  </div>
+
+  <p>One of the striking facts about malformations and disease of the central nervous system is the
+  frequency of cystic formations; <i>spina bifida</i> is a well-known instance. These cysts are
+  merely epithelial non-nervous cysts formed from the epithelium of the central canal, difficult to
+  understand if the whole nerve tube is one and entirely nervous, either actually or potentially,
+  but natural and easy if we are really dealing with a simple epithelial tube on the outside of
+  which the nervous material was originally grouped. The cystic formation belongs naturally enough
+  to this tube, not to the nervous system.</p>
+
+  <p>Again, where animals such as lizards have grown a new tail, owing to the breaking off of the
+  original one, it is found that the central canal extends into this new tail for some distance, but
+  not <span class="pagenum" id="page51">{51}</span>the nervous material surrounding it; all the
+  nerves supplying the new tail arise from the uninjured spinal cord above, the central canal with
+  its lining layer of epithelial cells alone grows into the new-formed appendage.</p>
+
+  <p>To all intents and purposes the same thing is seen in the termination of the spinal cord in a
+  bird-embryo; more and more, as the end of the tail is approached, does the nervous matter of the
+  spinal cord grow less and less, until at last a naked central canal with its lining epithelium is
+  alone left to represent the so-called nerve-tube.</p>
+
+  <p>All these different methods of investigation lead irresistibly to the one conclusion that the
+  tubular nature of the central nervous system has been caused by the central nervous system
+  enclosing to a greater or less extent a pre-existing, non-nervous, epithelial tube.</p>
+
+  <p class="sp3">This must always be borne strictly in mind. The problem, therefore, which presents
+  itself is the comparison of these two factors separately, in order to find out the relationship of
+  the vertebrate to the invertebrate. The nervous system without the tube must be compared to other
+  nervous systems, and the tube must be considered apart from the nervous system.</p>
+
+  <p class="ac"><span class="sc">The Principle of Concentration and Cephalization.</span></p>
+
+  <p>The central nervous system of the vertebrate resembles that of all the Appendiculata in the
+  fact that it is composed of segments joined together which give origin to segmental nerves. There
+  is, however, a great difference between the two systems: the division into separate segments is
+  not obvious to the eye in the vertebrate nervous system, while in the invertebrate we can see that
+  it is composed of a series of separate pairs of ganglia joined together longitudinally by nervous
+  strands known as connectives and transversely by the nerve-commissures. Such a simple segmented
+  system is found in the segmented worms, and in the lower arthropods, such as Branchipus, no great
+  advance has been made on that of the annelid. In the higher forms, however, a greater and greater
+  tendency to fusion of separate ganglia exists, especially in the head-region, so that the
+  infra-&#x0153;sophageal ganglia, which, in the lower forms are as separate as those of the ventral
+  chain, in the higher forms are fused together to form a single nervous mass.</p>
+
+  <div><span class="pagenum" id="page52">{52}</span></div>
+
+  <p>This is the great characteristic of the advancement of the central nervous system among the
+  Invertebrata, its concentration in the region of the head. It may be called the principle of
+  cephalization, and is characteristic not only of higher organization in a group, but also of the
+  adult as distinguished from the larval form. Thus in the imago greater concentration is found than
+  in the caterpillar.</p>
+
+  <p>The segmented annelid type of nervous system consists of a supra-&#x0153;sophageal ganglion,
+  composed of the fused ganglia belonging to the pre-oral segments, and an infra-&#x0153;sophageal
+  chain of separate ganglia. With the concentration and modification around the mouth of the most
+  anterior locomotor appendages to form organs for prehension and mastication of food, a
+  corresponding concentration and fusion of the ganglia belonging to these segments takes place, so
+  that finally, in the higher annelids, and in most of the great arthropod group, a fusion of a
+  number of the most anterior ganglia has taken place to form the infra-&#x0153;sophageal
+  ganglion-mass.</p>
+
+  <p>The infra-&#x0153;sophageal ganglia which are the first to fuse are those which supply the most
+  anterior portion of the animal with nerves, and include always those anterior appendages which are
+  modified for mastication purposes. To this part the name <i>prosoma</i> has been given; in many
+  cases it forms a well-defined, distinct portion of the animal.</p>
+
+  <p>Succeeding this prosoma or masticatory region, there occurs in all gill-bearing arthropods a
+  respiratory region, in many cases more or less distinctly defined, which has received the name of
+  <i>mesosoma.</i> The rest of the body is called the <i>metasoma</i>.</p>
+
+  <p>In accordance with this nomenclature the central nervous system of many of the Arthropoda may
+  be divided as follows<span class="wnw">:&mdash;</span></p>
+
+  <p>1.  Pre-oral, or supra-&#x0153;sophageal ganglia.</p>
+
+  <p>2.  Infra-oral, or infra-&#x0153;sophageal ganglia and ventral chain, which consist of three
+  groups: prosomatic, mesosomatic, and metasomatic ganglia.</p>
+
+  <p>The infra-&#x0153;sophageal ganglion-mass, then, in most of the Arthropoda may be spoken of as
+  formed by the fusion of the prosomatic or mouth-ganglia, the mesosomatic and metasomatic remaining
+  separate and distinct. The number of ganglia which have fused may be observed by examination of
+  the embryo, in which it is easy to see indications of the individual ganglia or <i>neuromeres</i>,
+  although all such indication has disappeared in the adult; thus the <span class="pagenum"
+  id="page53">{53}</span>infra-&#x0153;sophageal ganglia of the cray-fish have been shown to be
+  constituted of six prosomatic ganglia.</p>
+
+  <p>In Fig. <a href="#fig25">25</a> I give figures of the central nervous system (with the
+  exception of the abdominal or metasomatic ganglia) of Branchipus, Astacus, Limulus, Scorpio,
+  Androctonus, Thelyphonus, and Ammoc&#x0153;tes. In all the figures the supra-&#x0153;sophageal
+  ganglia are lined horizontally, and their nerves shown, viz. optic (lateral eyes (II) and median
+  eyes (II&prime;)), olfactory (I) (first antennæ, camerostome, nose); then come the prosomatic
+  ganglia (dotted), with their nerves (A) supplying the mouth parts, and the second antennæ or
+  cheliceræ; then the mesosomatic (lined horizontally), with their nerves (B) supplying respiratory
+  appendages. These figures show that the concentrated brain mass around the &#x0153;sophagus of an
+  arthropod which has arrived at the stage of Astacus, is represented by the supra-&#x0153;sophageal
+  ganglia and the fused prosomatic ganglia.</p>
+
+  <p>The next stage in the evolution of the brain is seen in the gradual inclusion of the
+  mesosomatic ganglia, one after the other, into the infra-&#x0153;sophageal mass of the already
+  fused prosomatic ganglia. With this fusion is associated the loss of locomotion in these
+  mesosomatic appendages, and their entire subservience to the function of respiration. Dana urges
+  that cephalization is a consequence of functional alteration in the appendages, from organs of
+  locomotion to those of mastication and respiration. Whether this be true or not, it is certainly a
+  fact that in Limulus, the ganglion supplying the first mesosomatic appendage has fused with the
+  prosomatic, infra-&#x0153;sophageal mass. It is also a fact that the prosomatic appendages are the
+  organs of mastication, their basal parts being arranged round the mouth so as to act as foot-jaws,
+  while the mesosomatic appendages, though still free to move, have been reduced to such an extent
+  as to consist mainly of their basal parts, which are all respiratory in function, except in the
+  case of the first pair, where they carry the terminal ducts of the genital organs. In the next
+  stage, that, of the scorpion, in which the mesosomatic appendages have lost all power of free
+  locomotion, and have become internal branchiæ, another mesosomatic ganglion has fused with the
+  brain mass, while in Androctonus two of the branchial mesosomatic ganglia have fused; and finally,
+  in Thelyphonus and Phrynus, all the mesosomatic ganglia have coalesced with the fused prosomatic
+  ganglia, while the metasomatic ganglia have themselves fused together in the caudal region to form
+  what is known as the caudal brain.</p>
+
+  <div><span class="pagenum" id="page54">{54}</span></div>
+
+  <div class="ac w45 fcenter sp2">
+    <a href="images/fig025.png" id="fig25"><img style="width:100%" src="images/fig025.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p class="sp0"><span class="sc">Fig. 25.&mdash;Comparison of Invertebrate Brains from
+      Branchipus to Ammoc&#x0153;tes.</span></p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page55">{55}</span></div>
+
+  <p>The brain in these animals may be spoken of as composed of three parts&mdash;(1) the fused
+  supra-&#x0153;sophageal ganglia, (2) the fused prosomatic ganglia, and (3) the fused mesosomatic
+  ganglia. Such a brain is strictly homologous with the vertebrate brain, which also is built up of
+  three parts&mdash;(1) the part in front of the notochord, the prechordal or supra-infundibular
+  brain, which consists of the cerebral hemispheres, together with the basal and optic ganglia and
+  corresponds, therefore, to the supra-&#x0153;sophageal mass, with its olfactory and optic
+  divisions lying in front of the &#x0153;sophagus; (2 and 3) the epichordal brain, composed of (2)
+  a trigeminal and (3) a vagus division, of which the first corresponds strictly to the fused
+  prosomatic ganglia, and the second to the fused mesosomatic ganglia. Further, just as in the
+  embryo of an arthropod it is possible, with more or less accuracy, to see the number of neuromeres
+  or original ganglia which have fused to form the supra- and infra-&#x0153;sophageal portions of
+  its brain, so also in the embryo of a vertebrate we are able at an early stage to gain an
+  indication, more or less accurate, of the number of neuromeres which have built up the vertebrate
+  brain. The further consideration of these neuromeres, and the evidence they afford as to the
+  number of the prosomatic and mesosomatic ganglia which have formed the epichordal part of the
+  vertebrate brain, must be left to the chapter on the segmentation of the cranial nerves.</p>
+
+  <p class="sp3">The further continuation of this process of concentration of separate segments,
+  together with the fusion of the nervous system with the tube of the alimentary canal, leads in the
+  simplest manner to the formation of the spinal cord of the vertebrate from the metasomatic ganglia
+  of the ventral chain of the arthropod.</p>
+
+  <p class="ac"><span class="sc">The Antagonism between Cephalization and Alimentation.</span></p>
+
+  <p>This concentration of the nervous system in the head-region, together with an actual increase
+  in the bulk of the cephalic nervous masses, constitutes the great principle upon which the law of
+  upward progress or evolution in the animal kingdom is based, and it illustrates in a striking
+  manner the blind way in which natural selection works; for, as already explained, the central
+  nervous system arose as a ring round the mouth, in consequence of which, with the progressive
+  <span class="pagenum" id="page56">{56}</span>evolution of the animal kingdom, the &#x0153;sophagus
+  necessarily pierced the central nervous system at the cephalic end. At the same time, the very
+  fact that the evolution was progressive necessitated the concentration and increase of the nervous
+  masses in this very same &#x0153;sophageal region.</p>
+
+  <p>Progress on these lines must result in a crisis, owing to the inevitable squeezing out of the
+  food-channel by the increasing nerve-mass; and, indeed, the fact that such a crisis had in all
+  probability arisen at the time when vertebrates first appeared is apparent when we examine the
+  conditions at the present time.</p>
+
+  <p>Those invertebrates whose central nervous system is most concentrated at the cephalic end
+  belong to the arachnid group, among which are included the various living scorpion-like animals,
+  such as Thelyphonus, Androctonus, etc.</p>
+
+  <p>As already mentioned, the giants of the Palæostracan age were Pterygotus, Slimonia, etc., all
+  animals of the scorpion-type&mdash;in fact, sea-scorpions. Now, all these animals, spiders and
+  scorpions, without exception, are blood-suckers, and in all of them the concentrated cephalic mass
+  of nervous material surrounds an &#x0153;sophagus the calibre of which is so small that nothing
+  but a fluid pabulum can be taken into the alimentary canal; and even for that purpose a special
+  suctorial apparatus has in some species been formed on the gastric side of the &#x0153;sophagus
+  for the purpose of drawing blood through this exceedingly narrow tube.</p>
+
+  <p>In Fig. <a href="#fig25">25</a> this increasing antagonism between brain-power and
+  alimentation, as we pass from such a form as Branchipus to the scorpion, is illustrated, and in
+  Fig. <a href="#fig26">26</a> the relative sizes of the &#x0153;sophagus and the brain-mass
+  surrounding it is shown. The section shows that the food channel is surrounded by the white and
+  grey matter of the brain as completely as the central canal of the spinal cord of the vertebrate
+  is surrounded by the white and grey nervous material.</p>
+
+  <div class="ac w25 fcenter sp2">
+    <a href="images/fig026.png" id="fig26"><img style="width:100%" src="images/fig026.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 26.&mdash;Transverse Section through the Brain of a Young
+      Thelyphonus.</span></p>
+      <p class="sp0"><i>A</i>, supra-&#x0153;sophageal ganglia; <i>B</i>, infra-&#x0153;sophageal
+      ganglia; <i>Al</i>, &#x0153;sophagus.</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page57">{57}</span></div>
+
+  <p>Truly, at the time when vertebrates first appeared, the direction and progress of variation in
+  the Arthropoda was leading, owing to the manner in which the brain was pierced by the
+  &#x0153;sophagus, to a terrible dilemma&mdash;either the capacity for taking in food without
+  sufficient intelligence to capture it, or intelligence sufficient to capture food and no power to
+  consume it.</p>
+
+  <p>Something had to be done&mdash;some way had to be found out of this difficulty. The atrophy of
+  the brain meant degeneration and the reduction to a lower stage of organization, as is seen in the
+  Tunicata. The further development of the brain necessitated the establishment of a new method of
+  alimentation and the closure of the old &#x0153;sophagus, its vestiges still remaining as the
+  infundibular canal of the vertebrate, meant the enormous upward stride of the formation of the
+  vertebrate.</p>
+
+  <p>At first sight it might appear too great an assumption even to imagine the possibility of the
+  formation of a new gut in an animal so highly organized as an arthropod, but a little
+  consideration will, I think, show that such is not the case.</p>
+
+  <p>In the higher animals we are accustomed to speak of certain organs as vital and necessary for
+  the further existence of the animal; these are essentially the central nervous system, the
+  respiratory system, the circulatory system, and the digestive system. Of these four vital systems
+  the first cannot be touched without the chance of degeneration; but that is not the case with the
+  second. The passage from the fish to the amphibian, from the water-breathing to the air-breathing
+  animal, has actually taken place, and was effected by the modification of the swim-bladder to form
+  new respiratory organs&mdash;the lung; the old respiratory organs&mdash;the gills&mdash;becoming
+  functionless, but still persisting in the embryo as vestiges. The necessity arose in consequence
+  of the passage of the animal from water to land, and with this necessity nature found a means of
+  overcoming the difficulty; air-breathing vertebrates arose, and from the very fact of their being
+  able to extend over the land-surfaces, increased in numbers and developed in complexity in the
+  manner already sketched out.</p>
+
+  <p>For a respiratory system all that is required is an arrangement <span class="pagenum"
+  id="page58">{58}</span>by means of which blood should be brought to the surface, so as to
+  interchange its gases with those of the external medium; and it is significant to find that of all
+  vertebrates the Amphibia alone are capable of an effective respiration by means of the skin.</p>
+
+  <p>As to the circulatory system, it is exceedingly easily modified. An animal such as Amphioxus
+  has no heart; in some the heart is systemic, in others branchial; in some there are more than one
+  heart; in others there are contractile veins in addition to a heart. There is no difficulty here
+  in altering and modifying the system according to the needs of the individual.</p>
+
+  <p>For a digestive system all that is required is an arrangement for the digestion and absorption
+  of food, a mechanism which can arise easily if some of the cells of the skin possess digestive
+  power. Now Miss Alcock has shown that some of the surface-cells of crustaceans secrete a fluid
+  which possesses digestive powers, and she has also shown that certain of the cells in the skin of
+  Ammoc&#x0153;tes possess digestive power.</p>
+
+  <p>The difficulty, then, of forming a new digestive system in the passage from the arthropod to
+  the vertebrate is very much the same as the difficulty in forming a new respiratory system in the
+  passage from the water-breathing fish to the air-breathing amphibian&mdash;a change which does not
+  strike us as inconceivable, because we know it has taken place.</p>
+
+  <p>The whole argument so far leads to the conclusion that vertebrates arose from ancient forms of
+  arthropods by the formation of a new alimentary canal, and the enclosure of the old canal by the
+  growing central nervous system. If this conclusion is true, then it follows that we possess a
+  well-defined starting-point from which to compare the separate organs of the arthropod with those
+  of the vertebrate, and if, in consequence of such working hypothesis, each organ of the arthropod
+  is found in the vertebrate in a corresponding position and of similar structure, then the truth of
+  the starting-point is proved as fully as can possibly be expected by deductive methods. It is, in
+  fact, this method of comparative anatomy which has proved the descent of man from the ape, the
+  frog from the fish, etc.</p>
+
+  <p class="sp3">Let us, then, compare all the organs of such a low vertebrate as Ammoc&#x0153;tes
+  with those of an arthropod of the ancient type.</p>
+
+  <div><span class="pagenum" id="page59">{59}</span></div>
+
+  <p class="ac"><span class="sc">Life History of the Lamprey&mdash;not a Degenerate
+  Animal.</span></p>
+
+  <p>The striking peculiarity of the lamprey is its life-history. It lives in fresh water, spending
+  a large portion of its life in the mud during the period of its larval existence: then comes a
+  somewhat sudden transformation-stage, characterized, as in the lepidopterous larva, by a process
+  of histolysis, by which many of the larval tissues are destroyed and new ones formed, with the
+  result that the larval lamprey, or Ammoc&#x0153;tes, is transformed into the adult lamprey, or
+  Petromyzon. This transformation takes place in August, at all events in the neighbourhood of
+  Cambridge, and later in the year the transformed lamprey migrates to the sea, grows in size and
+  maturity, and returns to the river the following spring up to its spawning beds, where it spawns
+  and forthwith dies. How long it lives in the Ammoc&#x0153;tes stage is unknown; I myself have kept
+  some without transformation for four years, and probably they live in the rivers longer than that
+  before they change from their larval state. It is absolutely certain that very much the longest
+  part of the animal's life is spent in the larval stage, and that with the maturity of the sexual
+  organs and the production of the fertilized ova the life of the individual ends.</p>
+
+  <p>Now, the striking point of this transformation is that it produces an animal more nearly
+  comparable with higher vertebrates than is the larval form; in other words, the transformation
+  from larva to adult is in the direction of upward progress, not of degeneration. It is, therefore,
+  inaccurate to speak of the adult lamprey as degenerate from a higher race of fishes represented by
+  its larval form&mdash;Ammoc&#x0153;tes. Its transformation does not resemble that of the
+  tunicates, but rather that of the frog, so that, just as in the case of the tadpole, the
+  peculiarities of its larval form may be expected to afford valuable indications of its immediate
+  ancestry. The very peculiarities to which attention must especially be paid are those discarded at
+  transformation, and, as will be seen, these are essentially characteristic of the invertebrate and
+  are not found in the higher vertebrates. In fact, the transformation of the lamprey from the
+  Ammoc&#x0153;tes to the Petromyzon stage may be described as the casting off of many of its
+  ancestral invertebrate characters and the putting on of the characteristics of the vertebrate
+  type. It is this double individuality of the lamprey, together with its long-continued existence
+  in the larval form, which makes Ammoc&#x0153;tes more <span class="pagenum"
+  id="page60">{60}</span>valuable than any other living vertebrate for the study of the stock from
+  which vertebrates sprang.</p>
+
+  <p>Many authorities hold the view that the lamprey, like Amphioxus, must be looked upon as
+  degenerate, and therefore as no more suitable for the investigation of the problem of vertebrate
+  ancestry than is Amphioxus itself. This charge of degeneracy is based on the statement that the
+  lamprey is a parasite, and that the eyes in Ammoc&#x0153;tes are under the skin. The whole
+  supposition of the degeneracy of the Cyclostomata arose because of the prevailing belief of the
+  time that the earliest fishes were elasmobranchs, and therefore gnathostomatous. From such
+  gnathostomatous fishes the cyclostomes were supposed to have descended, having lost their jaws and
+  become suctorial in habit in consequence of their parasitism.</p>
+
+  <p>The charge of parasitism is brought against the lamprey because it is said to suck on to fishes
+  and so obtain nutriment. It is, however, undoubtedly a free-swimming fish; and when we see it
+  coming up the rivers in thousands to reach the spawning-beds, and sucking on to the stones on the
+  way in order to anchor itself against the current, or holding on tightly during the actual process
+  of spawning, it does not seem justifiable to base a charge of degeneration upon a parasitic habit,
+  when such so-called habit simply consists in holding on to its prey until its desires are
+  satisfied. If, of course, its suctorial mouth had arisen from an ancestral gnathostomatous mouth,
+  then the argument would have more force.</p>
+
+  <p>Dohrn, however, gives absolutely no evidence of a former <span class="correction"
+  title="Original reads 'gnathostomotous'">gnathostomatous</span> condition either in Petromyzon or,
+  in its larval state, Ammoc&#x0153;tes. He simply assumes that the Cyclostomata are degenerated
+  fishes and then proceeds to point out the rudiments of skeleton, etc., which they still possess.
+  Every point that Dohrn makes can be turned round; and, with more probability, it can be argued
+  that the various structures are the commencement of the skeletal and other structures in the
+  higher fishes, and not their degenerated remnants. Compare the life-history of the lamprey and of
+  the tunicate. In the latter case we look upon the animal as a degenerate vertebrate, because the
+  larval stage alone shows vertebrate characteristics; when transformation has taken place, and the
+  adult form is reached, the vertebrate characteristics have vanished, and the animal, instead of
+  reaching a higher grade, has sunk lower in the scale, the central nervous system especially having
+  lost all <span class="pagenum" id="page61">{61}</span>resemblance to that of the vertebrate. In
+  the former case a transformation also takes place, a marvellous transformation, characterized by
+  two most striking facts. On the one hand, the resulting animal is more like a higher vertebrate,
+  for, by the formation of new cartilages, its cranial skeleton is now comparable with that of the
+  higher forms, and the beginnings of the spinal vertebræ appear; by the increased formation of
+  nervous material, its brain increases in size and complexity, so as to compare more closely with
+  higher vertebrate brains; its eyes become functional, and its branchiæ are so modified,
+  simultaneously with the formation of the new alimentary canal in the cranial region, that they now
+  surround branchial pouches which are directly comparable to those of higher vertebrates. On the
+  other hand, the transformation process is equally characterized by the throwing off of tissues and
+  organs, one and all of which are comparable in structure and function with corresponding
+  structures in the Arthropoda&mdash;the thyroid of the Ammoc&#x0153;tes, the tentacles, the
+  muco-cartilage, the tubular muscles, all these structures, so striking in the Ammoc&#x0153;tes
+  stage, are got rid of at transformation. Here is the true clue. Here, in the throwing off of
+  invertebrate characters, and the taking on of a higher vertebrate form, especially a higher brain,
+  not a lower one, Petromyzon proclaims as clearly as is possible that it is not a degenerate
+  elasmobranch, but that it has arisen from Ammoc&#x0153;tes-like ancestors, even though Myxine,
+  Amphioxus, and the tunicates be all stages on the downward grade from those same
+  Ammoc&#x0153;tes-like ancestors.</p>
+
+  <p class="sp3">As to the eyes, they are functional in the adult form and as serviceable as in any
+  fish. There is no sign of degeneracy; it is only possible to speak of a retarded development which
+  lasts through the larval stage.</p>
+
+  <p class="ac"><span class="sc">Comparison of Brain of Ammoc&#x0153;tes with that of an
+  Arthropod.</span></p>
+
+  <p>Seeing that the steady progress of the development of the central nervous system is the most
+  important factor in the evolution of animals, it follows that of all organs of the body, the
+  central nervous system must be most easily comparable with that of the supposed ancestor. I will,
+  therefore, start by comparing the brain of Ammoc&#x0153;tes with that of arthropods, especially of
+  Limulus and of the scorpion-group.</p>
+
+  <div><span class="pagenum" id="page62">{62}</span></div>
+
+  <p>The supra-infundibular portion of the brain in vertebrates corresponds clearly to the
+  supra-&#x0153;sophageal portion of the invertebrate brain in so far that in both cases here is the
+  seat of the will. Voluntary action is as impossible to the arthropod deprived of its
+  supra-&#x0153;sophageal ganglia as to the vertebrate deprived of its cerebrum. It corresponds,
+  also, in that from it arise the nerves of sight and smell and no other nerves; this is also the
+  case with the supra-&#x0153;sophageal ganglia, for from a portion of these ganglia arise the
+  nerves to the eyes and the nerves to the first antennæ, of which the latter are olfactory in
+  function. Thus, in the accompanying figure, taken from Bellonci, it is seen that the
+  supra-&#x0153;sophageal ganglia consist of a superior segment corresponding to the cerebrum, a
+  middle segment from which arise the nerves to the lateral eyes and to the olfactory antennæ,
+  corresponding to the basal ganglia of the brain and the optic lobes, and, according to Bellonci,
+  of an inferior segment from which arise the nerves to the second pair of antennæ. This last
+  segment is not supra-&#x0153;sophageal in position, but is situated on the &#x0153;sophageal
+  commissures. It has been shown by Lankester and Brauer in Limulus and the scorpion to be in
+  reality the first ganglion of the infra-&#x0153;sophageal series, and not to belong to the
+  supra-&#x0153;sophageal group.</p>
+
+  <div class="ac w35 fcenter sp2">
+    <a href="images/fig027.png" id="fig27"><img style="width:100%" src="images/fig027.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 27.</span>&mdash;<span class="sc">The Brain of</span>
+      <i>Sphæroma serratum</i>. (After <span class="sc">Bellonci</span>.)</p>
+      <p class="sp0"><i>Ant. I.</i> and <i>Ant. II.</i>, nerves to 1st and 2nd antennæ.
+      <i>f.br.r.</i>, terminal fibre layer of retina; <i>Op. g. I.</i>, first optic ganglion; <i>Op.
+      g. II.</i>, second optic ganglion; <i>O.n.</i>, optic nerve-fibres forming an optic
+      chiasma.</p>
+    </div>
+  </div>
+
+  <p>Further, in Limulus, in the scorpion-group, and in all the extinct <span class="pagenum"
+  id="page63">{63}</span>Eurypteridæ&mdash;in fact, in the Palæostraca generally&mdash;there are two
+  median eyes in addition to the lateral eyes, which were innervated from these ganglia.</p>
+
+  <p>In Ammoc&#x0153;tes, then, if the supra-infundibular portion of the brain really corresponds to
+  the supra-&#x0153;sophageal of the palæostracan group, we ought to find, as indeed is the case, an
+  optic apparatus consisting of two lateral eyes and two median eyes, innervated from the
+  supra-infundibular brain-mass, and an olfactory apparatus built up on the same lines as in the
+  scorpion-group, also innervated from this region. If, in addition, it be found that those two
+  median eyes are degenerate eyes of the same type as the median eyes of Limulus and the
+  scorpion-group, then the evidence is so strong as to amount to a proof of the correctness of the
+  theory. This evidence is precisely what has been obtained in recent years, for the vertebrate did
+  possess two median eyes in addition to the two lateral ones, and these two median eyes are
+  degenerate eyes of the type found in the median eyes of arthropods and are not of the vertebrate
+  type. Moreover, as ought also to be the case, they are most evident, and one of the pair is most
+  nearly functional in the lowest perfect vertebrate, Ammoc&#x0153;tes.</p>
+
+  <p>Of all the discoveries made in recent years, the discovery that the pineal gland of the
+  vertebrate brain was originally a pair of median eyes is by far the most important clue to the
+  ancestry of the vertebrate, for not only do they correspond exactly in position with the median
+  eyes of the invertebrates, but, being already degenerate and functionless in the lowest
+  vertebrate, they must have been functional in a pre-vertebrate stage, thus giving the most direct
+  clue possible to the nature of the pre-vertebrate stage. It is especially significant that in
+  Limulus they are already partially degenerated. What, then, ought to be the structure and relation
+  to the brain of the median and lateral eyes of the vertebrate if they originated from the
+  corresponding organs of some one or other member of the palæostracan group?</p>
+
+  <p class="sp3">This question will form the subject of the next chapter.</p>
+
+  <p class="ac"><span class="sc">Summary.</span></p>
+
+  <div class="bq1 smaller sp5">
+    <p>The object of this book is to attempt to find out from what group of invertebrates the
+    vertebrate arose; no attempt is made to speculate upon the causes of variation by means of which
+    evolution takes place.</p>
+    <div><span class="pagenum" id="page64">{64}</span></div>
+    <p>A review of the animal kingdom as a whole leads to the conclusion that the upward development
+    of animals from an original c&#x0153;lenterate stock, in which the central nervous system
+    consists of a ring of nervous material surrounding the mouth, has led, in consequence of the
+    elaboration of the central nervous system, to a general plan among the higher groups of
+    invertebrates in the topographical arrangement of the important organs. The mouth is situated
+    ventrally, and leads by means of the &#x0153;sophagus into an alimentary canal which is situated
+    dorsally to the central nervous system. Thus the &#x0153;sophagus pierces the central nervous
+    system and divides it into two parts, the supra-&#x0153;sophageal ganglia and the
+    infra-&#x0153;sophageal ganglia. This is an almost universal plan among invertebrates, but
+    apparently does not hold for vertebrates, for in them the central nervous system is always
+    situated dorsally and the alimentary canal ventrally, and there is no piercing of the central
+    nervous system by an &#x0153;sophagus.</p>
+    <p>Yet a remarkable resemblance exists between the central nervous system of the vertebrate and
+    that of the higher invertebrates, of so striking a character as to compel one school of
+    anatomists to attempt the derivation of vertebrates from annelids. Now, the central nervous
+    system of vertebrates forms a hollow tube, and a diverticulum of this hollow tube, known as the
+    infundibulum, passes to the ventral surface of the brain in the very position where the
+    &#x0153;sophagus would have been if that brain had belonged to an annelid or an arthropod. This
+    school of anatomists therefore concluded that this infundibular tube represented the original
+    invertebrate &#x0153;sophagus which had become closed and no longer opened into the alimentary
+    canal owing to the formation of a new mouth in the vertebrate. As, however, the alimentary canal
+    of the vertebrate is ventral to the central nervous system, and not dorsal, as in the
+    invertebrate, it follows that the remains of the original invertebrate mouth into which the
+    &#x0153;sophagus (in the vertebrate the infundibular tube) must have opened must be searched for
+    on the dorsal side of the vertebrate; and so the theory was put forward that the vertebrate had
+    arisen from the annelid by the reversal of surfaces, the back of the one animal becoming the
+    front of the other.</p>
+    <p>The difficulties in the way of accepting such reversal of surfaces have proved insuperable,
+    and another school has arisen which suggests that evolution has throughout proceeded on two
+    lines, the one forming groups of animals in which the central nervous system is pierced by the
+    food-channel and the gut therefore lies dorsally to it, the other in which the central nervous
+    system always lies dorsally to the alimentary canal and is not pierced by it. In both cases the
+    highest products of the evolution have become markedly segmented animals, in the former,
+    annelids and arthropods; in the latter, vertebrates. The only evidence on which such theory is
+    based is the existence of low forms of animals, known as the <i>Enteropneusta</i>, the best
+    known example of which is called <i>Balanoglossus</i>; they are looked upon as aberrant annelid
+    forms by many observers.</p>
+    <p>This theory does not attempt to explain the peculiarities of the tube of the vertebrate
+    central nervous system, or to account for the extraordinary resemblance between the structure
+    and arrangement of the central nervous systems of vertebrates and of the highest invertebrate
+    group.</p>
+    <p>Neither of these theories is satisfactory or has secured universal acceptance. The problem
+    must be considered entirely anew. What are the guiding principles in this investigation?</p>
+    <div><span class="pagenum" id="page65">{65}</span></div>
+    <p>The evolution of animal life on this earth can clearly, on the whole, be described as a
+    process of upward progress culminating in the highest form&mdash;man; but it must always be
+    remembered that whole groups of animals such as the Tunicata have been able to survive owing to
+    a reverse process of degeneration.</p>
+    <p>If there is one organ more than another which increases in complexity as evolution proceeds,
+    which is the most essential organ for upward progress, surely it is the central nervous system,
+    especially that portion of it called the brain. This consideration points directly to the origin
+    of vertebrates from the most highly organized invertebrate group&mdash;the Arthropoda&mdash;for
+    among all the groups of animals living on the earth in the present day they alone possess a
+    central nervous system closely comparable with that of vertebrates. Not only has an upward
+    progress taken place in animals as a whole, but also in the tissues themselves a similar
+    evolution is apparent, and the evidence shows that the vertebrate tissues resemble more closely
+    those of the arthropod than of any other invertebrate group.</p>
+    <p>The evidence of geology points to the same conclusion, for the evidence of the rocks shows
+    that before the highest mammal&mdash;man&mdash;appeared, the dominant race was the mammalian
+    quadruped, from whom the highest mammal of all&mdash;man&mdash;sprung; then comes, in Mesozoic
+    times, the age of reptiles which were dominant when the mammal arose from them. Preceding this
+    era we find in Carboniferous times that the amphibian was dominant, and from them the next
+    higher group&mdash;the reptiles&mdash;arose. Below the Carboniferous come the Devonian strata
+    with their evidence of the dominance of the fish, from whom the amphibian was directly evolved.
+    The evidence is so clear that each succeeding higher form of vertebrate arose from the highest
+    stage reached at the time, as to compel one to the conclusion that the fishes arose from the
+    race which was dominant at the time when the fishes first appeared. This brings us to the
+    Silurian age, in which the evidence of the rocks points unmistakably to the sea-scorpions,
+    king-crabs, and trilobites as being the dominant race. It was preceded by the great trilobite
+    age, and the whole period, from the first appearance of the trilobite to the time of dwindling
+    away of the sea-scorpions, may be designated the Palæostracan age, using the term Palæostraca to
+    include both trilobites and the higher scorpion and king-crab forms evolved from them. The
+    evidence of geology then points directly and strongly to the origin of vertebrates from the
+    Palæostraca&mdash;arthropod forms which were not crustacean and not arachnid, but gave origin
+    both to the modern-day crustaceans and arachnids. The history of the rocks further shows that
+    these ancient fishes, when they first appeared, resembled in a remarkable manner members of the
+    palæostracan group, so that again and again palæontologists have found great difficulty in
+    determining whether a fossil is a fish or an arthropod. Fortunately, there is still alive on the
+    earth one member of this remarkable group&mdash;the Limulus, or King-Crab. On the vertebrate
+    side the lowest non-degenerate vertebrate is the lamprey, or Petromyzon, which spends a large
+    portion of its existence in a larval stage, known as the Ammoc&#x0153;tes stage of the lamprey,
+    because it was formerly considered to be a separate species and received the name of
+    Ammoc&#x0153;tes. The larval stages of any animal are most valuable for the study of ancestral
+    problems, so that it is most fortunate for the solution of the ancestry of vertebrates that
+    Limulus on the one side and Ammoc&#x0153;tes on the other are <span class="pagenum"
+    id="page66">{66}</span>available for thorough investigation and comparison. There are no
+    trilobites still alive, but in Branchipus and Apus we possess the nearest approach to the
+    trilobite organization among living crustaceans.</p>
+    <p>So strongly do all these different lines of argument point to the origin of vertebrates from
+    arthropods as to make it imperative to reconsider the position of that school of anatomists who
+    derived vertebrates from annelids by reversing the back and front of the animal. Let us not turn
+    the animal over, but re-consider the position, the infundibular tube of the vertebrate still
+    representing the &#x0153;sophagus of the invertebrate, the cerebral hemispheres and basal
+    ganglia the supra-&#x0153;sophageal ganglia, the <i>crura cerebri</i> the &#x0153;sophageal
+    commissures, and the infra-infundibular part of the brain the infra-&#x0153;sophageal ganglia.
+    It is immediately apparent that just as the invertebrate &#x0153;sophagus leads into the large
+    cephalic stomach, so the infundibular tube leads into the large cavity of the brain known as the
+    third ventricle, which, together with the other ventricles, forms in the embryo a large anterior
+    dilated part of the neural tube. In the arthropod this cephalic stomach leads into the straight
+    narrow intestine; in the vertebrate the fourth ventricle leads into the straight narrow canal of
+    the spinal cord. In the arthropod the intestine terminates in the anus; in the vertebrate embryo
+    the canal of the spinal cord terminates in the anus by way of the neurenteric canal. Keep the
+    animal unreversed, and immediately the whole mystery of the tubular nature of the central
+    nervous system is revealed, for it is seen that the nervous matter, which corresponds bit by bit
+    with that of the arthropod, has surrounded to a greater or less extent and amalgamated with the
+    tube of the arthropod alimentary canal, and thus formed the so-called central nervous system of
+    the vertebrate.</p>
+    <p>The manner in which the nervous material has invaded the walls of the tube is clearly shown
+    both by the study of the comparative anatomy of the central nervous system in the vertebrate and
+    also by its development in the embryo.</p>
+    <p class="sp0">This theory implies that the vertebrate alimentary canal is a new formation
+    necessitated by the urgency of the case, and, indeed, there was cause for urgency, for the
+    general plan of the evolution of the invertebrate from the c&#x0153;lenterate involved the
+    piercing of the anterior portion of the central nervous system by the &#x0153;sophagus, while,
+    at the same time, upward progress meant brain-development; brain-development meant concentration
+    of nervous matter at the anterior end of the animal, with the result that in the highest
+    scorpion and spider-like animals the brain-mass has so grown round and compressed the food-tube
+    that nothing but fluid pabulum can pass through into the stomach; the whole group have become
+    blood-suckers. These kinds of animals&mdash;the sea-scorpions&mdash;were the dominant race when
+    the vertebrates first appeared: here in the natural competition among members of the dominant
+    race the difficulty must have become acute. Further upward evolution demanded a larger and
+    larger brain with the ensuing consequence of a greater and greater difficulty of food-supply.
+    Nature's mistake was rectified and further evolution secured, not by degeneration in the
+    brain-region, for that means degradation not upward progress, but by the formation of a new
+    food-channel, in consequence of which the brain was free to develop to its fullest extent. Thus
+    the great and mighty kingdom of the Vertebrata was evolved with its culminating
+    organism&mdash;man&mdash;whose massive brain with all its possibilities could never have been
+    evolved if he had still been <span class="pagenum" id="page67">{67}</span>compelled to pass the
+    whole of his food through the narrow &#x0153;sophageal tube, still existent in him as the
+    infundibular tube. This, then, is the working hypothesis upon which this book is written. If
+    this view is right, that the Vertebrate was formed from the Palæostracan without any reversal of
+    surfaces, but by the amalgamation of the central nervous system and alimentary canal, then it
+    follows that we have various fixed points of comparison in the central nervous systems of the
+    two groups of animals from which to search for further clues. It further follows that from such
+    starting-point every organ of importance in the body of the arthropod ought to be visible in the
+    corresponding position in the vertebrate, either as a functional or rudimentary organ. The
+    subsequent chapters will deal with this detailed comparison of organs in the arthropod and
+    vertebrate respectively.</p>
+  </div>
+
+  <div><span class="pagenum" id="page68">{68}</span></div>
+
+  <p class="ac">CHAPTER II</p>
+
+  <p class="ac"><i>THE EVIDENCE OF THE ORGANS OF VISION</i></p>
+
+  <div class="bq1 it1 smaller sp3">
+    <p class="sp0">Different kinds of eye.&mdash;Simple and compound retinas.&mdash;Upright and
+    inverted retinas.&mdash;Median eyes.&mdash;Median or pineal eyes of Ammoc&#x0153;tes and their
+    optic ganglia.&mdash;Comparison with other median eyes.&mdash;Lateral eyes of vertebrates
+    compared with lateral eyes of crustaceans.&mdash;Peculiarities of the lateral eye of the
+    lamprey.&mdash;Meaning of the optic diverticula.&mdash;Evolution of vertebrate
+    eyes.&mdash;Summary.</p>
+  </div>
+
+  <p class="ac"><span class="sc">The Different Kinds of Eye.</span></p>
+
+  <p>In all animals the eyes are composed of two parts. 1. A set of special sensory cells called the
+  retina. 2. A dioptric apparatus for the purpose of forming an image on the sensory cells. The
+  simplest eye is formed from a modified patch of the surface-epithelium; certain of the hypodermal
+  cells, as they are called, elongate, and their cuticular surface becomes bulged to form a simple
+  lens. These elongated cells form the retinal cells, and are connected with the central nervous
+  system by nerve-fibres which constitute an optic nerve; the cells themselves may contain
+  pigment.</p>
+
+  <p>The more complicated eyes are modifications of this type for the purpose of making both the
+  retina and the dioptric apparatus more perfect. According to a very prevalent view, these
+  modifications have been brought about by invaginations of the surface-epithelium. Thus if ABCD
+  (Fig. <a href="#fig28">28</a>) represents a portion of the surface-epithelium, the chitinous
+  cuticle being represented by the dark line, with the hypodermal cells beneath, and if the part C
+  is modified to form an optic sense-plate, then an invagination occurring between A and B will
+  throw the retinal sense-cells with the optic nerve further from the surface, and the layers B and
+  A between the retina and the source of light will be available for the formation of the dioptric
+  apparatus. The lens is now formed from the cuticular surface of A, and the <span class="pagenum"
+  id="page69">{69}</span>hypodermal cells of A elongate to form the layer known by the name of
+  corneagen, or vitreogen, the cells of B remaining small and forming the pre-retinal layer of
+  cells. The large optic nerve end-cells of the retinal layer, C, take up the position shown in the
+  figure, and their cuticular surface becomes modified to form rods of varying shape called
+  rhabdites, which are attached to the retinal cells. Frequently the rhabdites of neighbouring cells
+  form definite groups, each group being called a rhabdome. Whatever shape they take it is
+  invariably found that these little rods (bacilli), or rhabdites, are modifications of the
+  cuticular surface of the cells which form the retinal layer. Also, as must necessarily be the case
+  from the method of formation, the optic nerve arises from the nuclear end of the retinal cells,
+  never from the bacillary end. As in the case first mentioned, so in this case, the light strikes
+  direct upon the bacillary end of the retinal cells; such eyes, therefore, are eyes with an
+  <i>upright retina</i>.</p>
+
+  <div class="ac w45 fcenter sp3">
+    <a href="images/fig028.png" id="fig28"><img style="width:100%" src="images/fig028.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p class="sp0"><span class="sc">Fig. 28.&mdash;Diagram of Formation of an Upright Simple
+      Retina.</span></p>
+    </div>
+  </div>
+
+  <p>It may happen that the part invaginated is the optic sense-plate itself, as would be the case
+  if in the former figure, instead of C, the part B was modified to form a sense-plate. This will
+  give rise to an eye of a character different from the former (Fig. <a href="#fig29">29</a>). The
+  optic nerve-fibres now lie between the source of light and the retinal end-cells, the layer A as
+  before forms the cuticular lens, and its hypodermal cells elongate to form the corneagen; there is
+  no pre-retinal layer, but, on the contrary, a post-retinal layer, C, called the tapetum, and, as
+  is seen, the light passes through the retinal layer to the <span class="pagenum"
+  id="page70">{70}</span>tapetum. The cuticular surface of the retinal cells forming the rods or
+  bacilli is directed towards the tapetal layer away from the source of light, and the nuclei of the
+  retinal cells are pre-bacillary in position, in contradistinction to the upright eye, where they
+  are post-bacillary. The retinal end-cells are devoid of pigment, the pigment being in the tapetal
+  layer.</p>
+
+  <p>Such an eye, in contradistinction to the former type, is an eye with an <i>inverted retina</i>;
+  but still the same law holds as in the former case&mdash;the optic nerve-fibres enter at the
+  nuclear ends of the cells, and the rods are formed from the cuticular surface.</p>
+
+  <p>In these eyes the pigmented tapetal layer is believed to act as a looking-glass; the dioptric
+  apparatus throws the image on to its shiny surface, from whence it is reflected directly on to the
+  rods, which are in close contact with the tapetum. A similar process has been suggested in the
+  case of the mammalian lateral eye, with its inverted retina. Johnson describes the post-retinal
+  pigmented layer as being frequently coloured and shiny, and imagines that it reflects the image
+  directly back on to the rods.</p>
+
+  <div class="ac w50 fcenter sp2">
+    <a href="images/fig029.png" id="fig29"><img style="width:100%" src="images/fig029.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 29.&mdash;Diagram of Formation of an Inverted Simple
+      Retina.</span></p>
+      <p class="sp0">The arrow shows the direction of the source of light in this as in the
+      preceding figure. In both figures the cuticular rhabdites are represented by thick black
+      lines.</p>
+    </div>
+  </div>
+
+  <p>Thus we see that eyes can be placed in different categories, <i>e.g.</i> those with an upright
+  retina and those with an inverted retina; also, according to the presence or absence of a tapetum,
+  eyes have been grouped as tapetal or non-tapetal. All the eyes considered so far are called simple
+  eyes, or ocelli; and a number of ocelli may be <span class="pagenum"
+  id="page71">{71}</span>contiguous though separate, as in the lateral eyes of the scorpion. They
+  may, however, come into close contact and form one single, large, compound eye. Such ocelli, in a
+  very large number of cases, retain each its own dioptric apparatus, and therefore the external
+  appearance of the compound eye represents not a single lens, but a large number of facets, as is
+  seen in the eyes of insects. Owing to these differences, eyes have been divided into simple and
+  compound, and into facetted and non-facetted.</p>
+
+  <p>Yet another complication occurs in the formation of eyes, which is, perhaps, the most important
+  of all: the retinal portion of the eye, instead of consisting of simple retinal cells, with their
+  accompanying rhabdites, may include within itself a portion of the central nervous system.</p>
+
+  <p>The rationale of such a formation is as follows: The external covering of the body is formed by
+  a layer of external epithelial cells&mdash;the ectodermal cell-layer&mdash;and an underlying
+  neural layer, of which the latter gives origin to the central nervous system. As development
+  proceeds, this central nervous system sinks inwards, leaving as its connection with the ectoderm
+  the sensory nerves of the skin. That part of the neural layer which underlies the optic plate
+  forms the optic ganglion, and when the central nervous system leaves the surface to take up its
+  deeper position, the strand of nerve-fibres known as the optic nerve, is left connecting it with
+  the retinal cells as seen in Figs. 28, 29. It may, however, happen that part of the optic ganglion
+  remains at the surface, in close connection with the retinal end-cells, when the rest of the
+  central nervous system sinks inwards. The retina of such an eye is composed of the combined optic
+  ganglion and retinal end-cells; the strand of nerve-fibres which is left as the connection between
+  it and the rest of the brain, which is also called the optic nerve, is not a true peripheral
+  nerve, as in the first case, but rather a tract of fibres connecting two parts of the brain, of
+  which one has remained at the periphery. Such a retina, in contradistinction to the first kind,
+  may be called a <i>compound retina</i>.</p>
+
+  <p>The optic ganglion, as seen in eyes with a simple retina, consists of a cortical layer of
+  small, round nerve-cells, and an internal medulla of fine nerve-fibres, which form a thick network
+  known as 'Punctsubstanz,' or in modern terminology, 'Neuropil.' Fibres which pass into this
+  'neuropil' from other parts of the brain connect them with the optic ganglion.</p>
+
+  <div><span class="pagenum" id="page72">{72}</span></div>
+
+  <p>At the present time, owing to the researches of Golgi, Ramón y Cajal, and others, the nervous
+  system is considered to be composed of a number of separate nerve-units, called neurones, each
+  neurone consisting of a nerve-cell with its various processes; one of these&mdash;the
+  neuraxon&mdash;constitutes the nerve-fibre belonging to that nerve-cell, the other
+  processes&mdash;the dendrites&mdash;establish communication with other neurones. The place where
+  these processes come together is called a synapse, and the tangle of fine fibres formed at a
+  number of synapses forms the 'neuropil.'</p>
+
+  <div class="ac w45 fcenter sp2">
+    <a href="images/fig030.png" id="fig30"><img style="width:100%" src="images/fig030.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 30.&mdash;Diagram of Formation of an Upright Compound
+      Retina.</span></p>
+      <p class="sp0"><i>ABCD</i>, as in Fig. <a href="#fig28">28</a>. <i>Op. g. I.</i> and <i>Op. g.
+      II.</i>, two optic ganglia which combine to form the retinal ganglion, <i>Rt. g.</i></p>
+    </div>
+  </div>
+
+  <p>When, therefore, a compound retina is formed by the amalgamation of the ectodermal
+  part&mdash;the retinal cells proper&mdash;with the neurodermic part&mdash;to which the name
+  'retinal ganglion' may be given,&mdash;such a retina consists of neuropil substance and
+  nerve-cells, as well as the retinal end-cells. In all such compound retinas, the retinal ganglion
+  is not single, but two optic ganglia at least are included in it, so that there are two sets of
+  nerve-cells and two synapses are always formed; one between the retinal end-cells and the neurones
+  of the first optic ganglion, which may be called the ganglion of the retina, the other between the
+  first and second ganglia, which, seeing that the neuraxons of its cells form the optic nerve, may
+  be called the ganglion of the optic nerve. The 'neuropil' formed by these synapses forms the
+  molecular layers of the compound retina, and the cells themselves form the nuclear layers. Thus an
+  upright compound retina, formed in the same way as the upright simple retina, would be illustrated
+  by Fig. <a href="#fig30">30</a>.</p>
+
+  <div><span class="pagenum" id="page73">{73}</span></div>
+
+  <p>Further, in precisely the same way as in the case of the simple retina, such a compound retina
+  may be upright or inverted. Thus, in the lateral eyes of crustaceans and insects, a compound
+  retina of this kind is formed, which is upright; while in the vertebrates the compound retina of
+  the lateral eyes is inverted.</p>
+
+  <p>The compound retina of vertebrates is usually described as composed of a series of layers,
+  which may be analyzed into their several components as follows<span
+  class="wnw">:&mdash;</span></p>
+
+  <table class="sp2 mc nothand" title="Layers of a compound retina"
+  summary="Layers of a compound retina">
+    <tr>
+      <td class="pr0" rowspan="3">Layer of rods and cones<br/>
+      External nuclear layer<br/>
+      External molecular layer<br/>
+      Internal nuclear layer<br/>
+      Internal molecular layer<br/>
+      Optic nerve-cell layer<br/>
+      Layer of optic nerve fibres</td>
+      <td class="vmi pl0 pr0"><img src="images/rbrace2.png" style="height:4.5ex; width:1em;"
+      alt="brace" /></td>
+      <td class="vmi pl0 pr0">retina proper</td>
+      <td class="vmi pl0 pr0"><img src="images/rbrace1.png" style="height:3.3ex; width:1em;"
+      alt="brace" /></td>
+      <td class="vmi pl0 pr0" colspan="3">Ectodermic part</td>
+    </tr>
+    <tr>
+      <td class="vmi pl0 pr0"><img src="images/rbrace2.png" style="height:4.5ex; width:1em;"
+      alt="brace" /></td>
+      <td class="vmi pl0 pr0">ganglion of retina</td>
+      <td class="vmi pl0 pr0" rowspan="2"><img src="images/rbrace3.png" style="height:7.0ex;
+      width:1em;" alt="brace" /></td>
+      <td class="vmi pl0 pr0" rowspan="2">retinal<br/>
+      ganglion</td>
+      <td class="vmi pl0 pr0" rowspan="2"><img src="images/rbrace2.png" style="height:4.5ex;
+      width:1em;" alt="brace" /></td>
+      <td class="vmi pl0 pr0" rowspan="2">neurodermic<br/>
+      part</td>
+    </tr>
+    <tr>
+      <td class="vmi pl0 pr0"><img src="images/rbrace2.png" style="height:4.5ex; width:1em;"
+      alt="brace" /></td>
+      <td class="vmi pl0 pr0">ganglion of optic nerve</td>
+    </tr>
+  </table>
+
+<!-- Trimmed down version for handheld -->
+
+  <table class="sp3 mc handonly" title="Layers of a compound retina"
+  summary="Layers of a compound retina">
+    <tr class="ba">
+      <td>Layer of rods and cones</td>
+      <td rowspan="3" class="vmi">retina proper</td>
+      <td rowspan="3" colspan="2" class="vmi">Ectodermic part</td>
+    </tr>
+    <tr class="ba">
+      <td>External nuclear layer</td>
+    </tr>
+    <tr class="ba">
+      <td rowspan="2">External molecular layer</td>
+    </tr>
+    <tr class="ba">
+      <td rowspan="3" class="vmi">ganglion of retina</td>
+      <td rowspan="6" class="vmi">retinal<br/>
+      ganglion</td>
+      <td rowspan="6" class="vmi">neurodermic<br/>
+      part</td>
+    </tr>
+    <tr class="ba">
+      <td>Internal nuclear layer</td>
+    </tr>
+    <tr class="ba">
+      <td rowspan="2">Internal molecular layer</td>
+    </tr>
+    <tr class="ba">
+      <td rowspan="3" class="vmi">ganglion of optic nerve</td>
+    </tr>
+    <tr class="ba">
+      <td>Optic nerve-cell layer</td>
+    </tr>
+    <tr class="ba">
+      <td>Layer of optic nerve fibres</td>
+    </tr>
+  </table>
+
+<!-- End of trimmed down version for handheld -->
+
+  <p>The difference between the development of these two types of eye&mdash;those with a simple
+  retina and those with a compound retina&mdash;has led, in the most natural manner, to the
+  conception that the retina is developed, in the higher animals, sometimes from the cells of the
+  peripheral epidermis, sometimes from the tissue of the brain&mdash;two modes of development termed
+  by Balfour 'peripheral' and 'cerebral.' An historical survey of the question shows most
+  conclusively that all investigators are agreed in ascribing the origin of the simple retina to the
+  peripheral method of development, the retina being formed from the hypodermal cells by a process
+  of invagination, while the cerebral type of development has been described only in the development
+  of the compound retina. The natural conclusion from this fact is that, in watching the development
+  of the compound retina, it is more difficult to differentiate the layers formed from the epidermal
+  retinal cells and those formed from the epidermal optic ganglion-cells, than in the case of the
+  simple retina, where the latter cells withdraw entirely from the surface. This is the conclusion
+  to which Patten has come, and, indeed, judging from the text-book of Korschelt and Heider, it is
+  the generally received opinion of the day that, as far as the Appendiculata are concerned, the
+  retina, in the true sense&mdash;the retinal end-cells, with their cuticular rods,&mdash;is formed,
+  in all cases, from the peripheral cells of the hypodermal layer, the cuticular rods being
+  modifications of the general cuticular surface of the body. The apparent cerebral development of
+  the crustacean <span class="pagenum" id="page74">{74}</span>retina, as quoted from Bobretsky by
+  Balfour, is therefore in reality the development of the retinal ganglion, and not of the retina
+  proper.</p>
+
+  <p class="sp3">There is, I imagine, a universal belief that the natural mode of origin of a
+  sense-organ, such as the eye, must always have been from the cells forming the external surface of
+  the animal, and that direct origin from the central nervous system is <i>a priori</i> most
+  improbable. It is, therefore, a matter of satisfaction to find that the evidence for the latter
+  origin has universally broken down, with the single exception of the eyes of vertebrates and their
+  degenerated allies; a fact which points strongly to the probability that a reconsideration of the
+  evidence upon which the present teaching of the origin of the vertebrate eye is based will show
+  that here, too, a confusion has arisen between that part formed from the epidermal surface and
+  that from the optic ganglion.</p>
+
+  <p class="ac"><span class="sc">The Median or Pineal Eyes.</span></p>
+
+  <p>Undoubtedly, in recent times, the most important clue to the ancestry of vertebrates has been
+  given by the discovery that the so-called pineal gland in the vertebrate brain is all that remains
+  of a pair of median or pineal eyes, the existence of which is manifest in the earliest
+  vertebrates; so that the vertebrate, when it first arose, possessed a pair of median eyes as well
+  as a pair of lateral eyes. The ancestor of the vertebrate, therefore, must also have possessed a
+  pair of median eyes as well as a pair of lateral eyes.</p>
+
+  <p>Very instructive, indeed, is the evidence with regard to these median eyes, for one of the
+  great characteristics of the ancient palæostracan forms is the invariable presence of a pair of
+  median eyes as well as a pair of lateral eyes. In the living representative of such
+  forms&mdash;Limulus&mdash;the pair of median eyes (Fig. <a href="#fig5">5</a>) is well shown, and
+  it is significant that here, according to Lankester and Bourne, these eyes are already in a
+  condition of degeneration; so also in many of the Palæostraca (Fig. <a href="#fig7">7</a>) the
+  lateral eyes are the large, well-developed eyes, while the median eyes resemble those of Limulus
+  in their insignificance.</p>
+
+  <p>We see, then, that in the dominant arthropod race at the time when the fishes first appeared,
+  the type of eyes consisted of a pair of well-developed lateral eyes and a pair of insignificant,
+  partially degenerated, median eyes. Further, according to all palæontologists, <span
+  class="pagenum" id="page75">{75}</span>in the best-preserved head-shields of the most ancient
+  fishes, especially well seen in the Osteostraci, in Cephalaspis, Tremataspis, Auchenaspis,
+  Keraspis, a pair of large, prominent lateral eyes existed, between which, in the mid-line, are
+  seen a pair of small, insignificant median eyes.</p>
+
+  <p>The evidence of the rocks, therefore, proves that the pair of median eyes which were originally
+  the principal eyes (Hauptaugen), had already, in the dominant arthropod group been supplanted by a
+  pair of lateral eyes, and had, in consequence, become small and insignificant, at the time when
+  vertebrates first appeared. This dwindling process thus initiated in the arthropod itself has
+  steadily continued ever since through the whole development of the vertebrates, with the result
+  that, in the highest vertebrates, these median or pineal eyes have become converted into the
+  pineal gland with its 'brain-sand.'</p>
+
+  <p>In the earliest vertebrate these median eyes may have been functional; they certainly were more
+  conspicuous than in later forms. Alone among living vertebrates the right median eye of
+  Ammoc&#x0153;tes is so perfect and the skin covering it so transparent that I have always felt
+  doubtful whether it may not be of use to the animal, especially when one takes into consideration
+  the undeveloped state of the lateral eyes in this animal, hidden as they are under the skin. Thus
+  the one living vertebrate which is comparable with these extinct fishes is the one in which one of
+  the pineal eyes is most well defined, most nearly functional.</p>
+
+  <p>Before passing to the consideration of the structure of the median eyes of Ammoc&#x0153;tes, it
+  is advisable to see whether these median eyes in other animals, such as arachnids and crustaceans,
+  belong to any particular type of eyes, for then assuredly the median eyes of Ammoc&#x0153;tes
+  ought to belong to the same type if they are derived from them.</p>
+
+  <p>In the specialized crustacean, as in the specialized vertebrate, the median eyes have
+  disappeared, at all events in the adult, but still exist in the primitive forms, such as
+  Branchipus, which resemble the trilobites in some respects. On the other hand, the median eyes
+  have persisted, and are well developed in the arachnids, both scorpions and spiders possessing a
+  well-developed pair. The characteristics of the median eyes must then be especially sought for in
+  the arachnid group.</p>
+
+  <p>Both scorpions and spiders possess many eyes, of which two are <span class="pagenum"
+  id="page76">{76}</span>always separate and median in position, while the others form lateral
+  groups; all these eyes possess a simple retina and a simple corneal lens. Grenacher was the first
+  to point out that in the spiders two very distinct types of eye are found. In the one the retina
+  is upright; in the other the retina is inverted, and the eye possesses a tapetal layer. The
+  distribution of these two types is most suggestive, for the inverted retina is always found in the
+  lateral eyes, never in the two median eyes; these always possess a simple upright retina.</p>
+
+  <p class="sp3">In the crustaceans, the lateral eyes differ also from the median eyes, but not in
+  the same way as in the arachnids; for here both types of eye possess an upright retina, but the
+  retina of the lateral eyes is compound, while that of the median eyes is simple. In other words,
+  the median eyes are in all cases eyes with a simple upright retina and a simple cuticular lens,
+  while the retina of the lateral eyes is compound or may be inverted, according as the animal in
+  question possesses crustacean or arachnid affinities. The lateral eye of the vertebrate,
+  possessing, as it does, an inverted compound retina, indicates that the vertebrate arose from a
+  stock which was neither arachnid nor crustacean, but gave rise to both groups&mdash;in fact, was a
+  member of the great palæostracan group. What, then, is the nature of the median eyes in the
+  vertebrate?</p>
+
+  <p class="ac"><span class="sc">The Median Eyes of Ammoc&#x0153;tes.</span></p>
+
+  <p>The evidence of Ammoc&#x0153;tes is so conclusive that I, for one, cannot conceive how it is
+  possible for any zoologist to doubt whether the parietal organ, as they insist on calling it, had
+  ever been an eye, or rather a pair of eyes.</p>
+
+  <p>Anyone who examines the head of the larval lamprey will see on the dorsal side, in the median
+  line, first, a somewhat circular orifice&mdash;the unpaired nasal opening; and then, tailwards to
+  this, a well-marked circular spot, where the skin is distinctly more transparent than elsewhere.
+  This spot coincides in position with the underlying dorsal pineal eye, which shines out
+  conspicuously owing to the glistening whiteness of its pigment. Upon opening the brain-case the
+  appearance as in Fig. <a href="#fig20">20</a> is seen, and the mass of the right <i>ganglion
+  habenulæ</i> (<i>G.H.R.</i>), as it has been called, stands out conspicuously as well as the right
+  or dorsal pineal eye (<i>Pn.</i>). Both eye and ganglion appear at first sight to be one-sided,
+  but further examination shows that a left <i>ganglion habenulæ</i> is present, though much smaller
+  than on <span class="pagenum" id="page77">{77}</span>the right side. In connection with this is
+  another eye-like organ&mdash;the left or ventral pineal eye,&mdash;much more aborted, much less
+  like an eye than the dorsal one; so also there are two bundles of peculiar fibres called Meynert's
+  bundles, which connect this region with the infra-infundibular region of the brain; of these, the
+  right Meynert's bundle is much larger than the left.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig031.jpg" id="fig31"><img style="width:100%" src="images/fig031.jpg" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 31.&mdash;One of a Series of Horizontal Sections through
+      the Head of Ammoc&#x0153;tes.</span></p>
+      <p class="sp0"><i>l.m.</i>, upper lip muscles; <i>m.c.</i>, muco-cartilage; <i>n.</i>, nose;
+      <i>na.c.</i>, nasal cartilage; <i>pn.</i>, right pineal eye and nerve; <i>g.h.r.</i>, right
+      <i>ganglion habenulæ</i>; <i>s.m.</i>, somatic muscles; <i>cr.</i>, membranous wall of
+      cranium; <i>ch.</i>, choroid plexus; <i>gl.</i>, glandular substance and pigment filling up
+      brain-case.</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page78">{78}</span></div>
+
+  <table class="mc tlf sp2 w60" summary="Group of illustrations" title="Group of illustrations">
+    <tr>
+      <td class="vbm ac" style="width:45%;"><a href="images/fig032.png" id="fig32"><img
+      style="width:100%" src="images/fig032.png" alt="" title=""/></a></td>
+      <td class="vbm ac" style="width:45%;"><a href="images/fig033.png" id="fig33"><img
+      style="width:100%" src="images/fig033.png" alt="" title=""/></a></td>
+    </tr>
+    <tr>
+      <td>
+        <div class="smaller aj it1">
+          <p class="ac"><span class="sc">Fig. 32.&mdash;Eye of Acilius Larva, with its Optic
+          Ganglion.</span></p>
+          <p class="sp0">On the right side the nerve end-cells have been drawn free from
+          pigment.</p>
+        </div>
+      </td>
+      <td>
+        <div class="smaller aj it1">
+          <p class="ac"><span class="sc">Fig. 33.&mdash;Pineal Eye of Ammoc&#x0153;tes, with
+          its</span> <i>Ganglion Habenulæ.</i></p>
+          <p class="sp0">On the left side the eye is drawn as it appeared in the section. On the
+          right side I have removed the pigment from the nerve end-cells, and drawn the eye as, in
+          my opinion, it would appear if it were functional.</p>
+        </div>
+      </td>
+    </tr>
+  </table>
+
+  <p>This difference between right and left indicates a greater degeneration on the left side, and
+  points distinctly to a close relationship between the nerve-masses known as <i>ganglia
+  habenulæ</i> and the median eyes. In my opinion this ganglion is, in part, at all events, the
+  optic ganglion of the median eye on each side. It is built up on the same type as the optic
+  ganglia of invertebrate simple eyes, with a cortex of small round cells and a medulla of fine
+  nerve-fibres. Into this ganglion, on the right side, there passes a very well-defined
+  nerve&mdash;the nerve of the dorsal eye. The eye itself with its nerve, <i>pn.</i>, and its optic
+  ganglion, <i>g.h.r.</i>, is beautifully shown by means of a horizontal section through the head of
+  Ammoc&#x0153;tes (Fig. <a href="#fig31">31</a>). Originally, as described by Scott, the eye stood
+  vertically <span class="pagenum" id="page79">{79}</span>above its optic ganglion, and presented an
+  appearance remarkably like Fig. <a href="#fig32">32</a>, which represents one of the simple eyes
+  and optic ganglia of a larva of Acilius as described by Patten; then, with the forward growth of
+  the upper lip, the right pineal eye was dragged forward and its nerve pulled horizontally over the
+  <i>ganglion habenulæ</i>. For this reason the eye, nerve, and ganglion are better shown in a
+  nearly horizontal than in a transverse section.</p>
+
+  <p>The optic nerve belonging to this eye is most evident and clearly shown in Fig. <a
+  href="#fig31">31</a>, and in the series of consecutive sections which follow upon this section; no
+  doubt can arise as to the structure in question having been the nerve of the eye, even though, as
+  is possible, it does not contain any functional nerve-fibres.</p>
+
+  <div class="ac w50 fcenter sp2">
+    <a href="images/fig034.png" id="fig34"><img style="width:56%" src="images/fig034.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p><span class="sc">Fig. 34.&mdash;Horizontal Section through Brain of Ammoc&#x0153;tes, to
+      show the Left, or Ventral Pineal Eye.</span></p>
+      <p class="sp0"><i>pn.<sub>2</sub></i>, left or ventral pineal eye; <i>pn.<sub>1</sub></i>,
+      last remnant of right, or dorsal pineal eye; <i>g.h.r.</i>, right <i>ganglion habenulæ</i>;
+      <i>g.h.l.<sub>1</sub></i>, <i>g.h.l.<sub>3</sub></i>, parts of left <i>ganglion habenulæ</i>;
+      <i>pi.</i>, fold of <i>pia mater</i> which separates the left <i>ganglion habenulæ</i> from
+      the left pineal eye; <i>f.</i>, strands of nerve-fibres connecting the left eye with its
+      ganglion, <i>g.h.l.<sub>3</sub></i>; <i>V<sub>3</sub></i>, third ventricle; <i>V.aq.</i>,
+      ventricle of aquæduct.</p>
+    </div>
+  </div>
+
+  <p>The second, ventral or left, eye, belonging to the left ganglion habenulæ is very different in
+  appearance, being much less evidently an eye. Fig. <a href="#fig34">34</a> is one of the same
+  series of horizontal sections as Fig. <a href="#fig31">31</a>, <i>pn.<sub>1</sub></i> being the
+  last remnant of the right, or dorsal, eye, while <i>pn.<sub>2</sub></i> shows the left, or
+  ventral, eye with its connection with the left <i>ganglion habenulæ</i>.</p>
+
+  <div><span class="pagenum" id="page80">{80}</span></div>
+
+  <p class="sp3">In a series of sections I have followed the nerve of the right pineal eye to its
+  destination, as described in my paper in the <i>Quarterly Journal of Microscopical Science</i>,
+  and have found that it enters into the <i>ganglion habenulæ</i> just as the nerve to any simple
+  eye enters into its optic ganglion. This nerve, as I have shown, is a very distinct, well-defined
+  nerve, with no admixture of ganglion-cells or of connective tissue, very different indeed to the
+  connection between the left pineal eye and its optic ganglion. Here there is no defined nerve at
+  all; but the cells of the <i>ganglion habenulæ</i> stretch right up to the remains of the eye
+  itself. Seeing, then, that both the eye and ganglion on this side have reached a much further
+  grade of degeneration than on the right side, it may be fairly concluded that the original
+  condition of these two median eyes is more nearly represented by the right eye, with its
+  well-defined nerve and optic ganglion, than by the left eye, or by the eyes in lizards and other
+  animals which do not show so well-defined a nerve as is possessed by Ammoc&#x0153;tes. Quite
+  recently Dendy has examined the two median eyes in the New Zealand lamprey <i>Geotria
+  australis</i>. In this species the second eye is much better defined than in the European lamprey,
+  and its connection with the <i>ganglion habenulæ</i> is more nerve-like. In neither eye, however,
+  is the nerve so clean cut and isolated as is the nerve of the dorsal, or right, eye in the
+  Ammoc&#x0153;tes stage of <i>Petromyzon Planeri</i>; in both, cells resembling those of the cortex
+  of the <i>ganglion habenulæ</i> and connective tissues are mixed up with the nerve-fibres which
+  pass from each eye to its respective optic ganglion.</p>
+
+  <p class="ac"><span class="sc">The Right Pineal Eye of Ammoc&#x0153;tes.</span></p>
+
+  <p>The optic fibres of the right median eye of Ammoc&#x0153;tes are connected with a well-defined
+  retina, the limits of which are defined by the white pigment so characteristic of this eye. This
+  pigment is apparently calcium phosphate, which still remains as the 'brain-sand' of the human
+  pineal gland. The cells, which are hidden by this pigment, were described by me in 1890 as the
+  retinal end-cells with large nuclei. In 1893, Studniçka examined them more closely, and concluded
+  that the retinal cells are of two kinds: the one, nerve end-cells, the sensory cells proper; the
+  other, pigmented epithelial cells, which surround the sense-cells. The sense-cells may contain
+  some of the white pigment, but not so much as the other cells. Similarly, in the <span
+  class="pagenum" id="page81">{81}</span>median eyes of Limulus, Lankester and Bourne find it
+  difficult to determine how far the retinal end-cells contain pigment and how far that pigment
+  really is in the cells surrounding these nerve end-cells.</p>
+
+  <p>The interior of the eye presents the appearance of a cavity in shape like a cornucopia, the
+  stalk of which terminates at the place where the nerve enters. This cavity is not empty, but the
+  posterior part of it is filled with the termination of the nerve end-cells of the retina, as
+  pointed out by me and confirmed by Studniçka. These terminations are free from pigment, and
+  contain strikingly translucent bodies, which I have described in my paper in the <i>Quarterly
+  Journal</i>, and called rhabdites, for they present the same appearance and are situated in the
+  same position as are many of the rhabdites on the terminations of the retinal end-cells of
+  arthropod eyes. Studniçka has also seen these appearances, and figures them in his second paper on
+  the nerve end-cells of the pineal eye of Ammoc&#x0153;tes.</p>
+
+  <p>Up to this point the following conclusions may be drawn<span class="wnw">:&mdash;</span></p>
+
+  <div class="bq1 sp2">
+    <p>1.  Ammoc&#x0153;tes possesses a pair of median eyes, just as was the case with the most
+    ancient fishes, and with the members of the contemporary palæostracan group.</p>
+    <p>2.  The retina of one of these eyes is well-defined and upright, not inverted, and therefore
+    in this respect agrees with that of all median eyes.</p>
+    <p>3.  The presence of nerve end-cells, with pigment either in them or in cells around them, to
+    the unpigmented ends of which translucent bodies resembling rhabdites are attached, is another
+    proof that this retina agrees with that of the median eyes of arthropods.</p>
+    <p class="sp0">4.  The simple nature of the nerve with its termination in an optic ganglion
+    closely resembling in structure an arthropod optic ganglion, together with Studniçka's statement
+    that the nerve end-cells pass directly into the nerve, points directly to the conclusion that
+    this retina is a simple, not a compound, retina, and that it therefore in this respect also
+    agrees with the retina of all median eyes.</p>
+  </div>
+
+  <p>With respect to this last conclusion, neither I myself nor Studniçka have been able to see any
+  definite groups of cells between the nerve end-cells and the optic nerve such as a compound retina
+  necessitates.</p>
+
+  <div><span class="pagenum" id="page82">{82}</span></div>
+
+  <p>On the other hand, Dendy describes in the New Zealand lamprey, <i>Geotria australis</i>, a
+  cavity where the nerve enters into the eye, which he calls the atrium. This cavity is distinct
+  from the main cavity of the eye, and is separated from it by a mass of cells similar in appearance
+  to those of the cortex of the <i>ganglion habenulæ</i>. In these two eyes then, groups of cells,
+  resembling in appearance those belonging to an optic ganglion, exist in the eyes themselves. This
+  atrium is evidently that part of the central cavity which I have called the handle of the
+  cornucopia in the European lamprey, and the very fact that it is separated from the rest of the
+  central cavity is evidence that we are dealing here with a later stage in the history of the
+  pineal eyes than in the case of the Ammoc&#x0153;tes of <i>Petromyzon Planeri</i>. Taking also
+  into consideration the continuity of the mass of small ganglion-cells which surround this atrium
+  with the cells of the <i>ganglion habenulæ</i> by means of the similar cells scattered along the
+  course of the nerve, and also bearing in mind the fact already stated that in the more degenerate
+  left eye of Ammoc&#x0153;tes the cells of the <i>ganglion habenulæ</i> extend right up to the eye
+  itself, it seems more likely than not that these cells do not represent the original optic
+  ganglion of a compound retina, but rather the subsequent invasion, by way of the pineal nerve, of
+  ganglion-cells belonging to a portion of the brain. In the last chapter it has been suggested that
+  the presence of the trochlear or fourth cranial nerve has given rise to the formation of the
+  cerebellum by a similar spreading.</p>
+
+  <p>There is certainly no appearance in the least resembling a compound retina such as is seen in
+  the vertebrate or crustacean lateral eye. In the median eyes of scorpions and of Limulus, cells
+  are found within the capsule of the eye among the nerve-fibres and the nerve end-cells. These are
+  especially numerous in the median eyes of Limulus, as described by Lankester and Bourne, and are
+  called by them intrusive connective tissue cells. The meaning of these cells is not, to my mind,
+  yet settled. It is sufficient for my purpose to point out that the presence of cells interneural
+  in position among the nerve end-cells of the retina of the median eyes of Ammoc&#x0153;tes is more
+  probable than not, on the assumption that the retina of these eyes is built up on the same plan as
+  that of the median eyes in Limulus and the scorpions.</p>
+
+  <p class="sp3">It is further to be borne in mind that these specimens of <i>Geotria</i> worked at
+  by Dendy were in the 'Velasia' stage of the New Zealand <span class="pagenum"
+  id="page83">{83}</span>lamprey, and correspond, therefore, more nearly to the Petromyzon than to
+  the Ammoc&#x0153;tes stage of the European lamprey.</p>
+
+  <p class="ac"><span class="sc">The Dioptric Apparatus.</span></p>
+
+  <p>Besides the retina, all eyes possess a dioptric apparatus. What is the evidence as to its
+  nature in these vertebrate median eyes? Lankester and Bourne have divided the eyes of scorpions
+  and Limulus into two kinds, monostichous and diplostichous. In the first the retinal cells are
+  supposed to give rise to not only rhabdites but also the cuticular chitinous lens, so that the eye
+  is one-layered; in the second the lens is formed by a well-marked hypodermal layer, in front of
+  the retina, composed of elongated cells, so that these eyes are two-layered or diplostichous. The
+  lateral eyes, according to them, are all monostichous, but the median eyes are diplostichous.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig035.png" id="fig35"><img style="width:100%" src="images/fig035.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 35.&mdash;Eye of Acilius Larvæ.</span> (After <span
+      class="sc">Patten</span>.)</p>
+      <p class="sp0"><i>l.</i>, chitinous lens; <i>c.</i>, corneagen; <i>pr.</i>, pre-retinal layer;
+      <i>rh.</i>, rhabdites; <i>ret.</i>, retinal end-cells.</p>
+    </div>
+  </div>
+
+  <p>This distinction is not considered valid by other observers. Thus, <span class="pagenum"
+  id="page84">{84}</span>as already indicated, Patten looks on all these eyes as three-layered, and
+  states that in all cases a corneagen or vitreogen layer exists, which gives origin to the lens.
+  For my own part I agree with Patten, but we are not concerned here with the lateral eyes. It is
+  sufficient to note that all observers are agreed that the median eyes are characterized by this
+  well-marked cell-layer, the so-called vitreous or corneagen layer of cells.</p>
+
+  <div class="ac w25 fcenter sp2">
+    <a href="images/fig036.png" id="fig36"><img style="width:100%" src="images/fig036.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p><span class="sc">Fig. 36.&mdash;Eye of Hydrophilus Larva, with the Pigment over the Retinal
+      End-cells.</span></p>
+      <p class="sp0"><i>l.</i>, chitinous lens; <i>c.</i>, corneagen; <i>pr.</i>, pre-retinal layer;
+      <i>rh.</i>, rhabdites; <i>ret.</i>, retinal end-cells.</p>
+    </div>
+  </div>
+
+  <p>This layer (<i>c.</i>, Fig. <a href="#fig35">35</a>) is composed of much-elongated cells of the
+  hypodermal layer, in each of which the large nucleus is always situated towards the base of the
+  cell. The space between it and the retina contains, according to Patten the cells of the
+  pre-retinal layer <i>(pr.)</i>. These may be so few and insignificant as to give the impression
+  that the vitreous layer is immediately adjacent to the retina (<i>ret.</i>).</p>
+
+  <p>Let us turn now to the right pineal eye of Ammoc&#x0153;tes (Fig. <a href="#fig37">37</a>) and
+  see what its further structure is. The anterior part of the eye is free from pigment, and is
+  composed, as is seen in hæmatoxylin or carmine specimens, of an inner layer of nuclei which are
+  frequently arranged in a wavy line. From this nucleated layer, strands of tissue, free from
+  nuclei, pass to the anterior edge of the eye.</p>
+
+  <p>In the horizontal longitudinal sections it is seen that these strands are confined to the
+  middle of the eye. On each side of them the nuclear layer reaches the periphery, so that if we
+  consider these strands to represent long cylindrical cells, as described by Beard, then the
+  anterior wall may be described as consisting of long cylindrical cells, which are flanked on
+  either side by shorter cells of a similar kind. The nuclei at the base of these cylindrical cells
+  are not all alike. We see, in the first place, large nuclei resembling the large nuclei belonging
+  to the nerve end-cells; these are the nuclei of <span class="pagenum" id="page85">{85}</span>the
+  long cylindrical cells. We see also smaller nuclei in among these larger ones, which look like
+  nuclei of intrusive connective tissue, or may perhaps form a distinct layer of cells, situated
+  between the cells of the anterior wall and the terminations of the nerve end-cells already
+  referred to.</p>
+
+  <p>These elongated cells are in exactly the same position and present the same appearance as the
+  cells of the corneagen layer of any median eye. Like the latter they are free from pigment and
+  never show with osmic staining any sign of the presence of translucent rhabdite-like bodies, such
+  as are seen in the termination of the retinal cells, and like the latter their nuclei are at the
+  base. The resemblance between this layer and the corneagen cells of any median eye is absolute.
+  Between it and the terminations of the retinal cells there exists some ill-defined material
+  certainly containing cells which may well correspond to Patten's pre-retinal layer of cells.</p>
+
+  <p>Retina, corneagen, nerve, optic ganglion, all are there, all in their right position, all of
+  the right structure, what more is needed to complete the picture?</p>
+
+  <div class="ac w25 fcenter sp2">
+    <a href="images/fig037.png" id="fig37"><img style="width:100%" src="images/fig037.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p class="sp0"><span class="sc">Fig. 37.&mdash;Pineal Eye of Ammoc&#x0153;tes, with its</span>
+      <i>Ganglion Habenulæ</i>.</p>
+    </div>
+  </div>
+
+  <p>In order to complete the dioptric apparatus a lens is necessary. Where, then, is the lens in
+  these pineal eyes? In all the arachnid eyes, whether median or lateral, the lens is a single
+  corneal lens composed of the external cuticle, which is thickened over the corneagen cells. This
+  thickened cuticle is composed of chitin, and is not cellular, but is dead material formed out of
+  the living underlying corneagen cells. Such a lens is in marked contrast to the lens of the
+  lateral vertebrate eye, which is formed by living cells themselves. This <span class="pagenum"
+  id="page86">{86}</span>thickening of the cuticular layer to form a lens could only exist as long
+  as that layer is absolutely external, so that the light strikes immediately upon it; for, if from
+  any cause the eye became situated internally, the place of such a lens must be filled by the
+  structures situated between it and the surface, and the thickened cuticle would no longer be
+  formed.</p>
+
+  <p>In all vertebrates these pineal eyes are separated from the external surface by a greater or
+  less thickness of tissues; in the case of Ammoc&#x0153;tes, as is seen in Fig. <a
+  href="#fig31">31</a>, the eye lies within the membranous cranial wall, and is attached closely to
+  it. The position, then, of the cuticular, or corneal lens, as it is often called, on the
+  supposition that this is a median eye of the arachnid type, is taken by the membranous cranium,
+  and, as I have described in my paper in the <i>Quarterly Journal</i>, on carefully lifting the eye
+  in the fresh condition from the cranial wall, it can be seen under a dissecting microscope that
+  the cranial wall often forms at this spot a lens-like bulging, which fits the shallow concavity of
+  the surface of the eye, and requires some little force to separate it from the eye.</p>
+
+  <p>As will appear in a subsequent chapter, this cranial wall has been formed by the growth,
+  laterally and dorsally, of a skeletal structure known by the name of the <i>plastron</i>. The last
+  part of it to be completed would be that part in the mid-dorsal line, where apparently, in
+  consequence of the insinking of the degenerating eyes, a dermal and subdermal layer already
+  intervened between the source of light and the eyes themselves.</p>
+
+  <p>When the membranous cranium was completed in the mid-dorsal region, it was situated here as
+  elsewhere just internally to the subdermal layer, and therefore enclosed the pineal eyes. This, to
+  my mind, is the reason why the pineal eyes, which, in all other respects, conform to the type of
+  the median eyes of an arachnid-like animal, do not possess a cuticular lens. Other observers have
+  attempted to make a lens out of the elongated cells of the anterior wall of the eye (my corneagen
+  layer), but without success.</p>
+
+  <p>Studniçka, who calls this layer the <i>pellucida</i>, does not look upon it as the lens, but
+  considers, strangely enough, that the translucent appearances at the ends of each nerve end-cell
+  represent a lens for that cell, so that every nerve end-cell has its own lens. Still more strange
+  is it that, holding this view, he should yet consider these knobs <span class="pagenum"
+  id="page87">{87}</span>to be joined by filaments to the cells in the anterior wall of the eye, a
+  conception fatal to the action of such knobs as lenses.</p>
+
+  <p class="sp3">The discovery that the vertebrate possesses, in addition to the lateral eyes, a
+  pair of median eyes, which are most conspicuous in the lowest living vertebrate, together with the
+  fact that such eyes are built up on the same plan as the median eyes of living crustaceans or
+  arachnids, not only with respect to the eye itself but also to its nerve and optic ganglion,
+  constitutes a fact of the very greatest importance for any theory of the origin of vertebrates;
+  especially in view of the further fact, that similar eyes in the same position are found not only
+  in all the members of the Palæostraca, but also in all those ancient forms (classed as fishes)
+  which lived at that time. At one and the same moment it proves the utter impossibility of
+  reversing dorsal and ventral surfaces, points in the very strongest manner to the origin of the
+  vertebrate from some member or other of the palæostracan group, and insists that the advocates of
+  the origin of vertebrates from the Hemichordata, etc., should give an explanation of the presence
+  of these two median eyes of a more convincing character than that given here.</p>
+
+  <p class="ac"><span class="sc">The Lateral Eyes.</span></p>
+
+  <p>Turning now to the consideration of the lateral eyes, we see that these eyes in the arachnids
+  often possess an inverted retina, in the crustaceans always an upright retina. In the arachnids
+  they possess a simple retina, while in the crustaceans their retina is compound; so that in the
+  latter case the so-called optic nerve is in reality a tract of fibres connecting together the
+  brain-region with a variable number of optic ganglia, which have been left at the periphery in
+  close contact with the retinal cells, when the brain sunk away from the superficial epithelial
+  covering.</p>
+
+  <p>There is, then, this difference between the lateral eyes of crustaceans and arachnids, that the
+  retina of the former is compound, but never inverted, while that of the latter may be inverted,
+  but is always simple.</p>
+
+  <p>The retina of the lateral eyes of the vertebrate resembles both of these, for it is compound,
+  as in the crustacean, and inverted as in the arachnid.</p>
+
+  <p>It must always be borne in mind that in the palæostracan epoch <span class="pagenum"
+  id="page88">{88}</span>the dominant race was neither crustacean nor arachnid, but partook of the
+  characters of both; also, as is characteristic of dominance, there was very great variety of form,
+  so that it seems more probable than not that some of these forms may have combined the arachnid
+  and crustacean characteristics to the extent of possessing lateral eyes with an inverted yet
+  compound retina. A certain amount of evidence points in this direction. As already stated, the
+  compound retina which characterizes the vertebrate lateral eyes is characteristic of all facetted
+  eyes, and in the trilobites facetted lateral eyes are commonly found. From this it may be
+  concluded that many of the trilobites possessed eyes with a compound retina. There have, however,
+  been found in certain species, e.g. <i>Harpes vittatus</i> and <i>Harpes ungula</i>, lateral eyes
+  which were not facetted, and are believed by Korschelt and Heider to be of an arachnid nature.
+  They say, "Palæontologists have appropriately described them as ocelli, although, from a
+  zoological point of view, they do not deserve this name, having most probably arisen in a way
+  similar to that conjectured in connection with the lateral eyes of scorpions." If this conjecture
+  is right, then in these forms the retina may have been inverted, but because they belonged to the
+  trilobite group, the retina was most probably compound, so that here we may have had the
+  combination of the arachnid and crustacean characteristics. On the other hand, in some forms of
+  Branchipus, and many of the Gammaridæ, a single corneal lens is found in conjunction with an eye
+  of the crustacean type, so that a non-facetted lateral eye, found in a fossil form, would not
+  necessarily imply the arachnid type of eye with the possibility of an inverted retina. Whatever
+  may be the ultimate decision upon these particular forms, the striking fact remains, that both in
+  the vertebrate and in the arachnid the median eyes possess a simple upright retina, while the
+  lateral eyes possess an inverted retina, and that both in the vertebrate and the crustacean the
+  median eyes possess a simple upright retina, while the lateral eyes possess a compound retina.</p>
+
+  <p>The resemblance of the retina of the lateral eyes of vertebrates to that of the lateral eyes of
+  many arthropods, especially crustaceans, has been pointed out by nearly every one who has worked
+  at these invertebrate lateral eyes. The foundation of our knowledge of the compound retina is
+  Berger's well-known paper, the results of which are summed up by him in the following two main
+  conclusions.</p>
+
+  <div><span class="pagenum" id="page89">{89}</span></div>
+
+  <p>1.  The optic ganglion of the Arthropoda consists of two parts, of which the one stands in
+  direct inseparable connection with the facetted eye, and together with the layer of retinal rods
+  forms the retina of the facetted eye, while the other part is connected rather with the brain, and
+  is to be considered as an integral part of the brain in the narrower sense of the word.</p>
+
+  <div class="ac w30 fcenter sp3">
+    <a href="images/fig038.png" id="fig38"><img style="width:100%" src="images/fig038.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 38.&mdash;The Retina of Musca.</span> (After <span
+      class="sc">Berger</span>.)</p>
+      <p class="sp0"><i>Br.</i>, brain; <i>O.n.</i>, optic nerve; <i>n.l.o.g.</i>, nuclear layer of
+      ganglion of optic nerve; <i>m.l.</i>, molecular layer (Punktsubstanz); <i>n.l.r.g.i.</i> and
+      <i>n.l.r.g.o.</i>, inner and outer nuclear layers of the ganglion of the retina;
+      <i>f.br.r.</i>, terminal fibre-layer of retina; <i>r.</i>, layer of retinal end-cells
+      (indicated only).</p>
+    </div>
+  </div>
+
+  <p>2.  In all arthropods examined by him, the retina consists of five layers, as follows<span
+  class="wnw">:&mdash;</span></p>
+
+  <div class="poem">
+    <p>(1)  The layer of rods and their nuclei.</p>
+    <p>(2)  The layer of nerve-bundles.</p>
+    <p>(3)  The nuclear layer.</p>
+    <p>(4)  The molecular layer.</p>
+    <p>(5)  The ganglion cell layer.</p>
+  </div>
+
+  <p>Berger passes under review the structure and arrangement of the optic ganglion in a large
+  number of different groups of arthropods, and concludes that in all cases one part of the optic
+  ganglion is always closely attached to the visual end-cells, and this combination he calls the
+  retina. On the other hand, the nerve-fibres which connect the peripheral part of the optic
+  ganglion with the brain, the so-called optic nerve, are by no means homologous in the different
+  groups; for in some cases, as in many of the stalk-eyed crustaceans, the whole optic ganglion is
+  at the periphery, while in others, as in the Diptera, only the retinal ganglion is at the
+  periphery, and the nerve-stalk connects this with the rest of the optic ganglion, the latter being
+  fused with the main brain-mass. In the Diptera, in fact, according to Berger, the optic nerve
+  <span class="pagenum" id="page90">{90}</span>and retina are most nearly comparable to those of the
+  vertebrate. For this reason I give Berger's picture of the retina of Musca (Fig. <a
+  href="#fig38">38</a>), in order to show the arrangement there of the retinal layers.</p>
+
+  <div class="ac w35 fcenter sp2">
+    <a href="images/fig039.png" id="fig39"><img style="width:100%" src="images/fig039.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 39.</span>&mdash;<span class="sc">The Brain of</span>
+      <i>Sphæroma serratum</i>. (After <span class="sc">Bellonci</span>.)</p>
+      <p class="sp0"><i>Ant. I.</i> and <i>Ant. II.</i>, nerves to 1st and 2nd antennæ.
+      <i>f.br.r.</i>, terminal fibre-layer of retina; <i>Op. g. I.</i>, first optic ganglion; <i>Op.
+      g. II.</i>, second optic ganglion; <i>O.n.</i>, optic nerve-fibres forming an optic
+      chiasma.</p>
+    </div>
+  </div>
+
+  <p>In Branchipus and other primitive Crustacea, Berger also finds the same retinal layers, but is
+  unable to distinguish in the brain the rest of the optic ganglion. Judging from Berger's
+  description of Branchipus, and Bellonci's of Sphæroma, it would almost appear as though the
+  cerebral part of the retina in the higher forms originated from two ganglionic enlargements, an
+  external and internal enlargement, as Bellonci calls them. The external ganglion (<i>Op. g.
+  I.</i>, Fig. <a href="#fig39">39</a>) may be called the ganglion of the retina, the cells of which
+  form the nuclear layer of the higher forms, and the internal ganglion (<i>Op. g. II.</i>, Fig. <a
+  href="#fig39">39</a>), from which the optic nerve-fibres to the brain arise, may therefore be
+  called the ganglion of the optic nerve. Bellonci describes how in this latter ganglion cells are
+  found very different to the small ones of the external ganglion or ganglion of the retina. So also
+  in Branchipus, judging from the pictures of Berger, Claus, and from my own observations
+  (<i>cf.</i> Fig. <a href="#fig46">46</a>, in which the double nature of the retinal ganglion is
+  indicated), the peripheral part of the optic ganglion&mdash;<i>i.e.</i> the retinal
+  ganglion&mdash;may be spoken <span class="pagenum" id="page91">{91}</span>of as composed of two
+  ganglia. The external of these is clearly the ganglion of the retina; its cells form the nuclear
+  layer, the striking character of which, and close resemblance to the corresponding layer in
+  vertebrates, is shown by Claus' picture, which I reproduce (Fig. <a href="#fig40">40</a>). The
+  internal ganglion with which the optic nerve is in connection contains large ganglion cells,
+  which, together with smaller ones, form the ganglionic layer of Berger.</p>
+
+  <p>The most recent observations of the structure of the compound retina of the crustacean eye are
+  those of Parker, who, by the use of the methylene blue method, and Golgi's method of staining, has
+  been able to follow out the structure of the compound retina in the arthropod on the same lines as
+  had already been done for the vertebrate. These two methods have led to the conclusion that the
+  arthropod central nervous system and the vertebrate central nervous system are built up in the
+  same manner&mdash;viz. by means of a series of ganglia connected together, each ganglion being
+  composed of nerve-cells, nerve-fibres, and a fine reticulated substance called by Leydig in
+  arthropods 'Punktsubstanz,' and known in vertebrates and in invertebrates at the present time as
+  'neuropil.' A further analysis resolves the whole system into a combination of groups of neurones,
+  the cells and fibres of which form the cells and fibres of the ganglia, while their dendritic
+  connections with the terminations of other neurones, together with the neuroglia-cells form the
+  'neuropil.' As is natural to expect, that part of the central nervous system which helps to form
+  the compound retina is built up in the same manner as the rest of the central nervous system.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig040.png" id="fig40"><img style="width:37%" src="images/fig040.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p><span class="sc">Fig. 40.&mdash;Bipolar Cells of Nuclear Layer in Retina of
+      Branchipus.</span> (After <span class="sc">Claus</span>.)</p>
+      <p class="sp0"><i>f.br.r.</i>, terminal fibre-layer of retina; <i>n.l.r.g.</i>, bipolar cells
+      of the ganglion of the retina = inner nuclear layer; <i>m.l.</i>, Punktsubstanz = inner
+      molecular layer; <i>b.m.</i>, basement membrane formed by neurilemma round central nervous
+      system.</p>
+    </div>
+  </div>
+
+  <p>Thus, according to Parker, the mass of nervous tissue which occupies the central part of the
+  optic stalk in Astacus is composed <span class="pagenum" id="page92">{92}</span>of four distinct
+  ganglia; the retina is connected with the first of these by means of the retinal fibres, and the
+  optic nerve extends proximally from the fourth ganglion to the brain. Each ganglion consists of
+  ganglion-cells, nerve-fibres, and 'neuropil,' and, in addition, supporting cells of a neuroglial
+  type. By means of the methylene blue method and the Golgi method, it is seen that the retinal
+  end-cells, with their visual rods, are connected with the fibres of the optic nerve by means of a
+  system of neurones, the synapses of which take place in and help to form the 'neuropil' of the
+  various ganglia. Thus, an impulse in passing from the retina to the brain would ordinarily travel
+  over five neurones, beginning with one of the first order and ending with one of the fifth. He
+  makes five neurones although there are only four ganglia, because he reckons the retinal cell with
+  its elongated fibre as a neurone of the first order, such fibre terminating in dendritic processes
+  which form synapses in the 'neuropil' of the first ganglion with the neurones of the second
+  order.</p>
+
+  <p>Similarly the neurones of the second order terminate in the 'neuropil' of the second ganglion,
+  and so on, until we reach the neurones of the fifth order, which terminate on the one hand in the
+  'neuropil' of the fourth ganglion, and on the other pass to the optic lobes of the brain by their
+  long neuraxons&mdash;the fibres of the optic nerve.</p>
+
+  <p>He compares this arrangement with that of Branchipus, Apus, Estheria, Daphnia, etc., and shows
+  that in the more primitive crustaceans the peripheral optic apparatus was composed, not of four
+  but of two optic ganglia, not, therefore, of five but of three neurones, viz.&mdash;</p>
+
+  <p>1.  The neurone of the first order&mdash;<i>i.e.</i> the retinal cell with its fibre
+  terminating in the 'neuropil' of the first optic ganglion (ganglion of the retina).</p>
+
+  <p>2.  The neurone of the second order, which terminates in the 'neuropil' of the second ganglion
+  (ganglion of the optic nerve).</p>
+
+  <p>3.  The neurone of the third order, which terminates in the optic lobes of the brain by means
+  of its neuraxons (the optic nerve).</p>
+
+  <p>We see, then, that the most recent researches agree with the older ones of Berger, Claus, and
+  Bellonci, in picturing the retina of the primitive crustacean forms as formed of two ganglia only,
+  and not of four, as in the specialized crustacean group the Malacostraca.</p>
+
+  <div><span class="pagenum" id="page93">{93}</span></div>
+
+  <p>The comparison of the arthropod compound retina with that of the vertebrate shows, as one would
+  expect upon the theory of the origin of vertebrates put forward in this book, that the latter
+  retina is built up of two ganglia, as in the more primitive less specialized crustacean forms. The
+  modern description of the vertebrate retina, based upon the Golgi method of staining, is exactly
+  Parker's description of the simpler form of crustacean retina in which the 'neuropil' of the first
+  ganglion is represented by the external molecular layer, and that of the second ganglion by the
+  internal molecular layer; the three sets of neurones being, according to Parker's terminology<span
+  class="wnw">:&mdash;</span></p>
+
+  <p>1.  The neurones of the first order&mdash;viz. the visual cells&mdash;the nuclei of which form
+  the external nuclear layer, and their long attenuated processes form synapses in the external
+  molecular layer with</p>
+
+  <p>2.  The neurones of the second order, the cells of which form the internal nuclear layer, and
+  their processes form synapses in the internal molecular layer with</p>
+
+  <p>3.  The neurones of the third order, the cells of which form the ganglionic layer and their
+  neuraxons constitute the fibres of the optic nerve which end in the optic lobes of the brain.</p>
+
+  <p class="sp3">Strictly speaking, of course, the visual cells with their elongated processes have
+  no right to be called neurones: I only use Parker's phraseology in order to show how closely the
+  two retinas agree even to the formation of synapses between the fine drawn-out processes of the
+  visual cells and the neurones of the ganglion of the retina.</p>
+
+  <p class="ac"><span class="sc">The Retina of the Lateral Eye of Ammoc&#x0153;tes.</span></p>
+
+  <p>As in the case of all other organs, it follows that if we are dealing here with a true genetic
+  relationship, then the lower we go in the vertebrate kingdom the more nearly ought the structure
+  of the retina to approach the arthropod type. It is therefore a matter of intense interest to
+  determine the nature of the retina in Ammoc&#x0153;tes in order to see whether it differs from
+  that of the higher vertebrates, and if so, whether such differences are explicable by reference to
+  the structure of the arthropod eye.</p>
+
+  <p>Before describing the structure of this retina it is necessary to clear away a remarkable
+  misconception, shared among others by <span class="pagenum" id="page94">{94}</span>Balfour, that
+  this eye is an aborted eye, and that it cannot be considered as a primitive type. Thus Balfour
+  says: "Considering the degraded character of the Ammoc&#x0153;te eye, evidence derived from its
+  structure must be received with caution," and later on, "the most interesting cases of partial
+  degeneration are those of Myxine and the Ammoc&#x0153;te. The development of such aborted eyes has
+  as yet been studied only in the Ammoc&#x0153;te, in which it resembles in most important features
+  that of other Vertebrata."</p>
+
+  <p>Again and again the aborted character of the eye is stated to be evidence of degeneration in
+  the case of the lamprey. What such a statement means, why the eye is in any way to be considered
+  as aborted, is to me a matter of absolute wonderment: it is true that in the larval form it lies
+  under the skin, but it is equally true that at transformation it comes to the surface, and is most
+  evidently as perfect an eye as could be desired. There is not the slightest sign of any
+  degeneration or abortion, but simply of normal development, which takes a longer time than usual,
+  lasting as it does throughout the life-time of the larval form.</p>
+
+  <p>Kohl, who has especially studied degenerated vertebrate eyes, discusses with considerable
+  fulness the question of the Ammoc&#x0153;tes eye, and concludes that in aborted eyes a retarded
+  development occurs, and this applies on the whole to Ammoc&#x0153;tes, "but with the important
+  difference that in this case the period of retarded development is not followed by a stoppage, but
+  on the contrary by a period of very highly intensified progressive development during the
+  metamorphosis," with the result that "the adult eye of <i>Petromyzon Planeri</i> does not diverge
+  from the ordinary type."</p>
+
+  <p>Referring in his summing up to this retarded development, he says: "Such reminiscences of
+  embryonic conditions are after all present here and there in normally developed organs, and by no
+  means entitle us to speak of abnormal development."</p>
+
+  <p>The evidence, then, is quite clear that the eye of Petromyzon, or, indeed, of the full-grown
+  Ammoc&#x0153;tes, is in no sense an abnormal eye, but simply that its development is slow during
+  the ammoc&#x0153;te stage. The retina of Petromyzon was figured and described by Langerhans in
+  1873. He describes it as composed of the following layers<span class="wnw">:&mdash;</span></p>
+
+  <div class="poem">
+    <p style="margin-left:0.35em">(1) <i>Membrana limitans interna.</i></p>
+    <p style="margin-left:0.35em">(2) Thick inner molecular layer.</p>
+    <p style="margin-left:0.35em">(3) Optic fibre layer.</p>
+    <p style="margin-left:0.35em">(4) Thick inner nuclear layer.</p>
+    <p style="margin-left:0.35em">(5) Peculiar double-layered ganglionic layer.</p>
+    <p style="margin-left:0.35em">(6) External molecular layer.</p>
+    <p style="margin-left:0.35em">(7) External nuclear layer.</p>
+    <p style="margin-left:0.35em">(8) <i>Membrana limitans externa.</i></p>
+    <p style="margin-left:0.35em">(9) Layer of rods.</p>
+    <p>(10) Pigment-epithelium.</p>
+  </div>
+
+  <div><span class="pagenum" id="page95">{95}</span></div>
+
+  <div class="ac w45 fcenter sp2">
+    <a href="images/fig041.png" id="fig41"><img style="width:100%" src="images/fig041.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 41.&mdash;Retina and Optic Nerve of Petromyzon. (After
+      Müller and Langerhans.)</span></p>
+      <p class="sp0">On the left side the Müllerian fibres and pigment-epithelium are represented
+      alone. The retina is divided into an epithelial part, <i>C</i> (the layer of visual
+      rod-cells), and a neurodermal or cerebral part which is formed of, <i>A</i>, the ganglion of
+      the optic nerve and, <i>B</i>, the ganglion of the retina. 1, int. limiting membrane; 2, int.
+      molecular layer with its two layers of cells; 3, layer of optic nerve fibres; 4, int. nuclear
+      layer; 5, double row of tangential fulcrum cells; 6, layer of terminal retinal fibres; 7, ext.
+      nuclear layer; 8, ext. limiting membrane; 9, layer of rods; 10, layer of pigment-epithelium.
+      <i>D</i>, axial cell layer (Axenstrang) in optic nerve. The layer 6 is drawn rather too
+      thick.</p>
+    </div>
+  </div>
+
+  <p>He points out especially the peculiarity of layer (2) (2, Fig. <a href="#fig41">41</a>), the
+  inner molecular, in which two rows of nuclei are arranged with great regularity, the one row
+  closely touching the <i>membrana limitans interna</i>, the other at the inner boundary of the
+  middle third of the <span class="pagenum" id="page96">{96}</span>molecular layer. Of these two
+  rows of nuclei, he describes the innermost as composed almost entirely of large nuclei belonging
+  to ganglion cells, while the outermost is composed mainly of distinctly smaller nuclei, which in
+  staining and appearance appear to belong not to nerve-cells but to the true reticular tissue of
+  the molecular layer.</p>
+
+  <p>He also draws special attention to the remarkable layer (5) (5, Fig. <a href="#fig41">41</a>),
+  which is not found in the retina of the higher vertebrates, the cells of which, in his opinion,
+  are of the nature of ganglion-cells.</p>
+
+  <p>W. Müller, in 1874, gave a most careful description of the eye of Ammoc&#x0153;tes and
+  Petromyzon, and traced the development of the retina; the subsequent paper of Kohl does not add
+  anything new, and his drawings are manifestly diagrams, and do not represent the appearances so
+  accurately as Müller's illustrations. In the accompanying figure (Fig. <a href="#fig41">41</a>) I
+  reproduce on the right-hand side Müller's picture of the retina of Petromyzon, but have drawn it,
+  as in Langerhans' picture, at the place of entry of the optic nerve.</p>
+
+  <p>From his comparison of this retina with a large number of other vertebrate retinas, he comes to
+  the conclusion that the retina of all vertebrates is divisible into</p>
+
+  <div class="bq1 sp2">
+    <p><i>A.</i> An ectodermal (epithelial) part consisting of the layer of the visual cells,
+    and</p>
+    <p class="sp0"><i>B.</i> A neurodermal (cerebral) part which forms the rest of the retina.</p>
+  </div>
+
+  <p>Further, Müller points out that the neuroderm gives origin throughout the central nervous
+  system to two totally different structures, on the one hand to the true nervous elements, on the
+  other to a system of supporting cells and fibres which cannot be classed as connective tissue, for
+  they do not arise from mesoblast, and are therefore called by him 'fulcrum-cells.' In the retina
+  he recognizes two distinct groups of such supporting structures&mdash;(1) a system of radial
+  fibres with well-marked elongated nuclei, which extend between the two limiting layers, and form
+  at their outer ends a membrane-like expansion which was originally the outer limit of the retina,
+  but becomes afterwards co-terminous with the <i>membrana limitans externa</i>, owing to the
+  piercing through it of the external limbs of the rods. This system, which is known by the name of
+  the radial Müllerian fibres (shown on the left-hand side of Fig. <a href="#fig41">41</a>), has no
+  connection with (2) the spongioblasts and neurospongium, which form a framework of neuroglia, in
+  which the terminations of the <span class="pagenum" id="page97">{97}</span>optic ganglion and of
+  the retinal ganglion ramify to form the molecular layers.</p>
+
+  <p>It is evident from Fig. <a href="#fig41">41</a> that the retina of Ammoc&#x0153;tes and
+  Petromyzon differs in a striking manner from the typical vertebrate retina. The epithelial part
+  (C) remains the same&mdash;viz. the visual rods, the external limiting membrane, and the external
+  nuclear layer; but the cerebral part, the retinal ganglion (A and B), is remarkably different. It
+  is true, it consists in the main of the small-celled mass known as the inner nuclear layer, and of
+  the reticulated tissue or 'neuropil' known as the inner molecular layer, just as in all other
+  compound retinal eyes; but neither the ganglion cell-layer nor the optic fibre-layer is clearly
+  defined as separate from this molecular layer; on the contrary, it is matter of dispute as to what
+  cells represent the ganglionic layer of higher vertebrates, and the optic fibres do not form a
+  distinct innermost layer, but pass into the inner molecular layer at its junction with the inner
+  nuclear layer. A comparison of this innermost part of the retina (A, Fig. 41), with the
+  corresponding part in Berger's picture of Musca (<i>n.l.o.g.</i>, Fig. <a href="#fig38">38</a>),
+  shows a most striking similarity between the two. In both cases the fibres of the optic nerve
+  (<i>O.n.</i>, Fig. <a href="#fig38">38</a>) which cross at their entrance pass into the 'neuropil'
+  of this part of the retinal ganglion, and are connected probably (though that is not proved in
+  either case) with the cells of the ganglionic layer. In both cases we find two well-marked
+  parallel rows of cells in this part of the retina, of which one, the innermost, is composed in
+  Ammoc&#x0153;tes of large ganglion-cells, and the other mainly of smaller, deeper staining cells
+  apparently supporting in function. Similarly, also, in Branchipus, as I conclude from my own
+  observations as well as from those of Berger and Claus, the ganglionic layer is composed partly of
+  true ganglion-cells and partly of supporting cells arranged in a distinct layer. This part, then,
+  of the retina of Ammoc&#x0153;tes is remarkably like that of a typical arthropod retina, and forms
+  that part of the retinal ganglion which may be called the ganglion of the optic nerve.</p>
+
+  <p>Next comes the ganglion of the retina (B, Fig. <a href="#fig41">41</a>) (Parker's first optic
+  ganglion), the cells of which form the small bipolar granule-cells of the inner nuclear layer;
+  granule-cells arranged in rows just as they are shown in Claus' picture of the same layer in the
+  retina of Branchipus (Fig. <a href="#fig40">40</a>), just as they are found in the cortical layers
+  of the optic ganglion of the pineal eye (<i>ganglion habenulæ</i>), in the <span class="pagenum"
+  id="page98">{98}</span>optic lobes and other parts of the Ammoc&#x0153;tes brain, or in the
+  cortical layers of the optic ganglia of all arthropods.</p>
+
+  <p>Between this small-celled nuclear layer (4, Fig. <a href="#fig41">41</a>) and the layer of
+  nuclei of the visual rod cells (7, Fig. <a href="#fig41">41</a>) (the external nuclear layer), we
+  find in the eye of Ammoc&#x0153;tes and Petromyzon two well-marked rows of cells of a most
+  striking character&mdash;viz. the two remarkably regular rows of large epithelial-like cells with
+  large conspicuous nuclei, which give the appearance of two opposing rows of limiting epithelium
+  (5, Fig. <a href="#fig41">41</a>), already mentioned in connection with the researches of
+  Langerhans and W. Müller. Here, then, is a striking peculiarity of the retina of the lamprey, and
+  according to Müller the obliteration of these two layers can be traced as we pass upwards in the
+  vertebrate kingdom. Among fishes, they are especially well seen in the perch; in the higher
+  vertebrates the whole layer is only a rudiment represented, he thinks, by the simple layer of
+  round cells which lies close against the inner surface of the layer of terminal fibres
+  (Nervenansätze), and is especially evident in birds and reptiles. In man and the higher mammals
+  they are probably represented by the horizontal cells of the outer part of the inner nuclear
+  layer.</p>
+
+  <p>Seeing, then, that they are most evident in Ammoc&#x0153;tes, and become less and less marked
+  in the higher vertebrates, it is clear that their origin cannot be sought among the animals higher
+  in the scale than Ammoc&#x0153;tes, but must, therefore, be searched for in the opposite
+  direction.</p>
+
+  <p>Müller describes them as forming a very conspicuous landmark in the embryology of the retina,
+  dividing it distinctly into two parts, an outer thinner, and an inner somewhat thicker part, the
+  zone formed by them standing out conspicuously on account of the size and regularity of the cells
+  and their lighter appearance when stained. Thus in his description of the retina of an
+  Ammoc&#x0153;tes 95 mm. in length, he says, "The layer of pale tangentially elongated cells formed
+  a double layer and produced the appearance of a pale, very characteristic zone between the outer
+  and inner parts of the retina."</p>
+
+  <p>Let us now turn to the retina of the crustacean and see whether there is any evidence there
+  that the retina is divisible into an outer and inner part, separated by a zone of
+  characteristically pale staining cells with conspicuous nuclei. The most elaborate description of
+  the development of the retina of Astacus is given by Reichenbach, <span class="pagenum"
+  id="page99">{99}</span>according to whom the earliest sign of the formation of the retina is an
+  ectodermic involution (Augen-einstülpung), which soon closes, so that the retinal area appears as
+  a thickening. In close contiguity to this thickening, the thickening of the optic ganglion arises,
+  so that that part of the optic ganglion which will form the retinal ganglion fuses with the
+  thickened optic plate and forms a single mass of tissue. Later on a fold (Augen-falte) appears in
+  this mass of tissue, in consequence of which it becomes divided into two parts. The lining walls
+  of this fold form a double row of cells, the nuclei of which are most conspicuous because they are
+  larger and lighter in colour than the surrounding nuclei, so that by this fold the retina is
+  divided into an outer and an inner wall, the line of demarcation being conspicuous by reason of
+  these two rows of large, lightly-staining nuclei.</p>
+
+  <p>Reichenbach is unable to say that this secondary fold is coincident with the primary
+  involution, and that therefore the junction between the two rows of large pale nuclei is the line
+  of junction between the retinal ganglion and the retina proper, because all sign of the primary
+  involution is lost before the secondary fold appears.</p>
+
+  <p>Parker compares the appearances in the lobster with Reichenbach's description in the crayfish,
+  and says that he finds only a thickening, no primary involution; at the same time he expressly
+  states that in the very early stages his material was deficient, and that he had not grounds
+  sufficient to warrant the statement that no involution occurs. He also finds that in the lobster
+  the ganglionic tissue which arises by proliferation is divided into an outer and inner part; the
+  separation is effected by a band of large, lightly-staining nuclei, which, in position and
+  structure, resemble the band figured by Reichenbach. According to Parker, then, the line of
+  separation indicated in the development by Reichenbach's outer and inner walls is not the line of
+  junction between the retina and the retinal ganglion, as Reichenbach was inclined to think, but
+  rather a separation of two rows of large ganglion-cells belonging to the retinal ganglion.</p>
+
+  <p>The similarity between these conspicuous layers of lightly-staining cells in Ammoc&#x0153;tes
+  and in crustaceans is remarkably close, and in both cases observers have found the same difficulty
+  in interpreting their meaning. In each case one group of observers looks upon them as
+  ganglion-cells, the other as supporting structures. Thus in the lamprey, Müller considers them to
+  belong to the supporting elements, while Langerhans and Kohl describe them as a double <span
+  class="pagenum" id="page100">{100}</span>layer of ganglion-cells. In the crustacean, Berger in
+  Squilla, Grenacher in Mysis, and Parker in Astacus, look upon them as supporting elements, while
+  Viallanes in Palinurus considers them to be true ganglionic cells.</p>
+
+  <p>Whatever the final interpretation of these cells may prove to be, we may, it seems to me,
+  represent an ideal compound retina of the crustacean type by combining the investigations of
+  Berger, Claus, Reichenbach, and Parker in the following figure.</p>
+
+  <div class="ac w50 fcenter sp2">
+    <a href="images/fig042.png" id="fig42"><img style="width:46%" src="images/fig042.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 42.&mdash;Ideal Diagram of the Layers in a Crustacean
+      Eye.</span></p>
+      <p class="sp0">The retina is divided into an epithelial part, <i>C</i> (the layer of retinular
+      cells and rhabdomes), and a neurodermal or cerebral part, which is formed of, <i>A</i>, the
+      ganglion of the optic nerve, and, <i>B</i>, the ganglion of the retina. 1, optic nerve fibres
+      which cross at their entrance into the retina; 2, int. molecular layer with its two rows of
+      cells; 3, int. nuclear layer; 4, Reichenbach's double row of large lightly-staining cells; 5,
+      layer of terminal retinal fibres; 6, ext. nuclear layer; 7, ext. limiting membrane; 8, layer
+      of crystalline cones; 9, cornea.</p>
+    </div>
+  </div>
+
+  <p>The comparison of this figure (Fig. <a href="#fig42">42</a>) with that of the Petromyzon retina
+  (Fig. <a href="#fig41">41</a>) shows how great is the similarity of the latter with the arthropod
+  type, and how the very points in which it deviates from the recognized vertebrate type are
+  explainable by comparison with that of the arthropod. The most striking difference between the
+  retinas in the two figures is that the layer of terminal nerve fibres (5, Fig. <a
+  href="#fig42">42</a>), which, after all, are only the elongated terminations of the retinal cells
+  belonging to Parker's neurones of the first order, is very much longer than in Petromyzon or in
+  any vertebrate, for the external molecular layer (6, Fig. <a href="#fig41">41</a>) (Müller's layer
+  of Nervenansätze) is very short and inconspicuous (in Fig. <a href="#fig41">41</a> it is drawn too
+  thick).</p>
+
+  <p>Turning from the retina to the fibres of the optic nerve we again find a remarkable
+  resemblance, for in Ammoc&#x0153;tes, as pointed out by <span class="pagenum"
+  id="page101">{101}</span>Langerhans and carefully figured by Kohl, a crossing of the fibres of the
+  optic nerve occurs as the nerve leaves the retina, just as is so universally the case in all
+  compound retinas. To this crossing Kohl has given the name <i>chiasma nervi optici</i>, in
+  distinction to the cerebral chiasma, which he calls <i>chiasma nervorum opticorum</i>. Further, we
+  find that even this latter chiasma is well represented in the arthropod brain; thus Bellonci in
+  Sphæroma, Berger, Dietl, and Krieger in Astacus, all describe a true optic chiasma, the only
+  difference in opinion being, whether the crossing of the optic nerves is complete or not.
+  Especially instructive are Bellonci's figures and description. He describes the brain of Sphæroma
+  as composed of three segments&mdash;a superior segment, the cerebrum proper, a middle segment, and
+  an inferior segment; the optic fibres, as is seen in Fig. <a href="#fig39">39</a>, after crossing,
+  pass direct into the middle segment, in the ganglia of which they terminate. From this segment
+  also arises the nerve to the first antenna of that side&mdash;<i>i.e.</i> the olfactory nerve. The
+  optic part, then, of this middle segment is clearly the brain portion of the optic ganglionic
+  apparatus, and may be called the optic lobes, in contradistinction to the peripheral part, which
+  is usually called the optic ganglion, and is composed of two ganglia, Op. g. I. and Op. g. II., as
+  already mentioned. These optic lobes are therefore homologous with the optic lobes of the
+  vertebrate brain.</p>
+
+  <p>The resemblance throughout is so striking as to force one to the conclusion that the retina of
+  the vertebrate eye is a compound retina, composed of a retina and retinal ganglion of the type
+  found in arthropods. From this it follows that the development of the vertebrate retina ought to
+  show the formation of (1) an optic plate formed from the peripheral epidermis and not from the
+  brain; (2) a part of the brain closely attached to this optic plate forming the retinal ganglion,
+  which remains at the surface when the rest of the optic ganglion withdraws; (3) an optic nerve
+  formed in consequence of this withdrawal, as the connection between the retinal and cerebral parts
+  of the optic ganglion.</p>
+
+  <p class="sp3">This appears to me exactly what the developmental process does show according to
+  Götte's investigations. He asserts that the retina arises from an optic plate, being the optical
+  portion of his 'Sinnes-platte.' At an early stage this is separated by a furrow (Furche) from the
+  general mass of epidermal cells which ultimately form the brain. This separation then vanishes,
+  and the retina and brain-mass <span class="pagenum" id="page102">{102}</span>become inextricably
+  united into a mass of cells, which are still situated at the surface. By the closure of the
+  cephalic plate and the withdrawal of the brain away from the surface, a retinal mass of cells is
+  left at the surface connected with the tubular central nervous system by the hollow optic
+  diverticulum or primary optic vesicle. If we regard only the retinal and nervous elements, and for
+  the moment pay no attention to the existence of the tube, Götte's observation that the true retina
+  has been formed from the optic plate (Sinnes-platte) to which the retinal portion of the brain
+  (retinal ganglion) has become firmly fixed, and that then the optic nerve has been formed by the
+  withdrawal of the rest of the brain (optic lobes), is word for word applicable to the description
+  of the development of the compound retina of the arthropod eye, as has been already stated.</p>
+
+  <p class="ac"><span class="sc">The Significance of the Optic Diverticula.</span></p>
+
+  <p>The origin of the retina from an optic epidermal plate in vertebrates, as in all other animals,
+  brings the cephalic eyes of all animals into the same category, and leaves the vertebrate eye no
+  longer in an isolated and unnatural position. In one point the retina of the vertebrate eye
+  differs from that of a compound retina of an invertebrate; in the former, a striking supporting
+  tissue exists, known as Müller's fibres, which is absent in the latter. This difference of
+  structure is closely associated with another of the same character as in the central nervous
+  system, viz. the apparent development of the nervous part from a tube. We see, in fact, that the
+  retinal and nervous arrangements of the vertebrate eye are comparable with those of the arthropod
+  eye, in precisely the same way and to the same extent as the nervous matter of the brain of the
+  vertebrate is comparable with the brain of the arthropod. In both cases the nervous matter is, in
+  structure, position, and function, absolutely homologous; in both cases there is found in the
+  vertebrate something extra which is not found in the invertebrate&mdash;viz. a hollow tube, the
+  walls of which, in the case of the brain, are utilized as supporting tissues for the nerve
+  structures. The explanation of this difference in the case of the brain is the fundamental idea of
+  my whole theory, namely, that the hollow tube is in reality the cephalic stomach of the
+  invertebrate, around which the nervous brain-matter was originally grouped in precisely the same
+  manner as in the invertebrate. What, then, are the optic diverticula?</p>
+
+  <div><span class="pagenum" id="page103">{103}</span></div>
+
+  <p>"The formation of the eye," as taught by Balfour, "commences with the appearance of a pair of
+  hollow outgrowths from the anterior cerebral vesicle. These outgrowths, known as the optic
+  vesicles, at first open freely into the cavity of the anterior cerebral vesicle. From this they
+  soon, however, become partially constricted, and form vesicles united to the base of the brain by
+  comparatively narrow, hollow stalks, the rudiments of the optic nerves."</p>
+
+  <p>"After the establishment of the optic nerves, there takes place (1) the formation of the lens,
+  and (2) the formation of the optic cup from the walls of the primary optic vesicle."</p>
+
+  <p>He then goes on to explain how the formation of the lens forms the optic cup with its double
+  walls from the primary optic vesicle, and says&mdash;</p>
+
+  <p>"Of its double walls, the inner, or anterior, is formed from the front portion, the outer, or
+  posterior, from the hind portion of the wall of the primary optic vesicle. The inner, or anterior,
+  which very speedily becomes thicker than the other, is converted into the retina; in the outer, or
+  posterior, which remains thin, pigment is eventually deposited, and it ultimately becomes the
+  tesselated pigment-layer of the choroid."</p>
+
+  <p>The difficulties in connection with this view of the origin of the eye are exceedingly great,
+  so great as to have caused Balfour to discuss seriously Lankester's suggestion that the eye must
+  have been at one time within the brain, and that the ancestor of the vertebrate was therefore a
+  transparent animal, so that light might get to the eye through the outer covering and the
+  brain-mass; a suggestion, the unsatisfactory nature of which Balfour himself confessed. Is there
+  really evidence of any part of either retina or optic nerve being formed from the epithelial
+  lining of the tube?</p>
+
+  <p>This tube is formed as a direct continuation of the tube of the central nervous system, and we
+  can therefore apply to it the same arguments as have been used in the discussion of the meaning of
+  the latter tube. Now, the striking point in the latter case is the fact that the lining membrane
+  of the central canal is in so many parts absolutely free from nervous matter, and so shows, as in
+  the so-called choroid plexuses, its simple, non-nervous epithelial structure. This also we find in
+  the optic diverticulum. Where there is no evidence of any invasion of the tube by nervous
+  elements, there it retains its simple non-nervous character of a tube composed of a single layer
+  of <span class="pagenum" id="page104">{104}</span>epithelial cells&mdash;viz. in that part of the
+  tube which, as Balfour says, remains thin, in which pigment is eventually deposited, and which
+  ultimately becomes the tesselated pigment-layer of the choroid. Nobody has ever suggested that
+  this pigment-layer is nervous matter, or ever was, or ever will be, nervous matter; it is in
+  precisely the same category as the membranous roof of the brain in Ammoc&#x0153;tes, which never
+  was, and never will be, nervous matter. Yet, according to the old embryology both in the case of
+  the eye and the brain, the pigment-layer and the so-called choroid plexuses are a part of the
+  tubular nervous system.</p>
+
+  <p>Turning now to the optic nerve, Balfour describes it as derived from the hollow stalk of the
+  optic vesicle. He says&mdash;</p>
+
+  <p>"At first the optic nerve is equally continuous with both walls of the optic cup, as must of
+  necessity be the case, since the interval which primarily exists between the two walls is
+  continuous with the cavity of the stalk. When the cavity within the optic nerve vanishes, and the
+  fibres of the optic nerve appear, all connection is ruptured between the outer wall of the optic
+  cup and the optic nerve, and the optic nerve simply perforates the outer wall, and becomes
+  continuous with the inner one."</p>
+
+  <p>In this description Balfour, because he derived the optic nerve fibres from the epithelial wall
+  of the optic stalk, of necessity supposed that such fibres originally supplied both the outer and
+  inner walls of the optic cup and, therefore, seeing that when the fibres of the optic nerve appear
+  they do not supply the outer wall, he supposes that their original connection with the outer wall
+  is ruptured, because a discontinuity of the epithelial lining takes place coincidently with the
+  appearance of the optic nerve-fibres, and, according to him, the optic nerve simply perforates the
+  outer wall and becomes continuous with the inner one. This last statement is very difficult to
+  understand. I presume he meant that some of the fibres of the optic nerve supplied from the
+  beginning the inner wall of the optic cup, but that others which originally supplied the outer
+  wall were first ruptured, then perforated the outer wall, and finally completed the supply to the
+  inner wall or retina.</p>
+
+  <p>This statement of Balfour's is the necessary consequence of his belief, that the epithelial
+  cells of the optic stalk gave rise to the fibres of the optic nerve. If, instead of this, we
+  follow Kölliker and His, who state that the optic nerve-fibres are formed outside the <span
+  class="pagenum" id="page105">{105}</span>epithelial walls of the optic stalk, and that the cells
+  of the latter form supporting structures for the nerve-fibres, then the position of the optic
+  nerve becomes perfectly simple and satisfactory without any rupturing of its connection with the
+  outer wall and subsequent perforation, for the optic nerve-fibres from their very first appearance
+  pass directly to supply the retina&mdash;<i>i.e.</i> the inner wall of the optic cup and nothing
+  else.</p>
+
+  <p>They pass, as is well known, without any perforation by way of the choroidal slit to the inner
+  surface of the inner wall (retina) of the optic cup; then, when the choroidal slit becomes closed
+  by the expansion of the optic cup, the optic nerve naturally becomes situated in the centre of the
+  base of the cup and spreads over its inner surface as that surface expands.</p>
+
+  <p>A section across the optic cup at an early stage at the junction of the optic stalk and optic
+  cup would be represented by the upper diagram in Fig. 43; at a later stage, when the choroidal
+  slit is closed, by the lower diagram.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig043.png" id="fig43"><img style="width:45%" src="images/fig043.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 43.&mdash;Diagram of the Relation of the Optic Nerve to
+      the Optic Cup.</span></p>
+      <p class="sp0">The upper diagram represents a stage before the formation of the choroidal
+      slit, the lower one the stage of closure of the choroidal slit. <i>R.</i>, retina;
+      <i>O.n.</i>, optic nerve; <i>p.</i>, pigment epithelium.</p>
+    </div>
+  </div>
+
+  <p>The evident truth of this manner of looking at the origin of the optic nerve is demonstrated by
+  the appearance of the optic nerve in Ammoc&#x0153;tes and Petromyzon. In the latter, although the
+  development is complete, and the eye, and consequently also the optic nerve-fibres, are fully
+  functional, there is still present in the axial core of the nerve a row of epithelial cells
+  (Axenstrang) which are altered so as to form supporting structures, in the same way as a row of
+  epithelial cells in the retina is altered to form the system of supporting cells known by the name
+  of the Müllerian fibres.</p>
+
+  <p>The origin of this axial core of cells is perfectly clear, as has been pointed out by W.
+  Müller. He says&mdash;</p>
+
+  <p>"The development of the optic nerve shows peculiarities in <span class="pagenum"
+  id="page106">{106}</span>Petromyzon of such a character as to make this animal one of the most
+  valuable objects for deciding the various controversial questions connected with the genesis of
+  its elements. The lumen of the stalk of the primary optic vesicle is obliterated quite early by a
+  proliferation of its lining epithelium. Also the original continuity of this epithelium with that
+  of the pigment-layer is at an early period interrupted at the point of attachment of the optic
+  stalk. This interruption occurs at the time when the fibres of the optic nerve first become
+  visible."</p>
+
+  <p>Further on he says&mdash;</p>
+
+  <p>"The epithelium of the optic stalk develops entirely into supporting cells, which in Petromyzon
+  fill up the original lumen and so form an axial core (Axenstrang) to the nerve-fibres which are
+  formed entirely outside them; the projections of these supporting cells are directed towards the
+  periphery, and so separate the bundles of the optic nerve-fibres. The mesodermal coat of the optic
+  stalk takes no part in this separation; it simply forms the connective tissue sheath of the optic
+  nerve. The development of the optic nerve in the higher vertebrates also obeys the same law, as I
+  am bound to conclude from my own observations."</p>
+
+  <p>The evidence, then, of Ammoc&#x0153;tes is very conclusive. Originally a tube composed of a
+  single layer of epithelial cells became expanded at the anterior end to form a bulb. On the
+  outside of this tube or stalk the fibres of the optic nerve make their appearance, arising from
+  the ganglion-cell layer of the retina, and, passing over the surface of the epithelial tube at the
+  choroidal fissure, proceed to the brain by way of the optic chiasma. Owing to the large number of
+  fibres, their crossing at the junction of the stalk with the bulb, and the narrowness at this
+  neck, the obliteration of the lumen of the tube which takes place in the stalk is carried out to a
+  still greater extent at this narrow part. The result of this is that all continuity of the
+  cell-layers of the original tube of the optic stalk with those of both the inner and outer walls
+  of the bulb is interrupted, and all that remains in this spot of the original continuous line of
+  cells which connected the tube of the stalk with that of the bulb are possibly some of the groups
+  of cells which are found scattered among the fibres of the optic nerve at their entrance into the
+  retina. Such separation of the originally continuous elements of the epithelial wall of the optic
+  stalk, which is apparent only at this neck of the nerve in Petromyzon, takes place <span
+  class="pagenum" id="page107">{107}</span>along the whole of the optic nerve in the higher
+  vertebrates, so that no continuous axial core of cells exist, but only scattered supporting
+  cells.</p>
+
+  <p>If further proof in support of this view be wanted, it is given by the evidence of physiology,
+  which shows that the fibres of the optic nerve are not different from other nerve-fibres of the
+  central nervous system, but that they degenerate when separated from their nerve-cell, and that
+  the nerve-cell of which the optic nerve-fibre is a process is the large ganglion-cell of the
+  ganglionic layer of the retina. The origin of the ganglionic layer of the retina cannot therefore
+  be separated from that of the optic nerve-fibres. If the one is outside the epithelial tube, so is
+  the other, and what holds true of the ganglionic layer must hold good of the rest of the retinal
+  ganglion and, from all that has been said, of the retina itself. We therefore come to the
+  conclusion that the evidence is distinctly in favour of the view, that the retina and optic nerve
+  in the true sense are structures which originally were outside a non-nervous tube, but, just like
+  the central nervous system as a whole, have amalgamated so closely with the elements of this tube
+  as to utilize them for supporting structures. One part of this non-nervous tube, its dorsal wall,
+  like the corresponding part of the brain-tube, still retains its original character, and by the
+  deposition of pigment has been pressed into the service of the eye to form the pigmented
+  epithelial layer.</p>
+
+  <p>We can, however, go further than this, for we know definitely in the case of the retina what
+  the fate of the epithelial cells lining this tube has been. They have become the system of
+  supporting structures known as Müllerian fibres.</p>
+
+  <p>The epithelial layer of the primary optic vesicle can be traced into direct continuity with the
+  lining epithelium of the brain cavity, as a single layer of epithelial cells in the core of the
+  optic nerve, forming the optic stalk, which, in consequence of close contact, becomes the
+  well-known axial layer of supporting cells. This epithelial layer of the optic stalk then expands
+  to form the optic bulb, the outer or dorsal wall of which still remains as a single layer of
+  epithelium and becomes the layer of pigment epithelium. This layer of epithelium becomes doubled
+  on itself by the approximation of the inner or ventral wall of the optic cup to the outer or
+  dorsal wall in consequence of the presence of the lens, and still remaining a single layer, forms
+  the <i>pars ciliaris retinæ</i>; then suddenly, at the <i>ora <span class="pagenum"
+  id="page108">{108}</span>serrata</i>, the single epithelial layer vanishes, and the layers of the
+  retina take its place. It has long been known, however, that even throughout the retina this
+  single epithelial layer still continues, being known as the fibres of Müller. This is how the fact
+  is described in the last edition of Foster's "Text-book of Physiology," p. 1308&mdash;</p>
+
+  <p>"Stretching radially from the inner to the outer limiting membrane in all regions of the retina
+  are certain peculiar-shaped bodies known as the radial fibres of Müller. Each fibre is the outcome
+  of the changes undergone by what was at first a simple columnar epithelial cell. The changes are,
+  in the main, that the columnar form is elongated into that of a more or less prismatic fibre, the
+  edges of which become variously branched, and that while the nucleus is retained the cell
+  substance becomes converted into neuro-keratin. And, indeed, at the <i>ora serrata</i> the fibres
+  of Müller may be seen suddenly to lose their peculiar features and to pass into the ordinary
+  columnar cells which form the <i>pars ciliaris retinæ</i>."</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig044.png" id="fig44"><img style="width:60%" src="images/fig044.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p><span class="sc">Fig. 44.&mdash;Diagram representing the Single-layered Epithelial Tube of
+      the Vertebrate Eye after Removal of the Nervous and Retinal Elements.</span></p>
+      <p class="sp0"><i>O.n.</i>, axial core of cells in optic nerve; <i>p.</i>, pigment epithelium;
+      <i>p.c.r., pars ciliaris retinæ</i>; <i>m.f.</i>, Müllerian fibres; <i>l.</i>, lens.</p>
+    </div>
+  </div>
+
+  <p>It is then absolutely clear that the essential parts of the eye may be considered as composed
+  of two parts&mdash;</p>
+
+  <p>1. A tube or diverticulum from the tube of the central nervous system, composed throughout of a
+  single layer of epithelium, which forms the supporting axial cells in the optic nerve, the pigment
+  epithelium and the Müllerian fibres of the retina. Such a tube would be represented by the
+  accompanying Fig. <a href="#fig44">44</a>, and the left side of Fig. <a href="#fig41">41</a>.</p>
+
+  <p>2. The retina proper with the retinal ganglion and the optic nerve-fibres as already described.
+  In this part supporting elements are found, just as in any other compound retina, of the nature of
+  neuroglia, which are independent of the Müllerian fibres.</p>
+
+  <div><span class="pagenum" id="page109">{109}</span></div>
+
+  <p>Of these two parts we have already seen that the second is to all intents and purposes a
+  compound retina of a crustacean eye, and seeing that the single-layered epithelial tube is
+  continuous with the single-layered epithelial tube of the central nervous system&mdash;<i>i.e.</i>
+  with the cephalic part of the gut of the arthropod ancestor&mdash;it follows with certainty that
+  the ancestor of the vertebrates must have possessed two anterior diverticula of the gut, with the
+  wall of which, near the anterior extremity, the compound retina has amalgamated on either side,
+  just as the infra-&#x0153;sophageal ganglia have amalgamated with the ventral wall of the main
+  gut-tube. In this way, and in this way alone, does the interpretation of the structure of the
+  vertebrate lateral eye harmonize in the most perfect manner with the rest of the conclusions
+  already arrived at.</p>
+
+  <p>The question therefore arises:&mdash;Have we any grounds for believing that the ancient forms
+  of primitive crustaceans and primitive arachnids, which were so abundant in the time when the
+  Cephalaspids appeared, possessed two anterior diverticula of the stomach, such as the
+  consideration of the vertebrate eye strongly indicates must have been the case?</p>
+
+  <p>The beautiful pictures of Blanchard, and his description, show how, on the arachnid side,
+  paired diverticula of the stomach are nearly universal in the group. Thus, although they are not
+  present in the scorpions, still, in the Thelyphonidæ, Phrynidæ, Solpugidæ, Mygalidæ, the most
+  marked characteristic of the stomach-region is the presence of four pairs of c&#x0153;cal
+  diverticula, which spread laterally over the prosomatic region. In the spiders the number of such
+  diverticula increases, and the whole prosomatic region becomes filled up with these tubes.
+  Blanchard considers that they form nutrient tubes for the direct nutrition of the organs in the
+  prosoma, especially the important brain-region of the central nervous system. He points out that
+  these animals are blood-suckers, and that, therefore, their food is already in a suitable form for
+  purposes of nutrition when it is taken in by them, so that, as it were, the anterior part of the
+  gut is transformed into a series of vessels or diverticula conveying blood directly to the
+  important organs in the prosoma, by means of which they obtain nourishment in addition to their
+  own blood-supply.</p>
+
+  <p>The universality of such diverticula among the arachnids makes it highly probable that their
+  progenitors did possess an alimentary canal with one or more pairs of anterior diverticula. In the
+  <span class="pagenum" id="page110">{110}</span>vertebrate, however, the paired diverticula are
+  associated with a compound retina, a combination which does not occur among living arachnids; we
+  must, therefore, examine the crustacean group for the desired combination, and naturally the most
+  likely group to examine is the Phyllopoda, especially such primitive forms as Branchipus and
+  Artemia, for it is universally acknowledged that these forms are the nearest living
+  representatives of the trilobites. If, therefore, it be found that the retina and optic nerve in
+  Artemia is in specially close connection with an anterior diverticulum of the gut on each side,
+  then it is almost certain that such a combination existed also in the trilobites.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig045.png" id="fig45"><img style="width:100%" src="images/fig045.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 45.&mdash;Section through one of the two Anterior
+      Diverticula of the Gut in Artemia and the Retinal Ganglion.</span></p>
+      <p class="sp0">The section is through the extreme anterior end of the diverticulum, thus
+      cutting through many of the columnar cells at right angles to their axis. <i>Al.</i>, gut
+      diverticulum; <i>rt. gl.</i>, retinal ganglion.</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page111">{111}</span></div>
+
+  <div class="ac w30 fcenter sp2">
+    <a href="images/fig046.png" id="fig46"><img style="width:100%" src="images/fig046.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p><span class="sc">Fig. 46.&mdash;The Brain, Eyes, and Anterior Termination of the Alimentary
+      Canal of Artemia, viewed from the Dorsal Aspect.</span></p>
+      <p class="sp0"><i>Br.</i>, brain; <i>l.e.</i>, lateral eyes; <i>c.e.</i>, median eyes;
+      <i>Al.</i>, alimentary canal.</p>
+    </div>
+  </div>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig047.png" id="fig47"><img style="width:100%" src="images/fig047.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p><span class="sc">Fig. 47.&mdash;A, The Formation of the Retina of the Eye of
+      Ammoc&#x0153;tes</span> (after <span class="sc">Scott</span>); <span class="sc">B, The
+      Formation of the Retina of the Eye of Ammoc&#x0153;tes, on my theory</span>.</p>
+      <p class="sp0"><i>R.</i>, retina; <i>l.</i>, lens; <i>O.n.</i>, optic nerve fibres;
+      <i>Al.</i>, cephalic end of invertebrate alimentary canal; <i>V.</i>, cavity of ventricles of
+      brain; <i>Al.d.</i>, anterior diverticulum of alimentary canal; <i>op.d.</i>, optic
+      diverticulum.</p>
+    </div>
+  </div>
+
+  <p>My friend Mr. W. B. Hardy has especially investigated the nervous system of Artemia. In the
+  course of his work he cut serial sections through the whole animal, and, as mentioned in my paper
+  in the <i>Journal of Anatomy and Physiology</i>, he discovered that the retinal ganglion of each
+  lateral eye is so closely attached to the end of the corresponding diverticulum of the gut that
+  the lining cells of the ventral part of the diverticulum form a lining to the retinal ganglion
+  (Fig. <a href="#fig45">45</a>). In this animal there are only two gut-diverticula, which are
+  situated most anteriorly. I have plotted out this series of sections by means of a camera lucida,
+  with the result that the retina appears as a bulging attached ventro-laterally to the extremity of
+  each gut-diverticulum, as is shown in Fig. <a href="#fig46">46</a>. It is instructive to compare
+  with this figure Scott's picture of the developing eye in Ammoc&#x0153;tes, where he figures the
+  retina as <span class="pagenum" id="page112">{112}</span>a bulging attached ventrally to the
+  extremity of the narrow tube of the optic diverticulum. In Fig. <a href="#fig47">47</a>, A, I
+  reproduce this figure of Scott, and by the side of it, Fig. <a href="#fig47">47</a>, B, I have
+  represented the origin of the vertebrate eye as I believe it to have occurred.</p>
+
+  <p>We see, then, this very striking fact, that in the most primitive of the Crustacea, not only
+  are there two anterior diverticula of the gut, but also the retinal ganglion of the lateral eye is
+  in specially close connection with the end of the diverticulum on each side. In fact, we find in
+  the nearest living representative of the trilobites a retina and retinal ganglion and optic nerve,
+  closely resembling that of the vertebrate, in close connection with an epithelial tube which has
+  nothing to do with the organ of sight, but is one of a pair of anterior gut-diverticula. It is
+  impossible to obtain more decisive evidence that the trilobites possessed a pair of
+  gut-diverticula surrounded to a greater or less extent by the retina and optic nerve of each
+  lateral eye.</p>
+
+  <p>Such anterior diverticula are commonly found in the lower Crustacea; they are usually known by
+  the name of liver-diverticula, but as they take no part in digestion, and, on the contrary,
+  represent that part of the gut which is most active in absorption, the term liver is not
+  appropriate, and it is therefore better to call them simply the pair of anterior diverticula. Our
+  knowledge of their function in Daphnia is given in a paper by Hardy and M&lsquo;Dougall, which
+  does not appear to be widely known. Hardy succeeded in feeding Daphnia with yolk of egg in which
+  carmine grains were mixed, and was able in the living animal to watch the whole process of
+  deglutition, digestion, and absorption. The food, which is made into a bolus, is moved down to the
+  middle region of the gut, and there digestion takes place. Then by an antiperistaltic movement the
+  more fluid products of the digestion-process are sent right forward into the two anterior
+  diverticula, where the single layer of columnar cells lining these diverticula absorbs these
+  products, the cells becoming thickly studded with fat-drops after a feed of yolk of egg. The
+  carmine particles, which were driven forward with the proteid- and fat-particles, are not
+  absorbed, but are at intervals driven back by contractions of the anterior diverticula to the
+  middle region of the gut.</p>
+
+  <p>These observations prove most clearly that the anterior diverticula have a special nutrient
+  function, being the main channels by which new nutrient material is brought into the body, and, as
+  <span class="pagenum" id="page113">{113}</span>pointed out by the authors, it is a remarkable
+  exception in the animal kingdom that absorption should occur in that portion of the gut which is
+  anterior to the part in which digestion occurs. In all these animals the two anterior diverticula
+  extend forwards over the brain, and, as we have seen in Artemia, the anterior extremity of each
+  one is so intimately related to a part of the brain&mdash;viz. the retinal ganglion&mdash;as to
+  form a lining membrane to that mass of nerve-cells. It follows, therefore, that the nutrient fluid
+  absorbed by the cells of this part of the gut-diverticulum must be primarily for the service of
+  the retinal ganglion. In fact, the relations of this anterior portion of the gut to the brain as a
+  whole suggest strongly that the marked absorptive function of this anterior portion of the gut
+  exists in order to supply nutrient material in the first place to the most vital, most important
+  organ in the animal&mdash;the brain and its sense-organs. This conclusion is borne out by the fact
+  that in these lower crustaceans the circulation of blood is of a very inefficient character, so
+  that the tissues are mainly dependent for their nutrition on the fluid immediately surrounding
+  them. It stands to reason that the establishment of the anterior portion of the gut as a nutrient
+  tube to the brain would necessitate a closer and closer application of the brain to that tube, so
+  that the process of amalgamation of the brain with the single layer of columnar epithelial cells
+  which constitutes the wall of the gut (which we see in its initial stage in the retinal ganglion
+  of Artemia), would tend rapidly to increase as more and more demands were made upon the brain,
+  until at last both the supra- and infra-&#x0153;sophageal ganglia, as well as the retinal ganglia
+  and optic nerves, were in such close intimate connection with the ventral wall of the anterior
+  portion of the gut and its diverticula as to form a brain and retina closely resembling that of
+  Ammoc&#x0153;tes.</p>
+
+  <p>Such an origin for the lateral eyes of the vertebrate explains in a simple and satisfactory
+  manner why the vertebrate retina is a compound retina, and why both retina and optic nerve have an
+  apparent tubular development.</p>
+
+  <p>At the same time one discrepancy still exists which requires consideration&mdash;viz. in no
+  arthropod eye possessing a compound retina is the retina inverted. All the known cases of
+  inversion among arthropods occur in eyes, the retina of which is simple, and are all natural
+  consequences of the process of invagination by which <span class="pagenum"
+  id="page114">{114}</span>the retina is formed. On the other hand, eyes with an inverted compound
+  retina are not entirely unknown among invertebrates, for the eyes of Pecten and of Spondylus
+  possess a retina which is inverted after the vertebrate fashion and still may be spoken of as
+  compound rather than simple. It is clear that an invagination, the effect of which is an inversion
+  of the retinal layer, would lead to the same result, whether the retinal optic nerves were short
+  or long, whether, in fact, a retinal ganglion existed or not. Undoubtedly the presence of the
+  retinal ganglion tends greatly to obscure any process of invagination, so that, as already
+  mentioned, many observers, with Parker, consider the retina of the crustacean lateral eye to be
+  formed by a thickening only, without any invagination, while Reichenbach says an obscure
+  invagination does take place at a very early stage. So in the vertebrate eye most observers speak
+  only of a thickening to form the retina, but Götte's observation points to an invagination of the
+  optic plate at an early stage. So also in the eye of Pecten, Korschelt and Heider consider that
+  the thickening, by which the retina is formed according to Patten, in reality hides an
+  invagination process by means of which, as Bütschli suggests, an optic vesicle is formed in the
+  usual manner. The retina is formed from the anterior wall of this vesicle, and is therefore
+  inverted.</p>
+
+  <p>The origin of the inverted retina of the vertebrate eye does not seem to me to present any
+  great difficulty, especially when one takes into consideration the fact that the retina is
+  inverted in the arachnid group, only in the lateral eyes. The inversion is usually regarded as
+  associated with the tubular formation of the vertebrate retina, and it is possible to suppose that
+  the retina became inverted in consequence of the involvement of the eye with the gut-diverticulum.
+  I do not myself think any such explanation is at all probable, because I cannot conceive such a
+  process taking place without a temporary derangement&mdash;to say the least of it&mdash;of the
+  power of vision, and as I do not believe that evolution was brought about by sudden, startling
+  changes, but by gradual, orderly adaptations, and as I also believe in the paramount importance of
+  the organs of vision for the evolution of all the higher types of the animal kingdom, I must
+  believe that in the evolution from the Arthropod to the Cephalaspid, the lateral eyes remained
+  throughout functional. I therefore, for my own part, would say that the inversion of the <span
+  class="pagenum" id="page115">{115}</span>retina took place before the complete amalgamation with
+  the gut-diverticulum, that, in fact, among the proto-crustacean, proto-arachnid forms there were
+  some sufficiently arachnid to have an inverted retina, and at the same time sufficiently
+  crustacean to possess a compound retina, and therefore a compound inverted retina after the
+  vertebrate fashion existed in combination with the anterior gut-diverticula. Thus, when the eye
+  and optic nerve sank into and amalgamated with the gut-diverticulum, neither the dioptric
+  apparatus nor the nervous arrangements would suffer any alteration, and the animal throughout the
+  whole process would possess organs of vision as good as before or after the period of
+  transition.</p>
+
+  <p>Further, not only the retina but also the dioptric apparatus of the vertebrate eye point to its
+  origin from a type that combined the peculiarities of the arachnids and the crustaceans. In the
+  former it is difficult to speak of a true lens, the function of a lens being undertaken by the
+  cuticular surface of the cells of the corneagen (Mark's 'lentigen'), while in the latter, in
+  addition to the corneal covering, a true lens exists in the shape of the crystalline cones.
+  Further, these crustacean lenses are true lenses in the vertebrate sense, in that they are formed
+  by modified hypodermal cells, and not bulgings of the cuticle, as in the arachnid. We see, in
+  fact, that in the compound crustacean eye an extra layer of hypodermal cells has become inserted
+  between the cornea and the retina to form a lens. So also in the vertebrate eye the lens is formed
+  by an extra layer of the epidermal cells between the cornea and the retina. The fact that the
+  vertebrate eye possesses a single lens, though its retina is composed of a number of ommatidia,
+  while the crustacean eye possesses a lens to each ommatidium, may well be a consequence of the
+  inversion of the vertebrate retina. It is most probable, as Korschelt and Heider have pointed out,
+  that the retina of the arachnid eyes is composed of a number of ommatidia, just as in the
+  crustacean eyes and in the inverted eyes it is probable that the image is focussed on to the
+  pigmented tapetal layer, and thence reflected on to the percipient visual rods. In such a method
+  of vision a single lens is a necessity, and so it must also be if, as I suppose, eyes existed with
+  an inverted compound retina. Owing to the crustacean affinities of such eyes, a lens would be
+  formed and the retina would be compound: owing to the arachnid affinities, the retina would be
+  inverted and the hypodermal cells which formed the lens would be massed <span class="pagenum"
+  id="page116">{116}</span>together to form a single lens, instead of being collected in groups of
+  four to form a series of crystalline cones.</p>
+
+  <p>To sum up: The study of the vertebrate eyes, both median and lateral, leads to most important
+  conclusions as to the origin of the vertebrates, for it shows clearly that whereas, as pointed out
+  in this and subsequent chapters, their ancestors possessed distinct arachnid characteristics, yet
+  that they cannot have been specialized arachnids, such as our present-day forms, but rather they
+  were of a primitive arachnid type, with distinct crustacean characteristics: animals that were
+  both crustacean and arachnid, but not yet specialized in either direction: animals, in fact, of
+  precisely the kind which swarmed in the seas at the time when the vertebrates first made their
+  appearance. In the opinion of the present day, the ancestral forms of the Crustacea, which were
+  directly derived from the Annelida, may be classed as an hypothetical group the Protostraca, the
+  nearest approach to which is a primitive Phyllopod.</p>
+
+  <p>"Starting from the Protostraca," say Korschelt and Heider, "according to the present condition
+  of our knowledge, we may, as has been already remarked, assume three great series of development
+  of the Arthropodan stock, by the side of which a number of smaller independent branches have been
+  retained. One of these series leads through the hypothetical primitive Phyllopod to the Crustacea;
+  the second through the Palæostraca (Trilobita, Gigantostraca, Xiphosura) to the Arachnida; the
+  third through forms resembling Peripatus to the Myriapoda and the Insecta. The Pantapoda and the
+  Tardigrada must probably be regarded as smaller independent branches of the Arthropodan
+  stock."</p>
+
+  <p class="sp3">To these "three great series of development of the Arthropodan stock" the evidence
+  of Ammoc&#x0153;tes shows that a fourth must be added, which, starting also from the Protostraca,
+  and closely connected with the second, palæostracan branch, leads through the Cephalaspidæ to the
+  great kingdom of the Vertebrata. Such a direct linking of the earliest vertebrates with the
+  Annelida through the Protostraca is of the utmost importance, as will be shown later in the
+  explanation of the origin of the vertebrate c&#x0153;lom and urinary apparatus.</p>
+
+  <div><span class="pagenum" id="page117">{117}</span></div>
+
+  <p class="ac"><span class="sc">Summary.</span></p>
+
+  <div class="bq1 smaller sp5">
+    <p>The most important discovery of recent years which gives a direct clue to the ancestry of the
+    vertebrates is undoubtedly the discovery that the pineal gland is all that remains of a pair of
+    median eyes which must have been functional in the immediate ancestor of the vertebrate, seeing
+    how perfect one of them still is in Ammoc&#x0153;tes. The vertebrate ancestor, then, possessed
+    two pairs of eyes, one pair situated laterally, the other median. In striking confirmation of
+    the origin of the vertebrate from Palæostracans it is universally admitted that all the
+    Eurypterids and such-like forms resembled Limulus in the possession of a pair of median eyes, as
+    well as of a pair of lateral eyes. Moreover, the ancient mailed fishes the Ostracodermata, which
+    are the earliest fishes known, are all said to show the presence of a pair of median eyes as
+    well as of a pair of lateral eyes. This evidence directly suggests that the structure of both
+    the median and lateral vertebrate eyes ought to be very similar to that of the median and
+    lateral arthropod eyes. Such is, indeed, found to be the case.</p>
+    <p>The retina of the simplest form of eye is formed from a group of the superficial epidermal
+    cells, and the rods or rhabdites are formed from the cuticular covering of these cells; the
+    optic nerve passes from these cells to the deeper-lying brain. This kind of retina may be called
+    a simple retina, and characterizes the eyes, both median and lateral, of the scorpion group.</p>
+    <p>In other cases a portion of the optic ganglion remains at the surface, when the brain sinks
+    inwards, in close contiguity to the epidermal sense-cells which form the retina; a tract of
+    fibres connects this optic ganglion with the underlying brain, and is known as the optic nerve.
+    Such a retina may be called a compound retina and characterizes the lateral eyes of both
+    crustaceans and vertebrates. Also, owing to the method of formation of the retina by
+    invagination, the cuticular surface of the retinal sense-cells, from which the rods are formed,
+    may be directed towards the source of light or away from it. In the first case the retina may be
+    called upright, in the second inverted.</p>
+    <p>Such inverted retinas are found in the vertebrate lateral eyes and in the lateral eyes of the
+    arachnids, but not of the crustaceans.</p>
+    <p>The evidence shows that all the invertebrate median eyes possess a simple upright retina, and
+    in structure are remarkably like the right median or pineal eye of Ammoc&#x0153;tes; while the
+    lateral eyes possess, as in the crustaceans, an upright compound retina, or, as in many of the
+    arachnids, a simple inverted retina. The lateral eyes of the vertebrates alone possess a
+    compound inverted retina.</p>
+    <p>This retina, however, is extraordinarily similar in its structure to the compound crustacean
+    retina, and these similarities are more accentuated in the retina of the lateral eye of
+    Petromyzon than that of the higher vertebrates.</p>
+    <p>The evidence afforded by the lateral eye of the vertebrate points unmistakably to the
+    conclusion that the ancestor of the vertebrate possessed both crustacean and arachnid
+    characters&mdash;belonged, therefore, to a group of animals which gave rise to both the
+    crustacean and arachnid groups. This is precisely the position of the Palæostracan group, which
+    is regarded as the ancestor of both the crustaceans and arachnids. <span class="pagenum"
+    id="page118">{118}</span>In two respects the retina of the lateral eyes of vertebrates differs
+    from that of all arthropods, for it possesses a special supporting structure, the Müllerian
+    fibres, which do not exist in the latter, and it is developed in connection with a tube, the
+    optic diverticulum, which is connected on each side with the main tube of the central nervous
+    system. These two differences are in reality one and the same, for the Müllerian fibres are the
+    altered lining cells of the optic diverticulum, and this tube has the same significance as the
+    rest of the tube of the nervous system; it is something which has nothing to do with the nervous
+    portion of the retina but has become closely amalgamated with it. The explanation is, word for
+    word, the same as for the tubular nervous system, and shows that the ancestor of the vertebrate
+    possessed two anterior diverticula of its alimentary canal which were in close relationship to
+    the optic ganglion and nerve of the lateral eye on each side. It is again a striking coincidence
+    to find that Artemia, which with Branchipus represents a group of living crustaceans most nearly
+    allied to the trilobites, does possess two anterior diverticula of the gut which are in
+    extraordinarily close relationship with the optic ganglia of the retina of the lateral eyes on
+    each side.</p>
+    <p class="sp0">The evidence of the optic apparatus of the vertebrate points most remarkably to
+    the derivation of the Vertebrata from the Palæostraca.</p>
+  </div>
+
+  <div><span class="pagenum" id="page119">{119}</span></div>
+
+  <p class="ac">CHAPTER III</p>
+
+  <p class="ac"><i>THE EVIDENCE OF THE SKELETON</i></p>
+
+  <div class="bq1 it1 smaller sp3">
+    <p class="sp0">The bony and cartilaginous skeleton considered, not the notochord.&mdash;Nature
+    of the earliest cartilaginous skeleton.&mdash;The mesosomatic skeleton of Ammoc&#x0153;tes; its
+    topographical arrangement, its structure, its origin in muco-cartilage.&mdash;The prosomatic
+    skeleton of Ammoc&#x0153;tes; the trabeculæ and parachordals, their structure, their origin in
+    white fibrous tissue.&mdash;The mesosomatic skeleton of Limulus compared with that of
+    Ammoc&#x0153;tes; similarity of position, of structure, of origin in muco-cartilage.&mdash;The
+    prosomatic skeleton of Limulus; the entosternite or plastron compared with the trabeculæ of
+    Ammoc&#x0153;tes; similarity of position, of structure, of origin in fibrous
+    tissue.&mdash;Summary.</p>
+  </div>
+
+  <p>The explanation of the two optic diverticula given in the last chapter accounts in the same
+  harmonious manner for every other part of the tube around which the central nervous system of the
+  vertebrate has been grouped. The tube conforms in all respects to the simple epithelial tube which
+  formed the alimentary canal of the ancient type of marine arthropods such as were dominant in the
+  seas when the vertebrates first appeared. The whole evidence so far is so uniform and points so
+  strongly in the direction of the origin of vertebrates from these ancient arthropods, as to make
+  it an imperative duty to proceed further and to compare one by one the other parts of the central
+  nervous system, together with their outgoing nerves in the two groups of animals.</p>
+
+  <p>Before proceeding to do this, it is advisable first to consider the question of the origin of
+  the vertebrate skeletal tissues, for this is the second of the great difficulties in the way of
+  deriving vertebrates from arthropods, the one skeleton being an endo-skeleton composed of
+  cartilage and bone, and the other an exo-skeleton composed of chitin. Here is a problem of a
+  totally different kind to that we have just been considering, but of so fundamental a character
+  that it must, if possible, be solved before passing on to the consideration of the cranial nerves
+  and the organs they supply.</p>
+
+  <div><span class="pagenum" id="page120">{120}</span></div>
+
+  <p class="sp3">Is there any evidence which makes it possible to conceive the method by which the
+  vertebrate skeleton may have arisen from the skeletal tissues of an arthropod? By the vertebrate
+  skeleton I mean the bony and cartilaginous structures which form the backbone, the cranio-facial
+  skeleton, the pectoral and pelvic girdles, and the bones of the limbs. I do not include the
+  notochord in these skeletal tissues, because there is not the slightest evidence that the
+  notochord played any part in the formation of these structures; the notochordal tissue is
+  something <i>sui generis</i>, and never gives rise to cartilage or bone. The notochord happens to
+  lie in the middle line of the body and is very conspicuous in the lowest vertebrate; with the
+  development of the backbone the notochord becomes obliterated more and more, until at last it is
+  visible in the higher vertebrates only in the embryo; but that obliteration is the result of the
+  encroachment of the growing bone-masses, not the cause of their growth. Although, then, the
+  notochord may in a sense be spoken of as the original supporting axial rod of the vertebrate, it
+  is so different to the rest of the endo-skeleton, has so little to do with it, that the
+  consideration of its origin is a thing apart, and must be treated by itself without reference to
+  the origin of the cartilaginous and bony skeleton.</p>
+
+  <p class="ac"><span class="sc">The Commencement of the Bony Skeleton in the Vertebrate.</span></p>
+
+  <p>What is the teaching of the vertebrate? What evidence is there as to the origin of the bony
+  skeleton in the vertebrate phylum itself?</p>
+
+  <p>The axial bony skeleton of the higher Mammalia consists of two parts, (1) the vertebral column
+  with its attached bony parts, and (2) the cranio-facial skeleton. Of these two parts, the bony
+  tissue of the first arises in the embryo from cartilage, of the second partly from cartilage,
+  partly from membrane.</p>
+
+  <p>In strict accordance with their embryonic origin is their phylogenetic origin: as we pass from
+  the higher vertebrates to the lower these structures can be traced back to a cartilaginous and
+  membranous condition, so that, as Parker has shown, the cranio-facial bony skeleton of the higher
+  vertebrates can be derived directly from a non-bony cartilaginous skeleton, such as is seen in
+  Petromyzon and the cartilaginous fishes.</p>
+
+  <p>Balfour, in his "Comparative Embryology," states that the <span class="pagenum"
+  id="page121">{121}</span>primitive cartilaginous cranium is always composed of the following
+  parts<span class="wnw">:&mdash;</span></p>
+
+  <p>1. A pair of cartilaginous plates on each side of the cephalic section of the notochord known
+  as the parachordals (<i>pa.ch.</i>, Fig. <a href="#fig49">49</a>; <i>iv.</i>, Fig. <a
+  href="#fig48">48</a>). These plates, together with the notochord (<i>ch.</i>) enclosed between
+  them, form a floor for the hind and mid-brain.</p>
+
+  <table class="mc tlf sp2 w55" summary="Group of illustrations" title="Group of illustrations">
+    <tr>
+      <td class="vbm ac" style="width:43%;"><a href="images/fig048.png" id="fig48"><img
+      style="width:100%" src="images/fig048.png" alt="" title=""/></a></td>
+      <td class="vbm ac" style="width:43%;"><a href="images/fig049.png" id="fig49"><img
+      style="width:100%" src="images/fig049.png" alt="" title=""/></a></td>
+    </tr>
+    <tr>
+      <td>
+        <div class="smaller aj it1">
+          <p><span class="sc">Fig. 48.&mdash;Embryo Pig, two-thirds of an inch long</span> (from
+          <span class="sc">Parker</span>), <span class="sc">Elements of Skull seen from
+          below</span>.</p>
+          <p class="sp0"><i>ch.</i>, notochord; <i>iv.</i>, parachordals; <i>au.</i>, auditory
+          capsule; <i>py.</i>, pituitary body; <i>tr.</i>, trabecula; <i>ctr.</i>, trabecular cornu;
+          <i>pn.</i>, pre-nasal cartilage; <i>ppg.</i>, palato-pterygoid tract; <i>mn.</i>,
+          mandibular arch; <i>th.h.</i>, first branchial arch; <i>VII.-XII.</i>, cranial nerves.</p>
+        </div>
+      </td>
+      <td>
+        <div class="smaller aj it1">
+          <p><span class="sc">Fig. 49.&mdash;Head of Embryo Dog-fish</span> (from <span
+          class="sc">Parker</span>), <span class="sc">Basal View of Cranium from above</span>.</p>
+          <p class="sp0"><i>ol.</i>, olfactory sacs; <i>au.</i>, auditory capsule; <i>py.</i>,
+          pituitary body; <i>pa.ch.</i>, parachordal cartilage; <i>tr.</i>, trabecula; <i>inf.</i>,
+          infundibulum; <i>pt.s.</i>, pituitary space; <i>e.</i>, eye.</p>
+        </div>
+      </td>
+    </tr>
+  </table>
+
+  <p>2. A pair of bars forming the floor for the fore-brain, known as the trabeculæ (<i>tr</i>).
+  These bars are continued forward from the parachordals. They meet posteriorly and embrace the
+  front end of the notochord, and after separating for some distance bend in again in such a way as
+  to enclose a space&mdash;the pituitary space (<i>pt.s.</i>). In <span class="pagenum"
+  id="page122">{122}</span>front of this space they remain in contact, and generally unite. They
+  extend forward into the nasal region (<i>pn.</i>).</p>
+
+  <p>3. The cartilaginous capsules of the sense organs. Of these the auditory (<i>au.</i>) and the
+  olfactory capsules (<i>ol.</i>) unite more or less intimately with the cranial walls; while the
+  optic capsules, forming the usually cartilaginous sclerotics, remain distinct.</p>
+
+  <p>The parachordals and notochord form together the basilar plate, which forms the floor for that
+  section of the brain belonging to the primitive postoral part of the head, and its extent
+  corresponds roughly to that of the basioccipital of the adult skull.</p>
+
+  <p>The trabeculæ, so far as their mere anatomical relations are concerned, play the same part in
+  forming the floor for the front cerebral vesicle as do the parachordals for the mid- and
+  hind-brain. They differ, however, from the parachordals in one important feature, viz. that except
+  at their hinder end they do not embrace the notochord. The notochord always terminates at the
+  infundibulum, and the trabeculæ always enclose a pituitary space, in which lies the infundibulum
+  (<i>inf.</i>) and the pituitary body (<i>py.</i>).</p>
+
+  <p>In the majority of the lower forms the trabeculæ arise quite independently of the parachordals,
+  though the two sets of elements soon unite.</p>
+
+  <p>The trabeculæ are usually somewhat lyre-shaped, meeting in front and behind, and leaving a
+  large pituitary space between their middle parts. Into this space the whole base of the fore-brain
+  primitively projects, but the space itself gradually becomes narrowed until it usually contains
+  only the pituitary body.</p>
+
+  <p>The trabecular floor of the brain does not long remain simple. Its sides grow vertically
+  upwards, forming a lateral wall for the brain, in which in the higher types, two regions may be
+  distinguished, viz. an alisphenoidal region behind, growing out from what is known as the
+  basisphenoidal region of the primitive trabeculæ, and an orbito-sphenoidal region in front,
+  growing out from the presphenoidal region of the trabeculæ. These plates form at first a
+  continuous lateral wall of the cranium. The cartilaginous walls which grow up from the trabecular
+  floor of the cranium generally extend upwards so as to form a roof, though almost always an
+  imperfect roof, for the cranial cavity.</p>
+
+  <p>The basi-cranial cartilaginous skeleton reduces itself always into trabeculæ and parachordals
+  with olfactory and auditory cartilaginous capsules.</p>
+
+  <div><span class="pagenum" id="page123">{123}</span></div>
+
+  <p>In addition, a branchial skeleton exists, which consists of a series of bars known as the
+  branchial bars, so situated as to afford support to the successive branchial pouches. An anterior
+  arch known as the mandibular arch (Fig. <a href="#fig50">50</a>, <i>Mn.</i>), placed in front of
+  the hyo-mandibular cleft, and a second arch, known as the hyoid arch (<i>Hy.</i>), placed in front
+  of the hyo-branchial cleft, are developed in all types; the succeeding arches are known as the
+  true branchial arches (<i>Br.</i>), and are only fully developed in the Ichthyopsida. In all cases
+  of jaw-bearing (gnathostomatous) vertebrates the first arch has become a supporting skeleton for
+  the mouth (Fig. <a href="#fig51">51</a>), and in the higher vertebrates in combination with the
+  second or hyoid arch takes part in the formation of the ear-bones.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig050.png" id="fig50"><img style="width:62%" src="images/fig050.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 50.&mdash;Head of Embryo Dog-fish, eleven lines
+      long.</span> (From <span class="sc">Parker</span>.)</p>
+      <p class="sp0"><i>Tr.</i>, trabecula; <i>Mn.</i>, mandibular cartilage; <i>Hy.</i>, hyoid
+      arch; <i>Br<sub>1</sub>.</i>, first branchial arch; <i>Na.</i>, olfactory sac; <i>E.</i>, eye;
+      <i>Au.</i>, auditory capsule; <i>Hm.</i>, hemisphere; <i>C<sub>1</sub></i>,
+      <i>C<sub>2</sub></i>, <i>C<sub>3</sub></i>, cerebral vesicles.</p>
+    </div>
+  </div>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig051.png" id="fig51"><img style="width:100%" src="images/fig051.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 51.&mdash;Skull of Adult Dog-fish, Side View.</span> (From <span
+      class="sc">Parker</span>.)</p>
+      <p class="sp0"><i>cr.</i>, cranium; <i>Br.</i>, branchial arches; <i>Mn.</i> + <i>Hy.</i>,
+      mandibular and hyoid arches.</p>
+    </div>
+  </div>
+
+  <p>The true branchial arches persist, to a certain extent, in the Amphibia, and become still more
+  degenerated in the Amniota (reptiles, birds, and mammals) in correlation with the total
+  disappearance of a branchial respiration at all periods of their life. <span class="pagenum"
+  id="page124">{124}</span>Their remnants become more or less important parts of the hyoid bone, and
+  are employed solely in support of the tongue.</p>
+
+  <p>In no single animal is there any evidence that the foremost arch, the mandibular, is a true
+  branchial arch. As low down as the Elasmobranchs it becomes divided into two elements which form
+  respectively the upper and lower jaws; the hyoid arch, on the other hand, although it has altered
+  its form and acquired the secondary function of supporting the mandibular arch, still retains its
+  respiratory function.</p>
+
+  <p>The evidence afforded by the mode of formation of the skeletal tissues of vertebrates down to
+  the Elasmobranchs indicates that the primitive cranial skeleton arose from two paired basal
+  cartilages, the parachordals and trabeculæ, to which were attached respectively cartilaginous
+  cases enclosing the organs of hearing and smell. In addition, the branchial portion of the cranial
+  region was provided with cartilaginous bars arranged serially for the support of the branchiæ,
+  with the exception of the foremost, the mandibular bar, which formed supporting tissues for the
+  mouth&mdash;the upper and lower jaws.</p>
+
+  <p>Just as in past times the spinal nerves and the segments they supplied were supposed to
+  represent the type on which the original vertebrate was built, so also the spinal vertebræ
+  afforded the type of the segmented skeleton, and the anatomists of those days strove hard to
+  resolve the cranio-facial skeleton into a series of modified vertebræ. Owing especially to the
+  labours of Huxley, who showed that the segmentation in the head-region was essentially a
+  segmentation due to the presence of branchial bars, this conception was finally laid to rest and
+  nowadays it is admitted to be hopeless to resolve the cranium into vertebral segments. Still,
+  however, the vertebrate is a segmented animal and its segmented nature is visible in the cranial
+  region, so far as the skeletal tissues are concerned, in the shape of the series of branchial and
+  visceral bars.</p>
+
+  <p>To this segmentation the name of 'branchiomeric' has been given, while that due to the presence
+  of vertebræ is called 'mesomeric.'</p>
+
+  <p>As we have seen, the internal bony skeleton of the vertebrate commences as a cartilaginous and
+  membranous skeleton. For this reason the preservation of such skeletons is impossible in the
+  fossil form, unless the cartilage has become impregnated with lime salts, so that there is but
+  little hope of ever obtaining traces of such <span class="pagenum"
+  id="page125">{125}</span>structures in the fossils of the Silurian age either among the vertebrate
+  or invertebrate remains. Fortunately for this investigation there are still living on the earth
+  two representatives of that age; on the invertebrate side Limulus, and on the vertebrate side
+  Ammoc&#x0153;tes.</p>
+
+  <p>The Elasmobranchs represent the most primitive of the gnathostomatous vertebrates. Below them
+  come the Agnatha, known as the cyclostomatous fishes or Marsipobranchii, the lampreys (Petromyzon)
+  and the hag-fishes (Myxine).</p>
+
+  <p>The skeleton of Petromyzon (Fig. <a href="#fig52">52</a>) consists of a cranio-facial skeleton
+  composed of a cartilaginous unsegmented cranium, with the basal trabeculæ and parachordals and a
+  series of branchial and visceral cartilaginous bars forming the so-called branchial basket-work;
+  to these must be added auditory and nasal capsules. In contradistinction to this elaborate
+  cranio-facial skeleton, the spinal vertebral skeleton is represented only by segmentally arranged
+  small pieces of cartilage formed in the connective tissue dissepiments between segmented sheets of
+  body-muscles (myotomes).</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig052.png" id="fig52"><img style="width:100%" src="images/fig052.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 52.&mdash;Skeleton of Petromyzon.</span> (From <span
+      class="sc">Parker</span>.)</p>
+      <p class="sp0"><i>na.</i>, nasal capsule; <i>au.</i>, auditory capsule; <i>nc.</i>,
+      notochord.</p>
+    </div>
+  </div>
+
+  <p>But Petromyzon is derived from Ammoc&#x0153;tes by a remarkable process of transformation, and
+  a most important part of that transformation is the formation of new cartilaginous structures.
+  Thus we see that in Ammoc&#x0153;tes there is no sign of a cartilaginous vertebral column; at
+  transformation the rudimentary vertebræ of Petromyzon are formed. In Ammoc&#x0153;tes the
+  brain-case is a simple fibrous membranous covering; at transformation this becomes cartilaginous.
+  In Ammoc&#x0153;tes there are no cartilaginous structures corresponding to the sub-ocular arches;
+  these are all formed at transformation. It follows, that we can trace back the bony skeleton of
+  the vertebrate head to the skeleton of Ammoc&#x0153;tes, and we may therefore conclude <span
+  class="pagenum" id="page126">{126}</span>that the primitive cartilaginous skeleton of the
+  vertebrate consisted of the following structures (Fig. <a href="#fig53">53</a>, B), viz. the
+  branchial bars forming a basket-work, the trabeculæ and parachordals, the auditory and nasal
+  capsules&mdash;a clear proof that the cranial skeleton is older than the spinal. Of these
+  structures the branchial bars are the only evidently segmented parts.</p>
+
+  <div class="ac w50 fcenter sp3">
+    <a href="images/fig053.png" id="fig53"><img style="width:50%" src="images/fig053.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 53.&mdash;Comparison of Cartilaginous Skeleton of Limulus
+      and Ammoc&#x0153;tes.</span></p>
+      <p>A, Diagram of cartilaginous skeleton of Limulus. <i>Soft cartilage</i>, entapophysial
+      ligaments, deep black; branchial bars simply hatched; <i>hard cartilage</i>, lateral trabeculæ
+      of entosternite, netted; <i>Ph.</i>, position of pharynx.</p>
+      <p class="sp0">B, Diagram of cartilaginous skeleton of Ammoc&#x0153;tes. <i>Soft
+      cartilage</i>, sub-chordal cartilaginous bands, deep black; branchial basket-work (first
+      formed part), simply hatched; <i>hard cartilage</i>, cranio-facial skeleton, trabeculæ,
+      parachordals and auditory capsules, netted; <i>Inf.</i>, position of tube of infundibulum (old
+      &#x0153;sophagus).</p>
+    </div>
+  </div>
+
+  <p class="ac"><span class="sc">The Soft Cartilage of the Branchial Skeleton of
+  Ammoc&#x0153;tes.</span></p>
+
+  <p>The study of Ammoc&#x0153;tes gives yet another clue to the nature of the earliest skeleton,
+  for these two marked groups of cartilage&mdash;the branchial and basi-cranial&mdash;are
+  characterized by a difference in structure as well as a difference in topographical position. J.
+  Müller was the first to point out that these two sets of cartilages differ in appearance and
+  constitution, and he gave to them the name of yellow and grey cartilage. Parker has described them
+  fully under the terms soft and hard cartilage, terms which Schaffer has also used, and I shall
+  also make use of them here. The whole of the branchial cartilaginous skeleton is composed of soft
+  cartilage, while the basi-cranial skeleton, consisting of trabeculæ, parachordals, and auditory
+  capsule, is composed <span class="pagenum" id="page127">{127}</span>of hard cartilage, the only
+  soft cartilage in this region being that which forms the nasal capsule, not represented in Fig. <a
+  href="#fig53">53</a>, B.</p>
+
+  <p>These two groups of cartilage arise independently, so that at first the basi-cranial system is
+  quite separate from the branchial, and only late in the history of the animal is a junction
+  effected between the branchial system and the trabeculæ and parachordals, an initial separation
+  which is especially striking when we consider that in this animal all the cartilaginous structures
+  of any one system are continuous: there is no sign of anything in the nature of joints.</p>
+
+  <p>Of these two main groups, the branchial cartilages are formed first in the embryo, a fact which
+  suggests that they are the most primitive of the vertebrate cartilages, and that, therefore, the
+  first true formation of cartilage in the invertebrate ancestor may be looked for in the shape of
+  bars supporting the branchial mechanism. The evidence of the origin of the cartilaginous
+  structures in Ammoc&#x0153;tes is given by Shipley in the following words<span
+  class="wnw">:&mdash;</span></p>
+
+  <p>"The branchial bases are the first part of the skeleton to appear. They arise about the 24th
+  day as straight bars of cartilage, lying external and slightly posterior to the branchial
+  vessel.</p>
+
+  <p>"The first traces of the basi-cranial skeleton appear on the 30th day as two rods of
+  cartilage&mdash;the trabeculæ."</p>
+
+  <p>Our attention must, in the first place, be directed to this branchial basket-work of
+  Ammoc&#x0153;tes.</p>
+
+  <p>Underlying the skin of Ammoc&#x0153;tes in the branchial region is situated the sheet of
+  longitudinal body-muscles, divided into a series of segments or myotomes, which forms the somatic
+  muscles so characteristic of all fishes. This muscular sheet is depicted on the left-hand side of
+  Fig. <a href="#fig54">54</a>. It does not extend over the lower lip or over that part in the
+  middle line where the thyroid gland is situated. In these parts a sheet of peculiar tissue known
+  by the name of muco-cartilage lies immediately under the skin, covering over the thyroid gland and
+  lower lip. The somatic muscular sheet with the superjacent skin can be stripped off very easily
+  owing to the vascularity and looseness of the tissue immediately underlying it. When this is done
+  the branchial basket-work comes beautifully into view as is seen on the right-hand side of Fig. <a
+  href="#fig54">54</a>. It forms a cage within which the branchiæ and their muscles lie entirely
+  concealed.</p>
+
+  <p>This is the great characteristic of this most primitive form of the branchial cartilaginous
+  bars and distinguishes it from the branchial <span class="pagenum" id="page128">{128}</span>bars
+  of other higher fishes, in that it forms a system of cartilages which lie external to the
+  branchiæ&mdash;an extra-branchial system.</p>
+
+  <p>This branchial basket-work is simpler in Ammoc&#x0153;tes than in Petromyzon, and its actual
+  starting-point consists of a main transverse bar corresponding to each branchial segment; from
+  this transverse bar the system of longitudinal bars by which the basket-work is formed has sprung.
+  These transverse bars arise from a cartilaginous longitudinal rod, situated close against the
+  notochord on each side. These rods may be called the subchordal cartilaginous bands (Fig. <a
+  href="#fig53">53</a>), and, according to the observations of Schneider and others, each subchordal
+  band does not form at first a continuous cartilaginous rod, but the cartilage is conspicuous only
+  at the places where the transverse bars arise. In the youngest Ammoc&#x0153;tes examined by
+  Schaffer, he could find no absolute discontinuity of the cartilage except between the first two
+  transverse bars, but he says that the thinning between the transverse bars was so marked as to
+  make it highly probable that at an earlier stage there was discontinuity. The whole system of
+  branchial bars and subchordal rods is at first absolutely disconnected from the cranial system of
+  trabeculæ and parachordals, and only later do the two systems join.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig054.png" id="fig54"><img style="width:47%" src="images/fig054.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 54.&mdash;Ventral View of Head Region of Ammoc&#x0153;tes.</span></p>
+      <p class="sp0"><i>Th.</i>, thyroid gland; <i>M.</i>, lower lip, with its muscles.</p>
+    </div>
+  </div>
+
+  <p>These observations on Ammoc&#x0153;tes lead most definitely to the conclusion that the
+  starting-point of the whole cartilaginous skeleton of the vertebrate consisted of a series of
+  transverse cartilaginous bars, for the purpose of supporting branchial segments; these were
+  connected with two axial longitudinal cartilaginous rods, which at first contained cartilage only
+  near the places of junction of the branchial <span class="pagenum" id="page129">{129}</span>bars.
+  This system may be called the mesosomatic skeleton, as it is entirely confined to the branchial or
+  mesosomatic region.</p>
+
+  <p class="sp3">In addition to this primitive cartilaginous framework, which was formed for the
+  support of the mesosomatic or respiratory segments, but at a slightly later period in the
+  phylogenetic history, a separate cartilaginous system was formed for the support of the prosomatic
+  segments, viz. the trabeculæ and parachordals with the auditory capsules: a system which was at
+  first entirely separated from the mesosomatic, and, as we shall see, is more advanced in structure
+  than the branchial system. Later still, the story is completed at the time of transformation to
+  Petromyzon by the formation of the simple cartilaginous skull and the rudimentary vertebræ, the
+  structure of which is also of a more advanced type.</p>
+
+  <p class="ac"><span class="sc">The Structure of the Soft Branchial Cartilage.</span></p>
+
+  <p>Having considered the topographical position of the primitive branchial cartilaginous skeleton,
+  we may now inquire, What was its structure and how was it formed?</p>
+
+  <p>In the higher vertebrates various forms of cartilage are described, viz. hyaline,
+  fibro-cartilage, elastic cartilage, and parenchymatous cartilage. Of these, the parenchymatous
+  cartilage is looked upon as the most primitive form, because it preserves without modification the
+  characters of embryonic cartilage.</p>
+
+  <p>Embryology, then, would lead to the belief that the earliest form of cartilage in the
+  vertebrate kingdom ought to be of this type, viz. large cells, each of which is enclosed in a
+  simple capsule, so that the capsules of the cells form the whole of the matrix, and thus form a
+  simple homogeneous honeycomb-structure, in the alveoli of which the cartilage-cells lie singly.
+  If, then, the branchial cartilages of Ammoc&#x0153;tes are, as has just been argued, the
+  representatives of the cartilaginous skeleton of the primitive vertebrate, it is reasonable to
+  suppose that they should resemble in structure this embryonic cartilage. Such is undoubtedly the
+  case: all observers who have described the branchial basket-work of Ammoc&#x0153;tes or Petromyzon
+  have been struck with the extremely primitive character of the cartilage, and the last observer
+  (Schaffer) describes it as composed of thin walls of homogeneous material, in which there are no
+  lines of separation, which form a simple honeycomb-structure, in the alveoli <span class="pagenum"
+  id="page130">{130}</span>of which the separate cells lie singly. These branchial cartilages are
+  each surrounded by a layer of perichondrium, and in Fig. <a href="#fig55">55</a>, A, I give a
+  picture of a section of a portion of one of the bars.</p>
+
+  <div class="ac w50 fcenter sp2">
+    <a href="images/fig055.jpg" id="fig55"><img style="width:100%" src="images/fig055.jpg" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p class="sp0"><span class="sc">Fig. 55.&mdash;A, Branchial Cartilage of Ammoc&#x0153;tes,
+      stained with Thionin. B, Branchial Cartilage of Limulus, stained with Thionin.</span></p>
+    </div>
+  </div>
+
+  <p class="sp3">Hence we see that structurally as well as topographically the branchial bars of
+  Ammoc&#x0153;tes justify their claim to be considered as the origin of the vertebrate
+  cartilaginous framework.</p>
+
+  <p class="ac"><span class="sc">On the Structure of the Muco-cartilage in
+  Ammoc&#x0153;tes.</span></p>
+
+  <p>We can, however, go further than this, and ask how this cartilage itself is formed in
+  Ammoc&#x0153;tes? The answer is most definite, most instructive and suggestive, for in all cases
+  this particular kind of cartilage is formed from, or at all events in, a peculiar fibrous tissue,
+  which was called by Schneider "Schleim-Knorpel," or muco-cartilage, a tissue which is
+  distinguishable from other connective tissues, not only by its structural peculiarities, but also
+  by its strong affinity for all dyes which differentiate mucoid or chondro-mucoid substances.</p>
+
+  <p>This muco-cartilage is thus described by Schneider:&mdash;The perichondrium in Ammoc&#x0153;tes
+  is not confined to the true cartilaginous structures, but extends itself in the form of thin
+  plates in definite directions. Between these plates of perichondrium a peculiar tissue (Fig. <a
+  href="#fig56">56</a>)&mdash;the muco-cartilage&mdash;exists, consisting of fibrillæ, whose
+  direction is mainly at right angles to the planes of the perichondrial plates, with star-shaped
+  cells in among them, and with the spaces between the fibrillæ filled up with a semi-fluid
+  mass.</p>
+
+  <div><span class="pagenum" id="page131">{131}</span></div>
+
+  <p>From this tissue all the primitive cartilages which resemble the branchial bars are formed,
+  either by the invasion of chondroblasts from the surrounding perichondrium, or by the
+  proliferation and encapsulation of the cells of the muco-cartilage itself.</p>
+
+  <div class="ac w25 fcenter sp2">
+    <a href="images/fig056.png" id="fig56"><img style="width:100%" src="images/fig056.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p class="sp0"><span class="sc">Fig. 56.&mdash;Section of Muco-cartilage from Dorsal
+      Head-plate of Ammoc&#x0153;tes.</span></p>
+    </div>
+  </div>
+
+  <p>This very distinctive tissue&mdash;the muco-cartilage&mdash;is of very great importance in all
+  questions of the origin of the skeletal tissues. In all descriptions of the skeletal tissues it
+  has been practically disregarded until recent years when, besides my own observations, its
+  distribution has been mapped out by Schaffer. Thus Parker, in his well-known description of the
+  skeleton of the marsipobranch fishes, does not even mention its existence. Its importance is shown
+  by its absolute disappearance at transformation and its non-occurrence in any of the higher
+  vertebrates. It is entirely confined to the head-region, and its distribution there is most
+  suggestive, for, as will be described fully later on, it forms a skeleton which both in structure
+  and position resembles very closely the head-shields of cephalaspidian fishes. At the present part
+  of my argument its more immediate interest lies in the method of tracing this tissue. For this
+  purpose I made use of the micro-chemical reaction of thionin, a dye which, as shown by Hoyer,
+  stains all mucin-containing substances a bright purple. Schaffer made use of a corresponding
+  basophil stain, hæmalum. When stained with thionin, the matrix, or ground-substance of the
+  branchial cartilages as well as the matrix or semi-fluid substance in which the fibrils of the
+  muco-cartilaginous cells are embedded take on a deep purple colour, while the fibrous material of
+  the cranial walls and other connective tissue strands, such as the perichondrium, are coloured
+  light blue. Muco-cartilage, then, may be described as a peculiar form of connective tissue which
+  differs from other connective tissue not only in its appearance but in <span class="pagenum"
+  id="page132">{132}</span>its chemical composition, for unlike white fibrous tissue it contains a
+  large amount of mucin, and this tissue is the forerunner of the earliest cartilaginous vertebrate
+  skeleton, the branchial bars of Ammoc&#x0153;tes.</p>
+
+  <p>The conclusions to which we are led by the study of the structure, position, and mode of origin
+  of these primitive cartilages of Ammoc&#x0153;tes may be thus summed up<span
+  class="wnw">:&mdash;</span></p>
+
+  <p>1.  The immediate ancestor of the vertebrate must have possessed a peculiar fibrous
+  tissue&mdash;the ground-substance of which stained deep purple with thionin&mdash;in which
+  cartilage arose.</p>
+
+  <p>2.  The cartilage so formed was not like hyaline cartilage, but resembled in a striking manner
+  parenchymatous cartilage.</p>
+
+  <p class="sp3">3.  This cartilage was situated partly in two axial longitudinal bands, partly as
+  transverse bars, which supported the branchial apparatus.</p>
+
+  <p class="ac"><span class="sc">The Prosomatic or Basi-cranial Skeleton of
+  Ammoc&#x0153;tes.</span></p>
+
+  <p>Before searching for any evidence of a similar tissue in any invertebrate group, it is
+  advisable to consider the other portion of the cartilaginous skeleton of Ammoc&#x0153;tes, which
+  consists of the trabeculæ, parachordals and auditory capsules&mdash;the basi-cranial
+  skeleton&mdash;and is composed of hard, not soft cartilage.</p>
+
+  <p>This basi-cranial skeleton represented in Fig. <a href="#fig53">53</a>, B, is confined to the
+  region of the notochord, the cranial walls being composed entirely of a white fibrous membrane. It
+  is separated at first entirely from the sub-chordal portion of the branchial basket-work, and is
+  composed of a foremost part, the trabeculæ (<i>Tr.</i>), and of a hindermost part, the
+  parachordals (<i>Pr.ch.</i>), which are characterized by the attachment on each side of the large
+  auditory capsule (<i>Au.</i>). In Ammoc&#x0153;tes the trabecular bars are continuous with the
+  parachordals, the junction being marked by a small lateral projection on each side, which at
+  transformation is seen to play an important part in the formation of the sub-ocular arch. The
+  trabecular bar lies close against the notochord on each side up to its termination; it then bends
+  away from the middle line and curves round until it meets its fellow on the opposite side, thus
+  forming, as it were, the head of a racquet of which the notochord forms the splice in the handle.
+  The strings of the racquet are represented by a thin membrane, in the centre of which the position
+  of the infundibulum (<i>Inf.</i>) of the <span class="pagenum" id="page133">{133}</span>brain can
+  be clearly seen. In an earlier stage of Ammoc&#x0153;tes the two trabecular horns do not meet, but
+  are separated by connective tissue, which afterwards becomes cartilaginous.</p>
+
+  <p>As far, then, as the topography of this basi-cranial skeleton is concerned, the striking points
+  are&mdash;the shape of the trabecular portion, diverging as it does around the infundibulum, and
+  the presence on the parachordal portion of the two large auditory capsules.</p>
+
+  <p>These two points indicate, on the hypothesis that infundibulum and &#x0153;sophagus are
+  convertible terms, that two supporting structures of a cartilaginous nature must have existed in
+  the ancestor of the vertebrate, the first of which surrounded the &#x0153;sophagus, and the second
+  was in connection with its auditory apparatus.</p>
+
+  <div class="ac w45 fcenter sp3">
+    <a href="images/fig057.jpg" id="fig57"><img style="width:100%" src="images/fig057.jpg" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="sp0"><span class="sc">Fig. 57.&mdash;A, Cartilage of Trabeculæ of Ammoc&#x0153;tes,
+      stained with Hæmatoxylin and Picric Acid. B, Nests of Cartilage Cells in Entosternite of
+      Hypoctonus, stained with Hæmatoxylin and Picric Acid.</span></p>
+    </div>
+  </div>
+
+  <p class="ac"><span class="sc">Structure of the Hard Cartilages.</span></p>
+
+  <p>The structure of this hard cartilage of the trabeculæ and auditory capsules resembles that of
+  the soft, in so far that it consists of large cells with a comparatively small amount of
+  intercellular substance. Schaffer, who has described it lately, considers that it is a nearer
+  approach to hyaline cartilage than the soft, but yet cannot be called hyaline cartilage in the
+  usual sense of the term. Its peculiarities and its differences from the soft are especially well
+  seen by its staining reactions. I have myself been particularly struck with the effect of
+  picrocarmine or combined hæmatoxylin and picric acid <span class="pagenum"
+  id="page134">{134}</span>staining (Fig. <a href="#fig57">57</a>). In the case of the soft
+  cartilage the capsular substance stains respectively a brilliant red or blue, while that of the
+  hard cartilage is coloured a deep yellow, so that the junction between the parachordals and the
+  branchial cartilages is beautifully marked out. Then, again, with thionin, which gives so marked a
+  reaction in the case of the soft cartilage, the hard cartilage of the auditory capsule is not
+  stained at all, and in the trabeculæ the deep purple colour is confined to the mucoid
+  cement-substance between the capsules, just as Schaffer has stated. The same kinds of reactions
+  have been described by Schaffer: thus by double staining with hæmalum-eosin the hard cartilage
+  stains red, the soft blue; and he points out that even with over-staining by hæmalum the auditory
+  capsule remains colourless, just as I have noticed with thionin. He infers, precisely as I have
+  done from the thionin reaction, that chondro-mucoid, which is so marked a constituent of the soft
+  cartilage and of the muco-cartilage, is absent or present in but slight quantities in the hard
+  cartilage. Similarly, he points out that double staining with trop&#x0153;olin-methyl-violet
+  stains the hard cartilage a bright orange colour, and the soft cartilage a violet.</p>
+
+  <p>The evidence, then, shows clearly that a marked chemical difference exists between these two
+  cartilages, which may be expressed by saying that the one contains very largely a basophil
+  substance, which we may speak of as belonging to the class of chondro-mucoid substances, while the
+  other contains mainly an oxyphil substance, probably a chondro-gelatine substance.</p>
+
+  <p>We may perhaps go further and attribute this difference of composition to a difference of
+  origin; for whereas the soft cartilage is invariably formed in a special tissue, the
+  muco-cartilage, which shows by its reaction how largely it is composed of a mucoid substance, the
+  hard cartilage is certainly, in the case of the cartilage of the cranium where its origin has been
+  clearly made out, formed in the membranous tissue of the cranium of
+  Ammoc&#x0153;tes&mdash;<i>i.e.</i> in a tissue which stains light blue with thionin, and contains
+  a gelatinous rather than a mucoid substratum.</p>
+
+  <p>The best opportunity of finding out the mode of origin of the hard cartilage is afforded at the
+  time of transformation, when so much of this kind of cartilage is formed anew. Unfortunately, it
+  is very difficult to obtain the early transformation stages, consequently we cannot be said to
+  possess any really exhaustive and <span class="pagenum" id="page135">{135}</span>definite account
+  of how the new cartilages are formed. Bujor, Kaensche, and Schaffer all profess to give a more or
+  less definite account of their formation, and the one striking impression left on the mind of the
+  reader is how their descriptions vary. In one point only are they agreed, and in that I also agree
+  with them, viz. the manner in which the new cranial walls are formed. Schaffer describes the
+  process as the invasion of chondroblasts into the homogeneous fibrous tissue of the cranial walls.
+  Such chondroblasts not only form the cartilaginous framework, but also assimilate the fibrous
+  tissue which they invade, so that finally all that remains of the original fibrous matrix in which
+  the cartilage was formed are these lines of cement-substance between the groups of cartilage
+  cells, which, containing some basophil material, are marked out, as already mentioned (Fig. <a
+  href="#fig57">57</a>).</p>
+
+  <p>We may therefore conclude, from the investigation of Ammoc&#x0153;tes, that the front part of
+  the basi-cranial skeleton arose as two trabecular bars, to which muscles were attached, situated
+  bilaterally with respect to the central nervous system. These bars were composed of tendinous
+  material with a gelatinous rather than a mucoid substratum, in which nests of cartilage-cells were
+  formed, the cartilaginous material formed by these cells being of the hard variety, not staining
+  with thionin, and staining yellow with picro-carmine, etc. By the increase of such nests and the
+  assimilation of the intermediate fibrous material, the original fibro-cartilage was converted into
+  the close-set semi-hyaline cartilage of the trabeculæ and auditory capsules, in which the fibrous
+  material still marks out by its staining-reaction the limits of the cell-clusters.</p>
+
+  <p>Such I gather to be Schaffer's conclusions, and they are certainly borne out by my own and Miss
+  Alcock's observations. As far as we have had an opportunity of observing at present, the first
+  process at transformation appears to consist of the invasion of the fibrous tissue of the cranial
+  wall by groups of cells which form nests of cells between the fibrous strands. These nests of
+  cells form round themselves capsular material, and thus form cell-territories of cartilage, which
+  squeeze out and assimilate the surrounding fibrous tissue, until at last all that remains of the
+  original fibrous matrix is the lines of cement-substance which mark out the limits of the various
+  cell-groups.</p>
+
+  <p>At present I am inclined to think that both soft and hard cartilage originate in a very similar
+  manner, viz. by the formation of capsular <span class="pagenum" id="page136">{136}</span>material
+  around the invading chondroblasts, and that the difference in the resulting cartilage is mainly
+  due to the difference in chemical composition of the matrix of the connective tissue which is
+  invaded. Thus the difference may be formulated as follows<span class="wnw">:&mdash;</span></p>
+
+  <p>The hard cartilage is formed by the invasion of chondroblasts into a fibrous tissue, which
+  contains a gelatinous rather than a mucoid substratum, in contradistinction to the soft cartilage
+  which is formed, probably also by the invasion of chondroblasts, in a tissue&mdash;the
+  muco-cartilage&mdash;which contains a specially mucoid substratum.</p>
+
+  <p class="sp3">Such, then, is the very clearly defined starting-point of the vertebrate
+  skeleton&mdash;two distinct formations of different histological and chemical structure,&mdash;the
+  one forming a segmented branchial skeleton, the other a non-segmented basi-cranial skeleton.</p>
+
+  <p class="ac"><span class="sc">The Cartilaginous Skeleton of Limulus.</span></p>
+
+  <p>Among the whole of the invertebrates at present living on the earth, is there any sign of an
+  internal cartilaginous skeleton that will give a direct clue to the origin of the primitive
+  vertebrate skeleton? The answer to this question is most significant: only one animal among all
+  those at present known possesses a cartilaginous skeleton, which is directly comparable with that
+  of Ammoc&#x0153;tes, and here the comparison is very close&mdash;only one animal among the
+  thousands of living invertebrate forms, and that animal is the only representative still surviving
+  of the palæostracan group, which was the dominant race when the vertebrate first made its
+  appearance. The Limulus, or king-crab, possesses a segmented branchial internal cartilaginous
+  skeleton (Fig. <a href="#fig53">53</a>, A), made up of the same kind of cartilage as the branchial
+  skeleton of Ammoc&#x0153;tes, confined to the mesosomatic or branchial region, just as in
+  Ammoc&#x0153;tes, forming, as in Ammoc&#x0153;tes, cartilaginous bars supporting the branchiæ, and
+  these bars are situated externally to the branchiæ, as in Ammoc&#x0153;tes. In addition this
+  animal possesses a basi-cranial internal semi-cartilaginous unsegmented plate known as the
+  entosternite or plastron situated, with respect to the &#x0153;sophagus, similarly to the position
+  of the trabeculæ with respect to the infundibulum in Ammoc&#x0153;tes. Moreover, the cartilaginous
+  cells in this tissue differ from those in the branchial region, in precisely the same manner as
+  the hard cartilage differs from the soft in Ammoc&#x0153;tes.</p>
+
+  <div><span class="pagenum" id="page137">{137}</span></div>
+
+  <p class="sp3">This plastron, it is true, is found in other animals, all of which are members of
+  the scorpion tribe, except in one instance, and this, strikingly enough, is the crustacean
+  Apus&mdash;a strange primitive form, which is acknowledged to be the nearest representative of the
+  Trilobita still living on the earth. None of these forms, however, possess any sign of an internal
+  cartilaginous branchial skeleton, such as is possessed by Limulus. Scorpions, Apus, Limulus, are
+  all surviving types of the stage of organization which had been reached in the animal world when
+  the vertebrate first appeared.</p>
+
+  <p class="ac"><span class="sc">The Mesosomatic or Respiratory Skeleton of Limulus, composed of
+  Soft Cartilage.</span></p>
+
+  <p>Searching through the literature of the histology of the cartilaginous tissues in invertebrate
+  animals, to see whether any cartilage had been described similar to that seen in the branchial
+  cartilages of Ammoc&#x0153;tes, and whether such cartilage, if found, arose in a fibrous tissue
+  resembling muco-cartilage, I was speedily rewarded by finding, in Ray Lankester's article on the
+  tropho-skeletal tissues of Limulus, a picture of the cartilage of Limulus, which would have passed
+  muster for a drawing of the branchial cartilage of Ammoc&#x0153;tes. This clue I followed out in
+  the manner described in my former paper in the <i>Journal of Anatomy and Physiology</i>, and
+  mapped out the topography of this remarkable tissue.</p>
+
+  <p>Limulus, like other water-dwelling arthropods, breathes by means of gills attached to its
+  appendages. These gill-bearing appendages are confined to the mesosomatic region, as is seen in
+  Fig. <a href="#fig59">59</a>; and these appendages are very different to the ordinary locomotor
+  appendages, which are confined to the prosomatic region. Each appendage, as is seen in Fig. <a
+  href="#fig58">58</a>, consists mainly of a broad, basal part, which carries the gill-book on its
+  under surface; the distal parts of the appendage have dwindled to mere rudiments and still exist,
+  not for locomotor purposes, but because they carry on each segment organs of special importance to
+  the animal (see Chapter XI.). As is seen in Fig. <a href="#fig58">58</a>, the basal parts of each
+  pair of appendages form a broad, flattened paddle, by means of which the animal is able to swim in
+  a clumsy fashion. Very striking and suggestive is the difference between these gill-bearing
+  mesosomatic appendages and the non-gill-bearing locomotor appendages of the prosoma.</p>
+
+  <div><span class="pagenum" id="page138">{138}</span></div>
+
+  <div class="ac w45 fcenter sp2">
+    <a href="images/fig058.png" id="fig58"><img style="width:100%" src="images/fig058.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p><span class="sc">Fig. 58.&mdash;Transverse Section through the Mesosoma of Limulus, to show
+      the Anterior (A) and the Posterior (B) Surfaces of a Mesosomatic or Branchial
+      Appendage.</span></p>
+      <p class="sp0">In each figure the branchial cartilaginous bar, <i>Br.C.</i>, has been exposed
+      by dissection on one side. <i>Ent.</i>, entapophysis; <i>Ent.l.</i>, entapophysial ligament
+      cut across; <i>Br.C.</i>, branchial cartilaginous bar, which springs from the entapophysis;
+      <i>H.</i>, heart; <i>P.</i>, pericardium; <i>Al.</i>, alimentary canal; <i>N.</i>, nerve cord;
+      <i>L.V.S.</i>, longitudinal venous sinus; <i>Dv.</i>, dorso-ventral somatic muscle;
+      <i>Vp.</i>, veno-pericardial muscle.</p>
+    </div>
+  </div>
+
+  <p>At the base of each of these appendages, where it is attached to the body of the animal, the
+  external chitinous surface is characterized by a peculiar stumpy, rod-like marking, and upon
+  removing the chitinous covering, this surface-appearance is seen to correspond to a well-marked
+  rod of cartilage (<i>Br.C.</i>), which extends from the body <span class="pagenum"
+  id="page139">{139}</span>of the animal well into each appendage. This bar of cartilage arises on
+  each side from the corresponding entapophysis (<i>Ent.</i>), which is the name given to a
+  chitinous spur which projects a short distance (Fig. <a href="#fig58">58</a>, B) into the animal
+  from the dorsal side, for the purpose of giving attachment to various segmental muscles. These
+  entapophyses are formed by an invagination of the chitinous surface on the dorsal side and are
+  confined to the mesosomatic region, so that the mesosomatic carapace indicates, by the number of
+  entapophyses, the number of segments in that region, in contradistinction to the prosomatic
+  carapace, which gives no indication on its surface of the number of its components.</p>
+
+  <p>Each entapophysis is hollow and its walls are composed of chitin; but from the apex of each
+  spur there stretches from spur to spur a band of tissue, called by Lankester the entapophysial
+  ligament (<i>Ent.l.</i>) (Fig. <a href="#fig58">58</a>), and in this tissue cartilage is formed.
+  Isolated cartilaginous cells, or rather groups of cells, are found here and there, but a
+  concentration of such groups always takes place at each entapophysis, forming here a solid mass of
+  cartilage, from which the massive cartilaginous bar of each branchial appendage arises.</p>
+
+  <p class="sp3">Further, not only is this cartilage exactly similar to parenchymatous cartilage, as
+  it occurs in the branchial cartilages of Ammoc&#x0153;tes, but also its matrix stains a brilliant
+  purple with thionin in striking contrast to the exceedingly slight light-blue colour of the
+  surrounding perichondrium. In its chemical composition it shows, as might be expected, that it is
+  a cartilage containing a very large amount of some mucin-body.</p>
+
+  <p class="ac"><span class="sc">The Muco-cartilage of Limulus.</span></p>
+
+  <p>The resemblance between this structure and that of the branchial bars of Ammoc&#x0153;tes does
+  not end even here, for, as already mentioned, the cartilage originates in a peculiar connective
+  tissue band, the entapophysial ligament, and this tissue bears the same relation in its chemical
+  reactions to the ordinary connective tissue of Limulus, as muco-cartilage does to the white
+  fibrous tissue of Ammoc&#x0153;tes. The white connective tissue of Limulus, as already stated,
+  resembles that of the vertebrate more than does the connective tissue of any other invertebrate,
+  and, similarly to that of Ammoc&#x0153;tes, does not stain, or gives only a light-blue tinge with
+  thionin. The tissue of <span class="pagenum" id="page140">{140}</span>the entapophysial ligament,
+  on the contrary, just like muco-cartilage, takes on an intense purple colour when stained with
+  thionin. It possesses a mucoid substratum, just as does muco-cartilage, and in both cases a
+  perfectly similar soft cartilage is born from it.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig059.jpg" id="fig59"><img style="width:100%" src="images/fig059.jpg" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 59.&mdash;Diagram of Limulus, to show the Nerves to the
+      Appendages (1-13) and the Branchial Cartilages.</span></p>
+      <p class="sp0">The branchial cartilages and the entapophysial ligaments are coloured blue, the
+      branchiæ red. <i>gl.</i>, generative and hepatic glands surrounding the central nervous system
+      and passing into the base of the flabellum (<i>fl.</i>).</p>
+    </div>
+  </div>
+
+  <p>One difference, however, exists between the branchial cartilages of these two animals; the
+  innermost axial layer of the branchial bar of Limulus is very apt to contain a specially hard
+  substance, apparently chalky in nature, so that it breaks up in sections, and gives the appearance
+  of a broken-down spongy mass; if, however, the tissue is first placed in a solution of
+  hydrochloric acid, it then cuts easily, and the whole tissue is seen to be of the same structure
+  throughout, the main difference being that the capsular spaces in the axial region are much larger
+  and much more free from cell-protoplasm than are those of the smaller younger cells near the
+  periphery.</p>
+
+  <div><span class="pagenum" id="page141">{141}</span></div>
+
+  <p>I have attempted in Fig. <a href="#fig53">53</a> to represent this close resemblance between
+  the segmented branchial skeleton of Limulus and of Ammoc&#x0153;tes, a resemblance so close as to
+  reach even to minute details, such as the thinning out of the cartilage in the subchordal bands
+  and entapophysial ligaments respectively between the places where the branchial bars come off.</p>
+
+  <div class="ac w45 fcenter sp2">
+    <a href="images/fig060.jpg" id="fig60"><img style="width:100%" src="images/fig060.jpg" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 60.&mdash;Diagram of Ammoc&#x0153;tes cut open to show the
+      Lateral System of Cranial Nerves</span> <i>V., VII., IX., X.</i>, <span class="sc">and the
+      Branchial Cartilages.</span></p>
+      <p class="sp0">The branchial cartilages and sub-chordal ligaments are coloured blue, the
+      branchiæ red. <i>gl.</i>, glandular substance surrounding the central nervous system and
+      passing into the auditory capsule with the auditory nerve (<i>VIII.</i>).</p>
+    </div>
+  </div>
+
+  <p>In Fig. <a href="#fig59">59</a> I have shown the prosoma and mesosoma of Limulus, and indicated
+  the nerves to the appendages together with the mesosomatic cartilaginous skeleton.</p>
+
+  <p class="sp3">In Fig. <a href="#fig60">60</a> I have drawn a corresponding picture of the
+  prosomatic and mesosomatic region of Ammoc&#x0153;tes with the corresponding nerves <span
+  class="pagenum" id="page142">{142}</span>and cartilages. In this figure the animal is supposed to
+  be slit open along the ventral mid-line and the central nervous system exposed.</p>
+
+  <p class="ac"><span class="sc">The Prosomatic Skeleton of Limulus, composed of Hard
+  Cartilage.</span></p>
+
+  <p>The rest of the primitive vertebrate skeleton arose in the prosomatic region, and formed a
+  support for the base of the brain. This skeleton was composed of hard cartilage, and arose in
+  white fibrous tissue containing gelatin rather than mucin.</p>
+
+  <p>Is there, then, any peculiar tissue of a cartilaginous nature in Limulus and its allies,
+  situated in the prosomatic region, which is entirely separate from the branchial cartilaginous
+  skeleton, which acts as a supporting internal framework, and contains a gelatinous rather than a
+  mucoid substratum?</p>
+
+  <p>It is a striking fact, common to the whole of the group of animals to which our inquiries,
+  deduced from the consideration of the structure of Ammoc&#x0153;tes, have, in every case, led us
+  in our search for the vertebrate ancestor, that they do possess a remarkable internal
+  semi-cartilaginous skeleton in the prosomatic region, called the entosternite or plastron, which
+  gives support to a large number of the muscles of that region; which is entirely independent of
+  the branchial skeleton, and differs markedly in its chemical reactions from that cartilage, in
+  that it contains a gelatinous rather than a mucoid substratum.</p>
+
+  <p>In Limulus it is a large, tough, median plate, fibrous in character, in which are situated rows
+  and nests of cartilage-cells. The same structure is seen in the plastron of Hypoctonus, of
+  Thelyphonus, and to a certainty in all the members of the scorpion group. Very different is the
+  behaviour of this tissue to staining from that of the branchial region. No part of the plastron
+  stains purple with thionin; it hardly stains at all, or gives only a very slight blue colour. In
+  its chemical composition there is a marked preponderance of gelatin with only a slight amount of a
+  mucin-body. In some cases, as in Hypoctonus (Fig. <a href="#fig57">57</a>, B) and Mygale, the
+  capsules of the cartilage-cells stain a deep yellow with hæmatoxylin and picric acid, while the
+  fibres between the cell-nests stain a blue-brown colour, partly from the hæmatoxylin, partly from
+  the picric acid.</p>
+
+  <p>All the evidence points to the plastron as resembling the basi-cranial skeleton of
+  Ammoc&#x0153;tes in its composition and in the origin <span class="pagenum"
+  id="page143">{143}</span>of its cells in a white fibrous tissue. What, then, is its topographical
+  position? It is in all cases a median structure lying between the cephalic stomach and the
+  infra-&#x0153;sophageal portion of the central nervous system, and in all cases it possesses two
+  anterior horns which pass around the &#x0153;sophagus and the nerve-masses which immediately
+  enclose the &#x0153;sophagus (Fig. <a href="#fig61">61</a>, A). These lateral horns, then, which
+  lie laterally and slightly ventral to the central nervous system, and are called by Ray Lankester
+  and Benham the sub-neural portion of the entosternite, are very nearly in exactly the position of
+  the racquet-shaped head of the trabeculæ in Ammoc&#x0153;tes. It is easy to see that, with a more
+  extensive growth of the nervous material dorsally, such lateral horns might be caused to take up a
+  still more ventral position. Now, these two lateral horns of the plastron of Limulus are continued
+  along its whole length so as to form two thickened lateral ridges, which are conspicuous on the
+  flat surface of the rest of this median plate. In other cases, as in the Thelyphonidæ, the
+  plastron consists mainly of these two lateral ridges or trabeculæ, as they might be called, and
+  Schimkéwitsch, who more than any one else has made a comparative study of the entosternite,
+  describes it as composed in these animals of two lateral trabeculæ crossed by three transverse
+  trabeculæ. I myself can confirm his description, and give in Fig. <a href="#fig61">61</a>, B, the
+  appearance of the entosternite of Thelyphonus or of Hypoctonus. The supra-&#x0153;sophageal
+  ganglia and part of the infra-&#x0153;sophageal ganglia fill up the space <i>Ph.</i>; stretching
+  over the rest of the infra-&#x0153;sophageal mass is a transverse trabecula, which is very thin;
+  then comes a space in which is seen the rest of the infra-&#x0153;sophageal mass, and then the
+  posterior part of the plastron, ventrally to which lies the commencement of the ventral
+  nerve-cord.</p>
+
+  <div class="ac w25 fcenter sp2">
+    <a href="images/fig061.png" id="fig61"><img style="width:100%" src="images/fig061.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 61.&mdash;A, Entosternite of Limulus; B, Entosternite of
+      Thelyphonus.</span></p>
+      <p class="sp0"><i>Ph.</i>, position of pharynx.</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page144">{144}</span></div>
+
+  <p>In these forms, in which the central nervous system is more concentrated towards the cephalic
+  end than in Limulus, the whole of the concentrated brain-mass is separated from the gut only by
+  this thin transverse band of tissue. Judging, then, from the entosternite of Thelyphonus, it is
+  not difficult to suppose that a continuation of the same growth of the brain-region of the central
+  nervous system would cause the entosternite to be separated into two lateral trabeculæ, which
+  would then take up the ventro-lateral position of the two trabeculæ of Ammoc&#x0153;tes.</p>
+
+  <p>On the other hand, it might be that two lateral trabeculæ, similar to those of Thelyphonus and
+  situated on each side of the central nervous system, were the original form from which, by the
+  addition of transverse fibres running between the gut and nervous system, the entosternite of
+  Thelyphonus and of the scorpions, etc., was formed. From an extensive consideration of the
+  entosternite in different animals, Schimkéwitsch has come to the conclusion that this latter
+  explanation is the true one. He points out that the lateral trabeculæ can be distinguished from
+  the transverse by their structure, being much more cellular and less fibrous, and the
+  cell-cavities more rounded, or, as I should express it, the two lateral trabeculæ are more
+  cartilaginous, while the transverse are more fibrous. Schimkéwitsch, from observations of
+  structure and from embryological investigations, comes to the conclusion that the entosternite was
+  originally composed of two parts&mdash;</p>
+
+  <p>1. A transverse muscle corresponding to the adductor muscle of the shell of certain
+  crustaceans, such as Nebalia.</p>
+
+  <p>2. A pair of longitudinal mesodermic tendons, which may have been formed originally out of a
+  number of segmentally arranged mesodermic tendons, and are crossed by the fibrils of the
+  transverse muscular bundles.</p>
+
+  <p>These paired tendons of the entosternite he considers to correspond to the intermuscular
+  tendons, situated lengthways, which are found in the ventral longitudinal muscles of most
+  arthropods.</p>
+
+  <p>It is clear from these observations of Schimkéwitsch, that the essential part of the
+  entosternite consists of two lateral trabeculæ, which were originally tendinous in nature and have
+  become of the nature of cartilaginous tissue by the increase of cellular elements in the matrix of
+  the tissue: these two trabeculæ function as supports for the attachment of muscles, which are
+  specially attached at certain places. At these places transverse fibres belonging to some <span
+  class="pagenum" id="page145">{145}</span>of the muscular attachments cross between the two
+  longitudinal trabeculæ, and so form the transverse trabeculæ.</p>
+
+  <p>I entirely agree with Schimkéwitsch that the nests of cartilage-cells are much more extensive
+  in, and indeed nearly entirely confined to, these two lateral trabeculæ in the entosternite of
+  Hypoctonus. Ray Lankester describes in the entosternite of Mygale peculiar cell-nests strongly
+  resembling those of Hypoctonus, and he also states that they are confined to the lateral portions
+  of the entosternite.</p>
+
+  <p>From this evidence it is easy to see that that portion of the basi-cranial skeleton known as
+  the trabeculæ may have originated from the formation of cartilage in the plastron or entosternite
+  of a palæostracan animal. Such an hypothesis immediately suggests valuable clues as to the origin
+  of the cranium and of the rest of the basi-cranial skeleton&mdash;the parachordals and the
+  auditory capsules. The former would naturally be a dorsal extension of the more membranous portion
+  of the plastron, in which, equally naturally, cartilaginous tissue would subsequently develop; and
+  the reason why it is impossible to reduce the cranium into a series of segments would be
+  self-evident, for even though, as Schimkéwitsch thinks, the plastron may have been originally
+  segmented, it has long lost all sign of segmentation. The latter would be derived from a second
+  entosternite of the same nature as the plastron, but especially connected with the auditory
+  apparatus of the invertebrate ancestor. The following out of these two clues will be the subject
+  of a future chapter.</p>
+
+  <p class="sp3">In our search, then, for a clue to the origin of the skeletal tissues of the
+  vertebrate we see again that we are led directly to the palæostracan stock on the invertebrate
+  side and to the Cyclostomata on that of the vertebrate; for in Limulus, the only living
+  representative of the Palæostraca, and in Limulus alone, we find a skeleton marvellously similar
+  to the earliest vertebrate skeleton&mdash;that found in Ammoc&#x0153;tes. Later on I shall give
+  reasons for the belief that the earliest fishes so far found, the Cephalaspidæ, etc., were built
+  up on the same plan as Ammoc&#x0153;tes, so that, in my opinion, in Limulus and in
+  Ammoc&#x0153;tes we actually possess living examples allied to the ancient fauna of the Silurian
+  times.</p>
+
+  <div><span class="pagenum" id="page146">{146}</span></div>
+
+  <p class="ac"><span class="sc">Summary.</span></p>
+
+  <div class="bq1 smaller sp2">
+    <p>The skeleton considered in this chapter is not the notochord, but that composed of cartilage.
+    The tracing downwards of the vertebrate bony and cartilaginous skeleton to its earliest
+    beginnings leads straight to the skeleton of the larval lamprey (Ammoc&#x0153;tes), in which
+    vertebræ are not yet formed, but the cranial and branchial skeleton is well marked.</p>
+    <p>The embryological and phylogenetic histories are in complete unison to show that the cranial
+    skeleton is older than the spinal, and this primitive branchial skeleton is also in harmony with
+    the laws of evolution, in that its structure, even in the adult lamprey (Petromyzon), never gets
+    beyond the stage characteristic of embryonic cartilage in the higher vertebrates.</p>
+    <p>The simplest and most primitive skeleton is that found in Ammoc&#x0153;tes and consists of
+    two parts: (1) a prosomatic, (2) a mesosomatic skeleton.</p>
+    <p>The prosomatic skeleton forms a non-segmented basi-cranial skeleton of the simplest
+    kind&mdash;the trabeculæ and the parachordals with their attached auditory capsules, just as the
+    embryology of the higher vertebrates teaches us must be the case. There in the free-living,
+    still-existent Ammoc&#x0153;tes we find the manifest natural outcome of the embryological
+    history in the shape of simple trabeculæ and parachordals, from which the whole complicated
+    basi-cranial skeleton of the higher vertebrates arose.</p>
+    <p>The mesosomatic skeleton, which is formed before the prosomatic, consisted, in the first
+    instance, of simple branchial bars segmentally arranged, which were connected together by a
+    longitudinal subchordal bar, situated laterally on each side of the notochord. These simple
+    branchial bars later on form the branchial basket-work, which forms an open-work cage within
+    which the branchiæ are situated.</p>
+    <p>The cartilages which compose these two skeletons respectively are markedly different in
+    chemical constitution, in that the first (hard cartilage) is mainly composed of chondro-gelatin,
+    the second (soft cartilage) of chondro-mucoid material.</p>
+    <p>The same kind of difference is seen in the two kinds of connective tissue which are the
+    forerunners of these two kinds of cartilage. Thus, the cranial walls in Ammoc&#x0153;tes are
+    formed of white fibrous tissue, an essentially gelatin-containing tissue; at transformation
+    these are invaded by chondro-blasts and the cartilaginous cranium, formed of hard cartilage,
+    results. On the other hand, the forerunner of the branchial soft cartilage is a very striking
+    and peculiar kind of connective tissue loaded with mucoid material, to which the name
+    muco-cartilage has been given.</p>
+    <p>The enormous interest of this muco-cartilage consists in the fact that it forms very
+    well-defined plates of tissue, entirely confined to the head-region, which are not found in any
+    higher vertebrate, not even in the adult form Petromyzon, for every scrap of the tissue as such
+    disappears at transformation.</p>
+    <p>It is this evidence of primitive non-vertebrate tissues, which occur in the larval but not in
+    the adult form, which makes Ammoc&#x0153;tes so valuable for the investigation of the origin of
+    vertebrates.</p>
+    <p class="sp0">The evidence, then, is extraordinarily clear as to the beginnings of the
+    vertebrate skeletal tissues.</p>
+  </div>
+
+  <div><span class="pagenum" id="page147">{147}</span></div>
+
+  <div class="bq1 smaller sp5">
+    <p>In the invertebrate kingdom true cartilage occurs but scantily. There is a cartilaginous
+    covering of the brain of cephalopods. It is never found in crabs, lobsters, bees, wasps,
+    centipedes, butterflies, flies, or any of the great group of Arthropoda, except, to a slight
+    extent, in some members of the scorpion group, and more fully in one single animal, the
+    King-crab or Limulus: a fact significant of itself, but still more so when the nature of the
+    cartilage and its position in the animal is taken into consideration, for the identity both in
+    structure and position of this internal cartilaginous skeleton with that of Ammoc&#x0153;tes is
+    extraordinarily great.</p>
+    <p>Here, in Limulus, just as in Ammoc&#x0153;tes, an internal cartilaginous skeleton is found,
+    composed of two distinct parts: (1) prosomatic, (2) mesosomatic. As in Ammoc&#x0153;tes, the
+    latter consists of simple branchial bars, segmentally arranged, which are connected together on
+    each side by a longitudinal ligament containing cartilage&mdash;the entapophysial ligament. This
+    cartilage is identical in structure and in chemical composition with the soft cartilage of
+    Ammoc&#x0153;tes, and, as in the latter case, arises in a markedly mucoid connective tissue. The
+    former, as in Ammoc&#x0153;tes, consists of a non-segmental skeleton, the plastron, composed of
+    a white fibrous connective tissue matrix, an essentially gelatin-containing tissue, in which are
+    found nests of cartilage cells of the hard cartilage variety.</p>
+    <p>This remarkable discovery of the branchial cartilaginous bars of Limulus, together with that
+    of the internal prosomatic plastron, causes the original difficulty of deriving an animal such
+    as the vertebrate from an animal resembling an arthropod to vanish into thin air, for it shows
+    that in the past ages when the vertebrates first appeared on the earth, the dominant arthropod
+    race at that time, the members of which resembled Limulus, had solved the question; for, in
+    addition to their external chitinous covering, they had manufactured an internal cartilaginous
+    skeleton. Not only so, but that skeleton had arrived, both in structure and position, exactly at
+    the stage at which the vertebrate skeleton starts.</p>
+    <p>What the precise steps are by which chitin-formation gives place to chondrin-formation are
+    not yet fully known, but Schmiedeberg has shown that a substance, glycosamine, is derivable from
+    both these skeletal tissues, and he concludes his observations in the following words: "Thus, by
+    means of glycosamine, the bridge is formed which connects together the chitin of the lower
+    animals with the cartilage of the more highly organized creations."</p>
+    <p class="sp0">The evidence of the origin of the cartilaginous skeleton of the vertebrate points
+    directly to the origin of the vertebrate from the Palæostraca, and is of so strong a character
+    that, taken alone, it may almost be considered as proof of such origin.</p>
+  </div>
+
+  <div><span class="pagenum" id="page148">{148}</span></div>
+
+  <p class="ac">CHAPTER IV</p>
+
+  <p class="ac"><i>THE EVIDENCE OF THE RESPIRATORY APPARATUS</i></p>
+
+  <div class="bq1 it1 smaller sp3">
+    <p class="sp0">Branchiæ considered as internal branchial appendages.&mdash;Innervation of
+    branchial segments.&mdash;Cranial region older than spinal.&mdash;Three-root system of cranial
+    nerves, dorsal, lateral, ventral.&mdash;Explanation of van Wijhe's segments.&mdash;Lateral mixed
+    root is appendage-nerve of invertebrate.&mdash;The branchial chamber of
+    Ammoc&#x0153;tes.&mdash;The branchial unit, not a pouch but an appendage.&mdash;The origin of
+    the branchial musculature.&mdash;The branchial circulation.&mdash;The branchial heart of the
+    vertebrate.&mdash;Not homologous with the systemic heart of the arthropod.&mdash;Its formation
+    from two longitudinal venous sinuses.&mdash;Summary.</p>
+  </div>
+
+  <p>The respiratory apparatus in all the terrestrial vertebrates is of the same kind&mdash;one
+  single pair of lungs. These lungs originate as a diverticulum of the alimentary canal. On the
+  other hand, the aquatic vertebrates breathe by means of a series of branchiæ, or gills, which are
+  arranged segmentally, being supported by the segmental branchial cartilaginous bars, as already
+  mentioned in the last chapter.</p>
+
+  <p>The transition from the gill-bearing to the lung-bearing vertebrates is most interesting, for
+  it has been proved that the lungs are formed by the modification of the swim-bladder of fishes;
+  and in a group of fishes, the Dipnoi, or lung-fishes, of which three representatives still exist
+  on the earth, the mode of transition from the fish to the amphibian is plainly visible, for they
+  possess both lungs and gills, and yet are not amphibians, but true fishes. But for the fortunate
+  existence of Ceratodus in Australia, Lepidosiren in South America, and Protopterus in Africa, it
+  would have been impossible from the fossil remains to have asserted that any fish had ever existed
+  which possessed at the same moment of time the two kinds of respiratory organs, although from our
+  knowledge of the development of the amphibian we might have felt sure that such a transitional
+  stage must have existed. Unfortunately, there is at present no likelihood of any corresponding
+  transitional stage being discovered <span class="pagenum" id="page149">{149}</span>living on the
+  earth in which both the dorsal arthropod alimentary canal and the ventral vertebrate one should
+  simultaneously exist in a functional condition; still it seems to me that even if Ceratodus,
+  Lepidosiren, and Protopterus had ceased to exist on the earth, yet the facts of comparative
+  anatomy, together with our conception of evolution as portrayed in the theory of natural
+  selection, would have forced us to conclude rightly that the amphibian stage in the evolution of
+  the vertebrate phylum was preceded by fishes which possessed simultaneously lungs and gills.</p>
+
+  <p class="sp3">In the preceding chapter the primitive cartilaginous vertebrate skeleton, as found
+  in Ammoc&#x0153;tes, was shown to correspond in a marvellous manner to the cartilaginous skeleton
+  of Limulus. In a later chapter I will deal with the formation of the cranium from the prosomatic
+  skeleton; in this chapter it is the mesosomatic skeleton which is of interest, and the
+  consideration of the necessary consequences which logically follow upon the supposition that the
+  branchial cartilaginous bars of Limulus are homologous with the branchial basket-work of
+  Ammoc&#x0153;tes.</p>
+
+  <p class="ac"><span class="sc">Internal Branchial Appendages.</span></p>
+
+  <p>Seeing that in both cases the cartilaginous bars of Limulus and Ammoc&#x0153;tes are confined
+  to the branchial region, their homology of necessity implies an homology of the two branchial
+  regions, and leads directly to the conclusion that the branchiæ of the vertebrate were derived
+  from the branchiæ of the arthropod, a conclusion which, according to the generally accepted view
+  of the origin of the respiratory region in the vertebrate, is extremely difficult to accept; for
+  the branchiæ of Limulus and of the Arthropoda in general are part of the mesosomatic appendages,
+  while the branchiæ of vertebrates are derived from the anterior part of the alimentary canal. This
+  conclusion, therefore, implies that the vertebrate has utilized in the formation of the anterior
+  portion of its new alimentary canal the branchial appendages of the palæostracan ancestor.</p>
+
+  <div><span class="pagenum" id="page150">{150}</span></div>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig062.png" id="fig62"><img style="width:47%" src="images/fig062.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 62.</span>&mdash;<i>Eurypterus.</i></p>
+      <p class="sp0">The segments and appendages on the right are numbered in correspondence with
+      the cranial system of lateral nerve-roots as found in vertebrates. <i>M.</i>, metastoma. The
+      surface ornamentation is represented on the first segment posterior to the branchial segments.
+      The opercular appendage is marked out by dots.</p>
+    </div>
+  </div>
+
+  <p>Let us consider dispassionately whether such a suggestion is <i>a priori</i> so impossible as
+  it at first appears. One of the principles of evolution is that any change which is supposed to
+  have taken place in the process of formation of one animal or group of animals from a lower group
+  must be in harmony with changes which are known to have occurred in that lower group. On the
+  assumption, therefore, that the vertebrate branchiæ represent the branchial portion of the
+  arthropod mesosomatic appendages which have sunk in and so become internal, we ought to find that
+  in members of this very group such inclusion of branchial appendages has taken place. This,
+  indeed, is exactly what we do find, for in all the scorpion tribe, which is acknowledged to be
+  closely related to Limulus, there are no external mesosomatic appendages, but in all cases these
+  appendages have sunk into the body, have disappeared as such, and retained only the vital part of
+  them&mdash;the branchiæ. In this way the so-called lung-books of the scorpion are formed, which
+  are in all respects homologous with the branchiæ or gill-books of Limulus. Now, as already
+  mentioned, the lords of creation in the palæostracan times were the sea-scorpions, which, as is
+  seen in Fig. <a href="#fig62">62</a>, resembled the land-scorpions of the present day in the
+  entire absence of any external appendages on the segments of the mesosomatic region. As they lived
+  in the sea, they must have breathed with gills, and those branchial appendages must have been
+  internal, just as in the land-scorpions of the present time. Indeed, markings have been found on
+  the internal side of the segments 1-5, Fig. <a href="#fig62">62</a>, which are supposed to
+  indicate branchiæ, and these segments are therefore supposed to have borne the branchiæ. Up to the
+  present time no indication of gill-slits has been found, and we cannot say with certainty how
+  these animals breathed. Further, in the Upper Silurian of Lesmahago, Lanarkshire, a scorpion
+  (<i>Palæophonus Hunteri</i>), closely resembling the modern scorpion, has been found, which, as
+  Lankester states, was in all probability aquatic, and not terrestrial in its habits. How it <span
+  class="pagenum" id="page151">{151}</span>breathed is unknown; it shows no signs of stigmata, such
+  as exist in the scorpion of to-day.</p>
+
+  <p>Although we possess as yet no certain knowledge of the position of the gill-openings in these
+  ancient scorpion-like forms, what we can say with certainty&mdash;and that is the important
+  fact&mdash;is, that at the time when the vertebrates appeared, a very large number of the dominant
+  arthropod race possessed internally-situated branchiæ, which had been directly derived from the
+  branchiæ-bearing appendages of their Limulus-like kinsfolk.</p>
+
+  <p class="sp3">This abolition of the branchiæ-bearing appendages as external organs of locomotion,
+  with the retention of the important branchial portion of the appendage as internal branchiæ, is a
+  very important suggestion in any discussion of the way vertebrates have arisen from arthropods;
+  for, if the same principle is of universal application, it leads directly to the conclusion that
+  whenever an appendage possesses an organ of vital importance to the animal, that organ will
+  remain, even though the appendage as such completely vanishes. Thus, as will be shown later,
+  special sense-organs such as the olfactory remain, though the animal no longer possesses antennæ;
+  the important excretory organs, the coxal glands, and important respiratory organs, the branchiæ,
+  are still present in the vertebrate, although the appendages to which they originally belonged
+  have dwindled away, or, at all events, are no longer recognizable as arthropod appendages.</p>
+
+  <p class="ac"><span class="sc">Innervation of Branchial Segments.</span></p>
+
+  <p>Passing from <i>a priori</i> considerations to actual facts, it is advisable to commence with
+  the innervation of the branchial segments; for, seeing that the foundation of the whole of this
+  comparative study of the vertebrate and the arthropod is based upon the similarity of the two
+  central nervous systems, it follows that we must look in the first instance to the innervation of
+  any organ or group of organs in order to find out their relationship in the two groups of
+  animals.</p>
+
+  <p>The great characteristic of the vertebrate branchial organs is their segmental arrangement and
+  their innervation by the vagus group of nerves, <i>i.e.</i> by the hindermost group of the cranial
+  segmental nerves. These cranial nerves are divided by Gegenbaur into two great groups&mdash;an
+  anterior group, the trigeminal, which supplies the muscles of mastication, and a posterior group,
+  the vagus, which is essentially <span class="pagenum" id="page152">{152}</span>respiratory in
+  function. Of these two groups, I will consider the latter group first.</p>
+
+  <p>In Limulus the great characteristic of the branchial region is its pronounced segmental
+  arrangement, each pair of branchial appendages belonging to a separate segment. This group of
+  segments forms the mesosoma, and these branchial appendages are the mesosomatic appendages.
+  Anterior to them are the segments of the prosoma, which bear the prosomatic or locomotor
+  appendages. The latter are provided at their base with gnathites or masticating apparatus, so that
+  the prosomatic group of nerves, like the trigeminal group in the vertebrate, comprises essentially
+  the nerves subserving the important function of mastication. As already pointed out, the
+  brain-region of the vertebrate is comparable to the supra-&#x0153;sophageal and
+  infra-&#x0153;sophageal ganglia of the invertebrate, and it has been shown (p. <a
+  href="#page54">54</a>) how, by a process of concentration and cephalization, the foremost region
+  of the infra-&#x0153;sophageal ganglia becomes the prosomatic region, and is directly comparable
+  to the trigeminal region in the vertebrate; while the hindermost region is formed from the
+  concentration of the mesosomatic ganglia, and is directly comparable to the medulla oblongata,
+  <i>i.e.</i> to the vagus region of the vertebrate brain.</p>
+
+  <p class="sp3">As far, then, as concerns the centres of origin of these two groups of nerves and
+  their exits from the central nervous system, they are markedly homologous in the two groups of
+  animals.</p>
+
+  <p class="ac"><span class="sc">Comparison of the Cranial and Spinal Segmental Nerves.</span></p>
+
+  <p>It has often been held that the arrangements of the vertebrate nervous system differ from those
+  of other segmented animals in one important particular. The characteristic of the vertebrate is
+  the origin of every segmental nerve from two roots, of which one contains the efferent fibres,
+  while the other possesses a sensory ganglion, and contains only afferent fibres. This arrangement,
+  which is found along the whole spinal cord of all vertebrates, is not found in the segmental
+  nerves of the invertebrates; and as it is supposed that the simpler arrangement of the spinal cord
+  was the primitive arrangement from which the vertebrate central nervous system was built up, it is
+  often concluded that the animal from which the vertebrate arose must have possessed a series of
+  nerve-segments, from each of which there arose bilaterally ventral (efferent) and dorsal
+  (afferent) roots.</p>
+
+  <div><span class="pagenum" id="page153">{153}</span></div>
+
+  <p>Now, the striking fact of the vertebrate segmental nerves consists in this, that, as far as
+  their structure and the tissues which they innervate are concerned, the cranial segmental nerves
+  are built up on the same plan as the spinal; but as far as concerns their exit from the central
+  nervous system they are markedly different. A large amount of ingenuity, it is true, has been
+  spent in the endeavour to force the cranial nerves into a series of segmental nerves, which arise
+  in the same way as the spinal by two roots, of which the ventral series ought to be efferent and
+  the dorsal series afferent, but without success. We must, therefore, consider the arrangement of
+  the cranial segmental nerves by itself, separately from that of the spinal nerves, and the problem
+  of the origin of the vertebrate segmental nerves admits of two solutions&mdash;either the cranial
+  arrangement has arisen from a modification of the spinal, or the spinal from a simplification of
+  the cranial. The first solution implies that the spinal cord arrangement is older than the
+  cranial, the second that the cranial is the oldest.</p>
+
+  <p>In my opinion, the evidence of the greater antiquity of the cranial region is overwhelming.</p>
+
+  <p>The evidence of embryology points directly to the greater phylogenetic antiquity of the cranial
+  region, for we see how, quite early in the development, the head is folded off, and the organs in
+  that region thereby completed at a time when the spinal region is only at an early stage of
+  development. We see how the first of the trunk somites is formed just posteriorly to the head
+  region, and then more and more somites are formed by the addition of fresh segments posteriorly to
+  the one first formed. We see how, in Ammoc&#x0153;tes, the first formed parts of the skeleton are
+  the branchial bars and the basi-cranial system, while the rudiments of the vertebræ do not appear
+  until the Petromyzon stage. We see how, with the elongation of the animal by the later addition of
+  more and more spinal segments, organs, such as the heart, which were originally in the head,
+  travel down, and the vagus and lateral-line nerves reach their ultimate destination. Again, we see
+  that, whereas the cranial nerves, viz. the ocular motor, the trigeminal, facial, auditory,
+  glossopharyngeal, and vagus nerves, are wonderfully fixed and constant in all vertebrates, the
+  only shifting being in the spino-occipital region, in fact, at the junction of the cranial and
+  spinal region, the spinal nerves, on the other hand, are not only remarkably variable in number in
+  different <span class="pagenum" id="page154">{154}</span>groups of animals, but that even in the
+  same animal great variations are found, especially in the manner of formation of the
+  limb-plexuses. Such marked meristic variation in the spinal nerves, in contrast to the fixed
+  character of the cranial nerves, certainly points to a more recent formation of the former
+  nerves.</p>
+
+  <p>Also the observations of Assheton on the primitive streak of the rabbit, and on the growth in
+  length of the frog embryo, have led him to the conclusion that, as in the rabbit so in the frog,
+  there is evidence to show that the embryo is derived from two definite centres of growth: the
+  first, phylogenetically the oldest, being a protoplasmic activity, which gives rise to the
+  anterior end of the embryo; the second, one which gives rise to the growth in length of the
+  embryo. This secondary area of proliferation coincides with the area of the primitive streak, and
+  he has shown, in a subsequent paper, by means of the insertion of sable hairs into the unincubated
+  blastoderm of the chick, that a hair inserted into the centre of the blastoderm appears at the
+  anterior end of the primitive streak, and subsequently is found at the level of the most anterior
+  pair of somites.</p>
+
+  <p>He then goes on to say&mdash;</p>
+
+  <p>"From these specimens it seems clear that all those parts in front of the first pair of
+  mesoblastic somites&mdash;that is to say, the heart, the brain and medulla oblongata, the
+  olfactory, optic, auditory organs and foregut&mdash;are developed from that portion of the
+  unincubated blastoderm which lies anterior to the centre of the blastoderm, and that all the rest
+  of the embryo is formed by the activity of the primitive streak area."</p>
+
+  <p>In other words, the secondary area of growth, <i>i.e.</i> the primitive streak area, includes
+  the whole of the spinal cord region, while the older primary centre of growth is coincident with
+  the cranial region.</p>
+
+  <p>In searching, then, for the origin of the segmental nerves, we must consider the type on which
+  the cranial nerves are arranged rather than that of the spinal nerves.</p>
+
+  <p>The first striking fact occurs at the spino-occipital region, where the spinal cord merges into
+  the medulla oblongata, for here in the cervical region we find each spinal segment gives origin to
+  three distinct roots, not two&mdash;a dorsal root, a ventral root, and a lateral root. This third
+  root gives origin to the spinal accessory nerve, and in the region of the medulla oblongata these
+  lateral roots merge directly into the roots of the vagus nerve; more anteriorly the same system
+  <span class="pagenum" id="page155">{155}</span>continues as the roots of the glossopharyngeal
+  nerve, as the roots of the facial nerve, and as a portion, especially the motor portion, of the
+  trigeminal nerve. Now, all these nerves belong to a well-defined system of nerves, as Charles
+  Bell<a id="NtA_1" href="#Nt_1"><sup>[1]</sup></a> pointed out in 1830, a system of nerves
+  concerned with respiration and allied mechanisms, such as laughing, sneezing, mastication,
+  deglutition, etc., nerves innervating a set of muscles of very different kind from the ordinary
+  body-muscles concerned with locomotion and equilibration. Also the centres from which these motor
+  nerves arise are well defined, and form cell-masses in the central nervous system, quite separate
+  from those which give origin to somatic muscles.</p>
+
+  <p>This original idea of Charles Bell, after having been ignored for so long a time, is now seen
+  to be a very right one, and it is an extraordinary thing that his enunciation of the dual nature
+  of the spinal roots, which was, to his mind, of subordinate importance, should so entirely have
+  overshadowed his suggestion, that in addition to the dorsal and ventral roots, a lateral system of
+  nerves existed, which were not exclusively sensory or exclusively motor, but formed a separate
+  system of respiratory nerves.</p>
+
+  <p>Further, anatomists divide the striated muscles of the body into two great natural groups,
+  characterized by a difference of origin and largely by a difference of appearance. The one set is
+  concerned with the movements of internal organs, and is called visceral, the other is derived from
+  the longitudinal sheet of musculature which forms the myotomes of the fish, and has been called
+  parietal or somatic. The motor nerves of these two sets of muscles correspond with the lateral or
+  respiratory and ventral roots respectively.</p>
+
+  <p>Finally, it has been shown that the segments of which a vertebrate is composed are recognizable
+  in the embryo by the segmented manner in which the musculature is laid down, and van Wijhe has
+  shown that in the cranial region two sets of muscles are laid down segmentally, thus forming a
+  dorsal and ventral series of commencing muscular segments. Of these the anterior segments of the
+  dorsal series give origin to the striated muscles of the eye which are innervated by the IIIrd
+  (oculomotor), IVth (trochlearis), and VIth (abducens) nerves, while the posterior segments give
+  origin to the <span class="pagenum" id="page156">{156}</span>muscles from the cranium to the
+  shoulder-girdle, innervated by the XIIth (hypoglossal) nerve. The ventral series of segments give
+  origin to the musculature supplied by the trigeminal, facial, glossopharyngeal, and vagus
+  nerves.</p>
+
+  <p>Also, the afferent or sensory nerves of the skin over the whole of this head-region are
+  supplied by the trigeminal nerve, while the afferent nerves to the visceral surfaces are supplied
+  by the vagus, glossopharyngeal and facial nerves.</p>
+
+  <p>In van Wijhe's original paper he arranged the segments belonging to the cranial nerves in the
+  following table<span class="wnw">:&mdash;</span></p>
+
+  <table class="sp2 mc bb nothand" title="Segments belonging to the cranial nerves"
+  summary="Segments belonging to the cranial nerves">
+    <tr>
+      <th class="vmi smaller bt bb">Segments.</th>
+      <th class="vmi smaller ba" colspan="2">Ventral nerve-roots and muscles<br/>
+      derived from myotomes.</th>
+      <th class="vmi smaller ba">Visceral clefts</th>
+      <th class="vmi smaller bt bb" colspan="2">Dorsal nerve-roots and muscles.</th>
+    </tr>
+    <tr class="br">
+      <td class="ac">1</td>
+      <td class="ac">III.</td>
+      <td class="al">M. rectus superior,<br/>
+      m. rectus internus,<br/>
+      m. rectus inferior,<br/>
+      m. obliquus inferior</td>
+      <td></td>
+      <td class="ac">V. N. opthalmicus<br/>
+      profundus</td>
+      <td class="br0"></td>
+    </tr>
+    <tr class="br">
+      <td class="ac">2</td>
+      <td class="ac">IV.</td>
+      <td class="al">M. obliquus superior</td>
+      <td class="vbm">1st Mandibular</td>
+      <td class="ac">V.</td>
+      <td class="ac br0">Masticating<br/>
+      muscles.</td>
+    </tr>
+    <tr class="br">
+      <td class="ac">3</td>
+      <td class="ac">VI.</td>
+      <td class="al">M. rectus externus</td>
+      <td></td>
+      <td class="ac">VII.<sub>1</sub></td>
+      <td class="ac br0" rowspan="2" style="padding:0">
+        <table title="Segments belonging to the cranial nerves"
+        summary="Segments belonging to the cranial nerves">
+          <tr>
+            <td class="vmi pl0 pr0"><img src="images/lbrace3.png" style="height:7.0ex; width:1em;"
+            alt="brace" /></td>
+            <td>Facial muscles<br/>
+            (VIII. is dorsal<br/>
+            branch of VII.)</td>
+          </tr>
+        </table>
+      </td>
+    </tr>
+    <tr class="br">
+      <td class="ac vmi">4</td>
+      <td class="ac vmi">&mdash;</td>
+      <td class="ac vmi">&mdash;</td>
+      <td class="vmi" style="padding:0">
+        <table title="Segments belonging to the cranial nerves"
+        summary="Segments belonging to the cranial nerves">
+          <tr>
+            <td class="vmi">2nd</td>
+            <td class="vmi pl0 pr0"><img src="images/lbrace2.png" style="height:4.5ex; width:1em;"
+            alt="brace" /></td>
+            <td>Hyoid<sub>1</sub><br/>
+            Hyoid<sub>2</sub></td>
+          </tr>
+        </table>
+      </td>
+      <td class="ac vmi">VII.<sub>2</sub></td>
+    </tr>
+    <tr class="br">
+      <td class="ac">5</td>
+      <td class="ac">&mdash;</td>
+      <td class="ac">&mdash;</td>
+      <td class="al pr0">3rd 1st Branchial</td>
+      <td class="ac">IX.</td>
+      <td class="ac br0" rowspan="5" style="padding:0">
+        <table title="Segments belonging to the cranial nerves"
+        summary="Segments belonging to the cranial nerves">
+          <tr>
+            <td class="vmi pl0 pr0"><img src="images/rbrace5.png" style="height:13.2ex; width:1em;"
+            alt="brace" /></td>
+            <td class="vmi pl0 pr0">Branchial and<br/>
+            visceral muscles.</td>
+          </tr>
+        </table>
+      </td>
+    </tr>
+    <tr class="br">
+      <td class="ac">6</td>
+      <td class="ac">&mdash;</td>
+      <td class="ac">&mdash;</td>
+      <td class="al pr0">4th 2nd <span class="hid">Brt</span>"</td>
+      <td class="ac">X.<sub>1</sub></td>
+    </tr>
+    <tr class="br">
+      <td class="ac">7</td>
+      <td class="ac">XII.</td>
+      <td class="al vmi" rowspan="3" style="padding:0">
+        <table title="Segments belonging to the cranial nerves"
+        summary="Segments belonging to the cranial nerves">
+          <tr>
+            <td class="vmi pl0 pr0"><img src="images/rbrace3.png" style="height:7.0ex; width:1em;"
+            alt="brace" /></td>
+            <td class="vmi pl0 pr0">Muscles from<br/>
+            cranium to<br/>
+            shoulder-girdle</td>
+            <td class="vmi pl0 pr0"><img src="images/lbrace3.png" style="height:7.0ex; width:1em;"
+            alt="brace" /></td>
+          </tr>
+        </table>
+      </td>
+      <td class="al pr0">5th 3rd <span class="hid">Bra</span>"</td>
+      <td class="ac">X.<sub>2</sub></td>
+    </tr>
+    <tr class="br">
+      <td class="ac">8</td>
+      <td class="ac">XII.</td>
+      <td class="al pr0">6th 4th <span class="hid">Bra</span>"</td>
+      <td class="ac">X.<sub>3</sub></td>
+    </tr>
+    <tr class="br">
+      <td class="ac">9</td>
+      <td class="ac">XII.</td>
+      <td class="al pr0">7th 5th <span class="hid">Bra</span>"</td>
+      <td class="ac">X.<sub>4</sub></td>
+    </tr>
+  </table>
+
+<!-- Left half for handhelds -->
+
+  <table class="sp2 bb w100 handonly" title="Segments belonging to the cranial nerves
+  (1 of 2)" summary="Segments belonging to the cranial nerves
+  (1 of 2)">
+    <tr>
+      <th class="vmi smaller bt bb">Segments.</th>
+      <th class="vmi smaller ba" colspan="2">Ventral nerve-roots and muscles<br/>
+      derived from myotomes.</th>
+      <th class="vmi smaller bt bb">Visceral clefts</th>
+    </tr>
+    <tr class="br">
+      <td class="ac">1</td>
+      <td class="ac">III.</td>
+      <td class="al">M. rectus superior,<br/>
+      m. rectus internus,<br/>
+      m. rectus inferior,<br/>
+      m. obliquus inferior</td>
+      <td class="br0"></td>
+    </tr>
+    <tr class="br">
+      <td class="ac">2</td>
+      <td class="ac">IV.</td>
+      <td class="al">M. obliquus superior</td>
+      <td class="br0 vbm">1st Mandibular</td>
+    </tr>
+    <tr class="br">
+      <td class="ac">3</td>
+      <td class="ac">VI.</td>
+      <td class="al">M. rectus externus</td>
+      <td class="br0"></td>
+    </tr>
+    <tr class="br">
+      <td class="ac vmi">4</td>
+      <td class="ac vmi">&mdash;</td>
+      <td class="ac vmi">&mdash;</td>
+      <td class="br0 vmi" style="padding:0">
+        <table title="Segments belonging to the cranial nerves
+        (1 of 2)" summary="Segments belonging to the cranial nerves
+        (1 of 2)">
+          <tr>
+            <td class="vmi">2nd</td>
+            <td class="vmi pl0 pr0"><img src="images/lbrace2.png" style="height:4.5ex; width:1em;"
+            alt="brace" /></td>
+            <td>Hyoid<sub>1</sub><br/>
+            Hyoid<sub>2</sub></td>
+          </tr>
+        </table>
+      </td>
+    </tr>
+    <tr class="br">
+      <td class="ac">5</td>
+      <td class="ac">&mdash;</td>
+      <td class="ac">&mdash;</td>
+      <td class="al br0 pr0">3rd 1st Branchial</td>
+    </tr>
+    <tr class="br">
+      <td class="ac">6</td>
+      <td class="ac">&mdash;</td>
+      <td class="ac">&mdash;</td>
+      <td class="al br0 pr0">4th 2nd <span class="hid">Brt</span>"</td>
+    </tr>
+    <tr class="br">
+      <td class="ac">7</td>
+      <td class="ac">XII.</td>
+      <td class="al vmi" rowspan="3" style="padding:0">
+        <table title="Segments belonging to the cranial nerves
+        (1 of 2)" summary="Segments belonging to the cranial nerves
+        (1 of 2)">
+          <tr>
+            <td class="vmi pl0 pr0"><img src="images/rbrace3.png" style="height:7.0ex; width:1em;"
+            alt="brace" /></td>
+            <td class="vmi pl0 pr0">Muscles from<br/>
+            cranium to<br/>
+            shoulder-girdle</td>
+            <td class="vmi pl0 pr0"><img src="images/lbrace3.png" style="height:7.0ex; width:1em;"
+            alt="brace" /></td>
+          </tr>
+        </table>
+      </td>
+      <td class="al br0 pr0">5th 3rd <span class="hid">Bra</span>"</td>
+    </tr>
+    <tr class="br">
+      <td class="ac">8</td>
+      <td class="ac">XII.</td>
+      <td class="al br0 pr0">6th 4th <span class="hid">Bra</span>"</td>
+    </tr>
+    <tr class="br">
+      <td class="ac">9</td>
+      <td class="ac">XII.</td>
+      <td class="al br0 pr0">7th 5th <span class="hid">Bra</span>"</td>
+    </tr>
+  </table>
+
+<!-- Right half for handhelds -->
+
+  <table class="sp3 bb w100 handonly" title="Segments belonging to the cranial nerves
+  (2 of 2)" summary="Segments belonging to the cranial nerves
+  (2 of 2)">
+    <tr>
+      <th class="vmi smaller bt bb br">Segments.</th>
+      <th class="vmi smaller bt bb" colspan="2">Dorsal nerve-roots and muscles.</th>
+    </tr>
+    <tr class="br">
+      <td class="ac">1</td>
+      <td class="ac">V. N. opthalmicus<br/>
+      profundus</td>
+      <td class="br0"></td>
+    </tr>
+    <tr class="br">
+      <td class="ac">2</td>
+      <td class="ac">V.</td>
+      <td class="ac br0">Masticating<br/>
+      muscles.</td>
+    </tr>
+    <tr class="br">
+      <td class="ac">3</td>
+      <td class="ac">VII.<sub>1</sub></td>
+      <td class="ac br0" rowspan="2" style="padding:0">
+        <table title="Segments belonging to the cranial nerves
+        (2 of 2)" summary="Segments belonging to the cranial nerves
+        (2 of 2)">
+          <tr>
+            <td class="vmi pl0 pr0"><img src="images/lbrace3.png" style="height:7.0ex; width:1em;"
+            alt="brace" /></td>
+            <td>Facial muscles<br/>
+            (VIII. is dorsal<br/>
+            branch of VII.)</td>
+          </tr>
+        </table>
+      </td>
+    </tr>
+    <tr class="br">
+      <td class="ac vmi">4</td>
+      <td class="ac vmi">VII.<sub>2</sub></td>
+    </tr>
+    <tr class="br">
+      <td class="ac">5</td>
+      <td class="ac">IX.</td>
+      <td class="ac br0" rowspan="5" style="padding:0">
+        <table title="Segments belonging to the cranial nerves
+        (2 of 2)" summary="Segments belonging to the cranial nerves
+        (2 of 2)">
+          <tr>
+            <td class="vmi pl0 pr0"><img src="images/rbrace5.png" style="height:13.2ex; width:1em;"
+            alt="brace" /></td>
+            <td class="vmi pl0 pr0">Branchial and<br/>
+            visceral muscles.</td>
+          </tr>
+        </table>
+      </td>
+    </tr>
+    <tr class="br">
+      <td class="ac">6</td>
+      <td class="ac">X.<sub>1</sub></td>
+    </tr>
+    <tr class="br">
+      <td class="ac">7</td>
+      <td class="ac">X.<sub>2</sub></td>
+    </tr>
+    <tr class="br">
+      <td class="ac">8</td>
+      <td class="ac">X.<sub>3</sub></td>
+    </tr>
+    <tr class="br">
+      <td class="ac">9</td>
+      <td class="ac">X.<sub>4</sub></td>
+    </tr>
+  </table>
+
+<!-- End of handheld split -->
+
+  <p>As is seen in the table, van Wijhe attempts to arrange the cranial segmental nerves into dorsal
+  and ventral roots, in accordance with the arrangement in the spinal region. In order to do this he
+  calls the Vth, VIIth, IXth, and Xth nerves dorsal roots, although they are not purely sensory
+  nerves, but contain motor fibres as well.</p>
+
+  <p>It is not accidental that he should have picked out for his dorsal roots the very nerves which
+  form Charles Bell's lateral series of roots, inasmuch as this system of lateral roots, apart from
+  dorsal and ventral roots, really is, as Charles Bell thought, an important separate system,
+  dependent upon a separate segmentation in the embryo of the musculature supplied by these roots.
+  This segmentation may receive the name of <i>visceral</i> or <i>splanchnic</i> in
+  contradistinction to <i>somatic</i>, since all the muscles without exception belong to the
+  visceral group of striated muscles.</p>
+
+  <div><span class="pagenum" id="page157">{157}</span></div>
+
+  <p>These observations of van Wijhe lead directly to the following conclusion. In the cranial
+  region there is evidence of a double set of segments, which may be called somatic and splanchnic.
+  The somatic segments, consisting of the outer skin and the body musculature, are <i>doubly</i>
+  innervated as are those of the spinal cord by a series of ventral motor roots, the oculomotor or
+  IIIrd nerve, the trochlear or IVth nerve, the abducens or VIth nerve, and the hypoglossal or XIIth
+  nerve, and by a series of dorsal sensory roots, the sensory part of the trigeminal or Vth nerve.
+  But the splanchnic segments are innervated by <i>single</i> roots, the vagus or Xth nerve,
+  glossopharyngeal or IXth nerve, facial or VIIth nerve, and trigeminal or Vth nerve, which are
+  mixed, containing both sensory and motor fibres, thus differing markedly from the arrangement of
+  the spinal nerves.</p>
+
+  <p class="sp3">From this sketch it follows that the arrangement seen in the spinal cord, would
+  result from the cranial arrangement if this third system of lateral roots were left out. Further,
+  since the cranial system is the oldest, we must search in the invertebrate ancestor for a
+  tripartite rather than a dual system of nerve-roots for each segment; a system composed of a
+  dorsal root supplying only the sensory nerves of the skin-surfaces, a lateral mixed root supplying
+  the system connected with respiration with both sensory and motor fibres, and a ventral root
+  supplying the motor nerves to the body-musculature.</p>
+
+  <p class="ac"><span class="sc">Comparison of the Appendage Nerves of Limulus and Branchipus to the
+  Lateral Root System of the Vertebrate.</span></p>
+
+  <p>If the argument used so far is correct, and this tripartite system of nerve-roots, as seen in
+  the cranial nerves of the vertebrate, really represents the original scheme of innervation in the
+  palæostracan ancestor, then it follows that each segment of Limulus ought to be supplied by three
+  nerves&mdash;(1), a sensory nerve supplying its own portion of the skin-surface of the prosomatic
+  and mesosomatic carapaces; (2), a lateral mixed nerve supplying exclusively the appendage of the
+  segment, for the appendages carry the respiratory organs; and (3), a motor nerve supplying the
+  body-muscles of the segment.</p>
+
+  <p>It is a striking fact that Milne-Edwards describes the nerve-roots in exactly this manner. The
+  great characteristic of the nerve-roots <span class="pagenum" id="page158">{158}</span>in Limulus
+  as in other arthropods is the large appendage-nerve, which is always a mixed nerve; in addition,
+  there is a system of sensory nerves to the prosomatic and mesosomatic carapaces, called by him the
+  epimeral nerves, which are purely sensory, and a third set of roots which are motor to the
+  body-muscles, and possibly also sensory to the ventral surface between the appendages.</p>
+
+  <p>Moreover, just as in the vertebrate central nervous system the centres of origin of the motor
+  nerves of the branchial segmentation are distinct from those of the somatic segmentation, so we
+  find, from the researches of Hardy, that a similar well-marked separation exists between the
+  centres of origin of the motor nerves of the appendages and those of the somatic muscles in the
+  central nervous system of Branchipus and Astacus.</p>
+
+  <p>In the first place, he points out that the nervous system of Branchipus is of a very primitive
+  arthropod type; that it is, in fact, as good an example of an ancient type as we are likely to
+  find in the present day; a matter of some importance in connection with my argument, since the
+  arthropod ancestor of the vertebrate, such as I am deducing from the study of Ammoc&#x0153;tes,
+  must undoubtedly have been of an ancient type, more nearly connected with the strange forms of the
+  trilobite era than with the crabs and spiders of the present day.</p>
+
+  <p>His conclusions with respect to Branchipus may be tabulated as follows<span
+  class="wnw">:&mdash;</span></p>
+
+  <p>1.  Each ganglion of the ventral chain is formed mainly for the innervation of the
+  appendages.</p>
+
+  <p>2.  Each ganglion is divided into an anterior and posterior division, which are connected
+  respectively with the motor and sensory nerves of the appendages.</p>
+
+  <p>3.  The motor nerves of the appendages arise as well-defined axis-cylinder processes of
+  nerve-cells, which are arranged in well-defined groups in the anterior division of the
+  ganglion.</p>
+
+  <p>4.  A separate innervation exists for the muscles and sensory surfaces of the trunk. The
+  trunk-muscles consist of long bundles, from which slips pass off to the skin in each segment; they
+  are thus imperfectly segmented. In accordance with this, a diffuse system of nerve-fibres passes
+  to them from certain cells on the dorsal surface of each lateral half of the ganglion. These
+  cell-groups are therefore very distinct from those which give origin to the motor <span
+  class="pagenum" id="page159">{159}</span>appendage-nerves, and, moreover, are not confined to the
+  ganglion, but extend for some distance into the interganglionic region of the nerve-cords which
+  connect together the ganglia of the ventral chain.</p>
+
+  <p>Hardy's observations, therefore, combined with those of Milne-Edwards, lead to the conclusion
+  that in such a primitive arthropod type as my theory postulates, each segment was supplied with
+  separate sensory and motor somatic nerves, and with a pair of nerves of mixed function, devoted
+  entirely to the innervation of the pair of appendages; that also, in the central nervous system,
+  the motor nerve-centres were arranged in accordance with a double set of segmented muscles in two
+  separate groups of nerve-cells. These nerve-cells in the one case were aggregated into
+  well-defined groups, which formed the centres for the motor nerves of the markedly segmented
+  muscles of the appendages, and in the other case formed a system of more diffused cells, less
+  markedly aggregated into distinct groups, which formed the centres for the imperfectly segmented
+  somatic muscles.</p>
+
+  <p>Such an arrangement suggests that in the ancient arthropod type a double segmentation existed,
+  viz. a segmentation of the body, and a segmentation due to the appendages. Undoubtedly, the
+  segments originally corresponded absolutely as in Branchipus, and every appendage was attached to
+  a well-defined separate body-segment. In, however, such an ancient type as Limulus, though the
+  segmentation may be spoken of as twofold, yet the number of segments in the prosomatic and
+  mesosomatic regions are much more clearly marked out by the appendages than by the divisions of
+  the soma; for, in the prosomatic region such a fusion of somatic segments to form the tergal
+  prosomatic carapace has taken place that the segments of which it is composed are visible only in
+  the young condition, while in the mesosomatic region the separate somatic segments, though fused
+  to form the mesosomatic carapace, are still indicated by the entapophysial indentations.</p>
+
+  <p>Clearly, then, if the mesosomatic branchial appendages of forms related to Limulus were reduced
+  to the branchial portion of the appendage, and that branchial portion became internal, just as is
+  known to be the case in the scorpion group, we should obtain an animal in which the <i>mesosomatic
+  region</i> would be characterized by a segmentation predominantly branchial, which might be
+  termed, as in vertebrates, the <i>branchiomeric segmentation</i>, but yet would show <span
+  class="pagenum" id="page160">{160}</span>indications of a corresponding somatic or <i>mesomeric
+  segmentation</i>. The nerve supply to these segments would consist of&mdash;</p>
+
+  <p>1.  The epimeral purely sensory nerves to the somatic surface, equivalent in the vertebrate to
+  the ascending root of the trigeminal.</p>
+
+  <p>2.  The mixed nerves to the internal branchial segments, equivalent in the vertebrate to the
+  vagus, glossopharyngeal, and facial.</p>
+
+  <p>3.  The motor nerves to the somatic muscles, equivalent in the vertebrate to the original
+  nerve-supply to the somatic muscles belonging to these segments, <i>i.e.</i> to the muscles
+  derived from van Wijhe's 4th, 5th, and 6th somites.</p>
+
+  <p>Further, the centres of origin of these appendage-nerves would form centres in the central
+  nervous system separate from the centres of the motor nerves to the somatic muscles, just as the
+  centres of origin of the motor parts of the facial, vagus, and glossopharyngeal nerves form groups
+  of cells quite distinct from the centres for the hypoglossal, abducens, trochlear, and oculomotor
+  nerves.</p>
+
+  <p>In fact, if the vertebrate branchial nerves are looked upon as the descendants of nerves which
+  originally supplied branchial appendages, then every question connected with the branchial
+  segmentation, with the origin and distribution of these nerves, receives a simple and adequate
+  solution&mdash;a solution in exact agreement with the conclusion that the vertebrate arose from a
+  palæostracan ancestor.</p>
+
+  <p>It would, therefore, be natural to expect that the earliest fishes breathed by means of
+  branchial appendages situated internally, and that the evidence for such appendages would be much
+  stronger in them than in more recent fishes.</p>
+
+  <p class="sp3">Although we know nothing of the nature of the respiratory apparatus in the extinct
+  fishes of Silurian times, we have still living, in the shape of Ammoc&#x0153;tes, a possible
+  representative of such types. If, then, we find, as is the case, that the respiratory apparatus of
+  Ammoc&#x0153;tes differs markedly from that of the rest of the fishes, and, indeed, from that of
+  the adult form or Petromyzon, and that that very difference consists in a greater resemblance to
+  internal branchial appendages in the case of Ammoc&#x0153;tes, then we may feel that the proof of
+  the origin of the branchial apparatus of the vertebrate from the internal branchial appendages of
+  the invertebrate has gained enormously.</p>
+
+  <div><span class="pagenum" id="page161">{161}</span></div>
+
+  <p class="ac"><span class="sc">The Respiratory Chamber of Ammoc&#x0153;tes.</span></p>
+
+  <p>In order to make clear the nature of the branchial segments in Ammoc&#x0153;tes, I have divided
+  the head-part of the animal by means of a longitudinal horizontal section into
+  halves&mdash;ventral and dorsal&mdash;as shown in Figs. 63 and 64. These figures are each a
+  combination of a section and a solid drawing. The animal was slit open by a longitudinal section
+  in the neighbourhood of the gill-slits, and each half was slightly flattened out, so as to expose
+  the ventral and dorsal internal surfaces respectively. The structures in the cut surface were
+  drawn from one of a series of horizontal longitudinal sections taken through the head of the
+  animal. These figures show that the head-region of Ammoc&#x0153;tes consists of two chambers, the
+  contents of which are different. In front, an oral or stomodæal chamber, which contains the velum
+  and tentacles, is enclosed by the upper and lower lips, and was originally separated by a septum
+  from the larger respiratory chamber, which contains the separate pairs of branchiæ. A glance at
+  the two drawings shows clearly that Rathke's original description of this chamber is the natural
+  one, for he at that time, looking upon <i>Ammoc&#x0153;tes branchialis</i> as a separate species,
+  described the branchial chamber as containing a series of paired gills, with the gill-openings
+  between consecutive gills. His branchial unit or gill, therefore, was represented by each of the
+  so-called diaphragms, which, as seen in Figs. 63, 64, are all exactly alike, except the first and
+  the last. Any one of these is represented in section in Fig. <a href="#fig65">65</a>, and
+  represents a branchial unit in Rathke's view and in mine. Clearly, it may be described as a
+  branchial appendage which projects into an open pharyngeal chamber, so that the series of such
+  appendages divides the chamber into a series of compartments, each of which communicates with the
+  exterior by means of a gill-slit, and with each other by means of the open space between opposing
+  appendages.</p>
+
+  <p>Each of these appendages possesses its own cartilaginous bar (<i>Br. cart.</i>), as explained
+  in Chapter III.; each possesses its own branchial or visceral muscles (coloured blue in Figs. 63
+  and 64), separated absolutely from the longitudinal somatic muscles (coloured dark red in Figs. 63
+  and 64) by a space (<i>Sp.</i>) containing blood and peculiar fat-cells, etc. Each possesses its
+  own afferent branchial blood-vessel from the ventral aorta, and its own efferent vessel to the
+  dorsal aorta (Fig. <a href="#fig65">65</a>, <i>a. br.</i> and <i>v. br.</i>). Each possesses its
+  own segmental nerve, which supplies its own branchial muscles and no others with motor fibres, and
+  sends sensory fibres to the general surface of each appendage, as also to the special sense-organs
+  in the shape of the epithelial pits (<i>S.</i>, Fig. <a href="#fig65">65</a>) arranged along the
+  free edges of the diaphragms; each of these nerves possesses its own ganglion&mdash;the
+  epibranchial ganglion.</p>
+
+  <div><span class="pagenum" id="page162">{162}</span></div>
+
+  <div class="ac w55 fcenter sp2">
+    <a href="images/fig063.jpg" id="fig63"><img style="width:100%" src="images/fig063.jpg" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 63.&mdash;Ventral half of Head-region of
+      Ammoc&#x0153;tes.</span></p>
+      <p class="sp0">Somatic muscles coloured red. Branchial and visceral muscles coloured blue.
+      Tubular constrictor muscles distinguished from striated constrictor muscles by simple
+      hatching. <i>Tent.</i>, tentacles; <i>Tent. m.c.</i>, muco-cartilage of tentacles; <i>Vel.
+      m.c.</i>, muco-cartilage of the velum; <i>Hy. m.c.</i>, muco-cartilage of the hyoid segment;
+      <i>Ps. br.</i>, pseudo-branchial groove; <i>Br. cart.</i>, branchial cartilages; <i>Sp.</i>,
+      space between somatic and splanchnic muscles; <i>Th. op.</i>, orifice of thyroid; <i>H.</i>,
+      heart.</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page163">{163}</span></div>
+
+  <div class="ac w50 fcenter sp2">
+    <a href="images/fig064.jpg" id="fig64"><img style="width:88%" src="images/fig064.jpg" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 64.&mdash;Dorsal half of Head-region of Ammoc&#x0153;tes.</span></p>
+      <p class="sp0"><i>Tr.</i>, trabeculæ; <i>Pit.</i>, pituitary space; <i>Inf.</i>, infundibulum;
+      <i>Ser.</i>, median serrated flange of velar folds.</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page164">{164}</span></div>
+
+  <table class="mc tlf sp2 w55" summary="Group of illustrations" title="Group of illustrations">
+    <tr>
+      <td class="vbm ac" style="width:45%;"><a href="images/fig065.png" id="fig65"><img
+      style="width:100%" src="images/fig065.png" alt="" title=""/></a></td>
+      <td class="vbm ac" style="width:45%;"><a href="images/fig066.png" id="fig66"><img
+      style="width:100%" src="images/fig066.png" alt="" title=""/></a></td>
+    </tr>
+    <tr>
+      <td>
+        <div class="smaller aj it1">
+          <p class="ac"><span class="sc">Fig. 65.&mdash;Section through Branchial Appendage of
+          Ammoc&#x0153;tes.</span></p>
+          <p class="sp0"><i>br. cart.</i>, branchial cartilage; <i>v. br.</i>, branchial vein; <i>a.
+          br.</i>, branchial artery; <i>b.s.</i>, blood-spaces; <i>p.</i>, pigment; <i>S.</i>,
+          sense-organ; <i>c.</i>, ciliated band; <i>E., I.</i>, external and internal borders; <i>m.
+          add.</i>, adductor muscle; <i>m.c.s.</i>, striated constrictor muscle; <i>m.c.t.</i>,
+          tubular constrictor muscle; <i>m.</i> and <i>m.v.</i>, muscles of valve.</p>
+        </div>
+      </td>
+      <td>
+        <div class="smaller aj it1">
+          <p class="ac"><span class="sc">Fig. 66.&mdash;Section through Branchial Appendage of
+          Limulus.</span></p>
+          <p class="sp0"><i>br. cart.</i>, branchial cartilage; <i>v.br.</i>, branchial vein;
+          <i>b.s.</i>, blood-spaces formed by branchial artery; <i>P.</i>, pigment;
+          <i>m<sub>1</sub></i>, posterior entapophysio-branchial muscle; <i>m<sub>2</sub></i>,
+          anterior entapophysio-branchial muscle; <i>m<sub>3</sub></i>, external branchial
+          muscle.</p>
+        </div>
+      </td>
+    </tr>
+  </table>
+
+  <p>The work of Miss Alcock has shown that the segmental branchial nerve supplies solely and
+  absolutely such an appendage or branchial <span class="pagenum" id="page165">{165}</span>segment,
+  and does not supply any portion of the neighbouring branchial segments. The nerve-supply in
+  Ammoc&#x0153;tes gives no countenance to the view that the original unit was a branchial pouch,
+  the two sides of which each nerve supplied, but is strong evidence that the original unit was a
+  branchial appendage, which was supplied by a <i>single</i> nerve with both motor and sensory
+  fibres.</p>
+
+  <p>Any observer having before him only this picture of the respiratory chamber of
+  Ammoc&#x0153;tes, upon which to base his view of a vertebrate respiratory chamber, would naturally
+  look upon the branchial unit of a vertebrate as a gilled appendage projecting into the open cavity
+  of the anterior part of the alimentary canal or pharynx. This is not, however, the usual
+  conception. The branchial unit is ordinarily described as a gill-pouch, which possesses two
+  openings or slits, an internal one into the lumen of the alimentary canal, and an external one
+  into the surrounding medium. This view is based upon embryological evidence of the following
+  character<span class="wnw">:&mdash;</span></p>
+
+  <p>The alimentary canal of all vertebrates forms a tube stretching the whole length of the animal;
+  the anterior part of this tube becomes pouched on each side at regular intervals, and the walls of
+  each pouch becoming folded form the respiratory surfaces or gills. The openings of these separate
+  pouches into the central lumen of the gut form the internal gill-pouch openings; the other
+  extremity of the pouch approaches the external surface of the animal, and finally breaks through
+  to form a series of external gill-pouch openings.</p>
+
+  <p>From the mesoblastic tissue, between each gill-pouch, there is formed a supporting
+  cartilaginous bar, to which are attached a system of branchial muscles, with their nerves and
+  blood-vessels. These cartilaginous bars, in all fishes above the Cyclostomata, form a supporting
+  framework for the internal gill-slit, so that the gills are situated externally to them; the more
+  primitive arrangement is, as already mentioned, a system of cartilaginous bars, extra-branchial in
+  position, so that the gills are situated internally to them.</p>
+
+  <p>From this description of the mode of formation of the respiratory apparatus in water-breathing
+  vertebrates the conception has arisen of the gill-pouch as the branchial unit, a conception which
+  is absolutely removed from all idea of a branchial unit such as is found in an arthropod, viz. an
+  appendage.</p>
+
+  <p>This conception of spaces as units pervades the whole of embryology, and is the outcome of the
+  gastrula theory&mdash;a theory which <span class="pagenum" id="page166">{166}</span>teaches that
+  all animals above the Protozoa are derived from a form which by invagination of its external
+  surface formed an internal cavity or primitive gut. From pouches of this gut other cavities were
+  said to be formed, called c&#x0153;lomic cavities, and thus arose the group of c&#x0153;lomatous
+  animals. To speak of the developmental history of animals in terms of spaces; to speak of the
+  atrophy of a cavity as though such a thing were possible, is, to my mind, the wrong way of looking
+  at the facts of anatomy. It resembles the description of a net as a number of holes tied together
+  with string, which is not usually considered the best method of description.</p>
+
+  <p>There are two ways in which a series of pouches can be formed from a simple tube without
+  folding, either by a thinning at regular intervals of the original tissue surrounding the tube, or
+  by the ingrowth into the tube of the surrounding tissue at regular intervals, thus&mdash;</p>
+
+  <div class="ac w45 fcenter sp2">
+    <a href="images/fig067.png" id="fig67"><img style="width:100%" src="images/fig067.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 67.&mdash;Diagrams to show the two methods of
+      Pouch-formation.</span></p>
+      <p class="sp0">A, by the thinning of the mesoblast at intervals. B, by the ingrowth of
+      mesoblast at intervals. <i>Ep.</i>, epiblast; <i>Mes.</i>, mesoblast; <i>Hy.</i>,
+      hypoblast.</p>
+    </div>
+  </div>
+
+  <p>In the first case (A) the formation of a pouch is the significant act, and therefore the
+  branchial segments might be expressed in terms of pouches. In the second case (B) the formation of
+  a pouch is <span class="pagenum" id="page167">{167}</span>brought about in consequence of the
+  ingrowth of the mesoblastic tissues at intervals; here, although the end-result is the same as in
+  the first case, the pouch-formation is only secondary, the true branchial unit is the mesoblastic
+  ingrowth.</p>
+
+  <p>The evidence all points directly to the second method of formation. Thus Shipley, in his
+  description of the development of the lamprey, says&mdash;</p>
+
+  <p>"The gill-slits appear to me to be the result of the ventral downgrowth of mesoblast taking
+  place only at certain places, these forming the gill-bars. Between each downgrowth the hypoblastic
+  lining of the alimentary canal remains in contact with the epiblast; here the gill-opening
+  subsequently appears about the twenty-second day."</p>
+
+  <p>Dohrn describes and gives excellent pictures of the growth of the diaphragms, as the
+  Ammoc&#x0153;tes grows in size, pictures which are distinctly reminiscent of the corresponding
+  illustrations given by Brauer of the growth of the internal gills in the scorpion embryo.</p>
+
+  <p>Another piece of evidence confirmatory of the view that the branchial segments are really of
+  the nature of internal appendages, as the result of which gill-pouches are formed, is given by the
+  presence in each of these branchial bars or diaphragms of a separate c&#x0153;lomic cavity. From
+  the walls of this cavity the branchial muscles and cartilaginous bar are formed.</p>
+
+  <p>Now, from an embryological point of view, the vertebrate shows that it is a segmented animal by
+  the formation of somites, which consist of a series of divisions of the c&#x0153;lom, of which the
+  walls form a series of muscular and skeletal segments. In the head-region, as already mentioned,
+  such c&#x0153;lomic divisions form two rows&mdash;a dorsal and a ventral set. From the walls of
+  the dorsal set the somatic musculature is formed. From those of the ventral set the branchial
+  musculature. From the latter also the branchial cartilaginous bars are formed. Thus Shipley, in
+  his description of the development of the lamprey, says: "The mesoblast between the gills arranges
+  itself into head-cavities, and the walls of these cavities ultimately form the skeleton of the
+  gill-arches."</p>
+
+  <p>Similarly, in the arthropod, the segments in the embryo are marked out by a series of
+  c&#x0153;lomic cavities and Kishinouye has described in Limulus a separate c&#x0153;lomic cavity
+  for every one of the mesosomatic or branchial segments, and he states that in Arachnida <span
+  class="pagenum" id="page168">{168}</span>the segmental c&#x0153;lomic cavities extend into the
+  limbs. These cavities both in the vertebrate and in the arthropod disappear before the adult
+  condition is reached.</p>
+
+  <p>The whole evidence thus points strongly to the conclusion that the true branchial segmental
+  units are the branchial bars or diaphragms, not the pouches between them.</p>
+
+  <p>It is possible to understand why such prominence has been given to the conception of the
+  branchial unit as a gill-pouch rather than as a gill-appendage, when the extraordinary change of
+  appearance in the respiratory chamber of the lamprey which occurs at transformation, is taken into
+  consideration. This change is of a very far-reaching character, and consists essentially of the
+  formation of a new alimentary canal in this region, whereby the pharyngeal chamber of
+  Ammoc&#x0153;tes is cut off posteriorly from the alimentary canal, and is confined entirely to
+  respiratory purposes, its original lumen now forming a tube called the bronchus, which opens into
+  the mouth and into a series of branchial pouches.</p>
+
+  <p>In Fig. <a href="#fig68">68</a> I give diagrammatic illustrations taken from Nestler's paper to
+  show the striking change which takes place at transformation, (A) representing three branchial
+  segments of Ammoc&#x0153;tes, and (B) the corresponding three segments of Petromyzon. The
+  corresponding parts in the two diagrams are shown by the cartilages (<i>br. cart.</i>), the
+  sense-organs (S), and the branchial veins (<i>V. br.</i>); the corresponding diaphragms are marked
+  by the figures 1, 2, 3 respectively. As is clearly seen, it is perfectly possible in the latter
+  case to describe the respiratory chamber, as Nestler has done, as divided into a series of
+  separate smaller chambers&mdash;the gill-pouches&mdash;by means of a series of diaphragms or
+  branchial bars. The surface of these gill-pouches is in part thrown into folds for respiratory
+  purposes, and each gill-pouch opens, on the one hand, into the bronchus (<i>Bro.</i>), and, on the
+  other, to the exterior by means of the gill-slit. The branchial unit in Petromyzon is, therefore,
+  according to Nestler and other morphologists, the folded opposed surfaces of two contiguous
+  diaphragms, and each one of the diaphragms is intersegmental between two gill-pouches.</p>
+
+  <p>Nestler then goes on to describe the arrangement in Ammoc&#x0153;tes in the same terms,
+  although there is no bronchus or gill-pouch, but only an open chamber into which these
+  gill-bearing diaphragms project, which open chamber serves both for the passage of food and <span
+  class="pagenum" id="page169">{169}</span>of the water for respiration. This is manifestly the
+  wrong way to look at the matter: the adult form is derived from the larval, not <i>vice versâ</i>,
+  and the transformation process shows exactly how the gills, in Rathke's sense, come together to
+  form the bronchus and so make the gill-pouches of Petromyzon.</p>
+
+  <p>When we bear in mind that almost all observers consider that the internal branchiæ of the
+  scorpion group are directly derived from branchial appendages of a kind similar to those of
+  Limulus, it is evident that a branchial appendage such as that of Ammoc&#x0153;tes might also have
+  arisen from such an appendage, because in various respects it is easier to compare the branchial
+  appendage of Ammoc&#x0153;tes, than that of the scorpion group, with that of Limulus.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig068.png" id="fig68"><img style="width:100%" src="images/fig068.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p><span class="sc">Fig. 68.&mdash;Diagram of three Branchial Segments of Ammoc&#x0153;tes (A)
+      compared with three Branchial Segments after Transformation (B) to show how the Branchial
+      Appendages of Ammoc&#x0153;tes form the Branchial Pouches of Petromyzon.</span> (After <span
+      class="sc">Nestler</span>.)</p>
+      <p class="sp0">In both figures the branchial cartilages (<i>br. cart.</i>), the branchial view
+      (<i>V. br.</i>), and the sense-organs (<i>S</i>), are marked out in order to show
+      corresponding points. The muscles, blood-spaces, branchial arteries, etc., of each branchial
+      segment are not distinguished, being represented a uniform black colour. <i>Bro.</i>, the
+      bronchus into which each gill-pouch opens.</p>
+    </div>
+  </div>
+
+  <p>In the case of the scorpions, various suggestions have been made as to the manner in which such
+  a conversion may have taken place. The most probable explanation is that given by Macleod, in
+  which <span class="pagenum" id="page170">{170}</span>each of the branchiæ of the scorpion group is
+  directly compared with the branchial part of the Limulus appendage which has sunk into and
+  amalgamated with the ventral surface.</p>
+
+  <p>According to this view, the modification which has taken place in transforming the branchial
+  Limulus-appendage into the branchial scorpion-appendage is a further stage of the process by which
+  the Limulus branchial appendage itself has been formed, viz. the getting rid of the free locomotor
+  segments of the original appendage, thus confining the appendage more and more to the basal
+  branchial portion. So far has this process been carried in the scorpion that all the free part of
+  the appendage has disappeared; apparently, also, the intrinsic muscles of the appendage have
+  vanished, with the possible exception of the post-stigmatic muscle, so that any direct comparison
+  between the branchial appendages of Limulus and the scorpions is limited to the comparison of
+  their branchiæ, their nerves, and their afferent and efferent blood-vessels.</p>
+
+  <p>In the case of Ammoc&#x0153;tes the comparison must be made not with air-breathing but with
+  water-breathing scorpions, such as existed in past ages in the forms of Eurypterus, Pterygotus,
+  Slimonia, and with the crowd of trilobite and Limulus-like forms which were in past ages so
+  predominant in the sea; forms in some of which the branchial appendages had already become
+  internal, but which, from the very fact of these forms being water-breathers, probably resembled,
+  in respect of their respiratory apparatus, Limulus rather than the present-day scorpion.</p>
+
+  <p class="sp3">On the assumption that the branchial appendages of Ammoc&#x0153;tes, like the
+  branchial appendages of the scorpion group, are to a certain extent comparable with those of
+  Limulus, it becomes a matter of great interest to inquire whether the mode in which respiration is
+  effected in Ammoc&#x0153;tes resembles most that of Limulus or of the scorpion.</p>
+
+  <p class="ac"><span class="sc">The Origin of the Branchial Musculature.</span></p>
+
+  <p>The difference between the movements of respiration in Limulus and those of the scorpions
+  consists in the fact that, although in both cases respiration is effected mainly by dorso-ventral
+  muscles, these muscles are not homologous in the two cases: in the former, the dorso-ventral
+  appendage-muscles are mainly concerned, in the latter, the dorso-ventral somatic muscles.</p>
+
+  <div><span class="pagenum" id="page171">{171}</span></div>
+
+  <p>The paper by Benham gives a full description of the musculature of Limulus, and according to
+  his arrangement the muscles are divided into two sets, longitudinal and dorso-ventral. Of the
+  latter, each mesosomatic segment possesses a pair of dorso-ventral muscles, attached to the
+  mid-ventral mesosomatic entochondrite, and to the tergal surface (Fig. <a href="#fig58">58</a>,
+  <i>Dv.</i>). These muscles are called by Benham the vertical mesosomatic muscles. I shall call
+  them the somatic dorso-ventral muscles, in contradistinction to the dorso-ventral muscles of the
+  branchial appendages. Of the latter, the two chief are the external branchial (Fig. <a
+  href="#fig66">66</a>, <i>m<sub>3</sub></i>) and the posterior entapophysio-branchial (Fig. <a
+  href="#fig66">66</a>, <i>m<sub>1</sub></i>); a third muscle is the anterior entapophysio-branchial
+  (Fig. <a href="#fig66">66</a>, <i>m<sub>2</sub></i>). Of these muscles, the posterior
+  entapophysio-branchial (<i>m<sub>1</sub></i>) is closely attached along the branchial
+  cartilaginous bar up to its round-headed termination on the anterior surface of the appendage. The
+  anterior entapophysio-branchial muscle (<i>m<sub>2</sub></i>) is attached to the branchial
+  cartilage near the entapophysis.</p>
+
+  <p>In the case of the scorpion, as described by Miss Beck, the branchial appendage has become
+  reduced to the branchiæ, and the intrinsic appendage-muscles have entirely disappeared, with the
+  possible exception of the small post-stigmatic muscle; on the other hand, the dorso-ventral
+  somatic muscles, which are clearly homologous with the corresponding muscles of Limulus, have
+  remained, and become the essential respiratory muscles.</p>
+
+  <p>Of these two possible types of respiratory movement it is quite conceivable that in the
+  water-breathing scorpions of olden times and in their allies, the dorso-ventral muscles of their
+  branchial appendages may have continued their <i>rôle</i> of respiratory muscles, and so have
+  given origin to the respiratory muscles of the ancestors of Ammoc&#x0153;tes.</p>
+
+  <p>The respiratory muscles of Ammoc&#x0153;tes are three in number, and have been described by
+  Nestler and Miss Alcock as the adductor muscle, the striated constrictor muscle, and the tubular
+  constrictor muscle (Fig. <a href="#fig65">65</a>, <i>m. add.</i>, <i>m.c.s.</i>, and
+  <i>m.c.t.</i>). Of these, the constrictor muscle (Fig. <a href="#fig71">71</a>, <i>m. con.
+  str.</i>) is in close contact with its cartilaginous bar, while the adductor (Fig. <a
+  href="#fig71">71</a>, <i>m. add.</i>) is attached to the cartilage only at its origin and
+  insertion, and the tubular muscles (Fig. <a href="#fig71">71</a>, <i>m. con. tub.</i>) have
+  nothing whatever to do with the cartilage at all, being attached ventrally to the connective
+  tissue in the neighbourhood <span class="pagenum" id="page172">{172}</span>of the ventral aorta
+  (<i>V.A.</i>), and dorsally to the mid-line between the dorsal aorta (<i>D.A.</i>) and the
+  notochord.</p>
+
+  <p>The close relationship of the constrictor muscle to the cartilaginous branchial bar does not
+  favour the surmise that this muscle is homologous with the dorso-ventral somatic muscle of the
+  scorpion. It is, however, directly in accordance with the view that this muscle is homologous with
+  one of the dorso-ventral appendage-muscles, such as the posterior entapophysio-branchial muscle
+  (<i>m<sub>1</sub></i>, Fig. <a href="#fig66">66</a>) of the Limulus appendage, especially when the
+  homology of the Ammoc&#x0153;tes branchial cartilage with the Limulus branchial cartilage is borne
+  in mind. I am, therefore, inclined to look upon the constrictor and adductor muscles of the
+  Ammoc&#x0153;tes branchial segment as more likely to have been derived from dorso-ventral muscles
+  which belonged originally to a branchial appendage, such as we see in Limulus, than from
+  dorso-ventral somatic muscles, such as the vertical mesosomatic muscles which are found both in
+  Limulus and scorpion. In other words, I am inclined to hold the view that the somatic
+  dorso-ventral muscles have disappeared in this region in Ammoc&#x0153;tes, while dorso-ventral
+  appendage-muscles have been retained, <i>i.e.</i> the exact reverse to what has taken place in the
+  air-breathing scorpion.</p>
+
+  <p>I am especially inclined to this view because of the manner in which it fits in with and
+  explains van Wijhe's results. Ever since Schneider divided the striated muscles of vertebrates
+  into parietal and visceral, such a division has received general acceptance and, as far as the
+  head-region is concerned, has received an explanation in van Wijhe's work; for Schneider's
+  grouping corresponds exactly to the two segmentations of the head-mesoblast, discovered by van
+  Wijhe, <i>i.e.</i> to the somatic and splanchnic striated muscles according to my nomenclature. Of
+  these two groups the splanchnic or visceral striated musculature, innervated by the Vth, VIIth,
+  IXth, and Xth nerves, which ought on this theory to be derived from the musculature of the
+  corresponding appendages, is, speaking generally, dorso-ventral in direction in Ammoc&#x0153;tes
+  and of the same character throughout; the somatic musculature, on the other hand, is clearly
+  divisible, in the head region, into two sets&mdash;a spinal and a cranial set. The somatic muscles
+  innervated by the spinal set of nerves, including in this term the spino-occipital or so-called
+  hypoglossal nerves, are in Ammoc&#x0153;tes most sharply defined from all the other muscles of the
+  body. They form the great dorsal and ventral longitudinal <span class="pagenum"
+  id="page173">{173}</span>body-muscles, which extend dorsally as far forward as the nose and are
+  developed embryologically quite distinctly from the others, being formed as muscle-plates
+  (Kästchen). On the other hand, the cranial somatic muscles are the eye-muscles, the formation of
+  which resembles that of the visceral muscles, and not of the spinal somatic. Their direction is
+  not longitudinal, but dorso-ventral; they cannot, in my opinion, be referred to the somatic
+  trunk-muscles, and must, therefore, form a separate group to themselves. Thus the striated
+  musculature of the Ammoc&#x0153;tes must be divided into (1) the visceral muscles; (2) the
+  longitudinal somatic muscles; and (3) the dorso-ventral somatic muscles. Of these the 1st, on the
+  view just stated, represent the original appendage-muscles; the 2nd belong to the spinal region,
+  and will be considered with that region; the 3rd represent the original segmental dorso-ventral
+  somatic muscles, which are so conspicuous in the musculature of the Limulus and the scorpion
+  group.</p>
+
+  <p>The discussion of this last statement will be given when I come to deal with the prosomatic
+  segments of Ammoc&#x0153;tes. I wish, here, simply to point out that van Wijhe has shown that the
+  eye-muscles develop from his 1st, 2nd, and 3rd dorsal mesoblastic segments, and therefore
+  represent the somatic muscles belonging to those segments, while no development of any
+  corresponding muscles takes place in the 4th, 5th, and 6th segments; so that if the eye-muscles
+  represent a group of dorso-ventral somatic muscles, such muscles have been lost in the 4th, 5th,
+  and 6th segments. The latter segments are, however, the glossopharyngeal and vagus segments, the
+  branchial musculature of which is derived from the ventral segments of the mesoderm. In other
+  words, van Wijhe's observations mean that the dorso-ventral somatic musculature has been lost in
+  the branchial or mesosomatic region, while the dorso-ventral appendage musculature has been
+  retained, and that, therefore, the mode of respiration in Ammoc&#x0153;tes more closely resembles
+  that of Limulus than of Scorpio.</p>
+
+  <p>In addition to these branchial muscles, another and very striking set of muscles is found in
+  the respiratory region of Ammoc&#x0153;tes&mdash;the so-called tubular muscles. These muscles are
+  of great interest, but as they are especially connected with the VIIth nerve, their consideration
+  is best postponed to the chapter dealing with that nerve.</p>
+
+  <p class="sp3">Also, in connection with the vagus group of nerves, special sense-organs are found
+  in the skin covering this mesosomatic region, the so-called epithelial pit-organs (<i>Ep.
+  pit.</i>, Fig. <a href="#fig71">71</a>). They, too, are of <span class="pagenum"
+  id="page174">{174}</span>great interest, but their consideration may also better be deferred to
+  the chapter dealing with those special sense-systems known as the lateral line and auditory
+  systems.</p>
+
+  <p class="ac"><span class="sc">Comparison of the Branchial Circulation in Ammoc&#x0153;tes and
+  Limulus.</span></p>
+
+  <p>Closely bound up with the respiratory system is the nature of the circulation of blood through
+  the gills. Before, therefore, proceeding to the consideration of the segments in front of those
+  which carry branchiæ, it is worth while to compare the circulation of the blood in the gills of
+  Limulus and of Ammoc&#x0153;tes respectively.</p>
+
+  <p>In all the higher vertebrates the blood circulates in a closed system of capillaries, which
+  unite the arterial with the venous systems. In all the higher invertebrates this capillary system
+  can hardly be said to exist; the blood is pumped from the arterial system into blood spaces or
+  lacunæ, and thus comes into immediate contact with the tissues. From these it is collected into
+  veins, and so returned to the heart. There is, in fact, no separate lymph-system in the higher
+  invertebrates; the blood-system and lymph-system are not yet differentiated from each other. This
+  also is the case in Ammoc&#x0153;tes; here, too, in many places the blood is poured into a lacunar
+  space, and collected thence by the venous system; a capillary system is only in its commencement
+  and a lymph-system does not yet exist. In this part of its vascular system Ammoc&#x0153;tes again
+  resembles the higher invertebrates more than the higher vertebrates.</p>
+
+  <p>This resemblance is still more striking when the circulation in the respiratory organs of the
+  two animals is compared. A branchial appendage is essentially an appendage whose vascular system
+  is arranged for the special purpose of aerating blood. In the higher vertebrates such a purpose is
+  attained by the pulmonary capillaries, in Limulus by the division of the posterior surface of the
+  basal part of the appendage into thin lamellar plates, the interior of each of which is filled
+  with blood. The two surfaces of each lamella are kept parallel to each other by means of fibrous
+  or cellular strands forming little pillars at intervals, called by Macleod "colonettes." A
+  precisely similar arrangement is found in the scorpion gill-lamella, as seen in Fig. <a
+  href="#fig69">69</a>, A, taken from Macleod. In Ammoc&#x0153;tes there are no well-defined
+  branchial capillaries, but the blood circulates, as in <span class="pagenum"
+  id="page175">{175}</span>the invertebrate gill, in a lamellar space; here, also, as Nestler has
+  shown, the opposing walls of the gill-lamella are held in position by little pillar-like cells, as
+  seen in Fig. <a href="#fig69">69</a>, B, taken from his paper.</p>
+
+  <p>In this representative of the earliest vertebrates the method of manufacturing an efficient
+  gill out of a lacunar blood-space is precisely the same as that which existed in Limulus and the
+  scorpion, and, therefore, as that which existed in the dominant invertebrate group at the time
+  when vertebrates first appeared. This similarity indicates a close resemblance between the
+  circulatory systems of the two groups of animals, and therefore, to the superficial inquirer,
+  would indicate an homology between the heart of the vertebrate and the heart of the higher
+  invertebrate; but the former is situated ventrally to the gut and the nervous system, while the
+  latter is composed of a long vessel which lies in the mid-dorsal line immediately under the
+  external dorsal covering. Indeed, this ventral position of the heart in the one group of animals
+  and its dorsal position in the other, combined with the corresponding positions of the central
+  nervous system, is one of the principal reasons why all the advocates of the origin of vertebrates
+  from the Appendiculata, with the single exception of myself, feel compelled to reverse the dorsal
+  and ventral surfaces in deriving the vertebrate from the invertebrate. But there is one most
+  important fact which ought to make us hesitate before accepting the homology of the dorsal heart
+  of the arthropod with the ventral heart of the vertebrate&mdash;The heart in all invertebrates is
+  a systemic heart, <i>i.e.</i> drives the arterial blood to the different organs of the body, and
+  then the veins carry it back to the respiratory organ, from whence it passes to the heart.</p>
+
+  <div class="ac w25 fcenter sp2">
+    <a href="images/fig069.png" id="fig69"><img style="width:100%" src="images/fig069.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p><span class="sc">Fig. 69.&mdash;Comparison of Branchial Lamellæ of Limulus and Scorpio with
+      Branchial Lamellæ of Ammoc&#x0153;tes.</span></p>
+      <p class="sp0">A, Branchial lamellæ of Scorpio (after Macleod); B, Branchial lamellæ of
+      Ammoc&#x0153;tes (after Nestler).</p>
+    </div>
+  </div>
+
+  <p>The only exception to this scheme is found in the vertebrate where the heart is essentially a
+  branchial heart, the blood being <span class="pagenum" id="page176">{176}</span>driven from the
+  heart to the ventral aorta, from which by the branchial arteries it is carried to the gills, and
+  then, after aeration, is collected into the dorsal aorta, whence it is distributed over the body.
+  The distributing systemic vessel is the dorsal aorta, not the heart which belongs essentially to
+  the ventral venous system. This constitutes a very strong reason for believing that the systemic
+  heart of the invertebrate is not homologous with the heart of the vertebrate. How, then, did the
+  vertebrate heart arise?</p>
+
+  <p>Let us first see how the blood is supplied to the gills in Limulus.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig070.png" id="fig70"><img style="width:100%" src="images/fig070.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p><span class="sc">Fig. 70.&mdash;Longitudinal Diagrammatic Section through the Mesosomatic
+      Region of Limulus, to show the origin of the Branchial Arteries.</span> (After <span
+      class="sc">Benham</span>.)</p>
+      <p class="sp0 ac"><i>L.V.S.</i>, longitudinal venous sinus, or collecting sinus; <i>a.
+      br.</i>, branchial arteries; <i>V.p.</i>, veno-pericardial muscles; <i>P.</i>,
+      pericardium.</p>
+    </div>
+  </div>
+
+  <p>In Limulus the blood flows into the lamellæ from sinuses or blood-spaces (<i>b.s.</i>, Fig. <a
+  href="#fig66">66</a>) at the base of each of the lamellæ, which sinuses are filled by a vessel
+  which may be called the branchial artery, since it is the afferent branchial vessel. On each side
+  of the middle line of the ventral surface of the body a large longitudinal venous sinus exists,
+  called by Milne-Edwards the venous collecting sinus, <i>L.V.S.</i>, (Fig. <a href="#fig70">70</a>
+  and Fig. <a href="#fig58">58</a>), which gives off to each of the branchial appendages on that
+  side a well-defined afferent branchial vessel&mdash;the branchial artery (<i>a. br.</i>). The
+  blood of the branchial artery flows into the blood-spaces between the anterior and posterior
+  laminæ of the appendage and thence into the gill-lamellæ, from which it is collected into an
+  efferent vessel or branchial vein, termed by Milne-Edwards the branchio-cardiac canal, which
+  carries it back to the dorsal heart. The position of the branchial artery and vein is shown in
+  Fig. <a href="#fig66">66</a>, which represents a section through the branchial appendage of
+  Limulus at right angles to the cartilaginous branchial bar (<i>br. cart.</i>), just as Fig. <a
+  href="#fig65">65</a> represents a section through the <span class="pagenum"
+  id="page177">{177}</span>branchial appendage of Ammoc&#x0153;tes at right angles to the
+  cartilaginous branchial bar.</p>
+
+  <p>Further, the observations of Blanchard, Milne-Edwards, Ray Lankester, and Benham concur in
+  showing that in both Limulus and the scorpion group a striking and most useful connection exists
+  between the heart and these two collecting venous sinuses, in the shape of a segmentally arranged
+  series of muscular bands (<i>V.p.</i>, Fig. 70 and Fig. <a href="#fig58">58</a>), attached, on the
+  one hand, to the pericardium, and on the other to the venous collecting sinus on each side. These
+  muscular bands, to which Lankester and Benham have given the name of 'veno-pericardial muscles,'
+  are so different in appearance from the rest of the muscular substance, that Milne-Edwards did not
+  recognize them as muscular, but called them 'brides transparentes.' Blanchard speaks of them in
+  the scorpion as 'ligaments contractiles,' and considers that they play an important part in
+  assisting the pulmonary circulation; for, he says, "en mettant a nu une portion du c&#x0153;ur, on
+  remarque que ces battements se font sentir sur les ligaments contractiles, et determinent sur les
+  poches pulmonaires une pression qui fait aussitot refluer et remonter le sang dans les vaisseaux
+  pneumocardiaques." Lankester, in discussing the veno-pericardial muscles of Limulus and of the
+  scorpions, says that these muscles probably contract simultaneously with the heart and are of
+  great importance in assisting the flow through the pulmonary system. More recently Carlson has
+  investigated the action of these muscles in the living Limulus and found that they act
+  simultaneously with the muscles of respiration.</p>
+
+  <p>Precisely the same arrangement of veno-pericardial muscles and of longitudinal venous
+  collecting sinuses occurs in the scorpions. It is one of the fundamental characters of the group,
+  and we may fairly assume that a similar arrangement existed in the extinct forms from which I
+  imagine the vertebrate to have arisen. The further consideration of this group of muscles will be
+  given in Chapter IX.</p>
+
+  <p>Passing now to the condition of the branchial blood-vessels of Ammoc&#x0153;tes, we see that
+  the blood passes into the gill-lamellæ from a blood-space in the appendage, which can hardly be
+  dignified by the name of a blood-vessel. This blood-space is supplied by the branchial artery
+  which arises segmentally from the ventral aorta (<i>V.A.</i>), as seen in Fig. <a
+  href="#fig71">71</a> (taken from Miss Alcock's paper). From the gill-lamellæ the blood is
+  collected into an efferent or branchial vein (<i>v. br.</i>), which <span class="pagenum"
+  id="page178">{178}</span>runs, as seen in Fig. <a href="#fig65">65</a>, along the free edge of the
+  diaphragm, and terminates in the dorsal aorta.</p>
+
+  <p>The ventral aorta is a single vessel near the heart, but at the commencement of the thyroid it
+  divides into two, and so forms two ventral longitudinal vessels, from which the branchial arteries
+  arise segmentally.</p>
+
+  <div class="ac w50 fcenter sp2">
+    <a href="images/fig071.jpg" id="fig71"><img style="width:72%" src="images/fig071.jpg" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p><span class="sc">Fig. 71.&mdash;Diagram constructed from a series of Transverse Sections
+      through a Branchial Segment, showing the arrangement and relative positions of the Cartilage,
+      Muscles, Nerves, and Blood-Vessels.</span></p>
+      <p class="sp0">Nerves coloured red are the motor nerves to the branchial muscles. Nerves
+      coloured blue are the internal sensory nerves to the diaphragms and the external sensory
+      nerves to the sense-organs of the lateral line system. <i>Br. cart.</i>, branchial cartilage;
+      <i>M. con. str.</i>, striated constrictor muscles; <i>M. con. tub.</i>, tubular constrictor
+      muscles; <i>M. add.</i>, adductor muscle; <i>D.A.</i>, dorsal aorta; <i>V.A.</i>, ventral
+      aorta; <i>S.</i>, sense-organs on diaphragm; <i>n. Lat.</i>, lateral line nerve; <i>X.</i>,
+      epibranchial ganglia of vagus; <i>R. br. prof. VII.</i>, <i>ramus branchialis profundus</i> of
+      facial; <i>J.v.</i>, jugular vein; <i>Ep. pit.</i>, epithelial pit.</p>
+    </div>
+  </div>
+
+  <p>From this description it is clear that the vascular supply of the branchial segment of
+  Ammoc&#x0153;tes would resemble most closely the vascular supply of the Limulus branchial
+  appendage, if the ventral aorta of the former was derived from two longitudinal veins, homologous
+  with the paired longitudinal venous sinuses of the latter.</p>
+
+  <div><span class="pagenum" id="page179">{179}</span></div>
+
+  <p class="sp3"><i>A priori</i>, such a derivation seems highly improbable; and yet it is precisely
+  the manner in which embryology teaches us that the heart and ventral aorta of the vertebrate have
+  arisen.</p>
+
+  <p class="ac"><span class="sc">The Origin of the Invertebrate Heart and the Origin of the
+  Vertebrate Heart.</span></p>
+
+  <p>Not only does the vertebrate heart differ from that of the invertebrate, in that it is
+  branchial while the latter is systemic, but also it is unique in its mode of formation in the
+  embryo. In the Appendiculata the heart is formed as a single organ in the mid-dorsal line by the
+  growth of the two lateral plates of mesoblast dorsalwards, the heart being formed where they meet.
+  In Mammalia and Aves, the heart and ventral aorta commence as a pair of longitudinal veins, one on
+  each side of the commencing notochord.</p>
+
+  <p>If the embryo be removed from the yolk, the surface of the embryo covering these two venous
+  trunks can be spoken of as the ventral surface of the embryo at that stage, and indeed we find
+  that in the present day there is an increasing tendency to speak of this surface as the ventral
+  surface of the embryo. Thus, Mitsukuri, in his studies of chelonian embryos, lays great stress on
+  the importance of surface views and when the embryo has been removed from the yolk, figures and
+  speaks of its ventral surface. So, also, Locy and Neal find that the best method of seeing the
+  early segments of the embryo is to remove the embryo from the yolk, and examine what they speak of
+  as a ventral view. At the period, then, before the formation of the throat, we may say that on the
+  ventral surface of the embryo a pair of longitudinal venous sinuses are found, one on each side of
+  the mid-ventral line, which are in the same position with respect to the mid-axis of the embryo as
+  are the longitudinal venous sinuses in Limulus.</p>
+
+  <p>The next step is the formation of the throat by the extension of the layers of the embryo
+  laterally to meet in the mid-line and so form the pharynx, with the consequence that a new ventral
+  surface is formed; these two veins, as is well known, travel round also, and, meeting together in
+  the new mid-ventral line, form the subintestinal vein, the heart, and the ventral aorta.</p>
+
+  <p>What is true of Mammalia and Aves, has been shown by P. Mayer to be true universally among
+  vertebrates, so that in all cases the heart and ventral aorta have arisen by the coalescence in
+  the new mid-ventral <span class="pagenum" id="page180">{180}</span>line of two longitudinal venous
+  channels, which were originally situated one on each side of the notochord, in what was then the
+  ventral surface of this part of the embryo. This history is especially instructive in showing how
+  the pharyngeal region is formed by the growing round of the lateral mesoblast, <i>i.e.</i> the
+  muscular and other mesoblastic tissues of the branchial segments, and how the two longitudinal
+  veins take part in this process. The phylogenetic interpretation of this embryological fact seems
+  to be, that the new ventral surface of the vertebrate in this region is formed, not only by the
+  branchial appendages, but also by the growth ventrally of that part of the original ventral
+  surface which covered each longitudinal venous sinus.</p>
+
+  <p>The following out of the consecutive clues, which one after the other arise in harmonious
+  succession as the necessary sequence of the original working hypothesis, brings even now into view
+  the manner in which the respiratory portion of the alimentary canal arose, and gives strong hints
+  as to the position of that part of the arthropod which gave origin to the notochord. Here I will
+  say no more at present, for the origin of the new alimentary canal of the vertebrate and of the
+  notochord will be more fittingly discussed as a whole, after all the other organs of the
+  vertebrate have been compared with the corresponding organs of the arthropod.</p>
+
+  <div class="ac w50 fcenter sp2">
+    <a href="images/fig072.png" id="fig72"><img style="width:44%" src="images/fig072.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p><span class="sc">Fig. 72.&mdash;Diagram (Upper Half of Figure) of the Original Position of
+      Veins (H) which come together to form the Heart of a Vertebrate.</span></p>
+      <p><i>C.N.S.</i>, central nervous system; <i>nc.</i>, notochord; <i>m.</i>, myotome.</p>
+      <p class="sp0">The lower half of figure shows comparative position of the longitudinal venous
+      sinus (<i>L.V.S.</i>) in Limulus. <i>C.N.S.</i>, central nervous system; <i>Al.</i>,
+      alimentary canal; <i>H.</i>, heart; <i>m.</i>, body-muscles.</p>
+    </div>
+  </div>
+
+  <p>The strong evidence that the vertebrate heart was formed from a pair of longitudinal venous
+  sinuses on the ventral side of the central canal, carries with it the conclusion that the original
+  single median dorsal heart of the arthropod is not represented in the vertebrate, <span
+  class="pagenum" id="page181">{181}</span>for the dorsal aorta cannot by any possibility represent
+  that heart.</p>
+
+  <p>Although it is not now functional the original existence of so important an organ as a dorsal
+  heart may have left traces of its former presence; if so, such traces would be most likely to be
+  visible in the lowest vertebrates, just as the median eyes are much more evident in them than in
+  the higher forms. In Fig. <a href="#fig58">58</a> the position of the dorsal heart is shown in
+  Limulus, and in Fig. <a href="#fig70">70</a> the shape and extent of this dorsal heart is shown.
+  It extends slightly into the prosomatic region, and thins down to a point there, runs along the
+  length of the animal and finally thins down to a point at the caudal end.</p>
+
+  <p>The heart is surrounded by a pericardium, from which at regular intervals a number of
+  dorso-ventral muscles pass, to be inserted into the longitudinal venous sinus on each side. These
+  veno-pericardial muscles are absolutely segmental with the mesosomatic segments, and are confined
+  to that region, with the exception of two pairs in the prosomatic region. Their homologies will be
+  discussed later.</p>
+
+  <p>Any trace of a heart such as we have just described must be sought for in Ammoc&#x0153;tes
+  between the central nervous system and the mid-line dorsally. Now, in this very position a large
+  striking mass of tissue is found, represented in section in Fig. <a href="#fig73">73</a>,
+  <i>f</i>. It forms a column of similar tissue along the whole mid-dorsal region, except at the two
+  extremities; it tapers away in the caudal region, and headwards grows thinner and thinner, so that
+  no trace of it is seen anterior to the commencement of the branchial region. It resembles in its
+  dorsal position, in its shape, and in its size a dorsal heart-tube such as is seen in Limulus and
+  elsewhere, but it differs from such a tube in its extension headwards. The heart-tube of Limulus
+  ceases at the anterior end of the mesosomatic region, this fat-column of Ammoc&#x0153;tes at the
+  posterior end. In its structure there is not the slightest sign of anything of the nature of a
+  heart; it is a solid mass of closely compacted cells, and the cells are all very full of fat,
+  staining intensely black with osmic acid. Nowhere else in the whole body of Ammoc&#x0153;tes is
+  such a column of fat to be found. It is not skeletogenous tissue with cells of the nature of
+  cartilage-cells, as Gegenbaur thought and as Balfour has depicted (Vol. II., Fig. 315) in his
+  'Comparative Embryology,' as though this tissue were a part of the vertebral column, but is simply
+  fat-cells, such as might easily have taken the place of some other previously existing organ.</p>
+
+  <div><span class="pagenum" id="page182">{182}</span></div>
+
+  <p>I do not know how to decide the question which thus arises. Supposing, for the sake of
+  argument, that this column of fat-cells has really taken the place of the original dorsal heart,
+  what criterion would there be as to this? The heart <i>ex hypothesi</i> having ceased to function,
+  the muscular tissue would not remain, and the space would be filled up, presumably with some form
+  of connective tissue. As likely as not, the connective tissue might take the form of fatty tissue,
+  the storage of fat being a physiological necessity to an animal, while at the same time no special
+  organ has been developed for such a purpose, but fat is being laid down in all manner of places in
+  the body.</p>
+
+  <p>This dorsal fat-column, as it is seen in Ammoc&#x0153;tes, is not found in the higher
+  vertebrates, so that it possesses, at all events, the significance of being a peculiarity of
+  ancient times before the vertebrate skeletal column was formed.</p>
+
+  <p>I mention it here in connection with my view as to the origin of vertebrates, because there it
+  is, in the very place where the dorsal heart ought to have been. For my own part, I should not
+  have expected that a muscular organ such as the heart would leave any trace of itself if it
+  disappeared, so that its absence in the dorsal region of the vertebrate does not seem to me in the
+  slightest degree to invalidate my theory.</p>
+
+  <div class="ac w30 fcenter sp3">
+    <a href="images/fig073.png" id="fig73"><img style="width:100%" src="images/fig073.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p><span class="sc">Fig. 73.&mdash;Section through the Notochord (<i>nc.</i>), the Spinal
+      Canal and the Fat-column (<i>f.</i>), of Ammoc&#x0153;tes, drawn from an Osmic
+      Preparation.</span></p>
+      <p class="sp0"><i>sp. c.</i>, spinal cord; <i>gl.</i>, glandular tissue filling the spinal
+      canal; <i>sk.</i>, Gegenbaur's skeletogenous cells; <i>p.</i>, pigment.</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page183">{183}</span></div>
+
+  <p class="ac"><span class="sc">Summary.</span></p>
+
+  <div class="bq1 smaller sp5">
+    <p>From the close similarity of structure and position between the branchial skeleton of Limulus
+    and of Ammoc&#x0153;tes, as given in the preceding chapter, it logically follows that the
+    branchiæ of Ammoc&#x0153;tes must be homologous with the branchiæ of Limulus. But the
+    respiratory apparatus of Limulus consists of branchial appendages. It follows, therefore, that
+    the branchiæ of Ammoc&#x0153;tes, and consequently of the vertebrates, must have been derived
+    from branchial appendages, and as they are internal, not external, such branchial appendages
+    must have been of the nature of 'sunk-in' branchial appendages. Such internal appendages are
+    characteristic of the scorpion tribe, and of, perhaps, the majority of the Palæostraca, for no
+    external respiratory appendages have been discovered in any of the sea-scorpions.</p>
+    <p>In the vertebrates&mdash;and it is especially well shown in Ammoc&#x0153;tes&mdash;a double
+    segmentation exists in the head-region, a body or somatic segmentation, and a branchial or
+    splanchnic segmentation, respectively expressed by the terms mesomeric and branchiomeric
+    segmentations. The nerves which supply the latter segments form a very well-marked group
+    (Charles Bell's system of lateral or respiratory nerves) which do not conform to the system of
+    spinal nerves, for they do not arise from separate motor and sensory roots, but are mixed nerves
+    from the very beginning.</p>
+    <p>The system of cranial segmental nerves is older than the spinal system, and cannot,
+    therefore, be derived from it, but can be arranged as a system supplying two segments, somatic
+    and splanchnic, which differ in the following way: Each somatic segment is supplied by two
+    roots, motor and sensory respectively, as in the spinal cord segments, while each splanchnic
+    segment possesses only one root, which is mixed in function.</p>
+    <p>The peculiarities of the grouping of the cranial segmental nerves, which have hitherto been
+    unexplained, immediately receive a straightforward and satisfactory explanation if the
+    splanchnic or branchiomeric segments owe their origin to a system of appendages after the style
+    of those of Limulus.</p>
+    <p>In Limulus and all the Arthropoda, the segmentation is double, being composed of (1) somatic
+    or body-segments, constituting the mesomeric segmentation; (2) appendage-segments, which, seeing
+    that they carry the branchiæ, constitute a branchiomeric segmentation. Similarly to the cranial
+    region of the vertebrate, the nerves which supply the somatic segments arise from separate
+    sensory and motor roots, while the single nerve which supplies each appendage contains all the
+    fibres for the appendage, both motor and sensory.</p>
+    <p>It follows from this that the branchial segments supplied by the vagus and glossopharyngeal
+    nerves ought to have arisen from appendages bearing branchiæ.</p>
+    <p>Although the evidence of such appendages has entirely disappeared in the higher vertebrates,
+    together with the disappearance of branchiæ, and is not strikingly apparent in the higher
+    gill-bearing fishes, yet in Ammoc&#x0153;tes, so great is the difference here from all other
+    fishes, it is natural to describe the pharyngeal or respiratory chamber as a chamber into which
+    a symmetrical series of respiratory appendages, the so-called diaphragms, are dependent. Each of
+    these appendages possesses its own mixed nerve, glossopharyngeal or vagus, <span class="pagenum"
+    id="page184">{184}</span>its own cartilage, its own set of visceral muscles, its own
+    sense-organs, just as do the respiratory appendages of Limulus.</p>
+    <p>The branchial unit in the vertebrate is not the gill-pouch, but the branchial bar or
+    appendage between the pouches. Embryology shows how each such appendage grows inwards, how a
+    c&#x0153;lomic cavity is formed in it, similarly to the ingrowing of the branchial appendage in
+    scorpions.</p>
+    <p>We do not know how the palæostracan sea-scorpions breathed; they resemble the scorpion of the
+    present day somewhat in form, but they are in many respects closely allied to Limulus. The
+    present-day scorpion is a land animal, and the muscles by which he breathes are dorso-ventral
+    somatic muscles, while those of Limulus are the appendage-muscles.</p>
+    <p>The old sea-scorpions very probably used their appendage-muscles after the Limulus fashion,
+    being water-breathers, even although their respiratory appendages were no longer free but sunk
+    in below the surface of the body. The probability that such was the case is increased after
+    consideration of the method of breathing in Ammoc&#x0153;tes, for the respiratory muscles of the
+    latter animal are directly comparable with the muscles of the respiratory appendages of Limulus,
+    and are not somatic. Even the gills themselves of Ammoc&#x0153;tes are built up in the same
+    fashion as are those of Limulus and the scorpions. The conception of the branchial unit as a
+    gill-bearing appendage, not a gill-pouch, immediately explains the formation of the vertebrate
+    heart, which is so strikingly different from that of all invertebrate hearts, in that it
+    originates as a branchial and not as a systemic heart, and is formed by the coalescence of two
+    longitudinal veins.</p>
+    <p class="sp0">The origin of these two longitudinal veins is immediately apparent if the
+    vertebrate arose from a palæostracan, for in Limulus and the whole scorpion tribe, in which the
+    heart is a systemic heart, the branchiæ are supplied with blood from two large longitudinal
+    venous sinuses, situated on each side of the middle line of the animal in an exactly
+    corresponding position to that of the two longitudinal veins, which come together to form the
+    heart and ventral aorta of the vertebrate. The consideration of the respiratory apparatus and of
+    its blood-supply in the vertebrate still further points to the origin of vertebrates from the
+    Palæostraca.</p>
+  </div>
+
+  <div><span class="pagenum" id="page185">{185}</span></div>
+
+  <p class="ac">CHAPTER V</p>
+
+  <p class="ac"><i>THE EVIDENCE OF THE THYROID GLAND</i></p>
+
+  <div class="bq1 it1 smaller sp3">
+    <p class="sp0">The value of the appendage-unit in non-branchial segments.&mdash;The double
+    nature of the hyoid segment.&mdash;Its branchial part.&mdash;Its thyroid part.&mdash;The double
+    nature of the opercular appendage.&mdash;Its branchial part.&mdash;Its genital
+    part.&mdash;Unique character of the thyroid gland of Ammoc&#x0153;tes&mdash;Its
+    structure.&mdash;Its openings.&mdash;The nature of the thyroid segment.&mdash;The uterus of the
+    scorpion.&mdash;Its glands.&mdash;Comparison with the thyroid gland of
+    Ammoc&#x0153;tes.&mdash;Cephalic genital glands of Limulus.&mdash;Interpretation of glandular
+    tissue filling up the brain-case of Ammoc&#x0153;tes.&mdash;Function of thyroid
+    gland.&mdash;Relation of thyroid gland to sexual functions.&mdash;Summary.</p>
+  </div>
+
+  <p>I have now given my reasons why I consider that the glossopharyngeal and vagus nerves were
+  originally the nerves belonging to a series of mesosomatic branchial appendages, each of which is
+  still traceable in the respiratory chamber of Ammoc&#x0153;tes, and gives the type-form from which
+  to search for other serially homologous, although it may be specially modified, segments.</p>
+
+  <p class="sp3">As long as the branchial unit consisted of the gill-pouch the segments of the
+  head-region were always referred to such units, hence we find Dohrn and Marshall picturing to
+  themselves the ancestor of vertebrates as possessing a series of branchial pouches right up to the
+  anterior end of the body. Marshall speaks of olfactory organs as branchial sense-organs; Dohrn of
+  the mouth as formed by the coalescence of gill-slits, of the trigeminal nerve as supplying
+  modified branchial segments, etc.; thus a picture of an animal is formed such as never lived on
+  this earth, or could be reasonably imagined to have lived on it. Yet Dohrn's conceptions of the
+  segmentation were sound, his interpretation only was in fault, because he was obliged to express
+  his segments in terms of the gill-pouch unit. Once abandon that point of view and take as the unit
+  a branchial appendage, then immediately we see that in the region in front of the branchiæ we may
+  still have segments <span class="pagenum" id="page186">{186}</span>homologous to the branchial
+  segments, originally characterized by the presence of appendages, but that such appendages need
+  never have carried branchiæ. The new mouth may have been formed by such appendages, which would
+  express Dohrn's suggestion of its formation by coalesced gill-slits; the olfactory organ may have
+  been the sense-organ belonging to an antennal appendage, which would be what Marshall really meant
+  in calling it a branchial sense-organ.</p>
+
+  <p class="ac"><span class="sc">The Facial Nerve and the Foremost Respiratory Segment.</span></p>
+
+  <p>This simple alteration of the branchiomeric unit from a gill-pouch to an appendage, which may
+  or may not bear branchiæ, immediately sheds a flood of light on the segmentation of the
+  head-region, and brings to harmony the chaos previously existing. Let us, then, follow out its
+  further teachings. Next anteriorly to the glossopharyngeal and vagus nerves comes the facial
+  nerve; a nerve which supplies the hyoid segment, or, rather, according to van Wijhe the two hyoid
+  segments, for embryologically there is evidence of two segments. As already mentioned, the facial
+  nerve is usually included in the trigeminal or pro-otic group of nerves, the opisthotic group
+  being confined to the glossopharyngeal and vagus. This inclusion of the facial nerve into the
+  pro-otic group of nerves forms one of the main reasons why this group has been supposed to have
+  originally supplied gill-pouch segments, for the hyoid segment is clearly associated with
+  branchiæ.</p>
+
+  <p>When, however, we examine Ammoc&#x0153;tes (<i>cf.</i> Figs. 63 and 64) it is clear that the
+  foremost of the segments forming the respiratory chamber, which must be classed with the rest of
+  the mesosomatic or opisthotic segments, is that supplied by the facial nerves.</p>
+
+  <p>An examination of this respiratory chamber shows clearly that there are six pairs of branchial
+  appendages or diaphragms, which are all exactly similar to each other. These are those already
+  considered, the foremost of which are supplied by the IXth or glossopharyngeal nerves. Immediately
+  anterior to this glossopharyngeal segment is seen in the figures the segment supplied by the VIIth
+  or facial nerves. It is so much like the segments belonging to the glossopharyngeal and vagus
+  nerves as to make it certain that we are dealing here with a branchial segment, composed of a pair
+  of branchial appendages similar to those in the other cases, except that the cartilaginous bar is
+  here replaced by a bar of muco-cartilage and the branchiæ are confined to the posterior part of
+  each appendage. The anterior portion is, as is seen in Fig. <a href="#fig74">74</a>, largely
+  occupied by blood-spaces, but in addition carries the ciliated groove (<i>ps. br.</i>) called by
+  Dohrn 'pseudo-branchiale Rinne.' This groove leads directly into the thyroid gland, which is a
+  large bilateral organ situated in the middle line, as seen in Fig. <a href="#fig80">80</a> and
+  Fig. <a href="#fig85">85</a>. As shown by Miss Alcock, the facial nerve supplies this thyroid
+  gland, as well as the posterior hyoid branchial segment, and, as pointed out by Dohrn, there is
+  every reason to consider this thyroid gland as indicative of a separate segment, especially when
+  van Wijhe's statement that the hyoid segment is in reality double is taken into account.</p>
+
+  <div><span class="pagenum" id="page187">{187}</span></div>
+
+  <div class="ac w50 fcenter sp2">
+    <a href="images/fig074.jpg" id="fig74"><img style="width:88%" src="images/fig074.jpg" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 74.&mdash;Ventral half of Head-region of
+      Ammoc&#x0153;tes.</span></p>
+      <p class="sp0">Somatic muscles coloured red. Branchial and visceral muscles coloured blue.
+      Tubular constrictor muscles distinguished from striated constrictor muscles by simple
+      hatching. <i>Tent.</i>, tentacles; <i>Tent. m.c.</i>, muco-cartilage of tentacles; <i>Vel.
+      m.c.</i>, muco-cartilage of the velum; <i>Hy. m.c.</i>, muco-cartilage of the hyoid segment;
+      <i>Ps. br.</i>, pseudo-branchial groove; <i>Br. cart.</i>, branchial cartilages; <i>Sp.</i>,
+      space between somatic and splanchnic muscles; <i>Th. op.</i>, orifice of thyroid; <i>H.</i>,
+      heart.</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page188">{188}</span></div>
+
+  <p class="sp3">The evidence, then, of Ammoc&#x0153;tes points directly to this conclusion: The
+  facial nerves represent the foremost of the mesosomatic group of nerves, and supply two segments,
+  which have amalgamated with each other. The most posterior of these, the hyoid segment, is a
+  branchial segment of the same character as those supplied by the vagus and glossopharyngeal
+  nerves; represents, therefore, the foremost pair of branchial appendages. The anterior or thyroid
+  segment, on the other hand, differs from the rest in that, instead of branchiæ, it carries the
+  thyroid gland with its two ciliated grooves. If this segment, which is the foremost of the
+  mesosomatic segments, also indicates a pair of appendages which carry the thyroid gland instead of
+  branchiæ, then it follows that this pair of appendages has joined together in the mid-line
+  ventrally and thus formed a single median organ&mdash;the thyroid gland. If, then, we find that
+  the foremost of the mesosomatic appendages in the Palæostraca was really composed of two pairs of
+  appendages, of which the most posterior carried branchiæ, while the anterior pair had amalgamated
+  in the mid-line ventrally, and carried some special organ instead of branchiæ, then the
+  accumulation of coincidences is becoming so strong as to amount to proof of the correctness of our
+  line of investigation.</p>
+
+  <p class="ac"><span class="sc">The First Mesosomatic Segment in Limulus and its Allies.</span></p>
+
+  <p>What, then, is the nature of the foremost pair of mesosomatic appendages in Limulus. They
+  differ from the rest of the mesosomatic appendages in that they do not carry branchiæ, and instead
+  of being <span class="pagenum" id="page189">{189}</span>separate are joined together in the
+  mid-line ventrally to form a single terminal plate-like appendage known as the operculum. On its
+  posterior surface the operculum carries the genital duct on each side.</p>
+
+  <p>So also in the scorpion group, the operculum is always found and always carries the genital
+  ducts.</p>
+
+  <p>A survey of the nature of the opercular appendage demonstrates the existence of three different
+  types&mdash;</p>
+
+  <p>1.  That of Limulus, in which the operculum is free, and carries only the terminations of the
+  genital ducts. In this type the duct on each side opens to the exterior separately (Fig. <a
+  href="#fig75">75</a>).</p>
+
+  <table class="mc tlf sp2 w55" summary="Group of illustrations" title="Group of illustrations">
+    <tr>
+      <td class="vbm ac" style="width:45%;"><a href="images/fig075.png" id="fig75"><img
+      style="width:100%" src="images/fig075.png" alt="" title=""/></a></td>
+      <td class="vbm ac" style="width:45%;"><a href="images/fig076.png" id="fig76"><img
+      style="width:100%" src="images/fig076.png" alt="" title=""/></a></td>
+    </tr>
+    <tr>
+      <td>
+        <div class="smaller aj it1">
+          <p class="sp0"><span class="sc">Fig. 75.&mdash;operculum of Limulus To Show the two
+          separate Genital Ducts.</span></p>
+        </div>
+      </td>
+      <td>
+        <div class="smaller ac">
+          <p><span class="sc">Fig. 76.&mdash;Operculum of Male Scorpion.</span></p>
+          <p class="sp0"><i>Ut.</i>, terminal chamber, or uterus.</p>
+        </div>
+      </td>
+    </tr>
+  </table>
+
+  <p>2.  The type of Scorpio, Androctonus, Buthus, etc., in which the operculum is not free, but
+  forms part of the ventral surface of the body-wall, but, like Limulus, carries only the
+  terminations of the genital ducts. In this type the duct on each side terminates in a common
+  chamber (vagina or uterus), which communicates with the exterior by a single external median
+  opening. This common chamber, or uterus (<i>Ut.</i>), extends the whole breadth of the operculum
+  (as seen in Fig. <a href="#fig76">76</a>), and is limited to that segment.</p>
+
+  <p>3. The type of Thelyphonus, Hypoctonus, Phrynus, and other members of the Pedipalpi, in which
+  the operculum forms a part of the ventral surface of the body wall, but no longer covers only the
+  termination of the genital apparatus. It really consists of two parts, a median anterior, which
+  covers the terminal genital apparatus, <span class="pagenum" id="page190">{190}</span>and a
+  lateral posterior, which covers the first pair of gills, or lung-books, as they are called. In
+  this type (Fig. <a href="#fig77">77</a>) the genital ducts terminate in a common chamber or
+  uterus, the nature of which will be further considered.</p>
+
+  <p>As has been pointed out by Blanchard, the terminal genital organs of the scorpions and the
+  Pedipalpi vary considerably in the different genera, especially the male genital organs. The
+  general type of structure is the same, and consists in both male and female of vasa deferentia,
+  which come together to form a common chamber before the actual opening to the exterior. This
+  common chamber has been called in the female scorpion the vagina, or in Thelyphonus the uterus. I
+  shall use the latter term, in accordance with Tarnani's work, and the corresponding chamber in the
+  male will be the <i>uterus masculinus</i>.</p>
+
+  <p>A considerable discussion has taken place about the method of action of the external genital
+  organs in the members of the scorpion tribe, into which it is hardly necessary to enter here. The
+  evidence points to the conclusion that in all these forms the operculum covers a median single
+  chamber or uterus, into which the genital ducts open on each side, the main channels of emission
+  being provided with a massive chitinous internal framework. We may feel certain that in the old
+  extinct sea-scorpions, Eurypterus, etc., a similar arrangement existed, and that therefore in them
+  also the median portion of the operculum covered a median chamber or uterus composed of the
+  amalgamation of the terminations of the two genital ducts, which were originally separate, as in
+  Limulus.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig077.png" id="fig77"><img style="width:62%" src="images/fig077.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 77.&mdash;Operculum and Following Segments Of Male
+      Thelyphonus.</span></p>
+      <p class="sp0">Opercular segment is marked out by thick black line. <i>Ut. Masc.</i>, uterus
+      masculinus; <i>Int. Op.</i>, internal opening of uterus into genital chamber; <i>Ext. Op.</i>,
+      common external opening to genital chamber (<i>Gen. Ch.</i>) and pulmonary chamber.</p>
+    </div>
+  </div>
+
+  <p>The observations of Schmidt, Zittel, and others show that the <span class="pagenum"
+  id="page191">{191}</span>operculum in the old extinct sea-scorpions, Eurypterus, Pterygotus, etc.,
+  belonged to the type of Thelyphonus, rather than to that of Limulus or Scorpio. In Fig. <a
+  href="#fig78">78</a> I give a picture from Schmidt of the ventral aspect of Eurypterus, and by the
+  side of it a picture of the isolated operculum. Schmidt considers that there were five
+  branchiæ-bearing segments constituting the mesosoma, the foremost of which formed the operculum.
+  Such operculum is often found isolated, and is clearly composed of two lateral appendages fused
+  together in the middle line, of such a nature as to form a median elongated tongue, which lies
+  between and separates the first three pairs of branchial segments. This median tongue, together
+  with the anterior and median portion of the operculum, concealed, in all probability, according to
+  Schmidt, the terminal parts of the genital organs, just as the median part of the operculum in
+  Phrynus and Thelyphonus conceals the complicated terminal portions of the genital organs. The
+  posterior part of the operculum, like that of Phrynus and Thelyphonus, carried the first pair of
+  branchiæ, so Schmidt thinks from the evidence of markings on some specimens.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig078.png" id="fig78"><img style="width:50%" src="images/fig078.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 78.</span>&mdash;<i>Eurypterus.</i></p>
+      <p class="sp0">The segments and appendages on the right are numbered in correspondence with
+      the cranial system of lateral nerve-roots as found in vertebrates. <i>M.</i>, metastoma. The
+      surface ornamentation is represented on the first segment posterior to the branchial segments.
+      The opercular appendage is marked out by dots.</p>
+    </div>
+  </div>
+
+  <p>Apparently an opercular appendage of this kind is in reality the result of a fusion of the
+  genital operculum with the first branchial appendage in forms such as the scorpion; for, in order
+  that the tergal plates may correspond in number with the sternal in Eurypterus, etc., it is
+  necessary to consider that the operculum is composed of two sternites joined together. Similarly
+  in Thelyphonus, Phrynus, etc., this numerical correspondence is only observed if the operculum is
+  looked upon as double.</p>
+
+  <p>A restoration of the mesosomatic region of Eurypterus, viewed <span class="pagenum"
+  id="page192">{192}</span>from the internal surface, might be represented by Fig. <a
+  href="#fig79">79</a>, in which the thick line represents the outline of the opercular segment, and
+  the fainter lines the succeeding branchial segments. The middle and anterior part of the opercular
+  segment carried the terminations of the genital organs; these I have represented, in accordance
+  with our knowledge of the nature of these organs in the present-day scorpions, as a median
+  elongated uterus, bilaterally formed, from which the genital ducts passed, probably as in Limulus,
+  towards a mass of generative gland in the cephalic region, and not as in Scorpio or Thelyphonus,
+  tailwards to the abdominal region.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig079.png" id="fig79"><img style="width:52%" src="images/fig079.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p><span class="sc">Fig. 79.&mdash;Diagram To indicate the probable nature of the Mesosomatic
+      Segments of Eurypterus.</span></p>
+      <p class="sp0">The opercular segment is marked out by the thick black line. The segments
+      <i>II.-VI.</i> bear branchiæ, and segment <i>I.</i> is supposed in the male to carry the
+      uterus masculinus (<i>Ut. Masc.</i>) and the genital ducts.</p>
+    </div>
+  </div>
+
+  <p class="sp3">It is possible that in Holm's representation of Eurypterus, Fig. 104, the genital
+  duct on each side is indicated.</p>
+
+  <p class="ac"><span class="sc">The Thyroid Gland of Ammoc&#x0153;tes.</span></p>
+
+  <p>If we compare this mesosomatic region of Eurypterus with that of Ammoc&#x0153;tes, the
+  resemblance is most striking, and gives a meaning to the facial nerve which is in absolute
+  accordance with the interpretation already given of the glossopharyngeal and vagus nerves. In both
+  cases the foremost respiratory or mesosomatic segment is double, the posterior lateral part alone
+  bearing the branchiæ, while the median and anterior part bore in the one animal the uterus and
+  genital ducts, in the other the thyroid gland and ciliated grooves. We are driven, therefore, to
+  the conclusion that this extraordinary and unique organ, the so-called thyroid gland of
+  Ammoc&#x0153;tes, which exists only in the larval condition and is got rid of as soon as the adult
+  sexual organs are formed, shows the very form and position of the uterus of this invertebrate
+  ancestor of Ammoc&#x0153;tes. What, then, is the nature of the thyroid gland in
+  Ammoc&#x0153;tes?</p>
+
+  <div><span class="pagenum" id="page193">{193}</span></div>
+
+  <p>Throughout the vertebrate kingdom it is possible to compare the thyroid gland of one group of
+  animals with that of another without coming across any very marked difference of structure right
+  down to and including Petromyzon. When, however, we examine Ammoc&#x0153;tes, we find that the
+  thyroid has suddenly become an organ of much more complicated structure, covering a much larger
+  space, and bearing no resemblance to the thyroid glands of the higher forms. At transformation the
+  thyroid of Ammoc&#x0153;tes is largely destroyed, and what remains of the gland in Petromyzon
+  becomes limited to a few follicles resembling those of other fishes. The structure and position of
+  this gland in Ammoc&#x0153;tes is so well known that it is unnecessary to describe it in detail.
+  For the purpose, however, of making my points clear, I give in Fig. <a href="#fig80">80</a> the
+  position and appearance of the thyroid gland (<i>Th.</i>) when the skin and underlying laminated
+  layer has been removed by the action of hypochlorite of soda. On the one side the ventral somatic
+  muscles have been removed to show the branchial cartilaginous basket-work.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig080.png" id="fig80"><img style="width:45%" src="images/fig080.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 80.&mdash;Ventral View of Head Region of Ammoc&#x0153;tes.</span></p>
+      <p class="sp0"><i>Th.</i>, thyroid gland; <i>M.</i>, lower lip, with its muscles.</p>
+    </div>
+  </div>
+
+  <p>The series of transverse sections in Fig. <a href="#fig81">81</a> represents the nature of the
+  organ at different levels in front of and behind the opening into the respiratory chamber; and in
+  Fig. <a href="#fig82">82</a> I have sketched the appearance of the whole gland, viewed so as to
+  show its opening into the respiratory chamber and its posterior curled-up termination.</p>
+
+  <div><span class="pagenum" id="page194">{194}</span></div>
+
+  <div class="ac w45 fcenter sp2">
+    <a href="images/fig081.png" id="fig81"><img style="width:100%" src="images/fig081.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 81.&mdash;Samples from a Complete Series of Transverse
+      Sections through the Thyroid Gland of Ammoc&#x0153;tes.</span></p>
+      <p class="sp0">Sections 1 and 2 are anterior to the thyroid opening, <i>Th. o.</i>; sections
+      3, 4, and 5 are through the thyroid opening; and section 6 is posterior to the thyroid opening
+      before the commencement of the curled portion.</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page195">{195}</span></div>
+
+  <p>The series of transverse sections (1-6, Fig. <a href="#fig81">81</a>) show that we are dealing
+  here with a central glandular chamber, C (Fig. <a href="#fig81">81</a> (6) and Fig. <a
+  href="#fig82">82</a>), which opens by the thyroid duct (<i>Th. o.</i>) into the pharyngeal
+  chamber, and is curled upon itself in its more posterior part. This central chamber divides,
+  anteriorly to the thyroid orifice, into two portions, A, A&prime; (Fig. <a href="#fig82">82</a>),
+  giving origin to two tubes, B, B&prime;, which lie close alongside of, and extend further back
+  than, the posterior limit of the curled portion of the central chamber, C. The structure of the
+  central chamber, C, and, therefore, of the separate coils, is given in both Schneider's and
+  Dohrn's pictures, and is represented in Fig. <a href="#fig81">81</a> (6), which shows the peculiar
+  arrangement and character of the glandular cells typical of this organ, and also the nature of the
+  central cavity, with the arrangement of the ciliated epithelium. The structure of each of the
+  lateral tubes, B, is different from that of the central chamber, in that only half the central
+  chamber is present in them, as is seen by the comparison of the tube B with the tube C in Fig. <a
+  href="#fig81">81</a> (5 and 6), so that we may look upon the central chamber, C, as formed of two
+  tubes, similar in structure to the tubes B, which have come together to form a single chamber by
+  the partial absorption of their walls, the remains of the wall being still visible as the septum,
+  which partially divides the chamber, C, into halves.</p>
+
+  <p>In the walls of each of these tubes is situated a continuous glandular line, the structure of
+  the glandular elements being specially characterized by the length of the cells, by the large
+  spherical nucleus situated at the very base of each cell, and by the way in which the cells form a
+  wedge-shaped group, the thin points of all the wedge-shaped cells coming together so as to form a
+  continuous line along the chamber wall. This free termination of the cells of the gland in the
+  lumen of the chamber constitutes the whole method for the secretion of the gland; there is no
+  duct, no alveolus, nothing but this free termination of the cells.</p>
+
+  <p>Moreover, sections through the portion A, A&prime; (Fig. <a href="#fig82">82</a>) show that
+  here, as in the central chamber, C, four of these glandular lines open into a common chamber, but
+  they are not the same four as in the case of the central chamber, for if we name these glandular
+  lines on the left side <i>a b, a&prime; b&prime;</i> (Fig. <a href="#fig81">81</a>), and on the
+  right side <i>c d, c&prime; d&prime;</i>, then the central chamber has opening into it the glands
+  <i>a b, c d</i>, while the chambers of A and A&prime; have opening into them respectively <i>a b,
+  a&prime; b&prime;</i>, and <i>c d, c&prime; d&prime;</i>. Further, the same series of sections
+  shows that the glands <i>a</i> and <i>b</i> are continuous with the glands <i>a&prime;</i> and
+  <i>b&prime;</i> respectively across the apex of A, and similarly on the other side, so that the
+  two glandular rows <i>a b</i> are continuous with the two glandular rows <i>a&prime; b&prime;</i>,
+  and we see that the <span class="pagenum" id="page196">{196}</span>cavity of the portion A or
+  A&prime; is formed by the bending over of the tube or horn, B or B&prime;, with the partial
+  absorption of the septum so formed between the tube and its bent-over part. If, then, we uncoil
+  the curled-up part of C, and separate the portion, B, on each side from the chamber, C, we see
+  that the so-called thyroid of Ammoc&#x0153;tes may be represented as in Fig. <a
+  href="#fig83">83</a>, <i>i.e.</i> it consists of a long, common chamber, C, which, for reasons
+  apparent afterwards, I will call the <i>palæo-hysteron</i>, which opens, by means of a large
+  orifice, into the respiratory or pharyngeal chamber. The anterior end of this chamber terminates
+  in two tubes, or horns, B, B&prime;, the structure of which shows that the median chamber, C, is
+  the result of the amalgamation of two such tubes, and consequently in this chamber, or
+  <i>palæo-hysteron</i>, the glandular lines are symmetrically situated on each side.</p>
+
+  <div class="ac w35 fcenter sp2">
+    <a href="images/fig082.png" id="fig82"><img style="width:100%" src="images/fig082.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 82.&mdash;Diagrammatic Representation of the so-called
+      Thyroid Gland of Ammoc&#x0153;tes.</span></p>
+      <p class="sp0"><i>C</i>, central chamber; <i>A, A&prime;</i>, anterior extremity; <i>B,
+      B&prime;</i>, posterior extremity; <i>Th. o.</i>, thyroid opening into respiratory chamber;
+      <i>Ps. br., Ps. br&prime;.</i>, ciliated grooves, Dohrn's pseudo-branchial grooves.</p>
+    </div>
+  </div>
+
+  <div class="ac w35 fcenter sp2">
+    <a href="images/fig083.png" id="fig83"><img style="width:100%" src="images/fig083.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="sp0"><span class="sc">Fig. 83.&mdash;Thyroid Gland as it would appear if the Central
+      Chamber were Uncurled and the Two Horns,</span> <i>B</i>, <i>B&prime;</i>, <span
+      class="sc">separated from the Central Chamber.</span></p>
+    </div>
+  </div>
+
+  <p>Any explanation, then, of the thyroid gland of Ammoc&#x0153;tes, must <span class="pagenum"
+  id="page197">{197}</span>take into account the clear evidence that it is composed of two tubes,
+  which have in part fused together to form an elongated central chamber, in part remain as horns to
+  that chamber, and that in its walls there exist lines of gland-cells of a striking and
+  characteristic nature.</p>
+
+  <p>Further, this central chamber, with its horns, is not a closed chamber, but is in communication
+  with the pharyngeal or respiratory chamber by three ways. In the first place, the central chamber,
+  as is well known, opens into the respiratory chamber by a funnel-shaped opening&mdash;the
+  so-called thyroid duct (<i>Th. o.</i>). In the second place, there exist two ciliated grooves
+  (<i>Ps. br.</i>, <i>Ps. br&prime;.</i>), the pseudo-branchial grooves of Dohrn, which have direct
+  communication with the thyroid chamber. The manner in which these grooves communicate with the
+  thyroid chamber has never, to my knowledge, been described previously to my description in the
+  <i>Journal of Physiology and Anatomy</i>; it is very instructive, for, as I have there shown, each
+  groove enters into the corresponding lateral horn, so that, in reality, there are three openings
+  into the thyroid chamber or palæo-hysteron&mdash;a median opening into the central chamber, and a
+  separate opening into each lateral horn.</p>
+
+  <p>The system of ciliated grooves on the inner ventral surface of the respiratory chamber of
+  Ammoc&#x0153;tes was originally described by Schneider as consisting of a single median groove,
+  which extends from the opening of the thyroid to the posterior extremity of the branchial chamber,
+  and a pair of grooves, or semi-canals, which, starting from the region of the thyroid orifice, run
+  headwards and diverge from each other, becoming more and more lateral, and more and more dorsal,
+  till they come together in the mid-dorsal pharyngeal line below the auditory capsules. The latter
+  are the pseudo-branchial grooves of Dohrn, of which I have already spoken. Schneider looked upon
+  the whole of this system as a single system, for he speaks of "a ciliated groove, which extends
+  from the orifice of the stomach (<i>i.e.</i> anterior intestine) to the orifice of the thyroid,
+  then divides into two, and runs forward right and left of the median ridge, etc." Dohrn rightly
+  separates the median ciliated groove posterior to the thyroid orifice (seen in Fig. <a
+  href="#fig81">81</a> (6)) from the paired pseudo-branchial grooves; the former is a shallow
+  depression which opens into the rim of the thyroid orifice, while the latter has a much more
+  intimate connection with the thyroid gland itself.</p>
+
+  <div><span class="pagenum" id="page198">{198}</span></div>
+
+  <p>A series of sections, such as is given in Fig. <a href="#fig81">81</a>, shows the relation of
+  this pair of ciliated grooves to the thyroid better than any elaborate description. In the first
+  place, it is clear that they remain separate up to their termination&mdash;they do not join in the
+  middle line to open into the thyroid duct; in the second place, they are separate from the thyroid
+  orifice&mdash;they do not terminate at the rim of the orifice, as is the case with the median
+  groove just mentioned, but continue on each side on the wall of the thyroid duct (Fig. <a
+  href="#fig81">81</a> (2)), gradually moving further and further away from the actual opening of
+  the duct into the pharyngeal chamber. During the whole of their course on the wall of the
+  funnel-shaped duct they retain the character of grooves, and are therefore open to the lumen of
+  the duct. The direction of the groove (<i>Ps. br.</i>) shifts as it passes deeper and deeper
+  towards the thyroid, until at last, as seen in Fig. <a href="#fig81">81</a> (3 and 4), it is
+  continuous with the narrow diverticulum of the turned-down single part of the thyroid (B), or
+  turned-down horn, as I have called it. In other words, the median chamber opens into the
+  pharyngeal or respiratory chamber by a single large, funnel-shaped opening, and, in addition, the
+  two ciliated grooves terminate in the lateral horns on each side, and only indirectly into the
+  central chamber, owing to their being semi-canals, and not complete canals. If they were
+  originally canals, and not grooves, then the thyroid of Ammoc&#x0153;tes would be derived from an
+  organ composed of a large, common glandular chamber, which opened into the respiratory chamber by
+  means of an extensive median orifice, and possessed anteriorly two horns, from each of which a
+  canal or duct passed headwards to terminate somewhere in the region of the auditory capsule.</p>
+
+  <p>Dohrn has pointed out that a somewhat similar structure and topographical arrangement is found
+  in Amphioxus and the Tunicata, the gland-cells being here arranged along the hypobranchial groove
+  to form the endostyle and not shut off to form a closed organ, as in the thyroid of
+  Ammoc&#x0153;tes. Dohrn concludes, in my opinion rightly, that the endostyle in the Tunicata and
+  in Amphioxus represents the remnants of the more elaborate organ in Ammoc&#x0153;tes, and that,
+  therefore, in order to explain the meaning of these organs in the former animals, we must first
+  find out their meaning in Ammoc&#x0153;tes. Dohrn, however, goes further than this; for just as he
+  considers Amphioxus and the Tunicata to have arisen by degeneration from an Ammoc&#x0153;tes-like
+  form, so he considers Ammoc&#x0153;tes to have arisen <span class="pagenum"
+  id="page199">{199}</span>from a degenerated Selachian; therefore, in order to be logical, he ought
+  to show that the thyroid of Ammoc&#x0153;tes is an intermediate downward step between the thyroid
+  of Selachians and that of Amphioxus and the Tunicates. Here, it seems to me, his argument utterly
+  breaks down; it is so clear that the thyroid of Petromyzon links on to that of the higher fishes,
+  and that the Ammoc&#x0153;tes thyroid is so immeasurably more complicated and elaborate a
+  structure than is that of Petromyzon, as to make it impossible to believe that the
+  Ammoc&#x0153;tes thyroid has been derived by a process of degeneration from that of the Selachian.
+  On the contrary, the manner in which it is eaten up at transformation and absolutely disappears in
+  its original form is, like the other instances mentioned, strong evidence that we are dealing here
+  with an ancestral organ, which is confined to the larval form, and disappears when the change to
+  the higher adult condition takes place. Dohrn's evidence, then, points strongly to the conclusion
+  that the starting-point of the thyroid gland in the vertebrate series is to be found in the
+  thyroid of Ammoc&#x0153;tes, which has given rise, on the one hand, to the endostyle of Amphioxus
+  and the Tunicata, and on the other, to the thyroid gland of Petromyzon and the rest of the
+  Vertebrata.</p>
+
+  <p>The evidence which I have just given of the intimate connection of the two pseudo-branchial
+  grooves with the thyroid chamber shows, to my mind, clearly that Dohrn is right in supposing that
+  morphologically these two grooves and the thyroid must be considered together. His explanation is
+  that the whole system represents a modified pair of branchial segments distinct from those
+  belonging to the VIIth and IXth nerves. The cavity of the thyroid and the pseudo-branchial grooves
+  are, therefore, according to him, the remains of the gill-pouches of this fused pair of branchial
+  segments, which no longer open to the surface, and the glandular tissue of the thyroid is derived
+  from the modified gill-epithelium. This view of Dohrn's, which he has urged most strongly in
+  various papers, is, I think, right in so far as the separateness of the thyroid segment is
+  concerned, but is not right, and is not proven, in so far as concerns the view that the thyroid
+  gland is a modified pair of gills.</p>
+
+  <p>We may distinctly, on my view, look upon the thyroid segment, with its ciliated grooves and its
+  covering plate of muco-cartilage, as a distinct paired segment, homologous with the branchial
+  segments, without any necessity of deriving the thyroid gland from a pair of gills.</p>
+
+  <div><span class="pagenum" id="page200">{200}</span></div>
+
+  <p>The evidence that such a median segment has been interpolated ventrally between the foremost
+  pairs of branchial segments is remarkably clear, for the limits ventrally of the branchial
+  segments are marked out on each side by the ventral border of the cartilaginous basket-work; and
+  it is well known, as seen in Fig. <a href="#fig80">80</a>, that whereas this cartilaginous
+  framework on the two sides meets together in the middle ventral line in the posterior branchial
+  region, it diverges in the anterior region so as to form a tongue-shaped space between the
+  branchial segments on the two sides. This space is covered over with a plate of muco-cartilage
+  which bears on its inner surface the thyroid gland.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig084.png" id="fig84"><img style="width:100%" src="images/fig084.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="sp0"><span class="sc">Fig. 84.&mdash;Diagram of (A) Ventral Surface and (B) Lateral
+      Surface of Ammoc&#x0153;tes, showing the arrangement of the Epithelial Pits on the Branchial
+      Region, and their innervation by <i>VII.</i>, the Facial, <i>IX.</i>, the Glossopharyngeal,
+      and <i>X<sup>1</sup></i>-<i>X<sup>6</sup></i>, the Vagus Nerves.</span></p>
+    </div>
+  </div>
+
+  <p>In addition to this evidence that we are dealing here with a ventral tongue-like segment
+  belonging to the facial nerve which is interpolated between the foremost branchial segments, we
+  find the most striking fact that at transformation the whole of this muco-cartilaginous plate
+  disappears, the remarkable thyroid gland of the <span class="pagenum"
+  id="page201">{201}</span>Ammoc&#x0153;tes is eaten up, and nothing is left except a small, totally
+  different glandular mass; and now the cartilaginous basket-work meets together in the middle line
+  in this region as well as in the more posterior region. In other words, the striking
+  characteristic of transformation here is the destruction of this interpolated segment, and the
+  resulting necessary drawing together ventrally of the branchial segments on each side.</p>
+
+  <div class="ac w35 fcenter sp2">
+    <a href="images/fig085.png" id="fig85"><img style="width:100%" src="images/fig085.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 85.&mdash;Facial Segment of Ammoc&#x0153;tes marked out by
+      Shading.</span></p>
+      <p class="sp0"><i>VII.</i> 1, thyroid part of segment; <i>VII.</i> 2, hyoid or branchial part;
+      3-9, succeeding branchial segments belonging to IXth and Xth nerves; <i>V</i>, the velar
+      folds; <i>Ps. br.</i>, Dohrn's pseudo-branchial groove; <i>Th. o.</i>, thyroid opening;
+      <i>C</i>, curled portion of thyroid.</p>
+    </div>
+  </div>
+
+  <p>Moreover, another most instructive piece of evidence pointing in the same direction is afforded
+  by the behaviour of the ventral epithelial <span class="pagenum" id="page202">{202}</span>pits, as
+  determined by Miss Alcock. Although there is no indication on the ventral surface of the skin of
+  any difference between the anterior and posterior portions of the respiratory region, yet when the
+  ventral rows of the epithelial pits supplied by each branchial nerve are mapped out, we see how
+  the most anterior ones diverge more and more from the mid-ventral line, following out exactly the
+  limits of the underlying muco-cartilaginous thyroid plate (Fig. <a href="#fig84">84</a>).</p>
+
+  <p class="sp3">The whole evidence strongly leads to the conclusion that the thyroid portion of the
+  facial segment was inserted as a median tongue between the foremost branchial segments on each
+  side, and that, therefore, the whole facial segment, consisting as it does of a thyroid part and a
+  hyoid or branchial part, may be represented as in Fig. 85, which is obtained by splitting an
+  Ammoc&#x0153;tes longitudinally along the mid-dorsal line, so as to open out the pharyngeal
+  chamber and expose the whole internal surface. The facial segment is marked out by shading lines,
+  the glosso-pharyngeal and vagus segments and the last of the trigeminal segments being indicated
+  faintly. The position of the thyroid gland is indicated by oblique lines, C being the curled
+  portion.</p>
+
+  <p class="ac"><span class="sc">The Uterus of the Scorpion Group.</span></p>
+
+  <p>Seeing how striking is the arrangement and the structure of the glandular tissue of this
+  thyroid, how large the organ is and how absolutely it is confined to Ammoc&#x0153;tes,
+  disappearing entirely as such at transformation, we may feel perfectly certain that a
+  corresponding, probably very similar, organ existed in the invertebrate ancestor of the
+  vertebrate; for the transformation process consists essentially of the discarding of invertebrate
+  characteristics and the putting on of more vertebrate characters; also, so elaborate an organ
+  cannot possibly have been evolved as a larval adaptation during the life of Ammoc&#x0153;tes. We
+  may therefore assert with considerable confidence that the thyroid gland was the
+  <i>palæo-hysteron</i>, and was derived from the uterus of the ancient palæostracan forms. If,
+  then, it be found that a glandular organ of this very peculiar structure and arrangement is
+  characteristic of the uterus of any living member of the scorpion group, then the confidence of
+  this assertion is greatly increased.</p>
+
+  <p>In Limulus, as already stated, the genital ducts open separately <span class="pagenum"
+  id="page203">{203}</span>on each side of the operculum, and do not combine to form a uterus; I
+  have examined them and was unable to find any glandular structure at all resembling that of the
+  thyroid gland of Ammoc&#x0153;tes. I then turned my attention to the organs of the scorpion, in
+  which the two ducts have fused to form a single uterus.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig086.png" id="fig86"><img style="width:100%" src="images/fig086.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 86.&mdash;Section through the Terminal Chamber or Uterus of the Male
+      Scorpion.</span></p>
+      <p class="sp0"><i>C</i>, cavity of chamber. A portion of the epithelial lining of the channels
+      of emission is drawn above the section of the uterus.</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page204">{204}</span></div>
+
+  <table class="mc tlf sp2 w50" summary="Group of illustrations" title="Group of illustrations">
+    <tr>
+      <td class="vbm ac" style="width:42%;"><a href="images/fig087.png" id="fig87"><img
+      style="width:100%" src="images/fig087.png" alt="" title=""/></a></td>
+      <td class="vbm ac" style="width:42%;"><a href="images/fig088.png" id="fig88"><img
+      style="width:100%" src="images/fig088.png" alt="" title=""/></a></td>
+    </tr>
+    <tr>
+      <td>
+        <div class="smaller aj it1">
+          <p class="sp0"><span class="sc">Fig. 87.&mdash;Longitudinal Section through three of the
+          Cones of the Uterine Glands of the Scorpion.</span></p>
+        </div>
+      </td>
+      <td>
+        <div class="smaller aj it1">
+          <p class="sp0"><span class="sc">Fig. 88.&mdash;Sagittal Section through the Uterine Gland
+          of Scorpion, showing the Internal Chitinous Surface</span> (<i>b</i>) <span class="sc">and
+          the Glandular Cones</span> (<i>a</i>) <span class="sc">cut through at various distances
+          from the Internal Surface</span>.</p>
+        </div>
+      </td>
+    </tr>
+  </table>
+
+  <p>I there found that both in the male and in the female the genital ducts on each side terminate
+  in a common chamber or uterus, which underlies the whole length of the operculum, and opens to the
+  exterior in the middle line, as shown in Fig. <a href="#fig76">76</a>. In transverse section, this
+  uterus has the appearance shown in Fig. <a href="#fig86">86</a>, <i>i.e.</i> it is a large tube,
+  evidently expansible, lined with a chitinous layer and epithelial cells belonging to the
+  chitinogenous layer, except in two symmetrical places, where the uniformity of the uterine wall is
+  interrupted by two large, remarkable glandular structures. The structure of these glands is better
+  shown by means of sagittal sections. They are composed of very long, wedge-shaped cells, each of
+  which possesses a large, round nucleus at the basal end of the cell (Fig. <a
+  href="#fig87">87</a>). These cells are arranged in bundles of about eight to ten, which are
+  separated from each other by connective tissue, the apex of each conical bundle being directed
+  into the cavity of the uterus; where this brush-like termination of the cells reaches the surface,
+  the chitinous layer is absent, so that this layer is, on surface view, seen (Fig. <a
+  href="#fig88">88</a> (<i>b</i>)) to be pitted with round holes over that part of the internal
+  surface of the uterus where these glands are situated. Each of these holes represents the
+  termination of one of these cone-shaped wedges of cells. If the section is cut across at right
+  angles to the axis of these cones, then its appearance is represented in Fig. <a
+  href="#fig88">88</a> (<i>a</i>), and shows well the arrangement of the blocks of cells, separated
+  from each other by connective tissue. When the section passes through the basal part of the cones,
+  and only in that case, then the nuclei of the cells appear, often in considerable numbers in one
+  section, as <span class="pagenum" id="page205">{205}</span>is seen in Fig. <a
+  href="#fig89">89</a>. In Fig. <a href="#fig88">88</a> the section shows at <i>b</i> the holes in
+  the chitin in which the cones terminate, and then a series of layers of sections through the cones
+  further and further away from their apices.</p>
+
+  <p>These conical groups of long cells, represented in Fig. <a href="#fig87">87</a>, form on each
+  side of the uterus a gland, which is continuous along its whole length, and thus forms a line of
+  secreting surface on each side, just as in the corresponding arrangement of the glandular
+  structures in the thyroid of Ammoc&#x0153;tes. This uterus and glandular arrangement is found in
+  both sexes; the gland is, however, more developed in the male than in the female scorpion.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig089.png" id="fig89"><img style="width:35%" src="images/fig089.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p class="sp0"><span class="sc">Fig. 89.&mdash;Transverse Section through the Basal Part of
+      the Uterine Glands of the Scorpion.</span></p>
+    </div>
+  </div>
+
+  <p>The resemblance between the structure of the thyroid of Ammoc&#x0153;tes and the uterus of the
+  scorpion is most striking, except in two respects, viz. the nature of the lining of the
+  non-glandular part of the cavity&mdash;in the one case ciliated, in the other chitinous&mdash;and
+  the place of exit of the cavity, the thyroid of Ammoc&#x0153;tes opening into the respiratory
+  chamber, while the uterus of Scorpio opens direct to the exterior.</p>
+
+  <div class="ac w45 fcenter sp2">
+    <a href="images/fig090.png" id="fig90"><img style="width:100%" src="images/fig090.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p class="sp0"><span class="sc">Fig. 90.&mdash;Section of Central Chamber of Thyroid of
+      Ammoc&#x0153;tes and Section of Uterus of Scorpion.</span></p>
+    </div>
+  </div>
+
+  <p>With respect to the first difference, the same difficulty is met <span class="pagenum"
+  id="page206">{206}</span>with in the comparison of the ciliated lining of the tube in the central
+  nervous system of vertebrates with the chitinous lining of the intestine in the arthropod. Such a
+  difference does not seem to me either unlikely or unreasonable, seeing that cilia are found
+  instead of chitin in the intestine of the primitive arthropod Peripatus. Also the worm-like
+  ancestors of the arthropods almost certainly possessed a ciliated intestine. Finally, the
+  researches of Hardy and McDougall on the intestine of Daphnia point directly to the presence of a
+  ciliated rather than a chitinous epithelial lining of the intestine in this animal&mdash;all
+  evidence pointing to the probability that in the ancient arthropod forms, derived as they were
+  from the annelids, the intestine was originally ciliated and not chitinous. It is from such forms
+  that I suppose vertebrates to have sprung, and not from forms like the living king-crabs,
+  scorpions, Apus, Branchipus, etc. I only use them as illustrations, because they are the only
+  living representatives of the great archaic group, from which the Crustacea, Arachnida, and
+  Vertebrata all took origin.</p>
+
+  <p>The second difference is more important, and is at first sight fatal to any comparison between
+  the two organs. How is it possible to compare the uterus of the scorpion, which opens on the
+  surface by an <i>external</i> genital opening, with the thyroid of Ammoc&#x0153;tes, which opens
+  by an <i>internal</i> opening into the respiratory chamber? However close may be the histological
+  resemblance of structure in the two cases, surely such a difference is too great to be accounted
+  for.</p>
+
+  <p>It is, however, to be remembered that the operculum of Scorpio covers only the terminal genital
+  apparatus, and does not, therefore, resemble the operculum of the presumed ancestor of
+  Ammoc&#x0153;tes, which, as already argued, must have resembled the operculum of Thelyphonus with
+  its conjoint branchial and genital apparatus, rather than that of Scorpio. Before, therefore,
+  making too sure of the insuperable character of this difficulty, we must examine the uterus of the
+  Pedipalpi, and see the nature of its opening.</p>
+
+  <p>The nature of the terminal genital organs in Thelyphonus has been described to some extent by
+  Blanchard, and more recently by Tarnani. The ducts of the generative organs terminate, according
+  to the latter observer, in the large uterus, which is found both in the male and female; he
+  describes the walls of the uterus in the female as formed of elongated glandular epithelium, with
+  a strongly-developed porous, chitinized intima. In the male, he says that the <span
+  class="pagenum" id="page207">{207}</span>epithelium of the uterus masculinus and its processes is
+  extraordinarily elongated, the chitin covering being thick. In these animals, then, the common
+  chamber or uterus into which the genital ducts empty, which, like the corresponding chamber in the
+  scorpion, occupies the middle region of the operculum, is a large and conspicuous organ. Further,
+  and this is a most striking fact, the <i>uterus masculinus</i> does not open direct to the
+  exterior, but into the genital cavity, "which lies above the uterus, so that the latter is
+  situated between the lower wall of the genital cavity and the outer integument." The opening,
+  therefore, of the uterus is not external but <i>internal</i>, into the large internal space known
+  as the genital cavity. The arrangement is shown in Fig. 91, taken from Tarnani's paper, which
+  represents a diagrammatic sagittal section through the exit of the male genital duct. Yet another
+  most striking fact is described by Tarnani. This genital cavity is continuous with the pulmonary
+  or gill cavities on each side, so that instead of a single opening for the genital products and
+  one on each side for each gill-pouch, as would be the case if the arrangement was of the same kind
+  as in the scorpion, there is a single large chamber, the genital chamber, common to both
+  respiratory and genital organs.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig091.png" id="fig91"><img style="width:62%" src="images/fig091.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p><span class="sc">Fig. 91.&mdash;Sagittal Median Diagrammatic Section through the Operculum
+      of the Male Thelyphonus.</span> (From <span class="sc">Tarnani</span>.)</p>
+      <p class="sp0">The thick line is the operculum, composed of two segments, <i>I.</i> and
+      <i>II.</i> <i>Ut. Masc.</i>, uterus masculinus; <i>Gen. Ch.</i>, genital chamber; <i>Int.
+      Op.</i>, internal opening; <i>Ext. Op.</i>, external opening common to the genital and
+      respiratory organs.</p>
+    </div>
+  </div>
+
+  <p>This genital chamber, according to Tarnani, opens to the exterior by a single median opening
+  between the operculum and the succeeding segment; similarly, a communication from side to side
+  exists between the second pair of gill-pouches. I have been able to examine <i>Hypoctonus
+  formosus</i> and <i>Thelyphonus caudatus</i>, and in both cases, in both male and female, the
+  opening to the exterior of the common chamber for respiration and for the genital products was
+  <span class="pagenum" id="page208">{208}</span>not a single opening, as described by Tarnani in
+  <i>Thelyphonus asperatus</i>, but on each side of the middle line, a round orifice closed by a
+  lid, like the nest of the trapdoor spider, led into the common genital chamber (<i>Gen. Ch.</i>)
+  into which both uterus and gills opened. In Fig. <a href="#fig77">77</a> I have endeavoured to
+  represent the arrangement of the genital and respiratory organs in the male Thelyphonus according
+  to Tarnani's and my own observations.</p>
+
+  <p>If we may take Thelyphonus as a sample of the arrangement in those scorpions in which the
+  operculum was fused with the first branchial appendage, among which must be included the old
+  sea-scorpions, then it is most significant that their uterus should open internally into a cavity
+  which was continuous with the respiratory cavity. Thus not only the structure of the gland, but
+  also the arrangement of the internal opening into the respiratory, or, as it became later, the
+  pharyngeal cavity, is in accordance with the suggestion that the thyroid of Ammoc&#x0153;tes
+  represents the uterus of the extinct Eurypterus-like ancestor.</p>
+
+  <p>Into this uterus the products of the generative organs were poured by means of the <i>vasa
+  deferentia</i>, so that there was not a single median opening or duct in connection with it, but
+  also two side openings, the terminations of the <i>vasa deferentia</i>. These are described by
+  Tarnani in Thelyphonus as opening into the two horns of the uterus, which thus shows its bilateral
+  character, although the body of the organ is median and single; these ducts then pass within the
+  body of the animal, dorsal to the uterus, towards the testes or ovaries as the case may be, organs
+  which are situated in these animals, as in other scorpions, in the abdomen, so that the direction
+  of the ducts from the generative glands to the uterus is headwards. If, however, we examine the
+  condition of affairs in Limulus, we find that the main mass of the generative material is
+  cephalic, forming with the liver that dense glandular mass which is packed round the
+  supra-&#x0153;sophageal and prosomatic ganglia, and round the stomach and muscles of the
+  head-region. From this cephalic region the duct passes out on each side at the junction of the
+  prosomatic and mesosomatic carapace to open separately on the posterior surface of the operculum,
+  near the middle line, as is indicated in Fig. <a href="#fig75">75</a>.</p>
+
+  <p class="sp3">We have, therefore, two distinct possible positions for the genital ducts among the
+  group of extinct scorpion-like animals, the one from the cephalic region to the operculum, and the
+  other from the abdominal region to the operculum.</p>
+
+  <div><span class="pagenum" id="page209">{209}</span></div>
+
+  <p class="ac"><span class="sc">The Generative Glands of Limulus and its Allies.</span></p>
+
+  <p>The whole argument, so far, has in every case ended with the conclusion that the original
+  scorpion-like form with which I have been comparing Ammoc&#x0153;tes resembled in many respects
+  Limulus rather than the present-day scorpions, and therefore in the case also of the generative
+  organs, with which the thyroid gland or palæo-hysteron was in connection, it is more probable that
+  they were cephalic in position rather than abdominal. If this were so, then the duct on each side,
+  starting from the median ventral uterus, would take a lateral and dorsal course to reach the huge
+  mass of generative gland lying within the prosomatic carapace, just as I have represented in the
+  figure of Eurypterus (Fig. <a href="#fig79">79</a>), a course which would take much the same
+  direction as the ciliated groove in Ammoc&#x0153;tes.</p>
+
+  <p>We ought, therefore, on this supposition, to expect to find the remains of the invertebrate
+  generative tissue, the ducts of which terminated in the thyroid, in the head-region, and not in
+  the abdomen.</p>
+
+  <p>Upon removal of the prosomatic carapace of Limulus, a large brownish glandular-looking mass is
+  seen, in which, if it happens to be a female, masses of ova are very conspicuous. This mass is
+  composed of two separate glands, the generative glands and the hepatico-pancreatic
+  glands&mdash;the so-called liver&mdash;and surrounds closely the central nervous system and the
+  alimentary canal. From the generative glands proceed the genital ducts to terminate on the
+  posterior surface of the operculum. From the liver ducts pass to the pyloric end of the cephalic
+  stomach, and carry the fluid by means of which the food is digested, for, in all these animals,
+  the active digesting juices are formed in the so-called liver, and not in the cells of the stomach
+  or intestine.</p>
+
+  <p>It is a very striking fact that the brain of Ammoc&#x0153;tes is much too small for the
+  brain-case, and that the space between brain and brain-case is filled up with a very peculiar
+  glandular-looking tissue, which is found in Ammoc&#x0153;tes and not elsewhere. Further, it is
+  also striking that in the brain of Ammoc&#x0153;tes there should still exist the remains of a tube
+  extending from the IVth ventricle to the surface at the <i>conus post-commissuralis</i>, which can
+  actually be traced right into this tissue on the outside of the brain (see Fig. <a
+  href="#fig13">13</a>, <i>a-e</i>, Pl. XXVI., in my paper in the <i>Quarterly Journal of
+  Microscopical Science</i>). <span class="pagenum" id="page210">{210}</span>This, in my opinion, is
+  the last remnant of one of the old liver-ducts which extended from the original stomach and
+  intestine into the cephalic liver-mass. This glandular-looking material is shown surrounding the
+  pineal eye and its nerve, in Fig. <a href="#fig31">31</a>, also in Fig. <a href="#fig22">22</a>,
+  and separately in Fig. <a href="#fig92">92</a>. It is composed of large cells, with a badly
+  staining nucleus, closely packed together with lines of pigment here and there between the cells;
+  this pigment is especially congregated at the spot where the so-called liver-duct loses itself in
+  this tissue. The protoplasm in these large cells does not stain well, and with osmic acid gives no
+  sign of fat, so that Ahlborn's description of this tissue as a peculiar arachnoideal fat-tissue is
+  not true; peculiar it certainly is, but fatty it is not.</p>
+
+  <div class="ac w20 fcenter sp2">
+    <a href="images/fig092.png" id="fig92"><img style="width:100%" src="images/fig092.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="sp0"><span class="sc">Fig. 92.&mdash;Drawing of the Tissue which surrounds the Brain
+      of Ammoc&#x0153;tes.</span></p>
+    </div>
+  </div>
+
+  <p>This tissue has been largely described as a peculiar kind of connective tissue, which is there
+  as packing material, for the purpose of steadying a brain too small for its case. On the face of
+  it such an explanation is unscientific; certainly for all those who really believe in evolution,
+  it is out of the question to suppose that a brain-case has been laid down in the first instance
+  too large for the brain, in order to provide room for a subsequent increase of brain; just as it
+  is out of the question to suppose that the nervous system was laid down originally as an
+  epithelial tube in order to provide for the further development of the nervous system by the
+  conversion of more and more of that tube into nervous matter. Yet this latter proposition has been
+  seriously put forward by professed believers in evolution and in natural selection.</p>
+
+  <p>This tissue bears no resemblance whatever to any form of connective tissue, either fatty or
+  otherwise. By every test this tissue tells as plainly as possible that it is a vestige of some
+  former organ, presumably glandular, which existed in that position; that it is not there as
+  packing material because the brain happened to be too small for its case, but that, on the
+  contrary, the brain is too small for its case, because the case, when it was formed, included this
+  organ as well as the brain; in other words, this tissue <span class="pagenum"
+  id="page211">{211}</span>is there because it is the remnant of the great glandular mass which so
+  closely surrounds the brain and alimentary canal in animals such as Limulus. In my paper in the
+  <i>Quarterly Journal of Microscopical Science</i>, in which I was comparing the tube of the
+  vertebrate nervous system with the alimentary canal of the invertebrate, I spoke of this tissue as
+  being the remnant of the invertebrate liver. At the same time the whole point of my argument was
+  that the glandular material surrounding the brain of Limulus was made up of two glands&mdash;liver
+  and generative gland&mdash;so that this tissue might be the remnant of either one or the other, or
+  both. All I desired, at that time, was to point out the glandular appearance of this so-called
+  packing tissue, which surrounded the brain-region of Ammoc&#x0153;tes, in connection with the fact
+  that the brain and alimentary canal of Limulus were closely surrounded with a glandular mass
+  composed partly of liver, partly of the generative gland. At present, I think these large cells
+  found round the brain in Ammoc&#x0153;tes are much more likely to be the remnant of the generative
+  gland than of the liver; the size of the cells and their arrangement recalls Owen's picture of the
+  generative gland in Limulus, and seeing how important all generative glands are in their capacity
+  of internal secreting glands, apart entirely from the extrusion of the ripe generative products,
+  and how unimportant is an hepato-pancreas when the alimentary canal is closed, it is much more
+  likely that of the two glands the former would persist longer than the latter. It may be that all
+  that is left of the old hepato-pancreas consists of the pigment so markedly found in between these
+  cells, especially at the place where the old liver-duct reaches the surface of the brain; just as
+  the only remnant of the two pineal eyes in the higher vertebrates is the remains of the pigment,
+  known as brain-sand, which still exists in the pineal gland of even the highest vertebrate. This,
+  however, is a mere speculation of no importance. What is important is the recognition of this
+  tissue round the brain as the remnant of the glandular mass round the brain of animals such as
+  Limulus. Still further confirmation of the truth of this comparison will be given when the origin
+  of the auditory organ comes up for discussion.</p>
+
+  <p>I conclude, therefore, from the evidence of Ammoc&#x0153;tes, that the generative glands in the
+  ancestral form were situated largely in the cephalic region, and suggest that the course and
+  direction of the ciliated pseudo-branchial grooves on each side indicate the direction of the
+  <span class="pagenum" id="page212">{212}</span>original opercular ducts by which the generative
+  products were conveyed to the uterine chamber, i.e. to the chamber of the thyroid gland, and
+  thence to the common genital and respiratory cavity, and so to the exterior.</p>
+
+  <p>It is easy to picture the sequence of events. First, the generative glands, chiefly confined to
+  the cephalic region, communicating with the exterior by separate ducts on the inner surface of the
+  operculum as in Limulus. Then, in connection with the viviparous habit, these two oviducts fused
+  together to form a single chamber, covered by the operculum, which opened out to the exterior by a
+  single opening as in Scorpio: or, in forms such as Eurypterus, in which the operculum had
+  amalgamated with the first branchial appendage and possessed a long, tongue-like ventral
+  projection, the amalgamated ducts formed a long uterine chamber which opened internally into the
+  genital chamber&mdash;a chamber which, as in Thelyphonus, was common with that of the two
+  gill-chambers, while at the same time the genital ducts from the cephalic generative material
+  opened into two uterine horns which arose from the anterior part of the uterus, as in
+  Thelyphonus.</p>
+
+  <p>Such an arrangement would lead directly to the condition found in Ammoc&#x0153;tes, if the
+  generative material around the brain lost its function, owing to a new exit for generative
+  products being formed in the posterior part of the body. The connection of the genital duct with
+  this cephalic gland being then closed and cut off by the brain-case, the position of the oviducts
+  would still be shown by the ciliated grooves opening into the folded-down thyroid tube,
+  <i>i.e.</i> the folded-down horns of the uterus; the uterus itself would remain as the main body
+  of the thyroid and still open by a conspicuous orifice into the common respiratory chamber. Next,
+  in the degeneration process, we may suppose that not only the oviducts opened out to form the
+  ciliated groove, but that the uterine chamber itself also opened out, and thus formed the
+  endostyle of Amphioxus and of the Tunicata.</p>
+
+  <p>It might seem at first sight improbable that a closed tube should become an open groove,
+  although the reverse phenomenon is common enough; the difficulty, however, is clearly not
+  considered great, for it is precisely what Dohrn imagines to have taken place in the conversion of
+  the thyroid of Ammoc&#x0153;tes into the endostyle of Amphioxus and the Tunicata; it is only
+  carrying on the same idea a stage further to see in the open, ciliated groove of Ammoc&#x0153;tes
+  the remains of the closed genital duct of Limulus and its allies.</p>
+
+  <div><span class="pagenum" id="page213">{213}</span></div>
+
+  <p class="sp3">Such is the conclusion to which the study of the thyroid gland in Ammoc&#x0153;tes
+  seems to me to lead, and one cannot help wondering why such an unused and rudimentary organ should
+  have remained after its original function had gone. Is it possible to find out its function in
+  Ammoc&#x0153;tes?</p>
+
+  <p class="ac"><span class="sc">The Function of the Thyroid Gland in Ammoc&#x0153;tes.</span></p>
+
+  <p>The thyroid gland has been supposed to secrete mucus into the respiratory chamber for the
+  purpose of entangling the particles of food, and so aiding in digestion. I see no sign of any such
+  function; neither by the thionin method, nor by any other test, have Miss Alcock and myself ever
+  been able to see any trace of mucous secretion in the thyroid, and, indeed, the thyroid duct is
+  always remarkably free from any sign of any secretion whatever. Not only is there no evidence of
+  any mucous secretion in the thyroid of the fully developed Ammoc&#x0153;tes, but also no necessity
+  for such secretion from Dohrn's point of view, for so copious a supply of mucus is poured out by
+  the glands of the branchiæ, along the whole pharyngeal tract, especially from the cells of the
+  foremost or hyoid gills, as to mix up with the food as thoroughly as can possibly be needed.
+  Further, too, the ciliated pharyngeal bands described by Schneider are amply sufficient to move
+  this mixed mass along in the way required by Dohrn. Finally, the evidence given by Miss Alcock is
+  absolutely against the view that the thyroid takes any part in the process of digestion, while, on
+  the other hand, her evidence directly favours the view that these glandular <i>branchial</i>
+  mucus-secreting cells play a most important part in the digestive process.</p>
+
+  <p>In Fig. <a href="#fig93">93</a>, A is a representation of the respiratory tissue of a normal
+  gill; B is the corresponding portion of the first or hyoid gill, in which, as is seen, the whole
+  of the respiratory epithelium is converted into gland-tissue of the nature of mucous cells.</p>
+
+  <p>To sum up, the evidence is clear and conclusive that the Ammoc&#x0153;tes possesses in its
+  pharyngeal chamber mucus-secreting glands, which take an active part in the digestive process,
+  which do not in the least resemble either in structure or arrangement the remarkable cells of the
+  thyroid gland, and that the experimental evidence that the latter cells either secrete mucus or
+  take any part in digestion is so far absolutely negative. It is, of course, possible, that they
+  <span class="pagenum" id="page214">{214}</span>may contain mucin in the younger developmental
+  stages, and therefore possible that they might at that stage secrete it; they certainly, however,
+  show no sign of doing so in their more adult condition, and cannot be compared in the very
+  faintest degree to the glandular cells of the pharyngeal region. It is also perfectly possible for
+  gland-cells belonging to a retrograde organ to become mucus-secreting, and so to give rise to the
+  cells of Amphioxus and the Tunicata.</p>
+
+  <div class="ac w30 fcenter sp2">
+    <a href="images/fig093.png" id="fig93"><img style="width:100%" src="images/fig093.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="sp0">Fig. 93.&mdash;<span class="sc">A, Portion of a Gill of Ammoc&#x0153;tes with
+      ordinary Respiratory Epithelium; B, Corresponding Portion of the First or Hyoid
+      Gill.</span></p>
+    </div>
+  </div>
+
+  <p>If, then, these cells were not retained for digestive purposes, what was their function? To
+  answer this question we must first know the function of the corresponding gland-cells in the
+  uterus of the scorpion, which undoubtedly secreted into the cavity of the uterus and took some
+  part in connection with the generative act, and certainly not with digestion. What the function of
+  these cells is or in what way they act I am unable at present to say. I can only suppose that the
+  reason why the thyroid gland has persisted throughout the vertebrate kingdom, after the generative
+  tissues had found a new outlet for their products in the body-cavity of the posterior region, is
+  because it possessed some important function in addition to that connected with the exit of the
+  products of the generative organs; a function which was essential to the well-being, or even to
+  the life of the animal. We do not know its function in the scorpion, or the nature of its
+  secretion in that animal. We know only that physiology at the present day has demonstrated clearly
+  that the actual external secretion of a gland may be by no means its most important function; in
+  addition, glands possess what is called an internal secretion, viz. a <span class="pagenum"
+  id="page215">{215}</span>secretion into the blood and lymph, and this latter secretion may be of
+  the most vital importance. Now, the striking fact forces itself prominently forward, that the
+  thyroid gland of the higher vertebrates is the most conspicuous example of the importance of such
+  internal secretion. Here, although ductless, we have a gland which cannot be removed without fatal
+  consequences. Here, in the importance of its internal secretion, we have a reason for the
+  continued existence of this organ; an organ which remains much the same throughout the Vertebrata
+  down to and including Petromyzon, but, as is seen at transformation, is all that remains of the
+  more elaborate, more extensive organ of Ammoc&#x0153;tes. Surely we may argue that it is this
+  second function which has led to the persistence of the thyroid, and that its original form,
+  without its original function, is seen in Ammoc&#x0153;tes, because that is a larval form, and not
+  a fully-developed animal. As soon as the generative organs of Petromyzon are developed at
+  transformation, all trace of its connection with a genital duct vanishes, and presumably its
+  internal secretory function alone remains.</p>
+
+  <p>Yet, strange to say, a mysterious connection continues to exist between the thyroid gland and
+  the generative organs, even up to the highest vertebrate. That the thyroid gland, situated as it
+  is in the neck, should have any sympathy with sexual functions if it was originally a gland
+  concerned with digestion is, to say the least of it, extremely unlikely, but, on the contrary,
+  likely enough if it originated from a glandular organ in connection with the sexual organs of the
+  palæostracan ancestor of the vertebrate.</p>
+
+  <p class="sp3">Freund has shown, and shown conclusively, that there is an intimate connection
+  between the condition of the thyroid gland and the state of the sexual organs, not only in human
+  beings, but also in numerous animals, such as dogs, sheep, goats, pigs, and deer. He points out
+  that the swelling of the gland, which occurs in consequence of sexual excitement (a fact mentioned
+  both in folk-lore tales and in poetical literature), and also the swelling at the time of puberty,
+  may both lead to a true goitrous enlargement; that most of the permanent goitres commence during a
+  menstrual period; that during pregnancy swelling of the thyroid is almost universal, and may
+  become so extreme as to threaten suffocation, or even cause death; that the period of puberty and
+  the climacteric period are the two maximal periods for the onset of goitre, and that exophthalmic
+  goitre especially is associated with a special disease connected with the uterus.</p>
+
+  <div><span class="pagenum" id="page216">{216}</span></div>
+
+  <p class="ac"><span class="sc">Summary.</span></p>
+
+  <div class="bq1 smaller sp5">
+    <p>Step by step in the preceding chapters the evidence is accumulating in favour of the origin
+    of vertebrates from a member of the palæostracan group. In a continuously complete and
+    harmonious manner the evidence has throughout been most convincing when the vertebrate chosen
+    for the purpose of my arguments has been Ammoc&#x0153;tes.</p>
+    <p>So many fixed points have been firmly established as to enable us to proceed further with
+    very great confidence, in the full expectation of being able ultimately to homologize the
+    Vertebrata with the Palæostraca even to minute details.</p>
+    <p>Perhaps the most striking and unexpected result of such a comparison is the discovery that
+    the thyroid gland is derived from the uterus of the palæostracan ancestor. Yet so clear is the
+    evidence that it is difficult to see how the homology can be denied.</p>
+    <p>In the one animal (Palæostraca) the foremost pair of mesosomatic appendages forms the
+    operculum, which always bears the terminal generative organs and is fused in the middle line. In
+    many forms, essentially in Eurypterus and the ancient sea-scorpions, the operculum was composed
+    of two segments fused together: an anterior one which carried the uterus, and a posterior one
+    which carried the first pair of branchiæ.</p>
+    <p>In the other animal (Ammoc&#x0153;tes) the foremost segments of the mesosomatic or
+    respiratory region, immediately in front of the glossopharyngeal segments, are supplied by the
+    facial nerve, and are markedly different from those supplied by the vagus and glossopharyngeal,
+    for the facial supplies two segments fused together; the anterior one, the thyroid segment,
+    carrying the thyroid gland, the posterior one, the hyoid segment, carrying the first pair of
+    branchiæ.</p>
+    <p>Just as in Eurypterus the fused segment, carrying the uterus on its internal surface, forms a
+    long median tongue which separates the most anterior branchial segments on each side, so also
+    the fused segment carrying the thyroid forms in Ammoc&#x0153;tes a long median tongue, which
+    separates the most anterior branchial segments on each side.</p>
+    <p>Finally, and this is the most conclusive evidence of all, this thyroid gland of
+    Ammoc&#x0153;tes is totally unlike that of any of the higher vertebrates, and, indeed, of the
+    adult form Petromyzon itself, but it forms an elaborate complicated organ, which is directly
+    comparable with the uterus and genital ducts of animals such as scorpions. Not only is such a
+    comparison valid with respect to its shape, but also with respect to its structure, which is
+    absolutely unique among vertebrates, and very different to that of any other vertebrate gland,
+    but resembles in a striking manner a glandular structure found in the uterus, both of male and
+    female scorpions.</p>
+    <p>The generative glands in Limulus, together with the liver-glands, form a large glandular
+    mass, situated in the head-region closely surrounding the central nervous system, so that the
+    genital ducts pass from the head-region tailwards to the operculum. In the scorpion they lie in
+    the abdominal region, so that their ducts pass headwards to the operculum.</p>
+    <p>Probably in the Palæostraca the generative mass was situated in the cephalic region as in
+    Limulus, and it is probable that the remnant of it still exists in <span class="pagenum"
+    id="page217">{217}</span>Ammoc&#x0153;tes in the shape of the peculiar large cells packed
+    together, with pigment masses in between them, which form such a characteristic feature of the
+    glandular-looking material, which fills up the space between the cranial walls and the central
+    nervous system.</p>
+    <p>Finally, the relationship which has been known from time immemorial to exist between the
+    sexual organs and the thyroid in man and other animals, and has hitherto been a mystery without
+    any explanation, may possibly be the last reminiscence of a time when the thyroid glands were
+    the uterine glands of the palæostracan ancestor.</p>
+    <p class="sp0">The consideration of the facial nerve, and the segments it supplies, still
+    further points to the origin of the Vertebrata from the Palæostraca.</p>
+  </div>
+
+  <div><span class="pagenum" id="page218">{218}</span></div>
+
+  <p class="ac">CHAPTER VI</p>
+
+  <p class="ac"><i>THE EVIDENCE OF THE OLFACTORY APPARATUS</i></p>
+
+  <div class="bq1 it1 smaller sp3">
+    <p class="sp0">Fishes divided into Amphirhinæ and Monorhinæ.&mdash;Nasal tube of the
+    lamprey.&mdash;Its termination at the infundibulum.&mdash;The olfactory organs of the scorpion
+    group.&mdash;The camerostome.&mdash;Its formation as a tube.&mdash;Its derivation from a pair of
+    antennæ.&mdash;Its termination at the true mouth.&mdash;Comparison with the olfactory tube of
+    Ammoc&#x0153;tes.&mdash;Origin of the nasal tube of Ammoc&#x0153;tes from the tube of the
+    hypophysis.&mdash;Direct comparison of the hypophysial tube with the olfactory tube of the
+    scorpion group&mdash;Summary.</p>
+  </div>
+
+  <p>In the last chapter I finished the evidence given by the consideration of the mesosomatic or
+  opisthotic nerves, and the segments they supplied. The evidence is strongly in accordance with
+  that of previous chapters, and not only confirms the conclusion that vertebrates arose from some
+  member of the Palæostraca, but helps still further to delimit the nature of that member. It is
+  almost startling to see how the hypothesis put forward in the second chapter, suggested by the
+  consideration of the nature of the vertebrate central nervous system and of the geological record,
+  has received stronger and stronger confirmation from the consideration of the vertebrate optic
+  apparatus, the vertebrate skeleton, the respiratory apparatus, and, finally, the thyroid gland.
+  All fit naturally into a harmonious whole, and give a feeling of confidence that a similar harmony
+  will be found upon consideration of the rest of the vertebrate organs.</p>
+
+  <p>Following naturally upon the segments supplied by the opisthotic (mesosomatic) cranial nerves,
+  we ought to consider now the segments supplied by the pro-otic (prosomatic) cranial nerves, i.e.
+  the segments belonging to the trigeminal nerve-group in the vertebrate, and in the invertebrate
+  the segments of the prosoma with their characteristic appendages. There are, however, in all
+  vertebrates in this foremost cranial region, in addition to the optic nerves, two other
+  well-marked nerves of special sense, the olfactory and the auditory. Of these, the former are in
+  the same class as the optic nerves, for they arise <span class="pagenum"
+  id="page219">{219}</span>in the vertebrate from the supra-infundibular nerve-mass, and in the
+  invertebrate from the supra-&#x0153;sophageal ganglia. The latter arise in the vertebrate from the
+  infra-infundibular nerve-mass, and, as the name implies, are situated in the region where the
+  pro-otic nerves are contiguous to the opisthotic, <i>i.e.</i> at the junction of the prosomatic
+  and mesosomatic nerve-regions.</p>
+
+  <p>The chapter dealing with the evidence given by the olfactory nerves and the olfactory apparatus
+  ought logically to have followed immediately upon the one dealing with the optic apparatus, seeing
+  that both these special sense-nerves belong to the supra-infundibular segments in the vertebrate
+  and to the supra-&#x0153;sophageal in the invertebrate.</p>
+
+  <p class="sp5">I did not deal with them in that logical sequence because it was necessary for
+  their understanding to introduce first the conception of modified appendages as important factors
+  in any consideration of vertebrate segments; a conception which followed naturally after the
+  evidence afforded by the skeleton in Chapter III., and by the branchial segments in Chapter IV.
+  So, too, now, although the discussion of the prosomatic segmentation ought logically to follow
+  immediately on that of the mesosomatic segmentation, I have determined to devote this chapter to
+  the evidence of the olfactory organs, because the arguments as to the segments belonging to the
+  trigeminal nerve-group are so much easier to understand if the position of the olfactory apparatus
+  is first made clear.</p>
+
+  <p>In all vertebrates the nose is double and opens into the pharynx, until we descend to the
+  fishes, where the whole group Pisces has been divided into two subsidiary groups, Monorhinæ and
+  Amphirhinæ, according as they possess a median unpaired olfactory opening, or a paired opening.
+  The Monorhinæ include only the Cyclostomata&mdash;the lampreys and hag-fishes.</p>
+
+  <p>In the lampreys the single olfactory tube ends blindly, while in the hag-fishes it opens into
+  the pharynx. In the lamprey, both in Petromyzon and Ammoc&#x0153;tes, the opening of this nasal
+  tube is a conspicuous object on the dorsal surface of the head in front of the transparent spot
+  which indicates the position of the right median eye. It is especially significant, as showing the
+  primitive nature of this median olfactory passage, that a perfectly similar opening in the <span
+  class="pagenum" id="page220">{220}</span>same position is always found in the dorsal head-shields
+  of all the Cephalaspidæ and Tremataspidæ, as will be explained more fully in Chapter X.</p>
+
+  <p>All the evidence points to the conclusion that the olfactory apparatus of the vertebrate
+  originated as a single median tube, containing the special olfactory sense-epithelium, which,
+  although median and single, was innervated by the olfactory nerve of each side. The external
+  opening of this tube in the lamprey is dorsal. How does it terminate ventrally?</p>
+
+  <p>The ventral termination of this tube is most instructive and suggestive. It terminates blindly
+  at the very spot where the infundibular tube terminates blindly and the notochord ends. After
+  transformation, when the Ammoc&#x0153;te becomes the Petromyzon, the tube still ends blindly, and
+  does not open into the pharynx as in Myxine; it, however, no longer terminates at the
+  infundibulum, but extends beyond it towards the pharynx.</p>
+
+  <p>This position of the nasal tube suggests that it may originally have opened into the tube of
+  the central nervous system by way of the infundibular tube. This suggestion is greatly enhanced in
+  value by the fact that in the larval Amphioxus the tube of the central nervous system is open to
+  the exterior, its opening being known as the anterior neuropore, and this anterior neuropore is
+  situated at the base of a pit, known as the olfactory pit because it is supposed to represent the
+  olfactory organ of other fishes.</p>
+
+  <p class="sp3">Following the same lines of argument as in previous chapters, this suggestion
+  indicates that the special olfactory organs of the invertebrate ancestor of the vertebrates
+  consisted of a single median olfactory tube or passage, which led directly into the
+  &#x0153;sophagus and was innervated, though single and median, by a pair of olfactory nerves which
+  arose from the supra-&#x0153;sophageal ganglia. Let us see what is the nature of the olfactory
+  organs among arthropods, and whether such a suggestion possesses any probability.</p>
+
+  <p class="ac"><span class="sc">The Olfactory Organs of the Scorpion Group.</span></p>
+
+  <p>At first sight the answer appears to be distinctly adverse, for it is well known that in all
+  the Insecta, Crustacea, and the large majority of Arthropoda, the first pair of antennæ, often
+  called the antennules, are olfactory in function, and these are free-moving, bilaterally <span
+  class="pagenum" id="page221">{221}</span>situated, independent appendages. Still, even here there
+  is the striking fact that the nerves of these olfactory organs always arise from the
+  supra-&#x0153;sophageal ganglia, although those to the second pair of antennæ arise from the
+  infra-&#x0153;sophageal ganglia, just as the olfactory nerves of the vertebrate arise from the
+  supra-infundibular brain-mass. Not only is there this similarity of position, but also a
+  similarity of structure in the olfactive lobes of the brain itself of so striking a character as
+  to cause Bellonci to sum up his investigations as follows<span class="wnw">:&mdash;</span></p>
+
+  <p>"The structure and connections of the olfactive lobes present the same fundamental plan in the
+  higher arthropods and in the vertebrates. In the one, as in the other, the olfactory fibres form,
+  with the connecting fibres of the olfactory lobes, a fine meshwork, which, consisting as it does
+  of separate groups, may each one be called an olfactory glomerulus."</p>
+
+  <p>He attributes this remarkable resemblance to a physiological necessity that similarity of
+  function necessitates similarity of structure, for he considers it out of the question to suppose
+  any near relationship between arthropods and vertebrates.</p>
+
+  <p>Truly an interesting remark, with the one fallacy that relationship is out of the question.</p>
+
+  <p>The evidence so far has consistently pointed to some member of the palæostracan group as the
+  ancestor of the vertebrates&mdash;a group which had affinities both to the crustaceans and the
+  arachnids; indeed, many of its members resembled scorpions much more than they resemble
+  crustaceans. The olfactory organs of the scorpions and their allies are, therefore, more likely
+  than any others to give a clue to the position of the desired olfactory organs. In these animals
+  and their allies paired olfactory antennæ are not present, either in the living land-forms or the
+  extinct sea-scorpions, for all the antennæ-like, frequently chelate, appendages seen in
+  Pterygotus, etc. (Fig. <a href="#fig8">8</a>), represent the cheliceræ, and correspond, therefore,
+  to the second pair of antennæ in the crustaceans.</p>
+
+  <p>What, then, represents the olfactory antennæ in the scorpions? The answer to this question has
+  been given by Croneberg, and very striking it is. The two olfactory antennæ of the crustacean have
+  combined together to form a hollow tube at the base of which the mouth of the animal is situated,
+  so that the food passes along this olfactory passage before it reaches the mouth. This organ is
+  often called after Latreille, the camerostome, sometimes the rostrum; it is naturally median in
+  position and appears, therefore, to be an unpaired organ; its paired <span class="pagenum"
+  id="page222">{222}</span>character is, of course, evident enough, for it is innervated by a pair
+  of nerves, and these nerves, as ought to be the case, arise from the supra-&#x0153;sophageal
+  ganglia. In Galeodes it is a conspicuously paired antennæ-like organ (Fig. <a
+  href="#fig94">94</a>).</p>
+
+  <p>Croneberg has also shown that this rostrum, or camerostome, arises embryologically as a pair of
+  appendages similar to the other appendages. This last observation of Croneberg has been confirmed
+  by Brauer in 1894, who describes the origin of the upper lip, as he calls it, in very similar
+  terms, without, however, referring to Croneberg's paper. Croneberg further shows that this
+  foremost pair of antennæ not only forms the so-called upper lip or camerostome, but also a lower
+  lip, for from the basal part of the camerostome there projects on each side of the pharynx a
+  dependent accessory portion, which in some cases fuses in the middle line, and forms, as it were,
+  a lower lip. The entosclerite belonging to this dependent portion is apparently the post-oral
+  entosclerite of Lankester and Miss Beck.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig094.png" id="fig94"><img style="width:62%" src="images/fig094.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 94.&mdash;Dorsal View of Brain and Camerostome of
+      Galeodes.</span></p>
+      <p class="sp0"><i>cam.</i>, camerostome; <i>pr. ent.</i>, pre-oral entosclerite; <i>l.l.</i>,
+      dependent portion of camerostome; <i>ph.</i>, pharynx; <i>al.</i>, alimentary canal; <i>n.
+      op.</i>, median optic nerves; <i>pl.</i>, plastron; <i>v.c.</i>, ventral nerve chain; 2, 3,
+      second and third appendages.</p>
+    </div>
+  </div>
+
+  <p>At the base of the tubular passage formed by this modified first pair of antennæ the true mouth
+  is found opening directly into the dilated pharynx, the muscles of which enable the act of suction
+  to be carried out. The narrow &#x0153;sophagus leads out from the pharynx and is completely
+  surrounded by the supra- and infra-&#x0153;sophageal nerve masses.</p>
+
+  <p>Huxley also describes the mouth of the scorpion in precisely the same position (<i>cf. o</i>,
+  Fig. <a href="#fig96">96</a>).</p>
+
+  <div><span class="pagenum" id="page223">{223}</span></div>
+
+  <p>In order to convey to my readers the antennæ-like character of the camerostome in Galeodes
+  (Fig. <a href="#fig101">101</a>), and its position, I give a figure (Fig. <a href="#fig94">94</a>)
+  of the organ from its dorsal aspect, after removal of the cheliceræ and their muscles. A side view
+  of the same organ is given in Fig. <a href="#fig95">95</a> to show the feathered termination of
+  the camerostome, and the position of the dependent accessory portion (<i>l.l.</i>) (Croneberg's
+  'untere Anhang') with its single long antenna-like feather. In both figures the alimentary canal
+  (<i>al.</i>) is seen issuing from the conjoined supra- and infra-&#x0153;sophageal mass.</p>
+
+  <p>As is seen in the figures, the bilateral character of the rostrum, as Croneberg calls it, is
+  apparent not only in its feathered extremity but also in its chitinous covering, the softer median
+  dorsal part (left white in figure) being bounded by two lateral plates of hard chitin, which meet
+  in the middle line near the extremity of the organ. In all the members of the scorpion group, as
+  is clearly shown in Croneberg's figures, the rostrum or camerostome is built up on the same plan
+  as in Galeodes, though the antenna-like character may not be so evident.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig095.png" id="fig95"><img style="width:100%" src="images/fig095.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 95.&mdash;Lateral View of Brain and Camerostome of
+      Galeodes.</span></p>
+      <p class="sp0"><i>gl. supr. &#x0153;s.</i>, supra-&#x0153;sophageal ganglion; <i>gl. infr.
+      &#x0153;s.</i>, infra-&#x0153;sophageal ganglion. The rest of the lettering same as in Fig. <a
+      href="#fig94">94</a>.</p>
+    </div>
+  </div>
+
+  <p>When we consider that the first pair of antennæ in the crustaceans are olfactory in function,
+  Croneberg's observations amount to this&mdash;</p>
+
+  <p>In the arachnids and their allies the first pair of antennæ form a pre-oral passage or tube,
+  olfactory in function; the small mouth, which opens directly into the pharynx, being situated at
+  the end of this olfactory passage.</p>
+
+  <div><span class="pagenum" id="page224">{224}</span></div>
+
+  <p>Croneberg's observations and conclusions are distinctly of very great importance in bringing
+  the arachnids into line with the crustaceans, and it is therefore most surprising that they are
+  absolutely ignored by Lankester and Miss Beck in their paper published in 1883, in which Latreille
+  only is mentioned with respect to this organ, and his term "camerostome," or upper lip, is used
+  throughout, in accordance with the terminology in Lankester's previous paper. That this organ is
+  not only a movable lip or tongue, but essentially a sense-organ, almost certainly of smell and
+  taste, as follows from Croneberg's conclusions, is shown by the series of sections which I have
+  made through a number of young Thelyphonus (Fig. <a href="#fig102">102</a>).</p>
+
+  <div class="ac w50 fcenter sp2">
+    <a href="images/fig096.png" id="fig96"><img style="width:100%" src="images/fig096.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p class="sp0"><span class="sc">Fig. 96.&mdash;Median Sagittal Section through a Young
+      Thelyphonus.</span></p>
+    </div>
+  </div>
+
+  <p>I give in Fig. <a href="#fig96">96</a> a sagittal median section through the head-end of the
+  animal, which shows clearly the nature of Croneberg's conception. At the front end of the body is
+  seen the median eye (<i>ce.</i>), <i>o</i> is the mouth, <i>Ph.</i> the pharynx, <i>&#x0153;s.</i>
+  the narrow &#x0153;sophagus, compressed between the supra-&#x0153;sophageal (<i>supr.
+  &#x0153;s.</i>) and infra-&#x0153;sophageal (<i>infr. &#x0153;s.</i>) brain mass, which opens into
+  the large alimentary canal (<i>Al.</i>); <i>Olf. pass.</i> is the olfactory passage to the mouth,
+  lined with thick-set, very fine hairs, which spring from the hypostome (<i>Hyp.</i>) as well as
+  from the large conspicuous camerostome (<i>Cam.</i>), which limits this tube anteriorly. The space
+  between the camerostome and the median eye is filled up by the massive cheliceræ, which are not
+  shown in this section, as they begin to appear in the <span class="pagenum"
+  id="page225">{225}</span>sections on each side of the median one. The muscles of the pharynx and
+  the muscles of the camerostome are attached to the pre-oral entosclerite (<i>pr. ent.</i>). The
+  post-oral entosclerite is shown in section as <i>post. ent.</i> The dorsal blood-vessel, or heart,
+  is indicated at <i>H.</i></p>
+
+  <p>In Fig. <a href="#fig97">97</a> I give a transverse section through another specimen of the
+  same litter, to show the nature of this olfactory tube when cut across. Both sections show most
+  clearly that we are dealing here with an elaborate sense-organ, the surface of which is partly
+  covered with very fine long hairs, partly, as is seen in the figure, is composed of long,
+  separate, closely-set sense-rods (<i>bat.</i>), well protected by the long hairs which project on
+  every side in front of them, which recall to mind Bellonci's figure of the 'batonnets olfactives'
+  on the antennæ of Sphæroma. Finally, we have the observation of Blanchard quoted by Huxley, to the
+  effect that this camerostome is innervated by nerves from the supra-&#x0153;sophageal ganglia
+  which are clearly bilateral, seeing that they arise from the ganglion on each side and then unite
+  to pass into the camerostome; in other words, paired olfactory nerves from the
+  supra-&#x0153;sophageal ganglia.</p>
+
+  <p>These facts demonstrate with wonderful clearness that in one group of the Arthropoda the
+  olfactory antennæ have been so modified as to form an olfactory tube or passage, which leads
+  directly into the mouth and so to the &#x0153;sophagus of the animal, and, strikingly enough, this
+  group, the Arachnida, is the very one to which the scorpions belong.</p>
+
+  <p class="sp3">If for any cause the mouth <i>o</i> in Fig. <a href="#fig96">96</a> were to be
+  closed, then the olfactory tube (<i>olf. pass.</i>) might still remain, owing to its importance as
+  the organ of smell, and the olfactory tube would terminate blindly at the very spot where the
+  corresponding tube does terminate in the vertebrate, according to the theory put forward in this
+  book.</p>
+
+  <p class="ac"><span class="sc">The Olfactory Tube of Ammoc&#x0153;tes.</span></p>
+
+  <p>In all cases where there is similarity of topographical position in the organs of the
+  vertebrate and arthropod we may expect also to find similarity of structure. At first sight it
+  would appear as though such similarity fails us here, for a cross-section of the olfactory tube in
+  Petromyzon represents an elaborate organ such as is shown in Fig. 98, very different in appearance
+  to the section across the olfactory passage of a young Thelyphonus given in Fig. <a
+  href="#fig97">97</a>.</p>
+
+  <div><span class="pagenum" id="page226">{226}</span></div>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig097.png" id="fig97"><img style="width:100%" src="images/fig097.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 97.&mdash;Transverse Section through the Olfactory Passage of a Young
+      Thelyphonus.</span></p>
+      <p class="sp0">1 and 2, sections of first and second appendages.</p>
+    </div>
+  </div>
+
+  <div class="ac w45 fcenter sp2">
+    <a href="images/fig098.png" id="fig98"><img style="width:100%" src="images/fig098.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 98.&mdash;Transverse Section through the Olfactory Passage of
+      Petromyzon.</span></p>
+      <p class="sp0"><i>cart.</i>, nasal cartilage.</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page227">{227}</span></div>
+
+  <p>As is seen, it is difficult to see any connection between these folds of olfactory epithelium
+  and the simple tube of the scorpion. But in the nose, as in all other parts of the head-region of
+  the lamprey, remarkable changes take place at transformation, and examination of the same tube in
+  Ammoc&#x0153;tes demonstrates that the elaborate structure of the adult olfactory organ is
+  actually derived from a much simpler form of organ, represented in Fig. <a href="#fig99">99</a>.
+  Here, in Ammoc&#x0153;tes, the section is no longer strikingly different from that of the
+  Thelyphonus organ, but, instead, most strikingly similar to it. Thus, again, it is shown that this
+  larval form of the lamprey gives more valuable information as to vertebrate ancestry than all the
+  rest of the vertebrates put together.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig099.png" id="fig99"><img style="width:100%" src="images/fig099.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 99.&mdash;Transverse Section through the Olfactory Passage of
+      Ammoc&#x0153;tes.</span></p>
+      <p class="sp0"><i>cart.</i>, nasal cartilage.</p>
+    </div>
+  </div>
+
+  <p>Still, even now the similarity between the two organs is not complete, for the tube in the
+  lamprey opens on to the exterior on the dorsal surface of the head, while in the scorpion tribe it
+  is situated ventrally, being the passage to the mouth and alimentary canal. In accordance with
+  this there is no sign of any opening on the dorsal carapace of any of the extinct sea-scorpions or
+  of the living land-scorpions, such as is so universally found in the cephalaspids, tremataspids,
+  and lampreys. Here is a discrepancy of an apparently serious character, yet so wonderfully does
+  the development of the individual recapitulate the development of the race, that this very
+  discrepancy becomes converted into a triumphant vindication of the <span class="pagenum"
+  id="page228">{228}</span>correctness of the theory advocated in this book, as soon as we turn our
+  attention to the development of this nasal tube in the lamprey.</p>
+
+  <p>We must always remember not only the great importance of a larval stage for the unriddling of
+  problems of ancestry, but also the great advantage of being able to follow more favourably any
+  clues as to past history afforded by the development of the larva itself, owing to the greater
+  slowness in the development of the larva than of the embryo. Such a clue is especially well marked
+  in the course of development of Ammoc&#x0153;tes according to Kupffer's researches, for he finds
+  that when the young Ammoc&#x0153;tes is from 5 to 7 mm. in length, some time after it has left the
+  egg, when it is living a free larval life, a remarkable series of changes takes place with
+  considerable rapidity, so that we may regard the transformation which takes place at this stage,
+  as in some degree comparable with the great transformation which occurs when the Ammoc&#x0153;tes
+  becomes a Petromyzon.</p>
+
+  <p>All the evidence emphasizes the fact that the latter transformation indicates the passage from
+  a lower into a higher form of vertebrate, and is to be interpreted phylogenetically as an
+  indication of the passage from the Cephalaspidian towards the Dipnoan style of fish. If, then, the
+  former transformation is of the same character, it would indicate the passage from the
+  Palæostracan to the Cephalaspid.</p>
+
+  <p>What is the nature of this transformation process as described by Kupffer?</p>
+
+  <p>It is characterized by two most important events. In the first place, up to this time the oral
+  chamber has been cut off from the respiratory chamber by a septum&mdash;the velum&mdash;so that no
+  food could pass from the mouth to the alimentary canal. At this stage the septum is broken
+  through, the oral chamber communicates with the respiratory chamber, and the velar folds of the
+  more adult Ammoc&#x0153;tes are left as the remains of the original septum. The other striking
+  change is the growth of the upper lip, by which the orifice of the nasal tube is transferred from
+  a ventral to a dorsal position. Fig. <a href="#fig100">100</a>, taken from Kupffer's paper,
+  represents a sagittal section through an Ammoc&#x0153;tes 4 mm. long; <i>l.l.</i> is the lower
+  lip, <i>u.l.</i> the upper lip, and, as is seen, the short oral chamber is closed by the septum,
+  <i>vel.</i> Opening ventrally is a tube called the tube of the hypophysis, <i>Hy.</i>, which
+  extends close up to the termination of the infundibulum. On the anterior surface of this tube is
+  the projection called by Kupffer the olfactory plakode. At this stage the upper lip grows with
+  great <span class="pagenum" id="page229">{229}</span>rapidity and thickens considerably, thus
+  forcing the opening of the hypophysial tube more and more dorsalwards, until at last, in the
+  full-grown Ammoc&#x0153;tes, it becomes the dorsal opening of the nasal tube, as already
+  described. Here, then, in the hypophysial tube we have the original position of the olfactory tube
+  of the vertebrate ancestor, and it is significant, as showing the importance of this organ, to
+  find that such a hypophysial tube is characteristic of the embryological development of every
+  vertebrate, whatever may be the ultimate form of the external nasal orifices.</p>
+
+  <p>The single median position of the olfactory organ in the Cyclostomata, in contradistinction to
+  its paired character in the rest of the vertebrates, has always been a stumbling-block in the way
+  of those who desired to consider the Cyclostomata as degenerated Selachians, for the origin of the
+  olfactory protuberance, as a single median plakode, seemed to indicate that the nose arose as a
+  single organ and not as a paired organ.</p>
+
+  <div class="ac w45 fcenter sp2">
+    <a href="images/fig100.png" id="fig100"><img style="width:100%" src="images/fig100.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 100.&mdash;Ganglia of the Cranial Nerves of an
+      Ammoc&#x0153;tes, 4 mm. in length, projected on to the Median Plane.</span> (After <span
+      class="sc">Kupffer</span>.)</p>
+      <p class="sp0"><i>A-B</i>, the line of epibranchial ganglia; <i>au.</i>, auditory capsule;
+      <i>nc.</i>, notochord; <i>Hy.</i>, tube of hypophysis; <i>Or.</i>, oral cavity; <i>u.l.</i>,
+      upper lip; <i>l.l.</i>, lower lip; <i>vel.</i>, septum between oral and respiratory cavities;
+      <i>V.</i>, <i>VII.</i>, <i>IX.</i>, <i>X.</i>, cranial nerves; <i>x.</i>, nerve with four
+      epibranchial ganglia.</p>
+    </div>
+  </div>
+
+  <p>On the other hand, the two olfactory nerves of Ammoc&#x0153;tes compare absolutely with the
+  olfactory nerves of other vertebrates, and force one to the conclusion that this median organ of
+  Ammoc&#x0153;tes arose from a pair of bilateral organs, which have fused in the middle line.</p>
+
+  <div><span class="pagenum" id="page230">{230}</span></div>
+
+  <div class="ac w45 fcenter sp2">
+    <a href="images/fig101.jpg" id="fig101"><img style="width:100%" src="images/fig101.jpg" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p class="sp0"><span class="sc">Fig. 101.</span>&mdash;<i>Galeodes.</i> (From the Royal
+      Natural History.)</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page231">{231}</span></div>
+
+  <p>The comparison of this olfactory organ with the camerostome gives a satisfactory reason for its
+  appearance in the lowest vertebrates as an unpaired median organ; equally so, the history of the
+  camerostome itself supplies the reason why the olfactory nerves are double, why the organ is in
+  reality a paired organ and not a single median one. Thus, in a sense, the grouping of the fishes
+  into Monorhinæ and Amphirhinæ has not much meaning, seeing that the olfactory organ is in all
+  cases double.</p>
+
+  <div class="ac w35 fcenter sp2">
+    <a href="images/fig102.png" id="fig102"><img style="width:100%" src="images/fig102.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p class="sp0"><span class="sc">Fig. 102.</span>&mdash;<i>Thelyphonus.</i> (From the Royal
+      Natural History.)</p>
+    </div>
+  </div>
+
+  <p class="sp3">The evidence of the olfactory organs in the vertebrate not only confirms, in a most
+  striking manner, the theory of the origin of the <span class="pagenum"
+  id="page232">{232}</span>vertebrate from the Palæostracan, but points indubitably to an origin
+  from a scorpion-like rather than a crustacean-like stock. To complete the evidence, it ought to be
+  shown that the ancient sea-scorpions did possess an olfactory passage similar to the modern
+  land-scorpions. The evidence on this question will come best in the next chapter, where I propose
+  to deal with the prosomatic appendages of the Palæostracan group.</p>
+
+  <p class="ac"><span class="sc">Summary.</span></p>
+
+  <div class="bq1 smaller sp5">
+    <p>The vertebrate olfactory apparatus commences as a single median tube which terminates
+    dorsally in the lamprey, and is supplied by the two olfactory nerves which arise from the
+    supra-infundibular portion of the brain. It is a long, tapering tube which passes ventrally and
+    terminates blindly at the infundibulum in Ammoc&#x0153;tes. The dorsal position of the nasal
+    opening is not the original one, but is brought about by the growth of the upper lip. The nasal
+    tube originally opened ventrally, and was at that period of development known as the tube of the
+    hypophysis.</p>
+    <p>The evidence of Ammoc&#x0153;tes thus goes to show that the olfactory apparatus started as an
+    olfactory tube on the ventral side of the animal, which led directly up to, and probably into,
+    the &#x0153;sophagus of the original alimentary canal of the palæostracan ancestor.</p>
+    <p>Strikingly enough, although in the crustaceans the first pair of antennæ form the olfactory
+    organs, no such free antennæ are found in the arachnids, but they have amalgamated to form a
+    tube or olfactory passage, which leads directly into the mouth and &#x0153;sophagus of the
+    animal.</p>
+    <p>This olfactory passage is very conspicuous in all members of the scorpion group, and, like
+    the olfactory tube of the vertebrate, is innervated by a pair of nerves, which resemble those
+    supplying the first pair of antennæ in crustaceans as to their origin from the
+    supra-&#x0153;sophageal ganglia.</p>
+    <p>This nasal passage, or tube of the hypophysis, corresponds in structure and in position most
+    closely with the olfactory tube of the scorpion group, the only difference being that in the
+    latter case it opens directly into the &#x0153;sophagus, while in the former, owing to the
+    closure of the old mouth, it cannot open into the infundibulum.</p>
+    <p>The evidence of the olfactory apparatus, combined with that of the optic apparatus, is most
+    interesting, for, whereas the former points indubitably to an ancestor having scorpion-like
+    affinities, the structure of the lateral eyes points distinctly to crustacean, as well as
+    arachnid, affinities.</p>
+    <p class="sp0">Taking the two together the evidence is extraordinarily strong that the
+    vertebrate arose from a member of the palæostracan group with marked scorpion-like
+    affinities.</p>
+  </div>
+
+  <div><span class="pagenum" id="page233">{233}</span></div>
+
+  <p class="ac">CHAPTER VII</p>
+
+  <p class="ac"><i>THE PROSOMATIC SEGMENTS OF LIMULUS AND ITS ALLIES</i></p>
+
+  <div class="bq1 it1 smaller sp3">
+    <p class="sp0">Comparison of the trigeminal with the prosomatic region.&mdash;The prosomatic
+    appendages of the Gigantostraca.&mdash;Their number and nature.&mdash;Endognaths and
+    ectognath.&mdash;The metastoma.&mdash;The coxal glands.&mdash;Prosomatic region of Eurypterus
+    compared with that of Ammoc&#x0153;tes.&mdash;Prosomatic segmentation shown by muscular markings
+    on carapace.&mdash;Evidence of c&#x0153;lomic cavities in Limulus.&mdash;Summary.</p>
+  </div>
+
+  <p>The derivation of the olfactory organs of the vertebrate from the olfactory antennæ of the
+  arthropod in the last chapter is confirmatory proof of the soundness of the proposition put
+  forward in Chapter IV., that the segmentation in the cranial region of the vertebrate was derived
+  from that of the prosomatic and mesosomatic regions of the palæostracan ancestor. Such a
+  segmentation implies a definite series of body-segments, corresponding to the mesomeric
+  segmentation of the vertebrate, and a definite series of appendages corresponding to the
+  splanchnic segmentation of the vertebrate.</p>
+
+  <p>Of the foremost segments belonging to the supra-&#x0153;sophageal region characterized by the
+  presence of the median eyes, of the lateral eyes, and of the olfactory organs, a wonderfully exact
+  replica has been shown to exist in the pineal eyes, the lateral eyes, and the olfactory organ of
+  the vertebrate, belonging, as they all do, to the supra-infundibular region.</p>
+
+  <p>Of the infra-&#x0153;sophageal segments belonging to the prosoma and mesosoma respectively, the
+  correspondence between the mesosomatic segments carrying the branchial appendages and the uterus,
+  with those in the vertebrate carrying the branchiæ and the thyroid gland respectively, has been
+  fully proved in previous chapters.</p>
+
+  <p>There remain, then, only the segments of the prosomatic region to be considered, a region
+  which, both in the vertebrate and invertebrate, is never respiratory in function but always
+  masticatory, such <span class="pagenum" id="page234">{234}</span>mastication being performed in
+  Limulus and its allies by the muscles which move the foot-jaws or gnathites, which are portions of
+  the prosomatic appendages specially modified for that purpose, and in the vertebrates by the
+  masticatory muscles, which are always innervated by the trigeminal or Vth cranial nerve. This
+  comparison implies that the motor part of the trigeminal nerve originally supplied the prosomatic
+  appendages.</p>
+
+  <p>The investigations of van Wijhe and of all observers since the publication of his paper prove
+  that in this trigeminal region, as in the vagus region, a double segmentation exists, of which the
+  ventral or splanchnic segments, corresponding to the appendages in the invertebrate, are supplied
+  by the trigeminal nerves, while the dorsal or somatic segments, corresponding to the somatic
+  segments in the invertebrate, are supplied by the IIIrd or oculomotor and the IVth or trochlear
+  nerves&mdash;nerves which supply muscles moving the lateral eyes.</p>
+
+  <p>In accordance, then, with the evidence afforded by the nerves of the branchial segments, it
+  follows that the muscles supplied by the motor part of the trigeminal ought originally to have
+  moved the appendages belonging to a series of prosomatic segments. On the other hand, the
+  eye-muscles ought to have belonged to the body-part of the prosomatic segments, and must therefore
+  have been grouped originally in a segmental series corresponding to the prosomatic appendages.</p>
+
+  <p>The evidence for and against this conclusion will be the subject of consideration in this and
+  the succeeding chapters. At the outset it is evident that any such comparison necessitates an
+  accurate knowledge of the number of the prosomatic segments in the Gigantostraca and of the nature
+  of the corresponding appendages.</p>
+
+  <p>In all this group of animals, the evidence as to the number of segments in either the
+  prosomatic or mesosomatic regions is given by&mdash;</p>
+
+  <p>1.  The number of appendages.</p>
+
+  <p>2.  The segmental arrangement of the muscles of the prosoma or mesosoma respectively.</p>
+
+  <p>3.  The segmental arrangement of the c&#x0153;lomic or head-cavities.</p>
+
+  <p>4.  The divisions of the central nervous system, or neuromeres, together with their outgoing
+  segmental nerves.</p>
+
+  <p>It follows, therefore, that if from any cause the appendages are not apparent, as is the case
+  in many fossil remains, or have dwindled <span class="pagenum" id="page235">{235}</span>away and
+  become insignificant, we still have the muscular, c&#x0153;lomic, and nervous arrangements left to
+  us as evidence of segmentation in these animals, just as in vertebrates.</p>
+
+  <p>In this prosomatic region, we find in Limulus the same tripartite division of the nerves as in
+  the mesosomatic region, so that the nerves to each segment may be classed as (1) appendage-nerve;
+  (2) sensory or dorsal somatic nerve, supplying the prosomatic carapace; (3) motor or ventral
+  somatic nerve, supplying the muscles of the prosoma, and containing possibly some sensory fibres.
+  The main difference between these two regions in Limulus consists in the closer aggregation of the
+  prosomatic nerves, corresponding to the concentration of the separate ganglia of origin in the
+  prosomatic region of the brain.</p>
+
+  <p>The number of prosomatic segments in Limulus is not evident by examination of the prosomatic
+  carapace, so that the most reliable guide to the segmentation of this region is given by the
+  appendages, of which one pair corresponds to each prosomatic segment.</p>
+
+  <p>The number of such segments, according to present opinion, is seven, viz.<span
+  class="wnw">:&mdash;</span></p>
+
+  <p>(1) The foremost segment, which bears the cheliceræ.</p>
+
+  <p>(2, 3, 4, 5, 6) The next five segments, which carry the paired locomotor appendages; and</p>
+
+  <p>(7) The last segment, to which belongs a small abortive pair of appendages, known by the name
+  of the chilaria, situated between the last pair of locomotor appendages and the operculum or first
+  pair of mesosomatic appendages. These appendages are numbered from 1-7 in the accompanying drawing
+  (Fig. <a href="#fig103">103</a>).</p>
+
+  <p>Of these seven pairs of appendages, the significance of the first and the last has been matter
+  of dispute. With respect to the first pair, or the cheliceræ, the question has arisen whether
+  their nerves belong to the infra-&#x0153;sophageal group, or are in reality
+  supra-&#x0153;sophageal.</p>
+
+  <p>It is instructive to observe the nature and the anterior position of this pair of appendages in
+  the allied sea-scorpions, especially in Pterygotus, where the only chelate organs are found in
+  these long, antennæ-like cheliceræ. In Slimonia and in Stylonurus they are supposed by Woodward to
+  be represented by the small non-chelate antennæ seen in Fig. <a href="#fig8">8</a>, B and C (p. <a
+  href="#page27">27</a>), taken from Woodward. If such is the case, then these figures show that a
+  pair of appendages is missing in each <span class="pagenum" id="page236">{236}</span>of these
+  forms, for they possess only five free prosomatic appendages instead of six, as in Limulus and in
+  Pterygotus. Similarly, Woodward only allowed five appendages for Pterygotus, so that his
+  restorations were throughout consistent. Schmidt, in <i>Pterygotus osiliensis</i> has shown that
+  the true number was six, not five, as seen in his restoration given in Fig. <a href="#fig8">8</a>,
+  A (p. <a href="#page27">27</a>).</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig103.png" id="fig103"><img style="width:100%" src="images/fig103.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 103.&mdash;Ventral Surface of Limulus.</span> (Taken from <span
+      class="sc">Kishinouye</span>.)</p>
+      <p class="sp0">The gnathic bases of the appendages have been separated from those of the other
+      side to show the promesosternite or endostoma (<i>End.</i>).</p>
+    </div>
+  </div>
+
+  <p>With respect to Eurypterus, Schmidt figures an exceedingly minute pair of antennæ between the
+  coxal joints of the first pair of appendages, thus making six pairs of appendages. Gerhard Holm,
+  however, in his recent beautiful preparations from Schmidt's specimens and others collected at
+  Rootziküll, has proved most conclusively that the cheliceræ of Eurypterus were of the same kind as
+  those of Limulus. I reproduce his figure (Fig. <a href="#fig104">104</a>) showing the small
+  chelate cheliceræ (1) overhanging the mouth orifice, just as in Limulus or in Scorpio.</p>
+
+  <div><span class="pagenum" id="page237">{237}</span></div>
+
+  <p>So, also, since Woodward's monograph, Laurie has discovered in <i>Slimonia acuminata</i> a
+  small median pair of chelate appendages exactly corresponding to the cheliceræ of Limulus, or of
+  Eurypterus, or of Scorpio. We may, therefore, take it for granted that such was also the case in
+  Stylonurus, and that the foremost pair of prosomatic appendages in all these extinct sea-scorpions
+  were in the same position and of the same character as the cheliceræ of the scorpions.</p>
+
+  <div class="ac w45 fcenter sp2">
+    <a href="images/fig104.png" id="fig104"><img style="width:100%" src="images/fig104.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p class="sp0"><span class="sc">Fig. 104.</span>&mdash;<i>Eurypterus Fischeri.</i> (From <span
+      class="sc">Holm</span>.)</p>
+    </div>
+  </div>
+
+  <p>In the living scorpion and in Limulus the nerves to this pair of appendages undoubtedly arise
+  from the foremost prosomatic ganglia, and the reason why they appear to belong to the
+  supra-&#x0153;sophageal brain-mass has been made clear by Brauer's investigations on the
+  embryology of Scorpio; for he has shown that the cheliceral ganglia shift from the ventral to the
+  dorsal side of the &#x0153;sophagus during development, thus becoming
+  pseudo-supra-&#x0153;sophageal, though in reality belonging to the infra-&#x0153;sophageal
+  ganglia. This cheliceral pair of appendages is, in all probability, homologous with the second
+  pair of antennæ in the crustacea.</p>
+
+  <div><span class="pagenum" id="page238">{238}</span></div>
+
+  <p>I conclude, then, that the cheliceræ must truly be included in the prosomatic group, but that
+  they stand in a somewhat different category to the rest of the prosomatic appendages, inasmuch as
+  they take up a very median anterior and somewhat dorsal position, and their ganglia of origin are
+  also exceptional in position.</p>
+
+  <p>Next for consideration come the chilaria (7 in Fig. <a href="#fig103">103</a>), which Lankester
+  did not consider to belong to appendages at all, but to be a peculiar pair of sternites. Yet their
+  very appearance, with their spinous hairs corresponding to those of the other gnathites and their
+  separate nerve-supply, all point distinctly to their being a modified pair of appendages, and,
+  indeed, the matter has been placed beyond doubt by the observations of Kishinouye, who has found
+  embryologically that they arise in the same way as the rest of the prosomatic appendages, and
+  belong to a distinct prosomatic segment, viz. the seventh segment. In accordance with this, Brauer
+  has found that in the scorpion there is in the embryo a segment, whose appendages degenerate,
+  which is situated between the segment bearing the last pair of thoracic appendages and the genital
+  operculum&mdash;a segment, therefore, comparable in position to the chilarial segment of
+  Limulus.</p>
+
+  <p>Coming now to the five locomotor appendages, we find that they resemble each other to a
+  considerable extent in most cases, with, however, certain striking differences. Thus in Limulus
+  they are chelate, with their basal joints formed as gnathites, except in the case of the fifth
+  appendage, in which the extremity is modified for the purpose of digging in the sand. In
+  Pterygotus, Slimonia, Eurypterus, the first four of these appendages are very similar, and are
+  called by Huxley and Woodward endognaths; in all cases they possess a basal part or sterno-coxal
+  process, which acts as a gnathite or foot-jaw, and a non-chelate tactile part, which possesses no
+  prehensile power, and in most cases could have had no appreciable share in locomotion, called by
+  Huxley and Woodward the palpus. These small palps were probably retractile, and capable of being
+  withdrawn entirely under the hood. The fifth appendage is usually different, being a large
+  swimming organ in Pterygotus, Eurypterus, and Slimonia (Figs. 8 and 104), and is known as the
+  ectognath.</p>
+
+  <p>Finally, in <i>Drepanopterus Bembycoides</i>, as stated by Laurie, all five locomotor
+  appendages are built up after the same fashion, the last one not being formed as a paddle-shaped
+  organ or elongated as <span class="pagenum" id="page239">{239}</span>in Stylonurus, but all five
+  possess no special locomotor or prehensile power. According to Laurie this is a specially
+  primitive form of the group.</p>
+
+  <p>It is significant to notice from this sketch that with the absence of special prehensile
+  terminations such as chelæ, or the absence of special locomotor functions such as walking or
+  swimming, these appendages tend to dwindle and become insignificant, taking up the position of
+  mere feelers round the mouth, and at the same time are concentrated and pressed closely together,
+  so that their appendage-nerves must also be close together.</p>
+
+  <p>This sketch therefore shows us that&mdash;</p>
+
+  <p>Of the six foremost prosomatic appendages, the cheliceræ and the four endognaths were, at the
+  time when the vertebrates first appeared, in very many cases dwindling away; the latter especially
+  no longer functioned as locomotor appendages, but were becoming more and more mere palps or
+  tentacles situated round the mouth, which could by no possibility afford any help to
+  locomotion.</p>
+
+  <p>On the contrary, the sixth pair of appendages&mdash;the ectognaths&mdash;remained powerful,
+  being modified in many cases into large oar-like limbs by which the animal propelled itself
+  through the water.</p>
+
+  <p>It is a striking coincidence that those ancient fishes, <span class="correction"
+  title="Original reads 'Ptericthys'.">Pterichthys</span> and Bothriolepis, should have possessed a
+  pair of large oar-like appendages.</p>
+
+  <p>At this time, then, in strong contrast to the endognaths, the ectognaths, or sixth pair of
+  appendages, remained strong and vigorous. What about the seventh pair, the chilaria of
+  Limulus?</p>
+
+  <p>Of all the prosomatic appendages these are the most interesting from the point of view of my
+  theory, for whereas in the scorpion of the present day they have dwindled away and left no trace
+  except in the embryo, in the sea-scorpions of old, far from dwindling, they had developed and
+  become a much more important organ than the chilaria of Limulus.</p>
+
+  <p>In all these animals a peculiarly striking and unique structure is found in this region known
+  by the name of the metastoma, or lip-plate (Figs. 8 and 104 (7)); it is universally considered to
+  be formed by the fusion of the two chilarial appendages.</p>
+
+  <p>All observers are agreed that this lip-plate was freely movable. Nieskowski considers that the
+  movement of the metastoma was entirely in a vertical direction, whereby the cleft which is seen
+  <span class="pagenum" id="page240">{240}</span>between the basal joints of all the pairs of
+  locomotor appendages could be closed from behind. Woodward says it no doubt represents the labium,
+  and served more effectually to enclose the posterior part of the buccal orifice, being found
+  exteriorly to the toothed edges of the ectognaths or maxillipedes. Schmidt agrees with Nieskowski,
+  and looks on the mestasoma as forming a lower lip within which the bases of the ectognaths
+  worked.</p>
+
+  <div class="ac w45 fcenter sp2">
+    <a href="images/fig105.png" id="fig105"><img style="width:100%" src="images/fig105.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p class="sp0"><span class="sc">Fig. 105.&mdash;Diagram of Sagittal Median Section through A,
+      Limulus, B, Eurypterus.</span></p>
+    </div>
+  </div>
+
+  <p>Quite recently Gerhard Holm has worked over again the very numerous specimens of <i>Eurypterus
+  Fischeri</i>, which are obtainable at Rootziküll, and has thrown new light on the relation of the
+  metastoma to the mouth-parts. His preparations show clearly that the true lower lip of Eurypterus
+  was not the metastoma, for when the metastoma is removed another plate (<i>End.</i>, Fig. <a
+  href="#fig105">105</a>, B) situated <span class="pagenum" id="page241">{241}</span>internally to
+  it is disclosed, which, in his view, corresponds to the sternite between the bases of the
+  pro-somatic appendages in Limulus, <i>i.e.</i> to the sternite called by Lankester, the
+  pro-mesosternite (<i>End.</i>, Fig. <a href="#fig103">103</a>). This inner plate formed with the
+  metastoma ((7) Fig. 105) and the ectognaths (6) a chamber closed posteriorly, within which the
+  bases of the ectognaths worked. In other words, the removal of the metastoma discloses in
+  Eurypterus the true anterior ventral surface of the animal which corresponds to that of Limulus,
+  or of the scorpion group, with its pro-mesosternite and laterally attached gnathites or
+  sterno-coxal processes. To this inner plate or pro-mesosternite Holm gives the name of
+  <i>endostoma</i>.</p>
+
+  <p>To the anterior edge of the endostoma a thinner membrane is attached which passes inwards in
+  the direction of the throat, and forms, therefore, the lower lip (<i>Hyp.</i>, Fig. <a
+  href="#fig105">105</a>, B) of the passage of the mouth (<i>olf. p.</i>). This membrane bears upon
+  its surface a tuft of hairs, which he thought were probably olfactory in function. Consequently,
+  in his preliminary communication, he describes this lower lip as forming, in all probability, an
+  olfactory organ; in his full communication he repudiates this suggestion, because he thinks it
+  unlikely that such an organ would be situated within the mouth. I feel sure that if Holm had
+  referred to Croneberg's paper, and seen how the true mouth in all the scorpion group is situated
+  at the base of an olfactory passage, he would have recognized that his first suggestion is in
+  striking accordance with the nature of the entrance to the mouth in other scorpions.</p>
+
+  <p>That Eurypterus also possessed a camerostome (<i>cam.</i>) seems to follow of necessity from
+  its evident affinities both with Limulus and the scorpions. We see, in fact, that the mouth of
+  these old sea-scorpions was formed after the fashion of Limulus, surrounded by masticatory organs
+  in the shape of foot-jaws, and yet foreshadowed that of the scorpion, so that an ideal sagittal
+  section of one of these old palæostracan forms would be obtained by the combination of actual
+  sagittal sections through Limulus and a member of the scorpion group, with, at the same time, a
+  due recognition of Holm's researches. Such a section is represented in Fig. <a
+  href="#fig105">105</a>, B, in which I have drawn the central nervous system and its nerves, the
+  median eyes (<i>C.E.</i>), the olfactory organs (<i>Cam.</i>), the pharynx (<i>Ph.</i>),
+  &#x0153;sophagus (<i>&#x0153;s.</i>), and alimentary canal (<i>Al.</i>), but have not tried to
+  indicate the lateral eyes. I have represented the prosomatic appendages by numbers (1-7), and
+  <span class="pagenum" id="page242">{242}</span>the foremost mesosomatic segments by numbers
+  (8-13). I have placed the four endognaths and the nerves going to them close together, and made
+  them small, mere tentacles, in recognition of the character of these appendages in Eurypterus, and
+  have indicated the position and size of the large ectognath, with its separate nerve, by (6). If
+  among the ancient Eurypterus-like forms, which were living at the time when vertebrates first
+  appeared, there were some in which the ectognaths also had dwindled to a pair of tentacles, then
+  such animals would possess a prosomatic chamber formed by a metastoma or accessory lip, within
+  which were situated five pairs of short tactile appendages or tentacles. If the vertebrate were
+  derived from such an animal, then the trigeminal nerve, as the representative of these prosomatic
+  appendage-nerves, ought to be found to supply the muscles of this accessory lip and of these five
+  pairs of tentacles in the lowest vertebrate.</p>
+
+  <p class="sp3">This prosomatic or oral chamber, as it might be called, was limited posteriorly by
+  the fused metastoma (7) and operculum (8), so that if in the same imaginary animal one imagines
+  that the gill-chambers, instead of being separate, are united to form one large respiratory
+  chamber, then, in such an animal, a prosomatic oral chamber, in which the prosomatic appendages
+  worked, would be separated from a mesosomatic respiratory chamber by a septum composed of the
+  conjoined basal portions of the mesosomatic operculum and the prosomatic metastoma, as indicated
+  in the diagram. In this septum the nerves to the last prosomatic appendage (equivalent to the last
+  part of the trigeminal in the vertebrate) and to the first mesosomatic (equivalent to the thyroid
+  part of the facial) would run, as shown in the figure, close together in the first part of their
+  course, and would separate when the ventral surface was reached, to pass headwards and tailwards
+  respectively.</p>
+
+  <p class="ac"><span class="sc">The Coxal Glands.</span></p>
+
+  <p>One more characteristic of these appendages requires mention, and that is the excretory glands
+  situated at the base of the four endognaths known as the coxal glands. These glands are the main
+  excretory organs in Limulus and the scorpions, and extend into the basal segments or coxæ of the
+  four endognaths, not into those of the ectognaths or the chilaria (or metastoma). Hence their
+  name, coxal <span class="pagenum" id="page243">{243}</span>glands; and, seeing the importance of
+  the excretory function, it is likely enough that they would remain, even when the appendages
+  themselves had dwindled away. With the concentration and dwindling of the endognaths these coxal
+  glands would also be concentrated, so that in the diagram (Fig. <a href="#fig105">105</a>) they
+  would rightly be grouped together in the position indicated (<i>cox. gl.</i>).</p>
+
+  <p class="sp3">Such a diagram indicates the position of all the important organs of the
+  head-region except the special organs for taste and hearing. These, for the sake of convenience, I
+  propose to take separately, in order at this stage of my argument not to overburden the simplicity
+  of the comparison I desire to make with too much unavoidable detail.</p>
+
+  <p class="ac"><span class="sc">The Prosomatic Region of Ammoc&#x0153;tes.</span></p>
+
+  <p>Let us now compare this diagram with that of the corresponding region in Ammoc&#x0153;tes and
+  see whether or no any points of similarity exist.</p>
+
+  <p>With respect to this region, as in so many other instances already mentioned, Ammoc&#x0153;tes
+  occupies an almost unique position among vertebrates, for the region supplied by the trigeminal
+  nerve&mdash;the prosomatic region&mdash;consists of a large oral chamber which was separated from
+  the respiratory chamber in the very young stage by a septum which is subsequently broken through,
+  and so the two chambers communicate.</p>
+
+  <p>This chamber is bounded by the lower lip ventrally, the upper lip and trabecular region
+  dorsally, and the remains of the septum or velum laterally and posteriorly. It contains a number
+  of tentacles arranged in pairs within the chamber so as to form a sieve-like fringe inside the
+  circular mouth; of these, the ventral pair are large, fused together, and attached to the lower
+  lip.</p>
+
+  <p>All the muscles belonging to this oral chamber are of the visceral type, and are innervated by
+  the trigeminal nerve. In accordance with the evidence obtained up to this point this means that
+  such an oral chamber was formed by the prosomatic appendages of the invertebrate ancestor,
+  similarly to the oral chamber just figured for Eurypterus.</p>
+
+  <p>This chamber in the full-grown Ammoc&#x0153;tes is not only open to the respiratory chamber,
+  but is bounded by the large upper lip (<i>U.L.</i>, Fig. <a href="#fig106">106</a>, D). On the
+  dorsal surface of this region, in front of the <span class="pagenum"
+  id="page244">{244}</span>pineal eye (<i>C.E.</i>), is the most conspicuous opening of the
+  olfactory tube (<i>Na.</i>), which olfactory tube passes from the dorsal region to the ventral
+  side to terminate blindly at the very spot where the infundibulum comes to the surface of the
+  brain. Here, also, is situated that extraordinary glandular organ known as the pituitary body
+  (<i>Pit.</i>). A sagittal section, then, in diagram form, of the position of parts in the
+  full-grown Ammoc&#x0153;tes, would be represented as in Fig. <a href="#fig106">106</a>, D.</p>
+
+  <p>But, as argued out in the last chapter, the diagram of the adult Ammoc&#x0153;tes must be
+  compared with that of a cephalaspidian fish; the diagram of the palæostracan must be compared with
+  the larval condition of Ammoc&#x0153;tes. In other words, Fig. <a href="#fig106">106</a>, B, must
+  be compared with Fig. <a href="#fig106">106</a>, C, which represents a section through the larval
+  Ammoc&#x0153;tes as it would appear if it reached the adult condition without any forward growth
+  of the upper lip or any breaking through of the septum between the oral and respiratory chambers.
+  The striking similarity between this diagram and that of Eurypterus becomes immediately manifest
+  even to the smallest details. The only difference between the two, except, of course, the
+  notochord, consists in the closure of the mouth opening (<i>o</i>), in Fig. <a
+  href="#fig106">106</a>, B, by which the olfactory passage (<i>olf. p.</i>) of the scorpion becomes
+  converted into the hypophysial tube (<i>Hy.</i>), Fig. <a href="#fig106">106</a>, C, and later
+  into the nasal tube (<i>Na.</i>), Fig. <a href="#fig106">106</a>, D, of the full-grown
+  Ammoc&#x0153;tes. That single closure of the old mouth is absolutely all that is required to
+  convert the Eurypterus diagram into the Ammoc&#x0153;tes diagram.</p>
+
+  <p>Such a comparison immediately explains in the simplest manner a number of anatomical
+  peculiarities which have hitherto been among the great mysteries of the vertebrate organization.
+  For not only do the median eyes (<i>C.E.</i>) correspond in position in the two diagrams, and the
+  infundibular tube (<i>Inf.</i>) and the ventricles of the brain (<i>C.C.</i>) correspond to the
+  &#x0153;sophagus (<i>&#x0153;s.</i>) and the cephalic stomach (<i>Al.</i>), as already fully
+  discussed; but even in the very place where the narrow &#x0153;sophagus opened into the wider
+  chamber of the pharynx (<i>Ph.</i>), there, in all the lower vertebrates, the narrow infundibular
+  tube opens into the wider chamber of the membranous <i>saccus vasculosus</i> (<i>sac. vasc.</i>).
+  This is the last portion of the membranous part of the tube of the central nervous system which
+  has not received explanation in the previous chapters, and now it is seen how simple its
+  explanation is, how natural its presence&mdash;it represents the old pharyngeal chamber of the
+  palæostracan ancestor.</p>
+
+  <div><span class="pagenum" id="page245">{245}</span></div>
+
+  <div class="ac w50 fcenter sp2">
+    <a href="images/fig106.png" id="fig106"><img style="width:100%" src="images/fig106.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p class="sp0"><span class="sc">Fig. 106.&mdash;Diagram of Sagittal Median Section through B,
+      Eurypterus; C, Larval Ammoc&#x0153;tes; D, Full-grown Ammoc&#x0153;tes.</span></p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page246">{246}</span></div>
+
+  <p>Next among the mysteries requiring explanation is the pituitary body, that strange glandular
+  organ always found so closely attached to the brain in the infundibular region that when it is
+  detached in taking out the brain it leaves the infundibular canal patent right into the IIIrd
+  ventricle. A comparison of the two diagrams indicates that such a glandular organ (<i>Pit.</i>),
+  Fig. <a href="#fig106">106</a>, C, was there because the coxal excretory glands (<i>cox. gl.</i>),
+  Fig. <a href="#fig106">106</a>, B, were in a similar position in the palæostracan
+  ancestor&mdash;that, indeed, the pituitary body is the descendant of the coxal glands.</p>
+
+  <p>Finally, the diagrams not only indicate how the mesosomatic appendage-nerves supplying in the
+  one case the operculum and the respiratory appendages correspond to the respiratory group of
+  nerves, VII., IX., X., supplying in the other case the thyroid, hyoid, and branchial segments, but
+  also that a similar correspondence exists between the prosomatic appendage-nerves in the one case
+  and the trigeminal nerve in the other; a correspondence which supplies the reason why in the
+  vertebrate a septum originally existed between an oral and respiratory chamber.</p>
+
+  <p>Such a comparison, then, leads directly to the suggestion that the trigeminal nerve originally
+  supplied the prosomatic appendages, such appendages being: 1. The metastoma, which has become in
+  Ammoc&#x0153;tes the lower lip supplied by the velar or mandibular branch of the trigeminal nerve
+  (7); 2. The ectognath, which has become the large median ventral tentacle, called by Rathke the
+  tongue, supplied by the tongue nerve (6); 3. The endognaths, which have been reduced to tentacles
+  and are supplied by the tentacular branch of the trigeminal nerve (2, 3, 4, 5).</p>
+
+  <p>I have purposely put these two diagrams of the larval Ammoc&#x0153;tes and of Eurypterus before
+  the minds of my readers at this early stage of my argument, so as to make what follows more
+  understandable. I propose now to consider fully each one of these suggestive comparisons, and to
+  see whether or no they are in accordance with the results of modern research.</p>
+
+  <p>In the first instance, the diagrams suggest that the trigeminal nerve originally supplied the
+  prosomatic appendages of the palæostracan ancestor, while the eye-muscle nerves supplied the
+  body-muscles of the prosoma.</p>
+
+  <div><span class="pagenum" id="page247">{247}</span></div>
+
+  <p>As these appendages did not carry any vital organs such as branchiæ, but were mainly locomotor
+  and masticatory in function, it follows that their disappearance as such would be much more
+  complete than that of the mesosomatic branchial appendages. Most probably, then, in the higher
+  vertebrates no trace of such appendages might be left; consequently the segmentation due to their
+  presence would be very obscure, so that in this region the very reverse of what is found in the
+  region of the vagus nerve would be the rule. There branchiomeric segmentation is especially
+  evident, owing to the persistence of the branchial part of the branchial appendages; here, owing
+  to the disappearance of the appendages, the segmentation is no longer branchiomeric, but
+  essentially mesomeric in consequence of the persistence of the somatic eye-muscles.</p>
+
+  <p class="sp3">In addition to the evidence of the appendages themselves, the number of prosomatic
+  segments is well marked out in all the members of the scorpion group by the divisions of the
+  central nervous system into well-defined neuromeres in accordance with the appendages, a
+  segmentation the reminiscence of which may still persist after the appendages themselves have
+  dwindled or disappeared. In accordance with this possibility we see that one of the most recent
+  discoveries in favour of a number of segments in the head-region of the vertebrate is the
+  discovery in the early embryo of a number of partial divisions in the brain-mass, forming a system
+  of cephalic neuromeres which may well be the rudiments of the well-defined cephalic neuromeres of
+  animals such as the scorpion.</p>
+
+  <p class="ac"><span class="sc">The Evidence of the Prosomatic Musculature.</span></p>
+
+  <p>Even if the appendages as such become obscure, yet their muscles might remain and show evidence
+  of their presence. The most persistent of all the appendage-muscles are the basal muscles which
+  pass from coxa to carapace and are known by the name of tergo-coxal muscles. They are large, well
+  marked, segmentally arranged muscles, dorso-ventral in direction, and, owing to their connecting
+  the limb with the carapace, are likely to be retained even if the appendage dwindles away.</p>
+
+  <p>The muscular system of Limulus and Scorpio has been investigated by Benham and Miss Beck under
+  Lankester's direction, and the conclusions to which Lankester comes are these&mdash;</p>
+
+  <div><span class="pagenum" id="page248">{248}</span></div>
+
+  <p>The simple musculature of the primitive animal from which both Limulus and the scorpions arose
+  consisted of&mdash;</p>
+
+  <div class="bq1 sp2">
+    <p>1. A series of paired longitudinal dorsal muscles passing from tergite to tergite of each
+    successive segment.</p>
+    <p>2. A similar series of paired longitudinal ventral muscles.</p>
+    <p>3. A pair of dorso-ventral muscles passing from tergite to sternite in each segment.</p>
+    <p>4. A set of dorso-ventral muscles moving the coxa of each limb in its socket.</p>
+    <p class="sp0">5. A pair of veno-pericardial muscles in each segment.</p>
+  </div>
+
+  <p>Of these groups of muscles, any one of which would indicate the number of segments, Groups 1
+  and 2 do not extend into the prosomatic region, and Group 5 extends only as far as the heart
+  extends in the case of both Limulus and the Scorpion group; so that we may safely conclude that in
+  the Palæostraca the evidence of somatic segmentation in the prosomatic region would be given, as
+  far as the musculature is concerned, by the dorso-ventral somatic muscles (Group 3), and of
+  segmentation due to the appendages by the dorso-ventral appendage musculature (Group 4).</p>
+
+  <p>Therefore, if, as the evidence so far indicates, the vertebrate has arisen from a palæostracan
+  stock, we should expect to find that the musculature of the somatic segments in the region of the
+  trigeminal nerve did not resemble the segmental muscles of the spinal region, was not, therefore,
+  the continuation of the longitudinal musculature of the body, but was dorso-ventral in position,
+  and that the musculature of the splanchic segments resembled that of the vagus region, where, as
+  pointed out in Chapter IV., the respiratory muscles arose from the dorso-ventral muscles of the
+  mesosomatic appendages. This is, of course, exactly what is found for the muscles which move the
+  lateral eyes of the vertebrate; these muscles, innervated by the IIIrd, IVth, and VIth nerves,
+  afford one of the main evidences of segmentation in this region, are always grouped in line with
+  the somatic muscles of spinal segments, and yet cannot be classed as longitudinal muscles. They
+  are dorso-ventral in direction, and yet belong to the somatic system; they are exactly what one
+  ought to find if they represent Group 3&mdash;the dorso-ventral body-muscles of the prosomatic
+  segments of the invertebrate ancestor.</p>
+
+  <p>The interpretation of these muscles will be given immediately; at present I want to pass in
+  review all the different kinds of evidence <span class="pagenum" id="page249">{249}</span>of
+  segmentation in this region afforded by the examination of the invertebrate, whether living or
+  fossil, so as to see what clues are left if the evidence of appendages fails us. I will take in
+  the first instance the evidence of segmentation afforded by the presence of the musculature of
+  Group 4, even when, as in the case of many fossils, no appendages have yet been found. In such
+  animals as Mygale and Phrynus the prosomatic carapace is seen to be marked out into a series of
+  elevations and depressions, and upon removing the carapace we see that these elevations correspond
+  with and are due to the large tergo-coxal muscles of the appendages; so that if such carapace
+  alone were found fossilized we could say with certainty: this animal possessed prosomatic
+  appendages the number of which can be guessed with more or less certainty by these indications of
+  segments on the carapace.</p>
+
+  <p>In those forms, then, which are only known to us in the fossil condition, in which no
+  prosomatic appendages have been found, but which possess, more or less clearly, radial markings on
+  the prosomatic carapace resembling those of Phrynus or Mygale, such radial markings may be
+  interpreted as due to the presence of prosomatic appendages, which are either entirely concealed
+  by the prosomatic carapace or dorsal head-plate, or were of such a nature as not to have been
+  capable of fossilization.</p>
+
+  <p>The group of animals in question forms the great group of animals, chiefly extinct, classified
+  by H. Woodward under the order of Merostomata. They are divided by him into the sub-order of
+  Eurypteridæ, which includes&mdash;(1) Pterygotus, (2) Slimonia, (3) Stylonurus, (4) Eurypterus,
+  (5) Adelophthalmus, (6) Bunodes, (7) Arthropleura, (8) Hemiaspis, (9) Exapinurus, (10)
+  Pseudoniscus; and the sub-order Xiphosura, which includes&mdash;(1) Belinurus, (2) Prestwichia,
+  (3) Limulus.</p>
+
+  <div><span class="pagenum" id="page250">{250}</span></div>
+
+  <div class="ac w30 fcenter sp2">
+    <a href="images/fig107.png" id="fig107"><img style="width:100%" src="images/fig107.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 107.</span>&mdash;<i>Phrynus Margine-Maculata.</i></p>
+      <p class="sp0"><i>Ce.</i>, median eyes; <i>le.</i>, lateral eyes; <i>glab.</i>, median plate
+      over brain; <i>Fo.</i>, fovea.</p>
+    </div>
+  </div>
+
+  <div class="ac w35 fcenter sp2">
+    <a href="images/fig108.png" id="fig108"><img style="width:100%" src="images/fig108.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 108.</span>&mdash;<i>Phrynus sp.</i> (?). <span class="sc">Carapace
+      removed.</span></p>
+      <p class="sp0"><i>cam.</i>, camerostome; <i>pl.</i>, plastron.</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page251">{251}</span></div>
+
+  <p>The evidence of the Xiphosura and of the Hemiaspidæ conclusively shows, in Woodward's opinion,
+  that the Merostomata are closely related to the Trilobita, and the Hemiaspidæ especially are
+  supposed to be intermediate between the trilobites and the king-crabs. They are characterized, as
+  also Belinurus and Prestwichia, by the absence of any prosomatic appendages, so that in these
+  cases, as is seen in Fig. <a href="#fig12">12</a> (p. <a href="#page30">30</a>), representing
+  <i>Bunodes lunula</i>, found in the Eurypterus layer at Rootziküll, we have an animal somewhat
+  resembling Limulus in which the prosomatic appendages have either dwindled away and are completely
+  hidden by the prosomatic carapace, or became so soft as not to be preserved in the fossilized
+  condition. The appearance of the prosomatic carapace is, to my mind, suggestive of the presence of
+  such appendages, for it is marked out radially, as is seen in the figure, in a manner resembling
+  somewhat the markings on the prosomatic carapace of Mygale or Phrynus; the latter markings, as
+  already mentioned, are due to the aponeuroses between the tergo-coxal muscles of the prosomatic
+  appendages which lie underneath and are attached to the carapace.</p>
+
+  <p>A very similar radial marking is shown by Woodward in his picture of <i>Hemiaspis
+  limuloides</i>, reproduced in Fig. 109, found in the Lower Ludlow beds at Leintwardine. This
+  species has yielded the most perfect specimens of the genus Hemiaspis, which is recognized as
+  differing from Bunodes by the possession of a telson.</p>
+
+  <p>It is striking to find that similar indications of segments have been found on the dorsal
+  surface of the head-region in many of the most ancient extinct fishes, as will be fully discussed
+  later on.</p>
+
+  <div class="ac w20 fcenter sp3">
+    <a href="images/fig109.jpg" id="fig109"><img style="width:100%" src="images/fig109.jpg" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig.109.</span>&mdash;<i>Hemiaspis limuloides.</i> (From <span
+      class="sc">Woodward</span>.)</p>
+      <p class="sp0"><i>gl.</i>, glabellum.</p>
+    </div>
+  </div>
+
+  <p class="ac"><span class="sc">The Evidence of C&#x0153;lomic Cavities.</span></p>
+
+  <p>In the head-region of the vertebrate, morphologists depend largely upon the embryonic divisions
+  of the mesoderm for the estimation of the number of segments, and, therefore, upon the number of
+  c&#x0153;lomic cavities in this region, the walls of which give origin to the striated muscles of
+  the head, so that the question of the number of segments depends very largely upon the origin of
+  the muscles from the walls of these head-cavities. It is therefore interesting to examine whether
+  a similar criterion of segmentation holds good in such a segmented <span class="pagenum"
+  id="page252">{252}</span>animal as Limulus, or in the members of the scorpion group, in which the
+  number of segments are known definitely by the presence of the appendages. In Limulus we know,
+  from the observations of Kishinouye, that a series of c&#x0153;lomic cavities are formed
+  embryologically in the various segments of the mesosoma and prosoma, in a manner exceedingly
+  similar to their mode of formation in the head-region of the vertebrate, and he has shown that in
+  the mesosoma a separate c&#x0153;lomic cavity exists for each segment, so that just as the
+  dorso-ventral somatic muscles are regularly segmentally arranged in this region, so are the
+  c&#x0153;lomic cavities, and we should be right in our estimation of the number of segments in
+  this region by the consideration of the numerical correspondence of these cavities with the
+  mesomatic appendages. Similarly, in the vertebrate, we find every reason to believe that a single,
+  separate head-cavity corresponds to each of the branchial segments in the opisthotic region, and
+  therefore we should estimate rightly the number of segments by the division of the mesoderm in
+  this region.</p>
+
+  <p>In the prosomatic region of Limulus, the dorso-ventral muscles are not arranged with such
+  absolute segmental regularity as in the mesosomatic region, and Kishinouye's observations show
+  that the c&#x0153;lomic cavities in this region do not correspond absolutely with the number of
+  prosomatic appendages. His words are<span class="wnw">:&mdash;</span></p>
+
+  <p>A pair of c&#x0153;lomic cavities appears in every segment except the segments of the 2nd, 3rd,
+  and 4th appendages, in which c&#x0153;lomic cavities do not appear at all. At least eleven pairs
+  of these cavities are produced. The eleventh pair belongs to the seventh abdominal segment.</p>
+
+  <p>The first pair of c&#x0153;lomic cavities is common to the cephalic lobe and the segment of the
+  first appendage (<i>i.e.</i> the cheliceræ).</p>
+
+  <p>The second c&#x0153;lomic cavity belongs to the segment of the fifth appendage. It is well
+  developed.</p>
+
+  <p>The ventral portion of the second c&#x0153;lomic cavity remains as the coxal gland.</p>
+
+<hr style="width:6em"/>
+
+  <p>Consequently, if we were to estimate the number of segments in this region by the number of
+  c&#x0153;lomic cavities we should not judge rightly, for we should find only four cavities and
+  seven appendages, as is seen in the following table<span class="wnw">:&mdash;</span></p>
+
+  <div><span class="pagenum" id="page253">{253}</span></div>
+
+  <table class="sp2 mc bt bb nothand" title="C&#x0153;lomic Cavities"
+  summary="C&#x0153;lomic Cavities">
+    <tr class="bb">
+      <th colspan="9" class="vmi smaller br">LIMULUS.</th>
+      <th colspan="2" class="vmi smaller">VERTEBRATE.</th>
+    </tr>
+    <tr class="bb">
+      <th colspan="3" class="vmi smaller">Segments.</th>
+      <th class="vmi smaller ba">Appendages.</th>
+      <th colspan="4" class="vmi smaller ba">Eurypterid appendages.</th>
+      <th class="vmi smaller ba">C&#x0153;lomic<br/>
+      cavities.</th>
+      <th colspan="2" class="vmi smaller">C&#x0153;lomic cavities.</th>
+    </tr>
+    <tr class="br">
+      <td rowspan="7" class="vmi br0 pr0"><img src="images/prosomatic.png" style="width:1.25em"
+      alt="Prosomatic"/></td>
+      <td rowspan="7" class="vmi pl0 pr0"><img src="images/lbrace7.png" style="height:19.0ex;
+      width:1em;" alt="brace" /></td>
+      <td class="ac">1</td>
+      <td class="al">Cheliceræ or 1st locomotor.</td>
+      <td class="br0"></td>
+      <td colspan="3" class="al">Cheliceræ</td>
+      <td class="ac">1</td>
+      <td rowspan="5" class="br0"></td>
+      <td class="al br0">Anterior</td>
+    </tr>
+    <tr class="br">
+      <td class="ac">2</td>
+      <td class="al">2nd locomotor</td>
+      <td rowspan="4" class="br0 vmi pl0 pr0"><img src="images/rbrace4.png" style="height:10.3ex;
+      width:1em;" alt="brace" /></td>
+      <td colspan="3" rowspan="4" class="al vmi">Endognaths</td>
+      <td rowspan="4" class="ac vmi">2</td>
+      <td rowspan="4" class="al br0 vmi">Premandibular</td>
+    </tr>
+    <tr class="br">
+      <td class="ac">3</td>
+      <td class="al">3rd <span class="hid">loco</span>"</td>
+    </tr>
+    <tr class="br">
+      <td class="ac">4</td>
+      <td class="al">4th <span class="hid">loco</span>"</td>
+    </tr>
+    <tr class="br">
+      <td class="ac">5</td>
+      <td class="al">5th <span class="hid">loco</span>"</td>
+    </tr>
+    <tr class="br">
+      <td class="ac">6</td>
+      <td class="al">6th <span class="hid">loco</span>"</td>
+      <td rowspan="2" class="br0"></td>
+      <td colspan="3" class="al">Ectognath</td>
+      <td class="ac">3</td>
+      <td rowspan="2" class="br0 vmi pl0 pr0"><img src="images/rbrace2.png" style="height:4.5ex;
+      width:1em;" alt="brace" /></td>
+      <td rowspan="2" class="al br0 vmi">Mandibular</td>
+    </tr>
+    <tr class="br">
+      <td class="ac">7</td>
+      <td class="al">Chilaria</td>
+      <td colspan="3" class="al">Metastoma</td>
+      <td class="ac">4</td>
+    </tr>
+    <tr class="br bt">
+      <td rowspan="7" class="vmi br0 pr0"><img src="images/mesosomatic.png" style="width:1.25em"
+      alt="Mesosomatic"/></td>
+      <td rowspan="7" class="vmi pl0 pr0"><img src="images/lbrace7.png" style="height:19.0ex;
+      width:1em;" alt="brace" /></td>
+      <td class="ac">8</td>
+      <td class="al">Operculum</td>
+      <td rowspan="2" class="br0 vmi pl0 pr0"><img src="images/rbrace2.png" style="height:4.5ex;
+      width:1em;" alt="brace" /></td>
+      <td rowspan="2" class="al vmi br0">Operculum</td>
+      <td rowspan="2" class="vmi pl0 pr0 br0"><img src="images/lbrace2.png" style="height:4.5ex;
+      width:1em;" alt="brace" /></td>
+      <td class="al">Genital</td>
+      <td class="ac">5</td>
+      <td rowspan="2" class="br0 vmi pl0 pr0"><img src="images/rbrace2.png" style="height:4.5ex;
+      width:1em;" alt="brace" /></td>
+      <td rowspan="2" class="al br0 vmi">Hyoid</td>
+    </tr>
+    <tr class="br">
+      <td class="ac">9</td>
+      <td class="al">1st branchial</td>
+      <td class="al">1st&nbsp;branchial</td>
+      <td class="ac">6</td>
+    </tr>
+    <tr class="br">
+      <td class="ac">10<span class="hid">0</span></td>
+      <td class="al">2nd <span class="hid">brat</span>"</td>
+      <td rowspan="5" class="br0"></td>
+      <td colspan="3" class="al">2nd branchial</td>
+      <td class="ac">7</td>
+      <td rowspan="5" class="br0"></td>
+      <td class="al br0">1st branchial</td>
+    </tr>
+    <tr class="br">
+      <td class="ac">11<span class="hid">0</span></td>
+      <td class="al">3rd <span class="hid">bran</span>"</td>
+      <td colspan="3" class="al">3rd <span class="hid">bran</span>"</td>
+      <td class="ac">8</td>
+      <td class="al br0">2nd <span class="hid">brat</span>"</td>
+    </tr>
+    <tr class="br">
+      <td class="ac">12<span class="hid">0</span></td>
+      <td class="al">4th <span class="hid">bran</span>"</td>
+      <td colspan="3" class="al">4th <span class="hid">bran</span>"</td>
+      <td class="ac">9</td>
+      <td class="al br0">3rd <span class="hid">bran</span>"</td>
+    </tr>
+    <tr class="br">
+      <td class="ac">13<span class="hid">0</span></td>
+      <td class="al">5th <span class="hid">bran</span>"</td>
+      <td colspan="3" class="al">5th <span class="hid">bran</span>"</td>
+      <td class="ac">10<span class="hid">0</span></td>
+      <td class="al br0">4th <span class="hid">bran</span>"</td>
+    </tr>
+    <tr class="br">
+      <td class="ac">14<span class="hid">0</span></td>
+      <td class="al">6th <span class="hid">bran</span>"</td>
+      <td colspan="3"></td>
+      <td class="ac">11<span class="hid">0</span></td>
+      <td class="br0"></td>
+    </tr>
+  </table>
+
+<!-- Trimmed down version for handhelds -->
+
+  <table class="sp2 w100 bt bb handonly" title="C&#x0153;lomic Cavities"
+  summary="C&#x0153;lomic Cavities">
+    <tr class="bb">
+      <th colspan="5" class="vmi smaller br">LIMULUS.</th>
+      <th class="vmi smaller">VERTEBRATE.</th>
+    </tr>
+    <tr class="bb">
+      <th colspan="2" class="vmi smaller plhs prhs">Segments.</th>
+      <th class="vmi smaller ba">Appendages.</th>
+      <th class="vmi smaller ba">Eurypterid<br/>
+      appendages.</th>
+      <th class="vmi smaller ba">C.c.</th>
+      <th class="vmi smaller">C&#x0153;lomic<br/>
+      cavities.</th>
+    </tr>
+    <tr class="br">
+      <td rowspan="7" class="vmi br0 pr0 pl0"><img src="images/prosomatic.png" style="width:1.25em"
+      alt="Prosomatic"/></td>
+      <td class="ar">1</td>
+      <td class="al">Cheliceræ or 1st locomotor.</td>
+      <td class="al">Cheliceræ</td>
+      <td class="ar">1</td>
+      <td class="al br0">Anterior</td>
+    </tr>
+    <tr class="br">
+      <td class="ar">2</td>
+      <td class="al">2nd locomotor</td>
+      <td rowspan="4" class="al vmi br0" style="padding:0">
+        <table class="w100" title="C&#x0153;lomic Cavities" summary="C&#x0153;lomic Cavities">
+          <tr>
+            <td class="pl0 pr0"><img src="images/rbrace5.png" style="height:12.0ex; width:1em;"
+            alt="brace" /></td>
+            <td class="pl0 vmi br">Endognaths</td>
+          </tr>
+        </table>
+      </td>
+      <td rowspan="4" class="ar vmi">2</td>
+      <td rowspan="4" class="al br0 vmi">Premandibular</td>
+    </tr>
+    <tr class="br">
+      <td class="ar">3</td>
+      <td class="al">3rd <span class="hid">loco</span>"</td>
+    </tr>
+    <tr class="br">
+      <td class="ar">4</td>
+      <td class="al">4th <span class="hid">loco</span>"</td>
+    </tr>
+    <tr class="br">
+      <td class="ar">5</td>
+      <td class="al">5th <span class="hid">loco</span>"</td>
+    </tr>
+    <tr class="br">
+      <td class="ar">6</td>
+      <td class="al">6th <span class="hid">loco</span>"</td>
+      <td class="al">Ectognath</td>
+      <td class="ar">3</td>
+      <td rowspan="2" class="al br0 vmi">Mandibular</td>
+    </tr>
+    <tr class="br">
+      <td class="ar">7</td>
+      <td class="al">Chilaria</td>
+      <td class="al">Metastoma</td>
+      <td class="ar">4</td>
+    </tr>
+    <tr class="br bt">
+      <td rowspan="7" class="vmi br0 pr0 pl0"><img src="images/mesosomatic.png" style="width:1.25em"
+      alt="Mesosomatic"/></td>
+      <td class="ar">8</td>
+      <td class="al">Operculum</td>
+      <td rowspan="2" class="al vmi br0" style="padding:0">
+        <table class="w100" title="C&#x0153;lomic Cavities" summary="C&#x0153;lomic Cavities">
+          <tr>
+            <td class="vmi prhs" rowspan="2">Opc.</td>
+            <td class="br plhs">Genital</td>
+          </tr>
+          <tr>
+            <td class="br plhs">1st<br/>
+            branchial</td>
+          </tr>
+        </table>
+      </td>
+      <td class="ar">5</td>
+      <td rowspan="2" class="al br0 vmi">Hyoid</td>
+    </tr>
+    <tr class="br">
+      <td class="ar vmi">9</td>
+      <td class="al">1st branchial</td>
+      <td class="ar vmi">6</td>
+    </tr>
+    <tr class="br">
+      <td class="ar">10</td>
+      <td class="al">2nd <span class="hid">brat</span>"</td>
+      <td class="al">2nd branchial</td>
+      <td class="ar">7</td>
+      <td class="al br0">1st branchial</td>
+    </tr>
+    <tr class="br">
+      <td class="ar">11</td>
+      <td class="al">3rd <span class="hid">bran</span>"</td>
+      <td class="al">3rd <span class="hid">bran</span>"</td>
+      <td class="ar">8</td>
+      <td class="al br0">2nd <span class="hid">brat</span>"</td>
+    </tr>
+    <tr class="br">
+      <td class="ar">12</td>
+      <td class="al">4th <span class="hid">bran</span>"</td>
+      <td class="al">4th <span class="hid">bran</span>"</td>
+      <td class="ar">9</td>
+      <td class="al br0">3rd <span class="hid">bran</span>"</td>
+    </tr>
+    <tr class="br">
+      <td class="ar">13</td>
+      <td class="al">5th <span class="hid">bran</span>"</td>
+      <td class="al">5th <span class="hid">bran</span>"</td>
+      <td class="ar">10</td>
+      <td class="al br0">4th <span class="hid">bran</span>"</td>
+    </tr>
+    <tr class="br">
+      <td class="ar">14</td>
+      <td class="al">6th <span class="hid">bran</span>"</td>
+      <td></td>
+      <td class="ar">11</td>
+      <td class="br0"></td>
+    </tr>
+  </table>
+
+<!-- End of trimmed down version for handhelds -->
+
+  <p>The second cavity would in reality represent four segments belonging to the 2nd, 3rd, 4th, 5th
+  locomotor appendages, <i>i.e.</i> the very four segments which in the Eurypteridæ are concentrated
+  together to form the endognaths, and we should be justified in putting this interpretation on it,
+  because, according to Kishinouye, its ventral portion forms the coxal gland, and, according to
+  Lankester, the coxal gland sends prolongations into the coxa of the 2nd, 3rd, 4th, 5th locomotor
+  appendages. Similarly in the vertebrate, we find three head-cavities in the region which
+  corresponds, on my theory, to the prosomatic region of Limulus, (1) the anterior cavity discovered
+  by Miss Platt, (2) the premandibular cavity, and (3) the mandibular cavity, which, if they
+  corresponded with the prosomatic c&#x0153;lomic cavities of Limulus, would represent not three
+  segments but seven segments, as follows:&mdash;the anterior cavity would correspond to the first
+  c&#x0153;lomic cavity, <i>i.e.</i> the cavity of the cheliceral segments in both Limulus and the
+  Eurypteridæ; the premandibular, to the second c&#x0153;lomic cavity, representing, therefore, the
+  2nd, 3rd, 4th, 5th prosomatic segments in Limulus and the endognathal segments in the Eurypteridæ;
+  and the mandibular to the 3rd and 4th c&#x0153;lomic cavities, representing the last locomotor and
+  chilarial segments in Limulus, <i>i.e.</i> the ectognathal and metastomal segments in the
+  Eurypteridæ.</p>
+
+  <div><span class="pagenum" id="page254">{254}</span></div>
+
+  <p>It is worthy of note that, in respect to their c&#x0153;lomic cavities, as in the position and
+  origin of their nerves in the central nervous system, the first pair of appendages, the cheliceræ,
+  retain a unique position, differing from the rest of the prosomatic appendages.</p>
+
+  <p class="sp3">In the table I have shown how the vertebrate c&#x0153;lomic cavities may be
+  compared with those of Limulus. The next question to consider is the evidence obtained by
+  morphologists and anatomists as to the number of segments supplied by the trigeminal nerve-group;
+  this question will be considered in the next chapter.</p>
+
+  <p class="ac"><span class="sc">Summary.</span></p>
+
+  <div class="bq1 smaller sp5">
+    <p>In Chapters IV. and V. I have dealt with the opisthotic segments of the vertebrate, including
+    therein the segments supplied by the facial nerve, and shown that they correspond to the
+    mesosomatic segments of the palæostracan; consequently the facial (VII.), glossopharyngeal
+    (IX.), and vagus (X.) nerves originally supplied the branchial and opercular appendages.</p>
+    <p>In this chapter the consideration of the pro-otic segments is commenced, that is, the
+    segments supplied by the trigeminal (V.) and the eye-muscle nerves (III., IV., VI.). I have
+    considered the VIth nerve with the rest of the eye-muscle nerves for convenience' sake, though
+    in reality it belongs to the same segment as the facial. Of these, that part of the trigeminal
+    which innervates the muscles of mastication corresponds to the splanchnic segments, while the
+    eye-muscle nerves belong to the corresponding somatic segments; but the pro-otic segments of the
+    vertebrate ought to correspond to the prosomatic segments of the invertebrate, just as the
+    opisthotic correspond to the mesosomatic. Therefore the motor part of the trigeminal ought to
+    supply muscles which originally moved the prosomatic appendages, while the eye-muscles ought to
+    have belonged to the somatic part of the same segments.</p>
+    <p>The first question considered is the number of segments which ought to be found in this
+    region. In Limulus, the Eurypteridæ, and the scorpions there are seven prosomatic segments which
+    carry (1) the cheliceræ, (2, 3, 4, 5) the four first locomotor appendages&mdash;the endognaths,
+    (6) the large special appendage&mdash;the ectognath&mdash;and (7) the appendages, which in
+    Limulus are known as the chilaria, and are small and insignificant, but in Eurypterus and other
+    forms grow forwards, fuse together, and form a single median lip to an accessory oral chamber,
+    which lip is known as the metastoma. Of these appendages the cheliceræ and endognaths tend to
+    dwindle away and become mere tentacles, while the large swimming ectognath and metastoma remain
+    strong and vigorous.</p>
+    <p>In this, the prosomatic region, the somatic segmentation is not characterized by the presence
+    of the longitudinal muscle segments, for they do not extend into this head-region, but only by
+    the presence of the segmental somatic <span class="pagenum"
+    id="page255">{255}</span>ventro-dorsal muscles. Among the muscles of the appendages the system
+    of large tergo-coxal muscles is especially apparent.</p>
+    <p>From these considerations it follows that the number of segments in this region in the
+    vertebrate ought to be seven; that the musculature supplied by the trigeminal nerve ought to
+    represent seven ventral or splanchnic segments, of which only the last two are likely to be
+    conspicuous; and that the musculature supplied by the eye-muscle nerves ought to be
+    dorso-ventral in direction, which it is, and represent seven dorsal or somatic segments.</p>
+    <p>A further peculiarity of this region, both in Limulus and the scorpions, is found in the
+    excretory organs which are known by the name of coxal glands, because they extend into the basal
+    joint, or coxa, of certain of the prosomatic limbs. The appendages so characterized are always
+    the four endognaths, and it follows that if these four endognaths lose their locomotor power,
+    become reduced in size, and concentrated together to form mere tentacles, then of necessity the
+    coxal glands will be concentrated together, and tend to form a glandular mass in the region of
+    the mouth; in fact, take up a position corresponding to that of the pituitary body in
+    vertebrates.</p>
+    <p>Taking all these facts into consideration, it is possible to construct a drawing of a
+    sagittal section through the head-region of Eurypterus, which will represent, with considerable
+    probability, the arrangement of parts in that animal. This can be compared with the
+    corresponding section through the head of Ammoc&#x0153;tes.</p>
+    <p>Now, as pointed out in the last chapter, the early stage of Ammoc&#x0153;tes is remarkably
+    different from the more advanced stage; at that time the septum between the oral and respiratory
+    chambers has not yet broken through, and the olfactory or nasal tube, known at this stage as the
+    tube of the hypophysis, is directed ventrally, not dorsally.</p>
+    <p>The comparison of the diagram of Eurypterus with that of the early stage of Ammoc&#x0153;tes
+    is remarkably close, and immediately suggests not only that the single nose of the former is
+    derived from the corresponding organ in the palæostracan, but that the pituitary body is derived
+    from the concentrated coxal glands, and the lower lip from the metastoma. The further working
+    out of these homologies will be discussed in the next chapter.</p>
+    <p>In addition to the evidence of segmentation afforded by the appendages, there are in this
+    region, in Limulus and the scorpion group, three other criteria of segmentation available to us,
+    if from any cause the evidence of appendages fails us. These are&mdash;</p>
+    <p>1. The number of neuromeres are marked out in this region of the brain more or less plainly,
+    especially in the young animal, just as they are also in the embryo of the vertebrate.</p>
+    <p>2. The segmentation is represented here, just as in the mesosomatic region, by two sets of
+    muscle-segments; the one <i>somatic</i>, consisting of the segmentally arranged dorso-ventral
+    muscles, the continuation of the group already discussed in connection with the mesosomatic
+    segmentation, and the other <i>appendicular</i> characterized by the tergo-coxal muscles. These
+    latter segmental muscles are especially valuable, for in such forms as Mygale, Phrynus, etc.,
+    their presence is indicated externally by markings on the prosomatic carapace, and thus
+    corresponding markings found on fossil carapaces or on dorsal head-shields can be <span
+    class="pagenum" id="page256">{256}</span>interpreted. These two sets of muscle-segments
+    correspond in the vertebrate to the somatic and splanchnic segmentations.</p>
+    <p class="sp0">3. In the vertebrate the segmentation in this region is indicated by the
+    c&#x0153;lomic or head-cavities, which are cavities formed in the mesoderm of the embryo, the
+    walls of which give origin to the striated muscles of the head. In Limulus corresponding
+    c&#x0153;lomic cavities are found, which are directly comparable with those found in the
+    vertebrate.</p>
+  </div>
+
+  <div><span class="pagenum" id="page257">{257}</span></div>
+
+  <p class="ac">CHAPTER VIII</p>
+
+  <p class="ac"><i>THE SEGMENTS BELONGING TO THE TRIGEMINAL NERVE-GROUP</i></p>
+
+  <div class="bq1 it1 smaller sp3">
+    <p class="sp0">The prosomatic segments of the vertebrate.&mdash;Number of segments belonging to
+    the trigeminal nerve-group.&mdash;History of cranial segments.&mdash;Eye-muscles and their
+    nerves.&mdash;Comparison with the dorso-ventral somatic muscles of the
+    scorpion.&mdash;Explanation of the oculomotor nerve and its group of muscles.&mdash;Explanation
+    of the trochlearis nerve and its dorsal crossing.&mdash;Explanation of the abducens
+    nerve.&mdash;Number of segments supplied by the trigeminal nerves.&mdash;Evidence of their motor
+    nuclei.&mdash;Evidence of their sensory ganglia.&mdash;Summary.</p>
+  </div>
+
+  <p>From the evidence given in the last chapter, combined with that given in Chapter IV., the
+  probability of the theory that the trigeminal group of nerves of the vertebrate have been derived
+  from the prosomatic group of nerves of the invertebrate can be put to the test by the answers to
+  the following morphological and anatomical questions<span class="wnw">:&mdash;</span></p>
+
+  <p>1. Do we find in the vertebrate two segmentations in this region corresponding to the two
+  segmentations in the branchial region, <i>i.e.</i> a somatic or dorsal series of segments, and a
+  splanchnic or ventral series of segments? The latter would not be branchial, but rather of the
+  nature of free tactile appendages; so that it is useless to look for or talk about gill-slits,
+  although such appendages, being serially homologous with the branchial mesosomatic appendages,
+  would readily give rise to the conception of branchial segments.</p>
+
+  <p>2. Is there morphological evidence that the trigeminal nerve is not the nerve belonging to a
+  single segment, or even to two segments, but is really a concentration of at least six, probably
+  seven, segmental nerves?</p>
+
+  <p>3. Is there morphological evidence that the oculomotor and trochlear nerves, which on all sides
+  are regarded as belonging to the trigeminal segments, are not single nerves corresponding each
+  <span class="pagenum" id="page258">{258}</span>to a single segment, but are the somatic motor
+  roots belonging to the same segments as those to which the trigeminal supplies the splanchnic
+  roots?</p>
+
+  <p>4. Do the mesoderm segments, which give origin to the eye-muscles, and therefore do the
+  head-cavities of this region, correspond with the trigeminal segments? Considering the
+  concentration of parts in this region and the difficulty already presented by the want of
+  numerical agreement between the prosomatic appendages and the prosomatic c&#x0153;lomic cavities
+  in Limulus, it may very probably be difficult to determine the actual number of the mesoderm
+  segments.</p>
+
+  <p>5. Is there anatomical evidence that the ganglion of origin of the motor part of the trigeminal
+  nerve is not a single ganglion, but a representative of many, probably seven?</p>
+
+  <p>6. Is there anatomical evidence that the ganglia of origin of the oculomotor and trochlear
+  nerves represent many ganglia?</p>
+
+  <p>7. Is there any evidence that the organs originally supplied by the motor part of the
+  trigeminal nerve are directly comparable with prosomatic appendages?</p>
+
+  <p class="sp3">It is agreed on all sides that in this region of the head there is distinct
+  evidence of double segmentation, the dorsal mesoderm segments giving origin to the eye-muscles,
+  and the ventral segments to the musculature innervated by the trigeminal nerve. Originally,
+  according to the scheme of van Wijhe, two segments only were recognized, the dorsal parts of which
+  were innervated by the IIIrd and IVth nerves respectively. Since his paper, the tendency has been
+  to increase the number of segments in this region, as is seen in the following sketch, taken from
+  Rabl, of the history of cranial segmentation.</p>
+
+  <p class="ac"><span class="sc">History of Cranial Segmentation.</span></p>
+
+  <p>The first attempt to deal with this question was made by Goethe and Oken. They considered that
+  the cranial skeleton was composed of a series of vertebræ, but as early as 1842 Vogt pointed out
+  that only the occipital segments could be reduced to vertebræ. In 1869, Huxley showed that
+  vertebræ were insufficient to explain the cranial segmentation, and that the nerves must be
+  specially considered. The olfactory and optic nerves he regarded as parts of the brain, not true
+  segmental nerves; the rest of the cranial nerves <span class="pagenum"
+  id="page259">{259}</span>were segmental, with special reference to branchial arches and clefts,
+  the facial, glossopharyngeal, and separate vagus branches supplying the walls of the various
+  branchial pouches. In a similar manner, the supra- and infra-maxillary branches of the trigeminal
+  were arranged on each side of the mouth, and the inner and outer twigs of the first (ophthalmic)
+  branch of the trigeminal on each side of the orbito-nasal cleft, the trabecular and the
+  supra-maxillary arches being those on each side of this cleft. Thus Huxley considered that there
+  was evidence of a series of pairs of ventral arches belonging to the skull, viz. the trabecular
+  and maxillary in front of the mouth, the mandibular, hyoid, and branchial arches behind, and that
+  the Vth, VIIth, IXth, and Xth nerves were segmental in relation to these arches and clefts.
+  Gegenbaur, in 1871 and 1872, considered that the branchial arches represented the lower arches of
+  cranial vertebræ, and therefore corresponded to lower arches in the spinal region, <i>i.e.</i> the
+  skull was composed of as many vertebræ as there are branchial arches. These vertebræ were confined
+  to the notochordal part of the skull, the prechordal part having arisen secondarily from the
+  vertebral part, while the number of vertebræ are at least nine, possibly more. The nerves which
+  could be homologized with spinal nerves were, he thought, divisible into two great
+  groups&mdash;(1) the trigeminal group, which included the eye-muscle nerves, the facial, and its
+  dorsal branch, the auditory; (2) the vagus group, which included the glossopharyngeal and
+  vagus.</p>
+
+  <p>Such was the outcome of the purely comparative anatomical work of Huxley and
+  Gegenbaur&mdash;work that has profoundly influenced all the views of segmentation up to the
+  present day.</p>
+
+  <p>Now came the investigations of the embryologists, of whom I will take, in the first instance,
+  Balfour, whose observations on the embryology of the Selachians led him to the conclusion that
+  besides the evidence of segmentation to be found in the cranial nerves and in the branchial
+  clefts, further evidence was afforded by the existence of head-cavities, the walls of which formed
+  muscles just as they do in the spinal region. He came to the conclusion that the first head-cavity
+  belonged to one or more pre-oral segments, of which the nerves were the oculomotor, trochlearis,
+  and possibly abducens; while there were seven post-oral segments, each with its head-cavity and
+  its visceral arch, of which the trigeminal, facial, glossopharyngeal, and the four parts of the
+  vagus were the respective nerves.</p>
+
+  <div><span class="pagenum" id="page260">{260}</span></div>
+
+  <p>Marshall, in 1882, considered that the cranial segments were all originally respiratory, and
+  that all the segmental nerves are arranged uniformly with respect to a series of gill-clefts which
+  have become modified anteriorly and have been lost, to a certain extent, posteriorly. He included
+  the olfactory nerves among the segmental nerves, and looked upon the olfactory pit, the
+  orbito-nasal lacrymal duct, the mouth, and the spiracle as all modified gill-slits, so that he
+  reckoned three pre-oral and oral segments belonging to the Ist, IIIrd, IVth, and Vth nerves, and
+  eight post-oral segments belonging respectively to the VIIth and VIth nerves, and to the IXth
+  nerve, and six segments belonging to the Xth nerve. He pointed out that muscles supplied by the
+  oculomotor nerve develop from the outer wall of the first head-cavity; not, however, the
+  <i>obliquus superior</i> and <i>rectus externus</i>, the latter originating probably from the
+  walls of the third cavity.</p>
+
+  <p>In the same year, 1882, came van Wijhe's well-known paper, in which he showed that the mesoderm
+  of the head in the selachian divided into two sets of segments, dorsal and ventral; that the
+  dorsal segments were continuous with the body-somites, and that the ventral segments formed the
+  lateral plates of mesoblast between each of the visceral and branchial pouches. He concluded that
+  the dorsal somites were originally nine in number, that each was supplied with a ventral
+  nerve-root, in the same way as the somites in the trunk, and that to each one a visceral pouch
+  corresponded, whose walls were supplied by the corresponding dorsal nerve-root; of these nine
+  segments, the ventral nerve-roots of the first three segments were respectively the oculomotor,
+  trochlearis, and abducens nerves. The next three segments possessed no definable ventral root or
+  muscles, and the seventh, eighth, and ninth segments possessed as ventral roots the hypoglossal
+  nerve, with its muscular supply. The corresponding dorsal nerve-roots were the trigeminal, facial,
+  auditory, glossopharyngeal and vagus nerves, the difference between cranial and spinal dorsal
+  roots being that the former contain motor fibres.</p>
+
+  <p>Ahlborn, in 1884, drew a sharp distinction between the segments of the mesoderm and those of
+  the endoderm. The former segmentation he called mesomeric, the latter branchiomeric. He considered
+  the two segmentations to be independent, and concluded that the branchiomeric was secondary to the
+  mesomeric, and therefore not of <span class="pagenum" id="page261">{261}</span>segmental value. As
+  to the segments of the mesoderm in the head, the three hindmost or occipital in Petromyzontidæ
+  remain permanently, and correspond to the three last segments in the selachian head. Of the
+  anterior mesoderm segments, he considered that there were originally six, and that there are six
+  typical eye-muscles in all Craniota, which have been compressed into three segments, as in
+  Selachia.</p>
+
+  <p>Froriep (1885) showed in sheep-embryos and in chicks that the hypoglossal nerve belongs to
+  three proto-vertebræ posterior to the vagus region, which were true spinal segments. He therefore
+  modified Gegenbaur's conceptions to this extent: that portion of the skull designated by Gegenbaur
+  as vertebral must be divided into two parts&mdash;a hind or occipital region, which is clearly
+  composed of modified vertebræ and is the region of the hypoglossal nerves, and a front region,
+  extending from the oculomotor to the accessorius nerves, which is characterized segmentally by the
+  formation of branchial arches, but in which there is no evidence that proto-vertebræ were ever
+  formed. He therefore divides the head-skeleton into three parts&mdash;</p>
+
+  <p>1. Gegenbaur's evertebral part&mdash;the region of the olfactory and optic nerves&mdash;which
+  cannot be referred to any metameric segmentation.</p>
+
+  <p>2. The pseudo-vertebral, pre-spinal, or branchial part, clearly shown to be segmented from the
+  consideration of the nerves and branchial arches, but not referable to proto-vertebræ&mdash;the
+  region of the trigeminal and vagus nerves.</p>
+
+  <p>3. The vertebral spinal part&mdash;the region of the hypoglossal nerves.</p>
+
+  <p>He further showed that the ganglia of the specially branchial nerves, the facial,
+  glossopharyngeal, and vagus, are at one stage in connection with the epidermis, so that these
+  parts of the epidermis represent sense-organs which do not develop; these organs probably belonged
+  to the lateral line system. As the connection takes place at the dorsal edge of the gill-slits,
+  they may also be called rudimentary branchial sense-organs.</p>
+
+  <p>Since this paper of Froriep's, it has been generally recognized, and Gegenbaur has accepted
+  Froriep's view, that the three hindmost metameres, which distinctly show the characteristics of
+  vertebræ, belong to the spinal and not to the cranial region, so that the metameric segmentation
+  of the cranial region proper has become <span class="pagenum" id="page262">{262}</span>more and
+  more associated with the branchial segmentation. Froriep's discovery of the rudimentary branchial
+  sense-organs as a factor in the segmentation question has led Beard to the conclusion that the
+  olfactory and auditory organs represent in a permanent form two of these rudimentary branchial
+  sense-organs. He therefore includes both the olfactory and auditory nerves in his list of cranial
+  segmental nerves, and makes eleven cranial branchial segments in front of the spinal segments
+  represented by the hypoglossal.</p>
+
+  <p>A still larger number of cranial segments is supposed to exist, according to the researches of
+  Dohrn and Killian, in the embryos of <i>Torpedo ocellata</i>. The former, holding to the view that
+  vertebrates arose from annelids, considered that the head was formed of a series of metameres, to
+  each one of which a mesoderm-segment, a gill-arch, a gill-cleft, a segmental nerve and vessel
+  belonged. He found in the front head-region of a Torpedo embryo, corresponding to van Wijhe's
+  first four somites, no less than twelve to fifteen mesoderm segments, and concluded, therefore,
+  that the eye-muscle nerves, especially the oculomotor, represented many segmental nerves, and were
+  not the nerves of single segments; so, also, that the inferior maxillary part of the trigeminal
+  and the hyoid nerve of the facial are probably not single nerves, but a fusion of several. Killian
+  comes to much the same conclusion as Dohrn, for he finds seventeen to eighteen separate mesoderm
+  segments in the head, of which twelve belong to the trigeminal and facial region.</p>
+
+  <p>Since Rabl's paper, a number of papers have appeared, especially from America, dealing with yet
+  another criterion of the original segmentation of the head, viz. a series of divisions of the
+  central nervous system itself, which are seen at a very early stage of development, and are called
+  neuromeres; the divisions in the cranial region being known as encephalomeres, and those of the
+  spinal region as myomeres. Locy's paper has especially brought these divisions into prominence as
+  a factor in the question of segmentation. They are essentially segments of the epiblast and not of
+  the mesoblast; they are conspicuous in very early stages, and appear to be in relation with the
+  cranial nerves, according to Locy. He recognizes in <i>Squalus acanthias</i>, in front of the
+  spino-occipital region, fourteen pairs of such encephalomeres and a median unsegmented
+  termination, which may represent one more pair fused in the middle line, making at least fifteen.
+  He distributes these fifteen segments as follows: <span class="pagenum"
+  id="page263">{263}</span>fore-brain three and unsegmented termination, mid-brain two, and
+  hind-brain nine.</p>
+
+  <p>Again, Kupffer, in his recent papers on the embryology of Ammoc&#x0153;tes, asserts that
+  especial information as to the number of primitive segments is afforded by the appearance in the
+  early stages of a series of epibranchial ganglia in connection with the cranial nerves, which
+  remain permanently in the case of the vagus nerves, but disappear in the case of pro-otic nerves.
+  He considers that the evidence points to the number of segments in the mid- and hind-brain region
+  as being primitively fifteen, viz. six segments belonging to the trigeminal and abducens group,
+  three segments belonging respectively to the facial, auditory, and glossopharyngeal, and six to
+  the vagus.</p>
+
+  <p class="sp3">From this sketch we see that the modern tendency is to make six segments at least
+  out of the region of the trigeminal nerves rather than two. In this region, as already mentioned,
+  the evidence of segmentation is based more clearly on the somatic than on the splanchnic segments.
+  We ought, therefore, in the first place, to consider the teaching of the eye-muscles and their
+  nerves and the c&#x0153;lomic cavities in connection with them, and see whether the hypothesis
+  that such muscles represent the original dorso-ventral somatic muscles of the palæostracan
+  ancestor is in harmony with and explains the facts of modern research.</p>
+
+  <p class="ac"><span class="sc">Eye-Muscles and their Nerves.</span></p>
+
+  <p>The only universally recognized somatic nerves belonging to these segments which exist in the
+  adult are the nerves to the eye-muscles, of which, according to van Wijhe, the oculomotor is the
+  nerve of the 1st segment, the trochlearis of the 2nd, and the abducens of the 3rd; while the
+  nerves and muscles belonging to the 4th and 5th segments, <i>i.e.</i> the 2nd facial and
+  glossopharyngeal segments respectively, show only the merest rudiments, and do not exist in the
+  adult. One significant fact appears in this statement of van Wijhe, and is accepted by all those
+  who follow him, viz. that the oculomotor nerve has equal segmental value with the trochlearis and
+  the abducens, although it supplies a number of muscles, each of which, on the face of it, has the
+  same anatomical value as the superior oblique or external rectus. Dohrn alone, as far as I know,
+  as already pointed out, insists upon the multiple character of the oculomotor nerve.</p>
+
+  <div><span class="pagenum" id="page264">{264}</span></div>
+
+  <p>As far as the anatomist is concerned, the evidence is becoming clearer and clearer that the
+  nucleus of the IIIrd nerve is a composite ganglion composed of a number of nuclei, each similar to
+  that of the trochlearis, so that if the trochlearis nucleus is a segmental motor nucleus, then the
+  oculomotor nucleus is a combined nucleus belonging to at least four segmental nerves, each of
+  which has the same value as that of the trochlearis.</p>
+
+  <p>The investigations of a number of anatomists, among whom may be mentioned Gudden, Obersteiner,
+  Edinger, Kölliker, Gehuchten, all lead directly to the conclusion that this oculomotor nucleus is
+  composed of a number of separate nuclei, of which the most anterior as also the Edinger-Westphal
+  nucleus contains small cells, while the others contain large cells. Thus Edinger divides the
+  origin of the oculomotor nerve into a small-celled anterior part and a larger posterior part, of
+  which the cells are larger and distinctly arranged in three groups&mdash;(1) dorsal, (2) ventral,
+  and (3) median. Between the anterior and posterior groups lies the Edinger-Westphal nucleus, which
+  is small-celled; naturally, the large-celled group is that which gives origin to the motor nerves
+  of the eye-muscles, the small-celled being possibly concerned with the motor nerves of the
+  pupillary and ciliary muscles. I may mention that Kölliker considers that the anterior lateral
+  nucleus has nothing to do with the oculomotor nerve, but is a group of cells in which the fibres
+  of the posterior longitudinal bundle and of the deep part of the posterior commissure
+  terminate.</p>
+
+  <p>These conclusions of Edinger are the outcome of work done in his laboratory by Perlia, who says
+  that in new-born animals the nucleus of origin of the oculomotor nerve is made up of a number of
+  groups quite distinct from each other, each group being of the same character as that of the
+  trochlearis. He finds the same arrangement in various mammals and birds. Further, he finds that
+  some of the fibres arise from the nucleus of the opposite side, thus crossing, as in the
+  trochlearis; these crossing fibres belong to the most posterior of the dorsal group of nuclei,
+  <i>i.e.</i> to the nerve to the inferior oblique muscle.</p>
+
+  <p>The evidence, therefore, points to the conclusion that the oculomotor nucleus is a multiple
+  nucleus, each part of which gives origin to one of the nerves of one of the eye-muscles.</p>
+
+  <p>Edinger says that such an array of clinical observations exists, <span class="pagenum"
+  id="page265">{265}</span>and of facts derived from post-mortem dissections, that one may venture
+  to designate the portion of the nucleus from which the innervation of each individual ocular
+  muscle comes. He gives Starr's table, the latest of these numerous attempts, begun by Pick.
+  According to Starr, the nuclei of the nerves to the individual muscles are arranged from before
+  backward, thus&mdash;</p>
+
+  <table class="sp2 mc" title="Order of nuclei according to Starr"
+  summary="Order of nuclei according to Starr">
+    <tr>
+      <td><i>m. sphincter iridis.</i></td>
+      <td><i>m. ciliaris.</i></td>
+    </tr>
+    <tr>
+      <td><i>m. levator palpebræ.</i></td>
+      <td><i>m. rectus internus.</i></td>
+    </tr>
+    <tr>
+      <td><i>m. rectus superior.</i></td>
+      <td><i>m. rectus inferior.</i></td>
+    </tr>
+    <tr>
+      <td class="pr2"><i>m. obliquus inferior.</i></td>
+    </tr>
+  </table>
+
+  <p>Further, the evidence of the well-known physiological experiments of Hensen and Völckers that
+  the terminal branches of the oculomotor nerve arise from a series of segments of the nucleus,
+  arranged more or less one behind the other in a longitudinal row, leads them to the conclusion
+  that the nuclei of origin are arranged as follows, proceeding from head to tail<span
+  class="wnw">:&mdash;</span></p>
+
+  <table class="sp2 mc" title="Order of nuclei according to Hensen and
+  Völckers" summary="Order of nuclei according to Hensen and
+  Völckers">
+    <tr>
+      <td rowspan="6">Nearest brain.</td>
+      <td>1.</td>
+      <td><i>m. ciliaris.</i></td>
+    </tr>
+    <tr>
+      <td>2.</td>
+      <td><i>m. sphincter iridis.</i></td>
+    </tr>
+    <tr>
+      <td>3.</td>
+      <td><i>m. rectus internus.</i></td>
+    </tr>
+    <tr>
+      <td>4.</td>
+      <td><i>m. rectus superior.</i></td>
+    </tr>
+    <tr>
+      <td>5.</td>
+      <td><i>m. levator palpebræ.</i></td>
+    </tr>
+    <tr>
+      <td>6.</td>
+      <td><i>m. rectus inferior.</i></td>
+    </tr>
+    <tr>
+      <td class="pr1">Most posterior.</td>
+      <td>7.</td>
+      <td><i>m. obliquus inferior.</i></td>
+    </tr>
+  </table>
+
+  <p>It is instructive to compare this arrangement of Hensen and Völckers with the arrangement of
+  the origin of these muscles from the premandibular cavity as given by Miss Platt.</p>
+
+  <p>Thus she states that the most posterior part of the premandibular cavity is cut off so as to
+  form a separate cavity, resembling, except in position, the anterior cavity; this separate, most
+  posterior part gives origin to the inferior oblique muscle. She then goes on to describe how the
+  dorsal wall of the remainder of the premandibular cavity becomes thickened, to form posteriorly
+  the rudiment of the inferior rectus and anteriorly the rudiments of the superior and internal
+  recti, a slight depression in the wall of the cavity separating these rudiments. The internal
+  rectus is the more median of the two anterior muscles. In other words, her evidence points not
+  only to a fusion of somites to form the premandibular cavity, but also to the arrangement of these
+  somites as follows, from head to tail: (1) internal rectus, (2) superior rectus, (3) inferior
+  rectus, (4) inferior <span class="pagenum" id="page266">{266}</span>oblique&mdash;an order
+  precisely the same as that of Hensen and Völckers, and of Starr.</p>
+
+  <p>I conclude, from the agreement between the anatomical, physiological, and morphological
+  evidence, that the IIIrd and IVth nerves contain the motor somatic nerves belonging to the same
+  segments as the motor trigeminal, in other words, to the prosomatic segments, so that the
+  eye-muscles, innervated by III. and IV., represent segmental muscles belonging to the prosoma.
+  Further, I conclude that originally there were seven prosomatic segments, the first of which is
+  represented by the anterior cavity described by Miss Platt, and does not form any permanent
+  muscles; that the next four belong to the premandibular cavity, and the muscles formed are the
+  superior rectus, internal rectus, inferior rectus, and inferior oblique; and that the last two
+  belong to the mandibular cavity, the muscles formed being Miss Platt's mandibular muscle and the
+  superior oblique. It is, to say the least of it, a striking coincidence that such an arrangement
+  of the c&#x0153;lomic cavities as here given should be so closely mimicked by the arrangement in
+  the prosomatic region of Limulus as already mentioned; it suggests inevitably that the
+  head-cavities of the vertebrate are nothing more than the prosomatic and mesosomatic segmental
+  c&#x0153;lomic cavities, as found in animals such as Limulus. In the table on p. <a
+  href="#page253">253</a>, I have inserted the segments in the vertebrate for comparison with those
+  of Limulus.</p>
+
+  <p>Before we can come to any conclusion as to the original position of these eye-muscles, it is
+  necessary to consider the VIth nerve and the external rectus muscle. This nerve and this muscle
+  belong to van Wijhe's 4th segment. The muscle is, therefore, the somatic segmental muscle
+  belonging to the same segment as the facial and is, in fact, a segmental muscle belonging not to
+  the prosoma, but to the mesosoma. Neal comes to the conclusion that the existing abducens is the
+  only root which remains permanent among a whole series of corresponding ventral roots belonging to
+  the opisthotic segments, and further points out that the external rectus was originally an
+  opisthotic muscle which has taken up a pro-otic position, or, translating this statement into the
+  language of Limulus, etc., it is a mesosomatic muscle which has taken up a prosomatic
+  position.</p>
+
+  <p>There is, however, another muscle&mdash;the <i>Retractor oculi</i>&mdash;belonging to the same
+  group which is innervated by the VIth nerve. Quite recently Edgeworth has shown that in birds and
+  reptiles this muscle <span class="pagenum" id="page267">{267}</span>belongs to the hyoid segment;
+  so that in this respect also the hyoid segment proclaims its double nature.</p>
+
+  <p>With respect to the external rectus muscle, Miss Platt has shown that the mandibular muscle is
+  formed close alongside the external rectus, so that the two are in close relationship as long as
+  the former exists.</p>
+
+  <p>Further, as already mentioned, the eye-muscles in Ammoc&#x0153;tes must be considered by
+  themselves; they do not belong in structure or position to the longitudinal somatic muscles
+  innervated by the spinal nerves; their structure is not the same as that of the tubular
+  constrictor or branchial muscles, but resembles that structure somewhat; their position is
+  dorso-ventral rather than longitudinal; they may be looked upon as a primitive type of somatic
+  muscles segmentally arranged, the direction of which was dorso-ventral.</p>
+
+  <p>Anderson also has shown that the time of medullation of the nerves supplying these muscles is
+  much earlier than that of the nerves belonging to the somatic trunk-muscles, their medullation
+  taking place at the same time as that of the motor nerves supplying the striated visceral muscles;
+  and Sherrington has observed that these muscles do not possess muscle-spindles, while all somatic
+  trunk-muscles do. Both these observations are strong confirmation of the view that the eye-muscles
+  must be classified in a different category to the ordinary somatic trunk muscle group.</p>
+
+  <p>What, then, is the interpretation of these various embryological and anatomical facts?</p>
+
+  <p>Remembering the tripartite division of each segmental nerve-group in Limulus into (1) dorsal or
+  sensory somatic nerve, (2) appendage-nerve, and (3) ventral somatic nerve, I venture to suggest
+  that the three nerves&mdash;the <i>oculomotorius</i>, the <i>trochlearis</i>, and the
+  <i>abducens</i>&mdash;represent the ventral somatic nerves of the prosoma, and partly also of the
+  mesosoma; that they are nerves, therefore, which may have originally contained sensory fibres, and
+  which still contain the sensory fibres of the eye-muscles themselves, as stated by Sherrington.
+  According to this suggestion, the eye-muscles are the sole survivors of the segmental
+  dorso-ventral somatic muscles, so characteristic of the group from which I imagine the vertebrates
+  to have sprung. In the mesosomatic region the dorso-ventral muscles which were retained were those
+  of the appendages and not of the mesosoma itself, because the presumed ancestor breathed after the
+  fashion of the water-breathing Limulus, by means of the dorso-ventral muscles of its <span
+  class="pagenum" id="page268">{268}</span>branchial appendages, and not after the fashion of the
+  air-breathing scorpion, by means of the dorso-ventral muscles of the mesosoma. The only
+  mesosomatic dorso-ventral muscles which were retained were those of the foremost mesosomatic
+  segments, <i>i.e.</i> those supplied by the VIth nerve, which were preserved owing to their having
+  taken on a prosomatic position and become utilized to assist in the movements of the lateral
+  eyes.</p>
+
+  <p>Let us turn now to the consideration of the corresponding musculature in Limulus and in the
+  scorpion group. These muscles constitute the markedly segmental muscles to which I have given the
+  name 'dorso-ventral somatic muscles.' They are most markedly segmental in the mesosomatic region,
+  both in Limulus and in Scorpio, each mesosomatic segment possessing a single pair of these
+  vertical mesosomatic muscles, as Benham calls them (<i>cf.</i> Fig. <a href="#fig58">58</a>
+  (<i>Dv.</i>)). In the prosomatic region the corresponding muscles are not so clearly defined in
+  Limulus; they are apparently attached to the plastron forming the group of plastro-tergal muscles.
+  From Benham's description it is sufficiently evident that they formed originally a single pair to
+  each prosomatic segment.</p>
+
+  <p>In Scorpio, according to Miss Beck, the dorso-ventral prosomatic muscles are situated near the
+  middle line on each side and form the following well-marked series of pairs of muscles, shown in
+  Fig. <a href="#fig110">110</a>, A, taken from her paper, and thus described by her<span
+  class="wnw">:&mdash;</span></p>
+
+  <p>1. The dorso-cheliceral-sternal muscle (61) is the most anterior of the dorso-ventral muscles.
+  It is very small, and is attached to the carapace near the median line anteriorly to the central
+  eyes.</p>
+
+  <p>2. The median dorso-preoral-entosclerite muscle (62) is a large muscle, between which and its
+  fellow of the opposite side the eyes are situated. It is attached dorsally to the carapace and
+  ventrally to the pre-oral entosclerite.</p>
+
+  <p>3. The anterior dorso-plastron muscle (63) is attached dorsally to the carapace in the middle
+  line, being joined to its fellow of the opposite side. They separate, and are attached ventrally
+  to the plastron. Through the arch thus formed the alimentary canal and the dorsal vessel pass.</p>
+
+  <p>4. The median dorso-plastron muscle (64) is attached dorsally to the posterior part of the
+  carapace. It runs forward on the anterior surface of the posterior flap of the plastron to the
+  body of the plastron, to which it is attached.</p>
+
+  <div><span class="pagenum" id="page269">{269}</span></div>
+
+  <div id="fig110"></div>
+
+  <table class="sp2 mc tlf w60" title="Figs. 110 A-C" summary="Figs. 110 A-C">
+    <tr>
+      <td class="w50 ac"><span class="ac w30 fcenter"><a href="images/fig110a.png"><img
+      style="width:100%" src="images/fig110a.png" alt="" title=""/></a></span></td>
+      <td class="w50 ac vmi smaller">
+        <p>A.</p>
+        <p class="sp0"><span class="sc">Dorso-ventral Muscles on Carapace of Scorpion.</span> (From
+        <span class="sc">Miss Beck.</span>)</p>
+      </td>
+    </tr>
+    <tr>
+      <td class="w50 ac"><span class="ac w30 fcenter"><a href="images/fig110b.png"><img
+      style="width:100%" src="images/fig110b.png" alt="" title=""/></a></span></td>
+      <td class="w50 ac vmi smaller">
+        <p>B.</p>
+        <p class="sp0"><span class="sc">Similar Muscles on Carapace of Eurypterus.</span></p>
+      </td>
+    </tr>
+    <tr>
+      <td class="w50 ac"><span class="ac w30 fcenter"><a href="images/fig110c.png"><img
+      style="width:100%" src="images/fig110c.png" alt="" title=""/></a></span></td>
+      <td class="w50 ac vmi smaller">
+        <p>C.</p>
+        <p><span class="sc">Similar Muscles on Head-Shield of a Cephalaspid.</span></p>
+        <p><i>l.e.</i>, lateral eyes; <i>c.e.</i>, central eyes; <i>Fro.</i>, narial opening.</p>
+        <p class="sp0">62-65 refer to Miss Beck's catalogue of the scorpion muscles.</p>
+      </td>
+    </tr>
+    <tr>
+      <td colspan="2" class="ac smaller"><span class="sc">Fig. 110.</span></td>
+    </tr>
+  </table>
+
+  <div><span class="pagenum" id="page270">{270}</span></div>
+
+  <p>To these may be added, owing to its attachment to the plastron,</p>
+
+  <p>5. The posterior dorso-plastron muscle (65). This is the first of the dorso-ventral muscles
+  attached to the mesosomatic tergites, being attached to the tergite of the first segment of the
+  mesosoma.</p>
+
+  <p>This muscle is of interest, in connection with the prosomatic dorso-ventral muscles, because it
+  is attached to the plastron, and runs a course in close contact with the muscle (64), the two
+  muscles being attached dorsally close together, on each side of the middle line, the one at the
+  very posterior edge of the prosomatic carapace, and the other at the very anterior edge of the
+  mesosomatic carapace.</p>
+
+  <p>Taking these muscles separately into consideration, it may be remarked with respect to (61)
+  that the cheliceral segment in its paired dorso-ventral muscles, as in its tergo-coxal muscles,
+  takes up a separate position isolated from the rest of the prosomatic segments.</p>
+
+  <p>Next comes (62) the median dorso-preoral-entosclerite muscle, which is strikingly different
+  from all the other dorso-ventral muscles in its large size and the extent of its attachment to the
+  dorsal carapace, according to Miss Beck's figures. The reason of its large size is clearly seen
+  upon dissection of the muscles in <i>Buthus</i>, for I find that, strictly speaking, it is not a
+  single muscle, but is composed of a series of muscle-bundles, separated from each other by
+  connective tissue. There are certainly three separate muscles included in this large muscle, which
+  are attached in a distinct series along the pre-oral entosclerite, and present the appearance
+  given in Fig. <a href="#fig110">110</a>, A, at their attachment to the prosomatic carapace. Of
+  this muscle-group the most anterior and the most posterior bundle are distinctly separate muscles;
+  I am not, however, clear whether the middle bundle represents one or two muscles.</p>
+
+  <p>This division of Miss Beck's muscle (62) into three or four muscles brings the prosomatic
+  region of the scorpion into line with the mesosomatic, and enables us to feel sure that a single
+  pair of dorso-ventral somatic muscles belongs to each prosomatic segment just as to each
+  mesosomatic, and, conversely, that each such single pair of muscles possesses segmental value in
+  this region as much as in the mesosomatic.</p>
+
+  <p>It is very striking to see how in all the Scorpionidæ, in which the two median eyes are the
+  principal eyes, this muscle group (62) on the two sides closely surrounds these two eyes, so that
+  with a fixed <span class="pagenum" id="page271">{271}</span>pre-oral entosclerite, a slight
+  movement of the eyes, laterally or anteriorly, owing to the flexibility of the carapace, might
+  result as the consequence of their contraction. But this cannot be the main object of these
+  muscles. The pre-oral entosclerite is firmly fixed to the camerostome, as is seen in Fig. <a
+  href="#fig94">94</a>, <i>pr. ent.</i>, so that the main object of these muscles is, as Huxley has
+  pointed out, the movement of this organ.</p>
+
+  <p>In order to avoid repetition of the long name given to this muscle group (62) by Miss Beck,
+  because of their position, and for other reasons which will appear in the sequel, I will call this
+  group of muscles the group of recti muscles. These recti muscles belong clearly to the segments
+  posterior to the first prosomatic or cheliceral segment, and represent certainly three, probably
+  four, of these segments, <i>i.e.</i> belong to the segments corresponding to the second, third,
+  fourth, and fifth prosomatic locomotor appendages&mdash;the endognaths of the old Eurypterids.</p>
+
+  <p>The next pair of muscles is the pair of anterior dorso-plastron muscles (63). This muscle-pair
+  evidently belongs to a segment posterior to the segments represented by the group already
+  discussed, and belongs, therefore, in all probability to the same segment as the sixth pair of
+  prosomatic appendages&mdash;the ectognaths of the old Eurypterids. This can be settled by
+  considering either the nerve-supply or the embryological development. In the Eurypteridæ it seems
+  most highly probable that the dorso-ventral muscles of each half of the segments belonging to the
+  endognaths should be compressed together and separate from the dorso-ventral muscle belonging to
+  the ectognathal segment, on account of the evident concentration and small size of the endognathal
+  segments in contradistinction to the separateness and large size of the ectognathal segment.</p>
+
+  <p>The striking peculiarity of this muscle-pair, which distinguishes it from all other muscles in
+  the scorpion, is the common attachment of the muscles of the two sides in the mid-dorsal line, so
+  that the pair of muscles forms an arch through which the alimentary canal and dorsal blood-vessel
+  pass.</p>
+
+  <p>The same dorso-ventral muscles are present in <i>Phrynus</i>, and in this animal the fibres of
+  this pair of muscles (63) actually interlace before the attachment to the prosomatic carapace, so
+  that the attachment of the muscle on each side overpasses the mid-dorsal line, and a true crossing
+  occurs. In Fig. <a href="#fig108">108</a> the position of this pair of <span class="pagenum"
+  id="page272">{272}</span>muscles is shown just posteriorly to the brain-mass. This muscle I will
+  call the oblique muscle.</p>
+
+  <p>Finally we come to the muscles (64) and (65), the median and posterior dorso-plastron muscles,
+  which run close together. Both muscles are attached to the plastron, and, therefore, to that
+  extent belong to the prosomatic region; they are attached dorsally close to the junction of the
+  prosoma and mesosoma. This position of the first mesosomatic dorso-ventral muscle belonging to the
+  opercular segment may be compared with the position of the first mesosomatic dorso-ventral muscle
+  in Limulus which has become attached to the prosomatic carapace; in both cases we see an
+  indication that the foremost pair of mesosomatic dorso-ventral somatic muscles tend to take up a
+  prosomatic position.</p>
+
+  <p>As to the pair of small muscles (64), I believe that they represent the dorso-ventral muscles
+  of the seventh prosomatic segment (if the pair of muscles (63) belongs to the segment of the sixth
+  locomotor prosomatic appendages), <i>i.e.</i> they belong to the chilarial segment or
+  metastoma.</p>
+
+  <p>I desire to draw especial attention to the fact that the dorso-ventral muscle (64), which
+  represents the seventh segment, always runs close alongside the dorso-ventral muscle (65), which
+  represents the first mesosomatic or opercular segment.</p>
+
+  <p>The comparison, then, of these two sets of facts leads to the following conclusions<span
+  class="wnw">:&mdash;</span></p>
+
+  <p>The foremost prosomatic or trigeminal segment stood separate and apart, being situated most
+  anteriorly; the musculature of this segment does not develop, so that the only evidence of its
+  presence is given by the anterior c&#x0153;lomic cavity. This corresponds, according to my scheme,
+  with the first or anterior c&#x0153;lomic cavity of Limulus, and therefore represents, as far as
+  the prosomatic appendages are concerned, the first prosomatic appendage-pair, or the cheliceræ;
+  the appendage-muscles being the muscles of the cheliceræ, and the dorso-ventral somatic muscles
+  the pair of dorso-cheliceral sternal muscles (61) in the scorpion. Both these sets of muscles,
+  therefore, dwindle and disappear in the vertebrate.</p>
+
+  <p>Then came four segments fused together to form the premandibular segment, the characteristic of
+  which is the apparent non-formation of any permanent musculature from the ventral
+  mesoderm-segments, and the formation of the eye-muscles innervated by the <span class="pagenum"
+  id="page273">{273}</span>oculomotor nerve from the dorsal mesoderm segments. These four segments
+  have been so fused together that van Wijhe looked upon them as a single segment, and the
+  premandibular cavity as the cavity of a single segment. They represent, according to my scheme,
+  the segments belonging to the endognaths, <i>i.e.</i> the second, third, fourth, fifth pairs of
+  prosomatic appendages; the premandibular cavity, therefore, represents the second c&#x0153;lomic
+  cavity in Limulus, which, according to Kishinouye, is the sole representative of the
+  c&#x0153;lomic cavities of the second, third, fourth, fifth prosomatic segments. The muscles
+  derived from the ventral mesoderm-segments represent the muscles of these appendages, which
+  therefore dwindle and disappear in the vertebrate, with the possible exception of the muscles
+  innervated by the descending root of the trigeminal. The muscles derived from the dorsal
+  mesoderm-segments, <i>i.e.</i> the eye-muscles supplied by the oculomotor nerve, represent the
+  dorso-ventral somatic muscles of these four segments, muscles which are represented in the
+  scorpion by the recti group of muscles, <i>i.e.</i> the median dorso-preoral-entosclerite muscles
+  (62).</p>
+
+  <p>Then came two segments, the mandibular, in which muscles are formed both from the ventral and
+  from the dorsal mesoderm-segments. From the former arose the main mass of muscles innervated by
+  the motor root of the trigeminal, from the latter the superior oblique muscle and the mandibular
+  muscle of Miss Platt, of which the former alone survives in the adult condition. These two
+  segments are looked upon as a single segment by van Wijhe, of which the mandibular cavity is the
+  c&#x0153;lomic cavity. They represent, according to my scheme, the segments belonging to the sixth
+  pair of prosomatic appendages or ectognaths, and the seventh pair, <i>i.e.</i> the chilaria or
+  metastoma.</p>
+
+  <p>The first part, then, of the mandibular cavity represents the third c&#x0153;lomic cavity in
+  Limulus and the muscles derived from the ventral mesoderm, in all probability the muscles of the
+  tongue in the lamprey (<i>cf.</i> Chap. IX.), which represents the ectognaths or sixth pair of
+  prosomatic appendages, while the muscles derived from the dorsal mesoderm, <i>i.e.</i> the
+  superior oblique muscles, represent the dorso-ventral somatic muscles of this segment, muscles
+  which are represented in the scorpion group by the pair of anterior dorso-plastron or oblique
+  muscles (63).</p>
+
+  <p>The second part of the mandibular cavity represents the 4th <span class="pagenum"
+  id="page274">{274}</span>c&#x0153;lomic cavity in Limulus and the muscles derived from the ventral
+  mesoderm, in all probability the muscles of the lower lip in the lamprey (<i>cf.</i> Chap. IX.),
+  which represents the metastoma; while the muscles derived from the dorsal mesoderm, <i>i.e.</i>
+  Miss Platt's pair of mandibular muscles, represent the dorso-ventral somatic muscles of this
+  segment, muscles which are represented in the scorpion group by the pair of median dorso-plastron
+  muscles (64).</p>
+
+  <p>In connection with this last pair of muscles we find that the external rectus in the vertebrate
+  represents the first dorso-ventral mesosomatic muscle in the scorpion, <i>i.e.</i> the posterior
+  dorso-plastron muscle (65), and, as already mentioned (p. <a href="#page267">267</a>), that it
+  always lies closely alongside the mandibular muscle, just as in the scorpion group muscle (65)
+  always lies alongside muscle (64).</p>
+
+  <p>In the invertebrate as well as in the vertebrate this muscle is a mesosomatic muscle which has
+  taken up a prosomatic position.</p>
+
+  <p>The question naturally arises, what explanation can be given of the fact that these
+  dorso-ventral muscles attached on each side of the mid-dorsal line to the prosomatic carapace
+  became converted into the muscles moving the eyeballs of the two lateral eyes? An explanation
+  which must take into account not only the isolated position of the abducens nerve, but also the
+  extraordinary course of the trochlearis. The natural and straightforward answer to this question
+  appears to me quite satisfactory, and I therefore venture to commend it to my readers.</p>
+
+  <p>I have argued the case out to myself as follows: The lateral eyes must have been originally
+  situated externally to the group of muscles innervated by the oculomotor nerve, for a sheet of
+  muscle representing the superior <i>internal</i> and inferior rectus muscles could only wrap round
+  the internal surface of each lateral eye; <i>i.e.</i> the arrangement of the muscle-sheet, as in
+  the scorpion, about two median eyes, is in the wrong position, for if those two eyes, which are
+  the main eyes in the scorpion, were to move outwards to become two lateral eyes, then such a
+  muscle-group would form a superior <i>external</i> and inferior rectus group. The evidence,
+  however, of Eurypterus and similar forms is to the effect that the lateral eyes became big and the
+  median eyes insignificant and degenerate. If, then, with the degeneration of the one and the
+  increasing importance of the other, these lateral eyes came near the middle line, then the
+  muscular group (62), which I have called the recti group, would naturally be pressed into their
+  <span class="pagenum" id="page275">{275}</span>service, and would form an internal and not an
+  external group of eye-muscles.</p>
+
+  <p>In Fig. <a href="#fig110">110</a>, A, taken from Miss Beck's paper, I have shown the relative
+  position of the eyes and the segmental dorso-ventral prosomatic muscles on the carapace of the
+  scorpion. In Fig. <a href="#fig110">110</a>, B, I have drawn the prosomatic carapace of
+  <i>Eurypterus Scouleri</i>, taken from Woodward's paper, with the eyes as represented there; in
+  this I have inserted the segmental dorso-ventral muscles as met with in the scorpion, thereby
+  demonstrating how, with the degeneration of the median eyes and the large size of the lateral
+  eyes, the recti muscles of the scorpion would approach the position of an internal recti group to
+  the lateral eyes, and so give origin to the group of muscles innervated by the oculomotor nerve.
+  In the Eurypterus these large eyes are large single eyes, not separate ocelli, as in the
+  scorpion.</p>
+
+  <p>All, then, that is required is that in the first formed fishes, which still possessed the
+  dorso-ventral muscles of their Eurypterid ancestors, the lateral eyes should be the important
+  organs of sight, large and near the mid-dorsal line. Such, indeed, is found to be the case. In
+  amongst the masses of Eurypterids found in the upper Silurian deposits at Oesel, as described by
+  Rohon, numbers of the most ancient forms of fish are found belonging to the genera Thyestes and
+  Tremataspis. The nature of the dorsal head-shields of these fishes is shown in Fig. <a
+  href="#fig14">14</a>, which represents the dorsal head-shield of <i>Thyestes verrucosus</i>, and
+  Fig. <a href="#fig111">111</a> that of <i>Tremataspis Mickwitzi</i>. They show how the two lateral
+  eyes were situated close on each side of the mid-dorsal line in these Eurypterus-like fishes, in
+  the very position where they must have been if the eye-muscles were derived from the dorso-ventral
+  somatic muscles of a Eurypterid ancestor.</p>
+
+  <div class="ac w20 fcenter sp2">
+    <a href="images/fig111.png" id="fig111"><img style="width:100%" src="images/fig111.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 111.&mdash;Dorsal Head-Shield of</span> <i>Tremataspis
+      Mickwitzi</i>. (From <span class="sc">Rohon</span>.)</p>
+      <p class="sp0"><i>Fro.</i>, narial opening; <i>l.e.</i>, lateral eyes; <i>gl.</i>, glabellum
+      plate over brain; <i>Occ.</i>, occipital spine.</p>
+    </div>
+  </div>
+
+  <p>In Lankester's words, one of the characteristics of the Osteostraci (Cephalaspis, Auchenaspis,
+  etc.), as distinguished from the Heterostraci (Pteraspis), are the large orbits placed near the
+  centre of the shield. The apparent exception of Thyestes mentioned by him is no <span
+  class="pagenum" id="page276">{276}</span>exception, for orbits of the same character have since
+  been discovered, as is seen in Rohon's figure (Fig. <a href="#fig14">14</a>). In Fig. <a
+  href="#fig110">110</a>, C, I give an outline of the frontal part of the head-shield of a
+  Cephalaspid, in which I have drawn the eye-muscles as in the other two figures.</p>
+
+  <p>Although all the members of the Osteostraci possess large lateral eyes towards the centre of
+  the head-shield, the other group of ancient fishes, the Heterostraci, are characterized by the
+  presence of lateral eyes far apart, situated on the margin of the head-shield on each side
+  (<i>cf.</i> Fig. 142, <i>o</i>, p. <a href="#page350">350</a>).</p>
+
+  <p>So, also, on the invertebrate side, the lateral eyes of Pterygotus and Slimonia are situated on
+  the margin of the prosomatic carapace, while those of Eurypterus and Stylonurus are situated much
+  nearer the middle line of the prosomatic carapace.</p>
+
+  <p>Next comes the question of the superior oblique muscle and the trochlearis nerve. Why does this
+  nerve (<i>n.IV.</i> in Fig. <a href="#fig106">106</a>, C and D) alone of all the nerves in the
+  body take the peculiar position it always does take? The only suggestion that I know of which
+  sounds reasonable and worth consideration is that put forward by Fürbringer, which is an
+  elaboration of the original suggestion of Hoffmann. Hoffmann suggested in 1889 that the
+  trochlearis nerve represented originally a nerve for a protecting organ of the pineal eye, which
+  became secondarily a motor nerve for the lateral eye as the pineal eye degenerated. Fürbringer
+  differs from Hoffmann in that he considers that the nerve was originally a motor nerve, and was
+  not transformed from sensory to motor, yet thinks Hoffmann's suggestion is in the right
+  direction.</p>
+
+  <p>He points out that the crossing of the trochlearis is not a crossing of fibres between two
+  centres in the central nervous system, but may be explained by the shifting of the peripheral
+  organ, <i>i.e.</i> the muscle, from one side to the other, and the nerve following this shift.
+  Consequently, says Fürbringer, the course of the nerve indicates the original position of the
+  muscle, and therefore he imagines that the ancestor of the superior oblique muscle was a muscle
+  the fibres of which were attached in the mid-dorsal line, and interlaced with those of the other
+  side, the two muscles thus forming an arch through which the nervous system with its central canal
+  passed. Then, for the sake of getting a more efficient pull, the crossing muscle-fibres became
+  more definitely attached to the opposite side of the middle line, and finally obtained a new
+  attachment on the opposite side, with the <span class="pagenum"
+  id="page277">{277}</span>obliteration of the muscular arch; the nerve on each side, following the
+  shifts of the muscle, naturally took up the position of the original muscular arch, and so formed
+  the trochlear nerve, with its dorsal crossing. This explanation of Fürbringer's was associated by
+  him with movements of the median pineal eyes, the length of their nerve, according to him, even
+  yet indicating their previous mobility. This assumption is not, it seems to me, necessary. The
+  length of the nerve is certainly no indication of mobility, for in Limulus and the scorpion group
+  the nerve to each median eye is remarkably long, yet these eyes are immovably fixed in the
+  carapace. All that is required is a pair of dorso-ventral muscles belonging to the segment
+  immediately following the group of segments represented by the oculomotor nerves, the fibres of
+  which should cross the mid-dorsal line at their attachment; for, seeing that the lateral eyes were
+  originally so near this position, it follows that such muscles might form part of the muscular
+  group belonging to the lateral eye without having previously moved the pineal eyes. In fact,
+  Fürbringer's explanation requires as starting-point that the pair of muscles which ultimately
+  become the superior oblique should have the exact position of the pair of dorso-ventral muscles in
+  the scorpion, called by Miss Beck the anterior dorso-plastron muscles (63), which I have named the
+  oblique muscles. Here, and here only, do we find an interlacement, across the mid-dorsal line, of
+  the fibres of attachment of the muscles on the two sides, in consequence of which this pair of
+  muscles is described by her as forming an arch encircling the alimentary canal and dorsal vessel.
+  If, then, as I have previously argued, the primitive plastron formed a pair of trabeculæ, and the
+  nervous system grew round the alimentary canal, such an arch would encircle the tubular central
+  nervous system of the vertebrate.</p>
+
+  <p>Still more striking is this pair of muscles (63) in Phrynus (Fig. 108), where we see how the
+  arch formed by them almost touches the posterior extremity of the supra-&#x0153;sophageal
+  brain-mass, crossing, therefore, over the beginning of the stomach region of the animal. The angle
+  formed by the arch is much more obtuse than that formed in Scorpio, so that an actual crossing of
+  the muscle-fibres has taken place at the point of attachment to the carapace. Also, only the part
+  nearest the carapace is muscular, the rest forming a long tendinous prolongation of the plastron
+  wall (the primordial cranium), as seen in the figure.</p>
+
+  <div><span class="pagenum" id="page278">{278}</span></div>
+
+  <p>This muscle-pair is, as it should be, the pair of dorso-ventral muscles belonging to the
+  segment immediately following on the group of segments represented by the recti muscles,
+  <i>i.e.</i> according to previous argument, the segment belonging to the sixth pair of locomotor
+  appendages or ectognaths; a muscle, therefore, which would arise in the vertebrate from the
+  mandibular, and not from the premandibular cavity. A similar muscle probably existed in Eurypterus
+  (<i>M.obl.</i> in Fig. <a href="#fig106">106</a>, B), and, as in the case of the formation of the
+  oculomotor group, derived from the recti group of the scorpion, would form the commencement of the
+  superior oblique muscle in Thyestes and Tremataspis.</p>
+
+  <div class="ac w45 fcenter sp2">
+    <a href="images/fig112.png" id="fig112"><img style="width:100%" src="images/fig112.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p><span class="sc">Fig. 112.&mdash;A, Diagram of Position of Oblique Muscle in Scorpion; B,
+      Diagram of Transition Stage; C, Diagram of Superior Oblique Muscle in Vertebrate.</span></p>
+      <p class="sp0 ac"><i>l.e.</i>, lateral eyes; <i>c.e.</i>, central eyes; <i>C.N.</i>, central
+      nervous system; <i>Al.</i>, alimentary canal; <i>c.</i>, <i>aqueductus Sylvii</i>.</p>
+    </div>
+  </div>
+
+  <p>It is instructive to notice that the original position of attachment of this muscle is
+  naturally posterior to that of the oculomotor group of muscles, and that Fürbringer, in his
+  description of the eye-muscles of Petromyzon, asserts that this muscle in this primitive
+  vertebrate <span class="pagenum" id="page279">{279}</span>form is not attached as in other
+  vertebrates, but is posterior to the other muscles, so that he calls it the posterior rather than
+  the superior oblique. The nature of the change by which the muscle known in the scorpion as the
+  anterior dorso-plastron muscle (63) was probably converted into the superior oblique muscle of the
+  vertebrate, is represented in the drawings Fig. <a href="#fig112">112</a>, in which also are
+  indicated the dwindling of the median eyes, and the progressive superiority of the lateral eyes,
+  as well as the transformation of the recti muscle-group of the scorpion into the muscles supplied
+  by the oculomotor nerve of the vertebrate.</p>
+
+  <p>With respect to the external rectus muscle, it follows naturally that if the muscles (64) and
+  (65) are to follow suit with the rest of the group and become attached to the lateral eyes, they
+  must take up an external position. These two muscles, which always run together, as seen in Fig.
+  <a href="#fig110">110</a>, A, the one belonging to the prosoma and the other to the mesosoma, are
+  represented by the mandibular muscle of Miss Platt and the external rectus, the former derived
+  from the walls of the last pro-otic head-cavity, the latter from the foremost of the opisthotic
+  head-cavities.</p>
+
+  <p class="sp3">Such, then, is the simple explanation of the origin of the eye-muscles which
+  follows from my theory, and we see that the successive alterations of the position of the orbit,
+  and, therefore, of the globe of the eye with its muscles, as we pass from Thyestes to man, is the
+  natural consequence of the growth of the frontal bone, <i>i.e.</i> of the brain.</p>
+
+  <p class="ac"><span class="sc">The Trigeminal Nerves and the Muscles supplied by them.</span></p>
+
+  <p>Turning now to the evidence as to the number of ventral segments, <i>i.e.</i> the motor and
+  sensory supply to the prosomatic appendages afforded by the trigeminal nerve, we must, I think,
+  come to the same conclusion as Dohrn, viz. that if there were originally seven dorsal or somatic
+  segments in this region represented by: 1, Anterior cavity, muscle lost; 2, 3, 4, 5, muscles of
+  the premandibular cavity, <i>sup. rectus</i>, <i>inf. rectus</i>, <i>int. rectus</i>, <i>inf.
+  oblique</i>, supplied by IIIrd nerve; 6, 7, muscles of the mandibular cavity, <i>sup. oblique</i>,
+  supplied by IVth nerve and muscle lost, there must have been also seven corresponding ventral or
+  splanchnic segments supplied by the trigeminal. At present the evidence for such segments is
+  nothing like so strong as for the corresponding somatic ones; there are, however, certain
+  suggestive <span class="pagenum" id="page280">{280}</span>facts which point distinctly in this
+  direction in connection with both the motor and sensory parts of the trigeminal. The origin of the
+  trigeminal motor fibres in the central nervous system is most striking. We may take it for granted
+  that a nucleus of cells giving origin to one or more segmental motor nerves will possess a greater
+  or less longitudinal extension in the central nervous system, according to the number of fused
+  separate segmental centres it represents. Thus a nucleus such as that of the IVth nerve or of the
+  facial is small and compact in comparison to the extensive conjoint nucleus of the vagus and
+  cranial accessory.</p>
+
+  <p>Upon examination of the motor nucleus of the trigeminal, we find a compact or well-defined
+  nucleus, the <i>nucl. masticatorius</i>, the nerves of which supply the masseter, temporal, and
+  other muscles, so that the anatomical evidence at first sight appears to bear out van Wijhe's
+  conclusion that the motor trigeminal supplies at most two segments. Further examination, however,
+  shows that this is not all, for the extraordinary so-called descending root of the Vth must be
+  taken into consideration in any question of the origin of the motor elements, just as the equally
+  striking ascending root enters into the consideration of the meaning of the sensory elements of
+  the Vth.</p>
+
+  <p>It is not necessary here to discuss the controversy as to whether this descending root is motor
+  or sensory. It is universally considered at present to be motor, and is believed to supply, as
+  Kölliker suggested, among other muscles, the <i>m. tensor tympani</i> and the <i>m. tensor veli
+  palati</i>. It is thus described by Obersteiner&mdash;</p>
+
+  <p>"From the region of the mid-brain the motor root receives an important addition of thick
+  fibres, which form the cerebral or descending root. The large, round vesicular cells from which
+  the fibres of the descending root arise form no single compact group, but are partly single,
+  partly arranged like little bunches of grapes, as far as the region of the anterior corpora
+  quadrigemina. The further we go brainwards, the smaller is the number of fibres. In the region of
+  the anterior corpora quadrigemina, the few cells of origin are found more and more median; so that
+  the uppermost trigeminal fibres descend in curves almost from the mid-line, as is shown by the
+  exceptional occurrence of one or more of the characteristic cells above the aqueduct. At the
+  height of the posterior commissure one finds the last of these trigeminal cells."</p>
+
+  <div><span class="pagenum" id="page281">{281}</span></div>
+
+  <p>The anatomy of the Vth nerve reveals, then, three most striking facts<span
+  class="wnw">:&mdash;</span></p>
+
+  <p>1. The motor nucleus of the Vth extends from the very commencement of the infra-infundibular
+  region to nearly the commencement of the nucleus of the VIIth; in other words, the motor nucleus
+  of the Vth extends through the whole prosomatic region, just as it must have done originally if
+  its motor nerves supplied the muscles of the prosomatic appendages. Such an extended range of
+  origin is indicative of the remains of an equally extended series of segmental centres or
+  ganglia.</p>
+
+  <p>2. Of these centres the caudalmost have alone remained large and vigorous, constituting the
+  <i>nucleus masticatorius</i>, which in the fish is divided into an anterior and posterior group,
+  thus indicating a double rather than a single nucleus; while the foremost ones have dwindled away
+  until they are represented only by the cells of the descending root, the muscles of these segments
+  being still represented by possibly the <i>tensor veli palati</i> and the other muscles innervated
+  from these cells.</p>
+
+  <p>3. The headmost of these cells takes up actually a position dorso-lateral to the central canal,
+  so that the groups on each side nearly come together in the mid-dorsal line; a very unique and
+  extraordinary position for a motor cell-group, but not improbable when we recall to mind Brauer's
+  assertion as to the shifting of the foremost prosomatic ganglion-cells of the scorpion from the
+  ventral to the dorsal side of the alimentary canal.</p>
+
+  <p>On the sensory side the evidence is also suggestive, the question here being not so much the
+  distribution of the sensory nerves as the number of ganglia belonging to each of the cranial
+  nerves.</p>
+
+  <p>With respect to this question, morphologists have come to the conclusion that there is a marked
+  difference between spinal and cranial nerves, in that whereas the posterior root-ganglia of the
+  spinal nerves arise from the central nervous system itself, <i>i.e.</i> from the neural crest, the
+  ganglia of the cranial nerves arise partly from the neural crest, partly from the proliferation of
+  cells on the surface of the animal; and because of the situation of these proliferating epidermal
+  patches over the gill-clefts in the case of the vagus and glossopharyngeal nerves, they have been
+  called by Froriep and Beard branchial sense-organs. Beard divides the cranial ganglia into two
+  sets, one connected with the neural ridges, called the neural ganglia, <span class="pagenum"
+  id="page282">{282}</span>and the other connected with the surface-cells, which he calls the
+  lateral ganglia. This second set corresponds to Kupffer's epibranchial ganglia. Now it is clear
+  that in the case of the vagus nerve, where, as is well shown in Ammoc&#x0153;tes, the nerve is not
+  a single segmental nerve, but is in reality made up of a number of nerves going to separate
+  branchial segments, the indication of such segments is not given by the main vagus ganglion or
+  neural ganglion, but by the series of lateral ganglia. So also it is argued in the case of the
+  trigeminal, that if in addition to the ganglion-cells arising from the neural crest separate
+  ganglion-masses are found in the course of development, in connection with proliferating patches
+  of the surface (plakodes, Kupffer calls them), then such isolated lateral ganglia are indications
+  of separate segments, just as in the case of the vagus, even though the separate segments do not
+  show themselves in the adult. So far the argument appears to me just, but the further conclusion
+  that the presence of such plakodes shows the previous existence of <i>branchial</i> sense-organs,
+  and, therefore, that such ganglia are <i>epibranchial</i> ganglia, indicating the position of a
+  lost gill-slit, is not justified by the premises. If, as I suppose, the trigeminal nerve supplied
+  a series of non-branchial appendages serially homologous with the branchial appendages supplied by
+  the vagus, then it is highly probable that the trigeminal should behave with respect to its
+  sensory ganglia similarly to the vagus nerve, without having anything to do with branchiæ.</p>
+
+  <p>Such plakodal ganglia, then, may give valuable indication of non-branchial segments as well as
+  of branchial segments. The researches of Kupffer on the formation of the trigeminal ganglia in
+  Ammoc&#x0153;tes are the chief attempt to find out from the side of the sensory ganglia the number
+  of segments originally belonging to the trigeminal. The nature and result of these researches is
+  described in my previous paper (<i>Journal of Anatomy and Physiology</i>, vol. xxxiv.), and it
+  will suffice here to state that he himself concludes that the trigeminal originally supplied five
+  at least, probably six, segments. As I have stated there, the evidence as given by him seems to me
+  to indicate even as many as seven segments.</p>
+
+  <p>In the full-grown Ammoc&#x0153;tes, as is well known, there are two distinct ganglia belonging
+  to the trigeminal, the one the ganglion of the <i>ramus ophthalmicus</i>, the other the main
+  ganglion.</p>
+
+  <p>According to Kupffer the larval Ammoc&#x0153;tes possesses three sets of ganglia, not two, for
+  between the foremost and hindmost ganglion <span class="pagenum" id="page283">{283}</span>he
+  describes a nerve (<i>x.</i>, Fig. <a href="#fig113">113</a>), with four epibranchial ganglia,
+  which do not persist as separate ganglia, but either disappear or are absorbed into the two main
+  ganglia (Fig. <a href="#fig113">113</a>). This discovery of Kupffer's is very suggestive, for, as
+  already stated, a transformation takes place when the Ammoc&#x0153;tes is 5 mm. long, so that the
+  arrangement of the parts before that period is distinctly more indicative of the ancestral
+  arrangement than any later one.</p>
+
+  <p>If we use the name plakodal ganglia to represent that part of these ganglia which was
+  originally connected with the skin, then Kupffer's researches assert that in the larval
+  Ammoc&#x0153;tes there were seven such plakodal ganglia, one in front belonging to the foremost
+  trigeminal ganglion, two behind, parts of the hindmost ganglion, and four in between, which do not
+  exist later as separate ganglia.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig113.png" id="fig113"><img style="width:100%" src="images/fig113.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 113.&mdash;Ganglia of the Cranial Nerves of an
+      Ammoc&#x0153;tes, 4 mm. in length, projected on to the Median Plane.</span> (After <span
+      class="sc">Kupffer</span>.)</p>
+      <p class="sp0"><i>A-B</i>, the line of epibranchial ganglia; <i>au.</i>, auditory capsule;
+      <i>nc.</i>, notochord; <i>Hy.</i>, tube of hypophysis; <i>Or.</i>, oral cavity; <i>u.l.</i>,
+      upper lip; <i>l.l.</i> lower lip; <i>vel.</i>, septum between oral and respiratory cavities;
+      <i>V.</i>, <i>VII.</i>, <i>IX.</i>, <i>X.</i>, cranial nerves; <i>x.</i>, nerve with four
+      epibranchial ganglia.</p>
+    </div>
+  </div>
+
+  <p>In accordance with the views put forward in this book, a possible interpretation of these
+  plakodal ganglia would be given as follows<span class="wnw">:&mdash;</span></p>
+
+  <p class="sp3">Beard, who, after Froriep, drew attention to this relation of the cranial ganglia
+  to special skin-patches, has compared them with the parapodial ganglia of annelids, <i>i.e</i>.
+  ganglia in connection with annelidan appendages; whether we are here obtaining a glimpse of the
+  far-off annelidan ancestry of both arthropods and vertebrates it would be premature at present to
+  say. It is natural enough to expect, on my view, to find evidence of annelidan ancestry in <span
+  class="pagenum" id="page284">{284}</span>vertebrate embryology (as has been so often asserted to
+  be the case), seeing that undoubtedly the Arthropoda are an advanced stage of Annelida; and,
+  indeed, the way is not a long one when we consider Beecher's evidence that the Trilobita belong to
+  the Phyllopoda, certainly a primitive crustacean group, which Bernard derives directly from the
+  annelid group Chætopoda. If, then, these plakodal ganglia indicate the former presence of
+  appendages, we obtain this result:&mdash;The foremost ganglion on each side possesses one plakodal
+  ganglion, and therefore indicates an anterior pair of appendages, possibly the cheliceræ. Then
+  comes the peculiar nerve with four plakodal ganglia indicating on each side four appendages close
+  together, possibly the endognaths. Then, finally, on each side, the second large ganglion with two
+  plakodal ganglia, indicating two pairs of appendages, possibly the ectognaths and the
+  metastoma.</p>
+
+  <p class="ac"><span class="sc">Summary.</span></p>
+
+  <div class="bq1 smaller sp5">
+    <p>The consideration of the history of the cranial segmentation shows that whereas, from the
+    commencement of that history, the evidence for two ventral segments supplied by the trigeminal
+    nerve is clear and unmistakable, later observers have tended more and more to increase the
+    number of these segments, until at the present time the evidence is in favour of at least six,
+    probably seven, as the number of segments supplied by the motor part of the trigeminal.</p>
+    <p>So, also, the original evidence for the number of dorsal or somatic segments limits the
+    number to three, innervated respectively by the oculomotor (III.), trochlear (IV.), and abducens
+    (VI.) nerves, or rather two, since the last nerve belongs to the facial segment. The muscles
+    which these three nerves supply are derived respectively from the walls of the premandibular,
+    mandibular, and hyoid c&#x0153;lomic cavities.</p>
+    <p>Later evidence points strongly to the conclusion that the oculomotor nerve and the
+    premandibular cavity represent not one segment but the fusion of four, while the mandibular
+    cavity represents two segments. In addition to these, Miss Platt has discovered a still more
+    anterior head-cavity, which she has named the anterior cavity, so that the pro-otic segments on
+    this reckoning are seven in number, viz.: (1) the anterior cavity, (2, 3, 4, 5) the
+    premandibular cavity, (6, 7) the mandibular cavity. The somatic muscles belonging to these
+    dorsal segments are the eye-muscles, which are all dorso-ventral in position, and are not the
+    same as the longitudinal somatic muscles, but belong to a distinct dorso-ventral segmental
+    group, the only representative of which at present known in the mesosomatic region is the
+    external rectus innervated by the VIth nerve.</p>
+    <p>These head-cavities, and these muscles of the vertebrate, resemble the corresponding cavities
+    and muscles of the invertebrate to an extraordinary <span class="pagenum"
+    id="page285">{285}</span>degree, so that it becomes easy to see how the dorso-ventral muscles of
+    the prosomatic segments of the latter have become converted into the eye-musculature of the
+    former. The most powerful proof of all that such a conversion has taken place is that a natural
+    and simple explanation is at once given of the extraordinary course taken by the IVth or
+    trochlear nerve. Ever since neurology began, the course of this nerve has arrested the attention
+    of anatomists. Why should just this one pair of nerve-roots of all those in the whole body be
+    directed dorsalwards instead of ventralwards, and cross each other in the valve of Vieussens,
+    each to supply a simple eye-muscle (the superior oblique) belonging to the other side? For
+    generations anatomists have wondered and found no solution, and yet, without any straining of
+    hypotheses, in consequence simply of the investigation of the anatomy of the corresponding pair
+    of muscles in the scorpion group, the solution is immediately apparent.</p>
+    <p>This pair of muscles alone, of all the musculature attached to the carapace, crosses the
+    mid-dorsal line to be attached to the other side, thus carrying its nerve with it to the other
+    side; by a continuation of the same process the relation of the trochlear to the superior
+    oblique muscle can be explained.</p>
+    <p class="sp0">The comparison of the eye-muscles of the vertebrate with the dorso-ventral
+    segmented muscles of the invertebrate makes the number and nature of the pro-otic segments much
+    clearer.</p>
+  </div>
+
+  <div><span class="pagenum" id="page286">{286}</span></div>
+
+  <p class="ac">CHAPTER IX</p>
+
+  <p class="ac"><i>THE PROSOMATIC SEGMENTS OF AMMOC&#x0152;TES</i></p>
+
+  <div class="bq1 it1 smaller sp3">
+    <p class="sp0">The prosomatic region in Ammoc&#x0153;tes.&mdash;The suctorial apparatus of the
+    adult Petromyzon.&mdash;Its origin in Ammoc&#x0153;tes.&mdash;Its derivation from
+    appendages.&mdash;The segment of the lower lip or metastomal segment.&mdash;The tentacular
+    segments.&mdash;The tubular muscles.&mdash;Their segmental arrangement.&mdash;Their peculiar
+    innervation.&mdash;Their correspondence with the system of veno-pericardial muscles in
+    Limulus.&mdash;The old mouth or palæostoma.&mdash;The pituitary gland.&mdash;Its comparison with
+    the coxal gland of Limulus.&mdash;Summary.</p>
+  </div>
+
+  <p class="sp3">In the last chapter it was seen not to be incompatible with both the anatomical and
+  morphological evidence to look upon the trigeminal nerves as having originally supplied the seven
+  prosomatic pairs of appendages of the invertebrate ancestor, the foremost of which, the cheliceræ,
+  and the four pairs of endognaths dwindled away and became insignificant, leaving as trace of their
+  former presence the descending root of the Vth nerve; while the two hindmost pairs, the ectognaths
+  and the chilaria, or metastoma, remained vigorous and developed, leaving as proof of their
+  presence the <i>nucleus masticatorius</i>. Evidence in favour of this suggestion and of the nature
+  of the dwindling process is afforded when we examine what the trigeminus does supply in
+  Ammoc&#x0153;tes. In all vertebrates this nerve supplies the great muscles of mastication which,
+  in all gnathostomatous fishes, move the jaws. The lowest fishes, the cyclostomes, possess no jaws;
+  they take in their food by attaching themselves to their prey and by means of rasping teeth
+  situated in serried rows within the circular mouth, combined with a powerful suctorial apparatus,
+  they suck the juices of the fish they feed upon. Not possessing jaws, they feed by suction on the
+  living animal, a method of feeding which gives them no more claim to be classed as parasitic
+  animals than the whole group of spiders which feed in a similar manner on living flies.</p>
+
+  <div><span class="pagenum" id="page287">{287}</span></div>
+
+  <p class="ac"><span class="sc">The Origin of the Suctorial Apparatus of Petromyzon.</span></p>
+
+  <p>This powerful suctorial apparatus is innervated entirely by the trigeminal nerve, so that here
+  in its muscular arrangements any original segmental arrangement of the muscles of mastication
+  might be expected to be visible. It consists of a large rod or piston, to which are attached
+  powerful longitudinal muscles; a large muscle, the basilar muscle, which assists the piston in
+  producing a vacuum, and annular muscles around the circular lip.</p>
+
+  <p>Turn now to the full-grown larval form, Ammoc&#x0153;tes, an animal in the case of
+  <i>Petromyzon Planeri</i> as large as the full-grown Petromyzon, and seek for this musculature.
+  There is, apparently, no sign of it, no suctorial apparatus whatever, only, as already mentioned,
+  an oral chamber bounded by the lower and upper lips and the remains of the septum between it and
+  the respiratory chamber&mdash;the velar folds. Attached to its walls a number of tentacles are
+  situated, which form a fringe around and within the mouth. Most extraordinary is the contrast here
+  between the larval and the adult stages; in the former, no sign of the suctorial apparatus, but
+  simply tentacles and velar folds; in the latter, no sign of tentacles or of velar folds, but a
+  massive suctorial apparatus.</p>
+
+  <p>In order, then, to understand the origin of the muscles of mastication, it is necessary to
+  study the changes which occur at transformation, and thus to find out how the suctorial apparatus
+  of the adult arises. This most important investigation has been undertaken by Miss Alcock, and
+  owing to the kindness of Mr. Millington, of Thetford, we have been able to obtain a better series
+  in the transformation process than has ever been obtained before. Miss Alcock has not yet
+  published her researches, but has allowed me to make use of some of her facts.</p>
+
+  <p>An enormous proliferation of muscular tissue takes place with great rapidity during this
+  transformation, which causes the disappearance of the tentacles, and gives origin to the suctorial
+  apparatus. The starting point of this proliferation can be traced back in all cases to little
+  groups of embryonic tissue found below the epithelial lining of the oral chamber in
+  Ammoc&#x0153;tes. Of these groups the most conspicuous one is situated at the base of the large
+  median ventral tentacles. Others are situated at the base of the tentacular ridge. Further,
+  although this extraordinary change takes place in the <span class="pagenum"
+  id="page288">{288}</span>peripheral organ, no marked difference occurs in the arrangement of the
+  nerves issuing from the trigeminal motor centre, no new nerves are formed to supply the new
+  muscles, but every motor nerve-fibre and the motor cell from which it arises increases enormously
+  in size, and these giant nerve-fibres thus formed split into innumerable filaments corresponding
+  with the proliferation of the muscular elements.</p>
+
+  <p>The clue, then, to the origin of the suctorial apparatus and of the nature of the original
+  organs supplied by the trigeminal is afforded in this case, as in all other similar inquiries, by
+  the central nervous system and its outgoing nerves. Here is always the citadel, the fixed seat of
+  government, here is 'headquarters,' from which the answers to all our inquiries must
+  originate.</p>
+
+  <div class="ac w50 fcenter sp3">
+    <a href="images/fig114.jpg" id="fig114"><img style="width:100%" src="images/fig114.jpg" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 114.&mdash;Distribution of Trigeminal Nerve in
+      Ammoc&#x0153;tes.</span></p>
+      <p class="sp0"><i>ps. br.</i>, pseudo-branchial groove; <i>met.</i>, nerve to lower lip, or
+      metastomal nerve; <i>t.</i>, nerve to tongue; <i>tent.</i>, nerve to tentacles. The mandibular
+      and internal maxillary nerves are coloured red; the purely sensory nerves to the external
+      surface are coloured black.</p>
+    </div>
+  </div>
+
+  <p class="ac"><span class="sc">The Trigeminal Nerve of Ammoc&#x0153;tes.</span></p>
+
+  <p>Striking is the answer. In Fig. <a href="#fig114">114</a>, Miss Alcock has drawn the
+  distribution of the trigeminal nerve as traced by her through a series of sections. It arises, as
+  is well known, from two separate ganglia, of which the foremost gives rise to a purely cutaneous
+  nerve, the ophthalmic nerve, and the hindmost to three nerves, the most posterior of which is
+  purely cutaneous and passes tailwards over the ventral branchial region, as shown in the figure;
+  the other two nerves, both <span class="pagenum" id="page289">{289}</span>of which contain motor
+  fibres, are called by Hatschek the mandibular and maxillary nerves. Of these the mandibular or
+  velar nerve (<i>met.</i>) is a large, conspicuous nerve, which arises so separately from the rest
+  of the trigeminal as almost to deserve the title of a separate nerve. When it leaves the large
+  posterior ganglion, it passes into the anterior part of the velum, runs along with the tubular
+  muscles, which it supplies, to the ventral surface as far as the junction of the lower lip with
+  the thyroid plate, and has not been followed further by Hatschek. Miss Alcock, however, by means
+  of serial sections, has traced it further, and shown that at this point it turns abruptly
+  headwards to terminate in the muscles of the lower lip. If, then, as suggested, the lower lip
+  represents the metastoma&mdash;the last pair of prosomatic appendages&mdash;then this mandibular
+  or velar nerve represents that segmental nerve.</p>
+
+  <p>The other nerve&mdash;the maxillary nerve of Hatschek&mdash;which constitutes the larger part
+  of the trigeminal, passes forwards from the ganglion, and at a point somewhere about the anterior
+  region of the eyeball, divides into two, an external (<i>black</i> in Fig. <a
+  href="#fig114">114</a>) and an internal (<i>red</i> in Fig. <a href="#fig114">114</a>) nerve. The
+  external branch is apparently entirely sensory, and supplies the external surfaces of the upper
+  and lower lips. The internal branch is mainly motor, and supplies the muscles of the upper lip; it
+  contains also the nerves of the tentacles.</p>
+
+  <p>The nerve to the median ventral tentacle (<i>t.</i>) or tongue leaves the internal division of
+  the maxillary immediately after its separation from the external; it runs ventralwards, and at the
+  same time passes internally until it reaches a position between the muco-cartilage and the
+  epithelium lining the cavity of the throat. It then turns, and passing posteriorly (towards the
+  tail) to the point where the median ventral tentacle is attached to the lower lip, it supplies
+  some very rudimentary-looking muscles which run from the tentacle to the adjoining surface, and no
+  doubt serve to move the tentacle from side to side. A portion of the nerve still continues to run
+  along the side of the median ventral ridge, as far back as the point where the muscles of the
+  hyoid segment pass round to the ventral side between the velum and the thyroid; in fact, this
+  small nerve passes along the whole length of the median ventral ridge.</p>
+
+  <p>This description shows that the trigeminal nerve divides itself into two groups: the one
+  represented black in the figure, which is purely cutaneous and sensory, corresponding, in the
+  main, according <span class="pagenum" id="page290">{290}</span>to my theory, to the epimeral
+  nerves of Limulus; the other coloured red, which supplies muscles belonging to the visceral or
+  splanchnic muscle-group, and contains also the nerves to the tentacles.</p>
+
+  <p>This latter group, which is formed by two distinct well-defined nerves, viz. the mandibular and
+  the internal branch of the maxillary, corresponds, according to my theory, to the amalgamated
+  nerves of the prosomatic appendages, and is clearly divisible into three distinct
+  nerves&mdash;</p>
+
+  <p>1. The lower lip-nerve or the metastomal nerve (<i>met.</i>).</p>
+
+  <p>2. The tongue-nerve (<i>t.</i>).</p>
+
+  <p>3. The nerve (<i>tent.</i>) to the upper lip and tentacles.</p>
+
+  <p>Of these three pairs of nerves it is suggested that the first pair were derived from the nerves
+  to the metastomal appendage. The second pair of nerves ought, on this theory, originally to have
+  supplied the pair of appendages immediately in front of the metastoma&mdash;that is, the pair of
+  ectognaths, and therefore the ventral pair of tentacles, known as the tongue, would represent the
+  last remnant of these ectognaths. Similarly, the other tentacles would represent the endognaths,
+  and therefore the third pair of nerves would represent the fused nerves to these concentrated
+  endognaths, which, in the Eurypterids, stand aloof from the ectognaths.</p>
+
+  <p class="sp3">Let us consider these three propositions separately. In the first place, have we
+  any right to attribute segmental value to the mandibular nerve? What evidence is there of segments
+  in this region in Ammoc&#x0153;tes?</p>
+
+  <p class="ac"><span class="sc">The Segment of the Lower Lip, or Metastomal Segment.</span></p>
+
+  <p>We have seen that in the branchial or mesosomatic region the segments corresponding to the
+  mesosomatic appendages were mapped out by means of their supporting or skeletal structures, their
+  segmental muscles, and their nervous arrangements, as well as by the arrangement of the branchiæ.
+  Similarly, the segments in front of the branchial region, corresponding to the prosomatic
+  appendages, ought to be definable by the same means, although, owing to the absence of branchiæ
+  and the greater concentration in this region, the separate segments would probably not be so
+  conspicuous.</p>
+
+  <p>The last segment considered was the segment belonging to the VIIth nerve corresponding to the
+  opercular appendages of the <span class="pagenum" id="page291">{291}</span>Eurypterid. The segment
+  immediately in front of this is the next for consideration, viz. that corresponding to the
+  chilarial appendages or metastoma; and as the basal part of this pair of appendages was fused with
+  the basal part of the operculum, the one cannot be discussed without the other; therefore, the
+  segment to which the lower lip belongs must be considered in connection with and not apart from
+  the thyro-hyoid segments already dealt with.</p>
+
+  <p>In Chapter V., p. <a href="#page188">188</a>, I stated that the supporting bars of the foremost
+  mesosomatic segments, the thyro-hyoid segments, differed from the cartilaginous bars of the
+  branchial segments, in that they were composed of muco-cartilage. Also in addition to the
+  muco-cartilaginous skeletal bars, a ventral plate of muco-cartilage exists in Ammoc&#x0153;tes
+  which covers over the thyroid gland.</p>
+
+  <p>Similarly in the prosomatic segments the skeletal bars are composed of muco-cartilage and the
+  ventral plate of muco-cartilage continues forward as the plate of the lower lip. It is of special
+  interest, in connection with the segments indicated by such supporting structures, to find that
+  this special tissue is entirely confined to the head-region, and disappears absolutely at
+  transformation, thus indicating the ancestral nature of the segments marked out by its
+  presence.</p>
+
+  <p>This muco-cartilaginous skeleton is the key to the whole position, and requires, therefore, to
+  be understood. It is of great importance, not only because it demonstrates the position of the
+  segments in Ammoc&#x0153;tes which characterized its invertebrate ancestor, but also because it
+  possesses a structure remarkably similar to that found in the head-plates of the most ancient
+  fishes. For the present I will confine myself to the consideration of this muco-cartilaginous
+  skeleton as evidence of the relationship of Ammoc&#x0153;tes to the Eurypterids, and in the next
+  chapter will show how absolutely the same skeleton corresponds to that of the Cephalaspidæ, so
+  that Ammoc&#x0153;tes is really a slightly modified Cephalaspid, the larval form of which was
+  Eurypterid in character.</p>
+
+  <div><span class="pagenum" id="page292">{292}</span></div>
+
+  <div class="ac w50 fcenter sp2">
+    <a href="images/fig115.jpg" id="fig115"><img style="width:88%" src="images/fig115.jpg" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 115.&mdash;Dorsal half of Head-region of
+      Ammoc&#x0153;tes.</span></p>
+      <p class="sp0"><i>Tr.</i>, trabeculæ; <i>Pit.</i>, pituitary space; <i>Inf.</i>, infundibulum;
+      <i>Ser.</i>, median serrated flange of velar folds.</p>
+    </div>
+  </div>
+
+
+  <div><span class="pagenum" id="page293">{293}</span></div>
+
+  <div class="ac w40 fcenter">
+    <a href="images/fig116.jpg" id="fig116"><img style="width:100%" src="images/fig116.jpg" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 116.&mdash;Horizontal Section through the Anterior Part of
+      Ammoc&#x0153;tes, immediately Ventrally to the Auditory Capsule.</span></p>
+      <p class="sp0"><i>sk<sub>1</sub></i>-<i>sk<sub>5</sub></i>, skeletal bars;
+      <i>m<sub>1</sub></i>-<i>m<sub>5</sub></i>, striated visceral muscles;
+      <i>mt<sub>1</sub></i>-<i>mt<sub>4</sub></i>, tubular muscles;
+      <i>br<sub>1</sub></i>-<i>br<sub>3</sub></i>, branchiæ; <i>tr.</i>, trabeculæ; <i>inf.</i>,
+      infundibulum; <i>ped.</i>, pedicle; <i>V.</i>, trigeminal nerve. Muco-cartilage, <i>red</i>;
+      soft cartilage, blue; hard cartilage, purple.</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page294">{294}</span></div>
+
+  <div class="ac w50 fcenter sp2">
+    <a href="images/fig117.jpg" id="fig117"><img style="width:100%" src="images/fig117.jpg" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 117.&mdash;Sagittal Lateral Section through the Anterior Part of
+      Ammoc&#x0153;tes.</span></p>
+      <p class="sp0">Lettering and colouring same as in Fig. <a href="#fig116">116</a>. <i>aud.</i>,
+      auditory capsule; <i>j.v.</i>, jugular vein.</p>
+    </div>
+  </div>
+
+  <p>In Chapter IV., Figs. 63, 64, I have given a representation of the ventral and dorsal views of
+  an Ammoc&#x0153;tes cut in half horizontally. Such a section shows with great clearness the series
+  of branchial appendages with their segmental muscles and cartilaginous bars which form the
+  branchial segments innervated by the IXth and Xth nerves, according to my view of the branchial
+  unit. As is seen (Fig. 64 or 115), the skeletal bar of the hyoid or opercular appendage, which is
+  clearly serially homologous with the other branchial bars, is composed of muco-cartilage, and not
+  of cartilage. If we follow this series of horizontal sections nearer to the origin of the
+  cartilaginous bars from the sub-chordal cartilaginous rod on each side of the notochord, we obtain
+  a picture, as in Fig. <a href="#fig116">116</a>, in which each branchial segment is defined by the
+  section of the branchial cartilaginous bar (<i>sk<sub>4</sub></i>, <i>sk<sub>5</sub></i>), by the
+  section of the separate branchiæ (<i>br<sub>2</sub></i>, <i>br<sub>3</sub></i>), and by the
+  separate segmental muscles arranged round each bar, these muscles being partly ordinary striated
+  (<i>m<sub>4</sub></i>, <i>m<sub>5</sub></i>), partly tubular (<i>mt<sub>3</sub></i>,
+  <i>mt<sub>4</sub></i>). The uppermost of these branchial segments shows the same arrangement;
+  (<i>sk<sub>3</sub></i>) is the branchial skeletal bar, which is now composed of muco-cartilage,
+  not cartilage; (<i>br<sub>1</sub></i>) is the branchiæ in the same situation as the others, but
+  here composed of glandular rather than of respiratory epithelium, while the ordinary striated
+  branchial muscles of this segment are marked as (<i>m<sub>3</sub></i>), being separated from the
+  tubular muscles of the segment (<i>mt<sub>2</sub></i>), owing to the large size of the blood-space
+  in which these latter muscles are lying. In front of this segment so defined we see again another
+  well-marked skeletal bar (<i>sk<sub>2</sub></i>) of muco-cartilage, evidently indicating a similar
+  segment anterior to the hyoid segment. In connection with this bar there are no branchiæ, but
+  again we see two sets of visceral muscles, the one ordinary striated, marked
+  (<i>m<sub>2</sub></i>), and the other tubular, marked (<i>mt<sub>1</sub></i>). Here, then, the
+  section indicates the existence of a segment of the same character as the posteriorly situated
+  branchial segments but belonging to a non-branchial region&mdash;a segment which would represent a
+  non-branchial appendage, the last, therefore, of the prosomatic appendages. Let us, then, follow
+  <span class="pagenum" id="page295">{295}</span>out these two segmental muco-cartilaginous bars and
+  their attendant muscles, and see to what sort of segments their investigation leads.</p>
+
+  <p>The bar which comes first for consideration (<i>sk<sub>3</sub></i>) arises immediately behind
+  the auditory capsule from the first branchial cartilage very soon after it leaves the sub-chordal
+  cartilaginous ligament; the soft cartilage of the sub-chordal ligament ceases abruptly in its
+  extension along the notochord at the place where the hard cartilage of the parachordal joins it,
+  and in a sense it may be said to leave the notochord at this place and pass into the basal part of
+  the first branchial bar. The most anterior continuation of this branchial system is this
+  muco-cartilaginous bar (<i>sk<sub>3</sub></i>), which passes forward and ventralwards, being
+  separated from the axial line by the auditory capsule (<i>cf.</i> Fig. 118, A, B, C). Its position
+  is well seen in a sagittal section, such as Fig. <a href="#fig117">117</a>. It follows absolutely
+  the line of the pseudo-branchial groove (<i>ps. br.</i>, Fig. <a href="#fig114">114</a>), and
+  ventrally joins the plate of muco-cartilage which covers the thyroid gland. It forms a thickened
+  border to this plate anteriorly, just as the branchial cartilaginous bars border it posteriorly.
+  In fact, it behaves with respect to the hyoid segment in a manner similar to the rest of the
+  cartilaginous bars with respect to their respective segments.</p>
+
+  <p>It represents, although composed of muco-cartilage, the cartilaginous bar of the operculum in
+  Limulus, which also forms the termination of the branchial cartilaginous system, as fully
+  explained in Chapter III.; it may therefore be called the opercular bar.</p>
+
+  <p>The next bar (<i>sk<sub>2</sub></i>) is extremely interesting, as we are now out of the
+  branchial or mesosomatic region, and into the region corresponding to the prosoma. It starts from
+  a cartilaginous projection made of hard cartilage, just in front of the auditory capsule, called
+  by Parker the 'pedicle of the pterygoid'&mdash;a projection (<i>ped.</i>) which defines the
+  posterior limit of the trabeculæ on each side, where they join on to the parachordals,&mdash;and
+  winding round and below the auditory capsule, joins the opercular bar (<i>cf.</i> Fig. <a
+  href="#fig118">118</a>), to pass thence into and form part of the muco-cartilaginous plate of the
+  lower lip. In the section figured (Fig. <a href="#fig116">116</a>), this projection of hard
+  cartilage is not directly continuous with (<i>sk<sub>2</sub></i>), owing to a slight curvature in
+  the bar; the next few sections show clearly the connection between (<i>ped.</i>) and
+  (<i>sk<sub>2</sub></i>), and consequently the complete separation by means of this bar of the
+  hyoid segment from the segment in front.</p>
+
+  <div><span class="pagenum" id="page296">{296}</span></div>
+
+  <div class="ac w50 fcenter sp2">
+    <a href="images/fig118.jpg" id="fig118"><img style="width:100%" src="images/fig118.jpg" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 118.&mdash;Skeleton of Head-Region of Ammoc&#x0153;tes. A,
+      Lateral View; B, Ventral View; C, Dorsal View.</span></p>
+      <p class="sp0">Muco-cartilage, <i>red</i>; soft cartilage, <i>blue</i>; hard cartilage,
+      <i>purple</i>. <i>sk<sub>1</sub></i>, <i>sk<sub>2</sub></i>, <i>sk<sub>3</sub></i>, skeletal
+      bars; <i>c.e.</i>, position of pineal eye; <i>na. cart.</i>, nasal cartilage; <i>ped.</i>,
+      pedicle; <i>cr.</i>, cranium; <i>nc.</i>, notochord.</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page297">{297}</span></div>
+
+  <p>In the figures, the hard cartilage is coloured purple, the soft cartilage blue, and the
+  muco-cartilage red, so that the position of this bar is well shown. This bar may be looked upon as
+  bearing the same relation to the muco-cartilaginous plate of the lower lip as the opercular bar
+  does to the muco-cartilaginous plate over the thyroid; and seeing that these two plates form one
+  continuous ventral head-shield of muco-cartilage (Fig. <a href="#fig118">118</a>, B), and also
+  that this bar fuses with the opercular bar, we may conclude that the segment represented by the
+  lower lip is closely connected with the hyoid or opercular segments. In other words, if the lower
+  lip arose from the metastoma, then this pair of skeletal bars might be called the metastomal bars,
+  which formed the supporting skeleton of the last pair of prosomatic appendages and, as is likely
+  enough, arose in connection with the posterior lateral horns of the plastron; these posterior
+  lateral horns, like the rest of the plastron, would give rise to hard cartilage, and so form in
+  Ammoc&#x0153;tes the two lateral so-called pterygoid projections.</p>
+
+  <p>In the branchial region the muscles which marked out each branchial segment were of two
+  kinds&mdash;ordinary striated visceral muscles and tubular muscles. Of these the former
+  represented the dorso-ventral muscles of the branchial appendages, while the latter formed a
+  separate group of dorso-ventral muscles with a separate innervation which may have been originally
+  the segmental veno-pericardial muscles so characteristic of Limulus and the scorpions. In Figs.
+  116, 117, the grouping of these muscles in each branchial segment is well shown, and it is
+  immediately seen that the hyoid segment possesses its group of striated visceral muscles
+  (<i>m<sub>3</sub></i>) supplied by the VIIth nerve in the same manner as the posterior groups, as
+  has already been pointed out by Miss Alcock in her previous paper. Passing to the segment in
+  front, Fig. <a href="#fig116">116</a> shows that the group of visceral muscles
+  (<i>m<sub>2</sub></i>) corresponds in relative position with respect to the metastomal bar to the
+  hyoid muscles with respect to the opercular bar or to the branchial visceral muscles with respect
+  to each branchial bar. What, then, is this muscular group? The series of sections show that these
+  are the dorso-ventral muscles belonging to the lower lip, which, as seen in Fig. <a
+  href="#fig119">119</a> (<i>M.</i>), form a well-marked muscular sheet, whose fibres interlace
+  across the mid-ventral line of the lower lip. This group of lower lip-muscles is very suggestive,
+  for these muscles arise, not from the trabeculæ, but from the front dorsal region of the cranium,
+  just in front of the two lateral <span class="pagenum" id="page298">{298}</span>eyes. In Fig. <a
+  href="#fig117">117</a> the dorsal part is seen cut across on its way to its dorsal attachment.
+  Such an origin is reminiscent of the tergo-coxal group of muscles, arising, as they do, from the
+  primordial cranium and the tergal carapace, and suggests at once that when the chilarial
+  appendages expanded to form a metastoma, their tergo-coxal muscles formed a sheet of muscles
+  similar to those of the lower lip of Ammoc&#x0153;tes, by which the movements of the metastoma
+  were effected. The posterior limit of these muscles ventrally marks out the junction of the
+  segment of the lower lip with that of the thyroid; in other words, indicates where the metastoma
+  had fused ventrally with the operculum (Fig. <a href="#fig117">117</a>).</p>
+
+  <div class="ac w20 fcenter sp2">
+    <a href="images/fig119.png" id="fig119"><img style="width:100%" src="images/fig119.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 119.&mdash;Ventral View Of Head-Region of
+      Ammoc&#x0153;tes.</span></p>
+      <p class="sp0"><i>Th.</i>, thyroid gland; <i>M.</i>, lower lip, with its muscles.</p>
+    </div>
+  </div>
+
+  <p>Besides the striated visceral muscles, each branchial segment possesses its own tubular
+  muscles, shown in Fig. <a href="#fig116">116</a> (<i>mt<sub>3</sub></i>) and
+  (<i>mt<sub>4</sub></i>). As the section shows, there is clearly a group of tubular muscle-fibres
+  belonging to the hyoid segment (<i>mt<sub>2</sub></i>), and also another group belonging to the
+  segment in front of the hyoid (<i>mt<sub>1</sub></i>); so that, judging from this section, each of
+  these segments possesses its own tubular musculature just as do the branchial segments, the
+  difference being that the tubular muscles are more separated from the striated visceral group than
+  in the true branchial segments, owing to the size of the blood-spaces surrounding them. What,
+  then, are these two groups of muscles? Tracing them in the series of sections, both groups are
+  seen to belong to the system of velar muscles, forming an anterior and a posterior group
+  respectively; and we see, further, that there is not the slightest trace of any tubular muscles
+  anterior to these muscles of the velum.</p>
+
+  <p>In the living Ammoc&#x0153;tes the velar folds on each side can be seen <span class="pagenum"
+  id="page299">{299}</span>to move synchronously with the movements of respiration, contracting at
+  each expiration, and thus closing the slit by which the oral and respiratory chambers communicate,
+  and so forcing the waters of respiration through the gill-slits, as described by Schneider. Such a
+  fact is clear evidence that these tubular muscles of the velar folds belong to the same series as
+  the tubular muscles of the branchial segments, so that if, as I have already suggested, the latter
+  muscles were originally the veno-pericardial muscles of segments corresponding to the branchial
+  appendages, then the former would represent the veno-pericardial muscles of the segments
+  corresponding to the opercular and metastomal appendages. What, then, are these velar folds, and
+  how is it that the tubular muscles of these two segments become the velar muscles? I will
+  consider, in the first instance, the posterior group of muscles (<i>mt<sub>2</sub></i>) in Fig. <a
+  href="#fig116">116</a>.</p>
+
+  <p>It has already been pointed out that the tubular muscles of the branchial segments are
+  dorso-ventral, but do not run with the ordinary constrictors, having separate attachments and
+  running part of their course internally to and part externally to the ordinary constrictors. At
+  first sight, as is usually stated, the hyoid segment does not appear to possess tubular muscles at
+  all. If, however, we follow the posterior group of velar muscles (<i>mt<sub>2</sub></i>), we see
+  (Fig. <a href="#fig117">117</a>) that they pass between the auditory capsule and the opercular bar
+  (<i>sk<sub>3</sub></i>) of muco-cartilage to reach the region of the jugular vein (<i>j.v.</i>)
+  posteriorly to the auditory capsule, so that their dorsal origin bears the same relation to the
+  hyoid segment as the dorsal attachment of the rest of the tubular muscles to their respective
+  segments. Further, these muscles run along the length of the velar fold, and are attached
+  ventrally on each side of the thyroid gland, so that their ventral attachment also corresponds in
+  position, as regards the hyoid segment, with the ventral attachment of the rest of the tubular
+  muscles as regards their respective segments.</p>
+
+  <p>This ventral attachment is shown in Fig. <a href="#fig119">119</a> on each side of the thyroid,
+  and in Fig. <a href="#fig120">120</a> (<i>mt<sub>2</sub></i>); while in Fig. <a
+  href="#fig117">117</a> the fibres are seen converging to this ventral position. In other words,
+  this large posterior muscle of the velar folds is a dorso-ventral muscle, and would actually take
+  the same position in the hyoid segment as the dorso-ventral tubular muscles in the other branchial
+  segments, if the velum were put back into its original position as the septum terminating the
+  branchial chamber. Conversely, the presence of these <span class="pagenum"
+  id="page300">{300}</span>hyoid tubular muscles in the velum gives evidence that the opercular
+  segment takes part in the formation of the septum, as already suggested.</p>
+
+  <p>Miss Alcock, in her paper, speaks of tubular muscles belonging to the hyoid segment, which are
+  attached to the muco-cartilage. Schaffer also speaks of certain tubular muscles belonging to the
+  velar group as piercing the muco-cartilage (<i>h. r. s.</i>) in his figures 24 and 25, <i>i.e.</i>
+  the metastomal bar, near its junction with the opercular bar. In my specimens there is a distinct
+  group of tubular muscles which pierce the opercular bar of muco-cartilage at its junction with the
+  metastomal bar, and pass into the posterior group of velar muscles. They clearly belong to the
+  hyoid segment, as Miss Alcock supposed, but are not attached to the muco-cartilage. It is possible
+  that they represent a different group to those already considered, and suggest the possibility
+  that this opercular or thyro-hyoid segment is double with respect to its original veno-pericardial
+  muscles as well as in other respects.</p>
+
+  <p>The anterior group of tubular muscles (<i>mt<sub>1</sub></i>, Figs. 116, 117) belonging to the
+  same segment as the metastomal bar must now be taken into consideration. Very different is their
+  origin to that of the posterior group: they arise close up against the eye, and have given rise to
+  Kupffer's and Hatschek's misconception that the superior oblique muscle of the eye arises from a
+  part of the velar musculature. Naturally, as Neal has pointed out, they have nothing to do with
+  the eye-muscles; the superior oblique muscle is plainly in its true place entirely apart from
+  these velar muscles, which form the foremost group of the segmental tubular muscles. They pass
+  into the anterior part of the velar folds and run round to the ventral side just in the same way
+  as does the posterior group. This anterior group of tubular muscles represents the
+  veno-pericardial muscle of the segment immediately in front of the opercular, <i>i.e.</i> the
+  metastomal segment, and is the foremost of these veno-pericardial muscles. Its presence shows that
+  the velar folds, formed as they were by the breaking down of the septum, are in reality part of
+  two segments, viz. the opercular and the metastomal, which have fused together in their basal
+  parts, and by such fusion have caused the inter-relationship between the VIIth and Vth nerves, so
+  apparent in the anatomy of the vertebrate cranial nerves.</p>
+
+  <p>A further piece of evidence that this anterior portion of the velum <span class="pagenum"
+  id="page301">{301}</span>belongs to the same segment as the lower lip is the fact that in addition
+  to the tubular muscles a single ordinary striated muscle is found in the velum which, like the
+  muscles of the lower lip, is innervated by this same mandibular nerve.</p>
+
+  <p>This muscle is attached laterally to the muco-cartilage of the metastomal bar
+  (<i>sk<sub>2</sub></i>) at its junction with the muco-cartilage of the lower lip, and spreads out
+  into a number of strands which are attached at intervals along the whole length of the free
+  anterior edge of the velum. It is the only non-tubular muscle belonging to the velum, and by its
+  contraction it draws the anterior portions of the velar folds apart from each other, and so opens
+  the slit between them, through which the food and mud must pass. Clearly from its position it does
+  not belong to the original tergo-coxal group of muscles as do those of the lower lip; it must have
+  been one of the intrinsic muscles of the metastoma itself.</p>
+
+  <p>This anterior portion of the velar folds affords yet another striking hint of the correctness
+  of my comparison of the lower lip segment of Ammoc&#x0153;tes with the chilaria of Limulus or the
+  metastoma of Eurypterus; for the most dorsal anterior portion, which at its attachment possesses a
+  wedge of muco-cartilage, forms a separate, well-defined, rounded basal projection marked
+  <i>Ser.</i> in Fig. <a href="#fig115">115</a>, and <i>B</i> in the accompanying Fig. <a
+  href="#fig120">120</a>. This is that part of the velar folds which comes together in the middle
+  line and closes the entrance into the respiratory chamber. The epithelial surface here is most
+  striking and suggestive, for it is markedly serrated, being covered with a large number of
+  closely-set projections or serræ. The serration of the surface here is of so marked a character
+  that Langerhans considered this part of the velar folds to act as a masticating organ, grinding
+  and rasping the food and mud which passed through the narrow slit. In fact, Langerhans supposed
+  that this portion of the velum acted in a manner closely resembling the action of the gnatho-bases
+  of the prosomatic appendages in Limulus or the Eurypteridæ.</p>
+
+  <p>This suggestion of Langerhans is surely most significant, considering that this somewhat
+  separate portion of the velum, to which he assigns such a function, is in the very place where the
+  gnathite portion of the metastomal appendages would have been situated if it were true that the
+  lower lip and anterior portion of the velum of Ammoc&#x0153;tes were derived from the
+  metastoma.</p>
+
+  <p>In addition to this marked serrated edge the whole surface of <span class="pagenum"
+  id="page302">{302}</span>the anterior portion of the velum is covered over with a scale-like or
+  tubercular pattern remarkably like the surface-ornamentation seen in many of the members of the
+  ancient group Eurypteridæ. In Fig. <a href="#fig121">121</a> I give a picture of this
+  surface-marking of the velum. It is striking to see that just as in the case of the invertebrate
+  this marking and these serræ are formed simply by the cuticular surface of the epithelial cells; a
+  surface which, according to Wolff, possibly contains chitin. The interpretation which I would give
+  of the velar folds is therefore as follows<span class="wnw">:&mdash;</span></p>
+
+  <p>They represent the fused basal parts of the opercular and metastomal appendages, the
+  gnatho-bases of the latter still retaining in a reduced degree their rasping surfaces, because,
+  owing to their position on each side of the opening into the respiratory chamber they were still
+  able to manipulate the food as it passed by them after the closure of the old mouth.</p>
+
+  <div class="ac w50 fcenter sp2">
+    <a href="images/fig120.png" id="fig120"><img style="width:66%" src="images/fig120.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 120.&mdash;Ammoc&#x0153;tes cut open in Mid-Ventral Line
+      to show Position of Velum; Velar Folds removed on one side.</span></p>
+      <p class="sp0"><i>tr.</i>, trabeculæ; vel., velum; <i>B.</i>, anterior gnathic portion of
+      velum; <i>ps. br.</i>, pseudo-branchial groove; <i>m<sub>2</sub></i>, muscles of lower lip
+      segment; <i>m<sub>3</sub></i>, muscles of thyro-hyoid segment; <i>mt<sub>2</sub></i>,
+      insertion of tubular muscles of velum near thyroid.</p>
+    </div>
+  </div>
+
+  <div class="ac w15 fcenter sp2">
+    <a href="images/fig121.png" id="fig121"><img style="width:100%" src="images/fig121.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="sp0"><span class="sc">Fig. 121.&mdash;Surface View of Anterior Surface of
+      Velum.</span></p>
+    </div>
+  </div>
+
+  <p>The whole evidence points irresistibly to the conclusion that the mandibular or velar nerve of
+  the trigeminal does supply a splanchnic <span class="pagenum" id="page303">{303}</span>segment
+  which is, in all respects, comparable with the segments supplied by the facial, glossopharyngeal,
+  and vagus nerves, except that it does not possess branchiæ. This simply means that the appendages
+  which these nerves originally supplied were prosomatic, not mesosomatic, and corresponded,
+  therefore, to the chilarial or metastomal appendages.</p>
+
+  <p class="sp3">A comparison of the ventral surface of Slimonia, as given in Fig. <a
+  href="#fig8">8</a>, p. <a href="#page27">27</a>, with that of Ammoc&#x0153;tes (Fig. <a
+  href="#fig119">119</a>), when the thyroid gland and lower lip muscles have been exposed to view,
+  enables the reader to recognize at a glance the correctness of this conclusion.</p>
+
+  <p class="ac"><span class="sc">The Tentacular Segments and the Upper Lip.</span></p>
+
+  <p>Anterior to this metastomal segment, Fig. <a href="#fig116">116</a> shows a group of visceral
+  muscles, <i>m<sub>1</sub></i>, and yet again a muco-cartilaginous bar, <i>sk<sub>1</sub></i>, but,
+  as already stated, no tubular muscles. These visceral muscles indicate the presence in front of
+  the lower lip-segment of one or more segments of the nature of appendages. The muscles in question
+  (<i>m<sub>1</sub></i>) are the muscles of the upper lip, the skeletal elements form a pair of
+  large bars of muco-cartilage (<i>sk<sub>1</sub></i>), which start from the termination of the
+  trabeculæ, and pass ventralwards to fuse with the muco-cartilaginous plate of the lower lip (Figs.
+  117 and 118). This large bar forms the tentacular ridge on each side, and gives small projections
+  of muco-cartilage into each tentacle. In addition to this tentacular bar, a special bar of
+  muco-cartilage exists for the fused pair of median tentacles, the so-called tongue, which extends
+  in the middle line along the whole length of the lower lip, being separated from the
+  muco-cartilaginous plate of the lower lip by the muscles of the lower lip. This tongue bar of
+  muco-cartilage joins with the muco-cartilage of the lower lip at its junction with the thyroid
+  plate, and also with the tentacular bar just before the latter joins the muco-cartilaginous plate
+  of the lower lip. This arrangement of the skeletal tissue suggests that the pair of tentacles
+  known as the tongue stand in a category apart from the rest of the tentacles; a suggestion which
+  is strongly confirmed by the separate character of its nerve-supply, as already mentioned.</p>
+
+  <p>For three reasons, viz. the separateness both of their nerve-supply and of their skeletal
+  tissue, and the importance they assume at transformation, this pair of ventral tentacles must, it
+  seems to me, be put <span class="pagenum" id="page304">{304}</span>into a separate category from
+  the rest of the tentacles. On the other hand, the innervation of the rest of the tentacles by a
+  single nerve which sends off a branch as it passes each one, together with the concentration of
+  their skeletal elements into a single bar, with projections into each tentacle, points directly to
+  the conclusion that these tentacles must be considered as a group, and not singly.</p>
+
+  <p>I suggest that these tentacles are the remains of the ectognaths and endognaths; the tongue
+  representing the two ectognaths, and the four tentacles on each side the four pairs of
+  endognaths.</p>
+
+  <p>As we see, this method of interpretation attributes segmental value to the tentacles, a
+  conclusion which is opposed to the general opinion of morphologists, who regard them as having no
+  special morphological importance, and certainly no segmental value. On the other hand, the
+  importance of the pair of ventral tentacles, the 'tongue' of Rathke, which lie in the mid-line of
+  the lower lip, has been shown by Kaensche, Bujor, and others, all of whom are unanimous in
+  asserting that at transformation they are converted into that large and important organ the piston
+  or tongue of the adult Petromyzon. It is supposed that the rest of the tentacles vanish at
+  transformation, being absorbed; they appear to me rather to take part in the formation of the
+  sucking-disc, so that I am strongly inclined to believe that the whole of the remarkable suctorial
+  apparatus of Petromyzon is derived from the tentacles of Ammoc&#x0153;tes. In other words, on my
+  view, a conversion of the prosomatic appendages into a suctorial apparatus takes place at
+  transformation, just as is frequently the case among the Arthropoda.</p>
+
+  <p>It is to the arrangement of the muscles that we look for evidence of segmental value. As long
+  as it was possible to look upon these tentacles as mere sensory feelers round the mouth entrance,
+  it was natural to deny segmental value to them. Matters are now, however, totally different since
+  Miss Alcock's discovery of the rudimentary muscles at the base of the tentacles and their
+  development at transformation. If these muscles represent some of the appendage muscles belonging
+  to the foremost prosomatic segments just as the ocular muscles represent the dorso-ventral somatic
+  muscles of those same segments, then we may expect ultimately to be able to give as good evidence
+  of segmentation in their case as I have been able to give in the case of these latter muscles; for
+  the two sets of muscles are curiously alike, seeing that the eye-muscles do not develop until
+  <span class="pagenum" id="page305">{305}</span>transformation, but throughout the Ammoc&#x0153;tes
+  stage remain in almost as rudimentary a condition as the tentacular muscles.</p>
+
+  <p>Another difficulty with respect to the tentacles is the determination of the number of them,
+  owing to the fact that in addition to what may be called well-defined tentacles a large number of
+  smaller tactile projections are found on the surface of the upper lip, as is seen in Fig. <a
+  href="#fig115">115</a>. In the very young condition, 7 or 8 mm. in length, it is easier to make
+  sure on this point. At this stage they may be spoken of as arranged in two groups: an anterior
+  small group and a posterior larger group. The anterior group consists of a pair of very small
+  tentacles and a very small median tentacle, all three situated quite dorsally in the front part of
+  the upper lip. The posterior group, which is separate from the anterior, consists of five pairs of
+  much larger tentacles, the most ventral pair in the mid-line ventrally on the lower lip being
+  fused together to form the large ventral median tentacle or tongue already mentioned. This pair,
+  according to Shipley, is markedly larger than the others. There are, therefore, five conspicuous
+  tentacles on each side, and in front of them a smaller pair and a small median dorsal one. In the
+  very young condition the accessory projections above-mentioned are not present, or at all events
+  are not conspicuous, and the tentacles are also markedly larger in comparison to the size of the
+  animal than in the older condition, where they have distinctly dwindled.</p>
+
+  <p>This posterior group of five conspicuous tentacles is the one which I suggest represents the
+  four endognaths and one ectognath. What the significance of the small anterior group is, I know
+  not. It is possible that the cheliceræ are represented here, for they are situated distinctly
+  anterior to the other group; I know, however, of no sign of a markedly separate innervation to
+  these most dorsal tentacles such as I should have expected to find if they represented the
+  cheliceræ.</p>
+
+  <p>The muscles of the upper lip, which distinctly belong to the visceral and not to the somatic
+  musculature, form part of the foremost segments, and in these muscles the tentacular nerve reaches
+  its final destination. From their innervation, then, they must have belonged to the same
+  appendages as the tentacles supplied by the tentacular nerve, <i>i.e.</i> to the endognaths. What
+  conclusion can we form as to the probable origin of the upper lip of Ammoc&#x0153;tes? Since the
+  oral chamber was formed by the forward growth of the metastoma, <i>i.e.</i> the lower lip of
+  Ammoc&#x0153;tes, it follows that the upper <span class="pagenum" id="page306">{306}</span>lip is
+  the continuation forwards of the original ventral surface of such an animal as Limulus or a member
+  of the scorpion group, where there is no metastoma, and corresponds to the endostoma, as Holm
+  calls it, of Eurypterus. This termination of the ventral surface in all these animals is made up
+  of two parts: (1) Of sternites composing the true median ventral surface of the body, called by
+  Lankester the pro- and meso-sternites; and (2) of the sterno-coxal processes of the foremost
+  prosomatic appendages, called in the case of Limulus gnathites, because they are the main agents
+  in triturating the food previously to its passage into the mouth. In Limulus, a conjoined
+  pro-mesosternite forms the median ventral wall to which the sterno-coxal processes are attached on
+  each side, and in Phrynus and Mygale a well-marked pro-sternite and meso-sternite are present,
+  forming the posterior limit of the olfactory opening. In Buthus and the true scorpions the
+  sterno-coxal processes of the 2nd, 3rd, and 4th prosomatic appendages take part in surrounding the
+  olfactory tubular passage; in Thelyphonus only the processes of the 2nd pair of prosomatic
+  appendages play such a part, the pro-sternite not being present (<i>cf.</i> Fig. <a
+  href="#fig97">97</a>).</p>
+
+  <p>Seeing, then, what a large share the sterno-coxal processes of one or more of these prosomatic
+  appendages plays in the formation of this endostoma, and seeing also that the nerve which supplies
+  the upper lip-muscles in Ammoc&#x0153;tes is the same as that supplying the tentacles which are
+  attached to the upper lip, it appears to me more probable than not that the muscles in question
+  are the vestiges of the sterno-coxal muscles. These muscles differ markedly in their attachments
+  from the muscles of the lower lip, for whereas the latter resemble the tergo-coxal group in their
+  extreme dorsal attachment, the former resemble the sterno-coxal group in their attachment to what
+  corresponds to the endostoma.</p>
+
+  <p>This interpretation of the meaning of the transformation process is in accordance with all the
+  previous evidence both from the side of the palæostracan as from the side of the vertebrate, for
+  it signifies that a dwindling process has taken place in the foremost of the original prosomatic
+  appendages&mdash;the cheliceræ and the endognaths; while, on the contrary, the ectognath and the
+  metastoma have continued to increase in importance right into the vertebrate stage. This process
+  is simply a continuation of what was already going on in the invertebrate stage, for whereas in
+  Eurypterus and other cases <span class="pagenum" id="page307">{307}</span>the cheliceræ and
+  endognaths had dwindled down to mere tentacles, the ectognath was the large swimming appendage,
+  and the metastoma was on the upward grade from the two insignificant chilaria of Limulus.</p>
+
+  <p>The transformation of these foremost appendages into a suctorial apparatus is very common among
+  the arthropods, as is seen in the transformation of the caterpillar into the butterfly, and it is
+  in accordance with the evidence that the main mass of that suctorial apparatus should be formed
+  from appendages corresponding to the ectognath and metastoma rather than from the four endognaths.
+  In all probability the <i>nucleus masticatorius</i> of the trigeminal nerve with its innervation
+  of the great muscles of mastication is evidence of the continued development of the musculature of
+  these two last prosomatic appendages, just as the descending root of the Vth demonstrates the
+  further disappearance of all that belongs to the foremost prosomatic appendages. As yet, however,
+  as far as I know, the musculature of the head-region of Petromyzon has not been brought into line
+  with that of other vertebrates, and until that comparative study has been completed it is
+  premature to discuss the exact position of the masticating muscles of the higher vertebrates.</p>
+
+  <p>The analysis of these tentacular segments belonging to the trigeminal nerve presents greater
+  difficulties than that of any of the other cranial segments, owing to the deficiency of our
+  knowledge of what occurs at transformation. Light is required not only on the origin of the new
+  muscles but also on the origin of the new and elaborate cartilages which are newly formed at this
+  time.</p>
+
+  <p>Miss Alcock has not yet worked out the origin of all these cartilages and muscles, so that we
+  are not yet in a position to analyze the trigeminal supply in Petromyzon into its component
+  appendage elements, an analysis which ought ultimately to enable us to determine from which
+  appendage-muscles the masticating muscles in the higher vertebrates have arisen. As far as the
+  muscles are concerned, she gives me the following information<span class="wnw">:&mdash;</span></p>
+
+  <p>The tongue-nerve supplies in Ammoc&#x0153;tes the rudimentary muscles which pass laterally from
+  the base of the large ventral tentacle to the wall of the throat, and even in Ammoc&#x0153;tes
+  must possess some power of moving that tentacle.</p>
+
+  <p>At transformation these muscles proliferate and develop enormously, and form the bulk of the
+  large basilar muscle which <span class="pagenum" id="page308">{308}</span>surrounds the throat
+  ventrally and laterally, and is the most bulky muscle in the suctorial apparatus.</p>
+
+  <p>The velar or mandibular nerve supplies in Ammoc&#x0153;tes the muscles of the lower lip. In
+  Petromyzon it supplies also the longitudinal muscles of the tongue. The tongue-cartilage first
+  develops in the region of the median ventral tentacle, and there the longitudinal tongue-muscles
+  first begin to develop, not from the rudimentary muscles in the tongue but from those in the lower
+  lip region.</p>
+
+  <p>In Ammoc&#x0153;tes the tentacular nerve supplies the rudimentary muscles in the tentacles and
+  the muscles of the upper lip. The latter disappear entirely at transformation, and in Petromyzon
+  the tentacular nerve supplies the circular, pharyngeal, and annular muscles, which are derived
+  from the rudimentary tentacular muscles.</p>
+
+  <p>For the convenience of my reader I append here a table showing my conception of the manner in
+  which the endognathal and ectognathal segments of the Palæostracan are represented in
+  Ammoc&#x0153;tes. It shows well the uniform manner in which all the individual segmental factors
+  have been fused together to represent the appearance of a single segment (van Wijhe's first
+  segment) in the case of the four endognathal segments, but have retained their individuality in
+  the case of the ectognathal segment.</p>
+
+  <table class="sp2 mc ba nothand" title="Endognathal and ectognathal segments"
+  summary="Endognathal and ectognathal segments">
+    <tr>
+      <th rowspan="2" class="smaller ba">V.&nbsp;Wijhe's<br/>
+      segments.</th>
+      <th rowspan="2" colspan="2" class="smaller ba">Eurypterid<br/>
+      segments.</th>
+      <th colspan="2" class="smaller">Appendages.</th>
+      <th rowspan="2" class="smaller ba">Appendage<br/>
+      nerves.</th>
+      <th rowspan="2" class="smaller ba">Skeletal<br/>
+      elements.</th>
+      <th rowspan="2" class="smaller ba">Somatic<br/>
+      motor<br/>
+      nerves.</th>
+      <th rowspan="2" class="smaller ba">Dorso-<br/>
+      ventral<br/>
+      segmental<br/>
+      muscles.</th>
+      <th rowspan="2" class="smaller ba">C&#x0153;lomic<br/>
+      cavities.</th>
+      <th rowspan="2" class="smaller ba">Coxal<br/>
+      glands.</th>
+    </tr>
+    <tr class="ba">
+      <th class="smaller">Eurypterid.</th>
+      <th class="smaller">Ammoc&#x0153;tes.</th>
+    </tr>
+    <tr>
+      <td class="ac vmi ba">1</td>
+      <td class="ac vmi bb bt pl2 pr0">2<br/>
+      3<br/>
+      4<br/>
+      5</td>
+      <td class="ac vmi bb bt pl0 pr0"><img src="images/rbrace4.png" style="height:9.5ex;
+      width:1em;" alt="brace" /></td>
+      <td class="ac vmi ba">4 Endognaths</td>
+      <td class="ac vmi ba">4 Tentacles</td>
+      <td class="ac vmi ba">1 Tentacular to 4 tentacles</td>
+      <td class="ac vmi ba">1 Tentacular bar to 4 tentacles</td>
+      <td class="ac vmi ba">1 Oculomotor supplying 4 muscles</td>
+      <td class="ac vmi ba">Sup. inf. int. rectus and inf. oblique</td>
+      <td class="ac vmi ba">1 Premandibular fusion of 4</td>
+      <td class="ac vmi ba">1 Pituitary body; fusion of 4 coxal glands</td>
+    </tr>
+    <tr>
+      <td class="ac vmi ba">2</td>
+      <td class="ac vmi pl2 pr0">6</td>
+      <td></td>
+      <td class="ac vmi ba">1 Ectognath</td>
+      <td class="ac vmi ba">1 Tongue</td>
+      <td class="ac vmi ba">1 Tongue nerve</td>
+      <td class="ac vmi ba">1 Tongue bar</td>
+      <td class="ac vmi ba">1 Trochlearis supplying 1 muscle</td>
+      <td class="ac vmi ba">Sup. oblique</td>
+      <td class="ac vmi ba">1 Mandibular</td>
+      <td class="ba"></td>
+    </tr>
+  </table>
+
+<!-- Transposed table for handhelds -->
+
+  <table class="sp2 mc ba w100 handonly" title="Endognathal and ectognathal segments"
+  summary="Endognathal and ectognathal segments">
+    <tr class="ba ac vmi">
+      <td class="smaller pt05 pb05">V.&nbsp;Wijhe's<br/>
+      segments.</td>
+      <td>1</td>
+      <td>2</td>
+    </tr>
+    <tr class="ba ac vmi">
+      <td class="smaller pt05 pb05">Eurypterid<br/>
+      segments.</td>
+      <td>2 3 4 5</td>
+      <td>6</td>
+    </tr>
+    <tr class="ba ac vmi">
+      <td class="smaller pt05 pb05">Eurypterid<br/>
+      Appendages.</td>
+      <td>4 Endognaths</td>
+      <td>1 Ectognath</td>
+    </tr>
+    <tr class="ba ac vmi">
+      <td class="smaller pt05 pb05">Ammoc&#x0153;tes<br/>
+      Appendages.</td>
+      <td>4 Tentacles</td>
+      <td>1 Tongue</td>
+    </tr>
+    <tr class="ba ac vmi">
+      <td class="smaller pt05 pb05">Appendage<br/>
+      nerves.</td>
+      <td>1 Tentacular to 4 tentacles</td>
+      <td>1 Tongue nerve</td>
+    </tr>
+    <tr class="ba ac vmi">
+      <td class="smaller pt05 pb05">Skeletal<br/>
+      elements.</td>
+      <td>1 Tentacular bar to 4 tentacles</td>
+      <td>1 Tongue bar</td>
+    </tr>
+    <tr class="ba ac vmi">
+      <td class="smaller pt05 pb05">Somatic<br/>
+      motor<br/>
+      nerves.</td>
+      <td>1 Oculomotor supplying 4 muscles</td>
+      <td>1 Trochlearis supplying 1 muscle</td>
+    </tr>
+    <tr class="ba ac vmi">
+      <td class="smaller pt05 pb05">Dorso-<br/>
+      ventral<br/>
+      segmental<br/>
+      muscles.</td>
+      <td>Sup. inf. int. rectus and inf. oblique</td>
+      <td>Sup. oblique</td>
+    </tr>
+    <tr class="ba ac vmi">
+      <td class="smaller pt05 pb05">C&#x0153;lomic<br/>
+      cavities.</td>
+      <td>1 Premandibular fusion of 4</td>
+      <td>1 Mandibular</td>
+    </tr>
+    <tr class="ba ac vmi">
+      <td class="smaller pt05 pb05">Coxal<br/>
+      glands.</td>
+      <td>1 Pituitary body; fusion of 4 coxal glands</td>
+      <td></td>
+    </tr>
+  </table>
+
+<!-- End of transposed table for handhelds -->
+
+  <div><span class="pagenum" id="page309">{309}</span></div>
+
+  <p class="ac"><span class="sc">The Tubular Muscles.</span></p>
+
+  <p>The only musculature innervated by the trigeminal nerve which remains for further discussion,
+  consists of those peculiar muscles found in the velum, known by the name of striated tubular
+  muscles. This group of muscles has already been referred to in Chapter IV., dealing with
+  respiration and the origin of the heart.</p>
+
+  <p>It is a muscular group of extraordinary interest in seeking an answer to the question of
+  vertebrate ancestry, for, like the thyroid gland, it bears all the characteristics of a survival
+  from a prevertebrate form, which is especially well marked in Ammoc&#x0153;tes. I have already
+  suggested in this chapter that the homologues of these muscles are represented in Limulus by the
+  veno-pericardial group of muscles. I will now proceed to deal with the evidence for this
+  suggestion.</p>
+
+  <p>The structure of the muscle-fibres is peculiar and very characteristic, so that wherever they
+  occur they are easily recognized. Each fibre consists of a core of granular protoplasm, in the
+  centre of which the nuclei are arranged in a single row. This core is surrounded by a margin of
+  striated fibrillæ, as is seen in Fig. <a href="#fig122">122</a>. Such a structure is
+  characteristic of various forms of striated muscle found in various invertebrates, such as the
+  muscle-fibre of mollusca. It is, as far as I know, found nowhere in the vertebrate kingdom, except
+  in Ammoc&#x0153;tes. At transformation these muscles entirely disappear, becoming fattily
+  degenerated and then absorbed.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig122.png" id="fig122"><img style="width:42%" src="images/fig122.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 122.&mdash;A Tubular Muscle-fibre of
+      Ammoc&#x0153;tes.</span></p>
+      <p class="sp0">A, portion of fibre seen longitudinally; B, transverse section of fibre (osmic
+      preparation); the black dots are fat-globules.</p>
+    </div>
+  </div>
+
+  <p>For all these reasons they bear the stamp of a survival from a prevertebrate form. This alone
+  would not make this tissue of any great importance, but when in addition these muscles are found
+  to be arranged absolutely segmentally throughout the whole of the branchial region, then this
+  tissue becomes a clue of the highest importance.</p>
+
+  <p>As mentioned in Chapter IV., the segmental muscles of respiration consist of the adductor
+  muscle and the two constrictor muscles&mdash;the <span class="pagenum"
+  id="page310">{310}</span>striated constrictor and the tubular constrictor. Of these muscles, both
+  the muscles possessing ordinary striation are attached to the branchial cartilaginous skeleton,
+  whereas the tubular constrictors have nothing to do with the cartilaginous basket-work, but are
+  attached ventrally in the neighbourhood of the ventral aorta.</p>
+
+  <p>These segmental tubular muscles are found also in the velar folds&mdash;the remains of the
+  septum or velum which originally separated the oral from the respiratory chamber. In the branchial
+  region they act with the other constrictors as expiratory muscles, forcing the water out of the
+  respiratory chamber. In the living Ammoc&#x0153;tes, the velar folds on each side can be seen to
+  move synchronously with the movements of respiration, contracting at each expiration; they thus
+  close the slit by which the oral and respiratory chambers communicate, and therefore, in
+  conjunction with the respiratory muscles, force the water of respiration to flow out through the
+  gill-slits, as described by Schneider.</p>
+
+  <p>These tubular muscles thus form a dorso-ventral system of muscles essentially connected with
+  respiration; they belong to each one of the respiratory segments, and are also found in the velum;
+  anterior to this limit they are not to be found. What, then, are these tubular muscles in the
+  velar folds? Miss Alcock has worked out their topography by means of serial sections, and, as
+  already fully explained, has shown that they form exactly similar dorso-ventral groups, which
+  belong to the two segments anterior to the purely branchial segments, <i>i.e.</i> to the facial or
+  hyoid segments and the lower lip-segment of the trigeminal nerve. If the velar folds could be put
+  back into their original position as a septum, then the hyoid or facial group of tubular muscles
+  would take up exactly the same position as those belonging to each branchial segment.</p>
+
+  <p>The presence of these two so clearly segmental groups of muscles in the velum&mdash;the one
+  belonging to the region of the trigeminal, the other to the region of the facial&mdash;is strong
+  confirmation of my contention that this septum between the oral and respiratory chambers was
+  caused by the fusion of the last prosomatic and the first mesosomatic appendages, represented in
+  Limulus by the chilaria and the operculum.</p>
+
+  <p>Yet another clue to the meaning of these muscles is to be found in their innervation, which is
+  very extraordinary and unexpected. Throughout the branchial region the striated muscles of each
+  segment <span class="pagenum" id="page311">{311}</span>are strictly supplied by the nerve of that
+  segment, and, as already described, each segment is as carefully mapped out in its innervation as
+  it is in any arthropod appendage. One exception occurs to this orderly, symmetrical arrangement: a
+  nerve arises in connection with the facial nerve, and passes tailwards throughout the whole of the
+  branchial region, giving off a branch to each segment as it passes. This nerve (<i>Br. prof.</i>,
+  Fig. <a href="#fig123">123</a>) is known by the name of the <i>ramus branchialis profundus</i> of
+  the facial, and its extraordinary course has always aroused great curiosity in the minds of
+  vertebrate anatomists. Miss Alcock, by the laborious method of following its course throughout a
+  complete series of sections, finds that each of the segmental branches which is given off, passes
+  into the tubular muscles of that segment (Fig. <a href="#fig124">124</a>). The tubular muscles
+  which belong to the velum, <i>i.e.</i> those belonging to the lower lip-segment and to the hyoid
+  segments, receive their innervation from the velar or mandibular nerve, and belong, therefore, to
+  the trigeminal, not to the facial, system.</p>
+
+  <div class="ac w45 fcenter sp2">
+    <a href="images/fig123.jpg" id="fig123"><img style="width:100%" src="images/fig123.jpg" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 123.&mdash;Diagram showing the Distribution of the Facial
+      Nerve.</span></p>
+      <p class="sp0">Motor branches, <i>red</i>; sensory branches, blue.</p>
+    </div>
+  </div>
+
+  <p>The evidence presented by these muscles is as follows<span class="wnw">:&mdash;</span></p>
+
+  <p>In the ancestor of the vertebrate there must have existed a segmentally arranged set of
+  dorso-ventral muscles of peculiar structure, concerned with respiration, and confined to the
+  mesosomatic segments and to the last prosomatic segment, yet differing from the other
+  dorso-ventral muscles of respiration in their innervation and their attachment.</p>
+
+  <p>Interpreting these facts with the aid of my theory of the origin of vertebrates, and
+  remembering that the homologue of the vertebrate ventral aorta in such a palæostracan as Limulus
+  is the longitudinal <span class="pagenum" id="page312">{312}</span>venous sinus, while the
+  opercular and chilarial segments are respectively the foremost mesosomatic and the last prosomatic
+  segments; they signify that the palæostracan ancestor must have possessed a separate set of
+  segmental dorso-ventral muscles confined to the branchial, opercular and chilarial or metastomal
+  segments, which, on the one hand, were respiratory in function, and on the other were attached to
+  the longitudinal venous sinus. Further, these muscles must all have received a nerve-supply from
+  the neuromeres belonging to the chilarial and opercular segments, an unsymmetrical arrangement of
+  nerves, on the face of it, very unlikely to occur in an arthropod.</p>
+
+  <div class="ac w50 fcenter sp2">
+    <a href="images/fig124.jpg" id="fig124"><img style="width:72%" src="images/fig124.jpg" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p><span class="sc">Fig. 124.&mdash;Diagram constructed from a series of Transverse Sections
+      through a Branchial Segment, showing the arrangement and relative positions of the Cartilage,
+      Muscles, Nerves, and Blood-Vessels.</span></p>
+      <p class="sp0">Nerves coloured red are the motor nerves to the branchial muscles. Nerves
+      coloured blue are the internal sensory nerves to the diaphragms and the external sensory
+      nerves to the sense-organs of the lateral line system. <i>Br. cart.</i>, branchial cartilage;
+      <i>M. con. str.</i>, striated constrictor muscles; <i>M. con. tub.</i>, tubular constrictor
+      muscles; <i>M. add.</i>, adductor muscle; <i>D.A.</i>, dorsal aorta; <i>V.A.</i>, ventral
+      aorta; <i>S.</i>, sense-organs on diaphragm; <i>n. Lat.</i>, lateral line nerve; <i>X.</i>,
+      epibranchial ganglia of vagus; <i>R. br. prof. VII.</i>, <i>ramus branchialis profundus</i> of
+      facial; <i>J.v.</i>, jugular vein; <i>Ep. pit.</i>, epithelial pit.</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page313">{313}</span></div>
+
+  <p>Is this prophecy borne out by the examination of Limulus? In the first place, these muscles
+  were dorso-ventral and segmental, and, referring back to Chapter VII., Lankester arranges the
+  segmental dorso-ventral muscles in three groups: (1) The dorso-ventral somatic muscles; (2) the
+  dorso-ventral appendage muscles; and (3) the veno-pericardial muscles. Of these the first group is
+  represented in the vertebrate by the muscles which move the eye, the second group by the striated
+  constrictor and adductor muscles and the muscles for the lower lip. There is, then, the
+  possibility of the third group for this system of tubular muscles.</p>
+
+  <p>Looking first at the structure of these muscles as previously described, so different are they
+  in appearance from the ordinary muscles of Limulus, that Milne-Edwards, as already stated, called
+  them "brides transparentes," and did not recognize their muscular character, while Blanchard
+  called them in the scorpion, "ligaments contractils."</p>
+
+  <p>Consider their attachment and their function. They are attached to the longitudinal sinus,
+  according to Lankester's observation, in such a way that the muscle-fibres form a hollow cone
+  filled with blood; when they contract they force this blood towards the gills, and thus act as
+  accessory or branchial hearts. According to Blanchard, in the scorpion they contract synchronously
+  with the heart; according to Carlson, in Limulus they contract with the respiratory muscles. In
+  Ammoc&#x0153;tes, where the respiration is effected after the fashion of Limulus, not of Scorpio,
+  the tubular muscles are respiratory in function.</p>
+
+  <p>Look at their limits. The veno-pericardial muscles in Limulus are limited by the extent of the
+  heart, they do not extend beyond the anterior limit of the heart. In Fig. <a href="#fig70">70</a>
+  (p. <a href="#page176">176</a>) two of these muscles are seen in front of the branchial region
+  also attached to the longitudinal venous sinus, although in front of the gill-region. In
+  Ammoc&#x0153;tes the upper limit of the tubular muscles is the group found in the velum; this most
+  anterior group belongs to a region in front of the branchial region&mdash;that of the
+  trigeminal.</p>
+
+  <p>Moreover, the supposition that the segmental tubular muscles belong throughout to the
+  veno-pericardial group gives an adequate reason why they do not occur in front of the velum; for,
+  as their existence is dependent upon the longitudinal collecting sinus in Limulus and Scorpio,
+  which is represented by the ventral aorta in <span class="pagenum"
+  id="page314">{314}</span>Ammoc&#x0153;tes, they cannot extend beyond its limits. Now, Dohrn
+  asserts that the ventral aorta terminates in the spiracular artery, which exists only for a short
+  time; and, in another place, speaking of this same termination of the ventral aorta, he states:
+  "Dass je eine vorderste Arterie aus den beiden primären Aesten des Conus arteriosus hervorgeht,
+  die erste Anlage der Thyroidea umfasst, in der Mesodermfalte des späteren Velums in die Höhe
+  steigt um in die Aorta der betreffenden Seite einzumunden." These observations show that the
+  vessel which in Ammoc&#x0153;tes represents the longitudinal collecting sinus in the Merostomata
+  does not extend further forwards than the velum, and in consequence the representatives of the
+  veno-pericardial muscles cannot extend into the segments anterior to the velum. One of the
+  extraordinary characteristics of these tubular muscles which distinguishes them from other
+  muscles, but brings them into close relationship with the veno-pericardial group, is the manner in
+  which the bundles of muscle-fibres are always found lying freely in a blood-space; this is clearly
+  seen in the branchial region, but most strikingly in the velum, the interior of which, apart from
+  its muco-cartilage, is simply a large lacunar blood-space traversed by these tubular muscles.</p>
+
+  <p>All these reasons point to the same conclusion: the tubular muscles in Ammoc&#x0153;tes are the
+  successors of the veno-pericardial system of muscles.</p>
+
+  <p>If this is so, then this homology ought to throw light on the extraordinary innervation of
+  these tubular muscles by the <i>branchialis profundus</i> branch of the facial nerve and the velar
+  branch of the trigeminal. We ought, in fact, to find in Limulus a nerve arising exclusively from
+  the ganglia belonging to the chilarial and opercular segments, which, instead of being confined to
+  those segments, traverses the whole branchial region on each side, and gives off a branch to each
+  branchial segment; this branch should supply the veno-pericardial muscle of that side.</p>
+
+  <p>Patten and Redenbaugh have traced out the distribution of the peripheral nerves in Limulus, and
+  have found that from each mesosomatic ganglion a segmental cardiac nerve arises which passes to
+  the heart and there joins the cardiac median nerve, or rather the median heart-ganglion, for this
+  so-called nerve is really a mass of ganglion-cells. In all the branchial segments the same plan
+  exists, each cardiac nerve belonging to that neuromere is strictly segmental. <span
+  class="pagenum" id="page315">{315}</span>Upon reaching the opercular and chilarial neuromeres an
+  extraordinary exception is found; the cardiac nerves of these two neuromeres are fused together,
+  run dorsally, and then form a single nerve called the pericardial nerve, which runs outside the
+  pericardium along the whole length of the mesosomatic region, and gives off a branch to each of
+  the cardiac nerves of the branchial neuromeres as it passes them.</p>
+
+  <p>This observation of Patten and Redenbaugh shows that the pericardial nerve of Limulus agrees
+  with the very nerve postulated by the theory, as far as concerns its origin from the chilarial and
+  opercular neuromeres, its remarkable course along the whole branchial region, and its segmental
+  branches to each branchial segment.</p>
+
+  <p>At present the comparison goes no further; there is no evidence available to show what is the
+  destination of these segmental branches of the pericardial nerve, and so far all evidence of their
+  having any connection with the veno-pericardial muscles is wanting. Carlson, at my request,
+  endeavoured in the living Limulus to see whether stimulation of the pericardial nerve caused
+  contraction of the veno-pericardial muscles, but was unable to find any such effect. On the
+  contrary, his experimental work indicated that each veno-pericardial muscle received its motor
+  supply from the corresponding mesosomatic ganglion. This is not absolutely conclusive, for if, as
+  Blanchard asserts in the case of the scorpion, a close connection exists between the action of
+  these muscles and of the heart, it is highly probable that their innervation conforms to that of
+  the heart. Now Carlson has shown that this cardiac nerve from the opercular and chilarial
+  neuromeres is an inhibitory nerve to the heart, while the segmental cardiac nerves belonging to
+  the branchial ganglia are the augmentor nerves of the heart.</p>
+
+  <p>His experiments, then, show that the motor nerves of the heart and of the veno-pericardial
+  muscles run together in the same nerves, but he says nothing of the inhibitory nerves to the
+  latter muscles. If they exist and if they are in accordance with those to the heart, then they
+  ought to run in the pericardial nerve, and would naturally reach the veno-pericardial muscles by
+  the segmental branches of the pericardial nerve.</p>
+
+  <p>Moreover, inhibitory nerves are, in certain cases, curiously associated with sensory fibres; so
+  that the nerve which corresponds <span class="pagenum" id="page316">{316}</span>to the pericardial
+  nerve, viz. the <i>branchialis profundus</i> of the facial, may be an inhibitory and sensory
+  nerve, and not motor at all. Miss Alcock's observations are purely histological; no physiological
+  experiments have been made.</p>
+
+  <p>At present, then, it does not seem to me possible to say that Carlson's experiments have
+  disproved <i>any</i> connection of the pericardial nerve with the veno-pericardial muscles. We do
+  not know what is the destination of its segmental branches; they may still supply the
+  veno-pericardial muscles even if they do not cause them to contract; they certainly do not appear
+  to pass directly into them, for they pass into the segmental cardiac nerves, and can only reach
+  the muscles in conjunction with their motor nerves. Such a course would not be improbable when it
+  is borne in mind how, in the frog, the augmentor nerves run with the inhibitory along the whole
+  length of the vagus nerve.</p>
+
+  <p>Until further evidence is given both as to the function of the segmental branches of the
+  pericardial nerve in the Limulus, and of the <i>branchialis profundus</i> in Ammoc&#x0153;tes, it
+  is impossible, I think, to consider that the phylogenetic origin of these tubular muscles is as
+  firmly established as is that of most of the other organs already considered. I must say, my own
+  bias is strongly in favour of looking upon them as the last trace of the veno-pericardial system
+  of muscles, a view which is distinctly strengthened by Carlson's statement that the latter system
+  contracts synchronously with the respiratory movements, for undoubtedly in Ammoc&#x0153;tes their
+  function is entirely respiratory. Then again, although at present there is no evidence to connect
+  the pericardial nerve in Limulus with this veno-pericardial system of muscles, yet it is
+  extraordinarily significant that in such animals as Limulus and Ammoc&#x0153;tes, in both of which
+  the mesosomatic or respiratory region is so markedly segmental, an intrusive nerve should, in each
+  case, extend through the whole region, giving off branches to each segment. Still more striking is
+  it that this nerve should arise from the foremost mesosomatic and the last prosomatic neuromeres
+  in Limulus&mdash;the opercular and chilarial segments&mdash;precisely the same neuromeres which
+  give origin to the corresponding nerve in Ammoc&#x0153;tes, for according to my theory of the
+  origin of vertebrates, the nerves which supplied the opercular and metastomal appendages have
+  become the facial nerve and the lower lip-branch of the trigeminal nerve.</p>
+
+  <div><span class="pagenum" id="page317">{317}</span></div>
+
+  <p class="sp3">With the formation of the vertebrate heart from the two longitudinal venous sinuses
+  and the abolition of the dorsal invertebrate heart, the function of these tubular muscles as
+  branchial hearts was no longer needed, and their respiratory function alone remained. The last
+  remnant of this is seen in Ammoc&#x0153;tes, for the ordinary striated muscles were always more
+  efficient for the respiratory act, and so at transformation the inferior tubular musculature was
+  got rid of, there being no longer any need for its continued existence.</p>
+
+  <p class="ac"><span class="sc">The Palæostoma, or Old Mouth.</span></p>
+
+  <p>The arrangement of the oral chamber in Ammoc&#x0153;tes is peculiar among vertebrates, and,
+  upon my theory, is explicable by its comparison with the accessory oral chamber which apparently
+  existed in Eurypterus. According to this explanation, the lower lip of the original vertebrate
+  mouth was formed by the coalescence of the most posterior pair of the prosomatic
+  appendages&mdash;the chilaria; from which it follows that the vertebrate mouth was not the
+  original mouth, but a new structure due to such a formation of the lower lip.</p>
+
+  <p>It is very suggestive that the direct following out of the original working hypothesis should
+  lead to this conclusion, for it is universally agreed by all morphologists that the present mouth
+  is a new formation, and Dohrn has argued strongly in favour of the mouth being formed by the
+  coalescence of a pair of gill-slits. Interpret this in the language of my theory, and immediately
+  we see, as already explained, gill-slits must mean in this region the spaces between appendages
+  which did not carry gills; the mouth, therefore, was formed by the coalescence of a pair of
+  appendages to form a lower lip just as I have pointed out.</p>
+
+  <p>Where, then, must we look for the palæostoma, or original mouth? Clearly, as already suggested,
+  it was situated at the base of the olfactory passage, and the olfactory passage or nasal tube of
+  Ammoc&#x0153;tes was originally the tube of the hypophysis, so that the following out of the
+  theory points directly to the tube of the hypophysis as the place where the palæostoma must be
+  looked for.</p>
+
+  <p>This conclusion is not only not at variance with the opinions of morphologists, but gives a
+  straightforward, simple explanation why the palæostoma was situated in the very place where they
+  are most inclined to locate it. Thus, if we trace the history of the question, <span
+  class="pagenum" id="page318">{318}</span>we see that Dohrn's original view of the comparison of
+  the vertebrate and the annelid led him to the conception that the vertebrate mouth was formed by
+  the coalescence of a pair of gill-slits, and that the original mouth was situated somewhere on the
+  dorsal surface and opened into the gut by way of the infundibulum and the tube of the hypophysis.
+  This, also, was Cunningham's view as far as the tube of the hypophysis was concerned. Beard, in
+  1888, holding the view that the vertebrates were derived from annelids which had lost their
+  supra-&#x0153;sophageal ganglia, and that, therefore, there was no question of an
+  &#x0153;sophageal tube piercing the central nervous system of the vertebrate, explained the close
+  connection of the infundibulum with the hypophysis by the comparison of the tube of the hypophysis
+  with the annelidan mouth, so that the infundibular or so-called nervous portion was a special
+  nervous innervation for the original throat, just as Kleinenberg had shown to be the case in many
+  annelids. Beard therefore called this opening of the hypophysial tube the old mouth, or
+  palæostoma. Recently, in 1893, Kupffer has also put forward the view that the hypophysial opening
+  is the palæostoma. basing this view largely upon his observations on Ammoc&#x0153;tes and
+  Acipenser.</p>
+
+  <div class="ac w50 fcenter sp2">
+    <a href="images/fig125.png" id="fig125"><img style="width:100%" src="images/fig125.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 125.&mdash;Diagram to show the Meeting of the Four Tubes
+      in such a Vertebrate as the Lamprey.</span></p>
+      <p class="sp0"><i>Nc.</i>, neural canal with its infundibular termination; <i>Nch.</i>,
+      notochord; <i>Al.</i>, alimentary canal with its anterior diverticulum; <i>Hy.</i>,
+      hypophysial or nasal tube; <i>Or.</i>, oral chamber closed by septum.</p>
+    </div>
+  </div>
+
+  <p>As is seen in Fig. <a href="#fig125">125</a>, the position of this palæostoma is a very
+  suggestive one. At this single point in Ammoc&#x0153;tes, four separate tubes terminate; here is
+  the end of the notochordal tube, the termination of the infundibulum, the blind end of the nasal
+  tube or tube <span class="pagenum" id="page319">{319}</span>of the hypophysis, and the pre-oral
+  elongation of the alimentary canal.</p>
+
+  <p>It is perfectly simple and easy for the olfactory tube to open into any one of the other three.
+  By opening into the infundibulum it reproduces the condition of affairs seen in the scorpion; by
+  opening into the gut it produces the actual condition of things seen in Myxine and other
+  vertebrates; by opening into the notochordal tube it would produce a transitional condition
+  between the other two.</p>
+
+  <p>The view held by Kupffer is that this nasal tube (tube of the hypophysis) opened into the
+  anterior diverticulum of the vertebrate gut, and was for this reason the original mouth-tube; then
+  a new mouth was formed, and this connection was closed, being subsequently reopened as in Myxine.
+  My view is that this tube originally opened into the infundibulum, in other words, into the
+  original gut of the palæostracan ancestor, and was for this reason the original mouth-tube, in the
+  same sense as the olfactory passage of the scorpion may be, and often is, called the mouth-tube.
+  When, with the breaking through of the septum between the oral and respiratory chambers, the
+  external opening of the oral chamber became a new mouth, the old mouth was closed but the
+  olfactory tube still remained, owing to the importance of the sense of smell. Subsequently, as in
+  Myxine and the higher vertebrates, it opened into the pharynx, and so formed the nose of the
+  higher vertebrates.</p>
+
+  <p class="sp3">It is not, to my mind, at all improbable that during the transition stage, between
+  its connection with the old alimentary canal, as in Eurypterus or the scorpions, and its blind
+  ending, as in Ammoc&#x0153;tes, the nasal tube opened into the tube of the notochord. This
+  question will be discussed later on when the probable significance of the notochord is
+  considered.</p>
+
+  <p class="ac"><span class="sc">The Pituitary Gland.</span></p>
+
+  <p>Turning back to the comparison of Fig. <a href="#fig106">106</a>, B, and Fig. <a
+  href="#fig106">106</a>, C, which represent respectively an imaginary sagittal section through an
+  Eurypterus-like animal and through Ammoc&#x0153;tes at a larval stage, all the points for
+  comparison mentioned on p. <a href="#page244">244</a> have now been discussed with the exception
+  of the suggested homology between the coxal glands of the one animal and the pituitary body of the
+  other.</p>
+
+  <div><span class="pagenum" id="page320">{320}</span></div>
+
+  <p>This latter gland undoubtedly arises posteriorly to the hypophysial tube, or Rathke's pouch (as
+  it is sometimes called), and, as already mentioned, is supposed by Kupffer to be formed from the
+  posterior wall of this pouch. More recently, as pointed out in Haller's paper, Nusbaum, who has
+  investigated this matter, finds that the glandular hypophysis is not formed from the walls of
+  Rathke's pouch, but from the tissue of the rudimentary connection or stalk between the two
+  premandibular cavities, which becomes closely connected with the posterior wall of Rathke's pouch,
+  and becoming cut off from the rest of the premandibular cavity on each side, becomes permanently a
+  part of the 'Hypophysis Anlage.'</p>
+
+  <p>The importance of Nusbaum's investigation consists in this, that he derives the glandular
+  hypophysis from the connecting stalk between the two premandibular cavities, and therefore from
+  the walls of the ventral continuation of this cavity on each side.</p>
+
+  <p>This may be expressed as follows:&mdash;</p>
+
+  <p>The c&#x0153;lomic cavity, known as the premandibular cavity, divides into a dorsal and a
+  ventral part; the walls of the dorsal part give origin to the somatic muscles belonging to the
+  oculomotor nerve, while the walls of the ventral part on each side form the connecting stalk
+  between the two cavities, and give origin to the glandular hypophysis.</p>
+
+  <p>Now, as already pointed out, the premandibular cavity is homologous with the 2nd prosomatic
+  c&#x0153;lomic cavity of Limulus, and this 2nd prosomatic c&#x0153;lomic cavity divides, according
+  to Kishinouye, into a dorsal and a ventral part; and, further, the walls of this ventral part form
+  the coxal gland. Both in the vertebrate, then, and in Limulus, we find a marked glandular tissue
+  in a corresponding position, and the conclusion is forced upon us that the glandular hypophysis
+  was originally the coxal gland of the invertebrate ancestor. As in all other cases already
+  considered, when the facts of topographical anatomy, of morphology and of embryology, all combine
+  to the same conclusion as to the derivation of the vertebrate organ from that of the invertebrate,
+  then there must be also a structural similarity between the two. What, then, is the nature of the
+  coxal gland in the scorpions and Limulus? Lankester's paper gives us full information on this
+  point as far as the scorpion and Limulus are concerned, and he shows that the coxal gland of
+  Limulus differs markedly from that of Scorpio in the size of the cells and in the <span
+  class="pagenum" id="page321">{321}</span>arrangement of the tubes. In Fig. <a
+  href="#fig126">126</a>, A, I give a picture of a piece of the coxal gland of Limulus taken from
+  Lankester's paper.</p>
+
+  <p>Turning now to the vertebrate, Bela Haller's paper gives us a number of pictures of the
+  glandular hypophysis from various vertebrates, and he especially points out the tubular nature of
+  the gland and its solidification in the course of development in some cases. In Fig. <a
+  href="#fig126">126</a>, B, I give his picture of the gland in Ammoc&#x0153;tes.</p>
+
+  <p>The striking likeness between Haller's picture and Lankester's picture is apparent on the face
+  of it, and shows clearly that the histological structure of the glands in the two cases confirms
+  the deductions drawn from their anatomical and morphological positions.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig126.png" id="fig126"><img style="width:100%" src="images/fig126.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 126.&mdash;A, Section of Coxal Gland of Limulus</span> (from <span
+      class="sc">Lankester</span>); <span class="sc">B, Section of Pituitary Body of
+      Ammoc&#x0153;tes</span> (from <span class="sc">Bela Haller</span>).</p>
+      <p class="sp0"><i>n.a.</i>, termination of nasal passage.</p>
+    </div>
+  </div>
+
+  <p>The sequence of events which gave rise to the pituitary body of the vertebrate was in all
+  probability somewhat as follows<span class="wnw">:&mdash;</span></p>
+
+  <p>Starting with the excretory glands of the Phyllopoda, known as shell-glands, which existed
+  almost certainly in the phyllopod Trilobite, we pass to the coxal gland of the Merostomata.
+  Judging from Limulus, these were coextensive with the coxæ of the 2nd, 3rd, 4th, and 5th locomotor
+  appendages. When these appendages became reduced in size and purely tactile they were compressed
+  and concentrated round the mouth region, forming the endognaths of the Merostomata; as a necessary
+  consequence of the concentration of the coxæ of the endognaths, the coxal gland also became
+  concentrated, <span class="pagenum" id="page322">{322}</span>and took up a situation close against
+  the pharynx, as represented in Fig. <a href="#fig106">106</a>, B. When, then, the old mouth
+  closed, and the pharynx became the <i>saccus vasculosus</i>, the coxal gland remained in close
+  contact with the <i>saccus vasculosus</i>, and became the pituitary body, thus giving the reason
+  why there is always so close a connection between the pituitary body and the infundibular
+  region.</p>
+
+  <p>Whatever was the condition of the digestive tracts at the transition stage between the
+  arthropod and the vertebrate, the original mouth-opening at the base of the olfactory tube was
+  ultimately closed. The method of its closure was exceedingly simple and evident. The membranous
+  cranium was in process of formation by the extension of the plastron laterally and dorsally; a
+  slight growth of the same tissue in the region of the mouth would suffice to close it and thus
+  separate the infundibulum from the olfactory tube. As evidence that such was the method of
+  closure, it is instructive to see how in Ammoc&#x0153;tes the glandular tissue of the pituitary
+  body is embedded in and mixed up with the tissue of this cranial wall; how the termination of the
+  nasal tube is embedded in this same thickened mass of the cranial wall&mdash;how, in fact, both
+  coxal gland and olfactory tube have become involved in the growth of the tissue of the plastron,
+  by means of which the mouth was closed.</p>
+
+  <p>I have now passed in review the nature of the evidence which justifies a comparison between the
+  segments supplied by the cranial nerves of the vertebrate and the prosomatic and mesosomatic
+  segments of the palæostracan. For the convenience of my readers I have put these conclusions into
+  tabular form (see p. 323), for all the segments as far as that supplied by the glossopharyngeal
+  nerves. In both vertebrate and invertebrate this is a fixed position, for in the former, however
+  variable may be the number of branchial segments which the vagus supplies, the second branchial
+  segment is always supplied by a separate nerve, the glossopharyngeal, and in the latter, though
+  the number of segments bearing branchiæ varies, the minimum number of such segments (as seen in
+  the Pedipalpi) is never less than two.</p>
+
+  <div><span class="pagenum" id="page323">{323}</span></div>
+
+  <p class="ac"><span class="sc">Table of Comparison of Corresponding Segments in the Eurypterids
+  and in Ammoc&#x0153;tes</span><br/>
+  (<i>i.e.</i> <span class="sc">in Cephalaspids</span>).</p>
+
+  <table class="sp2 w100 ba nothand" title="Corresponding Segments in Eurypterids
+  and Cephalaspids" summary="Corresponding Segments in Eurypterids
+  and Cephalaspids">
+    <tr>
+      <th rowspan="3" class="smaller ba vmi ac"><img src="images/supraoes.png" style="width:1.25em"
+      alt="supraoes"/></th>
+      <th colspan="5" class="smaller ba">Median Eyes.</th>
+      <th colspan="4" class="smaller ba">Pineal Eyes.</th>
+      <th colspan="2" class="smaller ba">Pineal Nerve.</th>
+      <th rowspan="3" class="smaller ba"><img src="images/suprainf.png" style="width:1.25em"
+      alt="suprainf"/></th>
+    </tr>
+    <tr>
+      <th colspan="5" class="smaller ba">Lateral Eyes.</th>
+      <th colspan="4" class="smaller ba">Lateral Eyes.</th>
+      <th colspan="2" class="smaller ba">II</th>
+    </tr>
+    <tr>
+      <th colspan="5" class="smaller ba">Camerostome.</th>
+      <th colspan="4" class="smaller ba">Olfactory Organ.</th>
+      <th colspan="2" class="smaller ba">I</th>
+    </tr>
+    <tr>
+      <td></td>
+      <td colspan="5" class="smaller ac ba">Invertebrate (Limulus or Eurypterid).</td>
+      <td colspan="7" class="smaller ac ba">Vertebrate (Ammoc&#x0153;tes or Cephalaspid).</td>
+    </tr>
+    <tr>
+      <th rowspan="12" class="smaller ba"><img src="images/infraoes.png" style="width:1.25em"
+      alt="infraoes"/></th>
+      <th rowspan="9" class="smaller ba"><img src="images/prosomatic.png" style="width:1.25em"
+      alt="prosomatic"/></th>
+      <th rowspan="2" class="smaller ba">Segments.</th>
+      <td colspan="2" class="smaller ac vmi ba">Appendages.</td>
+      <th rowspan="2" class="smaller ba">C&#x0153;lomic<br/>
+      cavities</th>
+      <th rowspan="2" class="smaller ba">Appendages.</th>
+      <th rowspan="2" class="smaller ba">Splanchnic<br/>
+      Nerves.</th>
+      <td rowspan="2" class="smaller ac vmi ba">Somatic<br/>
+      Segmental<br/>
+      Muscles.</td>
+      <th rowspan="2" class="smaller ba">Somatic<br/>
+      Nerves.</th>
+      <th rowspan="2" class="smaller ba">C&#x0153;lomic<br/>
+      Cavities.</th>
+      <th rowspan="2" class="smaller ba">V.&nbsp;Wijhe's<br/>
+      Segments.</th>
+      <th rowspan="12" class="smaller ba"><img src="images/infrainf.png" style="width:1.25em"
+      alt="infrainf"/></th>
+    </tr>
+    <tr>
+      <td class="smaller ac vmi ba">Limulus.</td>
+      <td class="smaller ac vmi ba">Eurypterid.</td>
+    </tr>
+    <tr>
+      <td class="smaller ac ba">1</td>
+      <td class="smaller ac ba">Cheliceræ</td>
+      <td class="smaller ac ba">Cheliceræ</td>
+      <td class="smaller ac ba">1</td>
+      <td class="smaller ac ba">&mdash;</td>
+      <td class="smaller ac ba">&mdash;</td>
+      <td class="smaller ac ba">&mdash;</td>
+      <td class="smaller ac ba">&mdash;</td>
+      <td class="smaller ac ba">Anterior.</td>
+      <td class="smaller ac ba">&mdash;</td>
+    </tr>
+    <tr>
+      <td class="smaller ac ba">2</td>
+      <td class="smaller ba">1st&nbsp;Locomotor</td>
+      <td rowspan="4" class="ac vmi ba" style="padding:0">
+        <table title="Corresponding Segments in Eurypterids
+        and Cephalaspids" summary="Corresponding Segments in Eurypterids
+        and Cephalaspids">
+          <tr>
+            <td class="vmi pl0 pr0"><img src="images/rbrace4.png" style="height:10.3ex; width:1em;"
+            alt="brace" /></td>
+            <td class="smaller vmi pl0">4&nbsp;Endognaths</td>
+          </tr>
+        </table>
+      </td>
+      <td rowspan="4" class="smaller ac vmi ba">2<br/>
+      Ventral part forms coxal gland.</td>
+      <td rowspan="4" class="smaller ac vmi ba">4 tentacles and upper lip.</td>
+      <td rowspan="4" class="smaller ac vmi ba">V<br/>
+      Tentacular and upper lip nerve.</td>
+      <td rowspan="4" class="smaller ac vmi ba">Muscles supplied by oculomotor nerve.</td>
+      <td rowspan="4" class="smaller ac vmi ba">III</td>
+      <td rowspan="4" class="smaller ac vmi ba">Premandibular.<br/>
+      Ventral part forms pituitary body.</td>
+      <td rowspan="4" class="smaller ac vmi ba">1</td>
+    </tr>
+    <tr>
+      <td class="smaller ac ba">3</td>
+      <td class="smaller ba">2nd <span class="hid">Loct</span>"</td>
+    </tr>
+    <tr>
+      <td class="smaller ac ba">4</td>
+      <td class="smaller ba">3rd <span class="hid">Loco</span>"</td>
+    </tr>
+    <tr>
+      <td class="smaller ac ba">5</td>
+      <td class="smaller ba">4th <span class="hid">Loco</span>"</td>
+    </tr>
+    <tr>
+      <td class="smaller ac vmi ba">6</td>
+      <td class="smaller vmi ba">5th <span class="hid">Loco</span>"</td>
+      <td class="smaller ac vmi ba">Ectognath</td>
+      <td class="smaller ac vmi ba">3</td>
+      <td class="smaller ac vmi ba">Tongue</td>
+      <td class="smaller ac vmi ba">V<br/>
+      Tongue nerve</td>
+      <td class="smaller ac vmi ba">Sup. oblique</td>
+      <td class="smaller ac vmi ba">IV</td>
+      <td class="smaller ac vmi ba">Mandibular</td>
+      <td rowspan="2" class="vmi ba" style="padding:0">
+        <table class="w100" title="Corresponding Segments in Eurypterids
+        and Cephalaspids" summary="Corresponding Segments in Eurypterids
+        and Cephalaspids">
+          <tr>
+            <td class="vmi pl0 pr0"><img src="images/rbrace4.png" style="height:9.5ex; width:1em;"
+            alt="brace" /></td>
+            <td class="smaller vmi pl0 ac w80">2</td>
+          </tr>
+        </table>
+      </td>
+    </tr>
+    <tr>
+      <td class="smaller ac vmi ba">7</td>
+      <td class="smaller ac vmi ba">Chilaria</td>
+      <td class="smaller ac vmi ba">Metastoma</td>
+      <td class="smaller ac vmi ba">4</td>
+      <td class="smaller ac vmi ba">Lower lip</td>
+      <td class="smaller ac vmi ba">V<br/>
+      Lower lip nerve</td>
+      <td class="ba"></td>
+      <td class="ba"></td>
+      <td class="smaller ac vmi ba">Mandibular</td>
+    </tr>
+    <tr>
+      <td rowspan="3" class="smaller ac vmi ba"><img src="images/mesosomatic.png"
+      style="width:1.25em" alt="mesosomatic"/></td>
+      <td class="smaller ac vmi ba">8</td>
+      <td class="smaller ac vmi ba">Operculum</td>
+      <td rowspan="2" class="vmi ba" style="padding:0">
+        <table class="w100" title="Corresponding Segments in Eurypterids
+        and Cephalaspids" summary="Corresponding Segments in Eurypterids
+        and Cephalaspids">
+          <tr>
+            <td class="smaller vmi pr0 ac w80">Genital</td>
+            <td rowspan="2" class="vmi pl0 pr0"><img src="images/rbrace3.png" style="height:7.0ex;
+            width:1em;" alt="brace" /></td>
+            <td rowspan="2" class="smaller vmi pl0 ac w80">Operculum</td>
+          </tr>
+          <tr>
+            <td class="smaller vmi pr0 pt1 pb1 ac w80">Branchial</td>
+          </tr>
+        </table>
+      </td>
+      <td class="smaller ac vmi ba">5</td>
+      <td class="smaller ac vmi ba">Thyroid</td>
+      <td rowspan="2" class="vmi ba" style="padding:0">
+        <table class="w100" title="Corresponding Segments in Eurypterids
+        and Cephalaspids" summary="Corresponding Segments in Eurypterids
+        and Cephalaspids">
+          <tr>
+            <td class="vmi pl0 pr0"><img src="images/rbrace3.png" style="height:7.0ex; width:1em;"
+            alt="brace" /></td>
+            <td class="smaller vmi pl0 ac w80">VII</td>
+          </tr>
+        </table>
+      </td>
+      <td rowspan="2" class="smaller ac vmi ba">Ext. rectus<br/>
+      Retract oculi</td>
+      <td rowspan="2" class="smaller ac vmi ba">VI</td>
+      <td class="smaller ac vmi ba">Hyoid<sub>1</sub></td>
+      <td class="smaller ac vmi ba">3</td>
+    </tr>
+    <tr>
+      <td class="smaller ac vmi ba">9</td>
+      <td class="smaller vmi ba">1st Branchial</td>
+      <td class="smaller ac vmi ba">6</td>
+      <td class="smaller ac vmi ba">Hyoid or 1st Branchial</td>
+      <td class="smaller ac vmi ba">Hyoid<sub>2</sub></td>
+      <td class="smaller ac vmi ba">4</td>
+    </tr>
+    <tr>
+      <td class="smaller ac vmi ba">10<span class="hid">0</span></td>
+      <td class="smaller vmi ba">2nd <span class="hid">Bra</span>"</td>
+      <td class="smaller ac vmi ba">2nd Branchial</td>
+      <td class="smaller ac vmi ba">7</td>
+      <td class="smaller ac vmi ba">Branchial</td>
+      <td class="smaller ac vmi ba">IX</td>
+      <td class="smaller ac vmi ba">&mdash;</td>
+      <td class="smaller ac vmi ba">&mdash;</td>
+      <td class="smaller ac vmi ba">2nd Branchial</td>
+      <td class="smaller ac vmi ba">5</td>
+    </tr>
+  </table>
+
+<!-- Left half for handheld -->
+
+  <table class="sp2 w100 handonly" title="Corresponding Segments in Eurypterids
+  and Cephalaspids (1 of 2)" summary="Corresponding Segments in Eurypterids
+  and Cephalaspids (1 of 2)">
+    <tr>
+      <th rowspan="3" class="smaller ba vmi ac"><img src="images/supraoes.png" style="width:1.25em"
+      alt="supraoes"/></th>
+      <th colspan="5" class="smaller ba">Median Eyes.</th>
+    </tr>
+    <tr>
+      <th colspan="5" class="smaller ba">Lateral Eyes.</th>
+    </tr>
+    <tr>
+      <th colspan="5" class="smaller ba">Camerostome.</th>
+    </tr>
+    <tr>
+      <td class="ba"></td>
+      <td colspan="5" class="smaller ac ba">Invertebrate (Limulus or Eurypterid).</td>
+    </tr>
+    <tr>
+      <th rowspan="12" class="smaller ba"><img src="images/infraoes.png" style="width:1.25em"
+      alt="infraoes"/></th>
+      <th rowspan="9" class="smaller ba"><img src="images/prosomatic.png" style="width:1.25em"
+      alt="prosomatic"/></th>
+      <th rowspan="2" class="smaller ba">Seg.</th>
+      <td colspan="2" class="smaller ac vmi ba">Appendages.</td>
+      <th rowspan="2" class="smaller ba">C&#x0153;lomic<br/>
+      cavities</th>
+    </tr>
+    <tr>
+      <td class="smaller ac vmi ba">Limulus.</td>
+      <td class="smaller ac vmi ba">Eurypterid.</td>
+    </tr>
+    <tr>
+      <td class="smaller ac ba">1</td>
+      <td class="smaller ac ba">Cheliceræ</td>
+      <td class="smaller ac ba">Cheliceræ</td>
+      <td class="smaller ac ba">1</td>
+    </tr>
+    <tr>
+      <td class="smaller ac ba">2</td>
+      <td class="smaller ba">1st&nbsp;Locomotor</td>
+      <td rowspan="4" class="ac vmi ba" style="padding:0">
+        <table title="Corresponding Segments in Eurypterids
+        and Cephalaspids (1 of 2)" summary="Corresponding Segments in Eurypterids
+        and Cephalaspids (1 of 2)">
+          <tr>
+            <td class="vmi pl0 pr0"><img src="images/rbrace4.png" style="height:10.3ex; width:1em;"
+            alt="brace" /></td>
+            <td class="smaller vmi pl0">4&nbsp;Endognaths</td>
+          </tr>
+        </table>
+      </td>
+      <td rowspan="4" class="smaller ac vmi ba">2<br/>
+      Ventral part&nbsp;forms coxal&nbsp;gland.</td>
+    </tr>
+    <tr>
+      <td class="smaller ac ba">3</td>
+      <td class="smaller ba">2nd <span class="hid">Loct</span>"</td>
+    </tr>
+    <tr>
+      <td class="smaller ac ba">4</td>
+      <td class="smaller ba">3rd <span class="hid">Loco</span>"</td>
+    </tr>
+    <tr>
+      <td class="smaller ac ba">5</td>
+      <td class="smaller ba">4th <span class="hid">Loco</span>"</td>
+    </tr>
+    <tr>
+      <td class="smaller ac vmi ba">6</td>
+      <td class="smaller vmi ba">5th <span class="hid">Loco</span>"</td>
+      <td class="smaller ac vmi ba">Ectognath</td>
+      <td class="smaller ac vmi ba">3</td>
+    </tr>
+    <tr>
+      <td class="smaller ac vmi ba">7</td>
+      <td class="smaller ac vmi ba">Chilaria</td>
+      <td class="smaller ac vmi ba">Metastoma</td>
+      <td class="smaller ac vmi ba">4</td>
+    </tr>
+    <tr>
+      <td rowspan="3" class="smaller ac vmi ba"><img src="images/mesosomatic.png"
+      style="width:1.25em" alt="mesosomatic"/></td>
+      <td class="smaller ac vmi ba">8</td>
+      <td class="smaller ac vmi ba">Operculum</td>
+      <td rowspan="2" class="vmi ba" style="padding:0">
+        <table class="w100" title="Corresponding Segments in Eurypterids
+        and Cephalaspids (1 of 2)" summary="Corresponding Segments in Eurypterids
+        and Cephalaspids (1 of 2)">
+          <tr>
+            <td class="smaller vmi pr0 ac w80">Genital</td>
+            <td rowspan="2" class="vmi pl0 pr0"><img src="images/rbrace3.png" style="height:7.0ex;
+            width:1em;" alt="brace" /></td>
+            <td rowspan="2" class="smaller vmi pl0 ac w80">Opc.</td>
+          </tr>
+          <tr>
+            <td class="smaller vmi pr0 pt1 pb1 ac w80">Branchial</td>
+          </tr>
+        </table>
+      </td>
+      <td class="smaller ac vmi ba">5</td>
+    </tr>
+    <tr>
+      <td class="smaller ac vmi ba">9</td>
+      <td class="smaller vmi ba">1st Branchial</td>
+      <td class="smaller ac vmi ba">6</td>
+    </tr>
+    <tr>
+      <td class="smaller ac vmi ba">10<span class="hid">0</span></td>
+      <td class="smaller vmi ba">2nd <span class="hid">Bra</span>"</td>
+      <td class="smaller ac vmi ba">2nd Branchial</td>
+      <td class="smaller ac vmi ba">7</td>
+    </tr>
+  </table>
+
+<!-- Right half for handheld -->
+
+  <table class="sp2 w100 handonly" title="Corresponding Segments in Eurypterids
+  and Cephalaspids (2 of 2)" summary="Corresponding Segments in Eurypterids
+  and Cephalaspids (2 of 2)">
+    <tr>
+      <td rowspan="5" class="ba"></td>
+      <th colspan="4" class="smaller ba">Pineal Eyes.</th>
+      <th colspan="2" class="smaller ba">Pineal Nerve.</th>
+      <th rowspan="3" class="smaller ba"><img src="images/suprainf.png" style="width:1.25em"
+      alt="suprainf"/></th>
+    </tr>
+    <tr>
+      <th colspan="4" class="smaller ba">Lateral Eyes.</th>
+      <th colspan="2" class="smaller ba">II</th>
+    </tr>
+    <tr>
+      <th colspan="4" class="smaller ba">Olfactory Organ.</th>
+      <th colspan="2" class="smaller ba">I</th>
+    </tr>
+    <tr>
+      <td colspan="7" class="smaller ac ba">Vertebrate (Ammoc&#x0153;tes or Cephalaspid).</td>
+    </tr>
+    <tr>
+      <th class="smaller ba">Apps.</th>
+      <th class="smaller ba">Splanchnic<br/>
+      Nerves.</th>
+      <td class="smaller ac vmi ba">S.S.M.</td>
+      <th class="smaller ba">Somatic<br/>
+      Nerves.</th>
+      <th class="smaller ba">C&#x0153;lomic<br/>
+      Cavities.</th>
+      <th class="smaller ba">VW.<br/>
+      S.</th>
+      <th rowspan="11" class="smaller ba"><img src="images/infrainf.png" style="width:1.25em"
+      alt="infrainf"/></th>
+    </tr>
+    <tr>
+      <td class="smaller ar ba">1</td>
+      <td class="smaller ac ba">&mdash;</td>
+      <td class="smaller ac ba">&mdash;</td>
+      <td class="smaller ac ba">&mdash;</td>
+      <td class="smaller ac ba">&mdash;</td>
+      <td class="smaller ac ba">Anterior.</td>
+      <td class="smaller ac ba">&mdash;</td>
+    </tr>
+    <tr>
+      <td class="smaller ar ba">2</td>
+      <td rowspan="4" class="smaller ac vmi ba">4 tentacles and upper lip.</td>
+      <td rowspan="4" class="smaller ac vmi ba">V<br/>
+      Tentacular and upper lip nerve.</td>
+      <td rowspan="4" class="smaller ac vmi ba">Muscles supplied by oculo-<br/>
+      motor nerve.</td>
+      <td rowspan="4" class="smaller ac vmi ba">III</td>
+      <td rowspan="4" class="smaller ac vmi ba">Pre-<br/>
+      mandibular.<br/>
+      Ventral part forms pituitary body.</td>
+      <td rowspan="4" class="smaller ac vmi ba">1</td>
+    </tr>
+    <tr>
+      <td class="smaller ar ba">3</td>
+    </tr>
+    <tr>
+      <td class="smaller ar ba">4</td>
+    </tr>
+    <tr>
+      <td class="smaller ar ba">5</td>
+    </tr>
+    <tr>
+      <td class="smaller ar vmi ba">6</td>
+      <td class="smaller ac vmi ba">Tongue</td>
+      <td class="smaller ac vmi ba">V<br/>
+      Tongue nerve</td>
+      <td class="smaller ac vmi ba">Sup. oblique</td>
+      <td class="smaller ac vmi ba">IV</td>
+      <td class="smaller ac vmi ba">Mandibular</td>
+      <td rowspan="2" class="smaller ac vmi ba">2</td>
+    </tr>
+    <tr>
+      <td class="smaller ar vmi ba">7</td>
+      <td class="smaller ac vmi ba">Lower lip</td>
+      <td class="smaller ac vmi ba">V<br/>
+      Lower lip nerve</td>
+      <td class="ba"></td>
+      <td class="ba"></td>
+      <td class="smaller ac vmi ba">Mandibular</td>
+    </tr>
+    <tr>
+      <td class="smaller ar vmi ba">8</td>
+      <td class="smaller ac vmi ba">Thyroid</td>
+      <td rowspan="2" class="smaller ac vmi ba">VII</td>
+      <td rowspan="2" class="smaller ac vmi ba">Ext. rectus<br/>
+      Retract oculi</td>
+      <td rowspan="2" class="smaller ac vmi ba">VI</td>
+      <td class="smaller ac vmi ba">Hyoid<sub>1</sub></td>
+      <td class="smaller ac vmi ba">3</td>
+    </tr>
+    <tr>
+      <td class="smaller ar vmi ba">9</td>
+      <td class="smaller ac vmi ba">Hyoid or 1st Branchial</td>
+      <td class="smaller ac vmi ba">Hyoid<sub>2</sub></td>
+      <td class="smaller ac vmi ba">4</td>
+    </tr>
+    <tr>
+      <td class="smaller ar vmi ba">10</td>
+      <td class="smaller ac vmi ba">Branchial</td>
+      <td class="smaller ac vmi ba">IX</td>
+      <td class="smaller ac vmi ba">&mdash;</td>
+      <td class="smaller ac vmi ba">&mdash;</td>
+      <td class="smaller ac vmi ba">2nd Branchial</td>
+      <td class="smaller ac vmi ba">5</td>
+    </tr>
+    <tr>
+      <td class="smaller" colspan="8">S.S.M. = Somatic Segmental Muscles. VW.S. = V. Wijhe's
+      Segments.</td>
+    </tr>
+  </table>
+
+<!-- End of split table for handheld -->
+
+  <div><span class="pagenum" id="page324">{324}</span></div>
+
+  <p class="ac"><span class="sc">Summary.</span></p>
+
+  <div class="bq1 smaller sp5">
+    <p>The general consideration of the evidence of the number of segments, and their nature in the
+    pro-otic region of the vertebrate, as given in the last chapter, is not incompatible with the
+    view that the trigeminal nerve originally supplied seven appendages, which appendages did not
+    carry branchiæ, but were originally used for purposes of locomotion as well as of
+    mastication.</p>
+    <p>Such appendages clearly no longer exist in the higher vertebrates, the muscles of mastication
+    only remaining; but in the earliest fish-forms they must have existed, as, indeed, is seen in
+    <span class="correction" title="Original reads 'Ptericthys'.">Pterichthys</span> and
+    Bothriolepis. Judging from all the previous evidence some signs of their existence may
+    reasonably be expected still to remain in Ammoc&#x0153;tes. Such is indeed the case.</p>
+    <p>In the adult Petromyzon the trigeminal nerve innervates specially a massive suctorial
+    apparatus, by means of which it holds on to other fishes, or to stones in the bottom of the
+    stream. There is here no apparent sign of appendages. Very great, however, is the difference in
+    the oral chamber of Ammoc&#x0153;tes; here there is no sign of any suctorial apparatus, but
+    instead, a system of tentacles, together with the remains of the septum or velum, which
+    originally closed off the oral from the respiratory chamber. These tentacles are the last
+    remnants of the original foremost prosomatic appendages of the palæostracan ancestor. Like the
+    lateral eyes they do not develop until the transformation comes, but during the whole larval
+    condition their musculature remains in an embryonic condition, and then from these embryonic
+    muscles the whole massive musculature of the suctorial apparatus develops; a sucking apparatus
+    derived from the modification of appendages, as so frequently occurs in the arthropods.</p>
+    <p>The study of Ammoc&#x0153;tes indicates that the velum and lower lip correspond to the
+    metastoma of the Eurypterid, <i>i.e.</i> the chilaria of Limulus, while the large ventral pair
+    of tentacles, called the tongue, correspond to the ectognaths of the Eurypterids, and probably
+    to the oar-like appendages of <span class="correction"
+    title="Original reads 'Ptericthys'.">Pterichthys</span> and Bothriolepis. From these two
+    splanchnic segments the suctorial apparatus in the main arises; the motor supply of these two
+    segments forms the mass of the trigeminal nerve-supply, and the nerves supplying them, the velar
+    nerve and the tongue-nerve, are markedly separate from the rest of the trigeminal nerve.</p>
+    <p>The rest of the tentacles present much less the sign of independent segments. In their
+    nerves, their muco-cartilaginous skeleton, and their rudimentary muscles, they indicate a
+    concentration and amalgamation, such as might be expected from the concentrated endognaths. The
+    continuation of the dwindling process, already initiated in the Eurypterid, would easily result
+    in the tentacles of Ammoc&#x0153;tes.</p>
+    <p>The nasal tube of Ammoc&#x0153;tes, which originates in the hypophysial tube, corresponds
+    absolutely in position and in its original structure, to the olfactory tube of a scorpion-like
+    animal. From this homology two conclusions of importance follow: (1) the old mouth, or
+    palæostoma, of the vertebrate was situated at the end of this tube, therefore, at the
+    termination of the infundibulum; (2) the upper lip, which by its growth, brings the olfactory
+    tube from a ventral to a dorsal position, was originally formed by the foremost sternites or
+    endostoma, or else by the sterno-coxal processes of the second pair of prosomatic appendages of
+    the palæostracan ancestor.</p>
+    <p>In strict accordance with the rest of the comparisons made in this region, the pituitary body
+    shows by similarity of structure, as well as of position, that it arose from the coxal glands,
+    which were situated at the base of the four endognaths.</p>
+    <div><span class="pagenum" id="page325">{325}</span></div>
+    <p>One after another, when once the clue has been found, all these mysterious organs of the
+    vertebrate, such as the pituitary and thyroid glands, fall harmoniously into their place as the
+    remnants of corresponding important organs in the palæostraca.</p>
+    <p>Yet another clue is afforded by the tubular muscles of Ammoc&#x0153;tes, that strange set of
+    non-vertebrate striated muscles, which are so markedly arranged in a segmental manner, which
+    disappear at transformation, and are never found in any of the higher vertebrates, for the
+    limits of their distribution correspond to the veno-pericardial muscles of Limulus.</p>
+    <p>Their nerve-supply in Ammoc&#x0153;tes is most extraordinary; for, although they are
+    segmentally arranged throughout the whole respiratory region, which is segmentally supplied by
+    the VIIth, IXth, and Xth nerves, and are found in front of this region only in one segment, that
+    of the lower lip, which is supplied by the velar branch of the Vth nerve, yet they are not
+    supplied segmentally, but only by the velar nerve and a branch of the VIIth, the <i>ramus
+    branchialis profundus</i>. This latter nerve extends throughout the respiratory region, and
+    gives off segmental branches to supply these muscles.</p>
+    <p class="sp0">It is also a curious coincidence that in such a markedly segmented animal as
+    Limulus, a nerve&mdash;the pericardial nerve&mdash;which arises from the nerves of the chilarial
+    and opercular segments, should pass along the whole respiratory region and give off branches to
+    each mesosomatic segment. It is strange, to say the least of it, that the chilarial or
+    metastomal and the opercular segments of Limulus should, on the theory advocated in this book,
+    correspond to the lower lip and hyoid segments of the vertebrate. At present the homology
+    suggested is not complete, for there is no evidence as yet that the veno-pericardial muscles
+    have anything to do with the pericardial nerve.</p>
+  </div>
+
+  <div><span class="pagenum" id="page326">{326}</span></div>
+
+  <p class="ac">CHAPTER X</p>
+
+  <p class="ac"><i>THE RELATIONSHIP OF AMMOC&#x0152;TES TO THE MOST ANCIENT FISHES&mdash;THE
+  OSTRACODERMATA</i></p>
+
+  <div class="bq1 it1 smaller sp3">
+    <p class="sp0">The nose of the Osteostraci.&mdash;Comparison of head-shield of Ammoc&#x0153;tes
+    and of Cephalaspis.&mdash;Ammoc&#x0153;tes the only living representative of these ancient
+    fishes.&mdash;Formation of cranium.&mdash;Closure of old mouth.&mdash;Rohon's primordial
+    cranium.&mdash;Primordial cranium of Phrynus and Galeodes.&mdash;Summary.</p>
+  </div>
+
+  <p>The shifting of the orifice of the olfactory passage, which led to the old mouth, from the
+  ventral to the dorsal side, as seen in the transformation of the ventrally situated hypophysial
+  tube of the young Ammoc&#x0153;tes, to the dorsally situated nasal tube of the full-grown
+  Ammoc&#x0153;tes, affords one of the most important clues in the whole of this story of the origin
+  of vertebrates; for, if Ammoc&#x0153;tes is the nearest living representative of the first-formed
+  fishes, then we ought to expect to find that the dorsal head-shield of such fishes is
+  differentiated from that of the contemporary Palæostraca by the presence of a median frontal
+  opening anterior to the eyes. Conversely, if such median nasal orifice is found to be a marked
+  characteristic of the group, in combination with lateral and median eyes, as in Ammoc&#x0153;tes,
+  then we have strong reasons for interpreting these head-shields by reference to the head of
+  Ammoc&#x0153;tes.</p>
+
+  <p class="sp3">The oldest known fishes belong to a large group of strange forms which inhabited
+  the Silurian and Devonian seas, classed together by Smith Woodward under the name of Ostracodermi.
+  These are divided into three orders: (1) the Heterostraci, including one family, the Pteraspidæ,
+  to which Pteraspis and Cyathaspis belong; (2) the Osteostraci, divisible into two families, the
+  Cephalaspidæ and Tremataspidæ, which include Cephalaspis, Eukeraspis, Auchenaspis or Thyestes, and
+  Tremataspis; and (3) the Antiarcha, with one family, the Astrolepidæ, including Astrolepis,
+  Pterichthys, and Bothriolepis. <span class="pagenum" id="page327">{327}</span>Of these, the first
+  two orders belong to the Upper Silurian, while the third is Devonian.</p>
+
+  <p class="ac"><span class="sc">The Dorsal Head-Shield of the Osteostraci.</span></p>
+
+  <p>Of the three orders above-named, the Heterostraci and Osteostraci are the oldest, and among
+  them the Cephalaspidæ have afforded the most numerous and best worked-out specimens. At
+  Rootziküll, in the island of &#x0152;sel, the form known as <i>Thyestes (Auchenaspis)
+  verrucosus</i> is especially plentiful, being found thickly present in among the masses of
+  Eurypterid remains, which give the name to the deposit. Of late years this species has been
+  especially worked at by Rohon, and many beautiful specimens have been figured by him, so that a
+  considerable advance has been made in our knowledge since Pander, Eichwald, Huxley, Lankester, and
+  Schmidt studied these most interesting primitive forms.</p>
+
+  <p>All observers agree that the head-region of these fishes was covered by a dorsal and ventral
+  head-shield, while the body-region was in most cases unknown, or, as in Eichwald's specimens, and
+  in the specimens figured in Lankester and Smith Woodward's memoirs, was made up of segments which
+  were not vertebral in character, but formed an aponeurotic skeleton, being the hardened
+  aponeuroses between the body-muscles. This body-skeleton, which possesses its exact counterpart in
+  Ammoc&#x0153;tes, will be considered more fully when I discuss the origin of the spinal region of
+  the vertebrate.</p>
+
+  <p>Of the two head-shields, ventral and dorsal, the latter is best known and characterizes the
+  group. It consists of a dorsal plate, with characteristic horns, which in <i>Thyestes
+  verrucosus</i> (Fig. <a href="#fig128">128</a>), as described by Rohon, is composed of two parts,
+  a frontal part and an occipital part (<i>occ.</i>), the occipital part being composed of segments,
+  and possessing a median ridge&mdash;the <i>crista occipitalis</i>. In Lankester's memoir and in
+  Smith Woodward's catalogue, a large number of known forms are described and delineated, and we may
+  perhaps say that in some of the forms, such as <i>Eukeraspis pustuliferus</i> (Fig. <a
+  href="#fig127">127</a>, B), the frontal part of the shield only is capable of preservation as a
+  fossil, while in Cephalaspis (Fig. <a href="#fig127">127</a>, A) not only the frontal part but a
+  portion of the occipital region is preserved, the latter being small in extent when compared with
+  the occipital region of Auchenaspis (Thyestes). Finally, in Tremataspis and Didymaspis, the whole
+  of both frontal <span class="pagenum" id="page328">{328}</span>and occipital region is capable of
+  preservation, the line of demarcation between these two regions being well marked in the latter
+  species.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig127.png" id="fig127"><img style="width:100%" src="images/fig127.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p class="sp0"><span class="sc">Fig. 127.&mdash;A, Dorsal Head-Shield of Cephalaspis</span>
+      (from <span class="sc">Lankester</span>); <span class="sc">B, Dorsal Head-Shield of
+      Keraspis</span> (from <span class="sc">Lankester</span>).</p>
+    </div>
+  </div>
+
+  <p>In the best preserved specimens of all this group of fishes a frontal median orifice is always
+  present; it appears in some specimens obscurely partially divided into two parts. Perhaps the best
+  specimen of all was obtained by Rohon at Rootziküll, and is thus described by him<span
+  class="wnw">:&mdash;</span></p>
+
+  <p>The frontal part of the dorsal head-plate carried (Fig. <a href="#fig128">128</a>) the two
+  orbits for the lateral eyes (<i>l.e.</i>), a marked frontal organ (<i>fro.</i>), and a median
+  depression (<i>gl.</i>), to which he gives the name parietal organ. The occipital part
+  (<i>occ.</i>) was clearly segmented, and carried, he thinks, the branchiæ. I reproduce Rohon's
+  figure of the frontal organ in Thyestes (Fig. 129); he describes it as a deeply sunk pit, divided
+  in the middle by a slit, which leads deeper in, he supposes, towards the central nervous
+  system.</p>
+
+  <div class="ac w25 fcenter sp2">
+    <a href="images/fig128.jpg" id="fig128"><img style="width:100%" src="images/fig128.jpg" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p><span class="sc">Fig. 128.</span>&mdash;<span class="sc">Dorsal Head-Shield of</span>
+      <i>Thyestes (Auchenaspis) verrucosus</i>. (From <span class="sc">Rohon</span>.)</p>
+      <p class="sp0"><i>Fro.</i>, narial opening; <i>l.e.</i>, lateral eyes; <i>gl.</i>, glabellum
+      or plate over brain; <i>Occ.</i>, occipital region.</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page329">{329}</span></div>
+
+  <p>A similar organ was described by Schmidt in Tremataspis, and considered by him to be a median
+  nose. Such also is the view of Jaekel, who points out that a median pineal eye exists between the
+  two lateral eyes in this animal, as in all other of these ancient fishes, so that this frontal
+  organ does not, as Patten thinks, represent the pineal eye. The whole of this group of fishes,
+  then, is characterized by the following striking characteristics<span
+  class="wnw">:&mdash;</span></p>
+
+  <p>1.  Two well-marked lateral eyes near the middle line.</p>
+
+  <p>2.  Between the lateral eyes, well-marked median eyes, very small.</p>
+
+  <p>3.  In front of the eye-region a median orifice, single.</p>
+
+  <p>In addition, behind the eye-region a median plate is always found, frequently different in
+  structure to the rest of the head-shield, being harder in texture&mdash;the so-called post-orbital
+  plate.</p>
+
+  <div class="ac w20 fcenter sp3">
+    <a href="images/fig129.jpg" id="fig129"><img style="width:100%" src="images/fig129.jpg" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="sp0"><span class="sc">Fig. 129.</span>&mdash;<span class="sc">Narial Opening and
+      Lateral Orbits of</span> <i>Thyestes Verrucosus</i>. (From <span class="sc">Rohon</span>.)</p>
+    </div>
+  </div>
+
+  <p class="ac"><span class="sc">Structure of Head-Shield of Cephalaspis compared with that of
+  Ammoc&#x0153;tes.</span></p>
+
+  <p>What is the structure of this head-shield? It has been spoken of as formed of bone because it
+  possesses cells, being thus unlike the layers of chitin, which are formed by underlying cells but
+  are not themselves cellular. At the same time, it is recognized on all sides that it has no
+  resemblance to bone-structure as seen in fossil remains of higher vertebrates. The latest and best
+  figure of the structure of this so-called bone is given in Rohon's paper already referred to. It
+  is, so he describes, clearly composed of fibrillæ and star-shaped cells, arranged more or less in
+  regular layers, with other sets of similar cells and fibrillæ arranged at right angles to the
+  first set, or at varying angles. The groundwork of this tissue, in which these cells and fibrils
+  are embedded, contained calcium salts, and so the whole tissue was preserved. In places, spaces
+  are found in it, in the deepest layer large medullary spaces; more superficially, ramifying spaces
+  which he considers to be vascular, and calls Haversian canals; the <span class="pagenum"
+  id="page330">{330}</span>star-like cells, however, are not arranged concentrically around these
+  spaces, as in true Haversian canals.</p>
+
+  <p>This structure is therefore a calcareous infiltration of a tissue with cells in it. Where is
+  there anything like it?</p>
+
+  <p>As soon as I saw Rohon's picture (Fig. <a href="#fig130">130</a>), I was astounded at its
+  startling resemblance to the structure of muco-cartilage as is seen in Fig. <a
+  href="#fig131">131</a>, taken from Ammoc&#x0153;tes. If such muco-cartilage were infiltrated with
+  lime salts, then the muco-cartilaginous skeleton of Ammoc&#x0153;tes would be preserved in the
+  fossil condition, and be comparable with that of Cephalaspis, etc.</p>
+
+  <table class="mc tlf sp2 w55" summary="Group of illustrations" title="Group of illustrations">
+    <tr>
+      <td class="vbm ac" style="width:45%;"><a href="images/fig130.png" id="fig130"><img
+      style="width:100%" src="images/fig130.png" alt="" title=""/></a></td>
+      <td class="vbm ac" style="width:45%;"><a href="images/fig131.png" id="fig131"><img
+      style="width:100%" src="images/fig131.png" alt="" title=""/></a></td>
+    </tr>
+    <tr>
+      <td>
+        <div class="smaller aj it1">
+          <p class="sp0"><span class="sc">Fig. 130.&mdash;Section of a Head-Plate of a
+          Cephalaspid.</span> (From <span class="sc">Rohon</span>.)</p>
+        </div>
+      </td>
+      <td>
+        <div class="smaller aj it1">
+          <p class="sp0"><span class="sc">Fig. 131.&mdash;Section of Muco-Cartilage from Dorsal
+          Head-Plate of Ammoc&#x0153;tes.</span></p>
+        </div>
+      </td>
+    </tr>
+  </table>
+
+  <p class="sp3">The whole structure is clearly remarkably like Rohon's picture of a section of the
+  head-plate of a Cephalaspid (Fig. <a href="#fig130">130</a>). In the latter case the matrix
+  contains calcium salts, in the former it is composed of the peculiar homogeneous mucoid tissue
+  which stains so characteristically with thionin. With respect to this calcification, it is
+  instructive to recall the calcification in the interior of the branchial cartilages of Limulus, as
+  described in Chapter III., for this example shows how easy it is to obtain a calcification in this
+  chondro-mucoid material. With respect to the medullary spaces and smaller spaces in this tissue,
+  as described by Rohon, I would venture to suggest that they need not all necessarily indicate
+  blood-vessels, for similar spaces would appear in the head-shield of Ammoc&#x0153;tes if its
+  muco-cartilage alone <span class="pagenum" id="page331">{331}</span>were preserved. Of these, some
+  would indicate the position of blood-vessels, such, for instance, as of the external carotid which
+  traverses this structure; but the largest and most internal spaces, resembling Rohon's medullary
+  spaces, would represent muscles, being filled up with bundles of the upper lip-muscles.</p>
+
+  <p class="ac"><span class="sc">The Muco-Cartilaginous Head-Shield of Ammoc&#x0153;tes.</span></p>
+
+  <p>The resemblance between the structure of the head-shield of Thyestes and the muco-cartilage of
+  Ammoc&#x0153;tes, is most valuable, for muco-cartilage is unique, occurs in no other vertebrate,
+  and every trace of it vanishes at transformation; it is essentially a characteristic of the larval
+  form, and must, therefore, in accordance with all that has gone before, be the remnant of an
+  ancestral skeletal tissue. The whole story deduced from the study of Ammoc&#x0153;tes would be
+  incomplete without some idea of the meaning of this tissue. So also, as already mentioned, the
+  skeleton of Ammoc&#x0153;tes is incomplete without taking this tissue into account. It is confined
+  entirely to the head-region; no trace of it exists posteriorly to the branchial basket-work. It
+  consists essentially of dorsal and ventral head-shields, connected together by the tentacular,
+  metastomal, and thyroid bars, as already described. The ventral shield forms the
+  muco-cartilaginous plate of the lower lip and the plate over the thyroid gland, so that the
+  skeleton ventrally is represented by Fig. <a href="#fig118">118</a>, B, which shows how the
+  cartilaginous bars of the branchial basket-work are separated from each other by this thyroid
+  plate. At transformation, with the disappearance of this muco-cartilaginous plate, the bars come
+  together in the middle line, as in the more posterior portion of the branchial basket-work.</p>
+
+  <p>The dorsal head-shield of muco-cartilage covers over the upper lip, sends a median prolongation
+  over the median pineal eyes and a lateral prolongation on each side as far as the auditory
+  capsules, giving the shape of the head-shield of muco-cartilage, as in Fig. 118, C.</p>
+
+  <p>Not only then is the structure of the head-shield of a Cephalaspid remarkably like the
+  muco-cartilage of Ammoc&#x0153;tes, but also its general distribution strangely resembles that of
+  the Ammoc&#x0153;tes muco-cartilage.</p>
+
+  <p>Now, these head-shields in the Cephalaspidæ and Tremataspidæ <span class="pagenum"
+  id="page332">{332}</span>vary very much in shape, as is seen by the comparison of Tremataspis and
+  Auchenaspis with Cephalaspis and Eukeraspis, and yet, undoubtedly, all these forms belong to a
+  single group, the Osteostraci.</p>
+
+  <p>The conception that Ammoc&#x0153;tes is the solitary living form allied to this group affords a
+  clue to the meaning of this variation of shape, which appears to me to be possible, if not indeed
+  probable. There is a certain amount of evidence given in the development of Ammoc&#x0153;tes which
+  indicates that the branchial region of its ancestors was covered with plates of muco-cartilage as
+  well as the prosomatic region.</p>
+
+  <p>The evidence is as follows:&mdash;</p>
+
+  <p>The somatic muscles of Ammoc&#x0153;tes form a continuous longitudinal sheet of muscles along
+  the length of the body, which are divided up by connective tissue bands into a series of imperfect
+  segments or myotomes. This simple muscular sheet can be dissected off along the whole of the
+  head-region of the animal, with the exception of the most anterior part, without interfering with
+  the attachments or arrangements of the splanchnic muscular system in the least. The reason why
+  this separation can be so easily effected is to be found in the fact that the two sets of muscles
+  are not attached to the same fascia. The sheet of fascia to which the somatic muscles are attached
+  is separated from the fascia which encloses the branchial cavity by a space (<i>cf.</i> Figs. 63
+  and 64) filled with blood-spaces and cells containing fat, in which space is also situated the
+  cartilaginous branchial basket-work. These branchial bars are closely connected with the branchial
+  sheet of fascia, and have no connection with the somatic fascia, their perichondrium forming part
+  of the former sheet. Upon examination, this space is seen to be mainly vascular, the blood-spaces
+  being large and frequently marked with pigment; but it also possesses a tissue of its own,
+  recognized as fat-tissue by all observers. The peculiarity of the cells of this tissue is their
+  arrangement; they are elongated cells arranged at right angles to the plates of fascia, just as
+  the fibres of the muco-cartilage are largely arranged at right angles to their limiting plates of
+  perichondrium. These cells do not necessarily contain fat; and when they do, the fat is found in
+  the centre of each cell, and does not push the protoplasm of the cell to the periphery, as in
+  ordinary fat cells.</p>
+
+  <div><span class="pagenum" id="page333">{333}</span></div>
+
+  <p>In Fig. <a href="#fig132">132</a>, B, I give a specimen of this tissue stained by osmic acid;
+  in Fig. <a href="#fig132">132</a>, A, I give a drawing of ordinary muco-cartilage taken from the
+  plate of the lower lip; and in Fig. <a href="#fig133">133</a>, A, a modification of the
+  muco-cartilage taken from the velum, which shows the formation of a tissue intermediate between
+  ordinary muco-cartilage and this branchial fat-tissue.</p>
+
+  <p>Further, in fully-grown specimens of Ammoc&#x0153;tes, in the region of undoubted
+  muco-cartilage, a fatty degeneration of the cells frequently appears, together with an increase in
+  the blood spaces,&mdash;the precursor, in fact, of the great change which overtakes this tissue
+  soon afterwards, at the time of transformation, when it is invaded by blood, and swept away,
+  except in those places where new cartilage is formed. I conclude, then, that the tissue of this
+  vascular space was originally muco-cartilage, which has degenerated during the life of the
+  Ammoc&#x0153;tes. The fact that in most cases undoubted muco-cartilage is to be found here and
+  there in this space, is strong confirmation of the truth of this conclusion.</p>
+
+  <div class="ac w30 fcenter sp2">
+    <a href="images/fig132.png" id="fig132"><img style="width:100%" src="images/fig132.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="sp0"><span class="sc">Fig. 132.&mdash;A, Muco-cartilage of Lower Lip</span>
+      (<i>Mc.</i>); <i>m.ph.</i>, muscle of lower lip; <i>m.sm.</i>, somatic muscle; <i>Cor.</i>,
+      laminated layer of skin. <span class="sc">B, Degenerated Muco-cartilage of Branchial
+      Region.</span> <i>F.</i>, fat layer; <i>P.</i>, pigment; <i>Bl.</i>, blood-space; <i>N.</i>,
+      somatic nerve; <i>m.br.</i>, branchial muscle; <i>m.sm.</i>, somatic muscle.</p>
+    </div>
+  </div>
+
+  <p>If this conclusion is correct, we may expect that it would be confirmed by the embryological
+  history of the tissue, and we ought to find that in much younger stages a homogeneous tissue of
+  the same nature as muco-cartilage fills up the spaces in the branchial <span class="pagenum"
+  id="page334">{334}</span>region, where in the Ammoc&#x0153;tes only blood and fat-containing cells
+  are present. For this purpose Shipley kindly allowed me to examine his series of sections through
+  the embryo at various ages. These specimens are very instructive, especially those stained by
+  osmic acid, which preserves the natural thickness of this space better than other staining
+  methods. At an age when the branchial cartilages are seen to be formed, when no fat-cells are
+  present, a distinctive tissue (Fig. <a href="#fig133">133</a>, B) is plainly visible in the velum
+  and at the base of the tentacles, in the very position where in the more advanced Ammoc&#x0153;tes
+  muco-cartilage exists. Taking, then, this tissue as our guide, the specimens show that the space
+  between the skin and the visceral muscles in which the cartilaginous basket-work lies is filled
+  with a similar material. At this stage a sheet of embryonic tissue occupies the position where,
+  later on, blood-spaces and fat-cells are found, and this tissue resembles that seen in the velum
+  and other places where muco-cartilage is afterwards found.</p>
+
+  <div class="ac w30 fcenter sp2">
+    <a href="images/fig133.png" id="fig133"><img style="width:100%" src="images/fig133.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="sp0"><span class="sc">Fig. 133.&mdash;A, Muco-Cartilage of Velum; B, Embryonic
+      Muco-Cartilage of Tentacular Bar.</span></p>
+    </div>
+  </div>
+
+  <p>I conclude, therefore, that originally the branchial or mesosomatic region was covered with a
+  dorsal plate of muco-cartilage, which carried on its under surface the dorsal part of the
+  branchial basket-work, and sprang from the central core of skeletogenous tissue around the
+  notochord; this plate was separated from the plate which covered this region ventrally by the
+  lateral grove in which the gill-slits are situated. The ventral plate carried on its under surface
+  the ventral part of the branchial basket-work, and was originally continuous with the plate over
+  the thyroid gland.</p>
+
+  <div><span class="pagenum" id="page335">{335}</span></div>
+
+  <div class="ac w50 fcenter sp2">
+    <a href="images/fig134.jpg" id="fig134"><img style="width:100%" src="images/fig134.jpg" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 134.&mdash;Skeleton of Head-Region of Ammoc&#x0153;tes. A,
+      Lateral View; B, Ventral View; C, Dorsal View.</span></p>
+      <p class="sp0">Muco-cartilage, <i>red</i>; soft cartilage, <i>blue</i>; hard cartilage,
+      <i>purple</i>. <i>sk<sub>1</sub></i>, <i>sk<sub>2</sub></i>, <i>sk<sub>3</sub></i>, skeletal
+      bars; <i>c.e.</i>, position of pineal eye; <i>na. cart.</i>, nasal cartilage; <i>ped.</i>,
+      pedicle; <i>cr.</i>, cranium; <i>nc.</i>, notochord.</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page336">{336}</span></div>
+
+  <p>In Fig. <a href="#fig134">134</a>, A, B, C, the cranial skeleton of Ammoc&#x0153;tes is
+  represented from the dorsal, ventral, and lateral aspects. The muco-cartilage is coloured red, the
+  branchial or soft cartilage blue, and the hard cartilage purple. The degenerated muco-cartilage of
+  the branchial region is represented as an uncoloured plate, on which the branchial basket-work
+  stands in relief. If it were restored to its original condition of muco-cartilage, it would
+  represent a uniform plate, on the <i>under</i> surface of which the basket-work would be situated;
+  and if it were calcified and made solid, the branchial basket-work would not show at all in these
+  figures.</p>
+
+  <p>Is it possible to find the reason why this skeletal covering has degenerated so early before
+  transformation, and why the thyroid plate remains intact until transformation? We see that all
+  that part which has degenerated is covered over by the somatic muscles,&mdash;by, in fact, muscles
+  which, being innervated by the foremost spinal nerves, belong naturally to the region immediately
+  following the branchial. I suggest, therefore, that the original skeletal covering of
+  muco-cartilage has remained intact only where it has not been invaded and covered over by somatic
+  muscles, but has been invaded by blood and undergone the same kind of degenerative change as
+  overtakes the great mass of this tissue at transformation wherever the somatic muscles have
+  overgrown it.</p>
+
+  <p>The covering somatic muscles in the branchial region form a dorsal and ventral group, of which
+  the latter is formed in the embryo much later than the former, the line of separation between the
+  two groups being the lateral groove, with its row of branchial openings. This groove ends at the
+  first branchial opening, but the ventral and dorsal somatic muscles continue further headwards. It
+  is instructive to see that, although the lateral groove terminates, the separation between the two
+  groups of muscles is still marked out by a ridge of muco-cartilage, represented in Fig. <a
+  href="#fig134">134</a>, A, which terminates anteriorly in the opercular bar.</p>
+
+  <p>Passing now to the prosomatic region, we find that here, too, the muco-cartilaginous external
+  covering is divisible into a dorsal and a ventral head-plate, the ventral head-plate being the
+  plate of the lower lip, and the dorsal head-plate the plate of muco-cartilage over the front part
+  of the head. The staining reaction with thionin maps out this dorsal head-plate in a most
+  beautiful manner, and shows that the whole of the upper lip-region in front of the nasal orifice
+  is one large plate of muco-cartilage, obscured largely by the invasion of the crossing muscles of
+  the upper lip, but left pure and uninvaded all around the nasal orifice, and where the upper and
+  lower lips come together. In addition to this foremost plate, a median tongue of muco-cartilage
+  covers over the pineal eye and fills up the <span class="pagenum" id="page337">{337}</span>median
+  depression between the two median dorsal somatic muscles. Also, two lateral cornua pass
+  caudalwards from the main frontal mass of muco-cartilage over the lateral eyes, forming the
+  well-known wedge which separates the dorsal and lateral portions of the dorso-lateral somatic
+  muscle. In fact, similarly to what we find in the branchial region, the muco-cartilaginous
+  covering can be traced with greater or less completeness only in those parts which are not covered
+  by somatic muscles.</p>
+
+  <p>In Fig. <a href="#fig134">134</a>, A, B, C, this striking muco-cartilaginous head-shield, both
+  dorsal and ventral, is shown. Seeing that the upper lip wraps round the lower one on each side,
+  and that this most ventral edge of the upper lip contains muco-cartilage, as is seen in Fig. <a
+  href="#fig117">117</a>, the dorsal head-shield of muco-cartilage ought, strictly speaking, to
+  extend more ventrally in the drawings. I have curtailed it in order not to interfere with the
+  representation of the lower lip and tentacular muco-cartilages.</p>
+
+  <p>From what has been said, it follows that the past history of the skeletal covering of the whole
+  head-region of Ammoc&#x0153;tes, both frontal and occipital, can be conjectured by means of the
+  ontogenetic history of the foremost myomeres.</p>
+
+  <p>Dohrn and all other observers are agreed that during the development of this animal a striking
+  forward growth of the foremost somatic myomeres takes place, so that, as Dohrn puts it, the
+  body-musculature has extended forwards over the gill-region, and at the same time the gill-region
+  has extended backwards. It is therefore probable that in the ancestral form the myotomes,
+  innervated by the first spinal nerves, immediately succeeded the branchial region. Judging from
+  Ammoc&#x0153;tes, the forward growth was at first confined to the dorsal region, and therefore
+  invaded the dorsal head-plate, the ventral musculature being distinctly a later growth. With
+  respect to this dorsal part of the myotomes, the first myotome is originally situated some
+  distance behind the auditory capsule, and then grows forward towards the nasal opening; the
+  lateral part, according to Hatschek, grows forward more quickly than the dorsal part, and splits
+  itself above and below the eye into a dorso-lateral part, which extends up to the olfactory
+  capsule, and a ventro-lateral part (<i>m. lateralis capitis</i> anterior, superior, and inferior),
+  thus giving rise to the characteristic appearance of the muco-cartilaginous head-shield of
+  Ammoc&#x0153;tes.</p>
+
+  <p>According, then, to the extent of the growth of these somatic <span class="pagenum"
+  id="page338">{338}</span>muscles, the shape of the muco-cartilaginous head-shield will vary, and
+  if it were calcified and then fossilized we should obtain fossil head-shields of widely differing
+  configuration, although such fossils might be closely allied to each other. This is just what is
+  found in this group. Let the muco-cartilage extend over the whole of the branchial region of
+  Ammoc&#x0153;tes, the resulting head-shield would be as in Fig. <a href="#fig135">135</a>, A; the
+  branchial bars below the muco-cartilaginous shield might or might not be evident, and the line
+  between the branchial and the trigeminal region might or might not be indicated. Such a
+  head-shield would closely resemble those of Didymaspis and Tremataspis respectively. Now suppose
+  the somatic musculature to encroach slightly on the branchial region and also laterally to the end
+  of the anterior branchial region, then we should obtain a shape resembling that of Thyestes (Fig.
+  <a href="#fig135">135</a>, B). Continue the same process further, the lateral muscle always
+  encroaching further than the median masses, until the whole or nearly the whole branchial region
+  is invested, and we get the head-shield of Cephalaspis (Fig. <a href="#fig135">135</a>, C);
+  further still, that of Keraspis, and yet still further, that of Ammoc&#x0153;tes (Fig. <a
+  href="#fig135">135</a>, D).</p>
+
+  <div class="ac w35 fcenter sp2">
+    <a href="images/fig135.png" id="fig135"><img style="width:100%" src="images/fig135.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 135.&mdash;Diagrams to show the different shapes of Head-Shields due
+      to the forward growth of the Somatic Musculature.</span></p>
+      <p class="sp0">A, Didymaspis; B, Auchenaspis; C, Cephalaspis; D, Ammoc&#x0153;tes.</p>
+    </div>
+  </div>
+
+  <p class="sp3">So close is this similarity, from the comparative point of view, between the dorsal
+  head-shield of the Osteostraci and the dorsal cephalic region of Ammoc&#x0153;tes that it
+  justifies us in taking Ammoc&#x0153;tes as the nearest living representative of such types; it is
+  justifiable, therefore, to interpret by means of Ammoc&#x0153;tes the position of other organs in
+  these forms. First and foremost is the hard plate <span class="pagenum"
+  id="page339">{339}</span>known as the post-orbital plate, so invariably found. In Fig. <a
+  href="#fig134">134</a>, C, I have inserted (<i>cr.</i>) the position of the membranous cranium of
+  Ammoc&#x0153;tes, and it is immediately evident that the primordial cranium of the Osteostraci
+  must occupy the exact position indicated by this median hard plate. For this very reason this
+  median plate would be harder than the rest in order to afford a better protection to the brain
+  underneath. This plate, because of its position, may well receive the same name as the similar
+  plate in the trilobite and various palæostracans and be called the glabellum.</p>
+
+  <p class="ac"><span class="sc">Evidence of Segmentation in the Head-Shield&mdash;Formation of
+  Cranium.</span></p>
+
+  <p>We may thus conceive the position of the nose, lateral eyes, median eyes, and cranium in these
+  old fishes. In addition, other indications of a segmentation in this head-region have been found.
+  The most striking of all the specimens hitherto discovered are some of <i>Thyestes verrucosus</i>,
+  discovered by Rohon, in which the dorsal shield has been removed, and so we are able to see what
+  that dorsal shield covered.</p>
+
+  <p>In Fig. <a href="#fig136">136</a>, I reproduce his drawing of one of his specimens from the
+  dorsal and lateral aspects. These drawings show that the frontal part of the shield covered a
+  markedly segmented part of the animal; five distinct segments are visible apart from the median
+  most anterior region. This segmented region is entirely confined to the prosomatic region,
+  <i>i.e.</i> to the region innervated by the trigeminal nerve. An indication of similar markings is
+  given in Lankester's figure of <i>Eukeraspis pustuliferus</i> (see Fig. <a href="#fig127">127</a>,
+  B), and, indeed, evidence of a segmentation under the antero-lateral border of the head-shield is
+  recognized at the present time, not only in the Cephalaspidæ, but also in the Pteraspidæ, as was
+  pointed out to me by Smith Woodward in the specimens at the British Museum. Also, in
+  <i>Cyathaspis</i>, Jaekel has drawn attention to markings of a similar segmental nature (Fig. <a
+  href="#fig137">137</a>).</p>
+
+  <p>There seems, then, little doubt but that these primitive fishes possessed something in this
+  region which was of a segmental character, and indicated at least five segments, probably
+  more.</p>
+
+  <p>Rohon entitles his discovery 'the segmentation of the primordial cranium.' It would, I think,
+  be better to call it the segmentation of <span class="pagenum" id="page340">{340}</span>the
+  anterior region of the head, for that is in reality what his figures show, not the segmentation of
+  the primordial cranium, which, to judge from Ammoc&#x0153;tes, was confined to the region of the
+  glabellum.</p>
+
+  <p>What is the interpretation of this appearance?</p>
+
+  <table class="mc tlf sp2 w55" summary="Group of illustrations" title="Group of illustrations">
+    <tr>
+      <td class="vbm ac" style="width:43%;"><a href="images/fig136.jpg" id="fig136"><img
+      style="width:100%" src="images/fig136.jpg" alt="" title=""/></a></td>
+      <td class="vbm ac" style="width:43%;"><a href="images/fig137.png" id="fig137"><img
+      style="width:100%" src="images/fig137.png" alt="" title=""/></a></td>
+    </tr>
+    <tr>
+      <td>
+        <div class="smaller aj it1">
+          <p class="sp0"><span class="sc">Fig. 136.&mdash;Lateral and Dorsal Views of the Frontal
+          and Occipital Regions of the Head-Shield of Thyestes, after Removal of the Outer
+          Surface.</span> (From <span class="sc">Rohon</span>.)</p>
+        </div>
+      </td>
+      <td>
+        <div class="smaller aj it1">
+          <p><span class="sc">Fig. 137.&mdash;Under Surface of Head-Shield of Cyathaspis.</span>
+          (From <span class="sc">Jaekel</span>.)</p>
+          <p class="sp0 ac"><i>A.</i>, lateral eyes; <i>Ep.</i>, median eyes.</p>
+        </div>
+      </td>
+    </tr>
+  </table>
+
+  <p>Any segmentation in the head-region must be indicative of segments belonging to the trigeminal
+  or prosomatic region, or of segments belonging to the vagus or mesosomatic region. Many
+  palæontologists, looking upon segmentation as indicative of gills and gill-slits, have attempted
+  to interpret such markings as branchial segments, regardless of their position. As the figures
+  show, they extend in front of the eyes and reach round to the front middle line, a position which
+  is simply impossible for gills, but points directly to a segmentation connected with the
+  trigeminal nerve. Comparison with Ammoc&#x0153;tes makes it plain enough that the markings in
+  question are prosomatic in position, and that the gill-region must be sought for in the place
+  <span class="pagenum" id="page341">{341}</span>where Schmidt and Rohon located it in Thyestes,
+  viz. the so-called occipital region.</p>
+
+  <p>This discovery of Rohon's is, in my opinion, of immense importance, for it indicates that, in
+  these early fishes, the prosomatic segmentation, associated with the trigeminal nerve, was much
+  more well-marked than in any fishes living in the present day. Why should it be more well-marked?
+  Turning to the palæostracan, it is very suggestive to compare the markings on their prosomatic
+  carapace with these markings. Again and again we find indications of segmentation in these fossils
+  similar to those seen in the ancient fishes. Thus in Fig. <a href="#fig138">138</a> I have put
+  side by side the palæostracan <i>Bunodes</i> and the fish <i>Thyestes</i>, both life size. In the
+  latter I have indicated Rohon's segments; in the former the markings usually seen.</p>
+
+  <p>From the evidence of Phrynus, Mygale, etc., as already pointed out, such markings in the
+  palæostracan fossils would indicate the position of the tergo-coxal muscles of the prosomatic
+  appendages, even though such appendages have not yet been discovered, and it is significant that
+  in all these cases there is a distinct indication of a median plate or glabellum in addition to
+  the segmental markings. Especially instructive is the evidence of Phrynus, as is seen by a
+  comparison of Figs. 107 and 108, which shows clearly that this median plate (<i>glab.</i>) covered
+  the brain-region, a brain-region which is isolated and protected from the tergo-coxal muscles by
+  the growth dorsalwards of the flanges of the plastron. In this way an incipient cranium of a
+  membranous character is formed, which helps to give attachment to these tergo-coxal muscles. As
+  such cranium is derived directly from the plastron, it is natural that it should ultimately become
+  cartilaginous, just as occurs when Ammoc&#x0153;tes becomes Petromyzon and the cartilaginous
+  cranium of the latter arises from the membranous cranium of the former. In Galeodes also the
+  growth dorsalwards of the lateral flanges of the plastron to form an incipient cranium in which
+  the brain lies is very apparent.</p>
+
+  <div class="ac w20 fcenter sp2">
+    <a href="images/fig138.png" id="fig138"><img style="width:100%" src="images/fig138.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p><span class="sc">Fig. 138.</span>&mdash;<span class="sc">A, Outline of</span> <i>Thyestes
+      Verrucosus</i> <span class="sc">with Rohon's Segments indicated; B, Outline of</span>
+      <i>Bunodes Lunula</i> <span class="sc">with Lateral Eyes inserted</span>.</p>
+      <p class="sp0 ac">Both figures natural size.</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page342">{342}</span></div>
+
+  <p>I venture, then, to suggest that in the Osteostraci the median hard plate or glabellum
+  protected a brain which was enclosed in a membranous cranium, very probably not yet complete in
+  the dorsal region&mdash;certainly not complete if the median pineal eyes so universally found in
+  these ancient fishes were functional&mdash;a cranium derived from the basal trabeculæ, in
+  precisely the same manner as we see it already in its commencement in Phrynus and other scorpions.
+  With the completion of this cranium and its conversion into cartilage, and subsequently into bone,
+  an efficient protection was afforded to the most vital part of the animal, and thus the hard
+  head-shield of the Palæostraca and of the earliest fishes was gradually supplanted by the
+  protecting bony cranium of the higher vertebrates.</p>
+
+  <p>Step by step it is easy to follow in the mind's eye the evolution of the vertebrate cranium,
+  and because it was evolved direct from the plastron, the impossibility of resolving it into
+  segments is at once manifest; for although the plastron was probably originally segmented, as
+  Schimkéwitsch thinks, all sign of such segmentation had in all probability ceased, before ever the
+  vertebrates first made their appearance on the earth.</p>
+
+  <p>It follows further, from the comparison here made, that those antero-lateral markings
+  indicative of segments, found so frequently in these primitive fishes, must be interpreted as due
+  not to gills but to aponeuroses, due to the presence of muscles which moved prosomatic appendages,
+  muscles which arose from the dorsal region in very much the same position as do the muscles of the
+  lower lip in Ammoc&#x0153;tes; the latter, as already argued, represent the tergo-coxal muscles of
+  the last pair of prosomatic appendages&mdash;the chilaria or metastoma. Such an interpretation of
+  these markings signifies that the first-formed fishes must have possessed prosomatic appendages of
+  a more definite character than the tentacles of Ammoc&#x0153;tes, something intermediate between
+  those of the palæostracan and Ammoc&#x0153;tes.</p>
+
+  <p class="sp3">For my part I should not be in the least surprised were I to hear that something of
+  the nature of appendages in this region had been found, especially in view of the well-known
+  existence of the pair of appendages in the members of the Asterolepidæ&mdash;large, oar-like
+  appendages which may well represent the ectognaths.</p>
+
+  <div><span class="pagenum" id="page343">{343}</span></div>
+
+  <p class="ac"><span class="sc">The Relationship of the Ostracoderms.</span></p>
+
+  <p>Of the three groups of fishes&mdash;the Heterostraci, the Osteostraci, and the
+  Antiarcha&mdash;the last is Devonian, and therefore the latest in time of the three, while the
+  earliest is the first group, as both Pteraspis and Cyathaspis have been found in lower levels of
+  the Silurian age than any of the Osteostraci, and, indeed, Cyathaspis has been discovered in
+  Sweden in the lower Silurian. This, the earliest of all groups of fishes, is confined to two forms
+  only&mdash;Pteraspis and Cyathaspis,&mdash;for Scaphaspis is now recognized to be the ventral
+  shield of Pteraspis.</p>
+
+  <p>Hitherto a strong tendency has existed in the minds both of the comparative anatomist and the
+  palæontologist to look on the elasmobranchs as the earliest fishes, and to force, therefore, these
+  strange forms of fish into the elasmobranch ranks. For this purpose the same device is often used
+  as has been utilized in order to account for the existence of the Cyclostomata, viz. that of
+  degeneration. The evidence I have put forward is very strongly in favour of a connection between
+  the cyclostomes and the cephalaspids, and agrees therefore with all the rest of the evidence that
+  the jawless fishes are more ancient than those which bore jaws&mdash;the Gnathostomata.</p>
+
+  <p>This is no new view. It was urged by Cope, who classified the Heterostraci, Osteostraci, and
+  Antiarcha under one big group&mdash;the Agnatha&mdash;from which subsequently the Gnathostomata
+  arose. Cope's arguments have not prevailed up to the present time, as is seen in the writings of
+  Traquair, one of the chief authorities on the subject in Great Britain. He is still an advocate of
+  the elasmobranch origin of all these earliest fishes, and claims that the latest discoveries of
+  the Silurian deposits (<i>Thelodus Pagei</i>) and other members of the C&#x0153;lolepidæ confirm
+  this view of the question.</p>
+
+  <p>This view may be summed up somewhat as follows:&mdash;</p>
+
+  <p>Cartilaginous jaws would not fossilize, and the Ostracoderms may have possessed them.</p>
+
+  <p>They may have degenerated from elasmobranchs just as the cyclostomes are supposed to have
+  degenerated.</p>
+
+  <p>Seeing that bone succeeds cartilage, the presence of bony shields in Cephalaspis, etc.,
+  indicates that their precursors were cartilaginous, presumably elasmobranch fishes.</p>
+
+  <p>Of these arguments the strongest is based on the supposed bony <span class="pagenum"
+  id="page344">{344}</span>covering of the Osteostraci, with the consequent supposition that their
+  ancestors possessed a cartilaginous covering. This argument is entirely upset, if, as I have
+  pointed out, the structure of the cephalaspid shield is that of muco-cartilage and not of bone. If
+  these plates are a calcified muco-cartilage, then the whole argument for their ancestry from
+  animals with a cartilaginous skeleton falls to the ground, for muco-cartilage is the precursor not
+  only of bone, but also of cartilage itself.</p>
+
+  <p>The evidence, then, points strongly in favour of Cope's view that the most primitive fishes
+  were Agnatha, after the fashion of cyclostomes, as is also believed by Smith Woodward, Bashford
+  Dean, and Jaekel.</p>
+
+  <p>Among living animals, as I have shown, the Limulus is the sole survivor of the palæostracan
+  type, and Ammoc&#x0153;tes alone gives a clue to the nature of the cephalaspid, <i>i.e.</i> the
+  osteostracan fish. Older than the latter is the heterostracan, Pteraspis, and Cyathaspis. Is it
+  possible from their structure to obtain any clue as to the actual passage from the palæostracan to
+  the vertebrate?</p>
+
+  <p>Here again, as in the case of the Osteostraci, a relationship to the elasmobranch has been
+  supposed, for the following reasons<span class="wnw">:&mdash;</span></p>
+
+  <p>The latest discoveries in the Silurian and Devonian deposits have brought to light strange
+  forms such as Thelodus and Drepanaspis, of which the latter from the Devonian must, according to
+  Traquair, be included in the Heterostraci. It possessed, as seen in Fig. <a
+  href="#fig139">139</a>, large plates, after the fashion of Pteraspis, and also many smaller
+  ones.</p>
+
+  <p>The former, from the upper Silurian, belongs to the C&#x0153;lolepidæ, and was covered over
+  with shagreen composed of small scutes, after the fashion of an elasmobranch. Traquair suggests
+  that Thelodus arose from the original elasmobranch stock; that by the fusion of scutes such a form
+  as Drepanaspis occurred, and, with still further fusion, Pteraspis.</p>
+
+  <p>There are always two ways of looking at a question, and it seems to me possible and more
+  probable to turn the matter round and to argue that the original condition of the surface-covering
+  was that of large plates, as in Pteraspis. By the subsequent splitting up of such plates,
+  Drepanaspis was formed, and later on, by further splitting, the elasmobranch, Thelodus being a
+  stage on the way to the formation of an elasmobranch, and not a backward stage from the
+  elasmobranch towards Pteraspis.</p>
+
+  <div><span class="pagenum" id="page345">{345}</span></div>
+
+  <p>This method of looking at the problem seems to me to be more in consonance with the facts than
+  the reverse; for, as pointed out by Jaekel, the fishes with large plates are the oldest, and in
+  Cyathaspis, the very oldest of all, the size of the plates is most conspicuous; he considers,
+  therefore, this preconceived view that large plates are formed by the fusion of small ones must
+  give way to the opposite belief.</p>
+
+  <div class="ac w35 fcenter sp2">
+    <a href="images/fig139.png" id="fig139"><img style="width:100%" src="images/fig139.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 139.&mdash;Drepanaspis. Ventral and Dorsal Aspects.</span> (After
+      <span class="sc">Lankester</span>.)</p>
+      <p class="sp0"><i>A.</i>, anus; <i>E.</i>, lateral eyes.</p>
+    </div>
+  </div>
+
+  <p>So also Rohon, as quoted by Traquair, who, in his first paper accepted Lankester's view that
+  the ridges of the pteraspidian shield were formed by the fusion of a linear arrangement of numbers
+  of placoid scales, suggests in his second paper that these ridges may have been the most primitive
+  condition of the dermal skeleton of the vertebrate, out of which, by differentiation, the dermal
+  denticles (placoid scales) of the selachian, as well as their modifications in the ganoids,
+  teleosteans, and amphibians, have arisen.</p>
+
+  <p>One thing is agreed upon on all sides; no sign of bone-corpuscles is to be found in this dermal
+  covering of Pteraspis. In the deeper layers are large spaces, the so-called pulp-cavities leading
+  into narrow canaliculi, the so-called dentine canals. The structure is <span class="pagenum"
+  id="page346">{346}</span>looked upon as similar to that of the pulp and dentine canals of many
+  fish-scales.</p>
+
+  <p>On the other hand, this dermal covering of Pteraspis has been compared by Patten with the
+  arrangement of the chitinous structure of certain parts of the external covering of Limulus, a
+  comparison which to my mind presents a great difficulty. The chitin-layers in Limulus are
+  <i>external</i> to the epidermal cells, being formed by them; the layers in Pteraspis which look
+  like chitin must have been <i>internal</i> to the epidermal layer; for each vascular canal which
+  passes from a pulp-cavity on its way to be distributed into the dentine canals of the ridge gives
+  off short side branches, which open directly into the groove between the ridges. If these canals
+  were filled with blood they could not possibly open directly into the open grooves between the
+  ridges; these openings must, therefore, have been covered over with an epithelial layer which
+  covered over the surface of the animal, and consequently the chitin-like structure must have been
+  internal to the epidermis, and not external, as on Patten's view. The comparison of this structure
+  with the dentine of fish-scales signifies the same thing, for in the latter the epidermis is
+  external to the dentine-plates, the hard skeletal structure is in the position of the cutis, not
+  of the cuticle.</p>
+
+  <p>The position appears to me to be this: the dermal cranial skeleton of vertebrates, whether it
+  takes the form of a bony skull or of the dorsal plates of a cephalaspid or a pteraspid is, in all
+  cases, not cuticular, <i>i.e.</i> is not an external formation of the epidermal cells, but is
+  formed in tissue of the nature of connective tissue underlying the epidermis. On the contrary, the
+  hard part of the head-carapace of the palæostracan is an external formation of the epidermal
+  cells.</p>
+
+  <p>If, then, this tissue of Pteraspis is not to be looked upon as chitin, how can we imagine its
+  formation? It is certainly not bone, for there are no bone-corpuscles; it is a very regular
+  laminated structure resembling in appearance chitin rather than anything else.</p>
+
+  <p>As in all cases of difficulty, turn to Ammoc&#x0153;tes and let us see what clue there is to be
+  found there. The skin of Ammoc&#x0153;tes is peculiar among vertebrates in many respects. It
+  consists of a number of epidermal cells, as in Fig. <a href="#fig140">140</a>, the varying
+  function of which need not be considered here, covered over with a cuticular layer which is
+  extraordinarily thick for the cuticle of a vertebrate skin; this cuticular layer is perforated
+  with fine canaliculi, through which the <span class="pagenum" id="page347">{347}</span>secretion
+  of the underlying cells passes, as is seen in Fig. <a href="#fig140">140</a>, A and B. This
+  cuticle corresponds to the chitinous covering of the arthropod, and like it is perforated with
+  canaliculi, and, according to Lwoff, possibly contains chitin. The epidermal cells rest on a thick
+  layer of most striking appearance (Fig. <a href="#fig141">141</a>), for it resembles, in an
+  extraordinary degree, when examined superficially, a layer of chitin; it is called the laminated
+  layer, and is characterized by the extreme regularity of the laminæ. This appearance is due, as
+  the observations of Miss Alcock show, to alternate layers of connective tissue fibres arranged at
+  right angles to each other, each fibre running a straight course and possessing its own nucleus.
+  Although the fibres in each layer are packed close together, they are sufficiently apart to form
+  with the fibres of the alternate layers a meshwork rather than a homogeneous structure, and thus
+  the surface view of this layer shows a regular network of very fine spaces through which
+  nerve-fibres and fluid pass. This layer is easily dissolved in a solution of hypochlorite of soda,
+  a fluid which dissolves chitin. Any one looking at Ammoc&#x0153;tes would say that the only part
+  of its skin which resembles chitin is this laminated layer, and therefore the only part of its
+  skin which would afford an indication of the nature of the skeleton of Pteraspis is this laminated
+  layer, which belongs to the cutis, and not to the cuticle. Yet another significant peculiarity of
+  this layer is its entire disappearance at transformation. Miss Alcock, in a research not yet
+  published, has shown that this layer is completely broken up and absorbed at transformation; the
+  cutis of Petromyzon is formed entirely anew, and no longer presents any regular laminated
+  character, but resembles rather the sub-epidermal connective tissue layer of the skin of higher
+  vertebrates. This laminated layer, then, just like the muco-cartilage, shows, by its complete
+  disappearance at transformation, its ancestral character.</p>
+
+  <div class="ac w25 fcenter sp2">
+    <a href="images/fig140.png" id="fig140"><img style="width:100%" src="images/fig140.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="sp0"><span class="sc">Fig. 140.&mdash;Epithelial Cells of Ammoc&#x0153;tes to show
+      the Canaliculi in the Thick Cuticle (B). A, Transverse Section through the Cuticle.</span></p>
+    </div>
+  </div>
+
+  <p>Very suggestive is the arrangement of the different skeletal <span class="pagenum"
+  id="page348">{348}</span>tissues in the head-region of Ammoc&#x0153;tes. Fig. <a
+  href="#fig141">141</a> represents a section through the head near the pineal eye. Most internally
+  is <i>a</i>, a section of the membranous cranium, then comes <i>b</i>, the muco-cartilaginous
+  skeleton, then <i>c</i>, the laminated layer, and finally <i>d</i>, the external cuticle. If in
+  Ammoc&#x0153;tes we possess an epitome of the history of the vertebrate, how would these layers be
+  represented in the past ages, supposing they could be fossilized?</p>
+
+  <div class="ac w25 fcenter sp2">
+    <a href="images/fig141.png" id="fig141"><img style="width:100%" src="images/fig141.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p><span class="sc">Fig. 141.&mdash;Section of Skin and Underlying Tissues in the Head-Region
+      of Ammoc&#x0153;tes.</span></p>
+      <p class="sp0"><i>a</i>, cranial wall; <i>b</i>, muco-cartilage; <i>c</i>, laminated layer;
+      <i>d</i>, external cuticular layer.</p>
+    </div>
+  </div>
+
+  <p>The most internal layer <i>a</i>, by the formation of cartilage and then bone, represents the
+  great mass of vertebrate fossils; the next layer <i>b</i>, by a process of calcification, as
+  previously argued, represents the head-shield of the Osteostracan fishes; while the cuticular
+  layer <i>d</i>, no longer thin, is the remnant of the Palæostracan head-carapace. Between these
+  two layers, <i>b</i> and <i>d</i>, lies the laminated layer <i>c</i>. Intermediate to the
+  Palæostracan and the Osteostracan comes the Heterostracan, with its peculiar head-shield&mdash;a
+  head-shield whose origin is more easily conceivable as arising from something of the nature of the
+  laminated layer than from any other structure represented in Ammoc&#x0153;tes.</p>
+
+  <p>My present suggestion, then, is this: the transition from the skeletal covering of the
+  Palæostracan to that of the highest vertebrates was brought about by the calcification of
+  successive layers from without inwards, all of which still remain in Ammoc&#x0153;tes and show how
+  the external chitinous covering of the arthropod was gradually replaced by the deep-lying internal
+  bony cranium of the higher vertebrates.</p>
+
+  <p>In Ammoc&#x0153;tes the layer which represents the covering of the <span class="pagenum"
+  id="page349">{349}</span>Palæostracan has already almost disappeared. At transformation the layers
+  representing the stage arrived at by the Heterostracan and the Osteostracan disappear; but the
+  stage representing the higher vertebrates, far from disappearing, by the formation of cartilage
+  reaches a higher stage and prepares the way for the ultimate stage of all&mdash;the formation of
+  the bony cranium.</p>
+
+  <p>So much for the evidence as to the nature of the structure of the head-shield of the
+  Pteraspidæ.</p>
+
+  <p>It suggests that these fishes were covered anteriorly with armoured plates derived from the
+  cutis layer of the skin, a layer which was specially thickened and very vascular, apparently, to
+  enable respiration to be very largely, if not entirely, effected by the surface of the body. It is
+  difficult to understand how the sea-scorpions breathed, and it is easy to see how the formation of
+  ventral and dorsal plates enclosing the mesosomatic appendages may at the outset have hindered the
+  action of the branchiæ. The respiratory chamber, according to my view, had at first the double
+  function of respiration and digestion. A new digestive apparatus was the pressing need at the
+  time; it would, therefore, be of distinct advantage to remove, as much as possible, the burden of
+  respiration from this incipient alimentary canal.</p>
+
+  <p>What can be said as to the shape of these ancient forms of fishes? Certain parts of them are
+  absolutely known, other parts are guesswork. They are known to have possessed a dorsal shield, a
+  ventral shield formerly looked upon as belonging to a separate species, called Scaphaspis, and a
+  spine attached to the dorsal shield. The rest of their configuration, as given in Smith Woodward's
+  restoration (Fig. <a href="#fig142">142</a>) is guesswork; the fish-like body with its scales, the
+  heterocercal tail, is based on the most insufficient evidence of something of the nature of scales
+  having being found near the head-plates.</p>
+
+  <p>The dorsal shield is characterized by a pair of lateral eyes situated on the edge of the
+  shield, not as in Cephalaspis near the middle line. In the middle line, where the rostrum meets
+  the large dorsal plate, median eyes were situated. But the slightest sign of any median single
+  nasal opening, such as is so characteristic of the head-shield of the Osteostraci and of
+  Ammoc&#x0153;tes has never been discovered. The olfactory organ must have been situated on the
+  ventral side as in the larval stage of Ammoc&#x0153;tes, or in the Palæostraca. Many of these
+  head-shields are remarkably well preserved, <span class="pagenum" id="page350">{350}</span>and it
+  is difficult to believe that an olfactory opening would not be seen if any such had existed, as it
+  does in Thyestes.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig142.png" id="fig142"><img style="width:100%" src="images/fig142.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p class="sp0"><span class="sc">Fig. 142.&mdash;Restoration of Pteraspis.</span> (After <span
+      class="sc">Smith Woodward</span>.)</p>
+    </div>
+  </div>
+
+  <p>The difficulty of interpreting these types is the difficulty of understanding their method of
+  locomotion; that is largely the reason why the spine has been placed as if projecting from the
+  back, and a fish-like body with a heterocercal tail-fin added. If, on the contrary, the spine is a
+  terminal tail-spine, then, as far as the fossilized remains indicate, the animal consisted of a
+  dorsal shield, a ventral shield, and a tail-spine, to which must be added two apparently lateral
+  pieces and a few scales. If the animal did not possess a flexible body with a tail-fin, but
+  terminated in a rigid spike after the fashion of a Limulus-like animal, then it must have moved by
+  means of <span class="pagenum" id="page351">{351}</span>appendages. At present we have not
+  sufficient evidence to decide this question.</p>
+
+  <p>That the animal crawled about in the mud by means of free appendages is by no means an
+  impossible view, seeing how difficult it is to find the remains of appendages in the fossils of
+  this far-back time, even when we are sure that they existed. Thus, for many generations, the
+  appendages of trilobites, which occur in such countless numbers, and in such great variety of
+  form, were absolutely unknown, until at last, in consequence of a fortunate infiltration by
+  pyrites, they were found by Beecher preserved down to the minutest detail. Even to this day no
+  trace of appendages has been found in such forms as Hemiaspis, Bunodes, Belinurus,
+  Prestwichia.</p>
+
+  <p>The whole question of the evidence of any prosomatic appendages in these ancient fishes is one
+  of very great interest, and of late years has been investigated by Patten. It has long been known
+  that forms such as Pterichthys and Bothriolepis possessed two large, jointed locomotor appendages,
+  and Patten has lately obtained better specimens of Bothriolepis than have ever been found before,
+  which show not only the general configuration of the fish, but also the presence of mandibles or
+  gnathites in the mouth-region resembling those of an arthropod. These mandibles had been seen
+  before (Smith Woodward), but Patten's specimens are more perfect than any previously described,
+  and cause him to conclude that these ancient fish were of the nature of arthropods rather than of
+  vertebrates.</p>
+
+  <p>Patten has also been able to obtain some excellent specimens of the under surface of the head
+  of Tremataspis, which, as evident in Fig. <a href="#fig143">143</a>, show the presence of a series
+  of holes, ranging on each side from the mouth-opening, in a semicircular fashion towards the
+  middle line. He considers that these openings indicate the attachments of appendages, in
+  opposition to other observers, such as Jaekel, who look upon them as gill-slits. To my mind, they
+  are not in the right position for gill-slits; they are certainly in a prosomatic rather than in a
+  mesosomatic position, and I should not be at all surprised if further research justified Patten's
+  position. So convinced is he of the presence of appendages in all these old forms, that he
+  considers them to be arthropods rather than vertebrates, although, at the same time, he looks upon
+  them as indicating the origin of vertebrates from arthropods. Here, perhaps, it is advisable to
+  say a few words on Patten's attitude towards this question.</p>
+
+  <div><span class="pagenum" id="page352">{352}</span></div>
+
+  <p>Two years after I had put forward my theory of the derivation of vertebrates from arthropods,
+  Patten published, in the <i>Quarterly Journal of Microscopical Science</i>, simultaneously with my
+  paper in that journal, a paper entitled "The Origin of Vertebrates from Arachnids." In this paper
+  he made no reference to my former publications, but he made it clear that there was an absolutely
+  fundamental difference between our treatment of the problem; for he took the old view that of
+  necessity there must be a reversal of surfaces in order that the internal organs should be in the
+  same relative positions in the vertebrate and in the invertebrate. He simply, therefore,
+  substituted Arachnid for Annelid in the old theory. Because of this necessity for the reversal of
+  surfaces he discarded the terms dorsal and ventral as indicative of the surfaces of an animal, and
+  substituted hæmal and neural, thereby hopelessly confusing the issue and making it often very
+  difficult to understand his meaning.</p>
+
+  <div class="ac w30 fcenter sp2">
+    <a href="images/fig143.png" id="fig143"><img style="width:100%" src="images/fig143.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p class="sp0"><span class="sc">Fig. 143.&mdash;Under-Surface of Head-Region in
+      Tremataspis.</span> (After <span class="sc">Patten</span>.)</p>
+    </div>
+  </div>
+
+  <p>He still holds to his original opinion, and I am still waiting to find out when the reversal of
+  surfaces took place, for his investigations lead him, as must naturally be the case, to compare
+  the dorsal (or, as he would call it, the hæmal) surface of Bothriolepis, of the Cephalaspidæ, and
+  of the Pteraspidæ with the dorsal surface of the Palæostraca.</p>
+
+  <p>All these ancient fishes are, according to him, still in the arthropod stage, have not yet
+  turned over, though in a peculiarly unscientific manner he argues elaborately that they must have
+  swum on their back rather than on their front, and so indicated the coming reversal. Because they
+  were arthropods they cannot have had a <span class="pagenum" id="page353">{353}</span>frontal
+  nose-organ; therefore, Patten looks upon the nose and the two lateral eyes of the Osteostraci as a
+  complex median eye, regardless of the fact that the median eyes already existed.</p>
+
+  <p class="sp3">Every atom of evidence Patten has brought forward, every new fact he has
+  discovered, confirms my position and makes his still more hopelessly confused. Keep the animal the
+  right side uppermost, and the evidence of the rocks confirms the transition from the Palæostracan
+  to the Cyclostome; reverse the surfaces, and the attempt to derive the vertebrate from the
+  palæostracan becomes so confused and hopelessly muddled as to throw discredit on any theory of the
+  origin of vertebrates from arthropods. For my own part, I fully expect that appendages will be
+  found not only in the Cephalaspidæ but also in the Pteraspidæ, and I hope Patten will continue his
+  researches with increasing success. I feel sure, however, his task will be much simplified if he
+  abandons his present position and views the question from my standpoint.</p>
+
+  <p class="ac"><span class="sc">Summary.</span></p>
+
+  <div class="bq1 smaller sp5">
+    <p>The shifting of the nasal tube from a ventral to a dorsal position, as seen in
+    Ammoc&#x0153;tes, is, perhaps, the most important of all clues in connection with the comparison
+    of Ammoc&#x0153;tes to the Palæostracan on the one hand, and to the Cephalaspid on the other;
+    for, whereas the exact counterpart of the opening of such a tube is always found on the dorsal
+    head-shield in all members of the latter group, nothing of the kind is ever found on the dorsal
+    carapace of the former group.</p>
+    <p>The reason for this difference is made immediately evident in the development of
+    Ammoc&#x0153;tes itself, for the olfactory tube originates as a ventral tube&mdash;the tube of
+    the hypophysis&mdash;in exactly the same position as the olfactory tube of the Palæostracan, and
+    later on in its development takes up a dorsal position.</p>
+    <p>In fact, Ammoc&#x0153;tes in its development indicates how the Palæostracan head-shield
+    became transformed into that of the Cephalaspid.</p>
+    <p>In another most important character Ammoc&#x0153;tes indicates its relationship to the
+    Cephalaspidæ, for it possesses an external skeleton or head-shield composed of muco-cartilage,
+    which is the exact counterpart of the so-called bony head-shield of the latter group; and still
+    more strikingly the structure of the cephalaspidian head-shield is remarkably like that of
+    muco-cartilage. In the one case, by the deposition of calcium salts, a hard external skeleton,
+    capable of being preserved as a fossil, has been formed; in the other, by the absence of the
+    calcium salts, a soft chondro-mucoid matrix, in which the characteristic cells and fibrils are
+    embedded, distinguishes the tissue.</p>
+    <p>The recognition that the head-shields of these most primitive fishes were not composed of
+    bone, but of muco-cartilage, the precursor of both cartilage and bone, immediately clears up in
+    the most satisfactory manner the whole <span class="pagenum" id="page354">{354}</span>question
+    of their derivation from elasmobranch fishes; for the main argument in favour of the latter
+    derivation is the exceedingly strong one that bone succeeds cartilage&mdash;not <i>vice
+    versâ</i>&mdash;therefore, these forms, since their head-shield is bony, must have arisen from
+    some other fishes with a cartilaginous skeleton, most probably of an elasmobranch nature.
+    Seeing, however, that the structure of their shields resembles muco-cartilage much more closely
+    than bone, and that Ammoc&#x0153;tes forms a head-shield of muco-cartilage closely resembling
+    theirs, there is no longer any necessity to derive the jawless fishes from the gnathostomatous;
+    but, on the contrary, we may look with certainty upon the Agnatha as the most primitive group
+    from which the others have been derived.</p>
+    <p>The history of the rocks shows that the group of fishes, Pteraspis and Cyathaspis, are older
+    than the Cephalaspidæ&mdash;come, therefore, phylogenetically between the Palæostraca and the
+    latter group. In this group the head-shields are of a very different character, without any sign
+    of any structure comparable with that of bone, and although they possessed both lateral and
+    median eyes, there is never in any case any trace of a dorsal nasal orifice. Their olfactory
+    passage, like that of the Palæostraca, must have been ventral.</p>
+    <p>The remarkable comparison which exists between the head-shields of Ammoc&#x0153;tes and
+    Cephalaspis, enables us to locate the position of the brain and cranium of the latter with
+    considerable accuracy, and so to compare the segmental markings found in many of these fossils
+    with the corresponding markings, found either in fossil Palæostraca or on the head-carapaces of
+    living scorpions and spiders, such as Phrynus and Mygale. In all cases the cranial region was
+    covered with a median plate, often especially hard, which corresponded to the glabellum of the
+    trilobite; the growth of the cranium can be traced from its beginnings as the upturned lateral
+    flanges of the plastron to the membranous cranium of Ammoc&#x0153;tes.</p>
+    <p>From such a comparison it follows that the segments, found in the antero-lateral region of
+    the head-shield, were not segments of the cranium, but of parts beyond the region of the
+    cranium, and from their position must have been segments supplied by the trigeminal nerve, and
+    not by the vagus group; segments, therefore, which did not indicate gills and gill-slits, but
+    muscles, innervated by the trigeminal nerve; muscles which, as indicated by the corresponding
+    markings on the carapace of Phrynus, Mygale, etc., were the tergo-coxal muscles of the
+    prosomatic appendages.</p>
+    <p class="sp0">The discovery of the nature of these appendages in the Pteraspidæ and
+    Cephalaspidæ, as well as in the Asterolepidæ (Pterichthys and Bothriolepis), is a problem of the
+    future, though in the latter, not only have the well-known oar-like appendages been long since
+    discovered, but Patten has recently found specimens of Bothriolepis which throw light on the
+    anterior masticating gnathite-like appendages which these ancient forms possessed.</p>
+  </div>
+
+  <div><span class="pagenum" id="page355">{355}</span></div>
+
+  <p class="ac">CHAPTER XI</p>
+
+  <p class="ac"><i>THE EVIDENCE OF THE AUDITORY APPARATUS AND THE ORGANS OF THE LATERAL LINE</i></p>
+
+  <div class="bq1 it1 smaller sp3">
+    <p class="sp0">Lateral line organs.&mdash;Function of this group of organs.&mdash;Poriferous
+    sense-organs on the appendages in Limulus.&mdash;Branchial sense-organs.&mdash;Prosomatic sense
+    organs.&mdash;Flabellum.&mdash;Its structure and position.&mdash;Sense-organs of
+    mandibles.&mdash;Auditory organs of insects and arachnids.&mdash;Poriferous chordotonal
+    organs.&mdash;Balancers of Diptera.&mdash;Resemblance to organs of
+    flabellum.&mdash;Racquet-organs of Galeodes.&mdash;Pectens of scorpions.&mdash;Large size of
+    nerve to all these special sense-organs.&mdash;Origin of parachordals and auditory
+    capsule.&mdash;Reason why VIIth nerve passes in and out of capsule.&mdash;Evidence of
+    Ammoc&#x0153;tes.&mdash;Intrusion of glandular mass round brain into auditory
+    capsule.&mdash;Intrusion of generative and hepatic mass round brain into base of
+    flabellum.&mdash;Summary.</p>
+  </div>
+
+  <p>When speaking of the tripartite arrangement of the cranial nerves, an arrangement which gave
+  the clue to the meaning of the cranial segments, I spoke of the trigeminal as supplying the
+  sensory nerves to the skin in the head-region, and I compared this dorsal system of afferent
+  nerves to the system of epimeral nerves in Limulus which supply the prosomatic and mesosomatic
+  carapaces of Limulus with sensory fibres. I compared the ventral system of eye-muscle nerves with
+  the system of nerves supplying the segmental dorso-ventral somatic muscles of the prosomatic
+  region, and I compared the lateral system of mixed nerves with the nerves supplying the prosomatic
+  and mesosomatic appendages of Limulus. I compared, also, the optic nerves and the olfactory nerves
+  with the corresponding nerves in the same invertebrate group. My readers will see at once that one
+  well-marked group of nerves&mdash;the auditory and lateral line system&mdash;has been entirely
+  omitted up to the present, it has not even been mentioned in the scheme of the cranial segments; I
+  have purposely reserved its consideration until now, because the organs these nerves supply,
+  though situated in the skin, are of such a special character <span class="pagenum"
+  id="page356">{356}</span>as to form a category by themselves. These nerves cannot be classed among
+  the afferent nerves of the skin any more than the nerves of the optic and olfactory apparatus;
+  they require separate consideration. A very extensive literature has grown up on the subject of
+  this system of lateral line sense-organs and their innervation, the outcome of which is decisively
+  in favour of this system being classed with the sense-organs supplied by the auditory nerve, so
+  that in endeavouring to understand the position of the auditory nerve, we must always bear in mind
+  that any theory as to its origin must apply to the system of lateral line nerves as well.</p>
+
+  <p>Now, although the auditory apparatus is common to all vertebrates, the lateral line system is
+  not found in any land-dwelling animals; it belongs essentially to the fishes, and is, therefore,
+  an old system so far as concerns the vertebrate group. Its sense-organs are arranged along the
+  lateral line of the fish, and, in addition, on the head-region in three well-marked lines known as
+  the supra-orbital, infra-orbital, and mandibular line systems. These sense-organs lie in the skin
+  in a system of canals, and are innervated by a special nervous system different to that
+  innervating adjacent skin-areas. The great peculiarity of their innervation consists in the fact
+  that their nerves all belong to the branchial system of nerves; no fibres arise in connection with
+  the trigeminal, but all of them in connection with the facial, glossopharyngeal and vagus nerves.
+  In other words, although organs in the skin, their nerve-supply belongs to the lateral nervous
+  system which supplies splanchnic and not somatic segments, a system which, according to the theory
+  advanced in this book, originated in the nerves supplying appendages. The conclusion, therefore,
+  is that in order to obtain some clue as to the origin of the sense-organs of this system in the
+  assumed palæostracan ancestor, we must examine the mesosomatic appendages and see whether they
+  possess any special sense-organs of similar function.</p>
+
+  <p>Further, considering that the auditory organ is to be regarded as a specially developed member
+  of this system, we must especially look for an exceptionally developed organ in the region
+  supplied by the auditory nerve.</p>
+
+  <p>The question of the origin of this system of lateral line sense-organs possesses a special
+  interest for all those who attempt to obtain a solution of the origin of vertebrates, for the
+  upholders of the view that the vertebrates have descended from annelids have always <span
+  class="pagenum" id="page357">{357}</span>found its strongest support in the similarity of two sets
+  of segmental organs found in annelids and vertebrates. On the one hand, great stress was laid upon
+  the similarity of the segmental excretory organs in the two groups of animals, as will be
+  discussed later; on the other, of the similarity of the segmentally arranged lateral
+  sense-organs.</p>
+
+  <p>These lateral sense-organs of the annelids have been specially described by Eisig in the
+  Capitellidæ, and, according to Lang, "there are many reasons for considering these lateral organs
+  to be homologous with the dorsal cirri of the ventral parapodia of other Polychæta, and in the
+  family of the Glyceridæ we can follow, almost step by step, the transformation of the cirri into
+  lateral organs." Eisig describes them in the thoracic prebranchial region as slightly different
+  from those in the abdominal branchial region; in the latter region, the ventral parapodia are
+  gill-bearing, so that these lateral organs are in the branchial region closely connected with the
+  branchiæ, just as is also the case in the vertebrates. It is but a small step from the
+  gill-bearing ventral parapodia of the annelid to the gill-bearing appendages of the phyllopod-like
+  protostracan; so that if we assume that this is the correct line along which to search for the
+  origin of the vertebrate auditory apparatus, then, on my theory of the origin of the vertebrates
+  from a group resembling the Protostraca, it follows that special sense-organs must have existed
+  either on or in close connection with the branchial and prebranchial appendages of the
+  protostracan ancestor of the vertebrates, which would form an intermediate link between the
+  lateral organs of the annelids and the lateral and auditory organs of the vertebrates.</p>
+
+  <p>Further, these special sense-organs could not have been mere tactile hairs, but must have
+  possessed some special function, and their structure must have been compatible with that function.
+  Can we obtain any clear conception of the original function of this whole system of
+  sense-organs?</p>
+
+  <p>A large amount of experimental work has been done to determine the function of the lateral line
+  organs in fishes, and they have been thought at one time or another to be supplementary organs for
+  equilibration, organs for estimating pressure, etc. The latest experimental work done by Parker
+  points directly to their being organs for estimating slow vibrations in water in contradistinction
+  to the quicker vibrations constituting sound. He concludes that surface wave-movements, whether
+  produced by air moving on the water or <span class="pagenum" id="page358">{358}</span>solid bodies
+  falling into the water, are accompanied by disturbances which are stimuli for the lateral line
+  organs.</p>
+
+  <p>One of these segmental organs has become especially important and exists throughout the whole
+  vertebrate group, whether the animal lives on land or in water&mdash;this is the auditory organ.
+  Throughout, the auditory organ has a double function&mdash;the function of hearing and the
+  function of equilibration. If, then, this is, as is generally supposed, a specialized member of
+  the group, it follows that the less specialized members must possess the commencement of both
+  these functions, just as the experimental evidence suggests.</p>
+
+  <p>In our search, then, for the origin of the auditory organ of vertebrates, we must look for
+  special organs for the estimation of vibrations and for the maintenance of the equilibrium of the
+  animal, situated on the appendages, especially the branchial or mesosomatic appendages; and,
+  further, we must specially look for an exceptional development of such segmental organs at the
+  junction of the prosomatic and mesosomatic regions.</p>
+
+  <p>Throughout this book the evidence which I have put forward has in all cases pointed to the same
+  conclusion, viz. that the vertebrate arose by way of the Cephalaspidæ from some arthropod, either
+  belonging to, or closely allied to, the group called Palæostraca, of which the only living
+  representative is Limulus. If, then, my argument so far is sound, the appendages of Limulus, both
+  prosomatic and mesosomatic, ought to possess special sense-organs which are concerned in
+  equilibration or the appreciation of the depth of the water, or in some modification of such
+  function, and among these we might expect to find that somewhere at the junction of the prosoma
+  and mesosoma such sense-organs were specially developed to form the beginning of the auditory
+  organ.</p>
+
+  <p class="sp3">Now, it is a striking fact that the appendages of Limulus do possess special
+  sense-organs of a remarkable character, which are clearly not simply tactile. Thus Gegenbaur, as
+  already stated, has drawn attention to the remarkable branchial sense-organs of Limulus; and
+  Patten has pointed out that special organs, which he considers to be gustatory in function, are
+  present on the mandibles of the prosomatic appendages. I myself, as mentioned in my address to the
+  British Association at Liverpool in 1896, searched for some special sense-organ at the junction of
+  the prosoma and mesosoma, and was rewarded by finding that that extraordinary adjunct to the <span
+  class="pagenum" id="page359">{359}</span>last locomotor appendage, known as the flabellum, was an
+  elaborate sense-organ. I now propose to show that all these special sense-organs are constructed
+  on a somewhat similar plan; that the structure of the branchial sense-organs suggests that they
+  are organs for the estimation of water pressures; that among air-breathing arthropods
+  sense-organs, built up on a somewhat similar plan, are universally found, and are considered to be
+  of the nature of auditory and equilibration organs; and, what is especially of importance, in view
+  of the fact that the most prominent members of the Palæostraca were the sea-scorpions, that the
+  remarkable sense-organs of the scorpions known as the pectens belong apparently to the same
+  group.</p>
+
+  <p class="ac"><span class="sc">The Poriferous Sense-Organs of the Appendages in
+  Limulus.</span></p>
+
+  <p>On all the branchial appendages in Limulus, special sense-organs are found of a most
+  conspicuous character. They form in the living animal bluish convex circular patches, the
+  situation of which on the appendages is shown in Fig. <a href="#fig58">58</a>. These organs are
+  not found on the non-branchial operculum. Gegenbaur, who was the first to describe them, has
+  pointed out how the surface of the organ is closely set with chitinous goblets shaped as seen in
+  Fig. <a href="#fig144">144</a>, A, which do not necessarily project free on the surface, but are
+  extruded on the slightest pressure. Each goblet fits into a socket in the chitinous covering, and
+  is apparently easily protruded by variations of pressure from within. The whole surface of the
+  organ on the appendage is slightly bulged in the living condition, and the chitin is markedly
+  softer here than in the surrounding part of the limb. Each of these organs is surrounded by a
+  thick protection of strongly branching spines. On the surface of the organ itself no spines are
+  found, only these goblets, so that the surface-view presents an appearance as in Fig. <a
+  href="#fig144">144</a>, B. Each goblet possesses a central pore, which is the termination of a
+  very fine, very tortuous, very brittle chitinous tubule (<i>ch.t.</i>), which passes from the
+  goblet through the layers of the chitin into the subjacent tissue. The goblets vary considerably
+  in size, a few very large ones being scattered here and there. The fine chitinous tubule is
+  especially conspicuous in connection with these largest goblets. In the smaller ones there is the
+  same appearance of a pore and a commencing tube, but I have not been able to trace the tube
+  through the chitinous layers, as in the case of the larger goblets.</p>
+
+  <div><span class="pagenum" id="page360">{360}</span></div>
+
+  <table class="mc tlf sp2 w50" summary="Group of illustrations" title="Group of illustrations">
+    <tr>
+      <td class="vbm ac" style="width:44%;"><a href="images/fig144.png" id="fig144"><img
+      style="width:100%" src="images/fig144.png" alt="" title=""/></a></td>
+      <td class="vbm ac" style="width:44%;"><a href="images/fig145.png" id="fig145"><img
+      style="width:100%" src="images/fig145.png" alt="" title=""/></a></td>
+    </tr>
+    <tr>
+      <td>
+        <div class="smaller aj it1">
+          <p class="sp0"><span class="sc">Fig. 144.&mdash;A, A Goblet from one of the Branchial
+          Sense-organs of Limulus</span> (<i>ch.t.</i>, chitinous tubule); <span class="sc">B,
+          Surface View of a Portion of a Branchial Sense-Organ</span>.</p>
+        </div>
+      </td>
+      <td>
+        <div class="smaller aj it1">
+          <p class="sp0"><span class="sc">Fig. 145.&mdash;The Endognaths of Limulus pushed out of
+          the way on one side in order to show the position of the Flabellum</span> (<i>fl.</i>)
+          <span class="sc">projecting towards the crack between the Prosomatic and Mesosomatic
+          Carapaces</span>.</p>
+        </div>
+      </td>
+    </tr>
+  </table>
+
+  <p>Gegenbaur, in his picture, draws a straight tubule passing from every goblet among the fine
+  canaliculi of the chitin. He says they are difficult to see, except in the case of the larger
+  goblets. The tubule from the larger goblets is most conspicuous, and is in my sections always
+  tortuous, never straight, as represented by Gegenbaur. A special branch of the appendage-nerve
+  passes to these organs, and upon the fine branches of this nerve groups of ganglion-cells are
+  seen, very similar in appearance to the groups described by Patten on the terminal branches of the
+  nerves which supply the mandibular organs. At present I can see no mechanism by which the goblets
+  are extruded or returned into place. In the case of the Capitellidæ, Eisig describes retractor
+  muscles by means of which the lateral sense-organs are <span class="pagenum"
+  id="page361">{361}</span>brought below the level of the surface, and he imagines that the
+  protrusion is effected by hydraulic means, by the aid of the vascular system. In the branchial
+  sense-organs of Limulus there are no retractor muscles, and it seems to me that both retraction
+  and protrusion must be brought about by alterations of pressure in the vascular fluids. Certainly
+  the cavity of the organ is very vascular. If this be so, it seems likely enough that such an organ
+  should be a very delicate organ for estimating changes in the pressure of the external medium, for
+  the position of the goblets would depend on the relation between the pressure of the fluid inside
+  the organ and that on the surface of the appendage. Whether the chitinous tubule contains a
+  nerve-terminal or not I am unable to decide from my specimens, but, judging from Patten's
+  description of the similar chitinous tubules belonging to the mandibular organs, it is most highly
+  probable that these tubules also contain a fine terminal nerve-fibre.</p>
+
+  <p>These organs, then, represent segmental branchial sense-organs, of which it can be said their
+  structure suggests that they may be pressure-organs; but the experimental evidence is at present
+  wanting.</p>
+
+  <p>Passing now from the branchial to the prosomatic region, the first thing that struck me was the
+  presence of that most conspicuous projection at the base of the last locomotor appendage, which is
+  usually called the flabellum, and has been described by Lankester as an exopodite of this
+  appendage. It is jointed on to the most basal portion of the limb (<i>cf.</i> Fig. <a
+  href="#fig155">155</a>), and projects dorsally from the limb into the open slit between the
+  prosomatic and mesosomatic carapace, as is seen in Fig. <a href="#fig145">145</a> (<i>fl.</i>). Of
+  its two surfaces, the undermost is very convex and the uppermost nearly flat from side to side,
+  the whole organ being bent, so that when the animal is lying half buried in sand, entirely covered
+  over by the prosomatic and mesosomatic carapaces except along this slit between the two, the upper
+  flat or slightly convex surface of the flabellum is exposed to any movement of water through this
+  slit, and owing to its possessing a joint, the direction of the whole organ can be altered to a
+  limited extent. The whole of this flat upper surface is one large sense-organ of a striking
+  character, thus forming a great contrast to the convex under surface, which is remarkably free
+  from tactile spines or special sense-organs.</p>
+
+  <p>The nerve going to the flabellum is very large, almost as large as the nerve to the rest of the
+  appendage, and the very large majority <span class="pagenum" id="page362">{362}</span>of the
+  nerve-fibres turn towards the flat, uppermost side, where the sense-organ is situated. Between the
+  nerve-fibres (<i>n.</i>) and the chitinous surface containing the special sense-tubes masses of
+  cells (<i>gl.</i>) are seen, as in Fig. <a href="#fig146">146</a>, apparently nerve-cells, which
+  form a broad border between the nerve-fibres and the pigmented chitinogenous layer (<i>p.</i>). On
+  the opposite side, nothing of the sort intervenes between the pigmented layer and the blood-spaces
+  and nerve-fibres which constitute the central mass of the flabellum.</p>
+
+  <table class="mc tlf sp2 w50" summary="Group of illustrations" title="Group of illustrations">
+    <tr class="vmi">
+      <td class="vbm ac" style="width:46%;"><a href="images/fig146.png" id="fig146"><img
+      style="width:100%" src="images/fig146.png" alt="" title=""/></a>
+        <div class="smaller aj it1">
+          <p class="ac"><span class="sc">Fig. 146.&mdash;Section through Flabellum.</span></p>
+          <p class="sp0"><i>ch.</i>, chitinous layers; <i>s.o.</i>, sense-organs; <i>sp.</i>,
+          spike-organ; <i>p.</i>, pigment layer; <i>gl.</i>, ganglion cell layer; <i>bl.</i> and
+          <i>n.</i>, blood-spaces and nerves.</p>
+        </div>
+      </td>
+      <td class="vbm ac" style="width:46%;"><a href="images/fig147.png" id="fig147"><img
+      style="width:100%" src="images/fig147.png" alt="" title=""/></a>
+        <div class="smaller aj it1">
+          <p class="sp0"><span class="sc">Fig. 147.&mdash;Section parallel to the Surface of
+          Flabellum, showing the Porous Terminations of the Sense-Organs and the Arrangement of the
+          Canaliculi round them.</span></p>
+        </div>
+      </td>
+    </tr>
+  </table>
+
+  <p>At present I am inclined to look upon this mass of cells as constituting a large ganglion,
+  which extends over the whole length and breadth of the upper surface of the flabellum. At the same
+  <span class="pagenum" id="page363">{363}</span>time, my preparations are not sufficiently clear to
+  enable me to trace out the connections of these cells, especially their connections with the
+  special sense-organs.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig148.png" id="fig148"><img style="width:67%" src="images/fig148.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 148.&mdash;Section through the three Sense-Organs of
+      Flabellum.</span></p>
+      <p class="sp0"><i>bl.</i>, blood-spaces; <i>n.</i>, nerve; <i>gl.</i>, layer of
+      ganglion-cells; <i>p.</i>, pigment layer; <i>ch.</i>, 1, 2, 3, the three layers of chitin;
+      <i>ch.t.</i>, chitinous tubule in large tube of sense-organ; <i>cap.</i>, capitellum or
+      swollen extremity of large tube; <i>can.</i>, very fine porous canals or canaliculi of
+      chitin.</p>
+    </div>
+  </div>
+
+  <p>In Fig. <a href="#fig148">148</a> I give a magnified representation of a section through three
+  of these flabellar sense-organs. As is seen, the section divides itself into four zones: (1) the
+  chitinous layer (<i>ch.</i>); (2) the layer of pigment (<i>p.</i>) and hypodermal cells; (3) the
+  layer of ganglion-cells (<i>gl.</i>); and (4) the layer of nerve-fibres (<i>n.</i>) and
+  blood-spaces (<i>bl.</i>). The chitinous layer is composed of the usual three zones of the Limulus
+  surface&mdash;externally (Fig. <a href="#fig148">148</a>), a thin homogeneous layer, followed by a
+  thick layer of chitin (3), in which the fine vertical tubules or canaliculi are well marked; the
+  external portion (2) of this layer is differentiated from the rest by the presence of well-marked
+  horizontal layers in addition to the canaliculi.</p>
+
+  <p>In this chitinous layer the special sense-organs are found. They consist of a large tube which
+  passes through all the layers of the chitin except the thin homogeneous most external layer. <span
+  class="pagenum" id="page364">{364}</span>This tube is conical in shape, its base, which rests on
+  the pigmented layer, being so large and the organs so crowded together that a section of the
+  chitin across the base of the tubes gives the appearance of a honeycomb, the septa of which is all
+  that remains of the chitin. This large tube narrows down to a thin elongated neck as it passes
+  through the chitin, and then, at its termination, bulges out again into an oval swelling
+  (<i>cap.</i>) situated always beneath the homogeneous most external layer of chitin. Within this
+  tube a fine chitinous tubule (<i>ch. t.</i>) is situated similar to that seen in the branchial
+  sense-organs; it lies apparently free in the tube, not straight, but sinuous, and it passes right
+  through all the chitinous layers to open at the surface as a pore; in the last part of its course,
+  where it passes through the most external layer (1) of chitin, it lies always at right angles to
+  the surface.</p>
+
+  <p>If the flabellum be stained with methylene blue and acid fuchsin, then all the canaliculi in
+  the chitin show up as fine red lines, and present the appearance given in Fig. <a
+  href="#fig148">148</a>, and it is seen that each of the terminations of the tubules is surrounded
+  in the homogeneous layer of chitin by a thick-set circular patch of canaliculi which pass to the
+  very surface of the chitin, while the canaliculi in other parts terminate at the commencement of
+  the homogeneous layer and do not reach the surface. Further, the contents of the oval swelling,
+  and, indeed, of the tube as a whole, are stained blue, the chitinous tubule being either unstained
+  or slightly pink in colour. We see, then, that the chitinous tubule alone reaches the surface,
+  while the large tube, which contains the tubule, terminates in an oval swelling, which often
+  presents a folded or wrinkled appearance, as in Fig. <a href="#fig149">149</a> (see also Patten's
+  Fig. 1, Plate I.). This terminal bulging of the tube is reminiscent of the bulging in the
+  chitinous tubes of the lyriform organs of the Arachnida, as described by Gaubert, and of the
+  poriferous chordotonal organs in insects, as described by Graber (see Fig. <a
+  href="#fig150">150</a>). This terminal swelling is filled with a homogeneous refringent mass
+  staining blue with methylene blue, in which I have seen no trace of a nucleus; through this the
+  chitinous tubule makes its way without any sign of bulging on its part. Patten, in his description
+  of the sense-organs on the mandibles of Limulus, which are evidently the same in structure as
+  those on the flabellum, refers to this homogeneous mass as a coagulum. I doubt whether this is an
+  adequate description; it appears to me to stain rather more <span class="pagenum"
+  id="page365">{365}</span>readily than a blood-coagulum, yet in the sense of being structureless it
+  resembles a coagulum.</p>
+
+  <p>The enormous number of these organs crowded together over the whole flat surface of the
+  flabellum produces a very striking appearance when viewed on the surface. Such a view presents an
+  appearance resembling that of the surface-view of the branchial sense-organs; in both cases the
+  surface is covered with a great number of closely set circular plaques, in the centre of each of
+  which is seen a well-marked pore. The circular plaques in the case of the flabellum are much
+  smaller than those of the branchial sense-organs, and clearly are not protrusible as in the latter
+  organs, the appearance as of a plaque being due to the ring of thickly-set canaliculi round the
+  central tubule, as already described. When stained with methylene blue, the surface view of the
+  flabellum under a low power presents an appearance of innumerable circular blue masses, from each
+  of which springs a fine bent hair, terminating in a pore at the surface. The blue masses are the
+  homogeneous substance (<i>cap.</i>) of the bulgings seen through the transparent external layer of
+  chitin, and the hairs are the terminal part of the chitinous tubules. Patten has represented their
+  appearance in the mandibles in his Fig. 2, Plate I.</p>
+
+  <p>The large tubes in the chitin alter in shape according to their position. Those in the middle
+  of the sensory surface of the flabellum, in their course through the chitinous layers, are hardly
+  bent at all; as they approach the two lateral edges of this surface, their long thin neck becomes
+  bent more and more, the bending always being directed towards the middle of the surface (see Fig.
+  <a href="#fig146">146</a>); in this way the chitinous tubules increase more or less regularly in
+  length from the centre of the organ to the periphery. The large basal part of the conical tube
+  contains, besides the chitinous tubule, a number of nuclei which are confined to this part of the
+  tube; some of these nuclei look like those belonging to nerve-fibres, others are apparently the
+  nuclei of the chitinogenous membrane lining the tube. I have never seen any sign of nerve-cells in
+  the tube itself.</p>
+
+  <p>The only other kind of sense-organ I have found in connection with these sense-organs are a few
+  spike-like projections, the appearance of which is given in Fig. <a href="#fig149">149</a>. I have
+  always seen these in the position given in Fig. <a href="#fig146">146</a> (<i>sp.</i>),
+  <i>i.e.</i> at the junction of the surface which contains the sense-organs and the surface which
+  is free from them. They are, so far as I have seen, not very numerous; I have <span
+  class="pagenum" id="page366">{366}</span>not, however, attempted to examine the whole sense-organ
+  for the purpose of estimating their number and arrangement.</p>
+
+  <p>As is seen in Fig. <a href="#fig149">149</a>, they possess a fine tubule of the same character
+  as that of the neighbouring sense-organs, which apparently terminates at the apex of the
+  projecting spike. They appear to belong to the same group as the other poriferous sense-organs,
+  and are of special interest, because in their appearance they form a link between the latter and
+  the poriferous sense-organs which characterize the pecten of the scorpion (<i>cf.</i> Fig. <a
+  href="#fig152">152</a>, C).</p>
+
+  <div class="ac w25 fcenter sp2">
+    <a href="images/fig149.png" id="fig149"><img style="width:100%" src="images/fig149.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 149.&mdash;Spike-Organ of Flabellum.</span></p>
+      <p class="sp0"><i>ch.t.</i>, chitinous tubule.</p>
+    </div>
+  </div>
+
+  <p>Such, then, is the structure of this remarkable sense-organ of the flabellum, as far as I have
+  been able to work it out with the materials at my disposal. It is evident that the flabellar
+  organs, apart from the spike-organs, are of the same kind as those described by Patten on the
+  mandibles and chelæ of Limulus, and therefore it is most probable that the nerve-terminals in the
+  chitinous tubules, and the origin of the latter, are similar in the two sets of organs.</p>
+
+  <p>These organs, as Patten has described them, are situated in lines on the spines of the
+  mandibles of the prosomatic locomotor appendages, and are grouped closely together to form a
+  compact sense-organ on the surface of the inner mandible (Lankester's epicoxite) (<i>i.m.</i> in
+  Fig. <a href="#fig155">155</a>), so that a surface-view of the organ here gives the characteristic
+  appearance of these poriferous sense-patches. Precisely similar organs are found on the chilaria,
+  which are, in function at all events, simply isolated mandibles, to use Patten's terminology.</p>
+
+  <p>On the digging appendage (ectognath), as the comparison of Fig. <a href="#fig155">155</a>, A
+  and C, shows, the mandibular spines are almost non-existent, and the inner mandible or epicoxite
+  is not present, so that <span class="pagenum" id="page367">{367}</span>the special sense-organ of
+  this appendage is represented solely by the flabellum.</p>
+
+  <p>This sketch of the special sense-organs of Limulus shows that all the appendages of Limulus
+  possess special sense-organs, with the exception of the operculum. All these sense-organs are
+  formed on the same plan, in that they possess a fine chitinous tubule passing through the layers
+  of chitin into the underlying hypodermal and nervous tissues, which terminates on the surface in a
+  pore. The surface of the chitin where these pores are situated is perfectly smooth, although, in
+  the case of the branchial sense-organs, the goblet-shaped masses of chitin, each of which contains
+  a pore, are able to be pressed out beyond the level of the surface.</p>
+
+  <p>As to their functions, we unfortunately do not know much that is definite. Patten considers
+  that he has evidence of a gustatory function in the case of the mandibular organs, and suggests
+  also a temperature-sense in the case of some of these organs. The large organ of the flabellum and
+  the branchial organs he has not taken into consideration. The situation of these organs puts the
+  suggestion of any gustatory function, as far as they are concerned, out of the question; and I do
+  not think it probable that such large specialized organs would exist only for the estimation of
+  temperature, when one sees how, in the higher animals, the temperature-nerves and the nerves of
+  common sensation are universally distributed over the body. As already stated, the structure of
+  the branchial organs seems to me to point to organs for estimating varying pressures more than
+  anything else, and I am strongly inclined to look upon the whole set of organs as the derivatives
+  of the lateral sense-organs of annelids, such as are described by Eisig in the Capitellidæ. This
+  is Patten's opinion with respect to the mandibular organs; and from what I have shown, these
+  organs cannot be separated in type of structure from those of the flabellum and the branchial
+  sense-organs.</p>
+
+  <p>In our search, then, for the origin of the vertebrate auditory organ in Limulus and its allies,
+  we see so far the following indications<span class="wnw">:&mdash;</span></p>
+
+  <p>1.  The auditory organ of the vertebrate is regarded as a special organ belonging to a
+  segmentally arranged set of lateral sense-organs, whose original function was co-ordination and
+  equilibration.</p>
+
+  <p>2.  Such a set of segmentally arranged lateral sense-organs is found in annelids in connection
+  with the dorsal cirri of the ventral parapodia.</p>
+
+  <div><span class="pagenum" id="page368">{368}</span></div>
+
+  <p>3.  If, as has been supposed, there is a genetic connection between (1) and (2) and if, as I
+  suppose, the vertebrates did not arise from the annelids directly, but from a protostracan group,
+  then it follows that the lateral sense-organs, one of which gave rise to the auditory organ, must
+  have been situated on the protostracan appendages.</p>
+
+  <p>4.  In Limulus, which is the sole surviving representative of the palæostracan group, such
+  special sense-organs are found on both the prosomatic and mesosomatic appendages, and therefore
+  may be expected to give a direct clue to the origin of the vertebrate auditory organ.</p>
+
+  <p class="sp3">5.  Both from its position, its size, and its specialization, the flabellum,
+  <i>i.e.</i> an organ corresponding to the flabellum, must be looked upon as more likely to give a
+  direct clue to the origin of the auditory organ than the sense-organs of the branchial appendages,
+  or the so-called gustatory organs of the mandibles.</p>
+
+  <p class="ac"><span class="sc">The Auditory Organs of Arachnids and Insects.</span></p>
+
+  <p>The difficulty of the investigating these organs consists in the fact that so little is known
+  about them in those Arthropoda which live in the water; the only instance of any organ apparently
+  of the nature of an auditory organ, is the pair of so-called auditory sacs at the base of the
+  antennæ in various decapods. We are in a slightly better position when we turn to the land-living
+  arthropods; here the presence of stridulating organs in so many instances carries with it the
+  necessity of an organ for appreciating sound. It has now been shown that such stridulating organs
+  are not confined to the Insecta, but are present also in the scorpion group, and I myself have
+  added to their number by the discovery of a distinct stridulating apparatus in various members of
+  the Phrynidæ. We may then take it for granted that arachnids as well as insects hear. Where is the
+  auditory organ?</p>
+
+  <p>Many observers believe that certain surface-organs found universally among the spiders, to
+  which Gaubert has given the name of lyriform organs, are auditory in function. His investigations
+  show that they are universally present on the limbs and pro-meso-sternite of all spiders; that
+  they are present singly, not in groups, on the limbs of Thelyphonus, and that a group of them
+  exists on the second segment of each limb in the members of the Phrynus tribe. In the latter case
+  this organ is the most elaborate of all described by him.</p>
+
+  <div><span class="pagenum" id="page369">{369}</span></div>
+
+  <p>It is especially noticeable that they do not exist in Galeodes or in the scorpions, but in the
+  former special sense-organs are found in the shape of the so-called 'racquet-organs,' on the basal
+  segments of the most posterior pair of appendages, and also, according to Gaubert, on the
+  extremity of the palps and the first pair of feet, while in the latter they occur in the shape of
+  the pectens.</p>
+
+  <p>This observation of Gaubert suggests that the place of the lyriform organs in other arachnids
+  is taken in Galeodes by the racquet-organs, and in the scorpions by the pectens. Bertkau,
+  Schimkéwitsch, and Wagner, as quoted by Gaubert, all suggest that the lyriform organs of the
+  arachnids belong to the same group of sense-organs as the porous chordotonal organs of the
+  Insecta, sense-organs which have been found in every group of Insecta, and are generally regarded
+  as auditory organs. Gaubert does not agree with this, and considers the lyriform organs to be
+  concerned with the temperature-sense rather than with audition.</p>
+
+  <p>The chordotonal organs of insects have been specially studied by Graber. He divides them into
+  two groups, the poriferous and the non-poriferous, the former being characterized by the presence
+  of pores on the surface arranged in groups or lines. These poriferous chordotonal organs are
+  remarkably constant in position, being found only at the base of the wings on the subcostal ridge,
+  in marked contrast to the other group of chordotonal organs which are found chiefly on the
+  appendages in various regions. The striking character of this fixity of position of these organs
+  and the universality of their presence in the whole group, led Graber to the conclusion that in
+  these poriferous chordotonal organs we are studying a form of auditory apparatus which
+  characterized the ancestor of the insect-group. These organs are always well developed on the hind
+  wings, and in the large group of Diptera the auditory apparatus has usurped the whole of the
+  function of the wing; for the balancers or 'halteres,' as they are called, are the sole
+  representatives of the hind wings, and they are usually considered to be of the nature of auditory
+  organs. It is instructive to find that such an auditory organ serves not only for the purpose of
+  audition, but also as an organ of equilibration; thus Lowne gives the evidence of various
+  observers, and confirms it himself, that removal of the balancers destroys the power of orderly
+  flight in the animal.</p>
+
+  <p>A striking peculiarity of these organs in the Insecta, as described <span class="pagenum"
+  id="page370">{370}</span>by Graber, is the bulging of the porous canal near its termination (Fig.
+  <a href="#fig150">150</a>, C). This bulging is filled with a homogeneous, highly refractive
+  material, from which, according to Lowne, a chordotonal thread passes, to be connected with a
+  ganglion-cell and nerve. This sphere of refractive material he calls the 'capitellum' of the
+  chordotonal thread. The presence of this material produces in a surface view an appearance as of a
+  halo around the terminal plaque with its central pore; Graber has attempted to represent this by
+  the white area round the central area (in Fig. <a href="#fig150">150</a>, B). A very similar
+  appearance is presented by the surface view of the flabellum in those parts where the tube runs
+  straight to the surface, so that the refractive material which fills the oval bulging shines
+  through the overlying chitin and appears to surround the terminal plaque with a translucent
+  halo.</p>
+
+  <div class="ac w45 fcenter sp2">
+    <a href="images/fig150.png" id="fig150"><img style="width:100%" src="images/fig150.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="sp0"><span class="sc">Fig. 150</span> (from <span
+      class="sc">Graber</span>).&mdash;<span class="sc">A, Section of Subcostal Nervure of Hind Wing
+      of Dytiscus to show patch of Poriferous Organs</span> (<i>s.o.</i>). <span class="sc">B,
+      Surface View of Poriferous Organs; the White Space round each Organ indicates the deeper lying
+      Refringent Body which fills the bulging of the Canal seen in Transverse Section in
+      C.</span></p>
+    </div>
+  </div>
+
+  <p>Such a peculiarity must have a very definite meaning, and suggests that the canals in the
+  flabellum of Limulus and in the hind wings of insects belong to the same class of organ, the
+  chitinous tubule with its nerve-terminal in the former corresponding to the chordotonal thread in
+  the latter. One wonders whether this sphere of refractive material or 'capitellum' (to use Lowne's
+  phraseology) is so universally present in order to act as a damper upon the vibrations of the
+  chordotonal thread in the one case and of the <span class="pagenum"
+  id="page371">{371}</span>chitinous tubule in the other, just as the <i>membrana tectoria</i> and
+  the otoliths act in the case of the vertebrate ear.</p>
+
+  <p>Patten says that the only organs which seem to him to be comparable with the gustatory porous
+  organs of Limulus are the sense-organs in the extremities of the palps and of the first pair of
+  legs of Galeodes, as described by Gaubert. I imagine that he was thinking only of arachnids, for
+  the comparison of his drawings with those of Graber show what a strong family resemblance exists
+  between the poriferous sense-organs of Limulus and those of the insects. On the course of the
+  terminal nerve-fibres, between the nerve-cell and their entrance into the porous chitinous canal,
+  Graber describes the existence of rods or scolophores. On the course of the terminal fibres in the
+  Limulus organ, between the nerve-cells and their entrance into the porous chitinous canal, Patten
+  describes a spindle-shaped swelling, containing a number of rod-like thickenings among the fibrils
+  in the spindle, which present an appearance reminiscent of the rods described by Graber.</p>
+
+  <p class="sp3">It appears as though a type of sense-organ, characterized by the presence of pores
+  on the surface and a fine chitinous canal which opens at these pores, was largely distributed
+  among the Arthropoda. According to Graber, this kind of organ represents a primitive type of
+  sense-organ, which was probably concerned with audition and equilibration, and he expresses
+  surprise that similar organs have not been discovered among the Crustacea. It is, therefore, a
+  matter of great interest to find that so ancient a type of animal as Limulus, closely allied to
+  the primitive crustacean stock, <i>does</i> possess poriferous sense-organs upon its appendages
+  which are directly comparable with these poriferous chordotonal organs of the Insecta.</p>
+
+  <p class="ac"><span class="sc">The Pectens of Scorpions.</span></p>
+
+  <p>Among special sense-organs such as those with which I am now dealing, the pectens of scorpions
+  and the 'racquet-organs' of Galeodes must, in all probability, be classed. I have given my reasons
+  for this conclusion in my former paper.<a id="NtA_2" href="#Nt_2"><sup>[2]</sup></a> At present
+  such reasons are based entirely upon the structure of the organs; experimental <span
+  class="pagenum" id="page372">{372}</span>evidence as to their function is entirely wanting. With
+  respect to the pectens of the scorpion (Fig. <a href="#fig151">151</a>), it has been suggested
+  that they are of the nature of copulatory organs, a suggestion which may be dismissed without
+  hesitation, for they are not constructed after the fashion of claspers, but are simply elaborate
+  sense-organs, and, as such, are found equally in male or female. The only observer who has
+  hitherto specially studied the structure of the sense-organs in the pecten is, as far as I know,
+  Gaubert, and he describes their structure together with that of the sense-organs of the racquets
+  of Galeodes, in connection with the lyriform organs of arachnids, as though he recognized a family
+  resemblance between the three sets of organs.</p>
+
+  <div class="ac w35 fcenter sp2">
+    <a href="images/fig151.png" id="fig151"><img style="width:51%" src="images/fig151.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 151.&mdash;Under Surface of Scorpion
+      (Androctonus).</span></p>
+      <p class="sp0">The operculum is marked out with dots, and on each side of it is seen one of
+      the pectens.</p>
+    </div>
+  </div>
+
+  <p>The pecten of the scorpions is an elaborate sense-organ, or rather group of sense-organs, the
+  special organ being developed on each tooth of the comb; its surface, which is frequently
+  flattened, being directed backwards and inwards, when the axis of the pecten is horizontal at
+  right angles to the length of the body. The surface view of this part of the tooth resembles that
+  of the branchial organs or of the flabellum in Limulus, in that it is thickly covered with
+  circular patches, in the centre of which an ill-defined appearance as of a fine pore is seen. In
+  Fig. <a href="#fig152">152</a>, B, I give a sketch of the surface view of a part of the organ.</p>
+
+  <p>Transverse sections of a tooth of the comb of <i>Scorpio Europæus</i> present the appearance
+  given in Fig. <a href="#fig152">152</a>, A, and show that each of these circular patches is the
+  surface-view of a goblet-shaped chitinous organ, Fig. <a href="#fig152">152</a>, C, from the
+  centre of which a short, somewhat cylindrical chitinous spike projects. Within this spike, and
+  running through the goblet into the subjacent tissue, is a fine tubule. The series of goblets
+  gives rise to the appearance of the circular plaques on the surface-view, while the spike with its
+  tubule <span class="pagenum" id="page373">{373}</span>is the cause of the ill-defined appearance
+  of the central pore, just as the terminal pore is much less conspicuous on surface-view in the
+  spike-organs of the flabellum than in the purely poriferous organs, no part of which projects
+  beyond the level of the chitinous surface.</p>
+
+  <div class="ac w45 fcenter sp2">
+    <a href="images/fig152.png" id="fig152"><img style="width:100%" src="images/fig152.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 152.&mdash;A, Section through Tooth of Pecten of Scorpion; B, Surface
+      View of Sense-Organs; C, Goblet of Sense-Organ more highly magnified.</span></p>
+      <p class="sp0"><i>bl.</i> and <i>n.</i>, region of blood-spaces and nerves; <i>gl.</i>,
+      ganglion-cell layer; <i>ch.</i>, modified chitinous layer; <i>s.o.</i>, sense-organ.</p>
+    </div>
+  </div>
+
+  <p>The fine tubule is soon lost in the thickened but soft modification of the chitinous layer
+  (<i>ch.</i>) which is characteristic of the sense-organ; at all events, I have not succeeded in
+  tracing it through this layer with any more success than in the corresponding case of the tubules
+  belonging to the smaller goblets of the branchial sense-organ of Limulus already described.</p>
+
+  <p>At the base of the modified chitinous layer a series of cells is seen, many, if not all, of
+  which belong to the chitinogenous layer. Next to these is the marked layer of ganglion-cells
+  (<i>gl.</i>), similar to those seen in the flabellum of Limulus. The rest of the space in the
+  section of the tooth is filled up with nerves (<i>n.</i>) and blood-spaces (<i>bl.</i>) just as in
+  the section, Fig. <a href="#fig146">146</a>, of the flabellum of Limulus.</p>
+
+  <p>Gaubert does not appear to have seen the goblets at all clearly; <span class="pagenum"
+  id="page374">{374}</span>he describes them simply as conical eminences, and states that they
+  "recouvrent un pore analogue a celui des poils mais plus petit; il est rempli par le protoplasma
+  de la couche hypodermique." From the ganglion, according to him, nervous prolongations pass, which
+  traverse the chitinogenous layer and terminate at the base of the conical eminences. Each of these
+  prolongations "présente sur son trajet, mais un peu plus près du ganglion que de sa terminaison
+  périphérique, une cellule nerveuse fusiforme (<i>g.</i>) offrant, comme celles du ganglion, un
+  gros noyau." He illustrates his description with the following, Fig. <a href="#fig153">153</a>,
+  taken from his paper.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig153.png" id="fig153"><img style="width:37%" src="images/fig153.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 153</span> (from <span class="sc">Gaubert</span>).&mdash;<span
+      class="sc">Section of a Tooth of Pecten of Scorpion.</span></p>
+      <p class="sp0"><i>n.</i>, nerve; <i>gl.</i>, ganglion.</p>
+    </div>
+  </div>
+
+  <p>I have not been able to obtain any evidence of a fusiform nerve-cell on the course of the
+  terminal nerve-fibres as depicted by him; fusiform cells there are in plenty, as depicted in my
+  drawing, but none with a large nucleus resembling those of the main ganglion. In no case, either
+  in the flabellum or in the branchial organs of Limulus, or in the pecten-organs, have I ever seen
+  a ganglion-cell within the chitin-layer; all the nuclei seen there resemble those of the cells of
+  the hypodermis or else the elongated nuclei characteristic of the presence of nerve-fibres.
+  Gaubert's drawing is a striking one, and I have looked through my specimens to see whether there
+  was anything similar, but have hitherto failed to obtain any definite evidence of anything of the
+  kind.</p>
+
+  <p>I feel, myself, that an exhaustive examination of the structure and function of the pecten of
+  scorpions ought to be undertaken. At present I can only draw the attention of my readers to the
+  similarity of the arrangement of parts, and of the nature of the end-organs, in the sense-organs
+  of the flabellum of Limulus and of the pecten of the scorpion. In both cases the special
+  nerve-fibres terminate in a massive ganglion, situated just below the chitinogenous layer. In both
+  cases the terminal fibres from these ganglion-cells pass through the modified chitinous layer to
+  supply end-organs of a striking character; and although the end-organ of the pecten of the
+  scorpion does <span class="pagenum" id="page375">{375}</span>not closely resemble the majority of
+  the end-organs of the flabellum, yet it does resemble, on the one hand, the isolated poriferous
+  spikes found on the flabellum (Fig. <a href="#fig149">149</a>) and, on the other, the poriferous
+  goblets found on the sense-patches of the branchial appendages of Limulus (Fig. <a
+  href="#fig144">144</a>, A), so that a combination of these two end-organs would give an appearance
+  very closely resembling that of the pecten of the scorpion.</p>
+
+  <p>Finally, the special so-called 'racquet-organs' of Galeodes, which are found on the most basal
+  segments of the last pair of prosomatic appendages, ought also to be considered here. Gaubert has
+  described their structure, and shown how the nerve-trunk in the handle of the racquet splits up
+  into a great number of separate bundles, which spread out fan-shaped to the free edge of the
+  racquet; each of these separate bundles supplies a special sense-organ, which terminates as a
+  conical eminence on the floor of a deep groove, running round the whole free edge of the racquet.
+  This groove is almost converted into a canal, owing to the projection of its two sides. Gaubert
+  imagines that the sense-organs are pushed forward out of the groove to the exterior by the
+  turgescence of the whole organ; each of the nerve-fibres forming a bundle is, according to
+  Gaubert, connected with a nerve-cell before it reaches its termination.</p>
+
+  <p>This sketch of the special sense-organs on the appendages of Limulus, of the scorpions, of
+  Galeodes, and other arachnids, and their comparison with the porous chordotonal organs of insects,
+  affords reason for the belief that we are dealing here with a common group of organs, which,
+  although their nature is not definitely known, have largely been accredited with the functions of
+  equilibration and audition, a group of organs among which the origin of the auditory organ of
+  vertebrates must be sought for, upon any theory of the origin of vertebrates from arthropods.</p>
+
+  <p>Whenever in any animal these organs are concentrated together to form a special organ, it is
+  invariably found that the nerve going to this organ is very large, out of all proportion to the
+  size of the organ, and also that the nerve possesses, close to its termination in the organ, large
+  masses of nerve-cells. Thus, although the whole hind wing in the blow-fly has been reduced to the
+  insignificant balancers or 'halteres,' yet, as Lowne states, the nerves to them are the largest in
+  the body.</p>
+
+  <p>The pectinal nerve in the scorpion is remarkable for its size, and <span class="pagenum"
+  id="page376">{376}</span>so, also, is the nerve to the flabellum in Limulus, while the large size
+  of the auditory nerve in the vertebrate, in distinction to the size of the auditory apparatus, has
+  always aroused the attention of anatomists.</p>
+
+  <p>Throughout this book my attention has been especially directed to both Limulus and the scorpion
+  group in endeavouring to picture to myself the ancestor of the earliest vertebrates, because the
+  Eurypteridæ possessed such marked scorpion-like characteristics; so that in considering the origin
+  of a special sense-organ, such as the vertebrate auditory organ near the junction of the prosoma
+  and mesosoma, it seems to me that the presence of such marked special sense-organs as the
+  flabellum on the one hand and the pecten on the other, must both be taken into account, even
+  although the former is an adjunct to a prosomatic appendage, while the latter represents,
+  according to present ideas, the whole of a mesosomatic appendage.</p>
+
+  <p>From the point of view that the VIIIth nerve represents a segment immediately posterior to that
+  of the VIIth, it is evident that an organ in the situation of the pecten, immediately posterior to
+  the operculum, <i>i.e.</i> according to my view, posterior to the segment originally represented
+  by the VIIth nerve, is more correctly situated than an organ like the flabellum, which belongs to
+  a segment anterior to the operculum.</p>
+
+  <p class="sp3">On the other hand, from the point of view of the relationship between the scorpions
+  and the king-crabs, it is a possibly debatable question whether the pecten really belongs to a
+  segment posterior to the operculum. The position of any nerve in a series depends upon its
+  position of origin in the central nervous system, rather than upon the position of its peripheral
+  organ. Now, Patten gives two figures of the brain of the scorpion built up from serial sections.
+  In both he shows that the main portion of the pectinal nerve arises from a swelling, to which he
+  gives the name <i>ganglion nodosum</i>. This swelling arises on each side in close connection with
+  the origin of the most posterior prosomatic appendage-nerve, according to his drawings, and
+  posteriorly to such origin he figures a small nerve which he says supplies the distal parts of the
+  sexual organs. This nerve is the only nerve which can be called the opercular nerve, and
+  apparently arises posteriorly to the main part of the pectinal nerve. If this is so, it would
+  indicate that the pectens arose from sense-organs which were originally, like the flabella,
+  pre-opercular in position, but have shifted to a post-opercular position.</p>
+
+  <div><span class="pagenum" id="page377">{377}</span></div>
+
+  <p class="ac"><span class="sc">The Origin of the Parachordals and Auditory Cartilaginous
+  Capsule.</span></p>
+
+  <p>In addition to what I have already said, there is another reason why a special sense-organ such
+  as the pecten is suggestive of the origin of the vertebrate auditory organ, in that such a
+  suggestion gives a clue to the possible origin of the parachordals and auditory cartilaginous
+  capsules.</p>
+
+  <p>In the lower vertebrates the auditory organ is characterized by being surrounded with a
+  cartilaginous capsule which springs from a special part of the axial cartilaginous skeleton on
+  each side, known as the pair of parachordals. The latter, in Ammoc&#x0153;tes, form a pair of
+  cartilaginous bars, which unite the trabecular bars with the branchial cartilaginous basket-work.
+  They are recognized throughout the Vertebrata as distinct from the trabecular bars, thus forming a
+  separate paired cartilaginous element between the trabeculæ and the branchial cartilaginous
+  system, which of itself indicates a position for the auditory capsule between the prosomatic
+  trabeculæ and the mesosomatic branchial cartilaginous system.</p>
+
+  <p>The auditory capsule and parachordals when formed are made of the same kind of cartilage as the
+  trabeculæ, <i>i.e.</i> of hard cartilage, and are therefore formed from a gelatin-containing
+  tissue, and not from muco-cartilage. Judging from the origin already ascribed to the trabeculæ,
+  viz. their formation from the great prosomatic entochondrite or plastron, this would indicate that
+  a second entochondrite existed in the ancestor of the vertebrate in the region of the junction of
+  the prosoma and mesosoma, which was especially connected with the sense-organ to which the
+  auditory organ owes its origin. This pair of entochondrites becoming cartilaginous would give
+  origin to the parachordals, and subsequently to the auditory capsules, their position being such
+  that the nerve to the operculum would be surrounded at its origin by the growth of cartilage.</p>
+
+  <p>On this line of argument it is very significant to find that the scorpions do possess a second
+  pair of entochondrites, viz. the supra-pectinal entochondrites, situated between the nerve-cord
+  and the pectens, so that if the ancestor of the Cephalaspid was sufficiently scorpion-like to have
+  possessed a second pair of entochondrites and at the same time a pair of special sense-organs of
+  the nature either of <span class="pagenum" id="page378">{378}</span>the pectens or flabella, then
+  the origin of the auditory apparatus would present no difficulty.</p>
+
+  <p class="sp3">It is also easy to see that the formation of the parachordals from entochondrites
+  homologous with the supra-pectinal entochondrites, would give a reason why the VIIth or opercular
+  nerve is involved with the VIIIth in the formation of the auditory capsule, especially if the
+  special sense-organ which gave origin to the auditory organ was originally a pre-opercular
+  sense-organ such as the flabellum, which subsequently took up a post-opercular position like that
+  of the pecten.</p>
+
+  <p class="ac"><span class="sc">The Evidence of Ammoc&#x0153;tes.</span></p>
+
+  <p>As to the auditory apparatus itself, we see that the elaborate organ for hearing&mdash;the
+  cochlea&mdash;has been evolved in the vertebrate phylum itself. In the lowest vertebrates the
+  auditory apparatus tends more and more to resolve itself into a simple epithelial sac, the walls
+  of which in places bear auditory hairs projecting into the sac, and in part form otoliths. Such a
+  simple sac forms the early stage of the auditory vesicle in Ammoc&#x0153;tes, according to
+  Shipley; subsequently, by a series of foldings and growings together, the chambers of the ear of
+  the adult Petromyzon, as figured and described by Retzius, are formed. Further, we see that
+  throughout the Vertebrata this sac was originally open to the exterior, the auditory vesicle being
+  first an open pit, which forms a vesicle by the approximating of its sides, the last part to close
+  being known as the <i>recessus labyrinthicus</i>; in many cases, as in elasmobranchs, this part
+  remains open, or communicates with the exterior by means of the <i>ductus endolymphaticus</i>.</p>
+
+  <p>Judging, therefore, from the embryological evidence, it would appear that the auditory organ
+  originated as a special sense-organ, formed by modified epithelial cells of the surface, which
+  epithelial surface becoming invaginated, came to line a closed auditory vesicle under the surface.
+  This special sense-organ was innervated from a large ganglionic mass of nerve-cells, situated
+  close against the peripheral sense-cells, the axis-cylinder processes of which formed the sensory
+  roots of the nerve.</p>
+
+  <p>Yet another peculiarity of striking significance is seen in connection with the auditory organ
+  of Ammoc&#x0153;tes. The opening of the cartilaginous capsule towards the brain is a large one
+  (Fig. <a href="#fig154">154</a>), and <span class="pagenum" id="page379">{379}</span>admits the
+  passage not only of the auditory and facial nerves, but also of a portion of the peculiar tissue
+  which surrounds the brain. The large cells of this tissue, with their feebly staining nuclei and
+  the pigment between them, make them quite unmistakable; and, as I have already stated, nowhere
+  else in the whole of Ammoc&#x0153;tes is such a tissue found. When I first noticed these cells
+  within the auditory capsule, it seemed to me almost impossible that my interpretation of them as
+  the remnant of the generative and hepatic tissue which surrounds the brain of animals such as
+  Limulus could be true, for it seemed too unlikely that a part of the generative system could ever
+  have become included in the auditory capsule. Still, they are undoubtedly there; and, as already
+  argued with respect to the substance round the brain, they must represent some pre-existing tissue
+  which was functional in the ancestor of Ammoc&#x0153;tes. If my interpretation is right, this
+  tissue must be generative and hepatic tissue, and its presence in the auditory capsule immediately
+  becomes a most important piece of evidence, for it proves that the auditory organ must have been
+  originally so situated that a portion of the generative and hepatic mass surrounding the cephalic
+  region of the nervous system followed the auditory nerve to the peripheral sense-organ.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig154.png" id="fig154"><img style="width:100%" src="images/fig154.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 154.&mdash;Transverse Section through Auditory Capsules
+      and Brain of Ammoc&#x0153;tes.</span></p>
+      <p class="sp0"><i>Au.</i>, auditory organ; <i>VIII</i>, auditory nerve; <i>gl.</i>, ganglion
+      cells of VIIIth nerve; <i>Au. cart.</i>, cartilaginous auditory capsule; <i>gen.</i>, cells of
+      old generative tissue round brain and in auditory capsule; <i>bl.</i>, blood-vessels</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page380">{380}</span></div>
+
+  <p>Here there was a test of the truth of my theory ranking second only to the test of the median
+  eyes; the strongest possible evidence of the truth of any theory is given when by its aid new and
+  unexpected facts are brought to light. The theory said that in the group of animals from which the
+  vertebrates arose, a special sense-organ of the nature of an auditory organ must have existed on
+  the base of one of the appendages situated at the junction of the prosoma and mesosoma, and that
+  into this basal part of the appendage a portion of the cephalic mass of generative and hepatic
+  material must have made its way in close contiguity to the nerve of the special organ.</p>
+
+  <p>The only living example which nearly approaches the ancient extinct forms from which, according
+  to the theory, the vertebrates arose, is Limulus, and, as has already been shown, in this animal,
+  in the very position postulated by the theory, a large special sense-organ&mdash;the
+  flabellum&mdash;exists, which, as already stated, may well have given rise to a sense-organ
+  concerned with equilibration and audition. If, further, it be found that a diverticulum of the
+  generative and hepatic material does accompany the nerve of the flabellum in the basal part of the
+  appendage, then the evidence becomes very strong that the auditory organ of Ammoc&#x0153;tes,
+  <i>i.e.</i> of the ancient Cephalaspids, was derived from an organ homologous with the flabellum;
+  that, therefore, the material round the brain of Ammoc&#x0153;tes was originally generative and
+  hepatic material; that, in fact, the whole theory is true, for all the parts of it hang together
+  so closely that, if one portion is accepted, all the rest must follow. As pointed out in my
+  address at Liverpool, and at the meeting of the Philosophical Society at Cambridge, it is a most
+  striking fact that a mass of the generative and hepatic tissue does accompany the flabellar nerve
+  into the basal part of this appendage. Into no other appendage of Limulus is there the slightest
+  sign of any intrusion of the generative and hepatic masses; nowhere, except in the auditory
+  capsule, is there any sign of the peculiar large-celled tissue which surrounds the brain and upper
+  part of the spinal cord of Ammoc&#x0153;tes. The actual position of the flabellum on the basal
+  part of the ectognath is shown in Fig. <a href="#fig155">155</a>, A, and in Fig. <a
+  href="#fig155">155</a>, B, I have removed the chitin, to show the generative and hepatic tissue
+  (<i>gen.</i>) lying beneath.</p>
+
+  <p>The reason why, to all appearance, the generative and hepatic mass penetrates into the basal
+  part of this appendage only is apparent <span class="pagenum" id="page381">{381}</span>when we see
+  (as Patten and Redenbaugh have pointed out) to what part of the appendage the flabellum in reality
+  belongs.</p>
+
+  <div class="ac w45 fcenter sp2">
+    <a href="images/fig155.png" id="fig155"><img style="width:100%" src="images/fig155.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p><span class="sc">Fig. 155.&mdash;A, The Digging Appendage or Ectognath of Limulus; B, The
+      Middle Protuberance (2) of the Entocoxite opened, to show the Generative and Hepatic
+      Tissue</span> (<i>gen.</i>) <span class="sc">within it; C, One of the Prosomatic Locomotor
+      Appendages or Endognaths of Limulus, for comparison with A</span>.</p>
+      <p class="sp0 ac"><i>fl.</i>, flabellum; <i>cox.</i>, coxopodite; <i>ent.</i>, entocoxite;
+      <i>m.</i>, mandible; <i>i.m.</i>, inner mandible or epicoxite.</p>
+    </div>
+  </div>
+
+  <p>Patten and Redenbaugh, in their description of the prosomatic appendages of Limulus, describe
+  the segments of the limbs as (1) the dactylopodite, (2) the propodite, (3) the mero- and
+  carpo-podites, (4) the ischiopodite, (5) the basipodite, and (6) the coxopodite (<i>cox.</i> in
+  Fig. <a href="#fig155">155</a>). Still more basal than the coxopodite is situated the entocoxite
+  (<i>ent.</i> in Fig. <a href="#fig155">155</a>), which is composed of three sclerites <span
+  class="pagenum" id="page382">{382}</span>or sensory knobs, to use Patten's description. The middle
+  one of these three sclerites enlarges greatly in the digging appendage, and grows over the
+  coxopodite to form the base from which the flabellum springs. Thus, as they have pointed out, the
+  flabellum does not belong to the coxopodite of the appendage, but to the middle sensory knob of
+  the entocoxite. Upon opening the prosomatic carapace, it is seen that the cephalic generative and
+  hepatic masses press closely against the internal surface of the prosomatic carapace and also of
+  the entocoxite, so that any enlargement of one of the sensory knobs of the entocoxite would
+  necessarily be filled with a protrusion of the generative and hepatic masses. This is the reason
+  why the generative and hepatic material apparently passes into the basal segment of the ectognath,
+  and not into that of the endognaths; it does not really pass into the coxopodite of the appendage,
+  but into an enlarged portion of the entocoxite, which can hardly be considered as truly belonging
+  to the appendage. Kishinouye has stated that a knob arises in the embryo at the base of each of
+  the prosomatic locomotor appendages, but that this knob develops only in the last or digging
+  appendage (ectognath) forming the flabellum. Doubtless the median sclerites of the entocoxites of
+  the endognaths represent Kishinouye's undeveloped knobs.</p>
+
+  <p>I conclude, therefore, that the flabellum, together with its basal part, is an adjunct to the
+  appendage rather than a part of it, and might, therefore, easily remain as a separate and
+  well-developed entity, even although the appendage itself dwindled down to a mere tentacle.</p>
+
+  <p>The evidence appears to me very strong that the flabellum of Limulus and the pecten of
+  scorpions are the most likely organs to give a clue to the origin of the auditory apparatus of
+  vertebrates. At present both the Eurypterids and Cephalaspids have left us in the lurch; in the
+  former there is no sign of either flabellum or pecten; in the latter, no sign of any auditory
+  capsule beyond Rohon's discovery of two small apertures situated dorsally on each side of the
+  middle line in Tremataspis, which he considers to be the termination of the <i>ductus
+  endolymphaticus</i> on each side. In both cases it is probable, one might almost say certain, that
+  any such special sense-organ, if present, was not situated externally, but was sunk below the
+  surface as in Ammoc&#x0153;tes.</p>
+
+  <p class="sp3">The method by which such a sense-organ, situated externally on <span
+  class="pagenum" id="page383">{383}</span>the surface of the animal, comes phylogenetically to form
+  the lining wall of an internally situated membranous capsule is given by the ontogeny of this
+  capsule, which shows step by step how the sense-organ sinks in and forms a capsule, and finally is
+  entirely removed from the surface except as regards the <i>ductus endolymphaticus</i>.</p>
+
+  <p class="ac"><span class="sc">Summary.</span></p>
+
+  <div class="bq1 smaller sp5">
+    <p>The special apparatus for hearing is of a very different character from that for vision or
+    for smell, for its nerve belongs to the infra-infundibular group of nerves, and not to the
+    supra-infundibular, as do those of the other two special senses. Of the five special senses the
+    nerves for touch, taste, and hearing, all belong to the infra-infundibular segmental
+    nerve-groups. The invertebrate origin, then, of the vertebrate auditory nerve must be sought for
+    in the infra-&#x0153;sophageal segmental group of nerves, and not in the
+    supra-&#x0153;sophageal.</p>
+    <p>The organs supplied by the auditory nerve are only partly for the purpose of hearing; there
+    is always present also an apparatus&mdash;the semicircular canals&mdash;concerned with
+    equilibration and co-ordination of movements. Such equilibration organs are not confined to the
+    auditory nerve, but in the water-living vertebrates are arranged segmentally along the body,
+    forming the organs of the lateral line in fishes; the auditory organ is but one of these lateral
+    line organs, which has been specially developed.</p>
+    <p>These lateral line organs have been compared to similar segmental organs found in connection
+    with the appendages in worms, especially the respiratory appendages. In accordance with this
+    suggestion we see that they are all innervated from the region of the respiratory
+    nerves&mdash;the vagus, glosso-pharyngeal, and facial&mdash;nerves which originally supplied the
+    respiratory appendages of the palæostracan ancestor.</p>
+    <p>The logical conclusion is that the appendages of the Palæostraca possessed special
+    sense-organs concerned with the perception of special vibrations, especially in the mesosomatic
+    or respiratory region, and that somewhere at the junction of the prosoma and mesosoma, one of
+    these sense-organs was specially developed to form the origin of the vertebrate auditory
+    apparatus.</p>
+    <p>Impressed by this reasoning I made search for some specially striking sense-organ at the base
+    of one of the appendages of Limulus, at the junction of the prosoma and mesosoma, and was
+    immediately rewarded by the discovery of the extraordinary nature of the flabellum, which
+    revealed itself as an elaborate sense-organ supplied with a nerve out of all proportion to its
+    size. Up to this time no one had the slightest conception that this flabellum was a special
+    sense-organ; the discovery of its nature was entirely due to the logical following out of the
+    theory of the origin of vertebrates described in this book.</p>
+    <p>The structure of this large sense-organ is comparable with that of the sense-organs of the
+    pectens of the scorpion, and of many other organs found on the appendages of various members of
+    the scorpion group, of arachnids and <span class="pagenum" id="page384">{384}</span>other
+    air-breathing arthropods. Many of these organs, such as the lyriform organs of arachnids, and
+    the 'halteres' or balancers of the Diptera, are usually regarded as auditory and equilibration
+    organs.</p>
+    <p>On all the mesosomatic appendages of Limulus very remarkable sense-organs are found,
+    apparently for estimating pressures, which, when the appendages sank into the body to form with
+    their basal parts the branchial diaphragms of Ammoc&#x0153;tes, could easily be conceived as
+    remaining at the surface, and so giving rise to the lateral line organs.</p>
+    <p>Further confirmation of the view that an organ, such as the flabellum, must be looked upon as
+    the originator of the vertebrate auditory organ, is afforded by the extraordinary coincidence
+    that in Limulus a diverticulum of the generative and hepatic mass accompanies the flabellar
+    nerve into the basal part of the digging appendage, while in Ammoc&#x0153;tes, accompanying the
+    auditory nerve into the auditory capsule, there is seen a mass of cells belonging to that
+    peculiar tissue which fills up the space between the brain and the cranial walls, and has
+    already, on other grounds, been homologized with the generative and hepatic masses which fill up
+    the encephalic region of Limulus.</p>
+    <p class="sp0">For all these reasons special sense-organs, such as are found in the flabellum of
+    Limulus and in the pectens of scorpions, may be looked upon as giving origin to the vertebrate
+    auditory apparatus. In such case it is highly probable that the parachordals, with the auditory
+    capsules attached, arose from a second entochondrite of the same nature as the plastron; a
+    probability which is increased by the fact that the scorpion does possess a second
+    entochondrite, which, owing to its special relations to the pecten, is known as the
+    supra-pectinal entochondrite.</p>
+  </div>
+
+  <div><span class="pagenum" id="page385">{385}</span></div>
+
+  <p class="ac">CHAPTER XII</p>
+
+  <p class="ac"><i>THE REGION OF THE SPINAL CORD</i></p>
+
+  <div class="bq1 it1 smaller sp3">
+    <p class="sp0">Difference between cranial and spinal regions.&mdash;Absence of lateral
+    root.&mdash;Meristic variation.&mdash;Segmentation of c&#x0153;lom.&mdash;Segmental excretory
+    organs.&mdash;Development of nephric organs; pronephric, mesonephric,
+    metanephric.&mdash;Excretory organs of Amphioxus.&mdash;Solenocytes.&mdash;Excretory organs of
+    Branchipus and of Peripatus, appendicular and somatic.&mdash;Comparison of c&#x0153;lom of
+    Peripatus and of vertebrate.&mdash;Pronephric organs compared to coxal glands.&mdash;Origin of
+    vertebrate body-cavity (metac&#x0153;le).&mdash;Segmental duct.&mdash;Summary of formation of
+    excretory organs.&mdash;Origin of somatic trunk-musculature.&mdash;Atrial cavity of
+    Amphioxus.&mdash;Pleural folds.&mdash;Ventral growth of pleural folds and somatic
+    musculature.&mdash;Pleural folds of Cephalaspidæ and of Trilobita.&mdash;Significance of the
+    ductless glands.&mdash;Alteration in structure of excretory organs which have lost their duct in
+    vertebrates and in invertebrates.&mdash;Formation of lymphatic glands.&mdash;Segmental coxal
+    glands of arthropods and of vertebrates.&mdash;Origin of adrenals, pituitary body, thymus,
+    tonsils, thyroid, and other ductless glands.&mdash;Summary.</p>
+  </div>
+
+  <p>The consideration of the auditory nerve and the auditory apparatus terminates the comparison
+  between the cranial nerves of the vertebrate and the prosomatic and mesosomatic nerves of the
+  arthropod, and leaves us now free to pass on to the consideration of the vertebrate spinal nerves
+  and the organs they supply. Before doing so, it is advisable to pass in review the conclusions
+  already attained.</p>
+
+  <p>Starting with the working hypothesis that the central nervous system of the vertebrate has
+  arisen from the central nervous system of the arthropod, but has involved and enclosed the
+  alimentary canal of the latter in the process, so that there has been no reversal of surfaces in
+  the derivation of the one form from the other, we have been enabled to compare closely all the
+  organs of the head-region in the two groups of animals, and in no single case have we been
+  compelled to make any startling or improbable assumptions. The simple following out of this clue
+  has led in every case in the most natural <span class="pagenum" id="page386">{386}</span>manner to
+  the interpretation of all the organs in the head-region of the vertebrate from the corresponding
+  organs of the arthropod.</p>
+
+  <p>That it is possible to bring together all the striking resemblances between organs in the two
+  classes of animals, such as I have done in preceding chapters, has been ascribed to a perverted
+  ingenuity on my part&mdash;a suggestion which is flattering to my imaginative powers, but has no
+  foundation of fact. There has been absolutely no ingenuity on my part; all I have done is to
+  compare organs and their nerve-supply, as they actually exist in the two groups of animals, on the
+  supposition that there has been no turning over on to the back, no reversal of dorsal and ventral
+  surfaces. The comparison is there for all to read; it is all so simple, so self-evident that,
+  given the one clue, the only ingenuity required is on the part of those who fail to see it.</p>
+
+  <p>The great distinction that has arisen between the two head-regions is the disappearance of
+  appendages as such, never, however, of important organs on those appendages. If the olfactory
+  organs of the one group were originally situated on antennules, the olfactory organs still remain,
+  although the antennules as such have disappeared. The coxal excretory organs at the base of the
+  endognaths remain and become the pituitary body. A special sense-organ, such as the flabellum of
+  Limulus or the pecten of scorpion, remains and gives rise to the auditory organ. A special
+  glandular organ, the uterus in the base of the operculum, remains, and gives rise to the thyroid
+  gland. The branchiæ and sense-organs on the mesosomatic appendages remain, and even the very
+  muscles to a large extent. As will be seen later, the excretory organs at the base of the
+  metasomatic appendages remain. It is merely the appendage as such which vanishes either by
+  dwindling away, or by so great an alteration as no longer to be recognizable as an appendage.</p>
+
+  <p>This dwindling process was already in full swing before the vertebrate stage; it is only a
+  continuation of a previous tendency, as is seen in the dwindling of the prosomatic appendages in
+  the Merostomata and the inclusion of the branchiæ within the body of the scorpion. Already among
+  the Palæostraca, swimming had largely taken the place of crawling. The whole gradual
+  transformation from the arthropod to the vertebrate is associated with a transformation from a
+  crawling to a swimming animal&mdash;with the concomitant loss of locomotor appendages as such, and
+  the alteration of the shape of <span class="pagenum" id="page387">{387}</span>the animal into the
+  lithe fish-like form. The consideration of the manner in which this latter change was brought
+  about, takes us out of the cranial into the spinal region.</p>
+
+  <p>If we take Limulus as the only living type of the Palæostraca, we are struck with the fact that
+  the animal consists to all intents and purposes of prosomatic and mesosomatic regions only; the
+  metasoma consisting of the segments posterior to the mesosoma is very insignificant, so that the
+  large mass of the animal consists of what has become the head-region in the vertebrate; the spinal
+  region, which has become in the higher vertebrates by far the largest region of the body, can
+  hardly be said to exist in such an animal as Limulus. As to the Eurypterids and others, similar
+  remarks may be made, though not to the same extent, for in them a distinct metasoma does
+  exist.</p>
+
+  <p>In this book I have considered up to the present the cranial region as a system of segments,
+  and shown how such segments are comparable, one by one, with the corresponding segments in the
+  prosoma and mesosoma of the presumed arthropod ancestor.</p>
+
+  <p>In the spinal region such direct comparison is not possible, as is evident on the face of it;
+  for even among vertebrates themselves the spinal segments are not comparable one by one, so great
+  is the variation, so unsettled is the number of segments in this region. This meristic variation,
+  as Bateson calls it, is the great distinctive character of the spinal region, which distinguishes
+  it from the cranial region with its fixed number of nerves, and its substantive rather than
+  meristic variation. At the borderland, between the two regions, we see how the one type merges
+  into the other; how difficult it is to fix the segmental position of the spino-occipital nerves;
+  how much more variable in number are the segments supplied by the vagus nerves than those anterior
+  to them.</p>
+
+  <p>This meristic variation is a sign of instability, of want of fixedness in the type, and is
+  evidence, as already pointed out, that the spinal region is newer than the cranial. This
+  instability in the number of spinal segments does not necessarily imply a variability in the
+  number of segments of the metasoma of the invertebrate ancestor; it may simply be an expression of
+  adaptability in the vertebrate phylum itself, according to the requirements necessitated by the
+  conversion of a crawling into a swimming animal, and the subsequent conversion of the swimming
+  into a terrestrial or flying animal.</p>
+
+  <div><span class="pagenum" id="page388">{388}</span></div>
+
+  <p>However many may have been the original number of segments belonging to the spinal region, one
+  thing is certain&mdash;the segmental character of this region is remarkably clearly shown, not
+  only by the presence of the segmental spinal nerves, but also by the marked segmentation of the
+  mesoblastic structures. The question, therefore, that requires elucidation above all others is the
+  origin of the spinal mesoblastic segments, <i>i.e.</i> of the c&#x0153;lomic cavities of the
+  trunk-region, and the structures derived from their walls.</p>
+
+  <p>Proceeding on the same lines as in the case of the cranial segments, it is necessary in the
+  first instance to inquire of the vertebrate itself as to the scope of the problem in this region.
+  In addition to the variability in the number of segments so characteristic of the spinal region,
+  the complete absence in each spinal segment of a lateral root affords another marked difference
+  between the two regions. Here, except, of course, at the junction of the spinal and cranial
+  regions, each segmental nerve arises from two roots only, dorsal and ventral, and these roots are
+  separately sensory and motor, and not mixed in function as was the lateral root of each cranial
+  segment. Now, these lateral roots were originally the nerves supplying the prosomatic and
+  mesosomatic appendages with motor as well as sensory fibres. The absence, therefore, of lateral
+  roots in the spinal region implies that in the vertebrate none of the musculature belonging to the
+  metasomatic appendages has remained. Consequently, as far as muscles are concerned, the clue to
+  the origin of the spinal segments must be sought for in the segmentation of the body-muscles.</p>
+
+  <p>Here, in contradistinction to the cranial region, the segmentation is most marked, for the
+  somatic spinal musculature of all vertebrates can be traced back to a simple sheet of longitudinal
+  ventral and dorsal muscles, such as are seen in all fishes. This sheet is split into segments or
+  myotomes by transverse connective tissue septa or myo-commata; each myotome corresponding to one
+  spinal segment.</p>
+
+  <p>In addition to the evidence of segmentation afforded by the body-musculature in all the higher
+  vertebrates, similar evidence is given by the segmental arrangement of parts of the supporting
+  tissue to form vertebræ. Such segments have received the name of sclerotomes, and each sclerotome
+  corresponds to one spinal segment.</p>
+
+  <p>Yet another marked peculiarity of this region is the segmental arrangement of the excretory
+  organs. Just as our body-musculature <span class="pagenum" id="page389">{389}</span>has arisen
+  from the uniformly segmented simple longitudinal musculature of the lowest fish, so, as we pass
+  down the vertebrate phylum, we find more and more of a uniform segmental arrangement in the
+  excretory organs.</p>
+
+  <p class="sp3">The origin of all these three separate segmentations may, in accordance with the
+  phraseology of the day, be included in the one term&mdash;the origin of the spinal mesoblastic
+  segments&mdash;<i>i.e.</i> of the c&#x0153;lomic cavities of the trunk-region and the structures
+  derived from their walls.</p>
+
+  <p class="ac"><span class="sc">The Origin of the Segmental Excretory Organs.</span></p>
+
+  <p>Of these three clues to the past history of the spinal region, the segmentation manifested by
+  the presence of vertebræ is the least important, for in Ammoc&#x0153;tes there is no sign of
+  vertebræ, and their indications only appear at transformation. Especially interesting is the
+  segmentation due to the excretory organs, for the evidence distinctly shows that such excretory
+  organs have steadily shifted more and more posteriorly during the evolution of the vertebrate.</p>
+
+  <p>In Limulus the excretory organs are in the prosomatic region&mdash;the coxal glands; these
+  become in the vertebrate the pituitary body.</p>
+
+  <p>In Amphioxus the excretory organs are in the mesosomatic region, segmentally arranged with the
+  gills.</p>
+
+  <p>In vertebrates the excretory organs are in the metasomatic region posterior to the gills, and
+  are segmentally arranged in this region. Their investigation has demonstrated the existence of
+  three distinct stages in these organs: 1. A series of segmental excretory organs in segments
+  immediately following the branchial segments. This is the oldest of the three sets, and to these
+  organs the name of the <i>pronephros</i> is given. 2. A second series which extends more
+  posteriorly than the first, overlaps them to an extent which is not yet settled, and takes their
+  place; to them is given the name of the <i>mesonephros</i>. 3. A third series continuous with the
+  mesonephric is situated in segments still more posterior, supplants the mesonephros and forms the
+  kidneys of all the higher vertebrates. This forms the <i>metanephros</i>.</p>
+
+  <p>These three sets of excretory organs are not exactly alike in their origin, in that the
+  pronephric tubules are formed from a different portion of the c&#x0153;lomic walls to that from
+  which the meso- and <span class="pagenum" id="page390">{390}</span>metanephric tubules are formed,
+  and the former alone gives origin to a duct, which forms the basis for the generative and urinary
+  ducts, and is called the <i>segmental duct</i>. The mesonephric tubules, called also the Wolffian
+  body, open into this duct.</p>
+
+  <p>In order to make the embryology of these excretory organs quite clear, I will make use of van
+  Wijhe's phraseology and also of his illustrations. He terms the whole c&#x0153;lomic cavity the
+  <i>proc&#x0153;lom</i>, which is divisible into a ventral unsegmented part, the body-cavity or
+  <i>metac&#x0153;lom</i>, and a dorsal segmented part, the <i>somite</i>. This latter part again is
+  divided into a dorsal part&mdash;the <i>epimere</i>&mdash;and a part connecting the dorsal part
+  with the body-cavity, to which therefore he gives the name of <i>mesomere</i>.</p>
+
+  <p>The cavity of the epimere disappears, and its walls form the muscle and cutis plates of the
+  body. The part which forms the muscles is known as the <i>myotome</i>, which separates off from
+  the mesomere, leaving the latter as a blind sac&mdash;the
+  <i>mesoc&#x0153;lom</i>&mdash;communicating by a narrow passage with the body cavity or
+  <i>metac&#x0153;lom</i>. At the same time, from the mesomere is formed the <i>sclerotome</i>,
+  which gives rise to the skeletal tissues of the vertebræ, etc., so that van Wijhe's epimere and
+  mesomere together correspond to the original term, protovertebra, or somite of Balfour; and when
+  the myotome and sclerotome have separated off, there is still left the intermediate cell-mass of
+  Balfour and Sedgwick, <i>i.e.</i> the sac-like mesoc&#x0153;le of van Wijhe, the walls of which
+  give origin to the mesonephrotome or <i>mesonephros</i>. Further, according to van Wijhe, the
+  dorsal part of the unsegmented metac&#x0153;lom is itself segmented, but not, as in the case of
+  the mesoc&#x0153;le, with respect to both splanchnopleuric and somatopleuric walls. The
+  segmentation is manifest only on the somatopleuric side, and consists of a distinct series of
+  hollow somatopleuric outgrowths, called by him <i>hypomeres</i>, which give rise to the
+  <i>pronephros</i> and the segmental duct.</p>
+
+  <p>Van Wijhe considers that the whole metac&#x0153;lom was originally segmented, because in the
+  lower vertebrates the segmentation reaches further ventral-wards, so that in Selachia the
+  body-cavity is almost truly segmental. Also in the gill-region of Amphioxus the cavities which are
+  homologous with the body-cavity arise segmentally.</p>
+
+  <div><span class="pagenum" id="page391">{391}</span></div>
+
+  <div class="ac w45 fcenter sp2">
+    <a href="images/fig156.png" id="fig156"><img style="width:100%" src="images/fig156.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 156.&mdash;Diagrams to illustrate the Development of the
+      Vertebrate C&#x0153;lom.</span> (After <span class="sc">van Wijhe</span>.)</p>
+      <p class="sp0"><i>N.</i>, central nervous system; <i>Nc.</i>, notochord; <i>Ao.</i>, aorta;
+      <i>Mg.</i>, midgut. A, <i>My.</i>, myoc&#x0153;le; <i>Mes.</i>, mesoc&#x0153;le; <i>Met.</i>,
+      metac&#x0153;le; <i>Hyp.</i>, hypomere (pronephric). B and C, <i>My.</i>, myotome;
+      <i>Mes.</i>, mesonephros; <i>S.d.</i>, segmental duct (pronephric); <i>Met.</i>, body
+      cavity.</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page392">{392}</span></div>
+
+  <p>As is well known, Balfour and Semper were led, from their embryological researches, to compare
+  the nephric organs of vertebrates with those of annelids, and, indeed, the nature of the
+  vertebrate segmental excretory organs has always been the fact which has kept alive the belief in
+  the origin of vertebrates from a segmented annelid. These segmental organs thus compared were the
+  mesonephric tubules, and doubts arose, especially in the mind of Gegenbaur, as to the validity of
+  such a comparison, because the mesonephric tubules did not open to the exterior, but into a
+  duct&mdash;the segmental duct&mdash;which was an unsegmented structure opening into the cloaca;
+  also because the segmental duct, which was the excretory duct of the pronephros, was formed first,
+  and the mesonephric tubules only opened into it after it was fully formed. Further, the pronephros
+  was said to arise from an outbulging of the somatopleuric mesoblast, which extended over a limited
+  number of metameres, and was not segmental, but continuous. Gegenbaur and others therefore argued
+  that the original prevertebrate excretory organ was the pronephros and its duct, not the
+  mesonephros, from which they concluded that the vertebrate must have been derived from an
+  unsegmented type of animal, and not from the segmented annelid type.</p>
+
+  <p>Such a view, however, has no further reason for acceptance, as it was based on wrong premises,
+  for Rückert has shown that the pronephros does arise as a series of segmental nephric tubules, and
+  is not unsegmented. He also has pointed out that in Torpedo the anterior part of the pronephric
+  duct shows indications of being segmented, a statement fully borne out by the researches of Maas
+  on Myxine, who gives the clearest evidence that in this animal the anterior part of the pronephric
+  duct is formed by the fusion of a series of separate ducts, each of which in all probability once
+  opened out separately to the exterior.</p>
+
+  <p>Rückert therefore concludes that Balfour and Semper were right in deriving the segmental organs
+  of vertebrates from those of annelids, but that the annelid organs are represented in the
+  vertebrate, not by the mesonephric tubules, but by the pronephric tubules and their ducts, which
+  originally opened separately to the exterior. By the fusion of such tubules the anterior part of
+  the segmental duct was formed, while its posterior part either arose by a later c&#x0153;nogenetic
+  lengthening, or is the only remnant of a series of pronephric tubules which originally extended
+  the whole length of the body, as suggested also by Maas and Boveri. Rückert therefore supposed
+  that the mesonephric tubules were a secondary set of nephric organs, which were not necessarily
+  directly derived from the annelid nephric organs.</p>
+
+  <div><span class="pagenum" id="page393">{393}</span></div>
+
+  <p>At present, then, Rückert's view is the one most generally accepted&mdash;the original annelid
+  nephric organs are represented by the pronephric tubules and the pronephric duct, not by the
+  mesonephric tubules, which are a later formation. This latter statement would hold good if the
+  mesonephric tubules were found entirely in segments posterior to those containing the pronephric
+  tubules; such, however, is said not to be the case, for the two sets of organs are said to overlap
+  in some cases; even when they exist in the same segments, the former are said always to be formed
+  from a more dorsal part of the c&#x0153;lom than the pronephros, always to be a later formation,
+  and never to give any indication of communicating with the exterior except by way of the
+  pronephric duct.</p>
+
+  <p>The recent observations of Brauer on the excretory organs of the Gymnophiona throw great doubt
+  on the existence of mesonephric and pronephric tubules in the same segment. He criticizes the
+  observations on which such statements are based, and concludes that, as in Hypogeophis, the
+  nephrotome which is cut off after the separation of the sclero-myotome gives origin to the
+  pronephros in the more anterior regions, just as it gives origin to the mesonephros in the more
+  posterior regions. In fact, the observations of van Wijhe and others do not in reality show that
+  two excretory organs may be formed in one segment, the one mesonephric from the remains of the
+  mesomere and the other pronephric from the hypomere, but rather that in such cases there is only
+  one organ&mdash;the pronephros&mdash;part of which is formed from the mesomere and part from the
+  hypomere. Brauer goes further than this, and doubts the validity of any distinction between
+  pronephros and mesonephros, on the ground of the former arising from a more ventral part of the
+  proc&#x0153;lom than the latter; for, as he says, it is only possible to speak of one part of the
+  somite as being more ventral than another part when both parts are in the same segment; so that if
+  pronephric and mesonephric organs are never in the same segment, we cannot say with certainty that
+  the former arises more ventrally than the latter.</p>
+
+  <p>These observations of Brauer strongly confirm Sedgwick's original statement that the pronephric
+  and mesonephric organs are homodynamous organs, in that they are both derived from the original
+  serially situated nephric organs, the differences between them being of a subordinate nature and
+  not sufficient to force us to believe that the mesonephros is an organ of quite different origin
+  to the <span class="pagenum" id="page394">{394}</span>pronephros. So, also, Price, from his
+  investigations of the excretory organs of Bdellostoma, considers that in this animal both
+  pronephros and mesonephros are derived from a common embryonic kidney, to which he gives the name
+  <i>holonephros</i>.</p>
+
+  <p>Brauer also is among those who conclude that the vertebrate excretory organs were derived from
+  those of annelids; he thinks that the original ancestor possessed a series of similar organs over
+  the whole pronephric and mesonephric regions, and that the anterior pronephric organs, which alone
+  form the segmental duct, became modified for a larval existence&mdash;that their peculiarities
+  were adaptive rather than ancestral. This last view seems to me very far-fetched, without any
+  sufficient basis for its acceptance. According to the much more probable and reasonable view, the
+  pronephros represents the oldest and original excretory organs, while the mesonephros is a later
+  formation. Brauer's evidence seems to me to signify that the pronephros, mesonephros, and
+  metanephros are all serially homologous, and that the pronephros bears much the same relation to
+  the mesonephros that the mesonephros does to the metanephros. The great distinction of the
+  pronephros is that it, and it alone, forms the segmental duct.</p>
+
+  <p>We may sum up the conclusions at which we have now arrived as follows<span
+  class="wnw">:&mdash;</span></p>
+
+  <p>1.  The pronephric tubules and the pronephric duct are the oldest part of the excretory system,
+  and are distinctly in evidence for a few segments only in the most anterior part of the
+  trunk-region immediately following the branchial region. They differ also from the mesonephric
+  tubules by not being so clearly segmental with the myotomes.</p>
+
+  <p>2.  The mesonephric tubules belong to segments posterior to those of the pronephros, are
+  strictly segmental with the myotomes, and open into the pronephric duct.</p>
+
+  <p>3.  All observers are agreed that the two sets of excretory organs resemble each other in very
+  many respects, as though they arose from the same series of primitive organs, and, according to
+  Sedgwick and Brauer, no distinction of any importance does exist between the two sets of organs.
+  Other observers, however, consider that the pronephric organs, in part at all events, arise from a
+  part of the nephroc&#x0153;le more ventral than that which gives origin to the mesonephric organs,
+  and that this difference in position of origin, combined <span class="pagenum"
+  id="page395">{395}</span>with the formation of the segmental duct, does constitute a true
+  morphological distinction between the two sets of organs.</p>
+
+  <p>4. All the recent observers are in agreement that the vertebrate excretory organs strongly
+  indicate a derivation from the segmental organs of annelids.</p>
+
+  <p>The very strongest support has been given to this last conclusion by the recent discoveries of
+  Boveri and Goodrich upon the excretory organs of Amphioxus. According to Boveri, the nephric
+  tubules of Amphioxus open into the dorsal c&#x0153;lom by one or more funnels. Around each funnel
+  are situated groups of peculiar cells, called by him 'Fadenzellen,' each of which sends a long
+  process across the opening of the funnel. Goodrich has examined these 'Fadenzellen,' and found
+  that they are typical pipe-cells, or solenocytes, such as he has described in the nephridial
+  organs of various members of the annelid group Polychæta. Also, just as in the Polychæta, the
+  ciliated nephric tubule has no internal funnel-shaped opening into the c&#x0153;lom, but
+  terminates in these groups of solenocytes. "Each solenocyte consists of a cell-body and nucleus
+  situated at the distal free extremity of a delicate tube; the proximal end of the tube pierces the
+  wall of the nephridial canal and opens into its lumen. A single long flagellum arising from the
+  cells works in the tube and projects into the canal."</p>
+
+  <p>The exceedingly close resemblance between the organs of Amphioxus and those of Phyllodoce, as
+  given in his paper, is most striking, and, as he says, leads to the conclusion that the excretory
+  organs of Amphioxus are essentially identical with the nephridia of certain polychæte worms.</p>
+
+  <p>It is to me most interesting to find that the very group of annelids, the Polychæta, which
+  possess solenocytes so remarkably resembling those of the excretory organs of Amphioxus, are the
+  highest and most developed of all the Annelida. I have argued throughout that the law of evolution
+  consists in the origination of successive forms from the dominant group then alive, dominance
+  signifying the highest type of brain-power achieved up to that time. The highest type among
+  Annelida is found in the Chætopoda; from them, therefore, the original arthropod type must have
+  sprung. This original group of Arthropoda gave rise to the two groups of Crustacea and Arachnida,
+  in my opinion also to the Vertebrata, and, as already mentioned, it is convenient to give it a
+  generalized <span class="pagenum" id="page396">{396}</span>name, the Protostraca, from which
+  subsequently the Palæostraca arose.</p>
+
+  <p>The similarity between the excretory organs of Amphioxus and those of Phyllodoce suggests that
+  the protostracan ancestor of the vertebrates arose from the highest group of the
+  Chætopoda&mdash;the Polychæta. The evidence which I have already given points, however, strongly
+  to the conclusion that the vertebrate did not arise from members of the Protostraca near to the
+  polychæte stock, but rather from members in which the arthropod characters had already become well
+  developed&mdash;members, therefore, which were nearer the Trilobita than the Polychæta. Such early
+  arthropods would very probably have retained in part excretory organs of the same character as
+  those found in the original polychæte stock, and thus account for the presence of solenocytes in
+  the excretory organs of Amphioxus.</p>
+
+  <p>In connection with such a possibility, I should like to draw attention to the observations of
+  Claus and Spangenberg on the excretory organs of Branchipus&mdash;that primitive phyllopod, which
+  is recognized as the nearest approach to the trilobites at present living. According to Claus, an
+  excretory apparatus exists in the neighbourhood of each nerve-ganglion, and Spangenberg finds a
+  perfectly similar organ in the basal segment of each appendage&mdash;a system, therefore, of
+  excretory organs as segmentally arranged as those of Peripatus. Claus considers that although
+  these organs formed an excretory system, it is not possible to compare them with the annelid
+  segmental organs, because he thought the cells in question arose from ectoderm. Now, the striking
+  point in the description of the excretory cells in these organs, as described both by Claus and
+  Spangenberg, is that they closely resemble the pipe-cells or solenocytes of Goodrich; each cell
+  possesses a long tube-like projection, which opens on the surface. They appear distinctly to
+  belong to the category of flame-cells, and resemble solenocytes more than anything else. According
+  to Goodrich, the solenocyte is probably an ectodermal cell, so that even if it prove to be the
+  case, as Claus thought, that these pipe-cells of Branchipus are ectodermal, they would still claim
+  to be derived from the segmental organs of annelids, especially of the Polychæta, being, to use
+  Goodrich's nomenclature, true nephridial organs, as opposed to c&#x0153;lomostomes.</p>
+
+  <p>These observations of Claus and Spangenberg suggest not only that the primitive arthropod of
+  the trilobite type possessed segmental <span class="pagenum" id="page397">{397}</span>organs in
+  every segment directly derived from those of a polychæte ancestor, but also that such organs were
+  partly somatic and partly appendicular in position. Such a suggestion is in strict accord with the
+  observations of Sedgwick on the excretory organs of the most primitive arthropod known, viz.
+  Peripatus, where also the excretory organs, which are true segmental organs, are partly somatic
+  and partly appendicular. Further, the excretory organs of the Scorpion and Limulus group are again
+  partly somatic and partly appendicular, receiving the name of coxal glands, because there is a
+  ventral projection of the gland into the coxa of the corresponding appendage.</p>
+
+  <p>Judging from all the evidence available, it is probable that when the arthropod stock arose
+  from the annelids, simultaneously with the formation of appendages, the segmental somatic nephric
+  organs of the latter extended ventrally into the appendage, and thus formed a segmental set of
+  excretory organs, which were partly somatic, partly appendicular in position, and might therefore
+  be called coxal glands.</p>
+
+  <p>As already stated, all investigators of the origin of the vertebrate excretory organs are
+  unanimous in considering them to be derived from segmental organs of the annelid type. I naturally
+  agree with them, but, in accordance with my theory, would substitute the words "primitive
+  arthropod" for the word "annelid," for all the evidence I have accumulated in the preceding
+  chapters points directly to that conclusion. Further, the most primitive of the three sets of
+  vertebrate segmental organs&mdash;the pronephros, mesonephros, and metanephros&mdash;is
+  undoubtedly the pronephros; consequently the pronephric tubules are those which I consider to be
+  more directly derived from the coxal glands of the primitive arthropod ancestor. Such a derivation
+  appears to me to afford an explanation of the difficulties connected with the origin of the
+  pronephros and mesonephros respectively, which is more satisfactory than that given by the direct
+  derivation from the annelid.</p>
+
+  <p>The only living animal which we know of as at all approaching the most primitive arthropod type
+  is, as pointed out by Korschelt and Heider, Peripatus; and Peripatus, as is well known, possesses
+  a true c&#x0153;lom and true c&#x0153;lomic excretory organs in all the segments of the body.
+  Sedgwick shows that at first a true c&#x0153;lom, as typical as that of the annelids, is formed in
+  each segment of the body, and that then this c&#x0153;lom (which represents in the vertebrate van
+  Wijhe's pro-c&#x0153;lom) <span class="pagenum" id="page398">{398}</span>splits into a dorsal and
+  a ventral part. In the anterior segments of the body the dorsal part disappears (presumably its
+  walls give origin to the mesoblast from which the dorsal body-muscles arise), while the ventral
+  part remains and forms a nephroc&#x0153;le, giving origin to the excretory organs of the adult.
+  According to von Kennel, the cavity becomes divided into three spaces, which for a time are in
+  communication&mdash;a lateral (I.), a median (II.), and a dorso-median (III.). The dorso-median
+  portion becomes partitioned off, and this, as well as the greater part of the lateral portion,
+  which lies principally in the foot, is used up in providing elements for the formation of the
+  body- and appendage-muscles respectively and the connective tissue.</p>
+
+  <p>In Fig. <a href="#fig157">157</a> I reproduce von Kennel's diagram of a section across a
+  Peripatus embryo, in which I. represents the lateral appendicular part of the c&#x0153;lom, II.
+  the ventral somatic part, and III. the dorsal part which separates off from the ventral and
+  lateral parts, and, as its walls give origin largely to the body-muscles, may be called the
+  myoc&#x0153;le. The muscles of the appendages are formed from the ventral part of the original
+  proc&#x0153;lom, just as I have argued is the case with the muscles of the splanchnic segmentation
+  in vertebrates.</p>
+
+  <p>Sedgwick states that the ventral part of the c&#x0153;lom extends into the base of each
+  appendage, and there forms the end-sac of each nephric tubule, into which the nephric funnel
+  opens, thus forming a coxal gland; this end-sac or vesicle in the appendage is called by him the
+  internal vesicle (<i>i.v.</i>), because later another vesicle is formed from the ventral
+  c&#x0153;lom in the body itself, close against the nerve-cord on each side, which he calls the
+  external vesicle (<i>e.v.</i>). (<i>Cf.</i> Fig. <a href="#fig158">158</a>, taken from Sedgwick.)
+  This second vesicle is, according to him, formed later in the development from the nephric tubule
+  of the internal vesicle, so that it discharges its contents to the exterior by the same opening as
+  the original tubule. Of course, as he points out, the whole system of internal and external
+  vesicles and nephric tubules are all simply derivatives of the original ventral part of the
+  c&#x0153;lom or nephroc&#x0153;le.</p>
+
+  <div><span class="pagenum" id="page399">{399}</span></div>
+
+  <div class="ac w35 fcenter sp2">
+    <a href="images/fig157.png" id="fig157"><img style="width:100%" src="images/fig157.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 157.&mdash;Transverse Section of Peripatus Embryo.</span> (After
+      <span class="sc">von Kennel</span>.)</p>
+      <p class="sp0"><i>Al.</i>, alimentary canal; <i>N.</i>, nerve-cord; <i>App.</i>, appendage;
+      <i>I</i>, <i>II</i>, <i>III</i>, the three divisions (lateral, median, and dorso-median) of
+      the c&#x0153;lom.</p>
+    </div>
+  </div>
+
+  <div class="ac w35 fcenter sp2">
+    <a href="images/fig158.png" id="fig158"><img style="width:100%" src="images/fig158.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 158.&mdash;Section of Peripatus.</span> (After <span
+      class="sc">Sedgwick</span>.)</p>
+      <p class="sp0"><i>Al.</i>, alimentary canal; <i>N.</i>, nerve-cord; <i>App.</i>, appendage;
+      <i>i.v.</i>, internal, and <i>e.v.</i>, external vesicles of the segmented excretory tubule
+      (coxal gland).</p>
+    </div>
+  </div>
+
+  <p>Here, then, in Peripatus, and presumably, therefore, in members of the Protostraca, we see that
+  the original segmental organs of the annelid have become a series of nephric organs, which
+  extended into the base of the appendages, and may therefore be called coxal glands; also it is
+  clear, from Sedgwick's description, that if the appendages disappeared, the nephric organs would
+  still remain, not as coxal glands, but as purely somatic excretory glands. They would still be
+  homologous with the annelid segmental organs, or with the coxal glands, but would arise <i>in
+  toto</i> from a part of the ventral c&#x0153;lom or nephroc&#x0153;le, more dorsal than the former
+  appendicular part, because the appendages and their enclosed c&#x0153;lom are always situated
+  ventrally to the body. Again, according to Sedgwick, the nephric tubules are connected with two
+  c&#x0153;lomic vesicles, the one in the appendage the internal vesicle, and the other, the
+  so-called bladder, or the external vesicle, in the body itself, close against the nerve-cord.
+  Sedgwick appears to consider that either of these vesicles may form the end-sac of a nephric
+  tubule, for he discusses the question whether the single vesicle, which in each case gives origin
+  to the nephridia of the first three legs, corresponds to the internal or external vesicle. He
+  <span class="pagenum" id="page400">{400}</span>decides, it is true, in favour of the internal
+  vesicle, and therefore considers the excretory organ to be appendicular, <i>i.e.</i> a coxal
+  gland, in these segments as well as in those more posterior. Still, the very discussion shows that
+  in his opinion, at all events, the external vesicle might represent the end-sac of the tubule, in
+  the absence of the internal or appendicular vesicle.</p>
+
+  <p>Such an arrangement as Sedgwick describes in Peripatus is the very condition required to give
+  rise to the pronephric and mesonephric tubules, as deduced by me from the consideration of the
+  vertebrate, and harmonizes and clears up the controversy about the mesonephros and pronephros in
+  the most satisfactory manner. Both pronephros and mesonephros are seen to be derivatives of the
+  original annelid segmental organs, not directly from an annelid, but by way of an arthropodan
+  ancestor; the difference between the two is simply that the pronephric organs were coxal glands,
+  and indicate, therefore, the presence of the original metasomatic appendages, while the
+  mesonephric organs were homologous organs, formed in segments of later origin which had lost their
+  appendages. For this reason the pronephros is said to be formed, in part at least, from a portion
+  of the c&#x0153;lom situated more ventrally than the purely somatic part which gives rise to the
+  mesonephros. For this reason Sedgwick, Brauer, etc., can say that the mesonephros is strictly
+  homodynamous with the pronephros; while equally Rückert, Semon, and van Wijhe can say it is not
+  homodynamous, in so far that the two organs are not derived strictly from absolutely homologous
+  parts of the c&#x0153;lom. For this reason Semon can speak of the mesonephros as a dorsal
+  derivative of the pronephros, just as Sedgwick says that the external or somatic vesicle of
+  Peripatus is a derivative of the appendicular nephric organ. For this reason the pronephros, or
+  rather a part of it, is always derived from the somatopleuric layer, for, as is clear from Miss
+  Sheldon's drawing, the part of the c&#x0153;lom in Peripatus which dips into the appendage is
+  derived from the somatopleuric layer alone.</p>
+
+  <p>Such a c&#x0153;lom as that of Peripatus, Fig. <a href="#fig157">157</a>, would represent the
+  origin of the vertebrate c&#x0153;lom, and would therefore represent the proc&#x0153;lom of van
+  Wijhe. In strict accordance with this, we see that it separates into a dorsal part, the walls of
+  which give origin to the somatic muscles, or at all events to the great longitudinal dorsal
+  muscles of the animal, and a ventral part, which forms a nephroc&#x0153;le, <span class="pagenum"
+  id="page401">{401}</span>dips into the appendage, and gives origin to the muscles of the
+  appendage. In the vertebrate, after the somatic dorsal part or myoc&#x0153;le has separated off, a
+  ventral part is left, which forms a nephroc&#x0153;le in the trunk-region, and gives origin to the
+  splanchnic striated muscles in the cranial region, <i>i.e.</i> to the muscles which, according to
+  my theory, were once appendicular muscles. This ventral nephroc&#x0153;lic part is divisible in
+  the trunk into a segmented part, which forms the excretory organs proper, and an unsegmented part,
+  the metac&#x0153;le or true body-cavity of the vertebrate.</p>
+
+  <p>This comparison of the proc&#x0153;lom of the vertebrate and arthropod signifies that the
+  vertebrate metac&#x0153;le was directly derived by ventral downgrowth from the arthropod
+  nephroc&#x0153;le, so that if, as I suppose, the vertebrate nervous system represents the
+  conjoined nervous system and alimentary canal of the arthropod, then the vertebrate
+  metac&#x0153;le, or body-cavity, must have been originally confined to the region on each side of
+  the central nervous system, and from this position have spread ventrally, to enclose ultimately
+  the new-formed vertebrate gut. This means that the body-cavity (metac&#x0153;le) of the vertebrate
+  is not the same as the body-cavity of the annelid, but corresponds to a ventral extension of the
+  nephroc&#x0153;le, or ventral part of such body-cavity.</p>
+
+  <p>Such a phylogenetic history is most probable, because it explains most naturally and simply the
+  facts of the development of the vertebrate body-cavity; for the mesoblast always originates in the
+  neighbourhood of the notochord and central nervous system, and the lumen of the body-cavity always
+  appears first in that region, and then extends laterally and ventrally on each side until it
+  reaches the most ventral surface of the embryo, thus forming a ventral mesentery, which ultimately
+  disappears, and the body-cavity surrounds the gut, except for the dorsal mesentery. Thus Shipley,
+  in his description of the formation of the mesoblastic plates which line the body-cavity in
+  Ammoc&#x0153;tes, describes them as commencing in two bands of mesoblast situated on each side,
+  close against the commencing nervous system<span class="wnw">:&mdash;</span></p>
+
+  <p>"These two bands are separated dorsally by the juxtaposition of the dorsal wall of the
+  mesenteron and the epiblast, and ventrally by the hypoblastic yolk-cells which are in contact with
+  the epiblast over two-thirds of the embryo. Subsequently, but at a much later date, the mesoblast
+  is completed ventrally by the downgrowth on <span class="pagenum" id="page402">{402}</span>each
+  side of these mesoblastic plates. The subsequent downward growth is brought about by the cells
+  proliferating along the free ventral edge of the mesoblast, these cells then growing ventralwards,
+  pushing their way between the yoke-cells and epiblast."</p>
+
+  <p>The derivation of the vertebrate pronephric segmental organs from the metasomatic coxal glands
+  of a primitive arthropod would mean, if the segmental organs of Peripatus be taken as the type,
+  that such glands opened to the exterior on every segment, either at the base of the appendage or
+  on the appendage itself. It is taken for granted by most observers that the pronephric segmental
+  organs once opened to the exterior on each segment, and then, from some cause or other, ceased to
+  do so, and the separate ducts, by a process of fusion, came to form a single segmental duct, which
+  opened into the cloaca. Many observers have been led to the conclusion that the pronephric duct is
+  epiblastic in origin, although from its position in the adult, it appears far removed from all
+  epiblastic formations. However, at no time in the developmental history is there any clear
+  evidence of actual fusion of any part of the pronephric organ with the epidermis, and the latest
+  observer, Brauer, is strongly of opinion that there is never sufficiently close contact with the
+  epidermis to warrant the statement that the epiblastic cells take part in the formation of the
+  duct. All that can be said is, that the formation of the duct takes place at a time when the
+  pronephric diverticulum is in close propinquity to the epidermis, before the ventral downgrowth of
+  the myotome has taken place.</p>
+
+  <p>The formation of the anterior portion of the pronephric duct is, according to Maas in Myxine,
+  and Wheeler in Petromyzon, undoubtedly brought about by the fusion of a number of pronephric
+  tubules, which, according to Maas, are clearly seen in the youngest specimens as separate
+  segmental tubes; each of these tubules is supplied by a capillary network from a segmental branch
+  of the aorta, as in the tubules of Amphioxus according to Boveri, and does not possess a
+  glomerulus.</p>
+
+  <p>The posterior part of the duct into which the mesonephric tubules enter possesses also a
+  capillary network, which Maas considers to represent the original capillary network of a series of
+  pronephric tubules, the only remnant of which is the duct into which the mesonephric tubules open.
+  He therefore argues that the pronephric duct indicates a series of pronephric tubules, which
+  originally extended <span class="pagenum" id="page403">{403}</span>along the whole length of the
+  body, and were supplanted by the mesonephric tubules, which also belonged to the same
+  segments.</p>
+
+  <p>I also think that the paired appendages which have left the pronephric tubules as signs of
+  their past existence, existed originally, in the invertebrate stage, on every segment of the body.
+  But I do not consider that such a statement is at all equivalent to saying that such pairs of
+  tubules must have existed upon every one of the segments existing at the present day; for it seems
+  to me that Rückert is much more likely to be right when he says that in Selachians the duct
+  clearly does grow back, and is not formed throughout <i>in situ</i>; so that he gives a double
+  explanation of the formation of the duct&mdash;a palingenetic anterior part formed by the fusion
+  of the extremities of the original excretory tubules, to which a posterior c&#x0153;nogenetic
+  lengthening has been added.</p>
+
+  <p>It does not seem to me at all necessary that the immediate invertebrate ancestor of the
+  vertebrate should have possessed excretory organs which opened out separately to the exterior on
+  each segment; a fusion may already have taken place in the invertebrate stage, and so a single
+  duct have been acquired for a number of organs. Such a suggestion has been made by Rückert,
+  because of the fact discovered by Cunningham and E. Meyer, that the segmental organs of <i>Lanice
+  conchilega</i> are on each side connected together by a single strong longitudinal canal. I would,
+  however, go further than this and say, that even although the nephric organs of the polychæte
+  ancestor opened out on every segment, and although the primitive arthropodan ancestor derived from
+  such polychæte possessed coxal glands which opened out either on to or at the base of each
+  appendage, similarly to those of Peripatus, yet the immediate arthropodan ancestor, with its
+  palæostracan affinities, may already have possessed metasomatic coxal glands, all of which opened
+  into a single duct, with a single opening to the exterior.</p>
+
+  <p>Judging from Limulus, such was very probably the case, for Patten and Hazen have shown (1) that
+  the coxal glands of Limulus are segmental organs belonging to the prosomatic segments; (2) that
+  the organs belonging to the cheliceral and ectognathal segments are not developed; (3) that the
+  four glands belonging to the endognaths become connected together by a <i>stolon</i>, which
+  communicates with a single nephric duct, opening to the exterior on the basal segment of the 5th
+  prosomatic appendage (the last endognath). At <span class="pagenum" id="page404">{404}</span>no
+  time is there any evidence of any separate openings or any fusion with the ectoderm, such as might
+  indicate separate openings of these prosomatic coxal segmental organs. Thus we see that in
+  Limulus, which is presumably much nearer the annelid condition than the vertebrate, all evidence
+  of separate nephric ducts opening to the exterior on each prosomatic segment has entirely
+  disappeared, just as is the case in the metasomatic coxal glands (<i>i.e.</i> the pronephros) of
+  the vertebrate. What is seen in the prosomatic region of Limulus, and doubtless also of the
+  Eurypterids, may very probably have occurred in the metasomatic region of the immediate
+  invertebrate ancestors of the vertebrate, and so account for the single pronephric duct belonging
+  to a number of pronephric organs.</p>
+
+  <p>The interpretation of these various embryological investigations may be summed up as
+  follows<span class="wnw">:&mdash;</span></p>
+
+  <p>1.  The ancestor of the vertebrates possessed a pair of appendages on each segment; into the
+  base of each of these appendages the segmental excretory organ sent a diverticulum, thus forming a
+  coxal gland.</p>
+
+  <p>2.  Such coxal glands, even in the invertebrate stage, may have discharged into a common duct
+  which opened to the exterior most posteriorly.</p>
+
+  <p>3.  Then, from some cause, the appendages were rendered useless, and dwindled away, leaving
+  only the pronephric organs to indicate their former presence. At the end of this stage the animal
+  possessed vertebrate characteristics.</p>
+
+  <p>4.  For the purpose of increasing mobility, of forming an efficient swimming instead of a
+  crawling animal, the body-segments increased in number, always, as is invariably the case, by the
+  formation of new ones between those already formed and the cloacal region, and so of necessity
+  caused an elongation of the pronephric duct. Into this there now opened the ducts of the segmental
+  organs formed by recapitulation, those, therefore, belonging to the
+  body-segments&mdash;mesonephric&mdash;having nothing to do with appendages, for the latter had
+  already ceased to exist functionally, and would not, therefore, be repeated with each meristic
+  repetition.</p>
+
+  <p>This, so to speak, passive lengthening of the pronephric duct in consequence of the lengthening
+  of the early vertebrate body by the addition of metameres, each of which contained only
+  mesonephric and no pronephric tubules, is, to my mind, an example of a principle <span
+  class="pagenum" id="page405">{405}</span>which has played an important part in the formation of
+  the vertebrate, viz. that the meristic variation by which the spinal region of even the lowest of
+  existing vertebrates has been formed, has largely taken place in the vertebrate phylum itself, and
+  that such changes must be eliminated before we can picture to ourselves the pre-vertebrate
+  condition. As an example, I may mention the remarkable repetition of similar segments pictured by
+  Bashford Dean in Bdellostoma. Such repetition leads to passive lengthening of such parts as are
+  already formed but are not meristically repeated: such are the notochord, the vertebrate
+  intestine, the canal of the spinal cord, and possibly the lateral line nerve. The fuller
+  discussion of this point means the discussion of the formation of the vertebrate alimentary canal;
+  I will therefore leave it until I come to that part of my subject, and only say here that the
+  evidence seems to me to point to the conclusion that at the time when the vertebrate was formed,
+  the respiratory and cloacal regions were very close together, the whole of the metasoma being
+  represented by the region of the pronephros alone.</p>
+
+  <p>Here, as always, the evidence of Ammoc&#x0153;tes tends to give definiteness to our
+  conceptions, for Wheeler points out that up to a length of 7 mm. the pronephros only is formed;
+  there is no sign of the more posteriorly formed mesonephros. Now we know, as pointed out in
+  Chapter VI., p. <a href="#page228">228</a>, this is the time of Kupffer's larval stage of
+  Ammoc&#x0153;tes. This is the period during which the invertebrate stage is indicated in the
+  ontogeny, so that, in accordance with all that has gone before, this means that the metasoma of
+  the invertebrate ancestor was confined to the region of the pronephros.</p>
+
+  <p>Again, take Shipley's account of the development of Petromyzon. He says&mdash;</p>
+
+  <p>"The alimentary canal behind the branchial region may be divided into three sections.
+  Langerhans has termed these the stomach, midgut, and hindgut, but as the most anterior of these is
+  the narrowest part of the whole intestine, it would, perhaps, be better to call it
+  &#x0153;sophagus. This part of the alimentary canal lies entirely in front of the yolk, and is,
+  with the anterior region, which subsequently bears the gills, raised from the rest of the egg when
+  the head is folded off. It is supported by a dorsal mesentery, on each side of which lies the
+  head-kidney (pronephros)."</p>
+
+  <p>Further on he says&mdash;</p>
+
+  <div><span class="pagenum" id="page406">{406}</span></div>
+
+  <p>"The hindgut is smaller than the midgut; its anterior limit is marked by the termination of the
+  spiral valve, which does not extend into this region. The two segmental ducts open into it just
+  where it turns ventrally to open to the exterior by a median ventral anus. Its lumen is from an
+  early stage lined with cells which have lost their yolk, and it is in wide communication with the
+  exterior from the first. This condition seems to be, as Scott suggests, connected with the
+  openings of the ducts of the pronephros, for this gland is completed and seems capable of
+  functioning long before any food could find its way through the midgut, or, indeed, before the
+  stomodæum has opened."</p>
+
+  <p class="sp3">Is there no significance in this statement of Shipley? Even if it be possible to
+  find some special reason why the branchial and cloacal parts of the gut are freed from yolk and
+  lined with serviceable epithelium a long time before the midgut, why should a bit of the midgut,
+  which Shipley calls the &#x0153;sophagus, which is connected with the region of the pronephros and
+  not of the branchiæ, differ so markedly from the rest of the midgut? Surely the reason is that the
+  branchial region of the gut, the pronephric region of the gut, and the cloacal region of the gut,
+  belong to a different and earlier phase in the phylogenetic history of the Ammoc&#x0153;tes than
+  does the midgut between the pronephric and cloacal regions. This observation of Shipley fits in
+  with and emphasizes the view that the original animal from which the vertebrate arose consisted of
+  a cephalic and branchial region, followed by a pronephric and cloacal region; the whole
+  intermediate part of the gut, which forms the midgut, with its large lumen and spiral valve, and
+  belongs to the mesonephric region, being a later formation brought about by the necessity of
+  increasing the length of the body.</p>
+
+  <p class="ac"><span class="sc">The Origin of the Somatic Trunk-Musculature and the Formation of an
+  Atrial Cavity.</span></p>
+
+  <p>Next comes the question, why was the pronephros not repeated in the meristic repetition that
+  took place during the early vertebrate stage? What, in fact, caused the disappearance of the
+  metasomatic appendages, and the formation of the smooth body-surface of the fish?</p>
+
+  <p>The embryological evidence given by van Wijhe and others of the manner in which the original
+  superficially situated pronephros is <span class="pagenum" id="page407">{407}</span>removed from
+  the surface and caused to assume the deeper position, as seen in the later embryo, is perfectly
+  clear and uniform in all the vertebrate groups. The diagrams at the end of van Wijhe's paper,
+  which I reproduce here, illustrate the process which takes place. At first the myotome (Fig. <a
+  href="#fig159">159</a>, A) is confined to the dorsal region on each side of the spinal cord and
+  notochord. Then (Fig. <a href="#fig159">159</a>, B) it separates from the rest of the somite and
+  commences to extend ventrally, thus covering over the pronephros and its duct, until finally (Fig.
+  <a href="#fig159">159</a>, C) it reaches the mid-ventral line on each side, and the foundations of
+  the great somatic body-muscles are finally laid.</p>
+
+  <p>In order, therefore, to understand how the obliteration of the appendages took place, we must
+  first find out what is the past history of the myotomes. Why are they confined at first to the
+  dorsal region of the body, and extend afterwards to the ventral region, forcing by their growth an
+  organ that was originally external in situation to become internal?</p>
+
+  <p>In the original discussion at Cambridge, I was accused of violating the important principle
+  that in phylogeny we must look at the most elementary of the animals whose ancestors we seek, and
+  was told that the lowest vertebrate was Amphioxus, not Ammoc&#x0153;tes; that therefore any
+  argument as to the origin of vertebrates must proceed from the consideration of the former and not
+  the latter animal. My reply was then, and is still, that I was considering the cranial region in
+  the first place, and that therefore it was necessary to take the lowest vertebrate which possessed
+  cranial nerves and sense-organs of a distinctly vertebrate character, a criterion evidently not
+  possessed by Amphioxus. Such argument does not apply to the spinal region, so that, now that I
+  have left the cranial region and am considering the spinal, I entirely agree with my critics that
+  Amphioxus is likely to afford valuable help, and ought to be taken into consideration as well as
+  Ammoc&#x0153;tes. The distinction between the value of the spinal (including respiratory) and
+  cranial regions of Amphioxus for drawing phylogenetic conclusions is recognized by Boveri, who
+  says that, in his opinion, "Amphioxus shows simplicity and undifferentiation rather than
+  degeneration. If truly Amphioxus is somewhat degenerated, then it is so in its prehensile and
+  masticatory apparatus, its sense organs, and perhaps its locomotor organs, owing to its method of
+  living."</p>
+
+  <div><span class="pagenum" id="page408">{408}</span></div>
+
+  <div class="ac w45 fcenter sp2">
+    <a href="images/fig159.png" id="fig159"><img style="width:100%" src="images/fig159.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 159.&mdash;Diagrams to illustrate the Development of the
+      Vertebrate C&#x0153;lom.</span> (After <span class="sc">van Wijhe</span>.)</p>
+      <p class="sp0"><i>N.</i>, central nervous system; <i>Nc.</i>, notochord; <i>Ao.</i>, aorta;
+      <i>Mg.</i>, midgut. A, <i>My.</i>, myoc&#x0153;le; <i>Mes.</i>, mesoc&#x0153;le; <i>Met.</i>,
+      metac&#x0153;le; <i>Hyp.</i>, hypomere (pronephric). B and C, <i>My.</i>, myotome;
+      <i>Mes.</i>, mesonephros; <i>S.d.</i>, segmental duct (pronephric); <i>Met.</i>,
+      body-cavity.</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page409">{409}</span></div>
+
+  <p>Hatschek describes in Amphioxus how the c&#x0153;lom splits into a dorsal segmented portion,
+  the protovertebra, and a ventral unsegmented portion, the lateral plates. He describes in the
+  dorsal part the formation of myotome and sclerotome, as in the Craniota. Also, he describes how
+  the myotome is at first confined to the dorsal region in the neighbourhood of the spinal cord and
+  notochord, and subsequently extends ventrally, until, just as in Ammoc&#x0153;tes, the body is
+  enveloped in a sheet of somatic segmented muscles, the well-known myomeres.</p>
+
+  <p>The conclusion to be drawn from this is inevitable. Any explanation of the origin of the
+  somatic muscles in Ammoc&#x0153;tes must also be an explanation of the somatic muscles in
+  Amphioxus, and conversely; so that if in this respect Amphioxus is the more primitive and simpler,
+  then the condition in Ammoc&#x0153;tes must be looked upon as derived from a more primitive
+  condition, similar to that found in Amphioxus. Now, it is well <span class="correction"
+  title="Original reads 'know'">known</span> that a most important distinction exists between
+  Amphioxus and Ammoc&#x0153;tes in the topographical relation of the ventral portion of this
+  muscle-sheet, for in the former it is separated from the gut and the body-cavity by the atrial
+  space, while in the latter there is no such space. Fürbringer therefore concludes, as I have
+  already mentioned, that this space has become obliterated in the Craniota, but that it must be
+  taken into consideration in any attempt at formulating the nature of the ancestors of the
+  vertebrate.</p>
+
+  <p>Kowalewsky described this atrial space as formed by the ventral downgrowth of pleural folds on
+  each side of the body, which met in the mid-ventral line and enclosed the branchial portion of the
+  gut. According to this explanation, the whole ventral portion of the somatic musculature of the
+  adult Amphioxus belongs to the extension of the pleural folds, the original body-musculature being
+  confined to the dorsal region. This is expressed roughly on the external surface of Amphioxus by
+  the direction of the connective tissue septa between the myotomes (<i>cf.</i> Fig. <a
+  href="#fig162">162</a>, B). These septa, as is well known, bend at an angle, the apex of which
+  points towards the head. The part dorsal to the bend represents the part of the muscle belonging
+  to the original body; the part ventral to the bend is the pleural part, and represents the
+  extension into the pleural folds.</p>
+
+  <p>Lankester and Willey have attempted to give another explanation of the formation of the atrial
+  cavity; they look upon it as originating from a ventral groove, which becomes a canal by the
+  meeting of two <span class="pagenum" id="page410">{410}</span>outgrowths from the metapleure on
+  each side. This canal then extends dorsalwards on each side, and so forms the atrial cavity; the
+  metapleure still remains in the adult; the somatic muscles in the epipleure of the adult are the
+  original body-muscles, and not extensions into an epipleuric fold, for there is no such fold.</p>
+
+  <p>This explanation is a possible conception for the post-branchial portion of the atrium, but is
+  impossible for the branchial region; for, as Macbride points out, as must necessarily be the case,
+  the point of origin of the atrial wall is, in all stages of development, situated at the end of
+  the gill-slit. It shifts in position with the position of the gill-slit, but there can be no
+  backwards extension of the cavity. Macbride therefore agrees with Kowalewsky that the atrial
+  cavity is formed by the simultaneous ventral extension of pleural folds, and of the branchial part
+  of the original pharynx. Thus, in his summing up, he states: "In the larva practically the whole
+  sides and dorsal portion of the pharynx represent merely the hyper-pharyngeal groove and the
+  adjacent epithelium of the pharynx of the adult, the whole of the branchial epithelium of the
+  adult being represented by a very narrow strip of the ventral wall of the pharynx of the larva.
+  The subsequent disproportionate growth of this part of the pharynx of the larva, and of the
+  adjacent portion of the atrial cavity, has given the impression that the atrial cavity grew
+  upwards and displaced other structures, which is not the case."</p>
+
+  <p>Further, van Wijhe states that the atrium extends beyond the atriopore right up to the anus,
+  just as must have been the case if the pleural folds originally existed along the whole length of
+  the body. His words are: "Allerdings hat sich das Atrium beim <i>Amphioxus lanceolatus</i>
+  eigenthümlich ausgebildet, indem sich dasselbe durch den ganzen Rumpf bis an den Anus, d.h. bis an
+  die Wurzel des Schwanzes ausdehnt."</p>
+
+  <p>We get, therefore, this conception of the origin of the somatic musculature of the vertebrate.
+  The invertebrate ancestor possessed on each side, along the whole length of its body, a lateral
+  fold or pleuron which was segmented with the body, and capable of movement with the body, because
+  the dorsal longitudinal somatic muscles extended segmentally into each segment of the pleuron. By
+  the ventral extension of these pleural folds, not only was the smooth body-surface of the
+  vertebrate attained, but also the original appendages obliterated as such, leaving only as signs
+  of their existence the <span class="pagenum" id="page411">{411}</span>branchiæ, the pronephric
+  tubules, and the sense-organs of the lateral line system.</p>
+
+  <p>Such an explanation signifies that the somatic trunk-musculature of the vertebrate was derived
+  from the dorsal longitudinal musculature of the body of the arthropod, and not from the ventral
+  longitudinal musculature, and that therefore in the primitive arthropod stage the equivalent of
+  the myotome of the vertebrate did not give origin to the ventral longitudinal muscles of the
+  invertebrate ancestor. Now, as I have said, von Kennel states that in the proc&#x0153;lom of
+  Peripatus a dorsal part (III. in Fig. <a href="#fig157">157</a>) is cut off which gives origin to
+  the dorsal body-musculature, while the ventral part which remains (I. and II. in Fig. <a
+  href="#fig157">157</a>) gives origin in its appendicular portion (I.) to the muscles of the
+  appendage, and presumably in its ventral somatic portion (II.) to the ventral longitudinal muscles
+  of the body. This dorsal cut-off part might be called the myotome, in the same sense as the
+  corresponding part of the proc&#x0153;lom in the vertebrate is called the myotome. In both cases
+  the muscles derived from it form only a part of the voluntary musculature of the animal, and in
+  both cases the muscles in question are the dorsal longitudinal muscles of the body, to which must
+  be added the dorso-ventral body-muscles. Now, the whole of my theory of the origin of vertebrates
+  arose from the investigation of the structure of the cranial nerves, which led to the conception
+  that their grouping is not, like the spinal, a dual grouping of motor and sensory elements, but a
+  dual grouping to supply two sets of segments, characterized especially by the different
+  embryological origin of their musculature. The one set I called the somatic segmentation, because
+  the muscles belonging to it were the great longitudinal body-muscles; the other I called the
+  splanchnic segmentation, because its muscles were those connected with the branchial and visceral
+  arches. According to my theory, this latter segmentation was due to the segmentation of the
+  appendages in the invertebrate ancestor; and in previous chapters, dealing as they do with the
+  cranial region, attention was especially directed to the way in which the position of the striated
+  splanchnic musculature could be explained by a transformation of the prosomatic and mesosomatic
+  appendages. Now, I am dealing with the metasomatic region, in which it is true the appendages take
+  a very subordinate place, but still something corresponding to the splanchnic segments of the
+  cranial region might fairly be expected to exist, and I therefore <span class="pagenum"
+  id="page412">{412}</span>desire to emphasize what appears to me to be the fact, that the
+  musculature, which in the region of the trunk would correspond to that derived from the ventral
+  segmentation of the mesoblast in the region of the head, may have arisen not only from the
+  musculature of the appendages, but also from the ventral longitudinal musculature of the body of
+  the invertebrate ancestor, for it seems probable that this latter musculature had nothing to do
+  with the origin of the great longitudinal muscles of the vertebrate body, either dorsal or
+  ventral.</p>
+
+  <p>The way in which I imagine the obliteration of the atrial cavity to have taken place is
+  indicated in Fig. <a href="#fig160">160</a>, B, which is a modification of a section across a
+  trilobite-like animal as represented in Fig. <a href="#fig160">160</a>, A. As is seen, the pleural
+  folds on each side have nearly met the bulged-out ventral body-surface. A continuation of the same
+  process would give Fig. <a href="#fig160">160</a>, C, which is, to all intents and purposes, the
+  same as Fig. <a href="#fig159">159</a>, C, taken from van Wijhe, and shows how the segmental duct
+  is left in the remains of the atrial cavity. The lining walls of the atrial cavity are represented
+  very black, in order to indicate the presence of pigment, as indeed is seen in the corresponding
+  position in Ammoc&#x0153;tes. In these diagrams I have represented the median ventral surface as a
+  large bulged-out bag, without indicating any structures in it except the ventral extension of the
+  proc&#x0153;lom to form the metac&#x0153;lom. At present I will leave the space between the
+  central nervous system and the ventral mesentery blank, as in the diagrams; in my next chapter I
+  will discuss the possible method of formation within this blank space of the notochord and midgut.
+  Boveri considers that the obliteration of the atrial cavity in the higher vertebrates is not
+  complete, but that its presence is still visible in the shape of the pronephric duct. The evidence
+  of Maas and others that the duct is formed by the fusion of the pronephric tubules is, it seems to
+  me, conclusive against Boveri's view; but yet, as may be seen from my diagrammatic figures, the
+  very place where one would expect to find the last remnant of the atrial cavity is exactly where
+  the pronephric duct is situated. For my own part I should expect to find evidence of a former
+  existence of an atrial cavity rather in the pigment round the pronephros and its duct than in the
+  duct itself.</p>
+
+  <div><span class="pagenum" id="page413">{413}</span></div>
+
+  <div class="ac w50 fcenter sp2">
+    <a href="images/fig160.png" id="fig160"><img style="width:100%" src="images/fig160.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p><span class="sc">Fig. 160.&mdash;A, Diagram of Section through a Trilobite-like Animal; B,
+      Diagram to illustrate Suggested Obliteration of Appendages and the Formation of an Atrial
+      Cavity by the Ventral Extension of the Pleural Folds; C, Diagram to illustrate the Completion
+      of the Vertebrate Type by the Meeting of the Pleural Folds in the Mid-ventral Line and the
+      Obliteration of the Atrial Cavity.</span></p>
+      <p class="sp0"><i>Al.</i>, alimentary canal; <i>N.</i>, nervous system; <i>My.</i>, myotome;
+      <i>Pl.</i>, pleuron; <i>App.</i>, appendage; <i>Neph.</i>, nephroc&#x0153;le; <i>Met.</i>,
+      metac&#x0153;le; <i>S.d.</i>, segmental duct; <i>At.</i>, atrial chamber; <i>V.Mes.</i>,
+      ventral mesentery; <i>Mes.</i>, mesonephros. The dotted line represents the splanchnopleuric
+      mesoblast in all figures.</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page414">{414}</span></div>
+
+  <p>The conception that Amphioxus shows us how to account for the great envelope of somatic muscles
+  which wraps round the vertebrate body, in that the ancestor of the vertebrate possessed on each
+  side the body a segmented pleuron, is exactly in accordance with the theory of the origin of
+  vertebrates deduced from the study of Ammoc&#x0153;tes, as already set forth in previous chapters.
+  For we see that one of the striking characteristics of such forms as Bunodes, Hemiaspis, etc., is
+  the presence of segmented pleural flaps on each side of the main part of the body; and if we pass
+  further back to the great group of trilobites, we find in the most manifold form, and in various
+  degrees of extent, the most markedly segmented pleural folds. In fact, the hypothetical figure
+  (Fig. <a href="#fig160">160</a>, A) which I have deduced from the embryological evidence, might
+  very well represent a cross-section of a trilobite, provided only that each appendage of the
+  trilobite possessed an excretory coxal gland.</p>
+
+  <p>The earliest fishes, then, ought to have possessed segmented pleural folds, which were moved by
+  somatic muscles, and enveloped the body after the fashion of Ammoc&#x0153;tes and Amphioxus, and I
+  cannot help thinking that Cephalaspis shows, in this respect also, its relation to
+  Ammoc&#x0153;tes. It is well known that some of the fossil representatives of the Cephalaspids
+  show exceedingly clearly that these animals possessed a very well-segmented body, and it is
+  equally recognized that this skeleton is a calcareous, not a bony skeleton, and does not represent
+  vertebræ, etc. It is generally called an aponeurotic skeleton, meaning thereby that what is
+  preserved represents not dermal plates alone, or a vertebrate skeleton, but the calcified septa or
+  aponeuroses between a number of muscle-segments or myomeres, precisely of the same kind as the
+  septa between the myomeres in Ammoc&#x0153;tes. The termination of such septa on the surface would
+  give rise to the appearance of dermal plates or scutes, or the septa may even have been attached
+  to something of the nature of dermal plates. The same kind of picture would be represented if
+  these connective tissue dissepiments of Ammoc&#x0153;tes were calcified, and the animal then
+  fossilized. In agreement with this interpretation of the spinal skeleton of Cephalaspis, it may be
+  noted that again and again, in parts of these dissepiments, I have found in old specimens of
+  Ammoc&#x0153;tes nodules of cartilage formed, and at transformation it is in this very tissue that
+  the spinal cartilages are formed.</p>
+
+  <div><span class="pagenum" id="page415">{415}</span></div>
+
+  <div class="ac w45 fcenter sp2">
+    <a href="images/fig161.jpg" id="fig161"><img style="width:100%" src="images/fig161.jpg" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="sp0"><span class="sc">Fig. 161.</span>&mdash;<span class="sc">A, Facsimile of
+      Woodward's Drawing of a Specimen of</span> <i>Cephalaspis Murchisoni</i>, <span class="sc">as
+      seen from the side. The Cephalic Shield is on the Right and Caudal to it the Pleural Fringes
+      are well shown; B, Another Specimen of</span> <i>Cephalaspis Murchisoni</i> <span
+      class="sc">taken from the same block of Stone, showing the Dermoseptal Skeleton and in one
+      place the Pleural Fringes</span>, <i>bc.</i></p>
+    </div>
+  </div>
+
+  <p>Now, the specimens of Cephalaspis all show, as seen in Fig. <a href="#fig161">161</a>, that the
+  skeletal septa cover the body regularly, and then along one line are bent away from the body to
+  form, as it were, a fringe, or rather a free pleuron, which has been easily pushed at an angle to
+  the body-skeleton in the process of fossilization. Patten thinks that this fringed appearance is
+  evidence of a number of segmental appendages which were jointed to the corresponding
+  body-segments, and in the best specimen at the South Kensington Natural History Museum he thinks
+  such joints are clearly visible. He concludes, therefore, that the cephalaspids were arthropods,
+  and not vertebrates. I have also carefully examined this specimen, and do not consider that what
+  is seen resembles the joint of an arthropod appendage; the appearance is rather such as would be
+  produced if the line of attachment of Patten's appendages to the body were the place where the
+  pleural body folds became free from the body, and so with any pressure a <span class="pagenum"
+  id="page416">{416}</span>bending or fracture of the calcified plates would take place along this
+  line. There is, undoubtedly, an appearance of finish at the termination of these skeletal fringes,
+  as though they terminated in a definitely shaped spear-like point, just as is seen in the
+  trilobite pleuræ. This, again, to my mind, is rather evidence of pleural fringes than of true
+  appendages.</p>
+
+  <div class="ac w40 fcenter sp2">
+    <a href="images/fig162.png" id="fig162"><img style="width:100%" src="images/fig162.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p class="sp0"><span class="sc">Fig. 162.&mdash;A, Arrangement of Septa in
+      Ammoc&#x0153;tes</span> (<i>NC.</i>, position of notochord); <span class="sc">B, Arrangement
+      of Septa in Amphioxus</span>.</p>
+    </div>
+  </div>
+
+  <p>As already argued, I look upon Ammoc&#x0153;tes as the only living fish at all resembling the
+  cephalaspids; it is therefore instructive to compare the arrangement of this spinal dermo-septal
+  skeleton of Cephalaspis with that of the septa between the myomeres in the trunk-region of
+  Ammoc&#x0153;tes and Amphioxus. Such a skeleton in Ammoc&#x0153;tes would be represented by a
+  series of plates overlapping each other, arranged as in Fig. <a href="#fig162">162</a>, A, and in
+  Amphioxus as in Fig. <a href="#fig162">162</a>, B. I have lettered the corresponding parts of the
+  two structures by similar letters, <i>a</i>, <i>b</i>, <i>c</i>. Ammoc&#x0153;tes differs in
+  configuration from Amphioxus in that it possesses an extra dorsal (<i>a</i>, <i>d</i>) and an
+  extra ventral bend. Ammoc&#x0153;tes is a much rounder animal than Amphioxus, and both the dorsal
+  and ventral bends are on the extreme ventral and dorsal surfaces&mdash;surfaces which can hardly
+  be said to exist in Amphioxus. The part, then, of such an aponeurotic skeleton <span
+  class="pagenum" id="page417">{417}</span>in Ammoc&#x0153;tes which I imagine corresponds to
+  <i>b</i>, <i>c</i> in Amphioxus, and therefore would represent the pleural fold, is the part
+  ventral to the bend at <i>b</i>. In both the animals this bend corresponds to the position of the
+  notochord NC.</p>
+
+  <p>The skeleton of Cephalaspis compares more directly with that of Ammoc&#x0153;tes than that of
+  Amphioxus, for there is the same extra dorsal bend (Fig. <a href="#fig161">161</a>, <i>a</i>,
+  <i>d</i>) as in Ammoc&#x0153;tes; the lateral part of the skeleton again gives an angle <i>a</i>,
+  <i>b</i>, <i>c</i>; the part from <i>b</i> to <i>c</i> would therefore represent the pleural fold.
+  I picture to myself the sequence of events somewhat as follows<span
+  class="wnw">:&mdash;</span></p>
+
+  <p class="sp3">First, a protostracan ancestor, which, like Peripatus, possessed appendages on
+  every segment into which c&#x0153;lomic diverticula passed, forming a system of coxal glands; such
+  glands, being derived from the segmental organs of the Chætopoda, discharged originally to the
+  exterior by separate openings on each segment. It is, however, possible, and I think probable,
+  that a fusion of these separate ducts had already taken place in the protostracan stage, so that
+  there was only one external opening for the whole of these metasomatic coxal glands, just as there
+  is only one external opening for the corresponding prosomatic coxal glands of Limulus. Then, by
+  the ventral growth of pleural body-folds, such appendages became enclosed and useless, and the
+  coxal glands of the post-branchial segments, with their segmental or pronephric duct, were all
+  that remained as evidence of such appendages. This dwindling of the metasomatic appendages was
+  accompanied by the getting-rid of free appendages generally, in the manner already set forth, with
+  the result that a smooth fish-like body-surface was formed; then the necessity of increasing
+  mobility brought about elongation by the addition of segments between those last formed and the
+  cloacal region. Each of such new-formed segments was appendageless, so that its segmental organ
+  was not a coxal gland, but entirely somatic in position, and formed, therefore, a mesonephric
+  tubule, not a pronephric one. Such glands could no longer excrete to the exterior, owing to the
+  enclosing shell of the pleural folds; but the pronephric duct was there, already formed, and so
+  these nephric tubules opened into that, instead of, as in the case of the branchial slits, forcing
+  their way through the pleural walls when the atrium became closed.</p>
+
+  <div><span class="pagenum" id="page418">{418}</span></div>
+
+  <p class="ac"><span class="sc">The Meaning of the Ductless Glands.</span></p>
+
+  <p>If it is a right conception that the excretory organs of the protostracan group, which gave
+  origin to the vertebrates as well as to the crustaceans and arachnids, were of the nature of coxal
+  glands, then it follows that such coxal glands must have existed originally on every segment,
+  because they themselves were derived from the segmental organs of the annelids; it is therefore
+  worth while making an attempt to trace the fate of such segmental organs in the vertebrate as well
+  as in the crustacean and arachnid.</p>
+
+  <p>Such an attempt is possible, it seems to me, because there exists throughout the animal kingdom
+  striking evidence that excretory organs which no longer excrete to the exterior do not disappear,
+  but still perform excretory functions of a different character. Their cells still take up effete
+  or injurious substances, and instead of excreting to the exterior, excrete into the blood, forming
+  either ductless glands of special character, or glands of the nature of lymphatic glands.</p>
+
+  <p>The problem presented to us is as follows:&mdash;</p>
+
+  <p>The excretory organs of both arthropods and vertebrates arose from those of annelids, and were
+  therefore originally present in every segment of the body. In most arthropods and vertebrates they
+  are present only in certain regions; in the former case, as the coxal glands of the prosomatic or
+  head-region; in the latter, as the nephric glands of the metasomatic or trunk-region, and, in the
+  case of Amphioxus, of the mesosomatic or branchial region.</p>
+
+  <p>In the original arthropod, judging from Peripatus, they were present, as in the annelid, in all
+  the segments of the body, and formed coxal glands. Therefore, in the ancestors of the living
+  Crustacea and Arachnida, coxal glands must have existed in all the segments of the body, and we
+  ought to be able to find the vestiges of them in the mesosomatic or branchial and metasomatic or
+  abdominal regions of the body.</p>
+
+  <p>Similarly, in the vertebrates, derived, as has been shown, not from the annelids, but from an
+  arthropod stock, evidence of the previous existence of coxal glands ought to be manifested in the
+  prosomatic or trigeminal region, in the mesosomatic or branchial region, as well as in the
+  metasomatic or post-branchial region.</p>
+
+  <p>How does an excretory organ change its character when it ceases <span class="pagenum"
+  id="page419">{419}</span>to excrete to the exterior? What should we look for in our search after
+  the lost coxal glands?</p>
+
+  <p>The answer to these questions is most plainly given in the case of the pronephros, especially
+  in Myxine, where Maas has been able to follow out the whole process of the conversion of nephric
+  tubules into a tissue resembling that of a lymph-gland.</p>
+
+  <p>He states, in the first place, that the pronephros possesses a capillary network, which extends
+  over the pronephric duct, while the tubules of the mesonephros possess not only this capillary
+  network, equivalent to the capillaries over the convoluted tubules in the higher vertebrates, but
+  also a true glomerulus, in that the nephric segmental arteriole forms a coil (Knauel), and pushes
+  in the wall of the mesonephric tubule. He describes the pronephros of large adult individuals as
+  consisting of&mdash;</p>
+
+  <p>1.  Tubules with funnels which open into the pericardial c&#x0153;lom.</p>
+
+  <p>2.  A large capillary network (the glomus) at the distal end.</p>
+
+  <p>3.  A peculiar tissue (the 'strittige Gewebe' of the Semon-Spengel controversy), which Spengel
+  considers to be composed of the altered epithelium of pronephric tubules, while Semon looks on it
+  as an amalgamation of glomeruli.</p>
+
+  <p>Maas is entirely on the side of Spengel, and shows that this peculiar tissue is actually formed
+  by modified pronephric tubules, which become more and more lymphatic in character.</p>
+
+  <p>He says: "The pronephros consists of a number of nephric tubules, placed separately one behind
+  the other, which were originally segmental in character, each one of which is supplied by a
+  capillary network from a segmental branch of the aorta. The tubules begin with many mouths
+  (dorso-lateral and medial-ventral) in the pericardial cavity; on their other blind end they have
+  lost their original external opening, and there, in the cranial portion of the head-kidney, before
+  they have joined together to form a collecting duct, they, together with the vascular network, are
+  transformed into a peculiar adrenal-like tissue. The most posterior of the segmental capillary
+  nets retain their original character, and are concentrated into the separate capillary mass known
+  as the glomus."</p>
+
+  <p>Later on he says: "Further, the separate head-kidney is more and more removed in structure from
+  an excretory organ in the ordinary sense. One cannot, however, speak of it as an organ becoming
+  rudimentary; this is proved not only by the progressive transformation <span class="pagenum"
+  id="page420">{420}</span>of its internal tissue into a tissue of a very definite character, but
+  also by the cilia in its canals, and the steady increase in the number of its funnels. It appears,
+  therefore, to be the conversion of an excretory organ into an organ for the transference of fluid
+  out of the c&#x0153;lom into a special tissue, <i>i.e.</i> into its blood-sinus; in other words,
+  into an organ which must be classed as belonging to the lymph-system."</p>
+
+  <p>In exact correspondence with this transformation of a nephric tubule into a ductless gland of
+  the nature of a lymphatic gland, is the formation of the head-kidney in the Teleostea. Thus,
+  Weldon points out that, though the observations of Balfour left it highly probable that the
+  "lymphatic" tissue described by him was really a result of the transformation of part of the
+  embryonic kidney, he did not investigate the details of its development. This was afterwards done
+  by Emery, with the following results: "In those Teleostea which he has studied, Professor Emery
+  finds that at an early stage the kidney consists entirely of a single pronephric funnel, opening
+  into the pericardium, and connected with the segmental duct, which already opens to the exterior.
+  Behind this funnel, the segmental duct is surrounded by a blastema, derived from the intermediate
+  cell-mass, which afterwards arranges itself more or less completely into a series of solid cords,
+  attaching themselves to the duct. These develop a lumen, and become normal segmental tubules, but
+  it is, if I may be allowed the expression, a matter of chance how much of the blastema becomes so
+  transformed into kidney tubules, and how much is left as the 'lymphatic' tissue of Balfour, this
+  'lymphatic' tissue remaining either in the pronephros only, or in both pro- and <span
+  class="correction" title="Original reads 'neso-'.">meso</span>-nephros."</p>
+
+  <p>If we turn now to the invertebrates, we see also how close a connection exists between
+  lymphatic and phagocytic organs and excretory organs. The chief merit for this discovery is due to
+  Kowalewsky, who, taking a hint from Heidenhain's work on the kidney, in which he showed how easy
+  it was to find out the nature of different parts of the mammalian excretory organ by the injection
+  of different substances, such as a solution of ammoniated carmine, or of indigo-carmine, has
+  injected into a large number of different invertebrates various colouring matters, or litmus, or
+  bacilli, and thus shown the existence, not only of known excretory organs, but also of others,
+  lymphatic or lymphoid in nature, not hitherto suspected.</p>
+
+  <p>In all cases he finds that a phagocytic action with respect to solid <span class="pagenum"
+  id="page421">{421}</span>bodies is a property of the leucocytes, and that these leucocytes which
+  are found in the c&#x0153;lomic spaces of the Annelida, etc., are apparently derived from the
+  epithelium of such spaces. Also by the proliferation of such epithelium in places, <i>e.g.</i> the
+  septal glands of the terrestrial Oligochæta, segmental glandular masses of such tissue are formed
+  which take up the colouring matter, or the bacilli. In the limicolous Oligochæta such septal
+  glands are not found, but at the commencement of the nephridial organ, immediately following upon
+  the funnel, a remarkable modification of the nephridial wall takes place to form a large cellular
+  cavernous mass, the so-called filter, which in Euaxes is full of leucocytes; the cells are only
+  definable by their nuclei, and look like and act in the same way as the free leucocytes outside
+  this nephridial appendage. As G. Schneider points out, the whole arrangement is very like that
+  described by Kowalewsky in the leeches Clepsine and Nephelis, where, also immediately succeeding
+  the funnel of the nephridial organ, a large accessory organ is found, which is part of the
+  nephridium, and is called the nephridial capsule. This is the organ <i>par excellence</i> which
+  takes up the solid carmine-grains and bacilli, and apparently, from Kowalewsky's description,
+  contains leucocytes in large numbers. We see, then, that in such invertebrates, just as in the
+  vertebrate, modifications of the true excretory organ may give rise to phagocytic glands of the
+  nature of lymphatic glands. Further, these researches of Kowalewsky suggest in the very strongest
+  manner that whenever by such means new, hitherto unsuspected glands are discovered, such glands
+  must belong to the excretory system, <i>i.e.</i> must be derived from c&#x0153;lomic epithelium,
+  even when all evidence of any c&#x0153;lom has disappeared. Kowalewsky himself was evidently so
+  impressed with the same feeling that he heads one of his papers "The Excretory Organs of the
+  Pantopoda," although the organs in question had been discovered by him by this method, and
+  appeared as ductless glands with no external opening.</p>
+
+  <p>To my mind these observations of Kowalewsky are of exceeding interest, for it is immediately
+  clear that if the segmental organs of the annelids, which must have existed on all the segments of
+  the forefathers of the Crustacea and Arachnida (the Protostraca), have left any sign of their
+  existence in living crustaceans and arachnids, then such indication would most likely take the
+  form of lymphatic glands in the places where the excretory organs ought to have been.</p>
+
+  <p>Now, as already pointed out in Peripatus, such segmental organs <span class="pagenum"
+  id="page422">{422}</span>were formed by the ventral part of the c&#x0153;lom, and dipped
+  originally into each appendage. We know also that each segment of an arachnid embryo possesses a
+  c&#x0153;lomic cavity in its ventral part which extends into the appendage on each side; this
+  cavity afterwards disappears, and is said to leave no trace in the adult of any excretory coxal
+  gland derived from its walls. If, however, it is found that in the very position where such organ
+  ought to have been formed a segmentally arranged ductless gland is situated, the existence of
+  which is shown by its taking up carmine, etc., then it seems to me that in all probability such
+  gland is the modification of the original coxal gland.</p>
+
+  <p>This is what Kowalewsky has done. Thus he states that Metschnikoff had fed Mysis with
+  carmine-grains, and found tubules at the base of the thoracic feet coloured red with carmine. He
+  himself used an allied species, <i>Parapodopsis cornutum</i>, and found here also that the carmine
+  was taken up by tubules situated in the basal segments of the feet. In Nebalia, feeding
+  experiments with alizarin blue and carmine stained the antennal glands, and showed the existence
+  of glands at the base of the eight thoracic feet. These glands resemble the foot-glands of Mysis,
+  Parapodopsis, and Palæmon, and lie in the space through which the blood passes from the thoracic
+  feet, <i>i.e.</i> from the gills, to the heart. In Squilla also, in addition to the shell-glands,
+  special glands were discovered on the branchial feet on the path of the blood to the heart. These
+  glands form continuous masses of cells which constitute large compact glands at the base of the
+  branchial feet. Single cells of the same sort are found along the whole course of the branchial
+  venous canal, right up to the pericardium.</p>
+
+  <p>These observations show that the Crustacea possess not only true excretory organs in the shape
+  of coxal glands, <i>i.e.</i> antennary glands, shell-glands, etc., in the cephalic region, but
+  also a series of segmental glands situated at the base of the appendages, especially of the
+  respiratory appendages: a system, that is to say, of coxal glands which have lost their excretory
+  function, through having lost their external opening, but have not in consequence disappeared, but
+  still remain <i>in situ</i>, and still retain an important excretory function, having become
+  lymphatic glands containing leucocytes. Such glands are especially found in the branchial
+  appendages, and are called branchial glands by Cuénot, who describes them for all Decapoda.</p>
+
+  <p>Further, it is significant that the same method reveals the <span class="pagenum"
+  id="page423">{423}</span>existence in Pantopoda of a double set of glands of similar character,
+  one set in the basal segments of the appendage, and the other in the adjacent part of the
+  body.</p>
+
+  <p>In scorpions also, Kowalewsky has shown that the remarkable lymphatic organ situated along the
+  whole length of the nerve-cord in the abdominal region takes up carmine grains and bacilli; an
+  organ which in Androctonus does not form one continuous gland, but a number of separate,
+  apparently irregularly grouped, glandular bodies.</p>
+
+  <p>In addition to this median lymphatic gland, Kowalewsky has discovered in the scorpion a pair of
+  lateral glands, to which he gives the name of lymphoid glands, which communicate with the thoracic
+  body-cavity (<i>i.e.</i> the pseudoc&#x0153;le), are phagocytic, and, according to him, give
+  origin to leucocytes by the proliferation of their lining cells, thus, as he remarks, reminding us
+  of the nephridial capsules of Clepsine. These glands are so closely related in position to the
+  coxal glands on each side that he has often thought that the lumen of the gland communicated with
+  that of the coxal gland; he, however, has persuaded himself that there is no true communication
+  between the two glands. Neither of these organs appears to be segmental, and until we know how
+  they are developed it is not possible to say whether they represent fused segmental organs or
+  not.</p>
+
+  <p>The evidence, then, is very strong that in the Crustacea and Arachnida the original segmental
+  excretory organs do not disappear, but remain as ductless glands, of the nature of lymphatic
+  glands, which supply leucocytes to the system.</p>
+
+  <p>Further, the evidence shows that the nephric organs, or parts of the c&#x0153;lom in close
+  connection with these organs, may be transformed into ductless glands, which do not necessarily
+  contain free leucocytes as do lymph-glands, but yet are of such great importance as excretory
+  organs that their removal profoundly modifies the condition of the animal. Such a gland is the
+  so-called adrenal or suprarenal body, disease of which is a feature of Addison's disease; a gland
+  which forms and presumably passes into the blood a substance of remarkable power in causing
+  contraction of blood-vessels, a substance which has lately been prepared in crystalline form by
+  Jokichi Takamine, and called by him "adrenalin"; a gland, therefore, of very distinctly peculiar
+  properties, which cannot be regarded as rudimentary, but is of vital importance for the due
+  maintenance of the healthy state.</p>
+
+  <p>In the Elasmobranchs two separate glandular organs have been <span class="pagenum"
+  id="page424">{424}</span>called suprarenal; a segmental series of paired organs, each of which
+  possesses a branch from the aorta and a sympathetic ganglion, and an unpaired series in close
+  connection with the kidneys, to which Balfour gave the name of interrenal glands. Of these two
+  sets of glands, Swale Vincent has shown that the extract of the interrenals has no marked
+  physiological effect, in this respect resembling the extract of the cortical part of the mammalian
+  gland, while the extract of the paired segmental organs of the Elasmobranch produces the same
+  remarkable rise of blood-pressure as the extract of the medullary portion of the mammalian
+  gland.</p>
+
+  <p>The development also of these two sets of glands is asserted to be different. Balfour
+  considered that the suprarenals were derived from sympathetic ganglion-cells, but left the origin
+  of the interrenals doubtful. Weldon showed that the cortical part of the suprarenals in the lizard
+  was derived from the wall of the glomerulus of a number of mesonephric tubules. In Pristiurus, he
+  stated that the mesoblastic rudiment described by Balfour as giving origin to the interrenals is
+  derived from a diverticulum of each segmental tubule, close to the narrowing of its funnel-shaped
+  opening into the body-cavity. With respect to the paired suprarenals he was unable to speak
+  positively, but doubted whether they were derived entirely from sympathetic ganglia.</p>
+
+  <p>Weldon sums up the results of his observations by saying: "That all vertebrates except
+  Amphioxus have a portion of the kidney modified for some unknown purpose not connected with
+  excretion; that in Cyclostomes the pronephros alone is so modified, in Teleostei the pro- and part
+  of the meso-nephros; while in the Elasmobranchs and the higher vertebrates the mesonephros alone
+  gives rise to this organ, which has also in these forms acquired a secondary connection with
+  certain of the sympathetic ganglia."</p>
+
+  <p>Since Weldon's paper, a large amount of literature on the origin of the adrenals has appeared,
+  a summary of which, up to 1891, is given by Hans Rabl in his paper, and a further summary by
+  Aichel in his paper published in 1900. The result of the investigations up to this latter paper
+  may be summed up by saying that the adrenals, using this term to include all these organs of
+  whatever kind, are in all cases, partly at all events, derived from some part of the walls of
+  either the mesonephric or pronephric excretory organs, but that in addition a separate origin from
+  the sympathetic nervous system must <span class="pagenum" id="page425">{425}</span>be ascribed to
+  the medullary part of the organ and to the separate paired organs in the Elasmobranchs, which are
+  equivalent to the medullary part in other cases.</p>
+
+  <p>The evidence, then, of the transformation of the known vertebrate excretory organs&mdash;the
+  pronephros and the mesonephros&mdash;leads to the conclusion that in our search for the missing
+  coxal glands of the meso- and pro-somatic regions, we must look for either lymphatic glands, or
+  ductless glands of distinct importance to the body. I have already considered the question in the
+  prosomatic region, and have given my reasons why the pituitary gland must be looked upon as the
+  descendant of the arthropod coxal gland. In this case also the resulting ductless gland is still
+  of functional importance, for disease of it is associated with acromegaly. If, as is possible, it
+  is homologous with the Ascidian hypophysial gland, then it is confirmatory evidence that this
+  latter is said by Julin to be an altered nephridial organ.</p>
+
+  <p>Finally, I come to the mesosomatic or branchial region; and here, strikingly enough, we find a
+  perfectly segmental glandular organ of mysterious origin&mdash;the thymus gland&mdash;segmental
+  with the branchiæ, not necessarily with the myotomes, belonging, therefore, to the appendicular
+  system; and since the branchiæ represent, according to my theory, the basal part of the appendage,
+  such segmental glands would be in the position of coxal glands. Here, then, in the thymus may be
+  the missing mesosomatic coxal glands.</p>
+
+  <p>What, then, is the thymus?</p>
+
+  <p>The answer to this question has been given recently by Beard, who strongly confirms Kölliker's
+  original view that the thymus is a gland for the manufacture of leucocytes, and that such
+  leucocytes are directly derived from the epithelial cells of the thymus. Kölliker also further
+  pointed out that the blood of the embryo is for a certain period destitute of leucocytes. Beard
+  confirms this last statement, and says that up to a certain stage (varying from 10 to 16 mm. in
+  length of the embryo) the embryos of <i>Raja batis</i> have no leucocytes in the blood or
+  elsewhere. Up to this period the thymus-placode is well formed, and the first leucocytes can be
+  seen to be formed in it from its epithelial cells; then such formation takes place with great
+  rapidity, and soon an enormous discharge of leucocytes occurs from the thymus into the
+  tissue-spaces and blood. He therefore concludes that all lymphoid tissues in the body arise
+  originally from the thymus gland, <i>i.e.</i> from leucocytes discharged from the thymus.</p>
+
+  <div><span class="pagenum" id="page426">{426}</span></div>
+
+  <p>The segmental branchial glands, known by the name of thymus, are, according to this view, the
+  original lymphatic glands of the vertebrate; and it is to be noted that, in fishes and in
+  Amphibia, lymphatic glands, such as we know them in the higher mammals, do not exist; they are
+  characteristic of the higher stages of vertebrate evolution. In the lower vertebrates, the only
+  glandular masses apart from the cell-lining of the body-cavity itself, which give rise to
+  leucocyte-forming tissue, are these segmental branchial glands, or possibly also the modified
+  post-branchial segmental glands, known as the head-kidney in Teleostea, etc.</p>
+
+  <p>The importance ascribed by Beard to the thymus in the formation of leucocytes in the lowest
+  vertebrates would be considerably reduced in value if the branchial region of Ammoc&#x0153;tes
+  possessed neither thymus glands nor anything equivalent to them. Such, however, is not the case.
+  Schaffer has shown that in the young Ammoc&#x0153;tes masses of lymphatic glandular tissue are
+  found segmentally arranged in the neighbourhood of each gill-slit&mdash;tissue which soon becomes
+  converted into a swarming mass of leucocytes, and shows by its staining, etc., how different it is
+  from a blood-space. The presence of this thymus leucocyte-forming tissue, as described by
+  Schaffer, is confirmed by Beard, and I myself have seen the same thing in my youngest specimen of
+  Ammoc&#x0153;tes.</p>
+
+  <p>Further, the very methods by which Kowalewsky has brought to light the segmental lymph-glands
+  of the branchial region of the Crustacea, etc., are the same as those by which Weiss discovered
+  the branchial nephric glands in Amphioxus&mdash;excretory organs which Boveri considers to
+  represent the pronephros of the Craniota. In this supposition Boveri is right, in so far that both
+  pronephros and the tubules in Amphioxus belong to the same system of excretory organs; but I
+  entirely agree with van Wijhe that the region in Amphioxus is wrong. The tubules in Amphioxus
+  ought to be represented in the branchial region of the Craniota, not in the post-branchial region;
+  van Wijhe therefore suggests that further researches may homologize them with the thymus gland in
+  the Craniota, not with the pronephros. This suggestion of van Wijhe appears to me a remarkably
+  good one, especially in view of the position of the thymus glands in Ammoc&#x0153;tes and the
+  nephric branchial glands in Amphioxus. If, as I have pointed out, the atrial cavity of Amphioxus
+  has been closed in Ammoc&#x0153;tes by the apposition of <span class="pagenum"
+  id="page427">{427}</span>the pleural fold with the branchial body-surface, then the remains of the
+  position of the atrial chamber must exist in Ammoc&#x0153;tes as that extraordinary space between
+  the somatic muscles and the branchial basket-work filled with blood-spaces and modified
+  muco-cartilage. It is in this very space, close against the gill-slits, that the thymus glands of
+  Ammoc&#x0153;tes are found, in the very place where the nephric tubules of Amphioxus would be
+  found if its atrial cavity were closed completely. Instead, therefore, of considering with Boveri
+  that the branchial nephric tubules of Amphioxus still exist in the Craniota as the pronephros, and
+  that the atrial chamber has narrowed down to the pronephric duct, I would agree with van Wijhe
+  that the pronephros is post-branchial, and suggest that by the complete closure of the atrial
+  space in the branchial region the branchial nephric tubules have lost all external opening, and
+  consequently, as in all other cases, have changed into lymphatic tissue and become the segmental
+  thymus glands.</p>
+
+  <p>As van Wijhe himself remarks, the time is hardly ripe for making any positive statement about
+  the relationship between the thymus gland and branchial excretory organs. There is at present not
+  sufficient consensus of opinion to enable us to speak with any certainty on the subject, yet there
+  is so much suggestiveness in the various statements of different authors as to make it worth while
+  to consider the question briefly.</p>
+
+  <p>On the one hand, thymus, tonsils, parathyroids, epithelial cell-nests, and parathymus, are all
+  stated to be derivatives of the epithelium lining the gill-slits, and Maurer would draw a
+  distinction between the organs derived from the dorsal side of the gill-cleft and those derived
+  from the ventral side&mdash;the former being thymus, the latter forming the epithelial cell-nests,
+  <i>i.e.</i> parathyroids. The thymus in Ammoc&#x0153;tes, according to Schaffer, lies both ventral
+  and dorsal to the gill-cleft; Maurer thinks that only the dorsal part corresponds to the thymus,
+  the ventral part corresponding to the parathyroids, etc. Structurally, the thymus, parathyroids,
+  and the epithelial cell-nests are remarkably similar, so that the evidence appears to point to the
+  conclusion that, in the neighbourhood of the gill-slits, segmentally arranged organs of a
+  lymphatic character are situated, which give origin to the thymus, parathyroids, tonsils, etc.
+  Now, among these organs, <i>i.e.</i> among those ventrally situated, Maurer places the carotid
+  gland, so that, if he is right, the origin of the carotid gland <span class="pagenum"
+  id="page428">{428}</span>might be expected to help in the elucidation of the origin of the
+  thymus.</p>
+
+  <p>The origin of the carotid gland has been investigated recently by Kohn, who finds that it is
+  associated with the sympathetic nervous system in the same way as the suprarenals. He desires, in
+  fact, to make a separate category for such nerve-glands, or paraganglia, as he calls them, and
+  considers them all to be derivatives of the sympathetic nervous system, and to have nothing to do
+  with excretory organs. The carotid gland is, according to him, the foremost of the suprarenal
+  masses in the Elasmobranchs, viz. the so-called axillary heart.</p>
+
+  <p>In my opinion, nests of sympathetic ganglion-cells necessarily mean the supply of efferent
+  fibres to some organ, for all such ganglia are efferent, and also, if they are found in the organ,
+  would have been brought into it by way of the blood-vessels supplying the organ, so that Aichel's
+  statement of the origin of the suprarenals in the Elasmobranchs seems to me much more probable
+  than a derivation from nerve-cells. If, then, it prove that Aichel is right as to the origin of
+  the suprarenals, and Kohn is right in classifying the carotid gland with the suprarenals, then
+  Maurer's statements would bring the parathyroids, thymus, etc., into line with the adrenals, and
+  suggest that they represent the segmented glandular excretory organs of the branchial region, into
+  which, just as in the interrenals of Elasmobranchs, or the cortical part of the adrenals of the
+  higher vertebrates, there has been no invasion of sympathetic ganglion-cells.</p>
+
+  <p>Wheeler makes a most suggestive remark in his paper on Petromyzon: he thinks he has obtained
+  evidence of serial homologues of the pronephric tubules in the branchial region of
+  Ammoc&#x0153;tes, but has not been able up to the present to follow them out. If what he thinks to
+  be serial homologues of the pronephric tubules in the branchial region should prove to be the
+  origin of the thymus glands of Schaffer, then van Wijhe's suggestion that the thymus represents
+  the excretory organs of the branchial region would gain enormously in probability. Until some such
+  further investigation has been undertaken, I can only say that it seems to me most likely that the
+  thymus, etc., represent the lymphatic branchial glands of the Crustacea, and therefore represent
+  the missing coxal glands of the branchial region.</p>
+
+  <p>This, however, is not all, for the appendages of the mesosomatic region, as I have shown, do
+  not all bear branchiæ; the foremost or <span class="pagenum" id="page429">{429}</span>opercular
+  appendage carries the thyroid gland. Again, the basal part of the appendage is all that is left;
+  the thyroid gland is in position a coxal gland. It ought, therefore, to represent the coxal gland
+  of this appendage, just as the thymus, tonsils, etc., represent the coxal glands of the rest of
+  the mesosomatic appendages. In the thyroid gland we again see a ductless gland of immense
+  importance to the economy, not a useless organ, but one, like the other modified coxal glands,
+  whose removal involves far-reaching vital consequences. Such a gland, on my theory, was in the
+  arthropod a part of the external genital ducts which opened on the basal joint of the operculum.
+  What, then, is the opinion of morphologists as to the meaning of these external genital ducts?</p>
+
+  <p>In a note to Gulland's paper on the coxal glands of Limulus, Lankester states that the
+  conversion of an externally-opening tubular gland (coxal gland) into a ductless gland is the same
+  kind of thing as the history of the development of the suprarenal from a modified portion of
+  mesonephros, as given by Weldon. Further, that in other arthropods with glands of a tubular
+  character opening to the exterior at the base of the appendages, we also have coxal nephridia,
+  such as the shell-glands of the Entomostraca, green glands of Crustacea (antennary coxal gland);
+  and further on he writes: "When once the notion is admitted that ducts opening at the base of
+  limbs in the Arthropoda are possibly and even probably modified nephridia, we immediately conceive
+  the hypothesis that the genital ducts of the Arthropoda are modified nephridia."</p>
+
+  <p>So, also, Korschelt and Heider, in their general summing up on the Arthropoda, say: "In
+  Peripatus, where the nephridia appear, as in the Annelida, in all the trunk-segments, a
+  considerable portion of the primitive segments is directly utilized for the formation of the
+  nephridia. In the other groups, the whole question of the rise of the organs known as nephridia is
+  still undecided, but it may be mentioned as very probable that the salivary and anal glands of
+  Peripatus, the antennal and shell-glands of the Crustacea, the coxal glands of Limulus and the
+  Arachnida, as well as the efferent genital ducts, are derived from nephridia, and in any case are
+  mesodermal in origin."</p>
+
+  <p>The necessary corollary to this exceedingly probable argument is that glandular structures such
+  as the uterine glands of the scorpion already described, which are found in connection with these
+  terminal <span class="pagenum" id="page430">{430}</span>genital ducts, may be classed as modified
+  nephridial glands, and that therefore the thyroid gland of Ammoc&#x0153;tes, which, on the theory
+  of this book, arose in connection with the opercular genital ducts of the palæostracan ancestor,
+  represents the coxal glands of this fused pair of appendages. Such a gland, although its function
+  in connection with the genital organs had long disappeared, still, in virtue of its original
+  excretory function, persisted, and even in the higher vertebrates, after it had lost all semblance
+  of its former structure and become a ductless gland of an apparently rudimentary nature, still, by
+  its excretory function, demonstrates its vital importance even to the highest vertebrate.</p>
+
+  <p>By this simple explanation we see how these hitherto mysterious ductless glands, pituitary,
+  thymus, tonsils, thyroid, are all accounted for, are all members of a common stock&mdash;coxal
+  glands&mdash;which originally, as in Peripatus, excreted at the base of the prosomatic and
+  mesosomatic appendages, and are still retained because of the importance of their excretory
+  function, although ductless owing to the modification of their original appendages.</p>
+
+  <p>Finally, there is yet another organ in the vertebrate which follows the same law of the
+  conversion of an excretory organ into a lymphatic organ when its connection with the exterior is
+  obliterated, and that is the vertebrate body-cavity itself. According to the scheme here put
+  forth, the body-cavity of the vertebrate arose by the fusion of a ventral prolongation of the
+  original nephroc&#x0153;le on each side; prolongations which accompanied the formation of the new
+  ventral midgut, and by their fusion formed originally a pair of cavities along the whole length of
+  the abdomen, being separated from each other by the ventral mesentery of the gut. Subsequently, by
+  the ventral fusion of these two cavities, the body-cavity of the adult vertebrate was formed.</p>
+
+  <p>This is simply a statement of the known method of formation of the body-cavity in the embryo,
+  and its phylogenetic explanation is that the body-cavity of the vertebrate must be looked upon as
+  a ventral prolongation of the original ancestral body-cavity. Embryology clearly teaches that the
+  original body-cavity or somite was confined to the region of the notochord and central nervous
+  system, and there, just as in Peripatus, was divisible into a dorsal part, giving origin to the
+  myoc&#x0153;le, and a ventral part, forming the nephroc&#x0153;le. From this original
+  nephroc&#x0153;le are formed the pronephric excretory organs, the mesonephric excretory organs,
+  and the body-cavity.</p>
+
+  <div><span class="pagenum" id="page431">{431}</span></div>
+
+  <p class="sp3">That the vertebrate body-cavity was originally a nephroc&#x0153;le is generally
+  accepted, and its excretory function is shown by the fact that it communicates with the exterior
+  in all the lower vertebrates, either through abdominal pores or by way of nephridial funnels. Bles
+  has shown how largely these two methods of communicating with the exterior mutually exclude each
+  other. In the higher vertebrates both channels become closed, except in the case of the Fallopian
+  tubes, and thus, so to speak, the body-cavity becomes a ductless gland, still, however, with an
+  excretory function, but now, as in all other cases, forming a part of the lymphatic rather than of
+  the true excretory system.</p>
+
+  <p class="ac"><span class="sc">Summary.</span></p>
+
+  <div class="bq1 smaller sp5">
+    <p>The consideration of the formation of the vertebrate cranial region, as set forth in previous
+    chapters, indicates that the ancestor of the vertebrates was not an arachnid purely or a
+    crustacean purely, but possessed partly crustacean and partly arachnid characters. In order to
+    express this conclusion, I have used the term Protostraca, invented by Korschelt and Heider, to
+    indicate a primitive arthropod group, from which both arachnids and crustaceans may be supposed
+    to have arisen, and have therefore stated that the vertebrate did not arise directly from the
+    annelids, but from the Protostraca. Such an origin signifies that the origin of the excretory
+    organs of the vertebrate must not be looked for in the segmental organs of the annelid, but
+    rather in such modified annelid organs as would naturally exist in a primitive arthropod group.
+    The nature of such organs may be inferred, owing to the fortunate circumstance that so primitive
+    an arthropod as Peripatus still exists, and we may conclude that the protostracan ancestor
+    possessed in every segment a pair of appendages and a pair of c&#x0153;lomic cavities, which
+    extended into the base of these appendages. The ventral portion of each of these c&#x0153;lomic
+    cavities separated off from the dorsal and formed a nephroc&#x0153;le, giving origin to a
+    segmental excretory organ, which, seeing that its end-vesicle was in the base of the appendage,
+    and seeing also the nature of the known arachnid and crustacean excretory organs, may fitly be
+    termed a coxal gland. This, then, is the working hypothesis to explain the difficulties
+    connected with the origin of the pronephros and mesonephros&mdash;that the original segmental
+    organs were coxal glands, and therefore indicated the presence of appendages. This hypothesis
+    leads to the following conclusions<span class="wnw">:&mdash;</span></p>
+    <p>1. The coxal glands belonging to the post-branchial appendages of the invertebrate ancestor
+    are represented by the pronephric tubules, and existed over the whole metasomatic region.</p>
+    <p>2. Such glands discharged into a common duct&mdash;the pronephric duct&mdash;which opened
+    into the cloacal region, either in the protostracan stage, when the metasomatic appendages were
+    still in existence, just as the coxal glands of the prosomatic region in Limulus discharge into
+    a common duct, or else the pronephric duct was formed when the appendages were obliterated.</p>
+    <div><span class="pagenum" id="page432">{432}</span></div>
+    <p>3. The metasomatic appendages disappeared owing to their enclosure by pleural folds, which,
+    meeting in the mid-ventral line, not only caused the obliteration of the appendages, and gave a
+    smooth fish-like body-surface to the animal, but also caused the formation of an atrial
+    cavity.</p>
+    <p>4. Into these pleural folds the dorsal longitudinal muscles of the body extended, and
+    ultimately reached to the ventral surface, thus forming the somatic muscles of the vertebrate
+    body.</p>
+    <p>5. When the pleural folds had met in the mid-ventral line the animal had <span
+    class="correction" title="Original reads 'became'">become</span> a vertebrate, and was dependent
+    for its locomotion on the movements of these somatic muscles, and not on the movements of
+    appendages. Consequently, elongation of the trunk-region took place, for the purpose of
+    increasing mobility, by the formation of new metameres.</p>
+    <p>6. Each of such metameres possessed its own segmental excretory organ, formed in the same way
+    as the previous pronephric organs, but, as there were no appendages in these new-formed
+    segments, the excretory organs took on the characters of a mesonephros, not a pronephros, and
+    opened into the pronephric duct, because the direct way to the exterior was blocked by the
+    enveloping pleural folds.</p>
+    <p>7. The group of annelids from which the protostracan ancestor of the vertebrates arose was
+    the highest annelidan group, viz. the Polychæta, as shown by the nature of the excretory organs
+    in Amphioxus.</p>
+    <p>8. The coxal glands of the protostracan ancestor existed on all the segments, and were,
+    therefore, divisible into three groups, prosomatic, mesosomatic, and metasomatic; these three
+    groups of coxal glands still exist in the vertebrate as ductless glands.</p>
+    <p>9. The prosomatic coxal glands form the pituitary body.</p>
+    <p>10. The mesosomatic coxal glands form the thymus, thyroid, parathyroids, tonsils, etc.</p>
+    <p>11. The metasomatic coxal glands form the adrenals.</p>
+    <p>12. The proc&#x0153;lom of the vertebrate is the proc&#x0153;lom of the protostracan
+    ancestor, which splits into a dorsal part, the myoc&#x0153;le, and a ventral part, the
+    nephroc&#x0153;le. This latter part not only forms the pronephros and mesonephros, but also by a
+    ventral extension gives origin to the walls of the vertebrate body-cavity or
+    metac&#x0153;le.</p>
+    <p class="sp0">13. This ventral extension of the original nephroc&#x0153;le at first excreted to
+    the exterior, through abdominal pores, or through peritoneal funnels. When such paths to the
+    exterior became closed, it also became a ductless gland, belonging to the lymphatic system.</p>
+  </div>
+
+  <div><span class="pagenum" id="page433">{433}</span></div>
+
+  <p class="ac">CHAPTER XIII</p>
+
+  <p class="ac"><i>THE NOTOCHORD AND ALIMENTARY CANAL</i></p>
+
+  <div class="bq1 it1 smaller sp3">
+    <p class="sp0">Relationship between notochord and gut.&mdash;Position of unsegmented tube of
+    notochord.&mdash;Origin of notochord from a median groove.&mdash;Its function as an accessory
+    digestive tube.&mdash;Formation of notochordal tissue in invertebrates from closed portions of
+    the digestive tube.&mdash;Digestive power of the skin of Ammoc&#x0153;tes.&mdash;Formation of
+    new gut in Ammoc&#x0153;tes at transformation.&mdash;Innervation of the vertebrate
+    gut.&mdash;The three outflows of efferent nerves belonging to the organic system.&mdash;The
+    original close contiguity of the respiratory chamber to the cloaca.&mdash;The elongation of the
+    gut.&mdash;Conclusion.</p>
+  </div>
+
+  <p>In the previous chapters all the important organs of the arthropod have been found in the
+  vertebrate in their appropriate place, of similar structure, and innervated from corresponding
+  parts of the central nervous system. Such comparison is possible only as long as the ventral and
+  dorsal surfaces of the vertebrate correspond with the respective surfaces of the arthropod, and no
+  reversal is assumed. This method of comparative anatomy is the surest and most certain guide to
+  the relationship between two animals, and when the facts obtained by the anatomical method are so
+  strikingly confirmatory of the palæontological evidence, the combined evidence becomes so strong
+  as to amount almost to a certainty that vertebrates did arise from arthropods in the manner mapped
+  out in previous chapters, and not from a hypothetical group of animals, such as is postulated in
+  the theory of their origin from forms like Balanoglossus.</p>
+
+  <p class="sp3">The latter theory derives the alimentary canal of the vertebrate from that of the
+  invertebrate, and finds in the latter the commencement of the notochord. In the comparison which I
+  have made the alimentary canal of the invertebrate ancestor has become the tube of the central
+  nervous system of the vertebrate, and there is no sign of a notochord whatever. All the organs of
+  the arthropod have already been allocated; where the notochord is situated in the <span
+  class="pagenum" id="page434">{434}</span>vertebrate there is nothing but a gap in the
+  invertebrate, but the position of that gap can be settled with great accuracy from the previous
+  comparison of organs in the two groups. So, also, the alimentary canal of the vertebrate is from
+  the very nature of the case a new organ, yet, as has been shown in Chapter V., the comparison of
+  the respiratory organs in the two groups gives a strong suggestion of the manner in which such a
+  canal was formed.</p>
+
+  <p class="ac"><span class="sc">The Origin of the Notochord.</span></p>
+
+  <p>The time has now come to endeavour to frame a plausible theory of the method of formation of
+  the notochord and the new alimentary canal, and thus to complete the diagram on p. <a
+  href="#page413">413</a>. The comparative method is no longer available, for these structures are
+  both unrepresented as such in the arthropod; any suggested explanation, therefore, must be more
+  tentative, and cannot give the same feeling of certainty as is the case with all the organs
+  already considered. Our only chance of finding out the past history of the notochord lies in the
+  embryological method, in the hope that, according to the 'law of recapitulation,' the ancestral
+  history may be repeated in the ontogeny with sufficient clearness to enable some conclusion to be
+  drawn.</p>
+
+  <p>At the outset, one point comes out clearly&mdash;the close relationship between the notochord
+  and the vertebrate gut; they are both derived from the same layer, both parts of the same
+  structure. On this point all embryologists are agreed; it is expressed in such statements as, "the
+  notochord, as well as the alimentary canal, is formed from hypoblast"; "the notochord arises as a
+  thickening in the dorsal wall of the alimentary canal." The two structures are so closely
+  connected together that they must be considered together. If we can conjecture the origin of the
+  one, we may be sure that we have the clue to the origin of the other. The two together form the
+  one new organ which distinguishes the vertebrate from the arthropod, the only thing left which
+  requires explanation for the completion of this strange history.</p>
+
+  <p>What, then, is the notochord? What are its characteristics? In the highest vertebrates it is
+  conspicuous only in the embryo; with the development of the axial skeleton it is more and more
+  squeezed out of existence, until in the adult it is no longer visible. By the 'law of
+  recapitulation' this developmental history implies that, as we descend the vertebrate phylum, the
+  notochord ought to be more and <span class="pagenum" id="page435">{435}</span>more conspicuous,
+  more and more permanent during the life of the animal. Such is, indeed, found to be the case,
+  until at last, in the lowest vertebrates, such as the lamprey, and in forms like Amphioxus, the
+  notochord persists throughout the life of the animal as a large important axial supporting
+  rod.</p>
+
+  <p>This rod has a number of striking characteristics which distinguish it from all other
+  structures, and are the only means of guessing its probable origin. Its position in the body is
+  always the same in all vertebrates and is very significant, for it lies just ventrally to the
+  central nervous system, along nearly the whole length of the animal, not quite the whole length,
+  for it invariably terminates close to the place where the infundibulum comes to the surface of the
+  brain; it is, in fact, always confined to the infra-infundibular and spinal cord part of the
+  central nervous system. Interpreting this into the language of the arthropod, it means that a rod
+  was formed just ventrally to the nervous system, which extended the whole length of the
+  infra&#x0153;sophageal and ventral chain of ganglia, and terminated at the orifice of the mouth.
+  Moreover, this rod was unsegmented, for the notochord is devoid of segmentation.</p>
+
+  <p>At the anterior end the rod tapers to a point, as in Fig. <a href="#fig166">166</a>. In its
+  middle part it is very large and conspicuous, cylindrical in shape; its interior is filled with a
+  peculiar vacuolated tissue, different to any other known vertebrate tissue, which has therefore
+  received the name of notochordal tissue. Outside this is a thick sheath formed of many layers, of
+  which the external one gives the staining reactions of elastin, and is called the external elastic
+  layer. Between this sheath and the notochordal tissue a thin layer of lining cells, of normal
+  appearance, is conspicuous in Ammoc&#x0153;tes. These cells secrete the layers of the sheath, and
+  have originally, by proliferation, given rise to the notochordal tissue. In the notochord of
+  Ammoc&#x0153;tes there is no sign of either nerves, blood-vessels, or muscles.</p>
+
+  <p>The centre of the notochord presents the appearance of a slight slit, as though it had
+  originated from a tube, and that is the opinion now generally held, for its mode of formation in
+  the embryo is as that of a tube formed from an open groove, as will be explained immediately.</p>
+
+  <p>We may, then, conceive of the notochord as originally a tube lying in the mid-line just
+  ventrally to the central nervous system, and extending from the original mouth to the end of the
+  body. Translate this into the language of the arthropod and it denotes a tube on the <span
+  class="pagenum" id="page436">{436}</span>mid-ventral surface of the body, which extended from
+  mouth to anus. Such a tube might be formed from the mid-ventral surface as follows<span
+  class="wnw">:&mdash;</span></p>
+
+  <p>In Fig. <a href="#fig163">163</a>, A, the lining of the ventral surface between two appendages
+  is represented flat, in B is shown how the formation of a solid rod may arise from the bulging of
+  that ventral surface, and in C how a groove on that surface may lead to the formation of a tube
+  between the two appendages. The difference between a notochordal rod formed as in B from that in C
+  would be shown in the sheath, for in B the sheath would be formed from the cuticle of the lining
+  cells, and in C from the basement membrane. The structure of the sheath is in accordance with the
+  embryological evidence that the notochord is formed as a tube from a groove, as in C, and not as a
+  solid rod as in B, for it possesses a well-marked elastin layer, and elastin has never yet been
+  found as a constituent of any cuticular secretion, but invariably in connection with
+  basement-membranes.</p>
+
+  <div class="ac w35 fcenter sp2">
+    <a href="images/fig163.png" id="fig163"><img style="width:100%" src="images/fig163.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p class="sp0"><span class="sc">Fig. 163.&mdash;Diagram of two possible methods of the
+      Formation of a Notochord.</span></p>
+    </div>
+  </div>
+
+  <p>The position, then, of the notochord and its method of formation suggests that the mid-ventral
+  surface of the arthropod ancestor of the vertebrate formed a deep groove between the bases of all
+  the prosomatic, mesosomatic, and metasomatic appendages, which was subsequently converted into a
+  tube extending along the whole of the body between mouth and anus, and finally, by the
+  proliferation of its lining cells and their conversion into notochordal tissue, became the
+  notochordal rod of the vertebrate.</p>
+
+  <p>As already frequently stated, Apus and Branchipus are the two living arthropods which most
+  nearly resemble the extinct trilobites. The beautiful specimens of Triarthrus (Fig. <a
+  href="#fig165">165</a>) found by Beecher give an idea of the under surface of the trilobite such
+  as has never been obtained before, and demonstrate how closely the condition of things found in
+  Apus (Fig. <a href="#fig164">164</a>) was similar to that occurring in the trilobites. In both
+  cases the mid-ventral surface of the animal formed a deep groove which extended the whole length
+  of the <span class="pagenum" id="page437">{437}</span>animal; on each side of this groove in Apus
+  are closely set the gnatho-bases of the appendages, in such a manner that the groove can be easily
+  converted into a canal by the movements of these bases&mdash;a canal which, owing to the great
+  number of the appendages and their closeness to each other, can be completely and efficiently
+  closed.</p>
+
+  <table class="mc tlf sp2 w50" summary="Group of illustrations" title="Group of illustrations">
+    <tr>
+      <td class="vbm ac" style="width:44%;"><a href="images/fig164.png" id="fig164"><img
+      style="width:100%" src="images/fig164.png" alt="" title=""/></a></td>
+      <td class="vbm ac" style="width:44%;"><a href="images/fig165.png" id="fig165"><img
+      style="width:100%" src="images/fig165.png" alt="" title=""/></a></td>
+    </tr>
+    <tr>
+      <td>
+        <div class="smaller ac">
+          <p class="sp0"><span class="sc">Fig. 164.&mdash;Under-Surface of Apus.</span> (After <span
+          class="sc">Bronn</span>.)</p>
+        </div>
+      </td>
+      <td>
+        <div class="smaller aj it1">
+          <p class="sp0"><span class="sc">Fig. 165.</span>&mdash;<span class="sc">Under-Surface of a
+          Trilobite</span> (<i>Triarthrus</i>). (From <span class="sc">Beecher</span>.)</p>
+        </div>
+      </td>
+    </tr>
+  </table>
+
+  <p>All those who have seen Apus in the living state assert that this canal so formed is actually
+  used by the animal for feeding purposes. By the movements of the gnatho-bases food is passed up
+  from the hind end of the animal along the whole length of this ventral canal to the mouth, where
+  it is taken in and swallowed. In this way Apus has been seen to swallow its own eggs.</p>
+
+  <p>In the trilobites there is a similar deep channel formed by the mid-ventral surface, similar
+  gnatho-bases, and closely set appendages, and the membrane of this ventral groove was extremely
+  thin.</p>
+
+  <p>Here, then, in the very group of animals which were the progenitors of the presumed
+  palæostracan ancestor of the vertebrate&mdash;a group which is characterized by its extensive
+  prevalence and its <span class="pagenum" id="page438">{438}</span>enormous variety of form during
+  the great trilobite era&mdash;the formation of a mid-ventral canal out of this deep ventral groove
+  is seen to be not only easy to imagine, but most probable, provided that a necessity arose for
+  such a conversion.</p>
+
+  <p>For what purpose might such a tube have been formed? I would suggest that it might have acted
+  as an accessory food-channel, which was of sufficient value at the time to give some advantage in
+  the struggle for existence to those members of the group who were thus able to supplement their
+  intake of food, but at the same time was so inefficient that it was quickly superseded by the new
+  alimentary canal, and thus losing its temporary function, became solid, and was utilized to form
+  an axial supporting rod.</p>
+
+  <p>There is a very considerable amount of evidence in favour of the view that the notochord was
+  originally a digestive tube; in fact, as far as I know, this conclusion is universally accepted.
+  The evidence is based essentially upon its development and upon its structure. It is formed in the
+  vertebrate from the same layer as the alimentary canal, <i>i.e.</i> the hypoblast, and in
+  Amphioxus it commences as a groove in the dorsal wall of the future alimentary canal; this groove
+  then closes to form the tube of the notochord, and separates from the alimentary canal.
+  Embryologically, then, the notochord is looked upon as a tube formed directly from the alimentary
+  canal.</p>
+
+  <p>As regards its structure, its tissue is, as already stated, something <i>sui generis</i>.
+  Notochordal tissue has no resemblance to bone or cartilage, or any of the usual supporting
+  tissues. Such a tissue is not, however, entirely confined to the notochord of the vertebrates, but
+  tissue closely resembling it has been found not only in Amphioxus and the Tunicata, but in certain
+  other invertebrates, in the Enteropneusta (Balanoglossus, etc.), in Cephalodiscus, and in
+  Actinotrocha. In all these latter cases, such a tissue is invariably found in disused portions of
+  the alimentary canal; a diverticulum of the alimentary canal becomes closed, vacuolation of its
+  lining cells takes place, and a tissue resembling notochordal tissue is formed.</p>
+
+  <p>Owing to the notochord being invariably so striking and mysterious a feature of the lowest
+  vertebrates, the term vertebrate, which is inappropriate in the members of the group which do not
+  yet possess vertebræ, has been largely superseded by the term chordate, with the result of
+  attributing an undue preponderance to this tissue in any system of classification. Hence, wherever
+  any animal has been found <span class="pagenum" id="page439">{439}</span>with a tissue resembling
+  that of the notochord, enthusiasts have immediately jumped to the conclusion that a relationship
+  must exist between it and the chordate animals; and, accordingly, they have classified such
+  animals as follows: Amphioxus belongs to the group <i>Cephalochorda</i> because the notochord
+  projects beyond the central nervous system; the Tunicata are called <i>Urochorda</i> because it is
+  confined to the tail; the Enteropneusta, <i>Hemichorda</i>, because this tissue is confined to a
+  small diverticulum of the gut, and, finally, <i>Diplochorda</i> has been suggested for
+  Actinotrocha and Phoronis because two separate portions of the gut are transformed in this
+  way.</p>
+
+  <p>This exaggerated importance given to any tissue resembling in structure that of the notochord
+  is believed in by many of those who profess to be our teachers on this subject, the very men who
+  can deliberately shut their eyes to the plain reading of the story of the pineal eyes, and say,
+  "In our opinion this pineal organ was not an eye at all."</p>
+
+  <p>The only legitimate inference to be drawn from the similarity of structure between the
+  notochord and these degenerated gut-diverticula, is that the structure of the notochord may have
+  arisen in the same way, and that therefore the notochord may once have functioned as a gut. With
+  cessation of its function its cells became vacuolated, as in these other cases, and its lumen
+  became filled with notochordal tissue. This evidence strongly confirms the suggestion that the
+  notochord was once a digestive tube, but by no means signifies that such tissue, wherever found,
+  indicates the presence of a notochord.</p>
+
+  <p>In order to resemble a notochord, this tissue must possess not only a definite structure but a
+  definite position, and this position is a remarkably striking and suggestive one. The notochordal
+  tube is unsegmented, although the vertebrate is markedly segmented. But in all segmented animals
+  the only unsegmented tube which extends the whole length of the body, from mouth to anus, is
+  invariably the gut. In the vertebrate there are three such tubes: (1) the gut itself, (2) the
+  central canal of the nervous system, and (3) the notochordal tube.</p>
+
+  <p>The first is the present gut, the second the gut of the invertebrate ancestor, and the third
+  the tube in question.</p>
+
+  <p>These three unsegmented tubes, extending along the whole length <span class="pagenum"
+  id="page440">{440}</span>of the segmented animal, constitute the great peculiarity of the
+  vertebrate group; it is not the unsegmented notochord alone which requires explanation, but the
+  presence of three such tubes in the same animal. Any one of them might be the unsegmented gut of
+  the segmented animal. The most ventral tube is the actual gut of the present vertebrate; the most
+  dorsal&mdash;the neural canal&mdash;was, according to my view, the original gut of the
+  invertebrate ancestor; the middle one&mdash;the notochordal tube&mdash;was, in all probability,
+  also once a gut, formed at the time when the exigencies of the situation made it difficult for
+  food to pass along the original gut.</p>
+
+  <div class="ac w50 fcenter sp2">
+    <a href="images/fig166.png" id="fig166"><img style="width:100%" src="images/fig166.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p class="ac"><span class="sc">Fig. 166.&mdash;Diagram to show the Meeting of the Four Tubes
+      in such a Vertebrate as the Lamprey.</span></p>
+      <p class="sp0"><i>Nc.</i>, neural canal with its infundibular termination; <i>Nch.</i>,
+      notochord; <i>Al.</i>, alimentary canal with its anterior diverticulum; <i>Hy.</i>,
+      hypophysial or nasal tube; <i>Or.</i>, oral chamber closed by septum.</p>
+    </div>
+  </div>
+
+  <p>Yet another circumstance in favour of this suggestion is the very striking position of the
+  anterior termination of the notochord. It terminates at the point of convergence of three
+  structures<span class="wnw">:&mdash;</span></p>
+
+  <p>(1) The tube of the hypophysis or nasal tube.</p>
+
+  <p>(2) The infundibulum or old mouth-termination.</p>
+
+  <p>(3) The notochordal tube.</p>
+
+  <p>To these may be added, according to Kupffer, in the embryonic stage, the anterior diverticulum
+  of the gut (Fig. <a href="#fig166">166</a>).</p>
+
+  <p>This is a very significant point. Here originally, in the invertebrate stage, the olfactory
+  passage opened into the old mouth and &#x0153;sophagus. Here, finally, in the completed vertebrate
+  the same olfactory passage opens into the new pharynx. In the stage between the two it may well
+  have opened into an intermediate gut, the notochordal tube, its separation from which would leave
+  the end of the <span class="pagenum" id="page441">{441}</span>notochord blind, just as it had
+  already left the end of the infundibulum blind.</p>
+
+  <p>The whole evidence points to the derivation of the notochord from a ventral groove on the
+  surface of the animal, which closed to form a tube capable of acting as an accessory gut at the
+  critical period before the new gut was fully formed. The essentials of a gut tube are absorption
+  and digestion of food; is it likely that a tube formed as I have suggested would be efficient for
+  such purposes?</p>
+
+  <p>As far as absorption is concerned, no difficulty would arise. The gut of the arthropod is lined
+  with a thin layer of chitin, which is traversed, like all other chitinous surfaces, by fine
+  canaliculi. Through these canaliculi, absorption of fluid material takes place, from the gut to
+  the body. Similar canaliculi occur in the chitin covering the animal externally, so that, if such
+  external surface formed a tube, and food in the right condition for absorption passed along it,
+  absorption could easily take place through the chitinous surface. The evidence of Apus proves that
+  food does pass along such a tube in the open condition, and in the trilobites the chitinous
+  surface lining a similar groove was apparently very thin, a condition still more favourable to
+  such an absorption process.</p>
+
+  <p>At first sight the second essential of a gut-tube&mdash;the power of digestion&mdash;appears to
+  present an insuperable difficulty to this method of forming an accessory gut-tube, for it
+  necessitates the formation of a secretion capable of digesting proteid material by the external
+  cells of the body, whereas until recently it was supposed that such a function was confined to
+  cells belonging to the so-called hypoblastic layer. Experiments were made now years ago of turning
+  a Hydra inside out so that its internal layer should become external, and <i>vice versâ</i>, and
+  they were said to have been successful. Such an animal could go on living and absorbing and
+  digesting food, although its epiblastic surface was now its digestive internal surface. More
+  recent observations have shown that these experiments were fallacious. At night-time, when the
+  observer was not looking, the hydra reinverted itself, so that again its original digestive
+  surface was inside and it lived and prospered as before.</p>
+
+  <p>Another piece of evidence of somewhat similar kind, which has not as yet been discredited, is
+  seen in the Tunicata. In many of these, new individuals are formed from the parent by a process of
+  budding, and it has been proved that frequently the gut of the new <span class="pagenum"
+  id="page442">{442}</span>individual thus budded off arises not from the gut or hypoblastic layer
+  of the parent, but from the surface or epiblastic layer. Such gut so formed possesses as efficient
+  digestive powers as the gut of the parent.</p>
+
+  <p>The most remarkable evidence of all has been afforded by Miss Alcock's experiments. She
+  examined the different tissues of Ammoc&#x0153;tes for the express purpose of finding out their
+  power of digesting fibrin, with the result that the most active cells were those of the liver.
+  Next in activity came the extract of the lining cells of the respiratory chamber and of the skin.
+  The intestine itself when freed from the liver-secretion had very little digestive power; extracts
+  of muscle, nervous system, and thyroid gland had no power whatever, but the extract of the
+  skin-cells possessed a powerful digesting action.</p>
+
+  <p>Furthermore, it is not necessary to make an extract of the skin in order to obtain this
+  digestive fluid, for under the influence of chloroform the skin of Ammoc&#x0153;tes secretes
+  copiously, and this fluid thus secreted was found to possess strong digestive powers. So, also,
+  Miss Alcock has demonstrated the power of digesting fibrin in a similar secretion of the
+  epithelial cells lining the carapace of the crayfish. In both cases a very plausible reason for
+  the presence of a digestive ferment in a skin-secretion is found in the necessity of preventing
+  the growth of parasites, fungoid, or otherwise, especially in those parts where the animal cannot
+  keep itself clean by 'preening.' Thus in a crayfish, in which the &#x0153;sophageal commissures
+  had been cut, fungus was found to grow on the ventral side, but not on the dorsal carapace. The
+  animal was accustomed to keep its ventral surface clean by preening; owing to the paralysis it
+  could not do so, and consequently the fungus grew there. In the lamprey I found that wherever
+  there was a removal of the surface-epithelium, from whatever cause, that spot was immediately
+  covered with a fungoid growth, although in the intact lamprey the skin was invariably smooth and
+  clean.</p>
+
+  <p>I imagine, then, that this digestive power of the skin arose as a protective mechanism against
+  parasitic attacks; it is self-evident how a tube formed of such material must <i>ab initio</i> act
+  as a digestive tube.</p>
+
+  <p>In yet another respect this skin secretion of Ammoc&#x0153;tes is most instructive. The surface
+  of Ammoc&#x0153;tes is absolutely smooth, no scales <span class="pagenum"
+  id="page443">{443}</span>of any kind exist; this smoothness is due to the presence of a very
+  well-defined cuticular layer secreted by the underlying epithelial cells. This cuticle is very
+  much thicker than is usually found in vertebrates, and, strangely enough, has been thought to
+  contain chitin. Whether it really contains chitin or not I am unable to say, but it certainly
+  resembles a chitinous layer in one respect; it is perforated by innumerable very fine tubes or
+  canaliculi, along which, by appropriate staining, it is easy to see the secretion of the
+  underlying cell pass to the exterior (Fig. <a href="#fig140">140</a>). This marked digestive power
+  of the skin of Ammoc&#x0153;tes, together with the easy passage of the secretion through the thin
+  cuticular layer, renders it almost certain that a tube formed from the deep ventral groove of the
+  trilobite would, from the very first, act as a digestive as well as an absorbent tube; in other
+  words, the notochord as soon as formed was able to act as an accessory digestive tube.</p>
+
+  <p>This suggested origin of the notochord from a groove along the mid-ventral surface of the body
+  not only indicates a starting-point from a markedly segmented portion of the body, but also points
+  to its formation at a stage previous to the formation of the operculum by the fusion of the two
+  foremost mesosomatic appendages&mdash;indicates therefore its formation at a stage more nearly
+  allied to the trilobite than to the sea-scorpion. The chance of ever finding any direct evidence
+  of such a chordate trilobite stage appears to me exceedingly improbable, and I greatly fear that
+  this conception of the mode of formation of the notochord can never be put to direct proof, but
+  must always remain guesswork.</p>
+
+  <p>On the other hand, evidence of a kind in favour of its origin from a segmented part of the body
+  does exist, and that evidence has this special value, that it is found only in that most primitive
+  animal, Amphioxus.</p>
+
+  <p>This evidence is as follows:&mdash;</p>
+
+  <p class="sp3">At fairly regular intervals, the sheath of the notochord is interrupted on each
+  side of the mid-dorsal line by a series of holes, which penetrate the whole thickness of the
+  sheath. This dorsal part is pressed closely against the spinal cord, and through these holes
+  fibres appear to pass from the spinal cord to the interior of the notochord. So greatly do these
+  fibres present the appearance of ventral roots to the notochord, that Miss Platt looks upon them
+  as paired motor roots to the notochord, or at all events as once having been such motor <span
+  class="pagenum" id="page444">{444}</span>roots. Lwoff and Rolph both describe a direct
+  communication between the spinal cord and the notochord by means of fibres passing through these
+  holes, without however looking upon this connection as a nervous one. Joseph alone asserts that no
+  absolute connection exists, for the internal elastic layer of the notochord, according to him, is
+  not interrupted at these holes, and forms, therefore, a barrier between the fibres from the spinal
+  cord and those from the interior of the notochord. Still, whatever is the ultimate verdict as to
+  these fibres, the suggestive fact remains of the spaces in the notochordal sheath and of the
+  corresponding projecting root-like fibres from the spinal cord. The whole appearance gives the
+  impression of some former connection, or rather series of connections, between the spinal cord and
+  the notochord, such as would have occurred if nerves had once passed into the notochord. On the
+  other hand, such nerves were not arranged segmentally with the myotomes, for, according to Joseph,
+  in the middle of the animal ten to twelve such holes occur in one body-segment. In Apus the
+  appendages are more numerous than the body-segments, so that it is not necessary for a segmental
+  arrangement to coincide with that of the body-segments.</p>
+
+  <p class="ac"><span class="sc">The Origin of the Alimentary Canal.</span></p>
+
+  <p>In close connection with the notochord is the alimentary canal. Any explanation of the one must
+  be of assistance in explaining the other.</p>
+
+  <p>According to the prevalent embryological teaching, the body is formed of three layers,
+  epiblast, hypoblast, and mesoblast, and the gastræa theory of the origin of all Metazoa implies of
+  necessity that the formation of every individual commences with the formation of the gut. For this
+  reason the alimentary canal must in every case be regarded as the earliest formed organ, however
+  late in the development it may attain its finished appearance. Hence the notochord is spoken of as
+  developed from the mid-dorsal wall of the alimentary canal. It is possible to look at the question
+  the other way round, and suppose that the organ whose development is finished first is older than
+  the one still in process of making. In this case it would be more right to say a ventral extension
+  of the tissue, which gives rise to the notochord, takes place and forms the alimentary canal. It
+  is, to my mind, perfectly possible, and indeed probable, that <span class="pagenum"
+  id="page445">{445}</span>the formation of the vertebrate alimentary canal was a repetition of the
+  same process which had already led to the formation of the notochordal tube. The formation of the
+  anterior part of the alimentary canal in Ammoc&#x0153;tes at the time of transformation strongly
+  suggests the marked similarity of the two processes.</p>
+
+  <p>Of all the startling surprises which occur at transformation, this formation of a new anterior
+  gut is the most startling. From the oral chamber of Petromyzon two tubes start: the one leads into
+  the gill-chambers, is known as the bronchus, and is entirely concerned with respiration; the other
+  leads without a break from the mouth to the anus, has no connection with respiration, and is the
+  alimentary canal of the animal. Any one looking at Petromyzon would say that its alimentary canal
+  was absolutely non-respiratory in character. Before transformation, this kind of alimentary canal
+  commences at the end of the respiratory chamber; from here to the anus it is of the same character
+  as in Petromyzon, but in Ammoc&#x0153;tes the non-respiratory anterior part simply does not exist:
+  the whole anterior chamber is both respiratory and affords passage to food. This part of the
+  alimentary canal of the adult is formed anew. We see, then, here the formation of a part of the
+  alimentary canal taking place, not in an embryo full of yolk, but in a free-living, independent,
+  grown-up larval form in which all yolk has long since disappeared: a condition absolutely unique
+  in the vertebrate kingdom, but one which more than any other may be expected to give a clue to the
+  method of formation of a vertebrate gut.</p>
+
+  <p>The formation of this new gut can be easily followed at transformation, and was originally
+  described by Schneider. His statement has been confirmed by Nestler, and its absolute truth has
+  been demonstrated to me again and again by Miss Alcock, in her specimens illustrative of the
+  transformation process. First, in the mid-dorsal line of the respiratory chamber a distinct groove
+  is formed, the edges of which come together and form a solid rod. This solid rod blocks the
+  opening of the respiratory chamber into the mid-gut, so that during this period of the
+  transformation no food can pass out of the pharyngeal chamber. A lumen then begins to appear in
+  this solid rod at the posterior end, which steadily advances mouthwards until it opens into the
+  oral chamber and thus forms an open tube connecting the mouth with the gut.</p>
+
+  <p>Here, then, is the foundation of a new gut on very similar lines <span class="pagenum"
+  id="page446">{446}</span>to that of the notochord, by the conversion of a groove into a tube.
+  Still more suggestive is it to find that the tube so formed has no appearance whatever of
+  segmentation; it is as unsegmented as the rest of the gut, although, as is seen in Fig. <a
+  href="#fig62">62</a>, the dorsal wall of the respiratory chamber from which it arose is as
+  markedly segmented as any part of the animal. Here under our very eyes, in the course of a few
+  days or weeks, an object-lesson in the process of the manufacture of an alimentary canal is
+  carried out and completed, and the teaching of that lesson is that a gut-tube may be formed in the
+  same way as the notochordal tube, by the conversion of a grooved surface into a canal, and that
+  gut-tube so formed, like the notochord, loses all sign of segmentation, even although the original
+  grooved surface was markedly segmented.</p>
+
+  <p>The suggestion then is, that the new gut may have been formed by a repetition of the same
+  process which had already given origin to the notochord.</p>
+
+  <p class="sp3">Such a method of formation is not, in my opinion, opposed to the evidence given by
+  embryology, but in accordance with it; the discussion of this point will come best in the next
+  chapter, which treats of the embryological evidence as a whole, and will therefore be left till
+  then.</p>
+
+  <p class="ac"><span class="sc">The Evidence given by the Innervation of the Vertebrate Alimentary
+  Canal.</span></p>
+
+  <p>Throughout this investigation the one fixed landmark to which all other comparisons must be
+  referred, is the central nervous system, and the innervation of every organ has given the clue to
+  the meaning of that organ. So also it must be with the new alimentary canal; by its innervation we
+  ought to obtain some insight into the manner of its origination. In any organ the nerves which are
+  specially of value in determining its innervation, are of necessity the efferent or motor nerves,
+  for the limits of their distribution in the organ are much more easily determined than those of
+  the afferent or sensory nerves. The question therefore of primary importance in endeavouring to
+  determine the nature of the origin of the alimentary canal from its innervation is the
+  determination of the efferent supply to the musculature of its walls.</p>
+
+  <p>Already in previous chapters a commencement has been made in <span class="pagenum"
+  id="page447">{447}</span>this direction; thus the musculature of the oral chamber has been derived
+  directly from the musculature of the prosomatic appendages; the muscles which move the eyes from
+  the prosomatic and mesosomatic dorso-ventral somatic muscles; the longitudinal body-muscles from
+  the dorsal longitudinal somatic muscles of the arthropod; the muscles of respiration from the
+  dorso-ventral muscles of the mesosomatic appendages.</p>
+
+  <p>In all these cases we have been dealing with striated musculature and consequently with only
+  the motor nerves of the muscle; but the gut posterior to the pharyngeal or respiratory chamber
+  contains unstriped instead of striped muscle, and is innervated by two sets of nerves, those which
+  cause contraction and are motor, and those which cause relaxation and are inhibitory. It is by no
+  means certain that these two sets of nerves possess equal value from a morphological point of
+  view. The meaning of an inhibitory nerve is at present difficult to understand, and in this
+  instance, is rendered still more doubtful owing to the presence of Auerbach's plexus along the
+  whole length of the intestine&mdash;an elaborate system of nerve-cells and nerve-fibres situated
+  between the layers of longitudinal and circular muscles surrounding the gut-walls, which has been
+  shown by the recent experiments of Magnus, to constitute a special enteric nervous system.</p>
+
+  <p>One of the strangest facts known about the system of inhibitory nerves is their marked tendency
+  to leave the central nervous system at a different level to the corresponding motor nerves, as is
+  well known in the case of the heart, where the inhibitory nerve&mdash;the vagus&mdash;arises from
+  the medulla oblongata, while the motor nerve&mdash;the augmentor or accelerator&mdash;leaves the
+  spinal cord in the upper thoracic region. It is very difficult to obtain any idea of the origin of
+  such a peculiarity; I know of only one suggestive fact, which concerns the innervation of the
+  muscles which open and close the chela of the crayfish, lobster, etc. These muscles are
+  antagonistic to each other, and both possess inhibitory as well as motor nerves. The central
+  nervous system arrangements are of such a character that the contraction of the one muscle is
+  accompanied by the inhibition of its opposer, and the nerves which inhibit the contraction of the
+  one, leave the central nervous system with the nerves which cause the other to contract. Thus the
+  inhibitory and motor nerves of either the abductor (opener) or adductor (closer) muscles of the
+  crayfish claw do not leave the central nervous system together, but in separate nerves.</p>
+
+  <div><span class="pagenum" id="page448">{448}</span></div>
+
+  <p>If now for some cause the one set of muscles either disappeared, or were so altered as no
+  longer to present any appearance of antagonism, then there would be left a single set of muscles,
+  the inhibitory and motor nerves of which would leave the central nervous system at different
+  levels, and the older such systems might be, the greater would be the modification in the shape
+  and arrangements of parts in the animal, so that the two sets of fibres might ultimately arise
+  from very different levels.</p>
+
+  <p>As mentioned in the introductory chapter, the whole of this investigation into the origin of
+  vertebrates arose from my work on the system of efferent nerves which innervate the vascular and
+  visceral systems. One of the main points of that investigation was the proof that such nerves did
+  not leave the central nervous system uniformly along the whole length of it, but in three great
+  outflows, cranial, thoracico-lumbar, and sacral; there being two marked gaps separating the three
+  outflows, caused by the interpolation of the plexuses for the innervation of the anterior and
+  posterior limbs respectively. All these nerves are characterized by the presence of ganglion-cells
+  in their course to the periphery, they are, therefore, distinguished from ordinary motor nerves to
+  striated muscle in that their impulses pass through a ganglion-cell before they reach the
+  muscle.</p>
+
+  <p>The ganglia of the large middle thoracico-lumbar outflow constitute the ganglia of the
+  sympathetic system.</p>
+
+  <p>The functions of the nerves constituting these three outflows are very different, as I pointed
+  out in my original papers. Since then a large amount of further information has been obtained by
+  various observers, especially Langley and Anderson, which enable the following statements to be
+  made<span class="wnw">:&mdash;</span></p>
+
+  <p>All the nerves which cause contraction of the unstriped muscles of the skin, whether pilomotor
+  or not, all the nerves which cause secretion of sweat glands wherever situated, all the nerves
+  which cause contraction or augmentation of the action of muscles belonging to the vascular system,
+  all the nerves which are motor to the muscles belonging to all organs derived from the Wolffian
+  and Müllerian ducts, <i>e.g.</i> the uterus, ureters, urethra, arise from the thoracico-lumbar
+  outflow, never from the cranial or sacral outflows. It is essentially an efferent skin-system.</p>
+
+  <p>On the other hand, the latter two sets of nerves are concerned <span class="pagenum"
+  id="page449">{449}</span>with the supply of motor nerves to the alimentary canal; they form
+  essentially an efferent gut-system in contradistinction to the sympathetic or skin-system.</p>
+
+  <p>A marked distinction exists between these cranial and sacral nerves. The vagus never supplies
+  the large intestine, the sacral nerves never supply the small intestine. Associated with the large
+  intestine is the bladder, the whole system arising from the original cloacal region; the vagus
+  never supplies the bladder, its motor nerves belong to the sacral outflow. The motor nerves to the
+  ureters, to the urethra, and to the trigonal portion of the bladder between the ureters and the
+  urethra, do not arise from the sacral outflow, but from the thoracico-lumbar. These muscles belong
+  really to the muscles in connection with the Müllerian and Wolffian ducts and skin, not to the
+  cloacal region.</p>
+
+  <p>The motor innervation then of the alimentary canal reveals this striking and suggestive state
+  of affairs. The motor innervation of the whole of the small intestine arises from the cranial
+  region, and is immediately followed by an innervation from the sacral region for the whole of the
+  muscles of the cloaca. It thus indicates a head-region and a tail-region in close contiguity, the
+  whole of the spinal cord region between these two extremes being apparently unrepresented. Not,
+  however, quite unrepresented, for Elliott has shown recently that the ileo-colic valve at the
+  junction of the small and large intestine is in reality an ileo-colic sphincter muscle, and that
+  this muscle receives its motor nerves neither from the vagus nor from the sacral nerves, but from
+  the thoracico-lumbar outflow or sympathetic system. This may mean one of two things, either that a
+  band of fibres belonging to the skin-system has been added to the gut-musculature, for the purpose
+  of forming a sphincter at this spot, or that the region between the vagus territory and the cloaca
+  is represented by this small band of muscle. The second explanation seems to me the more probable
+  of the two. Between the mesosomatic region represented by the vagus, and the cloacal region, there
+  existed a small metasomatic region, represented by the pronephros, with its segmental duct, as
+  already discussed in Chapter XII. That part of the new alimentary canal which belonged to this
+  region is the short piece indicated by the ileo-colic sphincter, and innervated, therefore, from
+  the same region as the organs derived from the segmental duct.</p>
+
+  <p>Such innervation seems to me to suggest that originally the <span class="pagenum"
+  id="page450">{450}</span>vertebrate consisted, as far as its gut was concerned, of a prosomatic
+  and mesosomatic (branchial) region, close behind which came the cloaca and anus. Between the two
+  there was a short metasomatic region (possibly pronephric), so that the respiratory chamber did
+  not open directly into the cloaca.</p>
+
+  <p>Such an interpretation is, I think, borne out by the study of the most ancient forms of fish.
+  In Bothriolepis, according to Patten, and in Drepanaspis, according to Traquair, the cloacal
+  region and anus follow immediately upon the posterior end of the head-shield, <i>i.e.</i>
+  immediately after that region which presumably contained the branchiæ. Similarly, on the
+  invertebrate side, all those forms which resembled Limulus must have possessed a very short region
+  between the branchial and cloacal parts of the body. The original cloacal part of the vertebrate
+  gut may well have been the original cloaca of the arthropod, into which its intestine emptied
+  itself, especially when we see the tendency of the scorpion group of animals to form an accessory
+  cloacal pouch known as the stercoral pouch or pocket.</p>
+
+  <p>Again, it is striking to see how, in certain of the scorpion group, <i>e.g.</i> Thelyphonus and
+  Phrynus, there is a caudal massing of the central nerve-cells as well as a cephalic massing, so
+  that their central nervous system is composed of a cephalic and caudal brain. These two brains are
+  connected together by commissures extending the whole length of the body, in which I have been
+  unable to find any sign of ganglion-cells. What this caudal brain innervates I do not know; it is,
+  I think, a matter worth further investigation, especially as there are many indications in the
+  vertebrate that the lumbo-sacral region of the cord possesses higher functions than the thoracic
+  region.</p>
+
+  <p>The method of formation of the alimentary canal as indicated by its innervation is as
+  follows<span class="wnw">:&mdash;</span></p>
+
+  <p>In front an oral chamber, formed, as already pointed out, by the modification of the prosomatic
+  appendages, followed by a respiratory chamber, the muscles and branchiæ of which were the muscles
+  and branchiæ of the mesosomatic appendages. This mesosomatic, or branchial, part was in close
+  contiguity to the cloaca and anus, being separated from it only by a short tube formed in the
+  metasomatic or pronephric region.</p>
+
+  <p>I imagine that this connection was originally in the form of an <span class="pagenum"
+  id="page451">{451}</span>open groove, as already explained for both notochord and the anterior
+  part of the gut itself in Ammoc&#x0153;tes; an open groove formed from the mid-ventral surface of
+  the body, on each side of which were the remnants of the pronephric appendages. By the closure of
+  this groove ventrally, and the growing round of the pleural folds, as already suggested, the
+  remains of the pronephric appendages are indicated by the segmental duct and the form of the
+  vertebrate body is attained.</p>
+
+  <p>Even in the branchial region the same kind of thing must, I think, have occurred. The grooved
+  ventral surface became a tube, on each side of which were lying in regular order the in-sunk
+  branchial appendages, the whole being subsequently covered by the pleural folds to form an atrial
+  chamber. A tube thus formed from the grooved ventral surface would carry with it to the new
+  ventral surface the longitudinal venous sinuses, and thus form, in the way already suggested, the
+  heart and ventral aorta. Posterior to the heart in the pronephric region, the same process would
+  give rise to the sub-intestinal vein.</p>
+
+  <p>The evidence of comparative anatomy bears out most conclusively the suggestion that in the
+  original vertebrate the gut was mainly a respiratory chamber. In man and all mammals the oral
+  chamber opens into a small pharynx, followed by the &#x0153;sophagus, stomach and small intestine.
+  Of this whole length, a very small part is taken up by the pharynx, in which, in the embryo, the
+  branchial arches are found, showing that this represents the original respiratory part of the gut.
+  In the ordinary fish this branchial part is much more conspicuous, occupies a large proportion of
+  the gut, and in the lowest fishes, such as Ammoc&#x0153;tes and Amphioxus, the branchial region
+  extends over a large portion of the animal, while the intestine proper is a straight tube, the
+  length of which is insignificant in comparison with its length in the higher vertebrates.</p>
+
+  <p>Such a tube was able to act as a digestive tube, owing, as already pointed out, to the
+  digestive powers of the skin-epithelium, and I imagine at first the respiratory chamber, seeing
+  that it composed very nearly the whole of the gut, was at the same time the main digestive
+  chamber; even in Ammoc&#x0153;tes its digestive power is superior to that of the intestine
+  itself.</p>
+
+  <p>Just posterior to the branchial part a diverticulum of the gut was formed at an early stage, as
+  seen in Amphioxus, and provided the <span class="pagenum" id="page452">{452}</span>commencement of
+  the liver. This simple liver-diverticulum became the tubular liver of Ammoc&#x0153;tes, and
+  formed, curiously enough, not a glandular organ of the same character as the liver of the higher
+  vertebrates, but a hepato-pancreas, like the so-called liver of the arthropods, which also is a
+  special diverticulum of the gut, or rather the main true gut of the animal. In both cases the
+  liver is the chief agent in digestion, for in Ammoc&#x0153;tes the liver-extract is very much more
+  powerful in the digestion of proteids than the extract of any other organ tried by Miss Alcock.
+  Subsequently in the vertebrate the gastric and pancreatic glands arise and relieve the liver of
+  the burden of proteid digestion.</p>
+
+  <p class="sp3">It is, to my mind, somewhat significant that the liver on its first formation in
+  the vertebrate should have arisen as a digestive organ of the same character as the so-called
+  liver in the arthropods; whether it originally belonged to any separate segment is in our present
+  state of knowledge difficult to say.</p>
+
+  <p class="ac"><span class="sc">Conclusion.</span></p>
+
+  <p>In conclusion, I will endeavour to illustrate crudely the way in which, on my theory, the
+  notochord and vertebrate gut may have been formed, the agencies at work being in the main two,
+  viz. the dwindling of appendages as mere organs of locomotion, and the conversion of a ventral
+  groove into a tube.</p>
+
+  <p>I imagine that, among the Protostraca, forms were found somewhat resembling trilobites with
+  markedly polychætan affinities; which, like Apus, possessed a deep ventral groove from one end of
+  the body to the other, and also pleural fringes, as in many trilobites. This might be called the
+  Trilobite stage (Fig. <a href="#fig167">167</a>, A).</p>
+
+  <p>This groove became converted into a tube and so gave rise to the notochord, while the
+  appendages were still free and the pleuræ had not met to form a new ventral surface. This might be
+  called the Chordate Trilobite stage (Fig. <a href="#fig167">167</a>, B).</p>
+
+  <p>Then, passing from the protostracan to the palæostracan stage, the oral and respiratory
+  chambers were formed, not communicating with each other, in the manner described in previous
+  chapters, a ventral groove in the metasomatic region being the only connection between respiratory
+  chamber and cloaca. This might be called the Chordate Palæostracan stage (Fig. <a
+  href="#fig167">167</a>, C).</p>
+
+  <div><span class="pagenum" id="page453">{453}</span></div>
+
+  <div class="ac w50 fcenter sp2">
+    <a href="images/fig167.png" id="fig167"><img style="width:100%" src="images/fig167.png" alt=""
+    title=""/></a>
+    <div class="smaller aj it1">
+      <p><span class="sc">Fig. 167.&mdash;A, Diagram of Section through a Trilobite-like Animal; B,
+      Diagram to illustrate the Suggested Formation of the Notochord from a Ventral Groove; C,
+      Diagram to illustrate the Suggested Formation of the Post-Branchial Gut by the continuation of
+      the same process of Ventral Groove-Formation, combined with Obliteration of Appendages and
+      Growth of Pleural Folds; D, Diagram to illustrate the Completion of the Vertebrate Type by the
+      Meeting of the Pleural Folds in the Mid-Ventral Line with the Obliteration of the Atrial
+      Cavity and the Conversion of the Ventral Groove into the closed Alimentary Canal.</span></p>
+      <p class="sp0"><i>Al.</i>, alimentary canal; <i>N.</i>, nervous system; <i>My.</i>, myotome;
+      <i>Pl.</i>, pleuron; <i>App.</i>, appendage; <i>Neph.</i>, <span class="correction"
+      title="Original reads 'nephrocele'">nephroc&#x0153;le</span>; <i>Met.</i>, metac&#x0153;le;
+      <i>Sd.</i>, segmental duct; <i>Mes.</i>, mesonephros; <i>At.</i>, atrial chamber; <i>Nc.</i>,
+      notochord; <i>H.</i>, heart; <i>F.</i>, fat body; <i>Ng.</i>, notochordal groove. (These
+      diagrams are intended to complete the diagrams on p. <a href="#page413">413</a>, which, as
+      stated there, were purposely left incomplete.)</p>
+    </div>
+  </div>
+
+  <div><span class="pagenum" id="page454">{454}</span></div>
+
+  <p>Finally, with the conversion of this groove into a tube, the opening of the oral into the
+  respiratory chamber, and the formation of an atrium by the ventralwards growth of the pleural
+  folds, the formation of a Vertebrate was completed (Fig. <a href="#fig167">167</a>, D).</p>
+
+  <p>In my own mind I picture to myself an animal which possessed eurypterid and trilobite
+  characters combined, in which a notochordal tube had been formed in the way suggested, and a
+  respiratory chamber which communicated with the cloaca by means of a grooved channel along the
+  mid-ventral line of the metasomatic portion of the body. On each side of this channel were the
+  remains of the metasomatic appendages (pronephric). The whole was enveloped in the pleural folds,
+  which probably at this time did not yet meet in the middle line to form a new ventral surface.
+  This respiratory chamber, owing to the digestive power of the epidermis, assisted in the process
+  of alimentation to such an extent as to supersede the temporary notochordal tube, with the effect
+  of bringing about the conversion of the metasomatic groove into a closed canal, and so the
+  formation of an alimentary tube continuous with the respiratory chamber. The amalgamation of the
+  pleural folds ventrally completed the process, and so formed an animal resembling the
+  Cephalaspidæ, Ammoc&#x0153;tes, or Amphioxus.</p>
+
+  <p class="sp5">I have endeavoured in this chapter to make some suggestions upon the origin of the
+  notochord and of the vertebrate gut in accordance with my theory of the origin of vertebrates. I
+  feel, however, strongly that these suggestions are much more speculative than those put forward in
+  the previous chapters, and of necessity cannot give the same feeling of soundness as those based
+  directly upon comparative anatomy and histology. Still, the fact remains that the origin of the
+  notochord is at present absolutely unknown, and that my speculation that it may have originated as
+  an accessory digestive tube is at all events in accordance with the most widely spread opinion
+  that it arises in close connection with an alimentary canal.</p>
+
+  <div><span class="pagenum" id="page455">{455}</span></div>
+
+  <p class="ac">CHAPTER XIV</p>
+
+  <p class="ac"><i>THE PRINCIPLES OF EMBRYOLOGY</i></p>
+
+  <div class="bq1 it1 smaller sp3">
+    <p class="sp0">The law of recapitulation.&mdash;Vindication of this law by the theory advanced
+    in this book.&mdash;The germ-layer theory.&mdash;Its present position.&mdash;A physiological not
+    a morphological conception.&mdash;New fundamental law required.&mdash;Composition of adult
+    body.&mdash;Neuro-epithelial syncytium and free-living cells.&mdash;Meaning of the
+    blastula.&mdash;Derivation of the Metazoa from the Protozoa. Importance of the central nervous
+    system for Ontogeny as well as for Phylogeny.&mdash;Derivation of free-living cells from
+    germ-cells.&mdash;Meaning of c&#x0153;lom.&mdash;Formation of neural canal.&mdash;Gastrula of
+    Amphioxus and of Lucifer.&mdash;Summary.</p>
+  </div>
+
+  <p>In a discussion upon this theory of mine, which took place at Cambridge on November 25 and
+  December 2, 1895, it was said that such a theory was absolutely and definitely put out of court,
+  because it contravened the principles of embryology, was opposed, therefore, to our surest guide
+  in such matters; and the law was laid down with great assurance that no claim for genetic
+  relationship between two groups of animals can be allowed which is based upon topographical and
+  structural coincidences revealed by the study of the anatomy of two adult animals, however
+  numerous and striking they may be, if there are fundamental differences in the embryology of the
+  members of these two groups.</p>
+
+  <p>According to my theory the old gut of the arthropod still exists in the vertebrate as the
+  tubular lining of the central nervous system, and the vertebrate has formed a new gut. According
+  to the principles of embryology as held up to the present, in all animals above the Protozoa, the
+  different structures of the body arise from three definite embryonic layers, the epiblast,
+  mesoblast, and hypoblast, and in all cases the gut arises from the hypoblastic layer. In the
+  vertebrate the gut also arises from the hypoblast, while the neural canal is epiblastic. My
+  theory, then, makes the impossible assertion that what was hypoblast in the arthropod has become
+  epiblast in the vertebrate, and what was epiblast in the arthropod has become hypoblast in the
+  vertebrate. Such a conception is supposed to be so <span class="pagenum"
+  id="page456">{456}</span>absolutely impossible that it only requires to be stated to be dismissed
+  as an absurdity.</p>
+
+  <p>Against this opinion I claim boldly that my theory is not only not contrary to the principles
+  of embryology, but is mainly based upon the teachings of embryology. I wish here not to be
+  misunderstood. The great value of the study of embryology for questions of the sequence of the
+  evolution of animals is to be found in what is known as the Law of Recapitulation, which asserts
+  that every animal gives some indication in the stages of its individual development of its
+  ancestral history. Naturally enough it cannot pass through all the stages of its past history with
+  equal clearness, for what has taken millions of years to be evolved has to be compressed into an
+  evolution lasting only a few months or weeks, or even less.</p>
+
+  <p>When in the highest vertebrate a vestigial organ, such as the pineal gland, can be traced back
+  without leaving the vertebrate kingdom to a distinct median eye, such as is found in the lamprey,
+  that rudimentary organ is evidence of an organ which was functional in the earliest vertebrates or
+  their immediate ancestors. So it is generally with well defined vestigial organs found in the
+  adult animal; they always indicate an organ which was functional in the near ancestor.</p>
+
+  <p>Passing from the adult to the embryo we still find the same law. Here, also, vestigial organs
+  are met with, which may leave no trace in the adult, but indicate organs which were functional in
+  the near ancestor. Thus, but for embryology, we should have no certainty that the air-breathing
+  vertebrates had been derived from water-breathing fishes; the indication is not given by any close
+  resemblance between the formation of the embryos in their earliest stages, but by the formation of
+  vestigial gill-arches even in the embryos of the highest mammal.</p>
+
+  <p>For all questions of evolution the presence of vestigial organs in the embryo is the important
+  consideration, for they give an indication of near ancestry; the early formation of the embryo
+  concerns a much more remote ancestral period, all vestigial organs of which may well have been
+  lost and obscured by c&#x0153;nogenetic changes. Let us, then, consider the two things&mdash;the
+  vestigial organs and the early formation of the embryo&mdash;separately, and see how far my
+  opponents are justified in their statement that my theory contravenes the principles of
+  embryology.</p>
+
+  <div><span class="pagenum" id="page457">{457}</span></div>
+
+  <p>First, I will take the teachings of vestigial organs and the arrangement of organs found in the
+  vertebrate embryo. Here it is impossible to say that my theory is contrary to the teaching of
+  embryology, for as the previous chapters have shown again and again, the argument is based very
+  largely upon the facts of embryology. In the first place, the comparison which I have chiefly made
+  is a comparison between the larval form of a very low vertebrate and the arthropod group, a
+  comparison which exists only for the larval form, and not for the adult. The whole theory, then,
+  is based upon a developmental stage of the vertebrate, and not upon the anatomy of the adult.</p>
+
+  <p>Throughout the whole history it seems to me perfectly marvellous how completely the law of
+  recapitulation is vindicated by my theory of the origin of the vertebrate. The theory asserts that
+  the clue to the origin of vertebrates is to be found in the tubular nature of the central nervous
+  system of the vertebrate; in that the vertebrate central nervous system is in reality formed of
+  two things: (1) a central nervous system of the arthropod type, and (2) an epithelial tube in the
+  position of the alimentary canal of the arthropod.</p>
+
+  <p>Is it possible for embryology to recapitulate such a phylogenetic history more clearly than is
+  here the case? In order to avoid all possibility of our mistaking the clue, the nerve-tube in the
+  embryo always opens into the anus at its posterior end, while in the larval Amphioxus it is
+  actually still open to the exterior at the anterior end. The separateness of the tube from the
+  nervous system at its first origin is shown especially well in the frog, where, as Assheton has
+  pointed out, owing to the pigment in the cells of the external layer of epithelium, a pigmented
+  tube is formed, on the outside of which the nervous tissue is lying, and step by step the gradual
+  intermingling of the nerve-cells and the pigmented lining cells can be followed out.</p>
+
+  <p>Consider the shape of the nerve-tube when first formed in the vertebrate. At the cephalic end a
+  simple bulged-out tube with two simple anterior diverticula, which passes into a narrow straight
+  spinal tube; from this large cephalic bulging a narrow diverticulum, the infundibulum, passes to
+  the ventral surface of the forming brain. This tube is the embryological expression of the simple
+  dilated cephalic stomach, with its ventral &#x0153;sophagus and two anterior diverticula, which
+  opens into the straight intestine of the arthropod. Nay, more, by its very shape, and the
+  invariable presence of two anterior <span class="pagenum" id="page458">{458}</span>diverticula, it
+  points not only to an arthropod ancestry, but to a descent from a particular group of primitive
+  arthropods. Then comes the formation of the cerebral vesicles, and the formation of the optic cup,
+  telling us as plainly as can be how the invasion of nervous material over this simple cephalic
+  stomach and its diverticula has altered the shape of the original tube, and more and more enclosed
+  it with nervous elements.</p>
+
+  <p>So, too, in the spinal cord region. When the tube is first formed, it is a large tube, the
+  latero-ventral part of which presents two marked bulgings; connecting these two bulgings is the
+  anterior commissure. These two lateral bulgings, with their transverse commissure, represent, with
+  marked fidelity, the ventral ganglion-masses of the arthropod with their transverse commissure,
+  and occupy the same position with respect to the spinal tube, as the ganglion-masses do with
+  respect to the intestine in the arthropod. Then the further development shows how, by the
+  subsequent growth of the nervous material, the calibre of the tube is diminished in size, and the
+  spinal cord is formed.</p>
+
+  <p>Again, I say, is it possible to conceive that embryology should indicate the nature of the
+  origin of the vertebrate nervous system more clearly than it does?</p>
+
+  <p>It is the same with all the other organs. Take, for instance, the skeletal tissues. The study
+  of the vertebrate embryo asserts that the cartilaginous skeleton arose as simple branchial bars
+  and a simple cranio-facial skeleton, and also that the parenchymatous variety of cartilage
+  represents the embryonic form. Word for word, the early embryonic stage of the vertebrate skeleton
+  closely resembles the stage reached in the arthropod, as shown by Limulus, and again records,
+  unmistakably, the past history of the vertebrate.</p>
+
+  <p>So, too, with the whole of the prosomatic region; the situation of the old mouth, the manner in
+  which the nose of the cephalaspidian fishes arose from the palæostracan, are all shown with vivid
+  clearness by Kupffer's investigations of the early stage of Ammoc&#x0153;tes, while at the same
+  time the closure of the oral cavity by the septum shows how the oral chamber was originally
+  bounded by the operculum. Nay, further, the very formation of this chamber embryologically was
+  brought about by the forward growth of the lower lip, just as it must have been if the chilaria
+  grew forward to form the metastoma.</p>
+
+  <p>So, too, the study of the embryo teaches that the branchiæ arise as <span class="pagenum"
+  id="page459">{459}</span>ingrowths, that the heart arises as two longitudinal veins, just as the
+  theory supposes from the facts provided by Limulus and the scorpions. No indication of the origin
+  of the thyroid gland is given by the study of its structure in any adult vertebrate, but in the
+  larval form of the lamprey there is still preserved for us a most graphic record of its past
+  history.</p>
+
+  <p>The close comparisons which it is possible to make between the eye-muscles of the vertebrate
+  and the recti muscles of the scorpion group on the one hand, and between the pituitary and coxal
+  glands on the other, are based upon, or at all events are strikingly confirmed by, the study of
+  the c&#x0153;lomic cavities and the origin of these muscles in the two groups. In fact the
+  embryological evidence of the double segmentation in the head and the whole nature of the cranial
+  segments is one of the main foundation-stones on which the whole of my theory rests.</p>
+
+  <p>So it is throughout. Turn to the excretory organs&mdash;it is not the kidney of the adult
+  animal which leads direct to the excretory organs of the primitive arthropod, but the early
+  embryonic origin of that kidney.</p>
+
+  <p>So far from having put forward a theory which runs counter to the principles of embryology, I
+  claim to have vindicated the great Law of Recapitulation which is the foundation-stone of
+  embryological principles. My theory is largely based upon embryological facts, and its strength
+  consists in the manner in which it links together into one harmonious whole, the facts of
+  Embryology, Palæontology, Anatomy, and Physiology. Why, then, is it possible to assert that my
+  theory disregards the principles of embryology, when, as we have seen, embryology is proclaiming
+  as loudly as possible how the vertebrate arose? In my opinion, it is because the embryologists
+  have to a large extent gone wrong in their fundamental principles, and have attached more weight
+  to these faulty fundamental principles than to the obvious facts which, looked at thoughtfully,
+  could not have failed to suggest a doubt as to the correctness of these 'principles.'</p>
+
+  <p>The current laws of embryology upon which such weight is laid are based on the homology of the
+  germinal layers in all Metazoa, and state that in all cases after segmentation is finished a
+  blastula is formed, from which there arises a gastrula, formed of an internal layer, the
+  hypoblast, and an external layer, the epiblast; subsequently <span class="pagenum"
+  id="page460">{460}</span>between these arises a third layer, the mesoblast. These layers are
+  strictly morphological conceptions, and are stated to be homologous in all cases, so that the
+  hypoblast of one animal must be homologous to the hypoblast of another. In order, therefore, to
+  compare two adult animals for the purpose of finding kinship between them, it is necessary to find
+  whether parts such as the gut, which in both cases have the same function, arise from the same
+  germinal layer in the embryo. We can, in fact, have no certainty of kinship, even although the two
+  animals are built up as far as the adult state is concerned on a remarkably similar plan, unless
+  we can study their respective embryos and find out what parts arise from the hypoblast and what
+  from the epiblast. The homology of the germinal layers constitutes in all cases of disputed
+  relationship the court of final appeal. A new gut, therefore, in any animal can only be formed
+  from hypoblast, and any theory, such as that advocated in this book, which deals with the
+  formation of a new gut, and does not form that gut from pre-existing hypoblast, must of necessity
+  be wrong and needs no further consideration.</p>
+
+  <p>Such is the result of current conceptions&mdash;conceptions which to be valid must be based
+  upon an absolutely clear morphological definition of the formation of the germinal layers, a
+  definition not based on their subsequent history and function, but determined solely by the
+  uniformity of the manner of their origin.</p>
+
+  <p>What, then, is a germinal layer? How can we identify it when it first arises? What is the
+  morphological criterion by which hypoblast can be distinguished from epiblast, or mesoblast from
+  either?</p>
+
+  <p>This is the question put by Braem, in an admirable series of articles in the <i>Biologisches
+  Centralblatt</i>, and is one that must be answered by every worker who bases his views of the
+  process of evolution upon embryological investigation. As Braem points out, the germinal layers
+  are definable either from a morphological or physiological standpoint. In the one case they must
+  arise throughout on the same plan, and whatever be their fate in the adult, they must form at an
+  early stage structures strictly homologous in all animals. In the other case the criterion is
+  based on function, and the hypoblast, for instance, is that layer which is found afterwards to
+  form the definitive alimentary canal. There is no longer any morphological homology; such layers
+  are analogous; they may be, but are not necessarily, homologous. Braem gives a sketch of the
+  history of the views held on <span class="pagenum" id="page461">{461}</span>the germinal layers,
+  and shows how they were originally a purely physiological conception, and how gradually such
+  conception changed into a morphological one, with the result that what had up to that time been
+  looked upon as analogous structures became strictly homologous and of fundamental importance in
+  deciding the position of any animal in the whole animal series.</p>
+
+  <p>This change of opinion was especially due to the lively imagination of Haeckel, who taught that
+  the germinal layers of all Metazoa must be strictly homologous, because they were all derived from
+  a common ancestral stock, represented by a hypothetical animal to which he gave the name Gastræa;
+  an animal which was formed by the simple invagination of a part of the blastula, thus giving rise
+  to the original hypoblast and epiblast, and he taught that throughout the animal kingdom the
+  germinal layers were formed by such an invagination of a part of the blastula to form a simple
+  gastrula. If further investigation had borne out Haeckel's idea, if therefore the hypoblast was in
+  all cases formed as the invagination of a part of a single-layered blastula, then indeed the dogma
+  of the homology of the germinal layers would be on so firm a foundation that no speculation which
+  ran counter to it could be expected to receive acceptance; but that is just what has not taken
+  place. The formation of the gastrula by simple invagination of the single-layered blastula is the
+  exception, not the rule, and, as pointed out by Braem, is significantly absent in the earliest
+  Metazoa; in those very places where, on the Gastræa theory, it ought to be most conspicuous.</p>
+
+  <p>Braem discusses the question most ably, and shows again and again that in every case the true
+  criterion upon which it is decided whether certain cells are hypoblastic or not is not
+  morphological but physiological. The decision does not rest upon the answer to the question, Are
+  these cells in reality the invaginated cells of a single-celled blastula? but to the question, Do
+  these cells ultimately form the definitive alimentary canal? The decision is always based on the
+  function of the cells, not on their morphological position. Not only in Braem's paper, but
+  elsewhere, we see that in recent years the physiological criterion is becoming more and more
+  accepted by morphologists. Thus Graham Kerr, in his paper on the development of Lepidosiren, says:
+  "It seems to me quite impossible to define a layer as hypoblastic except by asking one or other of
+  the two questions: (1) Does it form the lining of an archenteric cavity? and (2) <span
+  class="pagenum" id="page462">{462}</span>Does it become a certain part of the definitive
+  epithelial lining of the gut?"</p>
+
+  <p>The appearance of Braem's paper was followed by a criticism from the pen of Samassa, who agrees
+  largely with Braem, but thinks that he presses the physiological argument too far. He considers
+  that morphological laws must exist for the individual development as well as for the phylogenetic,
+  and finishes his article with the following sentence, a sentence in which it appears to me he
+  expresses what is fast becoming the prevailing view: "Mit dem Satz, den man mitunter lesen kann:
+  'es muss doch auch für die Ontogenie allgemeine Gesetze geben' kann leicht Missbrauch getrieben
+  werden; diese allgemeinen Gesetze giebt es wohl, aber sie liegen nicht auf flacher Hand und bis zu
+  ihrer Erkenntnis hat es noch gute Wege; das eine kann man aber wohl heute schon sagen, die
+  Keimblätterlehre gehört zu diesen allgemeinen Gesetzen nicht."</p>
+
+  <p>I conclude, then, that we ought to go back to a time previous to that of Haeckel and ask
+  ourselves seriously the question, When we lay stress on the germinal layers and speak of this or
+  that organ arising from this or that germinal layer, are we thereby adding anything to the
+  knowledge that we already possess from the study of the anatomy and physiology of the adult body?
+  If by hypoblast we only mean the internal surface or alimentary canal and its glands, etc., and by
+  epiblast we mean the external surface or skin and its glands, etc., while mesoblast indicates the
+  middle structures between the other two, then I fail to see what advantages we obtain by using
+  Greek terms to express in the embryo what we express in English in the adult.</p>
+
+  <p>The evidence given by Braem, and it could be strengthened considerably, is conclusive against
+  the morphological importance of the theory of the germinal layers, and transfers the fundamental
+  importance of the early embryonic formation, from that of a three-layered embryo to that of a
+  single-layered embryo&mdash;the blastula&mdash;from which, in various ways, the adult animal has
+  arisen.</p>
+
+  <p>The derivation of both arthropod and vertebrate from such a single-layered animal is perfectly
+  conceivable, even though the gut of the latter is not homologous with the gut of the former. We
+  have seen that the teachings of embryology, as far as its later stages are concerned, afford one
+  of the main supports upon which this theory rests. What, therefore, is required to complete the
+  story is the way <span class="pagenum" id="page463">{463}</span>in which these later stages arise
+  from the blastula stage; here, as in all cases, the ontogenetic laws must be in harmony with the
+  phylogenetic; of the latter the most important is the steady development of the central nervous
+  system for the upward progress of the animal race. The study of comparative anatomy indicates the
+  central nervous system, not the gut, as the keystone of the edifice. So, also, it must be with
+  ontogeny; here also the central factor in the formation of the adult from the blastula ought to be
+  the formation of the central nervous system, not that of the gut.</p>
+
+  <p>Such, it appears to me, is the case, as may be seen from the following considerations.</p>
+
+  <p>The study of the development of any animal can be treated in two ways: either we can trace back
+  from the adult to the very beginning in the ovum, or we can trace forward from the fertilized egg
+  to the adult. Both methods ought to lead to the same result; the difference is, that in the first
+  case we are passing from the more known to the less known, and are expressing the unknown in terms
+  of the known. In the second case we are passing from the less known to the more known, and are
+  expressing the known in speculative terms, invented to explain the unknown. What has just been
+  said with respect to the germinal layers means that, however much we may study the embryo and try
+  to express the adult in terms of it, we finally come back to the first way of looking at the
+  question, and, starting with the adult, trace the continuity of function back to the first
+  formation of cells having a separate function.</p>
+
+  <p>Let us, then, apply this throughout, and see what are the logical results of tracing back the
+  various organs and tissues from the adult to the embryo.</p>
+
+  <p>The adult body is built up of different kinds of tissues, which fall naturally, from the
+  standpoint of physiology, into groups. Such groups are, in the first place&mdash;</p>
+
+  <div class="bq1 sp2">
+    <p>1. All those tissues which are connected with the central nervous system, including in that
+    group the nervous system itself.</p>
+    <p class="sp0">2. All those tissues which have no connection with the nervous system.</p>
+  </div>
+
+  <p>In the second group the physiologist places all germinal cells, all blood- and
+  lymph-corpuscles, all plasma-cells and connective tissue and its derivatives&mdash;in fact, all
+  free-living cells, whether in a free state or in a quiescent, so to speak encysted, condition,
+  such as is <span class="pagenum" id="page464">{464}</span>found in connective tissue. In the first
+  group the physiologist recognizes that the central nervous system is connected with all muscular
+  tissues, whether striped or unstriped, somatic or splanchnic, and that such connection is of an
+  intimate character. Further, all epithelial cells, either of the outer or inner surfaces, whether
+  forming special sense-organs and glands, such as the digestive and sweat-glands, or not, are
+  connected with the nervous system. Besides these structures, there is another set of organs as to
+  which we cannot speak definitely at present, which must be considered separately, viz. all the
+  cells, together with their derived organs, which line the body-spaces. Whatever may be the
+  ultimate decision as to this group of cells, it must fall into one or other of the two main
+  groups.</p>
+
+  <p>The members of these two groups are so interwoven with one another that either, if taken alone,
+  would still give the form of the body, so that, in a certain sense, we can speak of the body as
+  formed of two syncytia, separate from each other, but interlaced, of which the one forms a
+  continuous whole by means of cells connected together by a fluid medium or by solid threads formed
+  in such fluid medium, while the other does not form a syncytium in the sense that any cell of one
+  kind may be connected with any cell of another kind, but a syncytium of which all the different
+  elements are connected together only through the medium of the nervous system.</p>
+
+  <p>If we choose to speak of the body as made up of two syncytia in this way, we must at the same
+  time recognize the fundamental difference in character between them. In the one case the elements
+  are connected together only by what may be called non-living material; there is no direct
+  metabolic activity caused by the action of one cell over a more distant cell in consequence of
+  such connection, it is not a true syncytium; in the second case there is a living connection, the
+  metabolism of one part is directly influenced by the activity of another, and the whole utility of
+  the system depends upon such functional connection.</p>
+
+  <p>The tissues composing this second syncytium may be spoken of as the master-tissues of the body,
+  and we may express this conception of the building up of the body of the higher Metazoa by saying
+  that it is composed of a syncytial host formed of the master-tissues, which contains within its
+  meshes a system of free-living cells, none of which have any connection with the nervous system.
+  This syncytial <span class="pagenum" id="page465">{465}</span>host is in the adult composed of a
+  number of double elements, a nerve-cell element, and an epithelial element, such as muscle-cell,
+  gland-cell, etc., connected together by nerves; and if such connection is always present as we
+  pass from the adult to the embryo, if there is no period when, for example, the neural element
+  exists alone free from the muscle-cell, no period when the two can be seen to come together and
+  join, then it follows that when the single-layered blastula stage is reached, muscle-cell and
+  nerve-cell must have fused together to form a neuro-muscular cell. Similarly with all the other
+  neuro-epithelial organs; however far apart their two components may be in the adult, they must
+  come together and fuse in the embryo to form a neuro-epithelial element.</p>
+
+  <p>The close connection between muscle and nerve which has always been recognized by
+  physiologists, together with the origin of muscle from a myo-epithelial cell in Hydra and other
+  C&#x0153;lenterata, led the older physiologists to accept thoroughly Hensen's views of the
+  neuro-epithelial origin of all tissues connected with the central nervous system. Of late years
+  this conception has been largely given up owing to the statement of His that the nervous system
+  arises from a number of neuroblasts, which are entirely separate cells, and have at first no
+  connection with muscle-cells or any peripheral epithelial cells, but subsequently, by the
+  outgrowing of an axial fibre, find their way to the muscle, etc., and connect with it. I do not
+  think that His' statement by itself would have induced any physiologist to give up the conception
+  of the intimate connection of muscle and nerve, if the work of Golgi, Ramón y Cajal, and others
+  had not brought into prominence the neurone theory, <i>i.e.</i> that each element of the central
+  nervous system is an independent element, without real connection with any other element and
+  capable of influencing other cells by contact only. These two statements, emanating as they did
+  from embryological and anatomical studies respectively, have done much to put into the background
+  Hensen's conceptions of the syncytial nature of the motor, neural, and sensory elements, which
+  make up the master-tissues of the body, and have led to the view that all the elements of the body
+  are alike, in so far as they are formed of separate cells each leading an independent existence,
+  without any real intimate connection with each other.</p>
+
+  <p>The further progress of investigation is, it seems to me, bringing us back to the older
+  conception, for not only has the neuroblast theory <span class="pagenum"
+  id="page466">{466}</span>proved very difficult for physiologists to accept, but also Graham Kerr,
+  in his latest papers on the development of Lepidosiren, has shown that there is continuity between
+  the nerve-cell and the muscle-cell from the very first separation of the two sets of elements; in
+  fact, Hensen is right and His wrong in their respective interpretation of the earliest stages of
+  the connection between muscle and nerve. So also, it seems to me, the intimate connection between
+  the metabolism of the gland-cell, as seen in the submaxillary gland, and the integrity of its
+  nervous connection implies that the connection between nerve-cell and gland-cell is of the same
+  order as that between nerve-cell and muscle-cell. Graham Kerr also states in his paper that from
+  the very commencement there is, he believes, continuity between nerve-cell and epithelial cell,
+  but so far he has not obtained sufficiently clear evidence to enable him to speak positively on
+  this point.</p>
+
+  <p>Further, according to the researches of Anderson, the cells of the superior cervical ganglion
+  in a new-born animal will continue to grow healthily as long as they remain connected with the
+  periphery, even though entirely separated from the central nervous system by section of the
+  cervical sympathetic nerve, and conversely, when separated from the periphery, will atrophy, even
+  though still connected with the central nervous system. So, also, on the sensory side, Anderson
+  has shown that the ganglion-cells of the posterior root-ganglion will grow and remain healthy
+  after separation of the posterior roots in a new-born animal, but will atrophy if the peripheral
+  nerve is cut, even though they are still in connection with the central nervous system. Further,
+  although section of a posterior root in the new-born animal does not affect the development of the
+  nerve-cells in the spinal ganglion, and of the nerve-fibres connecting the posterior root-ganglion
+  with the periphery, it does hinder the development of that part of the posterior root connected
+  with the spinal ganglion.</p>
+
+  <p>These experiments of Anderson are of enormous importance, and force us, it seems to me, to the
+  same conclusion as that to which he has already arrived. His words are (p. 511): "I suggest,
+  therefore, that the section of peripheral nerves checked the development of motor and sensory
+  neurones, not because it blocked the passage of efferent impulses in the first case and the
+  reception of stimuli from the periphery in the second, but for the same reason in both cases,
+  <span class="pagenum" id="page467">{467}</span>viz. that the lesion disturbed the chemico-physical
+  equilibrium of an anatomically continuous (neuro-muscular or neuro-epithelial) chain of cells, by
+  separating the non-nervous from the nervous, and that the changes occurring in denervated muscle,
+  which I shall describe later (and possibly those in denervated skin), are in part due to the
+  reciprocal chemico-physical disturbance effected in these tissues by their separation from the
+  nervous tissues; also that the section of the posterior roots checked the development of those
+  portions of them still attached to the spinal ganglia, because the chemico-physical equilibrium in
+  those processes is maintained not only by the spinal ganglion-cells, but also by the intra-spinal
+  cells with which these processes are anatomically continuous."</p>
+
+  <p>What is seen so strikingly in the new-born animal can be seen also in the adult, and in
+  Anderson's paper references are given to the papers of Lugaro and others which lead to the same
+  conclusion.</p>
+
+  <p>These experiments seem to me distinctly to prove that the connection between the elements of
+  the peripheral organ and the proximate neurone is more than one of contact.</p>
+
+  <p>We can, however, go further than this, for, apart from the observations of Apathy, there is
+  direct physiological evidence that the vitality of other neurones besides the terminal neurones is
+  dependent upon their connection with the peripheral organ, even though their only connection with
+  the periphery is by way of the terminal neurone. Thus, as is seen from Anderson's experiments,
+  section of the cervical sympathetic nerve in a very young animal causes atrophy of many of the
+  cells in the corresponding intermedio-lateral tract, cells which I supposed gave origin to all the
+  vaso-constrictor, pilomotor, and sweat-gland nerves. A still more striking experiment given by
+  Anderson is the effect of the removal of the periphery upon the medullation of those efferent
+  fibres which arise from these same spinal cells, for, as he has shown, section of the nerves from
+  the superior cervical ganglion to the periphery in a very young animal delays the medullation in
+  the fibres of the cervical sympathetic&mdash;that is, in preganglionic fibres which are not
+  directly connected with the periphery but with the terminal neurones in the superior cervical
+  ganglion. So also on the afferent side a sufficiently extensive removal of sensory field will
+  cause atrophy of the cells of Clarke's column, so that, just as in the case of the primary
+  neurones, <span class="pagenum" id="page468">{468}</span>the secondary neurones show by their
+  degenerative changes the importance of their connection with the peripheral organs.</p>
+
+  <p>In this way I can conceive the formation of a series of both efferent and afferent relays in
+  the nervous system by proliferation of the original neural moiety of the neuro-epithelial
+  elements, every one of which is dependent upon its connection with the peripheral epithelial
+  elements for its due vitality, the whole system being a scheme for co-ordination of a larger and
+  larger number of peripheral elements. Thus the cells of the vasomotor centre are in connection
+  with the whole system of segmental vaso-constrictor centres in the lateral horns of the thoracic
+  region of the cord, so that to cause atrophy of these cells a very extensive removal of the
+  vascular system would be required. Each of the segmental centres in the cord supplies a number of
+  sympathetic segments, the connection with all of which would have to be cut in order to ensure
+  complete removal of the connection of each of its cells with the periphery, and finally each of
+  the cells in the sympathetic ganglia supplies a number of peripheral elements, all of which must
+  be removed to ensure complete severance.</p>
+
+  <p>Thus, if we take any arbitrary number, such as 4, to represent the number of peripheral
+  organ-elements with which each terminal neurone is connected, and suppose that each neurone has
+  proliferated into sets of 4, then a cell of the third order, such as a cell of the vasomotor
+  centre, would require the removal of 64 peripheral elements to cause its complete separation from
+  the periphery, one of the second order (a cell of the thoracic lateral horn) 16 elements, one of
+  the first order (a cell of a sympathetic ganglion) 4 elements.</p>
+
+  <p>Such intimate inter-relationship between the neurones, both afferent and efferent, and their
+  corresponding peripheral organs does not imply that all nerve-cells are necessarily as closely
+  dependent upon some connection with the periphery, for just as the proliferation of epithelial or
+  muscle-cells forms an epithelial or muscular sheet, the elements of which are so loosely, if at
+  all, connected together that their metabolism is in no way dependent upon such connection, so also
+  a similar proliferation of the neural elements may form connections between nerve-cell and
+  nerve-cell of a similarly loose nature.</p>
+
+  <p>It is this kind of proliferation which, in my opinion, would bind together the separate relays
+  of efferent and afferent neurones, and <span class="pagenum" id="page469">{469}</span>so give
+  origin to reflex actions at different levels. Such neurones would not be in the direct chain of
+  either the afferent or efferent neurones, and so not directly connected with the periphery, and
+  could therefore be removed without affecting the vitality of either the efferent or afferent chain
+  of neurones. In other words, the vitality of the cells on the efferent side ought not to be
+  dependent on the integrity of the reflex arc. With regard to the development of the anterior
+  roots, Anderson has shown that this is the case, for section of all the posterior roots conveying
+  afferent impulses from the lower limb in a new-born animal does not hinder the normal development
+  of the anterior roots supplying that limb. Also Mott, who originally thought that section of all
+  the posterior roots to a limb caused atrophy of the corresponding anterior roots, has now come to
+  the same conclusion as other observers, and can find no degeneration on the efferent side due to
+  removal of afferent impulses.</p>
+
+  <p>Again, the process of regeneration after section of a nerve is not in favour of the neuroblast
+  theory. There is no evidence that the cut end of a nerve can grow down and attach itself to a
+  muscular or epithelial element without the assistance of a nerve tube down which to grow. When the
+  cut nerves connected with the periphery degenerate, that applies only to the axis-cylinder and the
+  medullary sheath, not to the neurilemma; the connective tissue elements remain alive and form a
+  tube into which the growing axon finds its way, and so is conducted to the end-plate or end-organ
+  of the peripheral structure.</p>
+
+  <p>Possibly, as suggested by Mott and Halliburton, the products of degeneration of the
+  axis-cylinder and medullary sheath stimulate these connective tissue sheath-cells into active
+  proliferation, and so bring about the great multiplication of cells arranged as cell-chains, which
+  are so often erroneously spoken of as forming the young nerves. These sheath-cells are then
+  supposed to re-form and secrete a pabulum which is important for the process of regeneration of
+  the down-growing axis-cylinder and medullary sheath. Without such pabulum regeneration does not
+  take place, as is seen in the central nervous system, where the sheath of Schwann is absent.</p>
+
+  <p>Again, it is becoming more and more doubtful whether the peripheral terminations of nerves are
+  ever really free. As far as efferent nerves are concerned the nervous element may entirely <span
+  class="pagenum" id="page470">{470}</span>predominate over the muscular or glandular, as in the
+  formation of the electric organs of the Torpedo and Malapterurus, but still the final effect is
+  produced by the alteration of the muscle or gland-cell. On the afferent side especially free
+  nerve-terminations are largely recognized, or, as in Barker's book, nerves are spoken of as
+  arising in connective tissue. Thus the numerous kinds of special sense-organs, such as Pacinian
+  bodies, tendon-organs, genital corpuscles, etc., are all referred to by Barker under the heading
+  of "sensory nerve beginnings in mesoblastic tissues." Yet the type of these organs has been known
+  for a long time in the shape of Grandry's corpuscles or the tactile corpuscles in the duck's bill,
+  where it has been proved that the nerve terminates in special large tactile cells derived from the
+  surface-epithelium.</p>
+
+  <p>So also with all the others, further investigation tends to put them all in the same category,
+  all special sensory organs originating from a localized patch of surface-epithelium. Thus Anderson
+  has shown me in his specimens how the young Pacinian body is composed of rows of epithelial cells,
+  into each of which a twig from the nerve passes. He has also shown me how, in the case of the
+  tendon-organ, each nerve-fibre passes towards the attachment of the tendon and then bends back to
+  supply the tendon-organ, thus indicating, as he suggests, how the nest of epithelial cells has
+  wandered inwards from the surface to form the tendon-organ. Again, Meissner's corpuscles and
+  Herbst's corpuscles are evidently referable to the same class as those of Grandry and Pacini.</p>
+
+  <p>Yet another instance of the same kind is to be found in the chromatophores of the frog and
+  other animals which are under the influence of the central nervous system and yet have been
+  supposed by various observers to be pigmented connective tissue cells. The most recent work of Leo
+  Loeb and others has conclusively shown that such cells are invariably derived from the
+  surface-epithelium.</p>
+
+  <p>Finally, in fishes we find the special sense-organs of the lateral line and other accessory
+  sensory organs, all of which are indisputably formed from modified surface epithelial cells.</p>
+
+  <p>The whole of this evidence seems to me directly against Barker's classification of sensory
+  nerve-beginnings in mesoblastic tissues; in none of these cases are we really dealing with free
+  nervous tissue alone, the starting point is always a neuro-epithelial couple.</p>
+
+  <p>We may then, I would suggest, look upon the adult as formed of <span class="pagenum"
+  id="page471">{471}</span>a neural syncytium, which we may call the host, which carries with it in
+  its meshes a number of free cells not connected with the nervous system. If, then, we confine our
+  attention to the host and trace back this neural syncytium to its beginnings in the embryo, we see
+  that, from the very nature of the neuro-epithelial couple, each epithelial moiety must approach
+  nearer and nearer to its neural moiety, until at last it merges with it; the original
+  neuro-epithelial cell results, and we must obtain, as far as the host is concerned, a
+  single-layered blastula as the origin of all Metazoa. It follows, further, that there must always
+  be continuity of growth in the formation of the host, <i>i.e.</i> in the formation of the
+  neuro-epithelial syncytium; that therefore cells which have been previously free cannot settle
+  down and take part in its formation, as, for instance, in the case of the formation of any part of
+  the gut-epithelium or of muscle-cells from free-living cells.</p>
+
+  <p>Further, since the neural moiety is the one element common to all the different factors which
+  constitute the host, it follows that the convergence of each epithelial moiety to the neural
+  moiety, as we pass from the adult to the embryo, is a convergence of all outlying parts to the
+  neural moiety, <i>i.e.</i> to the central nervous system, if there is a concentrated nervous
+  system. Conversely, in the commencing embryo the place from which the spreading out of cells takes
+  place, <i>i.e.</i> from which growth proceeds, must be the position of the central nervous system,
+  if the nervous system is concentrated. If the nervous system is diffuse, and forms a general
+  sub-epithelial layer, then the growth of the embryo would take place over the whole surface of the
+  blastula.</p>
+
+  <p>Turning now to the consideration of the second group of tissues, those that are not connected
+  with the central nervous system, we find that they include among them such special cells as the
+  germinal cells, free cells of markedly phagocytic nature, and cells which were originally free and
+  phagocytic, but have settled down to form a supporting framework of connective tissue, and are
+  known as plasma-cells. In the embryo we find also in many cases free cells in the yolk, forming
+  more or less of a layer, which function as phagocytes and prepare the pabulum for the fixed cells
+  of the growing embryo; these cells are known by the name of vitellophags, and in meroblastic
+  vertebrate eggs form somewhat of a layer known by the name of periblast. Such cells must be
+  included in the second group, and, <span class="pagenum" id="page472">{472}</span>indeed, have
+  been said again and again to give origin to the free-living blood-corpuscles of the adult. In
+  other cases they are said to disintegrate after their work is done.</p>
+
+  <p>In the adult the free-living lymphocytes and hæmocytes reproduce themselves from already
+  existing free-living cells, but as we pass back to the embryo there comes a time, comparatively
+  late in the history of the embryo, when such free-living cells are not found in the fluids of the
+  body, and they are said to arise from the proliferation and setting free of cells which form a
+  lining epithelium. Such formation of leucocytes has been especially described in connection with
+  the lining epithelium of the c&#x0153;lomic cavities, as stated in Chapter XII., so that
+  anatomists look upon the origin of these free cells as being largely from the c&#x0153;lomic
+  epithelium, or mesothelium, as Minot calls it.</p>
+
+  <p>Then, again, the free cells which form the germinal cells can be traced back to a germinal
+  epithelium, which also is part of the c&#x0153;lom. Thus the suggestion arises that in the embryo
+  a cellular lining is formed to a c&#x0153;lomic cavity (mesothelium) composed of cells which have
+  no communication with the nervous system, and are capable of a separate existence as free
+  individuals, either in the form of germinal cells or of lymphocytes, hæmocytes, and plasma-cells,
+  so that these latter free cells may be considered as living an independent existence in the body,
+  and ministering to it in the same sense as the germ-cells live an independent existence in the
+  body. Again, the function of this mesothelium apart from the germ-cell is essentially excretory
+  and phagocytic. It is the cells of the excretory organs as well as the lymphocytes which pick up
+  carmine-grains when injected. It is the cells of the modified excretory organs, as mentioned in
+  Chapter XII., which, according to Kowalewsky and others, give origin to the free leucocytes.</p>
+
+  <p>We see, then, that the conception of a syncytial neuro-epithelial host holding in its meshes a
+  number of free cells leads directly to the questions: What is the c&#x0153;lom? To which category
+  does its lining membrane belong? and further, also, What is the origin of these free cells?</p>
+
+  <p>The Metazoa have been divided into two great groups&mdash;those which possess a c&#x0153;lom
+  (the C&#x0153;lomata; Lankester's C&#x0153;lomoc&#x0153;la) and those which do not
+  (C&#x0153;lenterata; Lankester's Enteroc&#x0153;la). As an example of the latter we may take
+  Hydra, because it is a very <span class="pagenum" id="page473">{473}</span>primitive form, and
+  because its development has been carefully worked out recently by Brauer.</p>
+
+  <p>In Hydra we find a dermal layer of cells and an inner layer of cells separated by a gelatinous
+  mass known as mesogl&#x0153;a; in this mass between the dermal and inner layers scattered cells
+  are found, the interstitial cells. Now, according to Brauer the position of the germ in Hydra is
+  the interstitial cell-layer. One cell of the ovarium becomes the egg-cell, the others have their
+  substance changed into yolk-grains, forming the so-called pseudo-cells, and as such afford pabulum
+  to the growing egg-cell. Thus we see that in between the dermal and gastral layer of cells a third
+  layer of cells is found, composed of free living germ-cells, some of which, by the formation of
+  yolk-granules, become degraded into pabulum for their more favoured kinsfolk. These interstitial
+  cells are said to arise from the dermal layer, or ectoderm, but clearly, as in other cases,
+  germ-cells constitute a class by themselves and cannot be spoken of as originating from
+  ectoderm-cells or from hypoderm-cells.</p>
+
+  <p>So also in Porifera, Minchin states: "In addition to the collared cells of the gastral layer,
+  and the various cell-elements of the dermal layer, the body-wall contains numerous wandering cells
+  or am&#x0153;bocytes, which occur everywhere among the cells and tissues. Though lodged
+  principally in the dermal layer, they are not to be regarded as belonging to it, but as
+  constituting a distinct class of cells by themselves. They are concerned probably with the
+  functions of nutrition and excretion, and from them arise the genital products." Further (p. 31):
+  "At certain seasons some of these cells become germ-cells; hence the wandering cells and the
+  reproductive cells may be included together under the general term archæocytes." Also (p. 51):
+  "The mesogl&#x0153;a is the first portion to appear as a structureless layer between the dermal
+  and gastral epithelia, and is probably a secretion of the former."</p>
+
+  <p>He also points out that in these, the very lowest of the Metazoa, the separate origin of these
+  archæocytes can be traced back to a very early period of embryonic life. Thus in <i>Clathrina
+  blanca</i> the ovum undergoes a regular and total cleavage, resulting in the formation of a hollow
+  ciliated blastula of oval form. At one point, the future posterior pole of the larva, are a pair
+  of very large granular cells with vesicular nuclei, which represent undifferentiated blastomeres
+  and are destined to give rise to the archæocytes, and, therefore, also to the <span
+  class="pagenum" id="page474">{474}</span>sexual cells of the adult. Thus, as he says, from the
+  very earliest period a distinction is made between the "tissue-forming" cells (my syncytial host)
+  and the archæocytes.</p>
+
+  <p>We see, then, that the origin of all these free-living cells can be traced back to the very
+  earliest of the Metazoa. Here between the dermal and gastral layers a gelatinous material, the
+  mesogl&#x0153;a is secreted by these layers. This material is non-living, non-cellular. In it live
+  free cells which may either be germ-cells, am&#x0153;bocytes, or 'collencytes' (connective tissue
+  cells). If this mesogl&#x0153;a were a fluid secretion, then we should have a tissue of the nature
+  of blood or lymph; if it were solid, then we should have the foundation of connective tissue,
+  cartilage, and bone.</p>
+
+  <p>From this primitive tissue it is easy to see how the special elements of the vascular,
+  lymphatic, and skeletal tissues gradually arose, the matrix being provided by the cells of the
+  syncytial host and the cellular elements by the archæocytes. In fact, we have no right to speak of
+  these lowest members of the Metazoa as not being triploblastic, as possessing nothing
+  corresponding to mesoblast, for in these free cells in the mesogl&#x0153;a we have the origin of
+  the mesenchyme of the higher groups. Thus Lankester, talking of mesenchyme, says: "I think we are
+  bound to bring into consideration here the existence in many C&#x0153;lentera of a tissue
+  resembling the mesenchyme of C&#x0153;lomoc&#x0153;la. In <span class="correction"
+  title="Original reads 'Scyphomedus&#x0153;'">Scyphomedusæ</span>, in Ctenophora, and in Anthozoa,
+  branched fixed and wandering cells are found in the mesogl&#x0153;a which seem to be the same
+  thing as a good deal of what is distinguished as mesenchyme in C&#x0153;lomoc&#x0153;la. These
+  appear to be derived from both the primitive layers; some produce spicules, others fibrous
+  substance, others again seem to be am&#x0153;bocytes with various functions. It appears to be
+  probable that, though it may be necessary to distinguish other elements in it, the mesenchyme of
+  C&#x0153;lomoc&#x0153;la is largely constituted by cells, which are the mother-cells of the
+  skeletotrophic group of tissues, and are destined to form connective tissues, blood-vessels, and
+  blood."</p>
+
+  <p>Thus we see that the earliest Metazoa were composed of a dermal and gastral epithelium, with a
+  sub-epithelial nervous system connecting the parts together, which formed, as it were, a host,
+  carrying around free living cells of varying function, all of which may be looked on as derived
+  from archæocytes, <i>i.e.</i> germ-cells. From these the c&#x0153;lomatous animals arose, and here
+  also we find, according to <span class="pagenum" id="page475">{475}</span>present-day opinion,
+  that the c&#x0153;lom arose in the first place in the very closest connection with the germ-cells
+  or gonads. Thus Lankester, in his review of the history of the c&#x0153;lom, states<span
+  class="wnw">:&mdash;</span></p>
+
+  <p>"The numerous embryological and anatomical researches of the past twenty years seem to me to
+  definitely establish the conclusion that the c&#x0153;lom is primarily the cavity, from the walls
+  of which the gonad cells (ova or spermata) develop, or which forms around those cells. We may
+  suppose the first c&#x0153;lom to have originated by a closing or shutting off of that portion of
+  the general archenteron of Enteroc&#x0153;la (C&#x0153;lentera), in which the gonads developed as
+  in Aurelia or as in Ctenophora. Or we may suppose that groups of gonad mother cells, having
+  proliferated from the endoderm, took up a position between it and the ectoderm, and there acquired
+  a vesicular arrangement, the cells surrounding the cavity in which liquid accumulated.</p>
+
+  <p>"The c&#x0153;lom is thus essentially and primarily (as first clearly formulated by Hatschek)
+  the perigonadial cavity or gonoc&#x0153;l, and the lining cells of gonadial chambers are
+  c&#x0153;lomic epithelium. In some few groups of C&#x0153;lomoc&#x0153;la the c&#x0153;loms have
+  remained small and limited to the character of gonoc&#x0153;ls. This seems to be the case in the
+  Nemertina, the Planarians, and other Platyhelmia. In some Planarians they are limited in number,
+  and of individually large size; in others they are numerous."</p>
+
+  <p>When Lankester says that "the lining cells of gonadial chambers are c&#x0153;lomic epithelium,"
+  that is equivalent to saying that the lining cells of the c&#x0153;lom form an epithelium which
+  was originally gonadial, provided that, as seems to me most probable, his second suggestion, of
+  the c&#x0153;lom being formed from gonadial mother-cells which have taken up an intermediate
+  position between endoderm and ectoderm and there acquired a vesicular arrangement, is the true
+  one. It does not seem to me possible to conceive of the gonads arising from cells of the epiblast
+  or of the hypoblast, in the sense that such cells are differentiated cells belonging to a layer
+  with a definite meaning. When we consider that the gonad gives origin to the whole of a new
+  individual, that in the protozoan ancestors of the Metazoa their ultimate aim and object was the
+  formation of gonads, it seems a wrong conception to speak of the gonads as formed from cells
+  belonging either to the gut-wall or to the external epithelium. The gonads must stand in a
+  category by themselves; they represent a whole, <span class="pagenum"
+  id="page476">{476}</span>while the other cells represent only a part; they cannot therefore be
+  derived from the latter. They may, and indeed do, give rise to cells of a subordinate character,
+  but they cannot rightly be spoken of as derived from such cells. The very fact mentioned by
+  Lankester, that in the lowest c&#x0153;lomatous Metazoa, the Platyhelminthes, the c&#x0153;loms
+  are limited to the character of simple gonoc&#x0153;ls, strongly points to the conclusion that all
+  the c&#x0153;lomic cells were originally of the nature of gonadial cells, and therefore
+  free-living and independent of the rest of the cells of the body. Whether the germ-cells appear,
+  as in Hydra, to be derived from the ectoblast, or, as is usually stated, from the endoblast, in
+  neither case ought they to be classed with the internal or external epithelium; they are
+  germ-cells, and the epithelium which they form is neither epiblastic nor hypoblastic, but
+  germinal, forming originally a simple gonoc&#x0153;le, afterwards, in the higher forms, the
+  c&#x0153;lom with its cells of various function. Thus, to quote again from Lankester, "The
+  c&#x0153;lomic fluid and the c&#x0153;lomic epithelium, as well as the floating corpuscles derived
+  from that epithelium, acquire special properties and importance over and above the original
+  functions subservient to the maturation of the gonadial cells ... the most important developments
+  of the c&#x0153;lom are in connection with the establishment of an exit for the generative
+  products through the body-wall to the outer world, and further in the specialization of parts of
+  its lining epithelium for renal excretory functions."</p>
+
+  <p>Such exits led very early to the formation of c&#x0153;lomoducts, which are true outgrowths of
+  the c&#x0153;lom itself (p. 14): "The c&#x0153;lomoducts and the gonoc&#x0153;ls of which they are
+  a part, frequently acquire a renal excretory function, and may retain both the function of genital
+  conduits and of renal organs, or may, where several pairs are present (metamerized or segmented
+  animals), subserve the one function in some segments of the body, and the other function in other
+  segments."</p>
+
+  <p>The origin of the c&#x0153;lom and its derivatives from a germinal membrane, as suggested by
+  Lankester, appears to me most probable, and, if true, it carries with it conclusions of
+  far-reaching importance, for it necessitates that all the cells which line true c&#x0153;lomic
+  cavities, and their derivatives, belong to the category of free-living cells, and are not
+  connected with the nervous system. The cells in question are essentially those which line serous
+  cavities and those which form excretory glands such as the kidneys. In the latter organ we ought
+  especially to be able to obtain a clear answer to this question, for is <span class="pagenum"
+  id="page477">{477}</span>it not a gland which secretes into a duct and might therefore be expected
+  to be innervated in the same way as other secretory glands? Although there is a strong <i>primâ
+  facie</i> presumption in favour of the existence of renal secretory nerves, yet according to the
+  universal opinion of physiologists no evidence in favour of such nerves has hitherto been found;
+  all the phenomena of excretion of urine consequent on nerve stimulation are explicable by the
+  action of nerves on the renal vessels, not on the renal cells. Not only is the physiological
+  evidence negative up to the present time, but also, I think, the histological. On the one hand,
+  Retzius has failed to find nerve-connections with kidney-cells; on the other, Berkley has obtained
+  such evidence with the Golgi method, but failed entirely with methylene blue. I do not myself
+  think that the evidence of the Golgi method alone is sufficient without corroboration by other
+  methods, and, in any case, Berkley's evidence does not show the nerve-fibres terminating in the
+  kidney-cells, in the same way as can be shown by modern methods to exist in the case of epithelial
+  cells of the surface, etc. Quite recently another paper on this subject has appeared by Smirnow,
+  who appears to have obtained better results than those given by Berkley.</p>
+
+  <p>Apart from these physiological and histological considerations, this question is also dependent
+  upon the nature of the development of the excretory organs, for, according to Lankester, all
+  excretory organs may be divided into the two classes of nephridial organs and c&#x0153;lomostomes,
+  of which the former are largely derived from epiblast. We should, therefore, expect to find
+  secretory nerves to nephridial organs, though possibly not to c&#x0153;lomostomes. The kidneys of
+  the Mammalia are supposed to be true c&#x0153;lomostomes, although, according to Goodrich's
+  researches, the excretory organs in Amphioxus are solenocytes, <i>i.e.</i> true nephridia.</p>
+
+  <p>As to the lining epithelium of the peritoneal, pleural, and pericardial
+  cavities&mdash;<i>i.e.</i> the mesothelium&mdash;there is no definite evidence that these cells
+  are provided with nerves. Such surfaces are remarkably insensitive in the healthy condition, and
+  the pain in such cavities is essentially a pressure phenomenon and referable to special
+  sense-organs, such as Pacinian bodies, etc., rather than to the mesothelium itself.</p>
+
+  <p>These sense-organs are identical in structure with those in the skin, and, as Anderson has
+  shown, the nerves of these organs <span class="pagenum" id="page478">{478}</span>medullate at the
+  same time as those in the skin, and both obtain their medullary sheaths earlier than any other
+  nerves, whether afferent or efferent. However difficult it may be to explain this fact, only one
+  conclusion seems to me possible&mdash;these Pacinian bodies, like the skin Pacinians, originate
+  from a nest of surface epithelial cells, a conclusion which is extremely probable on my theory of
+  the origin of vertebrates, but not, as far as I can see, on any other.</p>
+
+  <p>At the present moment the weight of evidence is, to my mind, in favour of the lining
+  endothelium of the c&#x0153;lomic cavities being composed of free cells, unconnected with the
+  nervous system rather than the reverse, but I must confess that the question is undecided. If it
+  be true that the c&#x0153;lomic lining is partly enteroc&#x0153;lic and partly gonoc&#x0153;lic,
+  as Lankester teaches, then it would be natural that its cells should be in connection with the
+  nervous system, to some extent at all events. This view is, however, based on very slender
+  foundations. If the mesothelium is composed of cells capable of becoming free, it cannot give rise
+  to the skeletal muscles, and it cannot therefore be right to speak of the skeletal muscles as
+  derived from the lining cells of a part of the primary c&#x0153;lom. The phylogenetic history of
+  the musculature of the different animals points strongly to its intimate connection with and
+  derivation from surface epithelial cells rather than from c&#x0153;lomic mesothelial cells. Thus
+  in the c&#x0153;lenterates, as seen in Hydra, the muscular layer arises directly from a
+  modification of the surface epithelial cells; and right up to the annelids, even to the highest
+  form in the Polychæta, we still see it stated that the musculature, both circular and
+  longitudinal, arises from the ectoderm. In the Oligochæta and Hirudinea, according to Bergh, there
+  are five rows of teloblasts on each side, of which four are ectodermic and give rise to the
+  nerve-ganglia and the circular muscles, while one is mesoblastic and forms the nephridial organs
+  and the longitudinal muscles. (The latter statement is, according to Bergh, well known, and is not
+  particularly shown by him. These longitudinal muscle-bands always lie close against the nervous
+  system at their first formation, and may well have been derived in connection with it.)</p>
+
+  <p>It is apparently only in the Vertebrata that the lining cells of the c&#x0153;lomic cavity are
+  definitely stated to give origin to the body-musculature, and taking into account on the one hand
+  the evidence of Graham Kerr as to the intimate connection between nerve-cell and <span
+  class="pagenum" id="page479">{479}</span>muscle-cell from the very beginning, and on the other the
+  manner in which all the skeletal muscles of the adult are lined with a lymphatic endothelium, I am
+  strongly inclined to believe that at the closing up of the myoc&#x0153;le, when the myomere
+  separates from the mesomere, the lining cells remain scattered in among the forming muscle-cells
+  and form the ultimate lymphatic tissue of the muscles. If this is really so, then the evidence in
+  favour of the mesothelium being composed of free cells not connected with the nervous system would
+  be much strengthened, for, on the one hand, an intimate relation exists between the connective
+  tissue cells and the endothelium of the roots of the lymphatic vessels, a relation which,
+  according to Virchow, has rendered it impossible to draw any sharp line of distinction between the
+  two; and, on the other, the lymphatic endothelium merges into the lining cells of the great serous
+  cavities of the body.</p>
+
+  <p>It is impossible to conceive of an animal possessing a nervous system which is not in
+  connection with sensory and muscular tissues; an isolated nerve-cell is a meaningless possession;
+  but it is equally natural to conceive of a germ-cell being isolated, capable of living an
+  independent existence. Such a difference between the two kinds of tissues must have existed from
+  the very commencement of the Metazoa, so that we must, it seems to me, imagine that in the
+  formation of the Metazoa from the Protozoa the whole of the body of the latter did not break up
+  into a mass of separate gonads, each capable of becoming a free-living protozoan similar to its
+  parent, but that a portion proliferated into a multinucleated syncytium while the remainder formed
+  the free-living gonads. This multinucleated syncytium, or host, as it might be called, would still
+  continue to exist for the purpose of carrying further afield the immortal gonads, which need no
+  longer be all shed at one time.</p>
+
+  <p>In such an animal as <i>Volvox globator</i> we have an indication of the very kind of animal
+  postulated as connecting the single-celled Protozoa and the multi-cellular Metazoa, for it
+  consists of a many-celled case which forms a hollow sphere, each of the cells being provided with
+  flagella for the purpose of locomotion of the sphere, except a certain number which are not
+  flagellated; the latter leave the case to swim freely in the fluid contained within the sphere,
+  and forming spermaries and ovaries, conjugate, maturate, and then are set free by the rupture of
+  the encircling locomotor host.</p>
+
+  <div><span class="pagenum" id="page480">{480}</span></div>
+
+  <p>This conception of the predecessors of the Metazoa being composed of a mortal host, holding
+  within itself the immortal sexual products, leads naturally to the idea of the separate
+  development of the host from that of the germ-cells <i>ab initio</i>, so that the study of the
+  development of the Metazoa means the study of two separate constituents of the metazoan
+  individual&mdash;on the one hand, the elaboration of the elements forming the syncytial host, on
+  the other, of those derived from the free-living independent germ-cells. The elaboration of the
+  host means the differentiation of the protoplasm into epithelial, muscular, and nervous elements,
+  by means of which the gonads were carried further afield and their nourishment as well as that of
+  the host ensured.</p>
+
+  <p>The <i>rôle</i> of the nervous system as the middleman between internal and external muscular
+  and epithelial surfaces was, I imagine, initiated from the very earliest time. The further
+  evolution of the host consisted in a greater and greater differentiation and elaboration of this
+  neuro-epithelial syncytium, with the result of a steadily increasing concentration and
+  departmental centralization of the main factor of the syncytium; in other words, it led to the
+  origin and elaboration of a central nervous system. In the interstices of this syncytium the
+  gonads were placed, and at first, doubtless, the life of the host ended when all the germ-cells
+  had been set free. 'Reproduce and die' was, I imagine, the law of the Metazoa at its earliest
+  origin, and throughout the ages, during all the changes of evolution, the reminiscence of such law
+  still manifests itself even up to the highest forms as yet reached. With the differentiation of
+  the syncytial host there came also differentiation of the free-living gonads, so that only some of
+  them attained to the perfection of independent existence, capable of continuing the species; while
+  others became subordinate to the first and provided them with pabulum, manufacturing within
+  themselves yolk-spherules, and thus in the shape of yolk-cells ministered to the developing
+  egg-cell. Thus arose a germinal epithelium of which only a few of the elements passed out of the
+  host as perfect individuals, the remainder being utilized for the nutrition of these few. Such
+  yolk-cells of the germinal epithelium would still, however, retain their character as free cells
+  totally independent of the syncytial host, and, situated as they were between the internal and
+  external epithelium, capable of am&#x0153;boid movement, would naturally have their phagocytic
+  action <span class="pagenum" id="page481">{481}</span>utilized either as yolk-cells for the
+  providing of pabulum to the egg-cell, or as excretory cells for the removal and rendering harmless
+  of deleterious products of all kinds. Thus the free cells of the body would become differentiated
+  into the three classes of germ-cells, yolk-cells, and excretory cells.</p>
+
+  <p>Further, the mass of gonads, which originally occupied so large a space within the interior of
+  the host, necessarily, as the tissues of the host differentiated more and more, took up less and
+  less space in proportion to the whole bulk of the host and formed a germinal mass of cells between
+  the outer and inner epithelial layers. This germinal mass formed an epithelium, some of the
+  members of which acted as scavengers for the inner and outer layers of the host, with the result
+  that fluid accumulated between the two parts of the germinal epithelium in connection respectively
+  with the external and internal epithelial surfaces of the host, and thus led to the formation of a
+  gonoc&#x0153;le, which, by obtaining an external opening, a c&#x0153;lomostome, gave origin to the
+  c&#x0153;lom.</p>
+
+  <p>Again, with the longer life of the host, the setting free of the gonads no longer necessitating
+  the destruction of the host, and also the gonads themselves requiring a longer and longer time to
+  be fed up to maturity, the bulk and complexity of the whole organism increased and special
+  supporting structures became a necessity. The host itself could and did provide these to a certain
+  extent by secretions from its epithelial elements, but the intermediate supports were provided by
+  the system of phagocytic cells utilizing the fluids of the body, at first in the shape of
+  plasma-cells able to move from place to place, then settling down to form a connective tissue
+  framework, and, later on, cartilage and bone.</p>
+
+  <p>So also were gradually evolved the whole of the endothelial structures; the lymph-cells,
+  blood-cells, etc., all having their origin from the free cells of the body, which themselves
+  originated in the extension of a germinal epithelium. Just as in a bee-hive the egg-cells may form
+  the fully developed sexual animal, whether drone or queen bee, or the asexual host of workers, so
+  in the body of the Metazoa the free cells may form either male or female germ-cells spermatozoa,
+  or ova, or a host of workers, scavengers, repairers, food-providers, all useful to the community,
+  all showing their common origin by their absolute independence of the nervous system.</p>
+
+  <p>Two points of great importance follow from this method of looking <span class="pagenum"
+  id="page482">{482}</span>at the problem. First, the evolution of the animal kingdom means
+  essentially the evolution of the host, for that is what forms the individual; secondly, as the
+  host is composed of a syncytium, the common factor of whose elements is the neural moiety, it
+  follows that the tissue of central importance for the evolution of the host must be, as indeed it
+  is, the nervous system. Further, seeing that the growth of the individual means the orderly
+  spreading out of the epithelial moiety away from the neural moiety, it follows that the germ-band
+  or germ-area from which growth starts must be in the position of the nervous system. If then, the
+  nervous system in the animal is a concentrated one, then the growth will emanate from the position
+  of such nervous system. If, on the other hand, the nervous system is diffused, then the growth
+  will also be diffused.</p>
+
+  <p>In this book I have throughout argued that the ancestors of vertebrates belonged to a great
+  group of animals which gave origin also to Limulus and scorpion-like animals; it is therefore
+  instructive to see what is the nature of the development of such animals. For this purpose I will
+  take the development of the scorpion, as given by Brauer, for he has worked out its development
+  with great thoroughness and care. His papers show that the segmentation is discoidal, and results
+  in an oval blastodermic area lying on a large mass of yolk. Very early there separates out in this
+  area genital cells and yolk-cells, which latter move freely into the yolk and prepare it into a
+  fluid pabulum for the nutrition of the cells of the embryonic shield or germ-band. These free
+  yolk-cells do not take part in the formation of the germinal layers, nor does the endoderm when
+  formed give origin to free yolk-cells.</p>
+
+  <p>The cells of the germ-band form a small compact area, in which by continual mitosis the cells
+  become more than one-layered, and soon it is found that those cells which lie close against the
+  fluid pabulum form a continuous layer and absorb the nutritious material for themselves and the
+  rest of the embryo. While this area is thus increasing in thickness by continuous development, the
+  group of genital cells remains always apart, increasing in number, but being always in a state of
+  isolation from the cells of the rest of the growing area. Thus from the very first Brauer's
+  observations on the development of the scorpion point to the formation of a syncytial host
+  containing separate genital cells. The continuous layer of cells against the fluid pabulum, which
+  is already functioning as a gut, and may <span class="pagenum" id="page483">{483}</span>therefore
+  be called hypoblast, spreads continuously over the yolk, as also does the surface epithelial
+  layer, or epiblast. Such spreading is always a continuous one for both surfaces, so that the yolk
+  is gradually enclosed by a continuous orderly growth from the germ-band, and not by the settling
+  down of free cells in the yolk here and there to form the gut-lining. This steady orderly
+  development proceeds owing to the nourishment afforded by the activity of the free cells or
+  vitellophags and the absorbing power of the hypoblast, a steady growth round the yolk which
+  results in the formation of the gut-tube, the outer covering and all the muscular and excretory
+  organs. Where, then, is this starting-point, this germ-band from which the whole embryo grows? It
+  forms the mid ventral area of the adult animal, it corresponds exactly to the position of the
+  central nervous system. The whole phenomenon of embryonic growth in the scorpion is exactly what
+  must take place on the argument deduced from the study of the adult that the animal arises as a
+  neuro-epithelial syncytium, and we see that that layer of cells which is situated next to the
+  food-material forms the alimentary tube. It is not a question whether such layer is ventral or
+  dorsal to the neural cells, but whether it is contiguous to or removed from the food-material.</p>
+
+  <p>Take, again, a meroblastic vertebrate egg as of the bird. Again we find free cells passing into
+  the yolk to act as vitellophags, the so-called periblast cells; again we see that the embryo
+  starts from a germ-band or embryonic shield, and spreads from there continuously and steadily;
+  again we see that the layer of cells which lies against the yolk absorbs the fluid pabulum for the
+  growing cells; again we see that the area from which the whole process of growth starts is that of
+  the central nervous system, and again we see that those cells which are contiguous to the food
+  form the commencing gut, and are therefore called hypoblast, though in this case they are ventral
+  not dorsal to the neural layer.</p>
+
+  <p>The comparison of these two processes shows that there is one common factor, one thing
+  comparable in the two, one thing that is homologous and is the essential in the formation of that
+  part of the animal which I have called the host, and that is the central nervous system. Whether
+  the epithelial layer which lies ventrally to it or the one that is dorsal forms the gut depends
+  upon the position of the food-mass. Where the food is, there will be the absorbing layer. <span
+  class="pagenum" id="page484">{484}</span>Where the food is not, there will be no gut formation,
+  whatever may have been the previous history of that layer. If, then, we suppose, as I do, that the
+  vertebrate arose from a scorpion-like animal without any reversal of dorsal and ventral surfaces,
+  and that the central nervous system remained the same in the two animals, then the comparison of
+  the development of the two embryos shows that the one would be derived from the other if the
+  yolk-mass shifted from the dorsal to the ventral side of the nervous system. This would leave the
+  dorsal epithelial layer of the original syncytium free from pabulum; it would no longer form the
+  definite gut, <i>but it would still tend to form itself in the same manner as before, would still
+  grow from a ventrally situated germ-band dorsalwards to form a tube, would recapitulate its past
+  history, and show how the alimentary canal of the arthropod became the neural canal of the
+  vertebrate</i>. Although this alimentary canal is formed in the same way as before, it is no
+  longer recognized as homologous with the scorpion's alimentary canal, but because it no longer
+  absorbs pabulum, and does not therefore form the definite gut, it is called an epiblastic tube,
+  and, in the words of Ray Lankester, has no developmental importance.</p>
+
+  <p>All the arthropods are built up on the same type, and in all the development may in its broad
+  outlines be referred to the type just mentioned. So also with the vertebrate group; in both cases
+  the position of the central nervous system determines the starting area of embryonic growth. In
+  both cases the absorbing layer shows the position of the definite gut. A concentrated nervous
+  system of this type is common to all the segmented animals from the annelids to the vertebrates,
+  and in all cases the germ-band which indicates the first formation of the embryo is in the
+  position of this nervous system.</p>
+
+  <p>As far as the embryo is concerned, there is no great difficulty in the conception that the
+  yolk-mass may have shifted from one side to the other in passing from the arthropod to the
+  vertebrate, for in the arthropod the embryo at first is surrounded by yolk and then passes to the
+  periphery of the egg. If it is permissible to speak of a dorsal and ventral surface to an egg, and
+  we may imagine the egg held with such dorsal surface uppermost, then the yolk would be situated
+  ventrally to the embryo, as in the vertebrate, if the protoplasmic cells of the embryo rose from
+  their central position to the surface through the yolk, while if they sank through the yolk, the
+  yolk would be situated dorsally to the embryo, as in the arthropod.</p>
+
+  <div><span class="pagenum" id="page485">{485}</span></div>
+
+  <p>In cases where there is no yolk, or very little, as in Lucifer and Amphioxus respectively, the
+  embryo is compelled to feed itself at a very early age; such embryos form a free-swimming pelagic
+  ciliated blastula, the invagination of which, for the purpose of collecting food material out of
+  the open sea, is the simplest method of obtaining nutriment. Here, as in other cases, it is the
+  physiological necessity which determines the method of formation of the gut, and such similarity
+  of appearance as exists between the gastrula of Lucifer and that of Amphioxus, by no means implies
+  that the gut of the adult Lucifer is homologous with the gut of Amphioxus.</p>
+
+  <p class="sp3">I have compared two meroblastic eggs of the two classes respectively, because the
+  scorpion's egg is meroblastic. I imagine that no real difficulty arises with respect to
+  holoblastic eggs, for the experiments of O. Hertwig and Samassa show that by centrifugalizing,
+  stimulating, and breaking down of large spheres the holoblastic amphibian egg may be converted
+  into a meroblastic one, and then development will proceed regularly, <i>i.e.</i> in this case also
+  the growth proceeds from the animal pole; the large cells of the vegetal pole, like the yolk-cells
+  of the meroblastic egg, manufacture pabulum for the growing syncytial host.</p>
+
+  <p class="ac"><span class="sc">Summary.</span></p>
+
+  <div class="bq1 smaller sp5">
+    <p>Any attempt to discover how vertebrates arose from invertebrates must be based upon the study
+    of Comparative Anatomy, of Palæontology, and of Embryology. The arguments and evidence put
+    forward in the preceding chapters show most clearly how the theory of the origin of vertebrates
+    from palæostracans is supported by the geological evidence, by the anatomical evidence, and by
+    the embryological evidence. Of the three the latter is the strongest and most conclusive, if it
+    be taken to include the evidence given by the larval stage of the lamprey.</p>
+    <p>The stronghold of embryology for questions of this sort is the Law of Recapitulation, which
+    asserts that the history of the race is recapitulated to a greater or less extent in the
+    development of the individual. In the previous chapters such recapitulation has been shown for
+    all the organs of the vertebrate body. In this respect, then, embryology has proved of the
+    greatest value in confirming the evidence of relationship between the palæostracan and the
+    vertebrate, given by anatomical and geological study.</p>
+    <p>There is, however, another side to embryology, which claims that the tissues of all the
+    Metazoa are built up on the same plan; that in all cases in the very early stage of the embryo
+    three layers are formed, the epiblast, mesoblast, and hypoblast; that in all animals above the
+    Protozoa these three layers are <span class="pagenum" id="page486">{486}</span>homologous, the
+    epiblast in all cases forming the external or skin-layer, the hypoblast the internal or
+    gut-layer.</p>
+    <p>Such a theory, therefore, as is advocated in this book, which turns the gut of the arthropod
+    into the neural canal of the vertebrate, and makes a new gut for the vertebrate from the
+    external surface must be wrong, as it flatly contradicts the fundamental germ-layer theory.</p>
+    <p>Of recent years grave doubts have been thrown upon the validity of this theory, doubts which
+    have increased in force year by year as more and more facts have been discovered which are not
+    in agreement with the theory. So much is it now discredited that any criticism against my
+    theory, which is based upon it, weighs nothing in the balance against the positive evidence of
+    recapitulation already stated. If the germ-layer theory is no longer credited, upon what
+    fundamental laws is embryology based?</p>
+    <p>In this chapter I have ventured to suggest a reply to this question, based on the uniformity
+    of the laws of growth throughout the existence of the individual.</p>
+    <p>In the adult animal the body is composed of two kinds of tissues, those which are connected
+    with or at all events are under the control of the nervous system, and those which are capable
+    of leading a free life independent of the nervous system. These two kinds of tissues can be
+    traced back from the adult to the embryo, and it is the task of embryology to find out how these
+    two kinds of tissue originate.</p>
+    <p>The following out of this line of thought leads to the conception that, throughout the
+    Metazoa, the body is composed of a host which consists of the master-tissues of the body, and
+    takes the form of a neuro-epithelial syncytium, within the meshes of which free living
+    independent organisms or cells live, so to speak, a symbiotic existence.</p>
+    <p>The evidence points to the origin of all these free cells from germ-cells, and thus leads to
+    the conception that the blastula stage of every embryo represents two kinds of cells, the one
+    which will form the mortal host being the locomotor neuro-epithelial cell, the other the
+    independent immortal symbiotic germ-cell. Such conception leads directly to the conclusion that
+    the blastula stage of every member of the Metazoa is the embryonic representation of a Protozoan
+    ancestor of the Metazoa; an ancestor, whose nature may be illustrated by such a living form as
+    <i>Volvox globator</i>, which, like a blastula, is composed of a layer of cells forming a hollow
+    sphere. These cells partly bear cilia, and so form a locomotor host, partly are of a different
+    character, and form male and female germ-cells. The latter leave the surface of the sphere, pass
+    as free individuals into its fluid contents, form spermaries and ovaries, and then by the
+    rupture of the mortal locomotor host pass out into the external medium, as free swimming young
+    Volvox.</p>
+    <p>It is of interest to note that such members of the Protozoa are among the most highly
+    developed of the members of this great group.</p>
+    <p>From such a beginning arose in orderly evolution, on the one hand, all the neuro-muscular and
+    neuro-epithelial structures of the body&mdash;the so-called master-tissues; on the other, the
+    germ-cells, the blood-corpuscles, lymph-corpuscles plasma and excretory cells, connective tissue
+    cells, cartilage and bone-cells, etc., all of them independent of the central nervous system,
+    all traceable to a modification of the original germ-cells.</p>
+    <div><span class="pagenum" id="page487">{487}</span></div>
+    <p>Such a view of the processes of embryology brings embryology into harmony with comparative
+    anatomy and phylogeny, for it makes the central nervous system and not the alimentary canal the
+    most important factor in the development of the host.</p>
+    <p>The growth of the individual, whether arthropod or vertebrate, spreads from the position of
+    the central nervous system, regardless of whether that position is a ventral or dorsal one with
+    respect to the yolk-mass. Where the pabulum is, there is the definite gut, the lining walls of
+    which are called in the embryo, hypoblast; but when the pabulum is no longer there, although a
+    tube is formed in the same manner as the alimentary canal of the arthropod, it is now called an
+    epiblastic tube, and is known as the neural tube of the vertebrate.</p>
+    <p>This is the great fallacy of the germ-layer theory, a fallacy which consists of an argument
+    in a vicious circle: thus the alimentary canal is homologous in all of the Metazoa, because it
+    is formed of hypoblast, but there is no definition of hypoblast, except that it is always that
+    layer which forms the definitive alimentary canal.</p>
+    <p>When, after the process of segmentation has been completed, a free swimming blastula results,
+    unprovided with any store of pabulum in the shape of yolk, then the same physiological necessity
+    causes such a form to obtain its nutriment from the surrounding medium. The simplest way to do
+    this is by a process of invagination, in consequence of which food particles are swept into the
+    invaginated part and then absorbed. For this reason in such cases true gastrulæ are formed, as
+    in the case of Amphioxus among the vertebrates, and Lucifer among the crustaceans; such a
+    formation does not in the least imply that the gut of the arthropod is homologous with that of
+    the vertebrate. The resemblance between the two is not a morphological one, but due to the same
+    physiological necessity. They are analogous formations, not homologous.</p>
+    <p class="sp0">The muscular tissues are found to be formed in close connection with the nervous
+    tissues, and in very many cases are described as formed from epiblast, so that there are strong
+    reasons for placing them in a special category of the so-called mesoblastic tissues. If they be
+    separated out, then it seems to me, the rest of the mesoblast would consist of the free-living
+    cells of the body, which are not connected with the central nervous system. In watching, then,
+    the formation of mesoblast, defined in this way, we are watching the separation out from the
+    master-tissues of the body of the independent skeletal and excretory cells.</p>
+  </div>
+
+  <div><span class="pagenum" id="page488">{488}</span></div>
+
+  <p class="ac">CHAPTER XV</p>
+
+  <p class="ac"><i>FINAL REMARKS</i></p>
+
+  <div class="bq1 it1 smaller sp3">
+    <p>Problems requiring investigation&mdash;</p>
+    <p>Giant nerve-cells and giant-fibres; their comparison in fishes and in arthropods; blood- and
+    lymph-corpuscles; nature of the skin; origin of system of unstriped muscles; origin of the
+    sympathetic nervous system; biological test of relationship.</p>
+    <p class="sp0">Criticism of Balanoglossus theory.&mdash;Theory of parallel
+    development.&mdash;Importance of the theory advocated in this book for all problems of
+    Evolution.</p>
+  </div>
+
+  <p>The discussion in the last chapter on the "Principles of Embryology" completes the evidence
+  which I am able to offer up to the present time in favour of my theory of the "Origin of
+  Vertebrates." There are various questions which I have left untouched, but still are well worth
+  discussion, and may be mentioned here. The first of these is the significance of the giant
+  nerve-cells and giant nerve-fibres so characteristic of the brain-region of the lower vertebrates.
+  In most fishes two very large cells are most conspicuous objects in any transverse section of the
+  <i>medulla oblongata</i> at the level of entrance of the auditory nerves. Each of these cells
+  gives off a number of processes, some of which pass in the direction of the auditory nerves and
+  one very large axis-cylinder process which forms a giant-fibre, known by the name of a Mauthnerian
+  fibre. Each Mauthnerian fibre crosses the middle line soon after its origin from the giant-cell,
+  and passes down the spinal cord on the opposite side right to the tail. Here, near the end of the
+  spinal cord, it breaks up into smaller fibres, which are believed by Fritsch and others to pass
+  out directly into the ventral roots to supply the muscles of the tail. Thus Bela Haller says: "The
+  Mauthnerian fibres are known to give origin to certain fibres which supply the ventral roots of
+  the last three spinal nerves, so that their terminal branches serve, in all probability, for the
+  innervation of the muscles of the tail-fin." They do not occur in the eel, according to Haller, or
+  in Silurus, according to Kölliker. <span class="pagenum" id="page489">{489}</span>Their absence in
+  those fishes, in which a well-developed tail-fin is also absent, increases the probability of the
+  truth of Fritsch's original conclusion that these giant-fibres are associated axis-cylinders for
+  certain definite co-ordinated movements of the fish, especially for the lateral movement of the
+  tail.</p>
+
+  <p>In Ammoc&#x0153;tes, instead of two Mauthnerian fibres, a number of giant-fibres are found.
+  They are called Müllerian fibres, and arise from giant-cells which are divisible into two groups.
+  The first group consists of three pairs situated headwards of the level of exit of the trigeminal
+  nerves. Two of these lie in front of the level of exit of the oculomotor nerves, and one pair is
+  situated at the same level as the origin of the oculomotor nerves. The second group consists of a
+  number of cells on each side at the level of the entrance of the fibres of the auditory
+  nerves.</p>
+
+  <p>The Müllerian fibres largely decussate, as described by Ahlborn, and then become the most
+  anterior portion of the white matter of the spinal cord, forming a group of about eight fibres on
+  each side (Fig. <a href="#fig73">73</a>). A few fibres are also found laterally, and slightly
+  dorsally, to the grey matter. These giant-fibres pass down the spinal cord right to the anal
+  region; their ultimate destination is unknown. Mayer considers that in the first part of their
+  course they correspond to those tracts of fibres known as the "posterior longitudinal bundles" in
+  other vertebrates. I imagine, therefore, that the spinal part of their course represents the two
+  antero-lateral descending tracts. The second group of giant-cells, which appears to have some
+  connection with the auditory nerves, may represent "Deiter's nucleus." The whole system is
+  probably the central nervous part of a co-ordination mechanism, which arises entirely in the
+  pro-otic or prosomatic region of the brain&mdash;the great co-ordinating and equilibrating region
+  <i>par excellence</i>.</p>
+
+  <p>If we turn now to the arthropod it is a striking coincidence that in the crayfish and in the
+  lobster the work of Retzius, of Celesia, of Allen, and of many others demonstrates the existence
+  of an equilibration-mechanism for the swimming movements of the tail-muscles, which is carried out
+  by means of giant-fibres. These giant-fibres are the axis-cylinder processes of giant-cells,
+  situated exclusively in the brain-region, and they run through the whole ventral ganglionic chain
+  in order to supply the muscles of the tail. In the ventral nerve-cord of the crayfish, according
+  to Retzius, two specially large <span class="pagenum" id="page490">{490}</span>giant-fibres exist,
+  each of which breaks up, at the last abdominal ganglion, into smaller fibres, which pass directly
+  out with the nerves to the tail-fin. Allen has shown that, in addition to these two specially
+  large giant-fibres, there are a number of others, some of which, similarly to the Müllerian fibres
+  of Ammoc&#x0153;tes, cross the middle line, while some do not. Each of these arises from a large
+  nerve-cell and passes to one or other of the last pair of abdominal ganglia. The latter fibres, he
+  says, send off collaterals, while the two specially large giant-fibres do not. The cells which
+  give origin to all these large, long fibres are situated in or in front of the prosomatic region
+  of the brain, similarly to the giant-cells, which give rise to the corresponding Müllerian fibres
+  of Ammoc&#x0153;tes. I do not know how far this system is represented in Limulus or Scorpio.</p>
+
+  <p>It is, to my mind, improbable that the Mauthnerian fibres pass out directly as motor fibres to
+  the muscles of the tail-fin; it is more likely that they are conducting paths between the
+  equilibration-mechanism in connection with the VIIIth nerve and the spinal centres for the
+  movements of the tail. Similarly, with respect to the arthropod, it is difficult to believe that
+  the motor fibres for the tail-muscles arise in the brain-region. In either case, the striking
+  coincidence remains that the movements of the tail-end of the body are regulated by means of
+  giant-fibres which arise from giant-cells in the head-region of the body in both the Arthropoda
+  and the lowest members of the Vertebrata.</p>
+
+  <p>The meaning of this system of giant-cells and giant-fibres in both classes of animals is well
+  worthy of further investigation.</p>
+
+  <p>Another important piece of comparative work which ought to help in the elucidation of this
+  problem is the comparison of the blood- and lymph-corpuscles of the vertebrate with those of the
+  invertebrate groups. As yet, I have not myself made any observations in this direction, and feel
+  that it is inadvisable to discuss the results of others until I know more about the facts from
+  personal observation.</p>
+
+  <p>The large and important question of the manner of formation of the vertebrate skin has only
+  been considered to a slight extent. A much more thorough investigation requires to be made into
+  the nature of the skin of the oldest fishes in comparison with the skin of Ammoc&#x0153;tes on the
+  one side, and of Limulus and the Palæostraca on the other.</p>
+
+  <p>The muscular system requires further investigation, not so much <span class="pagenum"
+  id="page491">{491}</span>the different systems of the striated voluntary musculature&mdash;for
+  these have been for the most part compared in the two groups of animals in previous
+  chapters&mdash;as the involuntary unstriped musculature, about which no word has been said. The
+  origin of the different systems of unstriped muscles in the vertebrate is bound up with the origin
+  of the sympathetic system and its relation to the cranial and sacral visceral systems. The reason
+  why I have not included in this book the consideration of the sympathetic nervous system is on
+  account of the difficulty in finding any such system in Ammoc&#x0153;tes. Also, so far as I know,
+  the distribution of unstriped muscle in Ammoc&#x0153;tes has not been worked out.</p>
+
+  <p>One clue has arisen quite recently which is of great importance, and must be worked out in the
+  future, viz. the extraordinary connection which exists between the action of the sympathetic
+  nervous system and the action of adrenalin. This substance, which is obtained from the medullary
+  part of the adrenal or suprarenal glands, when injected into an animal produces the same effects
+  as stimulation of the nerves, which belong to the lumbo-thoracic outflow of visceral nerves,
+  <i>i.e.</i> the system known as the sympathetic nervous system, which is distinct from both the
+  cranial and sacral outflows of visceral nerves. The similarity of its action to stimulation of
+  nerves is entirely confined to the nerves of this sympathetic system, and never resembles that of
+  either the cranial or sacral visceral nerves.</p>
+
+  <p>Another most striking fact which confirms the great importance of this connection between the
+  adrenals and the sympathetic nervous system from the point of view of the evolution of the latter
+  system is that the extract of the adrenals always produces the same effect as that of stimulation
+  of the nerves of the sympathetic system, whatever may be the animal from which the extract is
+  obtained. Thus adrenalin obtained from the elasmobranch fishes will produce in the highest mammal
+  all the effects known to occur upon stimulation of the nerves of its sympathetic system.</p>
+
+  <p>Further, the cells, which are always associated with the presence of this peculiar
+  substance&mdash;adrenalin&mdash;stain in a characteristic manner in the presence of chromic salts.
+  In Ammoc&#x0153;tes patches of cells which stain in this manner have been described in connection
+  with blood-vessels in certain parts, so that, although I know of no definite evidence of the
+  existence of cell-groups in Ammoc&#x0153;tes corresponding to the ganglia of the sympathetic
+  system in other vertebrates, it is <span class="pagenum" id="page492">{492}</span>possible that
+  further investigation into the nature and connection of these "chromaffine" cells may afford a
+  clue to the origin of the sympathetic nervous system. At present it is premature to discuss the
+  question further.</p>
+
+  <p>Finally, another test as to the kinship of two animals of different species must be considered
+  more fully than I have been able to do up to the present time. This test is of a totally different
+  nature to any put forth in previous pages. It is known as the "biological test" of relationship,
+  and is the outcome of pathological rather than of physiological or anatomical research. It is
+  possible that this test may prove the most valuable of all. At present we do not know sufficiently
+  its limitations and its sources of error, especially in the case of cold-blooded animals, to be
+  able to look upon it as decisive in a problem of the kind considered in this book.</p>
+
+  <p>The nature of this test is as follows: It has been found that the serum of the blood of another
+  animal, when injected in sufficient quantity into a rabbit, will cause such a change in the serum
+  of that rabbit's blood that when it is added to the serum of the other animal a copious
+  precipitate is formed, although the serum of normal rabbit's blood when mixed with that of another
+  animal will cause no precipitate whatever. This extraordinary production of a precipitate in the
+  one case and not in the other indicates the production of some new substance in the rabbit's serum
+  in consequence of the introduction of the foreign serum into the rabbit, which brings about a
+  precipitate when the rabbit's serum containing it is mixed with the serum originally injected. The
+  barbarous name "antibody" has been used to express this supposed substance in accordance with the
+  meaning of such a word as "antitoxin," which has been a long time in use in connection with
+  preventive remedies against pathogenic bacteria. Now, it is found that the rabbit's serum
+  containing a particular "antibody" will cause a precipitate only when added to the serum of the
+  blood of the animal from which the "antibody" was produced or to the serum of the blood of a
+  nearly related animal.</p>
+
+  <p>Further, if that animal is closely related a precipitate will be formed nearly as copious as
+  with the original serum, if more distantly related a cloudiness will occur rather than a
+  precipitate, and if the relationship is still more distant the mixture of the two sera will remain
+  absolutely clear. Thus this test demonstrates the close relationship of man to the anthropoid apes
+  and his more distant <span class="pagenum" id="page493">{493}</span>relationship to monkeys in
+  general. By this method very evident blood-relationships have been demonstrated, especially
+  between members of the Mammalia.</p>
+
+  <p>I therefore started upon an investigation into the possibility of proving relationship in this
+  way between Limulus and Ammoc&#x0153;tes, with the kind assistance of Mr. Graham Smith. I must
+  confess I was not sanguine of success, as I thought the distance between Limulus and
+  Ammoc&#x0153;tes was too great. Dr. Lee, of New York, kindly provided me with most excellent serum
+  of Limulus, and the first experiments showed that the anti-serum of Limulus gave a most powerful
+  precipitate with its own serum. Graham Smith then tried this anti-serum of Limulus with the serum
+  of Ammoc&#x0153;tes, and to his surprise, and mine, he obtained a distinct cloudiness, indicative
+  of a relationship between the two animals. This, however, is not considered sufficient, the
+  reverse experiment must also succeed. I therefore, with Graham Smith, obtained a considerable
+  amount of blood from the adult lampreys at Brandon, and produced an anti-serum of Petromyzon,
+  which gave some precipitate with its own serum, but not a very powerful one. This anti-serum tried
+  with Limulus gave no result whatever, but at the same time it gave no result with serum from
+  Ammoc&#x0153;tes, so that the experiment not only showed that Petromyzon was not related to
+  Limulus, but also was not related to its own larval form, which is absurd.</p>
+
+  <p>Considerable difficulties were encountered in preparing the Petromyzon anti-serum owing to the
+  extreme toxic character of the lamprey's serum to the rabbit; in this respect it resembled that of
+  the eel. It is possible that the failure of the lamprey's anti-serum was due to the necessity of
+  heating the serum sufficiently to do away with its toxicity before injecting it into the rabbit.
+  At this point the experiments have been at present left. It will require a long and careful
+  investigation before it is possible to speak decisively one way or the other. At present the
+  experiment is positive to a certain extent, and also negative; but the latter proves too much, for
+  it proves that the larva is not related to the adult.</p>
+
+  <p>Some day I hope this "biological test" will be of use for determining the relationships of the
+  Tunicata, the Enteropneusta, Amphioxus, etc., as well as of Limulus and Ammoc&#x0153;tes.</p>
+
+  <p>The origin of Vertebrates from a Palæostracan stock, as put forward in this book, gives no
+  indication of the systematic position <span class="pagenum" id="page494">{494}</span>of the
+  Tunicata or Enteropneusta. Neither the Tunicata nor Amphioxus can by any possibility be on the
+  direct line of ascent from the invertebrate to the vertebrate. They must both be looked upon as
+  persistent failures, relics of the time when the great change to the vertebrate took place. The
+  Enteropneusta are on a different footing; in their case any evidence of affinity with vertebrates
+  is very much more doubtful.</p>
+
+  <p>The observer Spengel, who has made the most exhaustive study of these strange forms, rejects
+  <i>in toto</i> any connection with vertebrates, and considers them rather as aberrant annelids.
+  The so-called evidence of the tubular central nervous system is worth nothing. There is not the
+  slightest sign of any tubular nervous system in the least resembling that of the vertebrate. It is
+  simply that in one place of the collar-region the piece of skin containing the dorsal nerve of the
+  animal, owing to the formation of the collar, is folded, and thus forms just at this region a
+  short tube. My theory explains in a natural manner every portion of the elaborate and complicated
+  tube of the vertebrate central nervous system. In the Balanoglossus theory the evolution of the
+  vertebrate tube in all its details from this collar-fold is simple guesswork, without any
+  reasonable standpoint. Similarly, the small closed diverticulum of the gut in Balanoglossus, which
+  is dignified with the name of "notochord," has no right to the name. As I have already said, it
+  may help to understand why the notochord has such a peculiar structure, but it gives no help to
+  understanding the peculiar position of the notochord. The only really striking resemblance is
+  between the gill-slits of Amphioxus and of the Enteropneusta. In this comparison there is a very
+  great difficulty, very similar to that of the original attempts to derive vertebrates from
+  annelids&mdash;the gill-slits open ventrally in the one animal and dorsally in the other. In both
+  animals an atrial cavity exists which is formed by pleural folds, and in these pleural folds the
+  gonads are situated so that the similarity of the two branchial chambers seems at first sight very
+  complete. In the Enteropneusta, however, there are certain forms&mdash;Ptychodera&mdash;in which
+  these pleural folds have not met in the mid-line in this branchial region, and in these it is
+  plainly visible that these folds, with their gonads, spring from the ventral mid-line and arch
+  over the dorsal region of the body. Equally clearly Amphioxus shows that its pleural folds, with
+  the gonads, spring from the dorsal side of the animal, <span class="pagenum"
+  id="page495">{495}</span>and grow ventralwards until they fuse in the ventral mid-line (<i>cf.</i>
+  Fig. <a href="#fig168">168</a>).</p>
+
+  <p>As far, then, as this one single striking similarity between Amphioxus and the Enteropneusta is
+  concerned it necessitates the reversal of dorsal and ventral surfaces to bring the two branchial
+  chambers into harmony.</p>
+
+  <div class="ac w45 fcenter sp2">
+    <a href="images/fig168.png" id="fig168"><img style="width:100%" src="images/fig168.png" alt=""
+    title=""/></a>
+    <div class="smaller ac">
+      <p><span class="sc">Fig. 168.&mdash;Diagram illustrating the Position of the Pleural Folds and
+      Gonads in Ptychodera (A) and Amphioxus (B) respectively.</span></p>
+      <p class="sp0"><i>Al.</i>, alimentary canal; <i>D.A.</i>, dorsal vessel; <i>V.A.</i>, ventral
+      vessel; <i>g.</i>, gonads; <i>NC.</i>, notochord; <i>C.N.S.</i>, central nervous system.</p>
+    </div>
+  </div>
+
+  <p>In a mud-dwelling animal, like Balanoglossus, which possesses no appendages, no special
+  sense-organs, it seems likely enough that ventral and dorsal may be terms of no particular
+  meaning, and consequently what is called ventral in Balanoglossus may correspond to what is dorsal
+  in Amphioxus; in this way the branchial regions of the two animals may be closely compared. Such
+  comparison, however, immediately upsets the whole argument of the vertebrate nature of
+  Balanoglossus based on the relative position of the central nervous system and gut, for now that
+  part of its nervous system which is looked upon as the central nervous system in Balanoglossus is
+  ventral to the gut, just as in a worm-like animal, and not dorsal to it as in a vertebrate.</p>
+
+  <p>There is absolutely no possibility whatever of making such a detailed comparison between
+  Balanoglossus and any vertebrate, as I have done between a particular kind of arthropod and
+  Ammoc&#x0153;tes. In the latter case not only the topographical anatomy of the organs in the two
+  animals is the same, but the comparison is valid even to microscopical structure. In the former
+  case the origin of almost all <span class="pagenum" id="page496">{496}</span>the vertebrate organs
+  is absolutely hypothetical, no clue is given in Balanoglossus, not even to the segmented nature of
+  the vertebrate. The same holds good with the evidence from Embryology and from Palæontology. I
+  have pointed out how strongly the evidence in both cases confirms that of Comparative Anatomy. In
+  neither case is the strength of the evidence for Balanoglossus in the slightest degree comparable.
+  In Embryology an attempt has been made to compare the origin of the c&#x0153;lom in Amphioxus and
+  in Balanoglossus. In Palæontology there is nothing, only an assumption that in the Cambrian and
+  Lower Silurian times a whole series of animals were evolved between Balanoglossus and the earliest
+  armoured fishes, which have left no trace, although they were able to hold their own against the
+  dominant Palæostracan race. The strangeness of this conception is that, when they do appear, they
+  are fully armoured, as in Pteraspis and Cephalaspis, and it is extremely hard luck for the
+  believers in the Balanoglossus theory that no intermediate less armoured forms have been found,
+  especially in consideration of the fact that the theory of the origin from the Palæostracan does
+  not require such intermediate forms, but finds that those already discovered exactly fulfil its
+  requirements.</p>
+
+  <p>One difficulty in the way of accepting the theory which I have advocated is perhaps the
+  existence of the Tunicata. I cannot see that they show any affinities to the Arthropoda, and yet
+  they are looked upon as allied to the Vertebrata. I can only conclude that both they and Amphioxus
+  arose late, after the vertebrate stock had become well established, so that in their degenerated
+  condition they give indications of their vertebrate ancestry and not of their more remote
+  arthropod ancestry.</p>
+
+  <p>In conclusion, the way in which vertebrates arose on the earth as suggested in this book
+  carries with it many important far-reaching conclusions with respect to the whole problem of
+  Evolution.</p>
+
+  <p>When the study of Embryology began, great hopes were entertained that by its means it would be
+  possible to discover the pedigree of every group of animals, and for this end all the stages of
+  development in all groups of animals were sought for and, as far as possible, studied. It was soon
+  found, however, that the interpretation of what was seen was so difficult, as to give rise to all
+  manner of views, depending upon the <span class="correction"
+  title="Original reads 'idiosyncracy'.">idiosyncrasy</span> of the observer. At his will he decided
+  whether any appearance was c&#x0153;nogenetic or palingenetic, <span class="pagenum"
+  id="page497">{497}</span>with the result that, in the minds of many, embryology has failed to
+  afford the desired clue.</p>
+
+  <p>At the same time, the geological record was looked upon as too imperfect to afford any real
+  help; it was said, and is said, that the Cambrian and pre-Cambrian periods were so immense, and
+  the animals discovered in the lower Silurian so highly organized, as to compel us to ascribe the
+  origination of all the present-day groups to this immense early period, the animals of which have
+  left no trace of their existence as fossils.</p>
+
+  <p>In consequence of, or at all events following upon, the supposed failure of embryology and of
+  geology to solve the problem of the sequence of evolution of animal life, a new theory has arisen,
+  which goes very near to the denial of evolution altogether. This is the theory of parallel
+  development. It discards the old picture of a genealogical tree with main branches arising at
+  different heights, these again branching and branching into smaller and smaller twigs, and
+  substitutes instead the picture of the ribs of a fan, every rib running independently of every
+  other, each group represented by a rib reaching its highest development on the circumference of
+  the fan and coming nearer and nearer to a common point at the handle of the fan. This point of
+  convergence, where all the groups ultimately meet, is so far back as to reach to the lowest living
+  organisms.</p>
+
+  <p>This, in my opinion, unscientific and inconceivable suggestion has arisen largely in
+  consequence of a conception which has become firmly fixed in the minds of very many writers on
+  this subject&mdash;the conception that in the evolution of every group, the higher members of the
+  group are the most specialized in the peculiarities of that group, and it is impossible to obtain
+  a new group with different peculiarities from such specialized members. If, then, a higher group
+  is to arise from a lower, it must arise from the generalized members of that lower group, in other
+  words, from the lowest members or those nearly akin to the next lower group.</p>
+
+  <p>Similarly, the highest members of this latter group are too specialized, and again we must go
+  to the more generalized members of the group. In this way each separate specialized group is put
+  on one side, and so the conception of parallel development comes into being.</p>
+
+  <p>The evidence given in this book dealing with the origin of vertebrates strikes at the
+  foundations of this belief, for it presents an <span class="pagenum"
+  id="page498">{498}</span>image of the sequence of evolution of animal forms in orderly upward
+  progress, caused by the struggle for existence among the members of the race dominant at the time,
+  which brought about the origin of the next higher group not from the lowest members of the
+  dominant group, but from some one of the higher members of that group.</p>
+
+  <p>The great factor in evolution has been throughout the growth of the central nervous system;
+  from that group of animals which possessed the highest nervous system evolved up to that time the
+  next higher group must have arisen.</p>
+
+  <p>In this way we can trace without a break, always following out the same law, the evolution of
+  man from the mammal, the mammal from the reptile, the reptile from the amphibian, the amphibian
+  from the fish, the fish from the arthropod, the arthropod from the annelid, and we may be hopeful
+  that the same law will enable us to arrange in orderly sequence all the groups in the animal
+  kingdom.</p>
+
+  <p>This very same law of the paramount importance of the development of the central nervous system
+  for all upward progress will, I firmly believe, lead to the establishment of a new and more
+  fruitful embryology, the leading feature of which will be, as suggested in the last chapter, not
+  the attempt to derive from the blastula three germ-layers common to all animals, but rather two
+  sets of organs&mdash;those which are governed by the nervous system and those which are
+  not&mdash;and thus by means of the development of the central nervous system obtain from
+  embryology surer indications of relationship than are given at present.</p>
+
+  <p>The great law of recapitulation, which asserts that the past history of the race is indicated
+  more or less in the development of each individual, a law which of late years has fallen somewhat
+  into disrepute, owing especially to the difficulty of interpreting the embryological history of
+  the vertebrate, is triumphantly vindicated by the theory put forward in this book. Each separate
+  vertebrate organ, one after the other, as shown in the last chapter, indicates in its development
+  the manner in which it arose from the corresponding organ of the arthropod. There is no failure in
+  the evidence of embryology, the failure is in the interpretation thereof.</p>
+
+  <p>So, too, my theory vindicates the geological method. There is no failure here; on the contrary,
+  the record of the rocks proclaims with startling clearness not only the sequence of evolution in
+  the <span class="pagenum" id="page499">{499}</span>vertebrate kingdom itself, but the origin of
+  the vertebrate from the most highly-developed invertebrate race.</p>
+
+  <p>The study of the comparative anatomy of organs down to the finest details has always been a
+  most important aid in finding out relationship between animals or groups of animals. My theory
+  endorses this view to the uttermost, and especially indicates the study of the central nervous
+  system and its outgoing nerves as that comparative study which is most likely to afford valuable
+  results.</p>
+
+  <p>As for the individual, so for the nation; as for the nation, so for the race; the law of
+  evolution teaches that in all cases brain-power wins. Throughout, from the dawn of animal life up
+  to the present day, the evidence given in this book suggests that the same law has always held. In
+  all cases, upward progress is associated with a development of the central nervous system.</p>
+
+  <p class="sp5">The law for the whole animal kingdom is the same as for the individual. "Success in
+  this world depends upon brains."</p>
+
+  <div><span class="pagenum" id="page501">{501}</span></div>
+
+  <p class="sp3 ac" style="margin-bottom:3.9ex;"><span class="larger">BIBLIOGRAPHY AND INDEX OF
+  AUTHORS</span></p>
+
+  <table class="sp5 mc w75" title="Bibliography and Index of Authors"
+  summary="Bibliography and Index of Authors">
+    <tr class="bb bt">
+      <th class="br smaller">Author's name.</th>
+      <th class="br smaller">Title of Paper.</th>
+      <th class="smaller w18">Pages of<br/>
+      reference.</th>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">AHLBORN</td>
+      <td class="it1p05">"Untersuchungen über das Gehirn der Petromyzonten"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page210">210</a>, <a
+      href="#page489">489</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Zeitsch. f. wiss. Zool.</i> Vol. 39. 1883</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Ueber die Segmentation des Wirbelthierkörpers"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page260">260</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Zeitsch. f. wiss. Zool.</i> Vol. 40. 1884</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">AICHEL</td>
+      <td class="it1p05">"Vergleichende Entwicklungsgeschichte und Stammesgeschichte der
+      Nebennieren"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page424">424</a>, <a
+      href="#page428">428</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Arch. f. Mikr. Anat.</i> Vol. 56. 1900</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">ALCOCK</td>
+      <td class="pb05"></td>
+      <td class="bl vbm pb05"><a href="#page135">135</a>, <a href="#page287">287</a>, <a
+      href="#page288">288</a>, <a href="#page289">289</a>, <a href="#page304">304</a>, <a
+      href="#page307">307</a>, <a href="#page347">347</a>, <a href="#page445">445</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"The Peripheral Distribution of the Cranial Nerves of
+      Ammoc&#x0153;tes"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page164">164</a>, <a href="#page171">171</a>, <a
+      href="#page177">177</a>, <a href="#page188">188</a>, <a href="#page202">202</a>, <a
+      href="#page297">297</a>, <a href="#page300">300</a>, <a href="#page310">310</a>, <a
+      href="#page311">311</a>, <a href="#page316">316</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Journ. of Anat. and Physiol.</i> Vol. 33. 1898</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"On Proteid Digestion in Ammoc&#x0153;tes"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page58">58</a>, <a href="#page213">213</a>, <a
+      href="#page442">442</a>, <a href="#page452">452</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Journ. of Anat. and Physiol.</i> Vol. 33. 1898</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">ALLEN</td>
+      <td class="it1p05">"Studies on the Nervous System of Crustacea"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page489">489</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 36. 1894</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">ANDERSON, H. K.</td>
+      <td class="pb05"></td>
+      <td class="bl vbm pb05"><a href="#page448">448</a>, <a href="#page470">470</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"The Nature of the Lesions which hinder the Development of Nerve-cells and
+      their Processes</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page466">466</a>, <a href="#page467">467</a>, <a
+      href="#page469">469</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Journ. of Physiol.</i> Vol. 28. 1902</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"On the Myelination of Nerve-fibres"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page467">467</a>, <a
+      href="#page477">477</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Report of the Brit. Assn.</i> 1898</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">APATHY</td>
+      <td class="it1p05">"Das leitende Element des Nervensystems und seine topographischen Beziehung
+      zu den Zellen"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page467">467</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Mitth. a. d. Zool. Stat. zu Neapel.</i> Vol. 12. 1896</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">ASSHETON</td>
+      <td class="it1p05">"On the Phenomenon of the Fusion of the Epiblastic Layers in the Rabbit and
+      in the Frog"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page42">42</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 37. 1894</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"An Experimental Examination into the Growth of the Blastoderm of the
+      Chick"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page154">154</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Proc. of Roy. Soc.</i> Vol. 60. 1896</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"><span class="pagenum" id="page502">{502}</span></td>
+      <td class="it1p05">"On the Growth in Length of the Frog Embryo"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page154">154</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 37. 1894</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"A Re-investigation into the Early Stages of the Development of the
+      Rabbit"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page154">154</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 37. 1894</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"The Primitive Streak of the Rabbit: the Causes which may determine its
+      Shape, and the part of the Embryo formed by its Activity"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page154">154</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 37. 1894</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">BALFOUR</td>
+      <td class="it1p05">'Comparative Embryology.' Vol. 2</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page73">73</a>, <a href="#page74">74</a>, <a
+      href="#page94">94</a>, <a href="#page103">103</a>, <a href="#page104">104</a>, <a
+      href="#page120">120</a>, <a href="#page181">181</a>, <a href="#page259">259</a>, <a
+      href="#page424">424</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">London. 1881. Macmillan &amp; Co.</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"On the Origin and History of the Urino-genital Organs of Vertebrates"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page390">390</a>, <a
+      href="#page392">392</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Journ. of Anat. and Physiol.</i> Vol. 10. 1876</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"On the Nature of the Organ in Adult Teleosteans and Ganoids, which is
+      usually regarded as the Head-kidney or Pronephros"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page420">420</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 22. 1882</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">BARKER</td>
+      <td class="it1p05">'The Nervous System'</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page470">470</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">London. 1901</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">BATESON</td>
+      <td class="it1p05">"The Ancestry of the Chordata"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page11">11</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 26. 1886</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">'Materials for the Study of Variation'</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page387">387</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">London. 1894</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">BEARD</td>
+      <td class="it1p05">"The System of Branchial Sense Organs and their Associated Ganglia in
+      Ichthyopsida"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page262">262</a>, <a href="#page281">281</a>, <a
+      href="#page283">283</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 26. 1885</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"The Development of the Peripheral Nervous System in Vertebrates"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page262">262</a>, <a href="#page281">281</a>, <a
+      href="#page283">283</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 29. 1888</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"The Old Mouth and the New"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page318">318</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Anat. Anzeiger.</i> 1888</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"The Source of Leucocytes and the True Function of the Thymus"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page425">425</a>, <a
+      href="#page426">426</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Anat. Anzeiger.</i> Vol. 18. 1900</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"The Parietal Eye of the Cyclostome Fishes"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page84">84</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 29. 1882</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">BECK <span class="smaller">AND</span> LANKESTER</td>
+      <td class="it1p05">"On the Muscular and Endo-skeletal Tissues of Scorpio"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page171">171</a>, <a href="#page222">222</a>, <a
+      href="#page224">224</a>, <a href="#page247">247</a>, <a href="#page268">268</a>-<a
+      href="#page277">277</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Trans. Zool. Soc.</i> Vol. 11. 1885</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">BEECHER</td>
+      <td class="it1p05">"Natural Classification of the Trilobites"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page283">283</a>, <a href="#page351">351</a>, <a
+      href="#page436">436</a>, <a href="#page437">437</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Amer. Journ. of Sci.</i> Ser. 4. Vol. 3. 1897</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">BELL, C.</td>
+      <td class="it1p05">'The Nervous System of the Human Body'</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page155">155</a>, <a href="#page156">156</a>, <a
+      href="#page183">183</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">London. 1830</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">BELLONCI</td>
+      <td class="it1p05">"Système Nerveux et Organes des sens du <i>Sphæroma serratum</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page62">62</a>, <a href="#page90">90</a>, <a
+      href="#page92">92</a>, <a href="#page101">101</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Archiv. Ital. de Biol.</i> Vol. 1. 1882</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Sur la structure et les rapports des lobes olfactives dans les Arthropods
+      superieurs et les Vertébrés"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page221">221</a>,<a href="#page225">225</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Archiv. Ital. de Biol.</i> Vol. 3. 1883</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">BENHAM <span class="smaller">AND</span> LANKESTER</td>
+      <td class="it1p05">"On the Muscular and Endo-skeletal Systems of Limulus"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page143">143</a>, <a href="#page171">171</a>, <a
+      href="#page176">176</a>, <a href="#page177">177</a>, <a href="#page247">247</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Trans. Zool. Soc.</i> Vol. 11. 1885</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05"><span class="pagenum" id="page503">{503}</span>
+        <p class="sp0">BERGER</p>
+      </td>
+      <td class="it1p05">"Untersuchungen über den Bau des Gehirns und der Retina der
+      Arthropoden"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page88">88</a>-<a href="#page92">92</a>, <a
+      href="#page97">97</a>, <a href="#page100">100</a>, <a href="#page101">101</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Arbeit. a. d. Zool. Instit. Wien.</i> Vol. 1. 1878</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">BERGH</td>
+      <td class="it1p05">"Neue Beiträge zur Embryologie der Anneliden"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page478">478</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Zeitsch. f. wiss. Zool.</i> Vol. 50. 1890</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">BERKLEY</td>
+      <td class="it1p05">"The Intrinsic Nerves of the Kidney"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page477">477</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Bulletin of the Johns Hopkins Hospital.</i> Vol. 4</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">BERNARD</td>
+      <td class="it1p05">'The Apodidæ: a Morphological Study'</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page284">284</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Nature Series.</i> 1892</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">BERTKAU</td>
+      <td class="it1p05">"Beiträge zur Kenntniss der Sinnesorgane der Spinnen. 1. Die Augen der
+      Spinnen"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page369">369</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Archiv. f. mikr. Anat.</i> Vol. 27. 1886</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">BIEDERMANN</td>
+      <td class="it1p05">'Electro-physiology'</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page20">20</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">Translated by F. A. Welby. London. 1896</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">BLANCHARD</td>
+      <td class="pb05 it1p05">Quoted by Huxley</td>
+      <td class="bl vbm pb05"><a href="#page225">225</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">'L'Organisation du Règne Animal. Arachnides'</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page109">109</a>, <a href="#page177">177</a>, <a
+      href="#page190">190</a>, <a href="#page206">206</a>, <a href="#page313">313</a>, <a
+      href="#page315">315</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">Paris. 1852</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">BLES</td>
+      <td class="it1p05">"The Correlated Distribution of Abdominal Pores and Nephrostomes in
+      Fishes"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page431">431</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Journ. of Anat. and Physiol.</i> Vol. 32. 1898</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">BOBRETSKY</td>
+      <td class="it1p05">'Development of Astacus and Palæmon'</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page74">74</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">Kiew. 1873</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">BOURNE <span class="smaller">AND</span> LANKESTER</td>
+      <td class="pb05 it1p05"><i>See</i> Lankester and Bourne.</td>
+      <td class="bl pb05"></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">BOVERI</td>
+      <td class="it1p05">"Die Nieren Canälchen des Amphioxus"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page392">392</a>, <a href="#page395">395</a>, <a
+      href="#page402">402</a>, <a href="#page407">407</a>, <a href="#page412">412</a>, <a
+      href="#page426">426</a>, <a href="#page427">427</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Zool. Jahrbuch.</i> Vol. 5. 1892</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">BRAEM</td>
+      <td class="it1p05">"Was ist ein Keimblatt"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page460">460</a>, <a href="#page461">461</a>, <a
+      href="#page462">462</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Biol. Centralblatt.</i> Vol. 15. 1895</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">BRAUER</td>
+      <td class="it1p05">"Beiträge zur Kenntniss der Entwicklungsgeschichte des Skorpions"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page62">62</a>, <a href="#page167">167</a>, <a
+      href="#page222">222</a>, <a href="#page237">237</a>, <a href="#page281">281</a>, <a
+      href="#page482">482</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Zeit. f. wiss. Zool.</i> Part I. Vol. 57. 1894<br/>
+      Part II. Vol. 59. 1895</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Beiträge zur Kenntniss der Entwicklung und Anatomie der Gymnophionen."
+      III.</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page393">393</a>, <a href="#page394">394</a>, <a
+      href="#page400">400</a>, <a href="#page402">402</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">"Die Entwicklung der Excretionsorgane" <i>Zool. Jahrbuch.</i> Vol.
+      16. 1902</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Ueber die Entwicklung von Hydra"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page473">473</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Zeit. f. wiss. Zool.</i> Vol. 52. 1891</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">BÜTSCHLI</td>
+      <td class="it1p05">"Notiz zur Morphologie des Auges der Muscheln"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page114">114</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Festschrift des Natur-hist-med. Vereins zu Heidelberg.</i>
+      1886</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">BUJOR</td>
+      <td class="it1p05">"Contribution a l'étude de la métamorphose de <i>l'Ammoc&#x0153;tes
+      branchialis</i> en <i>Petromyzon Planeri</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page135">135</a>, <a
+      href="#page304">304</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Revue Biologique du Nord de la France.</i> Vol. 3. 1891</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">CARLSON</td>
+      <td class="pb05"></td>
+      <td class="bl vbm pb05"><a href="#page177">177</a>, <a href="#page315">315</a>, <a
+      href="#page316">316</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">CELESIA</td>
+      <td class="it1p05">'Differenziamento della proprietà inibitoria e dei funzioni coordinatrici
+      nella catena gangliare dei crustacei decapodi'</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page489">489</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">Genoa. 1897</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">CLAUS</td>
+      <td class="it1p05">"Untersuchungen über den Organismus und Entwicklung von Branchipus und
+      Artemia"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page90">90</a>-<a href="#page92">92</a>, <a
+      href="#page97">97</a>, <a href="#page100">100</a>, <a href="#page396">396</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Arbeit a.d. Zool. Institut. Wien.</i> Vol. 6. 1886</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05"><span class="pagenum" id="page504">{504}</span>
+        <p class="sp0">COPE</p>
+      </td>
+      <td class="it1p05">"On the Phylogeny of the Vertebrata"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page343">343</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Proc. Amer. Philos. Soc.</i> Vol. 30. 1892</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">CRONEBERG</td>
+      <td class="it1p05">"Ueber die Mundtheile der Arachniden"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page221">221</a>-<a href="#page224">224</a>, <a
+      href="#page241">241</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Archiv. f. Naturgeschichte.</i> 1880</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">CUÉNOT</td>
+      <td class="it1p05">"Études sur le sang et les glandes lymphatiques dans la série animale; 2nd
+      partie; invertébrés"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page422">422</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Arch. d. Zool. exper. gen.</i> 2nd Ser. Vol. 9. 1891</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">CUNNINGHAM, J. T.</td>
+      <td class="it1p05">"The Significance of Kupffer's Vesicle, with Remarks on other Questions of
+      Vertebrate Morphology"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page318">318</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 25. 1885</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"The Nephridia of <i>Lanice conchilega</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page403">403</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Nature.</i> Vol. 36. 1887</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">DANA</td>
+      <td class="it1p05">"On Cephalization"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page53">53</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Mag. of Nat. Hist.</i> 1863</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">DEAN-BASHFORD</td>
+      <td class="it1p05">'Fishes, Living and Fossil'</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page344">344</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">New York. 1895</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"On the Embryology of <i>Bdellostoma Stouti</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page405">405</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Festschr. z. siebenzigsten Geburtstag. von C. v. Kupffer.</i>
+      Jena. 1899</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">DENDY</td>
+      <td class="it1p05">"On the Parietal Sense-organs and Associated Structures in the New Zealand
+      Lamprey (<i>Geotria australis</i>)"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page80">80</a>, <a href="#page82">82</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 51. 1907</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">DIETL</td>
+      <td class="it1p05">"Die Organisation des Arthropoden <span class="correction"
+      title="Original reads 'Gehirus'.">Gehirns</span>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page101">101</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Zeitsch. f. wiss. Zool.</i> Vol. 27. 1876</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">DOHRN</td>
+      <td class="it1p05">'Der Ursprung der Wirbelthiere und das Princip des Functionswechsels'</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page14">14</a>, <a href="#page60">60</a>, <a
+      href="#page185">185</a>, <a href="#page186">186</a>, <a href="#page317">317</a>, <a
+      href="#page318">318</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">Leipzig. 1875</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">Studien zur Urgeschichte des Wirbelthiere Körpers. VIII. "Die Thyroidea bei
+      Petromyzon, Amphioxus, und Tunicaten"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page188">188</a>, <a href="#page195">195</a>-<a
+      href="#page198">198</a>, <a href="#page199">199</a>, <a href="#page212">212</a>, <a
+      href="#page213">213</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Mitth. Zool. Stat. z. Neapel.</i> Vol. 6. 1886</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Neue Grundlagen zur Beurtheilung der Metamerie des Kopfes"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page262">262</a>, <a href="#page263">263</a>, <a
+      href="#page279">279</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Mitth. Zool. Stat. z. Neapel.</i> Vol. 9. 1890</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">Studien zur Urgeschichte des Wirbelthiere Körpers. XIII. "Ueber Nerven und
+      Gefässe bei Ammoc&#x0153;tes und <i>Petromyzon Planeri</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page167">167</a>, <a href="#page314">314</a>, <a
+      href="#page337">337</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Mitth. Zool. Stat. z. Neapel.</i> Vol. 8. 1888</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">DREVERMANN</td>
+      <td class="it1p05">"Ueber <i>Pteraspis dunensis</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page29">29</a>, <a href="#page30">30</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Zeitschr. d. Deutsch. Geol. Gesellschaft.</i> Vol. 56. 1904</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">EDGEWORTH</td>
+      <td class="it1p05">"The Development of the Head-muscles in <i>Gallus domesticus</i>, and the
+      Morphology of the Head-muscles in the Sauropsida"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page266">266</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 51. 1907</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">EDINGER</td>
+      <td class="it1p05">'Anatomy of Central Nervous System in Man and in Vertebrates'</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page17">17</a>, <a href="#page264">264</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">Translated by Hall. 1899</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">v. EICHWALD</td>
+      <td class="it1p05">"Die Thier- und Pflanzenreste des alten rothen Sandsteins und Bergkalks im
+      Nowgorodschen Gouvernement"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page327">327</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Bull. Sci. de l'Acad. Impér. d. St. Petersbourg.</i> 1840</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05"><span class="pagenum" id="page505">{505}</span>
+        <p class="sp0">EISIG</p>
+      </td>
+      <td class="it1p05">"Die Seiten-organe und becherförmigen Organe der Capitelliden"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page357">357</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Mitth. a. d. Zool. Stat. z. Neapel.</i> Vol. 1. 1879</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Capitelliden"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page357">357</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Faun. u. Flor. d. Golfes v. Neapel.</i> Vol. 16. 1887</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">ELLIOTT</td>
+      <td class="it1p05">"On the Innervation of the Ileo-colic Sphincter"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page449">449</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Journ. of Physiol.</i> Vol. 31. 1904</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">EMERY</td>
+      <td class="pb05 it1p05">Quoted by Weldon</td>
+      <td class="bl vbm pb05"><a href="#page420">420</a></td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">FOSTER, M.</td>
+      <td class="pb05 it1p05">Text-book of Physiology</td>
+      <td class="bl vbm pb05"><a href="#page108">108</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">FREUND</td>
+      <td class="it1p05">"Die Beziehungen der Schilddrüse zu den weiblichen Geschlechtsorganen"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page215">215</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Deutsch. Zeitsch. f. Chirugie.</i> Vol. 18. 1883</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">FRITSCH, G.</td>
+      <td class="it1p05">'Untersuchungen über den feineren Bau des Fischgehirns'</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page488">488</a>, <a
+      href="#page489">489</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">Berlin. 1878</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">FRORIEP</td>
+      <td class="it1p05">"Ueber Anlagen von Sinnesorganen am Facialis, Glossopharyngeus und Vagus,
+      über die genetische Stellung des Vagus zum Hypoglossus, und über die Herkunft der
+      Zungenmusculatur"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page261">261</a>, <a href="#page262">262</a>, <a
+      href="#page281">281</a>, <a href="#page283">283</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Arch. f. Anat. u. Physiol; Anat. Abtheil.</i> 1885</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">FÜRBRINGER, M.</td>
+      <td class="it1p05">'Ueber die Spino-occipetalen Nerven der Selachier und Holocephalen'</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page276">276</a>-<a href="#page278">278</a>, <a
+      href="#page409">409</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">Fest-schrift für Carl Gegenbaur. 1897</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">GAUBERT</td>
+      <td class="it1p05">'Recherches sur les organes des sens et sur les systèmes tegumentaire,
+      glandulaire et musculaire des appendices des arachnides'</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page364">364</a>, <a href="#page368">368</a>-<a
+      href="#page375">375</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">Paris. 1892</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">GEGENBAUR</td>
+      <td class="it1p05">"Anatomische Untersuchung eines Limulus"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page20">20</a>, <a href="#page358">358</a>-<a
+      href="#page360">360</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Abhandl. d. Naturforsch. Gesellsch. z. Halle.</i> Vol. 4.
+      1858</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Ueber die Skeletgewebe der Cyclostomen"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page181">181</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Jen. Zeitschrift.</i> Vol. 5. 1870</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">Untersuchungen zur vergleichende Anatomie der Wirbelthiere III. Heft. 'Das
+      Kopfskeletder Selachiern'</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page151">151</a>, <a href="#page259">259</a>, <a
+      href="#page261">261</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">Leipzig. 1872</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">'Grundriss der vergleichenden Anatomie'</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page392">392</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">Leipzig. 1878</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">v. GEHUCHTEN</td>
+      <td class="it1p05">"De l'origine du pathétique et de la racine supérieure du trijumeau"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page264">264</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Acad. d. Sci. Belg. Bulletin.</i> 3rd Ser. Vol. 29. 1895</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">GOETHE</td>
+      <td class="pb05"></td>
+      <td class="bl vbm pb05"><a href="#page258">258</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">GÖTTE</td>
+      <td class="it1p05">'Entwicklungsgeschichte der Unke'</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page101">101</a>, <a href="#page102">102</a>, <a
+      href="#page114">114</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">Leipzig. 1875</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">GOLGI</td>
+      <td class="pb05"></td>
+      <td class="bl vbm pb05"><a href="#page72">72</a>, <a href="#page465">465</a>, <a
+      href="#page477">477</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">GOODRICH</td>
+      <td class="it1p05">"On the Structure of the Excretory Organs of Amphioxus"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page395">395</a>, <a href="#page396">396</a>, <a
+      href="#page477">477</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 45. 1902</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"On the Nephridia of the <span class="correction"
+      title="Original reads 'Polych&#x0153;ta'.">Polychæta</span>." Parts I., II., III.</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page395">395</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vols. 40, 41, 43</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"On the Excretory Organs of Amphioxus"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page477">477</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Proc. Roy. Soc.</i> Vol. 69. 1902</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05"><span class="pagenum" id="page506">{506}</span>
+        <p class="sp0">GRABER</p>
+      </td>
+      <td class="it1p05">"Die <span class="correction" title="Original reads 'Chordo-tonalem'.">Chordo-tonalen</span>
+      Sinnesorgane und das Gehör der Insecten"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page364">364</a>, <a href="#page369">369</a>-<a
+      href="#page371">371</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Archiv. f. Mikr. Anat.</i> Vols. 20 and 21. 1882</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">GRENACHER</td>
+      <td class="it1p05">'Untersuchungen über das Sehorgan der Arthropoden'</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page76">76</a>, <a href="#page100">100</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">Göttingen. 1879</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">GUDDEN</td>
+      <td class="pb05 it1p05">Quoted in Obersteiner</td>
+      <td class="bl vbm pb05"><a href="#page264">264</a></td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">HAECKEL</td>
+      <td class="pb05"></td>
+      <td class="bl vbm pb05"><a href="#page461">461</a>, <a href="#page462">462</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">HALLER, BELA</td>
+      <td class="it1p05">"Untersuchungen über die Hypophyse und die Infundibulärorgane"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page320">320</a>, <a
+      href="#page321">321</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Morph. Jahrbuch.</i> Vol. 25. 1898</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Untersuchungen über das Rückenmark der Teleostier"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page488">488</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Morph. Jahrbuch.</i> Vol. 23. 1895</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">HARDY</td>
+      <td class="it1p05">"On the Histological Features and Physiological Properties of the
+      Post-&#x0153;sophageal Nerve-cord of the Crustacea"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page110">110</a>, <a
+      href="#page159">159</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Phil. Trans. Roy. Soc.</i> 1894. B.</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">HARDY <span class="smaller">AND</span> MACDOUGALL</td>
+      <td class="it1p05">"On the Structure and Functions of the Alimentary Canal of Daphnia"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page112">112</a>, <a
+      href="#page206">206</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Proc. Camb. Phil. Soc.</i> Vol. 8. 1893</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">HATSCHEK</td>
+      <td class="it1p05">"Die Metamerie des Amphioxus und des Ammoc&#x0153;tes"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page289">289</a>, <a href="#page300">300</a>, <a
+      href="#page337">337</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Anat. Anzeig.</i>, 7 Jahrgang, 1892. <i>Verhandl. d. Anat. Gesell.
+      in Wien</i>, p. 136</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Studien über Entwicklung des Amphioxus"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page407">407</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Arbeit. d. Zool. Inst. z. Wien.</i> Vol. 4. 1881</td>
+    </tr>
+    <tr>
+      <td class="br pb05"></td>
+      <td class="pb05 it1p05">Quoted by Lankester</td>
+      <td class="bl vbm pb05"><a href="#page475">475</a></td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">HAZEN</td>
+      <td class="pb05 it1p05"><i>See</i> Patten and Hazen.</td>
+      <td class="bl pb05"></td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">HEIDENHAIN</td>
+      <td class="pb05"></td>
+      <td class="bl vbm pb05"><a href="#page258">258</a>, <a href="#page259">259</a></td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">HEIDER</td>
+      <td class="pb05 it1p05"><i>See</i> Korschelt and Heider.</td>
+      <td class="bl pb05"></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">HENSEN</td>
+      <td class="it1p05">"Zur Entwicklung des Nervensystem"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page465">465</a>, <a
+      href="#page466">466</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Virchows Archiv.</i> Vol. 30. 1864</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">HENSEN <span class="smaller">AND</span> VÖLCKERS</td>
+      <td class="it1p05">&nbsp;</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page265">265</a>, <a
+      href="#page266">266</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Archiv. f. Opthalmol.</i> Vol. 24. 1878</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">HERTWIG, O., <span class="smaller">AND</span> SAMASSA</td>
+      <td class="pb05 it1p05">Quoted in Zeigler's 'Lehrbuch der vergleichenden
+      Entwicklungsgeschichte der niederen Wirbelthiere.' 1902</td>
+      <td class="bl vbm pb05"><a href="#page485">485</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">HIS</td>
+      <td class="it1p05">"Die Neuroblasten und deren Entstehung im embryonalen Mark"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page465">465</a>, <a
+      href="#page466">466</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Archiv. f. Anat. u. Physiol. Anat. Abth.</i> 1889</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">HOFFMANN</td>
+      <td class="it1p05">"Ueber die Metamerie des Nachhirns und Hinterhirns, und ihre Beziehung zu
+      den segmentalen Kopfnerven bei Reptilien embryonen"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page276">276</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Zool. Anzeiger.</i> Vol. 12. 1889</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">HOLM</td>
+      <td class="it1p05">"Ueber die Organisation des <i>Eurypterus Fischeri</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page192">192</a>, <a href="#page240">240</a>, <a
+      href="#page241">241</a>, <a href="#page306">306</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Mem. d. l'Acad. Imp. d. Sci. d. St. Petersbourg.</i> Vol. 8.
+      1898</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">HOYER</td>
+      <td class="it1p05">"Ueber den Nachweis des Mucins in Geweben Mittelst der Färbe-Methode"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page131">131</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Archiv. f. Mikr. Anat.</i> Vol. 36. 1890</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05"><span class="pagenum" id="page507">{507}</span>
+        <p class="sp0">HUXLEY</p>
+      </td>
+      <td class="pb05 it1p05">"Hunterian Lectures." 1869</td>
+      <td class="bl vbm pb05"><a href="#page124">124</a>, <a href="#page258">258</a>, <a
+      href="#page259">259</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"On the Structure of the Mouth and Pharynx of the Scorpion"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page222">222</a>, <a href="#page225">225</a>, <a
+      href="#page271">271</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 8. 1860</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"On the Anatomy and Affinities of the Genus Pterygotus"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page238">238</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Mem. of the Geol. Survey.</i> Monograph I. 1859</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"On Cephalaspis and Pteraspis"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page327">327</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. of Geol. Soc.</i> Vol. 14. 1858</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">JAEKEL</td>
+      <td class="it1p05">"Ueber Tremataspis und Patten's Ableitung der Wirbelthiere von
+      Arthropoden"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page329">329</a>, <a href="#page339">339</a>, <a
+      href="#page340">340</a>, <a href="#page351">351</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Protocoll der Deutschen Geolog. Gesellschaft</i>, p. 84; in
+      <i>Zeitsch. d. Deutschen Geologischen Gesellsch.</i> Vol. 55. 1903</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Ueber die Organisation und systematische Stellung der Asterolepiden"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page345">345</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Ibid.</i>, p. 41</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">JOHNSON</td>
+      <td class="it1p05">"Contributions to the Comparative Anatomy of the Mammalian Eye, chiefly
+      based on Opthalmoscopic Examination"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page70">70</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Phil. Trans. Roy. Soc. B.</i> Vol. 194. 1901</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">JOSEPH</td>
+      <td class="it1p05">"Ueber das Achsenskelett des Amphioxus"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page444">444</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Zeitsch. f. wiss. Zool.</i> Vol. 59. 1895</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">JULIN <span class="smaller">AND</span> VAN BENEDEN</td>
+      <td class="it1p05">Recherches sur l'Organisation des Ascidies simples. "Sur l'hypophyse,"
+      etc.</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page425">425</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Archives de Biologie.</i> Vol. 2. 1881</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">KAENSCHE</td>
+      <td class="it1p05">"Beiträge zur Kenntniss der Metamorphose des <i>Ammoc&#x0153;tes
+      branchialis</i> in <i>Petromyzon</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page135">135</a>, <a
+      href="#page304">304</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Schneider's Beiträge.</i> Vol. 2. 1890</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">v. KENNEL</td>
+      <td class="it1p05">"Entwickelungsgeschichte von <i>Peripatus Edwardsii</i> und <i>Peripatus
+      torquatus</i>." II. Theil</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page398">398</a>, <a href="#page399">399</a>, <a
+      href="#page411">411</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Arbeit. a. d. Zool. Zoot. Instit. Würzburg.</i> Vol. 8. 1888</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">KERR</td>
+      <td class="it1p05">"On some Points in the Early Development of Motor Nerve-trunks and Myotomes
+      in <i>Lepidosiren paradoxa</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page461">461</a>, <a href="#page466">466</a>, <a
+      href="#page478">478</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Trans. Roy. Soc. Edin.</i> Vol. 41. 1904</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">KILLIAN</td>
+      <td class="it1p05">"Zur Metamerie des Selachierkopfes"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page262">262</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Verhandl. d. Anat. Gesell. Versamml. in München.</i> 1891</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">KISHINOUYE</td>
+      <td class="it1p05">"On the Development of <i>Limulus longispina</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page167">167</a>, <a href="#page238">238</a>, <a
+      href="#page252">252</a>, <a href="#page253">253</a>, <a href="#page273">273</a>, <a
+      href="#page320">320</a>, <a href="#page382">382</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Journ. of Coll. of Sci., Tokio.</i> Vol. 5. 1891</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">KLEINENBERG</td>
+      <td class="pb05 it1p05">Quoted by Beard</td>
+      <td class="bl vbm pb05"><a href="#page318">318</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">v. KÖLLIKER</td>
+      <td class="it1p05">"Die obere Trigeminus-Wurzel"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page280">280</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Arch. f. Mikr. Anat.</i> Vol. 53. 1899</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">v. KÖLLIKER <span class="smaller">AND</span> TERTERJANZ</td>
+      <td class="pb05 it1p05">Handbuch der Gewebe-Lehre. 6th Auflage. 1893</td>
+      <td class="bl vbm pb05"><a href="#page264">264</a>, <a href="#page425">425</a>, <a
+      href="#page488">488</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">KOHL</td>
+      <td class="it1p05">"Rudimentäre Wirbelthieraugen"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page94">94</a>, <a href="#page96">96</a>, <a
+      href="#page99">99</a>, <a href="#page101">101</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Bibliotheca Zoologica. Leukart und Chun.</i> Vol. 4 and Vol.
+      5</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">KOHN</td>
+      <td class="it1p05">"Ueber den Bau und die Entwicklung der sogenannten Carotis-drüse"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page428">428</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Archiv. f. Mikr. Anat.</i> Vol. 56. 1900</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05"><span class="pagenum" id="page508">{508}</span>
+        <p class="sp0">KORSCHELT <span class="smaller">AND</span> HEIDER</p>
+      </td>
+      <td class="pb05 it1p05">'Text-book of the Embryology of the Invertebrates.' Translated by M.
+      Bernard. 1900. Part III. and Part IV.</td>
+      <td class="bl vbm pb05"><a href="#page27">27</a>, <a href="#page73">73</a>, <a
+      href="#page88">88</a>, <a href="#page114">114</a>-<a href="#page116">116</a>, <a
+      href="#page397">397</a>, <a href="#page429">429</a>, <a href="#page431">431</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">KOWALEWSKY</td>
+      <td class="it1p05">"Ein Beitrag zur Kenntniss der Excretionsorgane der Pantopoden"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page421">421</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Mem. d. l'Acad. d. Imp. d. Sci. d. St. Petersbourg.</i> Ser. VII.
+      Vol. 38. 1890</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Une nouvelle glande lymphatique chez le scorpion d'Europe"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page423">423</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Ibid.</i> Ser. VIII. Vol. 5. 1897</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Étude Biologique sur les Clepsines"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page421">421</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Ibid.</i> Ser. VIII. Vol. 5. 1897</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Ein Beitrag zur Kenntniss der Excretionsorgane"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page420">420</a>, <a href="#page422">422</a>, <a
+      href="#page472">472</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Biologisches Centralblatt.</i> 1889</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Weitere Studien über die Entwicklungsgeschichte des <i>Amphioxus
+      lanceolatus</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page409">409</a>, <a
+      href="#page410">410</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Archiv. f. Mikr. Anat.</i> Vol. 13. 1877</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">KRIEGER</td>
+      <td class="it1p05">"Ueber das Centralnervensystem des Flusskrebses</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page101">101</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Zeitsch. f. wiss. Zool.</i> Vol. 33. 1880</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">v. KUPFFER</td>
+      <td class="it1p05">
+        <p>'Studien zur vergleichenden Entwicklungsgeschichte des Kopfes der Kranioten.'</p>
+        <p class="sp0"><span class="gap" style="width:2em">&nbsp;</span>Heft. 1. 'Die Entwicklung
+        des Kopfes von <i>Acipenser</i>'</p>
+      </td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page318">318</a>, <a href="#page319">319</a>, <a
+      href="#page320">320</a>, <a href="#page440">440</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">München. 1893</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05"><span class="gap" style="width:2em">&nbsp;</span>Heft. 2. 'Die Entwicklung
+      des Kopfes von <i>Ammoc&#x0153;tes Planeri</i>'</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page300">300</a>, <a
+      href="#page440">440</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">München. 1894</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">
+        <p><span class="gap" style="width:2em">&nbsp;</span>Heft. 3. 'Die Entwicklung der Kopfnerven
+        von <i>Ammoc&#x0153;tes Planeri</i>.'</p>
+        <p class="sp0"><span class="gap" style="width:2em">&nbsp;</span>Dritter Abschnitt. 'Die
+        Metamorphose des larvalen Nervensystems des Kopfes'</p>
+      </td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page228">228</a>, <a href="#page263">263</a>, <a
+      href="#page282">282</a>, <a href="#page283">283</a>, <a href="#page405">405</a>, <a
+      href="#page458">458</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">München. 1895</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">LANG</td>
+      <td class="pb05 it1p05">'Text-book of Comparative Anatomy.' Translated by H. M. and M.
+      Bernard</td>
+      <td class="bl vbm pb05"><a href="#page357">357</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">LANGERHANS</td>
+      <td class="it1p05">"Untersuchungen über <i>Petromyzon Planeri</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page94">94</a>-<a href="#page101">101</a>, <a
+      href="#page301">301</a>, <a href="#page405">405</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Bericht v. d. Verhandl. d. Naturforsch. Gesellsch. z.
+      Freiburg.</i> 1873</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">LANGLEY</td>
+      <td class="pb05 it1p05">Schäfer's 'Text-book of Physiology.' Vol. 2. 1900</td>
+      <td class="bl vbm pb05"><a href="#page2">2</a>, <a href="#page3">3</a>, <a
+      href="#page448">448</a></td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">LANKESTER</td>
+      <td class="pb05 it1p05">Article "Vertebrata" in the 'Encyclopædia Britannica'</td>
+      <td class="bl vbm pb05"><a href="#page484">484</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"On the Skeleto-trophic Tissues and Coxal Glands of Limulus, Scorpio, and
+      Mygale</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page137">137</a>, <a href="#page139">139</a>, <a
+      href="#page253">253</a>, <a href="#page320">320</a>, <a href="#page321">321</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 24. 1884</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Limulus an Arachnid"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page62">62</a>, <a href="#page238">238</a>, <a
+      href="#page241">241</a>, <a href="#page306">306</a>, <a href="#page361">361</a>, <a
+      href="#page366">366</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 21. 1881</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">'Extinct Animals'</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page22">22</a>, <a href="#page150">150</a>, <a
+      href="#page345">345</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">London. Constable &amp; Co. 1906</td>
+    </tr>
+    <tr>
+      <td class="br pb05"></td>
+      <td class="pb05 it1p05">A treatise on Zoology. Edited by E. Ray Lankester. Part II. 'The
+      Entero-c&#x0153;la and the C&#x0153;lomoc&#x0153;la'</td>
+      <td class="bl vbm pb05"><a href="#page472">472</a>-<a href="#page478">478</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05"><span class="pagenum" id="page509">{509}</span>
+        <p class="sp0">LANKESTER <span class="smaller">AND</span> POWRIE</p>
+      </td>
+      <td class="it1p05">"A Monograph of the Fishes of the Old Red Sandstone of Britain." Part I.
+      "The Cephalaspidæ"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page29">29</a>, <a href="#page275">275</a>, <a
+      href="#page327">327</a>, <a href="#page339">339</a>, <a href="#page345">345</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Palæontographical Soc.</i> 1868</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">LANKESTER, BENHAM, <span class="smaller">AND</span>
+      BECK</td>
+      <td class="it1p05">"On the Muscular and Endo-skeletal Systems of Limulus and Scorpio, with
+      some Notes on the Anatomy and Generic Characters of Scorpions"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page177">177</a>, <a href="#page222">222</a>, <a
+      href="#page224">224</a>, <a href="#page313">313</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Trans. Zool. Soc.</i> Vol. 11. 1885</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">LANKESTER <span class="smaller">AND</span> BOURNE</td>
+      <td class="it1p05">"The Minute Structure of the Lateral and Central Eyes of Scorpio and
+      Limulus"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page74">74</a>, <a href="#page81">81</a>-<a
+      href="#page83">83</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 23</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">LANKESTER <span class="smaller">AND</span> WILLEY</td>
+      <td class="it1p05">"The Development of the Atrial Chamber of Amphioxus"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page409">409</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 31. 1890</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">LANKESTER <span class="smaller">AND</span> GULLAND</td>
+      <td class="it1p05">"Evidence in Favour of the View that the Coxal Gland of Limulus and of
+      other Arachnids is a Modified Nephridium"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page429">429</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 25. 1885</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">LATREILLE</td>
+      <td class="pb05"></td>
+      <td class="bl vbm pb05"><a href="#page221">221</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">LAURIE</td>
+      <td class="it1p05">"The Anatomy and Relations of the Eurypteridæ"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page237">237</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Trans. Roy. Soc. Edin.</i> Vol. 37. 1893</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"On a Silurian Scorpion and some Additional Eurypterid Remains from the
+      Pentland Hills</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page238">238</a>, <a
+      href="#page239">239</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Ibid.</i> Vol. 34. 1899</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">LEYDIG</td>
+      <td class="pb05"></td>
+      <td class="bl vbm pb05"><a href="#page91">91</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">LOCY</td>
+      <td class="it1p05">"Contributions to the Structure and Development of the Vertebrate
+      Head"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page179">179</a>, <a
+      href="#page262">262</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Journ. Morph.</i> Vol. 11. 1895</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">LOEB, LEO, <span class="smaller">AND</span> R. M.
+      STRONG</td>
+      <td class="it1p05">"On Regeneration in the Pigmented Skin of the Frog, and on the Character of
+      the Chromatophores"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page470">470</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Amer. Jour. of Anat.</i> Vol. 3. 1904</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">LOWNE</td>
+      <td class="it1p05">'The Anatomy, Physiology, Morphology, and Development of the Blow-fly'</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page369">369</a>, <a href="#page370">370</a>, <a
+      href="#page375">375</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">London. 1895</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">LUGARO</td>
+      <td class="pb05 it1p05">Quoted by Anderson</td>
+      <td class="bl vbm pb05"><a href="#page467">467</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">LWOFF</td>
+      <td class="it1p05">"Ueber den Zusammenhang von Markrohr und Chorda beim Amphioxus und ähnliche
+      Verbältnisse bei Anneliden"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page444">444</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Zeitsch. f. wiss. Zool.</i> Vol. 56. 1893</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">MAAS</td>
+      <td class="it1p05">"Ueber Entwicklungstadien der Vorniere und Urniere bei Myxine"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page392">392</a>, <a href="#page402">402</a>, <a
+      href="#page412">412</a>, <a href="#page419">419</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Zool. Jahrbuch.</i> Vol. 10. 1897</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">MACBRIDE</td>
+      <td class="it1p05">"Further Remarks on the Development of Amphioxus"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page410">410</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 43. 1900</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">McDOUGALL</td>
+      <td class="pb05 it1p05"><i>See</i> Hardy and McDougall.</td>
+      <td class="bl pb05"></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05"><span class="pagenum" id="page510">{510}</span>
+        <p class="sp0">MACLEOD</p>
+      </td>
+      <td class="it1p05">"Recherches sur la structure et la signification de l'appareil respiratoire
+      des Arachnides"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page169">169</a>, <a
+      href="#page174">174</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Archiv. de Biol.</i> Vol. 5. 1881</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">MAGNUS</td>
+      <td class="it1p05">"Versuche am überlebenden Dünndarm von Säugethieren"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page447">447</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Archiv. f. d. Ges. Physiologie.</i> Vols. 102, 103. 1904</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">MARK</td>
+      <td class="pb05"></td>
+      <td class="bl vbm pb05"><a href="#page115">115</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">MARSHALL</td>
+      <td class="it1p05">"On the Head-cavities and Associated Nerves of Elasmobranchs"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page185">185</a>, <a
+      href="#page186">186</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 21. 1881</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"The Segmental Value of the Cranial Nerves"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page260">260</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Journ. of Anat. and Physiol.</i> Vol. 16. 1882</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">MASTERMAN</td>
+      <td class="it1p05">"On the Diplochorda"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page16">16</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 43. 1900</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">MAURER</td>
+      <td class="it1p05">"Die Schilddrüse, Thymus und andere Schlundspaltenderivate bei den
+      Eidechse"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page427">427</a>, <a
+      href="#page428">428</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Morph. Jahrbuch.</i> Vol. 27. 1899</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">MAYER, F.</td>
+      <td class="it1p05">"Das Centralnervensystem von Ammoc&#x0153;tes"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page489">489</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Anat. Anzeig.</i> Vol. 13. 1897</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">MAYER, P.</td>
+      <td class="it1p05">"Ueber die Entwicklung des Herzens und der grossen Gefässstämme bei den
+      Selachiern"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page179">179</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Mitth. a. d. Zool. Stat. z. Neapel.</i> Vol. 7. 1887</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">METSCHNIKOW</td>
+      <td class="pb05 it1p05">Quoted by Kowalewsky</td>
+      <td class="bl vbm pb05"><a href="#page422">422</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">MEYER</td>
+      <td class="it1p05">"Studien über den Körperbau der Anneliden"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page403">403</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Mitth. a. d. Zool. Stat. z. Neapel.</i> Vol. 7. 1887</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">MILNE-EDWARDS</td>
+      <td class="it1p05">"Anatomie des Limules"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page157">157</a>, <a href="#page159">159</a>, <a
+      href="#page176">176</a>, <a href="#page177">177</a>, <a href="#page313">313</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Annales des Sciences Naturelles.</i> Ser. 5. Vol. 17. 1872</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">MINCHIN</td>
+      <td class="pb05 it1p05">A treatise on Zoology. Edited by Ray Lankester. Part II. "The Porifera
+      and C&#x0153;lenterata"</td>
+      <td class="bl vbm pb05"><a href="#page473">473</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">MITSUKURI</td>
+      <td class="it1p05">"On the Fate of the Blastopore, the Relations of the Primitive Streak, and
+      the Formation of the Posterior End of the Embryo in Chelonia," etc.</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page179">179</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Journ. Coll. Sci.</i> Tokyo. Vol. 10. 1896</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">MOTT</td>
+      <td class="pb05 it1p05">"Croonian Lectures of the Roy. Coll. of Physicians," 1900</td>
+      <td class="bl vbm pb05"><a href="#page469">469</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">MOTT <span class="smaller">AND</span> HALLIBURTON</td>
+      <td class="it1p05">"On the Chemistry of Nerve-degeneration"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page469">469</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Phil. Trans. Roy. Soc. B.</i> Vol. 194. 1901</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">MÜLLER, J.</td>
+      <td class="pb05"></td>
+      <td class="bl vbm pb05"><a href="#page1">1</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Vergleichende Anatomie der Myxinoiden"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page126">126</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Abhandl. d. Kgl. Akad. d. Wiss.</i> Berlin. 1834</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">MÜLLER, W.</td>
+      <td class="it1p05">"Ueber die Stammes Entwickelung des Sehorgans der Wirbelthiere"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page96">96</a>-<a href="#page100">100</a>, <a
+      href="#page105">105</a>, <a href="#page108">108</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">Festgabe C. Ludwig. Leipzig. 1874</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">NEAL</td>
+      <td class="it1p05">"The Segmentation of the Nervous System in <i>Squalus acanthias</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page179">179</a>, <a href="#page266">266</a>, <a
+      href="#page300">300</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Bull. of Mus. Comp. Zool.</i> Harvard. Vol. 31. 1898</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">NESTLER</td>
+      <td class="it1p05">"Beiträge zur Anatomie und Entwicklungsgeschichte von <i>Petromyzon
+      Planeri</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page168">168</a>, <a href="#page171">171</a>, <a
+      href="#page175">175</a>, <a href="#page445">445</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Archiv. f. Naturgesch. Jahrgang</i>, 56. Vol. I. 1890</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05"><span class="pagenum" id="page511">{511}</span>
+        <p class="sp0">NIESKOWSKI</p>
+      </td>
+      <td class="it1p05">"Der <i>Eurypterus Remipes</i> aus den obersilurischen Schichten der Insel
+      Oesel"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page26">26</a>, <a href="#page239">239</a>, <a
+      href="#page240">240</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Arch. f. d. Naturkunde Liv-Ehst-und Kurlands.</i> 1st Ser. Vol. 3.
+      1858</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">NUSBAUM, J.</td>
+      <td class="it1p05">"Einige neue Thatsachen zur Entwicklungsgeschichte des <i>Hypophysis
+      Cerebri</i> bei Säugethieren"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page320">320</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Anat. Anzeiger.</i> Vol. 12. 1896</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">OBERSTEINER</td>
+      <td class="pb05 it1p05">'Central Nervous System.' Translated by Hill. 1896</td>
+      <td class="bl vbm pb05"><a href="#page264">264</a>, <a href="#page280">280</a></td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">OKEN</td>
+      <td class="pb05"></td>
+      <td class="bl vbm pb05"><a href="#page258">258</a></td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">OWEN</td>
+      <td class="pb05 it1p05">"Essays on the Conario-Hypophysial Tract, and the Aspects of the Body
+      in Vertebrate and Invertebrate Animals"</td>
+      <td class="bl vbm pb05"><a href="#page14">14</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"On the Anatomy of the American King-crab (<i>Limulus polyphemus</i>)"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page211">211</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Trans. Linn. Soc.</i> Vol. 28. 1873</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">PANDER</td>
+      <td class="it1p05">'Monographie der fossilen Fische des Silurischen Systems des
+      russisch-baltischen Gouvernements'</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page327">327</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">St. Petersbourg. 1856</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">PARKER, G. H.</td>
+      <td class="it1p05">"The Retina and Optic Ganglia in Decapods, especially in Astacus"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page91">91</a>, <a href="#page93">93</a>, <a
+      href="#page97">97</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Mitth. a. d. Zool. Stat. z. Neapel.</i> Vol. 12. 1895</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"The Compound Eyes in Crustaceans"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page99">99</a>, <a href="#page100">100</a>, <a
+      href="#page114">114</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Bull. of Harvard Mus. of Comp. Zool.</i> Vol. 20. 1890</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"The Function of the Lateral-line Organs in Fishes"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page357">357</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Bull. of the Fisheries Bureau.</i> Washington. Vol. 24. 1904</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Studies on the Eyes of Arthropods"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page73">73</a>, <a href="#page79">79</a>, <a
+      href="#page83">83</a>-<a href="#page85">85</a>, <a href="#page114">114</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Journ. of Morphology.</i> Vols. 1 and 2. 1887 and 1889</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">PARKER, W. K.</td>
+      <td class="it1p05">"On the Skeleton of the Marsipobranch Fishes"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page120">120</a>, <a href="#page125">125</a>, <a
+      href="#page126">126</a>, <a href="#page131">131</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Phil. Trans. Roy. Soc.</i> 1883</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">PATTEN</td>
+      <td class="it1p05">"On the Origin of Vertebrates from Arachnids"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page352">352</a>, <a
+      href="#page353">353</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 31. 1890</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"On the Morphology and Physiology of the Brain and Sense-organs of
+      Limulus"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page358">358</a>-<a href="#page367">367</a>, <a
+      href="#page371">371</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 35. 1893</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"New Facts concerning Bothriolepis"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page32">32</a>, <a href="#page351">351</a>, <a
+      href="#page450">450</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Biological Bulletin.</i> Vol. 7. 1904</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"On the Structure and Classification of the Tremataspidæ"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page329">329</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Mem. d. l'Acad. Imp. d. Sci. de St. Petersbourg.</i> Vol. 13.
+      1903</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"On the Structure of the Pteraspidæ and Cephalaspidæ"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page415">415</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>The American Naturalist.</i> Vol. 37. 1903</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"On the Appendages of Tremataspis"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page351">351</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>The American Naturalist.</i> Vol. 37. 1903</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"On Structures Resembling Dermal Bones in Limulus"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page346">346</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Anat. Anzeig.</i> Vol. 9. 1894</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05"><span class="pagenum" id="page512">{512}</span>
+        <p class="sp0">PATTEN <span class="smaller">AND</span> HAZEN</p>
+      </td>
+      <td class="it1p05">"The Development of the Coxal Gland, etc., of <i>Limulus
+      Polyphemus</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page408">408</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Journ. of Morphol.</i> Vol. 16. 1900</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">PATTEN <span class="smaller">AND</span> REDENBAUGH</td>
+      <td class="it1p05">Studies on Limulus. II. "The Nervous System of <i>Limulus
+      Polyphemus</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page314">314</a>, <a href="#page315">315</a>, <a
+      href="#page381">381</a>, <a href="#page382">382</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Journ. of Morphol.</i> Vol. 16. 1900</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">PERLIA</td>
+      <td class="pb05 it1p05">Quoted by Edinger</td>
+      <td class="bl vbm pb05"><a href="#page264">264</a></td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">PICK</td>
+      <td class="pb05 it1p05"><span class="hid">Quo</span>"<span class="hid">ed by Ed</span>"<span
+      class="hid">nger</span></td>
+      <td class="bl vbm pb05"><a href="#page265">265</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">PLATT</td>
+      <td class="it1p05">"A Contribution to the Morphology of the Vertebrate Head, based on a Study
+      of <i>Acanthias vulgaris</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page253">253</a>, <a href="#page265">265</a>-<a
+      href="#page267">267</a>, <a href="#page273">273</a>, <a href="#page274">274</a>, <a
+      href="#page279">279</a>, <a href="#page284">284</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Journ. Morphol.</i> Vol. 5. 1891</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Fibres connecting the Central Nervous System and Chorda in Amphioxus"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page443">443</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Anat. Anzeig.</i> 1892</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">PRICE</td>
+      <td class="it1p05">"Development of the Excretory Organs of <i>Bdellostoma Stouti</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page394">394</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Zool. Jahrbuch.</i> Vol. 10. 1897</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">RABL</td>
+      <td class="it1p05">"Ueber die Metamerie des Wirbelthierkopfes"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page258">258</a>, <a
+      href="#page262">262</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Verhandl. der Anat. Gesellsch. Versamml. in Wien. Anat.
+      Anzeig.</i> 1892</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Die Entwicklung und Structur der Nebennieren bei den Vögeln"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page424">424</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Arch. f. mikr. Anat.</i> Vol. 38. 1891</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">RAMÓN Y. CAJAL</td>
+      <td class="pb05"></td>
+      <td class="bl vbm pb05"><a href="#page72">72</a>, <a href="#page465">465</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">RATHKE</td>
+      <td class="it1p05">"Anatomie des Querders"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page161">161</a>, <a href="#page169">169</a>, <a
+      href="#page304">304</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Naturforsch. Gesellsch. zu Dantzig.</i> Vol. 2. 1827</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">REDENBAUGH</td>
+      <td class="pb05 it1p05"><i>See</i> Patten and Redenbaugh.</td>
+      <td class="bl pb05"></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">REICHENBACH</td>
+      <td class="it1p05">"Entwicklungs-geschichte des Flusskrebses"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page98">98</a>-<a href="#page100">100</a>, <a
+      href="#page114">114</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Abhandl. d. Senckenbergischen Naturforsch. Gesellsch.</i> Vol. 14.
+      1886.</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">RETZIUS</td>
+      <td class="pb05 it1p05">'Biologische Untersuchungen.' Vol. 1. 1890. "Zur Kenntniss des
+      Nervensystem der Crustaceen"</td>
+      <td class="bl vbm pb05"><a href="#page20">20</a>, <a href="#page489">489</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">ROHON</td>
+      <td class="it1p05">Die Obersilurischen Fische von Oesel. 1st Theil. "Thyestidæ und
+      Tremataspidæ"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page32">32</a>, <a href="#page275">275</a>, <a
+      href="#page276">276</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Mem. d. l'Acad. Imp. d. Sci. d. St. Petersbourg.</i> 7th Ser. Vol.
+      38. 1892</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Weitere Mittheilungen über die Gattung <i>Thyestes</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page327">327</a>-<a href="#page330">330</a>, <a
+      href="#page339">339</a>-<a href="#page341">341</a>, <a href="#page382">382</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Bull. d. l'Acad. d. St. Petersbourg.</i> 5th Ser. Vol. 4.
+      1896</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">ROLPH</td>
+      <td class="it1p05">"Untersuchungen über den Bau des <i>Amphioxus lanceolatus</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page444">444</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Morphol. Jahrbuch.</i> Vol. 2. 1887</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">RÜCKERT, J.</td>
+      <td class="it1p05">"Entwicklung der Excretionsorgane"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page392">392</a>, <a href="#page393">393</a>, <a
+      href="#page400">400</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Merkel und Bonnet; Anat. Hefte.</i> Vol. 1. 1891.</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Ueber die Entstehung der Excretionsorgane bei Selachiern"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page403">403</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Archiv. f. Anatomie.</i> 1888</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">ST. HILAIRE</td>
+      <td class="it1p05">"Sur la Vertèbre"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page11">11</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>La Revue Encyclopédique.</i> 1822</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05"><span class="pagenum" id="page513">{513}</span>
+        <p class="sp0">SAMASSA</p>
+      </td>
+      <td class="it1p05">"Bemerkungen über die Methode der Vergleichenden
+      Entwicklungsgeschichte"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page462">462</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Biol. Centralblatt.</i> Vol. 18. 1898</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">SCHAFFER</td>
+      <td class="it1p05">"Ueber das Knorpelige Skelett von Ammoc&#x0153;tes"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page126">126</a>-<a href="#page135">135</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Zeitsch. f. wiss. Zool.</i> Vol. 61. 1896</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Ueber die Thymusanlage bei <i>Petromyzon Planeri</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page426">426</a>-<a href="#page428">428</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Sitzungsber. d. K. Akad d. Wiss. in Wien.</i> Vol. 103. 1894</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">SCHIMKÉWITSCH</td>
+      <td class="it1p05">"Sur la structure et sur la signification de l'Endosternite des
+      Arachnides"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page143">143</a>-<a href="#page145">145</a>, <a
+      href="#page342">342</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Zool. Anzeig.</i> 1893</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Anatomie de l'Epeire"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page369">369</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Ann. d. Sci. Nat.</i> Vol. 17. 1884</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">SCHMIDT</td>
+      <td class="it1p05">"Die Crustaceen-fauna der Eurypterenschichten von Rootziküll auf
+      Oesel"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page190">190</a>, <a href="#page191">191</a>, <a
+      href="#page236">236</a>, <a href="#page240">240</a>, <a href="#page329">329</a>, <a
+      href="#page341">341</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Mem. d'Acad. Imp. d. Sci. d. St. Petersbourg.</i> Vol. 31.
+      1883</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">SCHMIEDEBERG</td>
+      <td class="it1p05">"Ueber die chemische Zusammensetzung des Knorpels"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page147">147</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Arch. f. exper. Pathol. und Pharmak.</i> Vol. 28. 1891</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">SCHNEIDER, A.</td>
+      <td class="it1p05">'Beiträge zur Anatomie und Entwicklungsgeschichte der Wirbelthiere'</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page128">128</a>, <a href="#page130">130</a>, <a
+      href="#page172">172</a>, <a href="#page195">195</a>, <a href="#page197">197</a>, <a
+      href="#page213">213</a>, <a href="#page310">310</a>, <a href="#page445">445</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">Berlin. 1879</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">SCHNEIDER, G.</td>
+      <td class="it1p05">"Ueber phagocytäre Organe und Chloragogenzellen der Oligochæta"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page421">421</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Zeitsch. f. wiss. Zool.</i> Vol. 61. 1896</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">SCOTT</td>
+      <td class="it1p05">"Notes on the Development of Petromyzon"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page42">42</a>, <a href="#page78">78</a>, <a
+      href="#page111">111</a>, <a href="#page112">112</a>, <a href="#page406">406</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Journ. of Morphol.</i> Vol. 1. 1887</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">SEDGWICK</td>
+      <td class="it1p05">"A Monograph of the Development of <i>Peripatus capensis</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page397">397</a>-<a href="#page400">400</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Studies from the Morphological Laboratory, Cambridge.</i> Vol. 4.
+      1888</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Development of the Kidney in its Relation to the Wolffian Body in the
+      Chick"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page390">390</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 20. 1880</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Early Development of the Wolffian Duct and Anterior Wolffian Tubules in
+      the Chick; with some Remarks on the Vertebrate Excretory System"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page393">393</a>, <a href="#page394">394</a>, <a
+      href="#page400">400</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 21. 1881</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">SEMON</td>
+      <td class="it1p05">"Das Excretionssystem der Myxinoiden"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page400">400</a>, <a
+      href="#page419">419</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Festschrift f. Gegenbaur.</i> Leipzig. 1897</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">SEMPER</td>
+      <td class="it1p05">"Die Stammesverwandschaft der Wirbelthiere und Wirbellosen"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page390">390</a>, <a
+      href="#page392">392</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Arbeit. a. d. Zool. Zoot. Inst. Würzburg.</i> Vol. 2. 1875</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Das Urinogenitalsystem der Plagiostomen und seine Bedeutung für die
+      übrigen Wirbelthiere"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page390">390</a>, <a
+      href="#page392">392</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Ibid.</i> Vol. 2. 1875</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">SHELDON</td>
+      <td class="it1p05">"On the Development of <i>Peripatus Nova-Zealandiæ</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page400">400</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Studies from the Morphological Laboratory, Cambridge.</i> Vol. 4.
+      1889</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05"><span class="pagenum" id="page514">{514}</span>
+        <p class="sp0">SHERRINGTON</p>
+      </td>
+      <td class="it1p05">"On the Anatomical Constitution of the Nerves of Muscles"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page267">267</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Journ. of Physiol.</i> Vol. 17. 1894. <i>Proc. of Physiol.
+      Soc.</i> June 23</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">SHIPLEY</td>
+      <td class="pb05"></td>
+      <td class="bl vbm pb05"><a href="#page334">334</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"On some points in the Development of <i>Petromyzon fluviatilis</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page167">167</a>, <a href="#page305">305</a>, <a
+      href="#page378">378</a>, <a href="#page401">401</a>, <a href="#page405">405</a>, <a
+      href="#page406">406</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 27. 1887</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">v. SMIRNOW</td>
+      <td class="it1p05">"Ueber die Nervenendigungen in den Nieren der Säugethiere"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page477">477</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Anat. Anzeiger.</i> Vol. 19. 1901</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">SMITH, ELLIOT</td>
+      <td class="pb05"></td>
+      <td class="bl vbm pb05"><a href="#page17">17</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">SPANGENBERG</td>
+      <td class="it1p05">"Zur Kenntniss von <i>Branchipus stagnalis</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page396">396</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Zeitsch. f. wiss. Zool.</i> Vol. 25. 1875</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">SPENGEL</td>
+      <td class="it1p05">'Die Enteropneusten'</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page494">494</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">Berlin. 1893</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">STARR</td>
+      <td class="pb05 it1p05">Quoted by Edinger</td>
+      <td class="bl vbm pb05"><a href="#page265">265</a>, <a href="#page266">266</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">STUDNIÇKA</td>
+      <td class="it1p05">"Sur les organes pariétaux de <i>Petromyzon Planeri</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page80">80</a>, <a href="#page81">81</a>, <a
+      href="#page86">86</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Sitzungsber. d. K. Gesell. d. Wiss. in Prag.</i> 1893</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Ueber den feineren Bau der Parietalorgane von <i>Petromyzon
+      marinus</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page81">81</a>, <a href="#page86">86</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Sitzungsber. d. K. böhmischen Gesell. d. Wiss. Prag.</i> 1899</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">TAKAMINE</td>
+      <td class="it1p05">"The Isolation of the Active Principle of the Supra-renal Gland"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page423">423</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Journ. of Physiol.</i> Vol. 27. <i>Proc. of Physiol. Soc.</i>,
+      Dec. 14, 1901</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">TARNANI</td>
+      <td class="it1p05">"On the Anatomy of the Thelyphonides"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page190">190</a>, <a href="#page206">206</a>-<a
+      href="#page208">208</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Revue des Sciences Naturelles, St. Petersbourg.</i> 1890</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Die genitalen Organe der Thelyphonus"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page190">190</a>, <a href="#page206">206</a>-<a
+      href="#page208">208</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Biol. Centralblatt.</i> Vol. 9. 1889</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">TRAQUAIR</td>
+      <td class="it1p05">"Report on Fossil Fishes collected by the Geological Survey of Scotland in
+      the Silurian Rocks of the South of Scotland"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page343">343</a>-<a href="#page345">345</a>, <a
+      href="#page350">350</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Trans. Roy. Soc., Edin.</i> Vol. 39. 1899</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">VIALLANES</td>
+      <td class="it1p05">"Contribution à l'histologie du système nerveux des Invertébrés; la lame
+      ganglionnaire de la Langouste"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page100">100</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Ann. Sci. Nat.</i> Vol. 13</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">VINCENT, SWALE</td>
+      <td class="it1p05">"The Carotid Gland of Mammalia and its Relation to the Supra-renal Capsule,
+      with some Remarks upon Internal Secretion and the <span class="correction"
+      title="Original reads 'Phyogeny'.">Phylogeny</span> of the latter Organ"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page424">424</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Anat. Anzeiger.</i> Vol. 18. 1900</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Contributions to the Comparative Anatomy and Histology of the Supra-renal
+      Capsules"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page424">424</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Trans. Zool. Soc.</i> Vol. 14. 1897</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">VIRCHOW</td>
+      <td class="it1p05">"Transformation and Descent"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page479">479</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Journ. of Path. and Bacter.</i> Vol. 1. 1893</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">VOGT</td>
+      <td class="pb05"></td>
+      <td class="bl vbm pb05"><a href="#page258">258</a></td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">VÖLCKERS</td>
+      <td class="pb05 it1p05"><i>See</i> Hensen and Völckers.</td>
+      <td class="bl pb05"></td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">WAGNER</td>
+      <td class="pb05 it1p05">Quoted by Gaubert</td>
+      <td class="bl pb05"></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05"><span class="pagenum" id="page515">{515}</span>
+        <p class="sp0">WEISS</p>
+      </td>
+      <td class="it1p05">"Excretory Tubules in <i>Amphioxus Lanceolatus</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page426">426</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 31. 1890</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">WELDON</td>
+      <td class="it1p05">"On the Supra-renal Bodies of Vertebrates"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page420">420</a>, <a href="#page424">424</a>, <a
+      href="#page429">429</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Q. J. Micr. Sci.</i> Vol. 25. 1885</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Note on the Origin of the Supra-renal Bodies in Vertebrates"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page424">424</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Proc. Roy. Soc.</i> Vol. 37. 1884</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">WHEELER</td>
+      <td class="it1p05">"Development of the Urino-genital Organs of the Lamprey"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page402">402</a>, <a
+      href="#page405">405</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Zool. Jahrbuch.</i> Vol. 13. 1899</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">v. WIJHE</td>
+      <td class="it1p05">"Ueber die Mesodermsegmente des Rumpfes und die Entwicklung des
+      Excretionsystems bei Selachiern"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page155">155</a>-<a href="#page157">157</a>, <a
+      href="#page172">172</a>, <a href="#page173">173</a>, <a href="#page188">188</a>, <a
+      href="#page234">234</a>, <a href="#page258">258</a>, <a href="#page260">260</a>, <a
+      href="#page262">262</a>, <a href="#page263">263</a>, <a href="#page266">266</a>, <a
+      href="#page273">273</a>, <a href="#page280">280</a>, <a href="#page308">308</a>, <a
+      href="#page390">390</a>-<a href="#page393">393</a>, <a href="#page397">397</a>, <a
+      href="#page400">400</a>, <a href="#page406">406</a>-<a href="#page408">408</a>, <a
+      href="#page412">412</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Archiv. f. Mikr. Anat.</i> Vol. 33. 1889</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">"Beiträge zur Anatomie der Kopfregion des <i>Amphioxus
+      lanceolatus</i>"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page410">410</a>, <a href="#page426">426</a>-<a
+      href="#page428">428</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Petrus Camper. Deel.</i> 1; <i>Aflevering.</i> 2</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">WILLEY</td>
+      <td class="pb05 it1p05"><i>See</i> Lankester and Willey.</td>
+      <td class="bl pb05"></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">WOLFF</td>
+      <td class="it1p05">"Die Cuticula der Wirbelthierepidermis"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page302">302</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Jen. Zeitsch. f. Naturwissenschaft.</i> Vol. 23. 1889</td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br it1p05">WOODWARD, H.</td>
+      <td class="it1p05">"A Monograph of the British Fossil Crustacea, belonging to the order
+      Merostomata"</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page235">235</a>-<a href="#page240">240</a>, <a
+      href="#page249">249</a>, <a href="#page251">251</a>, <a href="#page275">275</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05"><i>Palæontographical Society.</i> 1878</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">WOODWARD, SMITH</td>
+      <td class="pb05"></td>
+      <td class="bl vbm pb05"><a href="#page339">339</a></td>
+    </tr>
+    <tr>
+      <td rowspan="2" class="br"></td>
+      <td class="it1p05">'Catalogue of Fossil Fishes in the British Museum.' Part II.</td>
+      <td rowspan="2" class="bl vbm pb05"><a href="#page29">29</a>, <a href="#page326">326</a>, <a
+      href="#page327">327</a>, <a href="#page344">344</a>, <a href="#page349">349</a>, <a
+      href="#page351">351</a></td>
+    </tr>
+    <tr>
+      <td class="ar pb05 pr05">London. 1891</td>
+    </tr>
+    <tr>
+      <td class="br pb05 it1p05">v. ZITTEL</td>
+      <td class="pb05 it1p05">Handbuch der Palæontologie</td>
+      <td class="bl vbm pb05"><a href="#page190">190</a></td>
+    </tr>
+  </table>
+
+  <div><span class="pagenum" id="page517">{517}</span></div>
+
+  <p class="ac" style="margin-bottom:2.9ex;"><span class="larger">GENERAL INDEX</span></p>
+
+  <p class="sp3 ac">[<i>The numbers in dark type refer to illustrations</i>]</p>
+
+  <div class="poem sp3">
+    <p>Acilius larva, eye of, <b><a href="#page78">78</a></b>, <b><a href="#page83">83</a></b></p>
+    <p>Acromegaly, <a href="#page425">425</a></p>
+    <p>Actinotrocha, <a href="#page438">438</a></p>
+    <p>Addison's disease, <a href="#page423">423</a></p>
+    <p>Adelopthalmus, <a href="#page249">249</a></p>
+    <p>Adrenalin, <a href="#page423">423</a>, <a href="#page491">491</a></p>
+    <p>Adrenals, <a href="#page423">423</a>, <a href="#page491">491</a></p>
+    <p>Agnathostomatous fishes, <a href="#page29">29</a>, <a href="#page343">343</a></p>
+    <p>Alimentary canal, <a href="#page433">433</a></p>
+    <p><span class="hid">Alim</span>"<span class="hid">ntary c</span>"<span class="hid">nal,</span>
+    Ammoc&#x0153;tes, <a href="#page168">168</a>, <a href="#page405">405</a>, <a
+    href="#page445">445</a></p>
+    <p><span class="hid">Alim</span>"<span class="hid">ntary c</span>"<span class="hid">nal,</span>
+    invertebrate, compared to tube of central nervous system of vertebrate, <a
+    href="#page43">43</a>, <a href="#page433">433</a></p>
+    <p><span class="hid">Alim</span>"<span class="hid">ntary c</span>"<span class="hid">nal,</span>
+    innervation of, <a href="#page447">447</a></p>
+    <p><span class="hid">Alim</span>"<span class="hid">ntary c</span>"<span class="hid">nal,</span>
+    origin of, <a href="#page444">444</a></p>
+    <p><span class="hid">Alim</span>"<span class="hid">ntary c</span>"<span class="hid">nal,</span>
+    position of vertebrate and invertebrate, <b><a href="#page10">10</a></b></p>
+    <p><span class="hid">Alim</span>"<span class="hid">ntary c</span>"<span class="hid">nal,</span>
+    possibility of formation of new, <a href="#page58">58</a></p>
+    <p><span class="hid">Alim</span>"<span class="hid">ntary c</span>"<span class="hid">nal,</span>
+    relationship between notochord and, <a href="#page434">434</a></p>
+    <p>Ammoc&#x0153;tes, <b><a href="#page32">32</a></b>, <b><a href="#page245">245</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> an ancestral type, <a
+    href="#page35">35</a>, <a href="#page309">309</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> alimentary canal, <a
+    href="#page168">168</a>, <a href="#page405">405</a>, <a href="#page445">445</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> auditory organ, <a
+    href="#page378">378</a>, <b><a href="#page379">379</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> brain, <a
+    href="#page39">39</a>, <b><a href="#page40">40</a></b>, <b><a href="#page41">41</a></b>, <a
+    href="#page45">45</a>, <b><a href="#page46">46</a></b>, <a href="#page48">48</a>, <b><a
+    href="#page54">54</a></b>, <a href="#page61">61</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> branchial appendages, <a
+    href="#page161">161</a>, <b><a href="#page162">162</a></b>, <b><a href="#page163">163</a></b>,
+    <b><a href="#page164">164</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, bra</span>"<span
+    class="hid">chial</span> basket-work, <b><a href="#page126">126</a></b>, <b><a
+    href="#page128">128</a></b>, <b><a href="#page296">296</a></b>, <a href="#page331">331</a>,
+    <b><a href="#page335">335</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, bra</span>"<span
+    class="hid">chial</span> chamber, <a href="#page161">161</a>, <a href="#page168">168</a>, <b><a
+    href="#page162">162</a></b>, <b><a href="#page163">163</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, bra</span>"<span
+    class="hid">chial</span> circulation in Limulus and, <a href="#page174">174</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, bra</span>"<span
+    class="hid">chial</span> diaphragms, <a href="#page161">161</a>, <a href="#page167">167</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, bra</span>"<span
+    class="hid">chial</span> lamellæ, <b><a href="#page175">175</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, bra</span>"<span
+    class="hid">chial</span> muscles, <a href="#page171">171</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, bra</span>"<span
+    class="hid">chial</span> nerves, <a href="#page164">164</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, bra</span>"<span
+    class="hid">chial</span> segments, <b><a href="#page178">178</a></b>, <b><a
+    href="#page312">312</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> cartilage, hard, <a
+    href="#page133">133</a>, <b><a href="#page133">133</a></b>, <b><a href="#page293">293</a></b>,
+    <b><a href="#page294">294</a></b>, <a href="#page377">377</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, cart</span>"<span
+    class="hid">lage,</span> muco, <a href="#page130">130</a>, <b><a href="#page131">131</a></b>, <a
+    href="#page291">291</a>, <b><a href="#page293">293</a></b>, <b><a href="#page294">294</a></b>,
+    <b><a href="#page296">296</a></b>, <b><a href="#page330">330</a></b>, <a
+    href="#page331">331</a>, <b><a href="#page333">333</a></b>, <b><a href="#page334">334</a></b>,
+    <b><a href="#page335">335</a></b>, <b><a href="#page338">338</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, cart</span>"<span
+    class="hid">lage,</span> soft, <a href="#page129">129</a>, <b><a href="#page130">130</a></b>,
+    <b><a href="#page293">293</a></b>, <b><a href="#page294">294</a></b>, <b><a
+    href="#page296">296</a></b>, <b><a href="#page335">335</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> degeneracy, evidence of,
+    <a href="#page59">59</a>, <a href="#page94">94</a>, <a href="#page343">343</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> development, <a
+    href="#page228">228</a>, <a href="#page458">458</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> digestion, <a
+    href="#page58">58</a>, <a href="#page442">442</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> epithelial cells of
+    gills, <b><a href="#page214">214</a></b></p>
+    <p><span class="pagenum" id="page518">{518}</span><span class="hid">Ammo</span>"<span
+    class="hid">&#x0153;tes,</span> epithelial cells of skin, <b><a href="#page347">347</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, epit</span>"<span
+    class="hid">elial</span> pits, <a href="#page173">173</a>, <b><a href="#page200">200</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> eye, <a
+    href="#page93">93</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, e</span>"<span
+    class="hid">e,</span> muscles, <a href="#page267">267</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, e</span>"<span
+    class="hid">e,</span> median or pineal, <a href="#page63">63</a>, <a href="#page75">75</a>, <a
+    href="#page76">76</a>, <b><a href="#page77">77</a></b>, <b><a href="#page78">78</a></b>, <a
+    href="#page80">80</a>, <b><a href="#page85">85</a></b>, <a href="#page86">86</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, e</span>"<span class="hid">e,
+    me</span>"<span class="hid">ian or pi</span>"<span class="hid">eal</span> left, <a
+    href="#page78">78</a>, <b><a href="#page79">79</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> fat-column, <a
+    href="#page181">181</a>, <b><a href="#page182">182</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, f</span>"<span
+    class="hid">t</span> in degenerated muco-cartilage, <b><a href="#page333">333</a></b>, <b><a
+    href="#page334">334</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> ganglia in embryo, <b><a
+    href="#page229">229</a></b>, <b><a href="#page283">283</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> gland-tissue round the
+    brain, <a href="#page209">209</a>, <b><a href="#page210">210</a></b>, <b><a
+    href="#page379">379</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> head-region, <b><a
+    href="#page128">128</a></b>, <b><a href="#page162">162</a></b>, <b><a
+    href="#page163">163</a></b>, <b><a href="#page193">193</a></b>, <b><a
+    href="#page293">293</a></b>, <b><a href="#page294">294</a></b>, <b><a
+    href="#page296">296</a></b>, <b><a href="#page298">298</a></b>, <b><a
+    href="#page335">335</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> head-shield, <a
+    href="#page329">329</a>, <a href="#page331">331</a>, <b><a href="#page338">338</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> liver, <a
+    href="#page442">442</a>, <a href="#page452">452</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> lymphatic glandular
+    tissue, <a href="#page426">426</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> Müllerian fibres, <a
+    href="#page489">489</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> muscles, eye, <a
+    href="#page173">173</a>, <a href="#page267">267</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, mu</span>"<span
+    class="hid">cles,</span> lip, lower, <a href="#page297">297</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, mu</span>"<span
+    class="hid">cles, li</span>"<span class="hid">,|</span> upper, <a href="#page305">305</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, mu</span>"<span
+    class="hid">cles,|</span> respiratory, <a href="#page171">171</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, mu</span>"<span
+    class="hid">cles,|</span> somatic, <a href="#page332">332</a>, <a href="#page336">336</a>, <a
+    href="#page409">409</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, mu</span>"<span
+    class="hid">cles,|</span> tubular, <a href="#page173">173</a>, <a href="#page298">298</a>, <b><a
+    href="#page309">309</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> nerves, cranial, <b><a
+    href="#page141">141</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, ne</span>"<span
+    class="hid">ves,</span> facial, <a href="#page186">186</a>, <b><a
+    href="#page311">311</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, ne</span>"<span
+    class="hid">ves,</span> glossopharyngeal, <a href="#page186">186</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, ne</span>"<span
+    class="hid">ves,</span> optic, <a href="#page105">105</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, ne</span>"<span
+    class="hid">ves,</span> trigeminal, <a href="#page282">282</a>, <a href="#page288">288</a>,
+    <b><a href="#page288">288</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, ne</span>"<span
+    class="hid">ves,</span> vagus, <a href="#page153">153</a>, <a href="#page173">173</a>, <a
+    href="#page186">186</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> nerve-fibres, medullation
+    of, <a href="#page20">20</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> notochord, <b><a
+    href="#page182">182</a></b>, <a href="#page435">435</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> olfactory tube, <a
+    href="#page219">219</a>, <a href="#page225">225</a>, <b><a href="#page227">227</a></b>, <a
+    href="#page317">317</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> oral chamber, <a
+    href="#page317">317</a>, <a href="#page243">243</a>, <a href="#page287">287</a>, <a
+    href="#page458">458</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> parasitism, <a
+    href="#page60">60</a>, <a href="#page286">286</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> pituitary, <b><a
+    href="#page321">321</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> prosomatic region, <a
+    href="#page243">243</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> pronephric duct, <a
+    href="#page402">402</a>, <a href="#page405">405</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> relationship to
+    Ostracodermata, <a href="#page326">326</a>, <a href="#page338">338</a>, <a
+    href="#page344">344</a>, <a href="#page414">414</a>, <a href="#page416">416</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> retina, <a
+    href="#page93">93</a>, <b><a href="#page111">111</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> skin, <a
+    href="#page58">58</a>, <a href="#page346">346</a>, <b><a href="#page348">348</a></b>, <a
+    href="#page442">442</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> skeleton, <a
+    href="#page125">125</a>, <b><a href="#page126">126</a></b>, <a href="#page132">132</a>, <a
+    href="#page291">291</a>, <b><a href="#page296">296</a></b>, <b><a
+    href="#page335">335</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> segments, comparison with
+    segments of Eurypterus, <a href="#page323">323</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, segm</span>"<span
+    class="hid">nts,</span> facial, <b><a href="#page201">201</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, segm</span>"<span
+    class="hid">nts,</span> hyoid, <a href="#page186">186</a>, <b><a href="#page201">201</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes, segm</span>"<span
+    class="hid">nts,</span> prosomatic, <a href="#page286">286</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> septa between myomeres,
+    <b><a href="#page416">416</a></b></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> tentacles of upper lip,
+    <a href="#page303">303</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> test, biological, to show
+    relationship with Limulus, <a href="#page493">493</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> thyroid, <a
+    href="#page192">192</a>, <b><a href="#page194">194</a></b>, <b><a href="#page196">196</a></b>,
+    <b><a href="#page205">205</a></b>, <a href="#page213">213</a>, <a href="#page430">430</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> transformation, <a
+    href="#page18">18</a>, <a href="#page59">59</a>, <a href="#page125">125</a>, <a
+    href="#page168">168</a>, <a href="#page193">193</a>, <a href="#page199">199</a>, <a
+    href="#page200">200</a>, <a href="#page220">220</a>, <a href="#page227">227</a>, <a
+    href="#page228">228</a>, <a href="#page287">287</a>, <a href="#page291">291</a>, <a
+    href="#page304">304</a>, <a href="#page307">307</a>, <a href="#page309">309</a>, <a
+    href="#page331">331</a>, <a href="#page336">336</a>, <a href="#page347">347</a>, <a
+    href="#page349">349</a>, <a href="#page389">389</a>, <a href="#page445">445</a></p>
+    <p><span class="hid">Ammo</span>"<span class="hid">&#x0153;tes,</span> velum, <a
+    href="#page228">228</a>, <a href="#page289">289</a>, <a href="#page298">298</a>, <b><a
+    href="#page302">302</a></b></p>
+    <p>Am&#x0153;bocytes, <a href="#page473">473</a></p>
+    <p>Amphibia, <a href="#page23">23</a>, <a href="#page345">345</a></p>
+    <p><span class="pagenum" id="page519">{519}</span>Amphioxus, <a href="#page33">33</a>, <a
+    href="#page407">407</a></p>
+    <p><span class="hid">Amphi</span>"<span class="hid">xus,</span> atrial cavity, <a
+    href="#page409">409</a></p>
+    <p><span class="hid">Amphi</span>"<span class="hid">xus,</span> branchial nephric glands, <a
+    href="#page426">426</a></p>
+    <p><span class="hid">Amphi</span>"<span class="hid">xus,</span> endostyle, <a
+    href="#page198">198</a>, <a href="#page212">212</a></p>
+    <p><span class="hid">Amphi</span>"<span class="hid">xus,</span> excretory organs, <a
+    href="#page389">389</a>, <a href="#page395">395</a>, <a href="#page477">477</a></p>
+    <p><span class="hid">Amphi</span>"<span class="hid">xus,</span> neuropore, <a
+    href="#page220">220</a>, <a href="#page457">457</a></p>
+    <p><span class="hid">Amphi</span>"<span class="hid">xus,</span> notochord, <a
+    href="#page435">435</a>, <a href="#page436">436</a>, <a href="#page443">443</a></p>
+    <p><span class="hid">Amphi</span>"<span class="hid">xus,</span> pleural folds, <b><a
+    href="#page495">495</a></b></p>
+    <p><span class="hid">Amphi</span>"<span class="hid">xus,</span> septa between myomeres, <b><a
+    href="#page416">416</a></b></p>
+    <p><span class="hid">Amphi</span>"<span class="hid">xus,</span> somatic muscles, <a
+    href="#page409">409</a></p>
+    <p><span class="hid">Amphi</span>"<span class="hid">xus,</span> yolk, <a
+    href="#page485">485</a></p>
+    <p>Androctonus, <a href="#page53">53</a>, <b><a href="#page54">54</a></b>, <b><a
+    href="#page372">372</a></b>, <a href="#page423">423</a></p>
+    <p>Annelids, lateral sense-organs, <a href="#page357">357</a>, <a href="#page367">367</a></p>
+    <p><span class="hid">Ann</span>"<span class="hid">lids,</span> nephric organs, <a
+    href="#page390">390</a></p>
+    <p><span class="hid">Ann</span>"<span class="hid">lids,</span> rigin of Arthropods from, <a
+    href="#page395">395</a></p>
+    <p><span class="hid">Ann</span>"<span class="hid">lids,</span> parapodal ganglia, <a
+    href="#page283">283</a></p>
+    <p><span class="hid">Ann</span>"<span class="hid">lids,</span> phagocytic glands, <a
+    href="#page421">421</a></p>
+    <p>Anthozoa, <a href="#page474">474</a></p>
+    <p>Antiarcha, <a href="#page29">29</a>, <a href="#page326">326</a>, <a
+    href="#page343">343</a></p>
+    <p>Antibody, <a href="#page492">492</a></p>
+    <p>Antitoxin, <a href="#page492">492</a></p>
+    <p>Anus, <a href="#page43">43</a>, <a href="#page457">457</a></p>
+    <p>Aponeuroses, <a href="#page327">327</a>, <a href="#page342">342</a>, <a
+    href="#page414">414</a></p>
+    <p>Apparatus, auditory, <a href="#page355">355</a></p>
+    <p><span class="hid">Appa</span>"<span class="hid">rtus,</span> dioptric, <a
+    href="#page83">83</a></p>
+    <p><span class="hid">Appa</span>"<span class="hid">rtus,</span> respiratory, <a
+    href="#page148">148</a></p>
+    <p><span class="hid">Appa</span>"<span class="hid">rtus,</span> suctorial, of Petromyzon, <a
+    href="#page287">287</a></p>
+    <p>Appendages, branchial, of Ammoc&#x0153;tes, <a href="#page161">161</a>, <b><a
+    href="#page162">162</a></b>, <b><a href="#page163">163</a></b>, <b><a
+    href="#page164">164</a></b></p>
+    <p><span class="hid">Appe</span>"<span class="hid">dages, bra</span>"<span class="hid">chial,
+    of</span> Limulus, <b><a href="#page164">164</a></b></p>
+    <p><span class="hid">Appe</span>"<span class="hid">dages, bra</span>"<span class="hid">chial,
+    of</span> internal, <a href="#page149">149</a></p>
+    <p><span class="hid">Appe</span>"<span class="hid">dages,</span> derivation of suctorial
+    apparatus of Petromyzon from, <a href="#page290">290</a></p>
+    <p><span class="hid">Appe</span>"<span class="hid">dages,</span> disappearance of, in
+    transformation of Arthropod into Vertebrate, <a href="#page386">386</a>, <b><a
+    href="#page413">413</a></b></p>
+    <p><span class="hid">Appe</span>"<span class="hid">dages,</span> evidence of, in prosomatic
+    region of ancient fishes, <a href="#page342">342</a></p>
+    <p><span class="hid">Appe</span>"<span class="hid">dages,</span> muscles, in Limulus and
+    Scorpion, <a href="#page247">247</a></p>
+    <p><span class="hid">Appe</span>"<span class="hid">dages,</span> prosomatic, of Gigantostraca,
+    <a href="#page234">234</a></p>
+    <p><span class="hid">Appe</span>"<span class="hid">dages,</span> Trilobites, <a
+    href="#page351">351</a></p>
+    <p>Apus, <b><a href="#page28">28</a></b>, <a href="#page137">137</a>, <a
+    href="#page436">436</a>, <b><a href="#page437">437</a></b></p>
+    <p>Arachnids, eyes, <a href="#page75">75</a>, <a href="#page87">87</a></p>
+    <p><span class="hid">Arac</span>"<span class="hid">nids,</span> diverticula of stomach, <a
+    href="#page109">109</a></p>
+    <p><span class="hid">Arac</span>"<span class="hid">nids,</span> lyriform organs, <a
+    href="#page364">364</a>, <a href="#page368">368</a></p>
+    <p><span class="hid">Arac</span>"<span class="hid">nids,</span> segmental excretory organs, <a
+    href="#page423">423</a></p>
+    <p>Archæocytes, <a href="#page473">473</a></p>
+    <p>Artemia, <i>v.</i> Branchipus</p>
+    <p>Arthropleura, <a href="#page249">249</a></p>
+    <p>Arthropoda, arrangement of organs, <b><a href="#page10">10</a></b></p>
+    <p><span class="hid">Arthr</span>"<span class="hid">poda,</span> evolution, <a
+    href="#page11">11</a></p>
+    <p><span class="hid">Arthr</span>"<span class="hid">poda,</span> excretory organs, <a
+    href="#page396">396</a>, <a href="#page418">418</a></p>
+    <p><span class="hid">Arthr</span>"<span class="hid">poda,</span> eyes, <a href="#page75">75</a>,
+    <a href="#page89">89</a></p>
+    <p><span class="hid">Arthr</span>"<span class="hid">poda,</span> giant-fibres, <a
+    href="#page489">489</a></p>
+    <p><span class="hid">Arthr</span>"<span class="hid">poda,</span> musculature, <a
+    href="#page411">411</a></p>
+    <p><span class="hid">Arthr</span>"<span class="hid">poda,</span> olfactory organs, <a
+    href="#page220">220</a></p>
+    <p><span class="pagenum" id="page520">{520}</span><span class="hid">Arthr</span>"<span
+    class="hid">poda,</span> resemblance to ancient fishes, <a href="#page29">29</a></p>
+    <p>Astacus, brain, <b><a href="#page54">54</a></b></p>
+    <p><span class="hid">Ast</span>"<span class="hid">cus,</span> digestive ferment in cells lining
+    the carapace, <a href="#page442">442</a></p>
+    <p><span class="hid">Ast</span>"<span class="hid">cus,</span> optic chiasma, <a
+    href="#page101">101</a></p>
+    <p><span class="hid">Ast</span>"<span class="hid">cus,</span> optic stalk, <a
+    href="#page91">91</a></p>
+    <p><span class="hid">Ast</span>"<span class="hid">cus,</span> etina, <a
+    href="#page98">98</a></p>
+    <p>Asterolepis, <a href="#page326">326</a>, <a href="#page342">342</a></p>
+    <p>Atrium, <a href="#page410">410</a></p>
+    <p>Auchenaspis (Thyestes), <b><a href="#page30">30</a></b>, <b><a href="#page31">31</a></b>, <a
+    href="#page75">75</a>, <a href="#page275">275</a>, <a href="#page326">326</a>, <a
+    href="#page327">327</a>, <b><a href="#page328">328</a></b>, <b><a
+    href="#page338">338</a></b></p>
+    <p>Auditory apparatus, <a href="#page355">355</a></p>
+    <p>Auerbach, plexus of, <a href="#page447">447</a></p>
+    <p>Aurelia, <a href="#page475">475</a></p>
+    <p>Autonomic nerves, <a href="#page3">3</a></p>
+    <p class="stanza">Balanoglossus, <a href="#page12">12</a>, <b><a href="#page12">12</a></b>, <a
+    href="#page433">433</a>, <a href="#page438">438</a>, <a href="#page494">494</a></p>
+    <p>Bdellostoma, <a href="#page394">394</a>, <a href="#page405">405</a></p>
+    <p>Belinurus, <a href="#page24">24</a>, <a href="#page249">249</a>, <a
+    href="#page351">351</a></p>
+    <p>Bird, rhomboidal sinus, <b><a href="#page46">46</a></b></p>
+    <p>Bladder, <a href="#page449">449</a></p>
+    <p><span class="hid">Bla</span>"<span class="hid">der,</span> swim, <a
+    href="#page148">148</a></p>
+    <p>Blastula, <a href="#page459">459</a>, <a href="#page471">471</a>, <a
+    href="#page473">473</a></p>
+    <p>Blood, <a href="#page463">463</a>, <a href="#page472">472</a>, <a href="#page474">474</a></p>
+    <p><span class="hid">Bl</span>"<span class="hid">od,</span> circulation, in Ammoc&#x0153;tes and
+    Limulus, <a href="#page174">174</a></p>
+    <p><span class="hid">Bl</span>"<span class="hid">od,</span> secretion of ductless glands into,
+    <a href="#page418">418</a></p>
+    <p>Bothriolepis, <a href="#page29">29</a>, <b><a href="#page32">32</a></b>, <a
+    href="#page239">239</a>, <a href="#page326">326</a>, <a href="#page351">351</a>, <a
+    href="#page450">450</a></p>
+    <p>Bone, <a href="#page344">344</a>, <a href="#page474">474</a>, <a href="#page481">481</a></p>
+    <p>Brain, Ammoc&#x0153;tes and Arthropod, <b><a href="#page54">54</a></b>, <a
+    href="#page61">61</a></p>
+    <p><span class="hid">Br</span>"<span class="hid">in,</span> and brain-case of Ammoc&#x0153;tes,
+    <b><a href="#page40">40</a></b>, <b><a href="#page41">41</a></b>, <b><a
+    href="#page46">46</a></b>, <a href="#page209">209</a></p>
+    <p><span class="hid">Br</span>"<span class="hid">in,</span> caudal, of Thelyphonus, <a
+    href="#page450">450</a></p>
+    <p><span class="hid">Br</span>"<span class="hid">in,</span> epithelial lining of, <a
+    href="#page38">38</a></p>
+    <p><span class="hid">Br</span>"<span class="hid">in,</span> roof, <a href="#page39">39</a></p>
+    <p><span class="hid">Br</span>"<span class="hid">in,</span> Sphæroma serratum, <b><a
+    href="#page62">62</a></b>, <b><a href="#page90">90</a></b></p>
+    <p><span class="hid">Br</span>"<span class="hid">in,</span> Thelyphonus, <b><a
+    href="#page56">56</a></b></p>
+    <p><span class="hid">Br</span>"<span class="hid">in,</span> ventricles, <a
+    href="#page4">4</a></p>
+    <p><span class="hid">Br</span>"<span class="hid">in,</span> vesicles, <a
+    href="#page48">48</a></p>
+    <p>Branchial basket-work of Ammoc&#x0153;tes, <b><a href="#page126">126</a></b>, <b><a
+    href="#page128">128</a></b>, <b><a href="#page296">296</a></b>, <a href="#page331">331</a>,
+    <b><a href="#page335">335</a></b></p>
+    <p>Branchipus, <b><a href="#page28">28</a></b></p>
+    <p><span class="hid">Bran</span>"<span class="hid">hipus,</span> brain, <a
+    href="#page51">51</a>, <b><a href="#page54">54</a></b></p>
+    <p><span class="hid">Bran</span>"<span class="hid">hipus,</span> eyes, lateral, <a
+    href="#page88">88</a></p>
+    <p><span class="hid">Bran</span>"<span class="hid">hipus, e</span>"<span class="hid">es,
+    lat</span>"<span class="hid">ral,</span> retina of, <b><a href="#page91">91</a></b>, <a
+    href="#page97">97</a></p>
+    <p><span class="hid">Bran</span>"<span class="hid">hipus, e</span>"<span class="hid">es,</span>
+    median, <a href="#page75">75</a></p>
+    <p><span class="hid">Bran</span>"<span class="hid">hipus,</span> excretory organs, <a
+    href="#page396">396</a></p>
+    <p><span class="hid">Bran</span>"<span class="hid">hipus,</span> (Artemia) diverticula of gut
+    and retinal ganglion, <b><a href="#page110">110</a></b>, <b><a href="#page111">111</a></b>, <a
+    href="#page113">113</a></p>
+    <p><span class="hid">Bran</span>"<span class="hid">hipus,</span> nerves of appendages, <a
+    href="#page157">157</a></p>
+    <p><span class="hid">Bran</span>"<span class="hid">hipus,</span> segmentation, <a
+    href="#page159">159</a></p>
+    <p><span class="hid">Bran</span>"<span class="hid">hipus,</span> resemblance to Trilobite, <a
+    href="#page436">436</a></p>
+    <p>Bunodes, <a href="#page24">24</a>, <b><a href="#page30">30</a></b>, <a
+    href="#page249">249</a>, <b><a href="#page341">341</a></b>, <a href="#page351">351</a>, <a
+    href="#page414">414</a></p>
+    <p>Bundle of Meynert, <a href="#page48">48</a>, <a href="#page77">77</a></p>
+    <p>Bundles, posterior longitudinal, <a href="#page489">489</a></p>
+    <p>Buthus, muscles, <a href="#page270">270</a></p>
+    <p class="stanza">Calcification in aponeuroses of Cephalaspis, <a href="#page414">414</a></p>
+    <p><span class="pagenum" id="page521">{521}</span><span class="hid">Calcifi</span>"<span
+    class="hid">ation</span> cartilage, <a href="#page140">140</a>,<a href="#page330">330</a></p>
+    <p><span class="hid">Calcifi</span>"<span class="hid">ation</span> successive layers of the
+    skin, <a href="#page348">348</a></p>
+    <p>Camerostome, <a href="#page221">221</a>, <b><a href="#page222">222</a></b>, <b><a
+    href="#page223">223</a></b>, <a href="#page224">224</a>, <a href="#page241">241</a>, <a
+    href="#page271">271</a></p>
+    <p>Canal, alimentary, formation of vertebrate, <a href="#page58">58</a>, <a
+    href="#page433">433</a>, <a href="#page446">446</a></p>
+    <p><span class="hid">Ca</span>"<span class="hid">al, alime</span>"<span class="hid">tary,</span>
+    innervation, <a href="#page447">447</a></p>
+    <p><span class="hid">Ca</span>"<span class="hid">al, alime</span>"<span class="hid">tary,</span>
+    relationships between notochord and, <a href="#page434">434</a></p>
+    <p><span class="hid">Ca</span>"<span class="hid">al, alime</span>"<span class="hid">tary,</span>
+    origin, <a href="#page444">444</a></p>
+    <p><span class="hid">Ca</span>"<span class="hid">al,</span> Haversian, <a
+    href="#page329">329</a></p>
+    <p><span class="hid">Ca</span>"<span class="hid">al,</span> central, of spinal cord, <a
+    href="#page405">405</a>, <a href="#page439">439</a>, <a href="#page455">455</a></p>
+    <p><span class="hid">Ca</span>"<span class="hid">al,</span> spinal, <b><a
+    href="#page182">182</a></b></p>
+    <p>Capsule, auditory, <a href="#page377">377</a>, <b><a href="#page379">379</a></b></p>
+    <p>Cartilage Ammoc&#x0153;tes, muco, <a href="#page127">127</a>, <a href="#page130">130</a>,
+    <b><a href="#page131">131</a></b>, <a href="#page200">200</a>, <a href="#page291">291</a>, <a
+    href="#page303">303</a>, <a href="#page330">330</a>, <b><a href="#page333">333</a></b>, <b><a
+    href="#page334">334</a></b>, <a href="#page344">344</a></p>
+    <p><span class="hid">Cart</span>"<span class="hid">ilge Ammo</span>"<span
+    class="hid">&#x0153;tes,</span> hard, <a href="#page133">133</a>, <b><a
+    href="#page133">133</a></b>, <a href="#page377">377</a></p>
+    <p><span class="hid">Cart</span>"<span class="hid">ilge Ammo</span>"<span
+    class="hid">&#x0153;tes,</span> soft, <a href="#page126">126</a>, <a href="#page129">129</a>,
+    <b><a href="#page130">130</a></b></p>
+    <p><span class="hid">Cart</span>"<span class="hid">ilge Ammo</span>"<span
+    class="hid">&#x0153;tes,</span> spinal cartilages, <a href="#page414">414</a></p>
+    <p><span class="hid">Cart</span>"<span class="hid">ilge</span> Hypoctonus, <b><a
+    href="#page133">133</a></b>, <a href="#page142">142</a></p>
+    <p><span class="hid">Cart</span>"<span class="hid">ilge</span> Limulus, hard, <a
+    href="#page142">142</a></p>
+    <p><span class="hid">Cart</span>"<span class="hid">ilge Lim</span>"<span class="hid">lus,</span>
+    muco, <a href="#page139">139</a></p>
+    <p><span class="hid">Cart</span>"<span class="hid">ilge Lim</span>"<span class="hid">lus,</span>
+    soft, <a href="#page20">20</a>, <b><a href="#page130">130</a></b>, <a
+    href="#page137">137</a></p>
+    <p><span class="hid">Cart</span>"<span class="hid">ilge</span> origin, <a
+    href="#page474">474</a>, <a href="#page481">481</a></p>
+    <p><span class="hid">Cart</span>"<span class="hid">ilge</span> staining reactions, <a
+    href="#page131">131</a>, <a href="#page133">133</a>, <a href="#page139">139</a>, <a
+    href="#page330">330</a>, <a href="#page336">336</a></p>
+    <p>Cavity, atrial, <a href="#page409">409</a>, <b><a href="#page413">413</a></b></p>
+    <p><span class="hid">Cai</span>"<span class="hid">ty,</span> c&#x0153;lomic, <a
+    href="#page167">167</a>, <a href="#page251">251</a>, <a href="#page266">266</a>, <a
+    href="#page320">320</a>, <a href="#page389">389</a>, <b><a href="#page391">391</a></b>, <b><a
+    href="#page408">408</a></b>, <a href="#page422">422</a>, <a href="#page430">430</a>, <a
+    href="#page472">472</a></p>
+    <p>Cells, free-living, <a href="#page463">463</a></p>
+    <p>Centre, vaso-motor, <a href="#page468">468</a></p>
+    <p>Cephalaspis, diverticula of gut, <a href="#page109">109</a></p>
+    <p><span class="hid">Ceph</span>"<span class="hid">laspis,</span> eyes, lateral, <a
+    href="#page75">75</a>, <a href="#page275">275</a></p>
+    <p><span class="hid">Ceph</span>"<span class="hid">laspis, e</span>"<span class="hid">es,</span>
+    median, <a href="#page75">75</a></p>
+    <p><span class="hid">Ceph</span>"<span class="hid">laspis,</span> head-shield, <a
+    href="#page327">327</a>, <b><a href="#page328">328</a></b>, <b><a href="#page330">330</a></b>,
+    <b><a href="#page338">338</a></b></p>
+    <p><span class="hid">Ceph</span>"<span class="hid">laspis,</span> muscles on head-shield, <b><a
+    href="#page269">269</a></b></p>
+    <p><span class="hid">Ceph</span>"<span class="hid">laspis,</span> resemblance to
+    Ammoc&#x0153;tes, <a href="#page145">145</a>, <a href="#page291">291</a>, <a
+    href="#page326">326</a>, <a href="#page329">329</a>, <b><a href="#page338">338</a></b>, <a
+    href="#page348">348</a>, <a href="#page414">414</a></p>
+    <p><span class="hid">Ceph</span>"<span class="hid">laspis, resem</span>"<span class="hid">lance
+    to</span> Arthropod, <a href="#page29">29</a></p>
+    <p><span class="hid">Ceph</span>"<span class="hid">laspis,</span> segmentation, <a
+    href="#page339">339</a></p>
+    <p>Ceratodus, <a href="#page148">148</a></p>
+    <p>Cephalization, <a href="#page51">51</a></p>
+    <p>Cephalodiscus, <a href="#page438">438</a></p>
+    <p>Cephalopod, <a href="#page23">23</a></p>
+    <p>Cerebellum, <a href="#page47">47</a>, <b><a href="#page50">50</a></b></p>
+    <p>Chætopoda, <a href="#page395">395</a></p>
+    <p>Chamber, oral, of Ammoc&#x0153;tes, <a href="#page243">243</a>, <a href="#page287">287</a>,
+    <a href="#page458">458</a></p>
+    <p>Cheliceræ, <a href="#page235">235</a></p>
+    <p>Chiasma, optic, <a href="#page101">101</a></p>
+    <p>Chilaria, <a href="#page235">235</a>, <a href="#page238">238</a>, <a href="#page291">291</a>,
+    <a href="#page301">301</a>, <a href="#page458">458</a></p>
+    <p>Chitin, <a href="#page85">85</a>, <a href="#page119">119</a>, <a href="#page139">139</a>, <a
+    href="#page205">205</a>, <a href="#page206">206</a>, <a href="#page302">302</a>, <a
+    href="#page329">329</a>, <a href="#page346">346</a>, <a href="#page359">359</a>, <a
+    href="#page440">440</a>, <a href="#page443">443</a></p>
+    <p>Cilia, <a href="#page206">206</a></p>
+    <p>Circulation, branchial, <a href="#page174">174</a></p>
+    <p>Cirri, <a href="#page357">357</a></p>
+    <p>Clarke's column, <a href="#page467">467</a></p>
+    <p>Clepsine, nephridial glands, <a href="#page423">423</a></p>
+    <p>Cochlea, <a href="#page378">378</a></p>
+    <p>C&#x0153;lenterata, <a href="#page465">465</a>, <a href="#page472">472</a></p>
+    <p>C&#x0153;lolepidæ, <a href="#page344">344</a></p>
+    <p><span class="pagenum" id="page522">{522}</span>C&#x0153;lom, <a href="#page167">167</a>, <a
+    href="#page251">251</a>, <a href="#page400">400</a>, <a href="#page472">472</a>, <a
+    href="#page481">481</a></p>
+    <p>C&#x0153;lomata, <a href="#page472">472</a></p>
+    <p>C&#x0153;lomoc&#x0153;la, <a href="#page472">472</a>, <a href="#page475">475</a></p>
+    <p>C&#x0153;lomostomes, <a href="#page477">477</a>, <a href="#page481">481</a></p>
+    <p>Colleneytes, <a href="#page474">474</a></p>
+    <p>Commissure, anterior, <a href="#page49">49</a></p>
+    <p><span class="hid">Comm</span>"<span class="hid">ssure,</span> &#x0153;sophageal, <a
+    href="#page14">14</a></p>
+    <p><span class="hid">Comm</span>"<span class="hid">ssure,</span> posterior, <a
+    href="#page48">48</a>, <a href="#page280">280</a></p>
+    <p>Comparison of brains of Ammoc&#x0153;tes and Arthropod, <a href="#page61">61</a></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of br</span>"<span class="hid">ins
+    of</span> invertebrate from Branchipus to Ammoc&#x0153;tes, <b><a href="#page54">54</a></b></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of br</span>"<span class="hid">ins
+    of</span> vertebrate, <b><a href="#page40">40</a></b></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of</span> branchial circulation in
+    Ammoc&#x0153;tes and Limulus, <a href="#page174">174</a></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of bran</span>"<span
+    class="hid">hial</span> lamellæ of Scorpion and Ammoc&#x0153;tes, <b><a
+    href="#page175">175</a></b></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of bran</span>"<span
+    class="hid">hial</span> segments of Ammoc&#x0153;tes and Petromyzon, <b><a
+    href="#page169">169</a></b></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of</span> Cephalaspidian and
+    Palæostracan fish, <a href="#page31">31</a></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of</span> C&#x0153;lom of Peripatus and
+    Vertebrate, <a href="#page400">400</a></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of</span> dermal covering of Pteraspis
+    with chitin of Limulus or dentine of fish scales, <a href="#page346">346</a></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of</span> entosternite or plastron of
+    Limulus with trabeculæ of Ammoc&#x0153;tes, <a href="#page145">145</a></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of</span> excretory organs of
+    vertebrates and invertebrates, <a href="#page389">389</a></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of</span> gut of Arthropod and tube of
+    central nervous system of Vertebrate, <a href="#page43">43</a>, <a href="#page244">244</a>, <a
+    href="#page433">433</a>, <a href="#page440">440</a>, <a href="#page455">455</a>, <a
+    href="#page457">457</a></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of</span> head-shield of Cephalaspis
+    and Ammoc&#x0153;tes, <a href="#page291">291</a>, <a href="#page329">329</a>, <b><a
+    href="#page338">338</a></b></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of</span> hypophysial tube with
+    olfactory tube of Arthropod ancestor, <a href="#page229">229</a></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of hypop</span>"<span class="hid">ysial
+    t</span>"<span class="hid">be</span> with position of palæostoma, <a href="#page317">317</a></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of</span> mesosomatic region of
+    Ammoc&#x0153;tes and Eurypterus, <a href="#page192">192</a></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of</span> muscles, branchial, of
+    Ammoc&#x0153;tes and appendage muscles of Scorpion, <a href="#page171">171</a>, <a
+    href="#page447">447</a></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of mu</span>"<span
+    class="hid">cles,</span> eye, of Vertebrate with dorso-ventral muscles of Scorpion, <a
+    href="#page267">267</a>, <a href="#page272">272</a>, <a href="#page459">459</a></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of mu</span>"<span
+    class="hid">cles,</span> of oral chamber of Ammoc&#x0153;tes and prosomatic musculature of
+    Limulus, <a href="#page247">247</a>, <a href="#page447">447</a></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of mu</span>"<span
+    class="hid">cles,</span> longitudinal body-muscles of Vertebrate and dorsal longitudinal muscles
+    of Arthropod, <a href="#page411">411</a>, <a href="#page447">447</a></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of</span> nerves, appendage of Limulus
+    and Branchipus to lateral root system of Vertebrate, <a href="#page157">157</a></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of ne</span>"<span
+    class="hid">ves,</span> cranial and spinal segmental, <a href="#page152">152</a></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of</span> nervous systems of Vertebrate
+    and Arthropod, <a href="#page36">36</a></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of</span> pineal gland of vertebrates
+    and median eyes of Arthropod, <a href="#page63">63</a>, <a href="#page456">456</a></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of</span> pituitary body and coxal
+    glands, <a href="#page246">246</a>, <a href="#page319">319</a>, <b><a
+    href="#page321">321</a></b></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of</span> prosoma and mesosoma of
+    Limulus and Ammoc&#x0153;tes, <b><a href="#page140">140</a></b>, <b><a
+    href="#page141">141</a></b></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of</span> prosomatic region of
+    Ammoc&#x0153;tes and Eurypterus, <a href="#page244">244</a>, <b><a
+    href="#page333">333</a></b></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of</span> retina in Ammoc&#x0153;tes
+    and Musca, <a href="#page97">97</a></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of re</span>"<span
+    class="hid">ina</span> compound in Arthropod and Vertebrate, <a href="#page87">87</a></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of</span> skeleton of Limulus and
+    Ammoc&#x0153;tes, <b><a href="#page126">126</a></b>, <a href="#page136">136</a></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of</span> sense-organs of Arthropod
+    appendages with auditory organs of Vertebrate, <a href="#page375">375</a></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of</span> thyroid with endostyle, <a
+    href="#page198">198</a></p>
+    <p><span class="hid">Comp</span>"<span class="hid">rison of thy</span>"<span class="hid">oid
+    w</span>"<span class="hid">th</span> uterus of Scorpion, <b><a href="#page205">205</a></b></p>
+    <p>Corneagen, <a href="#page69">69</a></p>
+    <p>Corpora quadrigemina, <a href="#page47">47</a></p>
+    <p><span class="pagenum" id="page523">{523}</span>Corpuscles, Pacinian, Herbst, Grandry, etc.,
+    <a href="#page470">470</a></p>
+    <p>Coxal glands, <a href="#page242">242</a>, <a href="#page246">246</a>, <a
+    href="#page319">319</a>, <b><a href="#page321">321</a></b>, <a href="#page389">389</a>, <a
+    href="#page398">398</a>, <a href="#page403">403</a>, <a href="#page429">429</a></p>
+    <p>Cranium, <a href="#page121">121</a>, <a href="#page145">145</a>, <a
+    href="#page339">339</a></p>
+    <p>Crayfish, <a href="#page442">442</a>, <a href="#page489">489</a></p>
+    <p>Crest, neural, <a href="#page281">281</a></p>
+    <p>Cromatophores of frog, <a href="#page470">470</a></p>
+    <p>Crura cerebri, <a href="#page14">14</a></p>
+    <p>Crustacea, first appearance, <a href="#page27">27</a></p>
+    <p><span class="hid">Crust</span>"<span class="hid">cea,</span> eyes, <a href="#page76">76</a>,
+    <a href="#page87">87</a></p>
+    <p><span class="hid">Crust</span>"<span class="hid">cea,</span> retina, <b><a
+    href="#page100">100</a></b></p>
+    <p><span class="hid">Crust</span>"<span class="hid">cea,</span> segmental glands, <a
+    href="#page422">422</a></p>
+    <p>Ctenophora, <a href="#page474">474</a></p>
+    <p>Cyathaspis, <a href="#page29">29</a>, <a href="#page326">326</a>, <b><a
+    href="#page340">340</a></b>, <a href="#page343">343</a></p>
+    <p>Cyclostomata, <a href="#page165">165</a>, <a href="#page229">229</a>, <a
+    href="#page343">343</a>, <a href="#page353">353</a>, <a href="#page424">424</a></p>
+    <p>Cysts, <a href="#page50">50</a></p>
+    <p class="stanza">Daphnia, <a href="#page112">112</a></p>
+    <p>Degeneration, <a href="#page17">17</a>, <a href="#page19">19</a>, <a href="#page59">59</a>,
+    <a href="#page74">74</a>, <a href="#page78">78</a>, <a href="#page94">94</a>, <a
+    href="#page107">107</a>, <a href="#page212">212</a>, <a href="#page309">309</a>, <a
+    href="#page333">333</a>, <a href="#page336">336</a>, <a href="#page343">343</a></p>
+    <p>Deiters' nucleus, <a href="#page489">489</a></p>
+    <p>Dendrites, <a href="#page72">72</a></p>
+    <p>Development, parallel, <a href="#page497">497</a></p>
+    <p><span class="hid">Devel</span>"<span class="hid">pment,</span> of two types of eye, <a
+    href="#page73">73</a></p>
+    <p><span class="hid">Devel</span>"<span class="hid">pment,</span> vertebrate retina, <a
+    href="#page101">101</a></p>
+    <p>Diaphragms, <a href="#page161">161</a>, <a href="#page167">167</a></p>
+    <p>Didymaspis, <a href="#page327">327</a>, <b><a href="#page338">338</a></b></p>
+    <p>Digestion, <a href="#page441">441</a></p>
+    <p>Dinosaurs, <a href="#page17">17</a></p>
+    <p>Dipnoans, <a href="#page23">23</a>, <a href="#page45">45</a>, <a href="#page148">148</a></p>
+    <p>Diptera, <a href="#page89">89</a>, <a href="#page369">369</a></p>
+    <p>Diverticula, optic, <a href="#page102">102</a></p>
+    <p>Dogfish, skull, <b><a href="#page121">121</a></b>, <b><a href="#page123">123</a></b></p>
+    <p>Drepanaspis, <a href="#page344">344</a>, <b><a href="#page345">345</a></b>, <a
+    href="#page450">450</a></p>
+    <p>Drepanopterus Bembycoides, <a href="#page238">238</a></p>
+    <p class="stanza">Ectognath, <a href="#page238">238</a>, <a href="#page242">242</a>, <a
+    href="#page271">271</a>, <a href="#page304">304</a>, <a href="#page342">342</a>, <b><a
+    href="#page381">381</a></b></p>
+    <p>Eel, <a href="#page488">488</a></p>
+    <p>Elasmobranchs, <a href="#page23">23</a>, <a href="#page343">343</a>, <a
+    href="#page423">423</a></p>
+    <p>Elastin, <a href="#page435">435</a></p>
+    <p>Embryo, head of dogfish, <b><a href="#page121">121</a></b>, <b><a
+    href="#page123">123</a></b></p>
+    <p><span class="hid">Em</span>"<span class="hid">ryo,</span> skull of pig, <b><a
+    href="#page121">121</a></b></p>
+    <p>Embryology, principles of, <a href="#page455">455</a></p>
+    <p>Encepalomeres, <a href="#page262">262</a></p>
+    <p>Endognath, <a href="#page238">238</a>, <a href="#page271">271</a>, <a
+    href="#page304">304</a>, <b><a href="#page381">381</a></b></p>
+    <p>Endostoma, <a href="#page241">241</a>, <a href="#page306">306</a></p>
+    <p>Endostyle, <a href="#page198">198</a>, <a href="#page212">212</a></p>
+    <p>Entapophysis of Limulus, <a href="#page139">139</a></p>
+    <p>Enteroc&#x0153;la, <a href="#page472">472</a></p>
+    <p>Enteropneusta, <a href="#page438">438</a>, <a href="#page494">494</a></p>
+    <p>Entochondrites, <a href="#page377">377</a></p>
+    <p>Entosclerite, <a href="#page222">222</a>, <a href="#page271">271</a></p>
+    <p>Entosternite, <b><a href="#page143">143</a></b></p>
+    <p>Epiblast, <a href="#page444">444</a>, <a href="#page445">445</a>, <a
+    href="#page459">459</a></p>
+    <p><span class="pagenum" id="page524">{524}</span>Epithelium cells of Ammoc&#x0153;tes, <b><a
+    href="#page347">347</a></b></p>
+    <p><span class="hid">Epith</span>"<span class="hid">lium</span> of central nervous system of
+    vertebrates, <a href="#page38">38</a>, <a href="#page457">457</a></p>
+    <p><span class="hid">Epith</span>"<span class="hid">lium of</span> c&#x0153;lomic spaces in
+    annelids, <a href="#page421">421</a></p>
+    <p><span class="hid">Epith</span>"<span class="hid">lium of</span> optic diverticula, <a
+    href="#page103">103</a></p>
+    <p><span class="hid">Epith</span>"<span class="hid">lium of</span> peritoneal, pleural, and
+    pericardial cavities, <a href="#page477">477</a></p>
+    <p><span class="hid">Epith</span>"<span class="hid">lium of</span> velum of Ammoc&#x0153;tes, <a
+    href="#page301">301</a>, <b><a href="#page302">302</a></b></p>
+    <p>Equilibration, <a href="#page358">358</a></p>
+    <p>Eukeraspis, <a href="#page326">326</a></p>
+    <p>Eurypterus, <b><a href="#page26">26</a></b>, <b><a href="#page150">150</a></b>, <b><a
+    href="#page191">191</a></b>, <b><a href="#page237">237</a></b></p>
+    <p><span class="hid">Eury</span>"<span class="hid">terus,</span> appendages, <a
+    href="#page150">150</a>, <a href="#page236">236</a>, <b><a href="#page237">237</a></b></p>
+    <p><span class="hid">Eury</span>"<span class="hid">terus,</span> classification, <a
+    href="#page249">249</a></p>
+    <p><span class="hid">Eury</span>"<span class="hid">terus,</span> comparison with
+    Ammoc&#x0153;tes, <a href="#page170">170</a>, <a href="#page323">323</a></p>
+    <p><span class="hid">Eury</span>"<span class="hid">terus,</span> diagram of sagittal median
+    section, <b><a href="#page240">240</a></b>, <b><a href="#page245">245</a></b></p>
+    <p><span class="hid">Eury</span>"<span class="hid">terus,</span> endostoma, <a
+    href="#page241">241</a>, <a href="#page306">306</a></p>
+    <p><span class="hid">Eury</span>"<span class="hid">terus,</span> eyes, <a
+    href="#page275">275</a></p>
+    <p><span class="hid">Eury</span>"<span class="hid">terus,</span> mesosomatic segments, <b><a
+    href="#page192">192</a></b></p>
+    <p><span class="hid">Eury</span>"<span class="hid">terus,</span> muscles of carapace, <a
+    href="#page269">269</a></p>
+    <p><span class="hid">Eury</span>"<span class="hid">terus,</span> operculum, <b><a
+    href="#page150">150</a></b>, <b><a href="#page190">190</a></b>, <a href="#page212">212</a></p>
+    <p>Evidence of alimentary canal, innervation, <a href="#page446">446</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of</span> auditory apparatus and lateral
+    line organs, <a href="#page355">355</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of</span> c&#x0153;lomic cavities in
+    Limulus, <a href="#page251">251</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of</span> degeneracy in Ammoc&#x0153;tes,
+    <a href="#page59">59</a>, <a href="#page94">94</a>, <a href="#page343">343</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of</span> embryology, cartilage, <a
+    href="#page20">20</a>, <a href="#page129">129</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span
+    class="hid">logy,</span> eye-muscles, <a href="#page263">263</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span
+    class="hid">logy,</span> excretory organs, <a href="#page390">390</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span
+    class="hid">logy,</span> heart, <a href="#page179">179</a>, <a href="#page451">451</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span
+    class="hid">logy,</span> nervous system, central, cerebral vesicles, <a href="#page48">48</a>,
+    <a href="#page458">458</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span class="hid">logy,
+    ner</span>"<span class="hid">ous sy</span>"<span class="hid">tem, ce</span>"<span
+    class="hid">tral,</span> epithelial tube, <a href="#page37">37</a>, <a href="#page42">42</a>, <a
+    href="#page102">102</a>, <a href="#page244">244</a>, <a href="#page433">433</a>, <a
+    href="#page455">455</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span class="hid">logy,
+    ner</span>"<span class="hid">ous sy</span>"<span class="hid">tem, ce</span>"<span
+    class="hid">tral,</span> neurenteric canal, <a href="#page37">37</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span class="hid">logy,
+    ner</span>"<span class="hid">ous sy</span>"<span class="hid">tem, ce</span>"<span
+    class="hid">tral,</span> neuropore, <a href="#page220">220</a>, <a href="#page457">457</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span class="hid">logy,
+    ner</span>"<span class="hid">ous sy</span>"<span class="hid">tem, ce</span>"<span
+    class="hid">tral,</span> optic diverticula, <a href="#page102">102</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span class="hid">logy,
+    ner</span>"<span class="hid">ous sy</span>"<span class="hid">tem, ce</span>"<span
+    class="hid">tral,</span> spinal cord, <a href="#page46">46</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span
+    class="hid">logy,</span> oral chamber, <a href="#page228">228</a>, <a href="#page242">242</a>,
+    <a href="#page243">243</a>, <a href="#page290">290</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span
+    class="hid">logy,</span> olfactory organ, <a href="#page220">220</a>, <b><a
+    href="#page227">227</a></b></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span
+    class="hid">logy,</span> palæostoma or old mouth, <a href="#page317">317</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span
+    class="hid">logy,</span> pineal or median eyes, <a href="#page15">15</a>, <a
+    href="#page63">63</a>, <a href="#page74">74</a>, <a href="#page456">456</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span
+    class="hid">logy,</span> pituitary body and coxal glands, <a href="#page246">246</a>, <a
+    href="#page319">319</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span
+    class="hid">logy,</span> thyroid, <a href="#page192">192</a>, <b><a
+    href="#page194">194</a></b></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span
+    class="hid">logy,</span> segmentation, double, of head, <a href="#page157">157</a>, <a
+    href="#page234">234</a>, <a href="#page258">258</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of embry</span>"<span
+    class="hid">logy,</span> skeleton, cranial, <a href="#page120">120</a>, <a
+    href="#page153">153</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of</span> nervous system, central, <a
+    href="#page8">8</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of</span> notochord, origin from
+    segmented region, <a href="#page443">443</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of</span> olfactory apparatus, <a
+    href="#page218">218</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of</span> organs of vision, <a
+    href="#page68">68</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of</span> palæontology, <a
+    href="#page20">20</a>, <a href="#page497">497</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of</span> pineal or median eyes, <a
+    href="#page74">74</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of</span> prosomatic musculature, <a
+    href="#page247">247</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of</span> respiratory apparatus, <a
+    href="#page148">148</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of</span> segmentation in head-shield, <a
+    href="#page339">339</a></p>
+    <p><span class="hid">Evide</span>"<span class="hid">ce of</span> skeleton, <a
+    href="#page119">119</a></p>
+    <p>Evolution, <a href="#page8">8</a>, <a href="#page15">15</a>, <a href="#page20">20</a>, <a
+    href="#page149">149</a>, <a href="#page482">482</a>, <a href="#page497">497</a></p>
+    <p><span class="pagenum" id="page525">{525}</span><span class="hid">Evol</span>"<span
+    class="hid">tion,</span> of brain in brain-case, <a href="#page210">210</a></p>
+    <p><span class="hid">Evol</span>"<span class="hid">tion, of</span> cranium of Vertebrate, <a
+    href="#page342">342</a></p>
+    <p><span class="hid">Evol</span>"<span class="hid">tion, of</span> excretory organs, <a
+    href="#page389">389</a></p>
+    <p><span class="hid">Evol</span>"<span class="hid">tion, of</span> eye of Vertebrate, <a
+    href="#page114">114</a></p>
+    <p><span class="hid">Evol</span>"<span class="hid">tion, of</span> nervous system, central, <a
+    href="#page34">34</a></p>
+    <p><span class="hid">Evol</span>"<span class="hid">tion, of</span> tissues, <a
+    href="#page19">19</a></p>
+    <p><span class="hid">Evol</span>"<span class="hid">tion, of</span> Vertebrate from Balanoglossus
+    and Amphioxus, <a href="#page33">33</a></p>
+    <p>Eyes, <a href="#page68">68</a></p>
+    <p><span class="hid">Ey</span>"<span class="hid">s,</span> lateral, <a href="#page87">87</a>,
+    <b><a href="#page105">105</a></b>, <b><a href="#page108">108</a></b></p>
+    <p><span class="hid">Ey</span>"<span class="hid">s,</span> median or pineal, <a
+    href="#page74">74</a>, <b><a href="#page77">77</a></b>, <b><a href="#page78">78</a></b>, <b><a
+    href="#page79">79</a></b></p>
+    <p class="stanza">Fat-cells in muco-cartilage, <a href="#page332">332</a></p>
+    <p>Fat-column of Ammoc&#x0153;tes, <a href="#page181">181</a>, <b><a
+    href="#page182">182</a></b></p>
+    <p>Fibres, Mauthnerian, <a href="#page488">488</a></p>
+    <p><span class="hid">Fib</span>"<span class="hid">rs,</span> Müllerian, of Ammoc&#x0153;tes
+    central nervous system, <a href="#page489">489</a></p>
+    <p><span class="hid">Fib</span>"<span class="hid">rs, Müll</span>"<span class="hid">rian,
+    of</span> retina, <a href="#page96">96</a>, <a href="#page107">107</a></p>
+    <p>Fishes, classification, <a href="#page218">218</a></p>
+    <p><span class="hid">Fis</span>"<span class="hid">es,</span> ancient, classification, <a
+    href="#page326">326</a>, <a href="#page343">343</a></p>
+    <p><span class="hid">Fis</span>"<span class="hid">es, ani</span>"<span class="hid">ent,</span>
+    cloacal region, <a href="#page450">450</a></p>
+    <p><span class="hid">Fis</span>"<span class="hid">es, ani</span>"<span class="hid">ent,</span>
+    dominance, <a href="#page23">23</a></p>
+    <p><span class="hid">Fis</span>"<span class="hid">es, ani</span>"<span class="hid">ent,</span>
+    eyes, <a href="#page75">75</a></p>
+    <p><span class="hid">Fis</span>"<span class="hid">es, ani</span>"<span class="hid">ent,</span>
+    head-shields. <i>See</i> Head-shields</p>
+    <p><span class="hid">Fis</span>"<span class="hid">es, ani</span>"<span class="hid">ent,</span>
+    pleural folds, <a href="#page414">414</a></p>
+    <p>Fissure, posterior, <a href="#page43">43</a></p>
+    <p>Fittest, survival of, <a href="#page16">16</a>, <a href="#page34">34</a></p>
+    <p>Flabellum, <a href="#page359">359</a>, <b><a href="#page360">360</a></b>, <b><a
+    href="#page362">362</a></b>, <b><a href="#page363">363</a></b>, <b><a
+    href="#page366">366</a></b></p>
+    <p>Folds, pleural, <a href="#page410">410</a>, <a href="#page414">414</a></p>
+    <p>Function of auditory organ, double, <a href="#page358">358</a></p>
+    <p><span class="hid">Funt</span>"<span class="hid">ion of</span> lateral line sense-organs, <a
+    href="#page357">357</a></p>
+    <p><span class="hid">Funt</span>"<span class="hid">ion of</span> nerves, <a
+    href="#page448">448</a></p>
+    <p><span class="hid">Funt</span>"<span class="hid">ion of</span> thyroid, <a
+    href="#page212">212</a>, <a href="#page215">215</a></p>
+    <p>Fusion of ganglia, <a href="#page52">52</a></p>
+    <p class="stanza">Galeodes, <a href="#page230">230</a></p>
+    <p><span class="hid">Gale</span>"<span class="hid">des,</span> brain, and camerostome, <b><a
+    href="#page222">222</a></b>, <b><a href="#page223">223</a></b></p>
+    <p><span class="hid">Gale</span>"<span class="hid">des,</span> primordial cranium, <a
+    href="#page341">341</a></p>
+    <p><span class="hid">Gale</span>"<span class="hid">des,</span> racquet-organs, <a
+    href="#page369">369</a>, <a href="#page375">375</a></p>
+    <p>Ganglia, infra&#x0153;sophageal, <a href="#page4">4</a>, <a href="#page12">12</a>, <a
+    href="#page14">14</a>, <a href="#page51">51</a>, <a href="#page221">221</a></p>
+    <p><span class="hid">Ganl</span>"<span class="hid">ia,</span> supra&#x0153;sophageal, <a
+    href="#page4">4</a>, <a href="#page12">12</a>, <a href="#page14">14</a>, <a
+    href="#page49">49</a>, <a href="#page52">52</a>, <a href="#page221">221</a>, <a
+    href="#page225">225</a></p>
+    <p><span class="hid">Ganl</span>"<span class="hid">ia,</span> origin of, of cranial and spinal
+    nerves, <a href="#page281">281</a></p>
+    <p>Ganglion, epibranchial, <a href="#page164">164</a>, <a href="#page282">282</a></p>
+    <p><span class="hid">Ganl</span>"<span class="hid">ion,</span> habenulæ, <a
+    href="#page48">48</a>, <a href="#page78">78</a></p>
+    <p><span class="hid">Ganl</span>"<span class="hid">ion,</span> optic of retina, <b><a
+    href="#page72">72</a></b>, <a href="#page89">89</a>, <a href="#page97">97</a></p>
+    <p><span class="hid">Ganl</span>"<span class="hid">ion,</span> of posterior root, <a
+    href="#page466">466</a></p>
+    <p><span class="hid">Ganl</span>"<span class="hid">ion,</span> cells of sympathetic system, <a
+    href="#page424">424</a>, <a href="#page428">428</a>, <a href="#page448">448</a></p>
+    <p>Ganoids, <a href="#page23">23</a>, <a href="#page345">345</a></p>
+    <p>Gastrula theory, <a href="#page165">165</a>, <a href="#page459">459</a></p>
+    <p>Genital corpuscles, <a href="#page470">470</a></p>
+    <p>Geological record, <a href="#page20">20</a></p>
+    <p><span class="hid">Geol</span>"<span class="hid">gical</span> strata, <b><a
+    href="#page22">22</a></b></p>
+    <p>Geotria australis, <a href="#page80">80</a></p>
+    <p>Germ-band, <a href="#page482">482</a></p>
+    <p><span class="pagenum" id="page526">{526}</span>Germ-cells, <a href="#page471">471</a></p>
+    <p>Giant-fibres, <a href="#page489">489</a></p>
+    <p>Gigantostraca, <a href="#page25">25</a>, <a href="#page234">234</a></p>
+    <p>Gills, <a href="#page148">148</a>, <a href="#page161">161</a>, <a href="#page185">185</a>, <a
+    href="#page214">214</a>, <a href="#page494">494</a></p>
+    <p>Glabellum, <a href="#page339">339</a></p>
+    <p>Glands, carotid, <a href="#page427">427</a></p>
+    <p><span class="hid">Gla</span>"<span class="hid">ds,</span> coxal, <a href="#page242">242</a>,
+    <a href="#page246">246</a>, <a href="#page319">319</a>, <b><a href="#page321">321</a></b>, <a
+    href="#page425">425</a>, <a href="#page429">429</a></p>
+    <p><span class="hid">Gla</span>"<span class="hid">ds,</span> ductless, <a
+    href="#page418">418</a></p>
+    <p><span class="hid">Gla</span>"<span class="hid">ds,</span> generative, of Limulus, <a
+    href="#page209">209</a></p>
+    <p><span class="hid">Gla</span>"<span class="hid">ds,</span> internal secretion of, <a
+    href="#page214">214</a></p>
+    <p><span class="hid">Gla</span>"<span class="hid">ds,</span> lymphatic, <a
+    href="#page418">418</a></p>
+    <p><span class="hid">Gla</span>"<span class="hid">ds,</span> pineal, <a href="#page15">15</a>,
+    <a href="#page63">63</a>, <a href="#page75">75</a>, <a href="#page456">456</a></p>
+    <p><span class="hid">Gla</span>"<span class="hid">ds,</span> pituitary, <a
+    href="#page244">244</a>, <a href="#page246">246</a>, <a href="#page319">319</a>, <a
+    href="#page425">425</a></p>
+    <p><span class="hid">Gla</span>"<span class="hid">ds,</span> segmental, of Crustacea, <a
+    href="#page422">422</a></p>
+    <p><span class="hid">Gla</span>"<span class="hid">ds,</span> submaxillary, <a
+    href="#page466">466</a></p>
+    <p><span class="hid">Gla</span>"<span class="hid">ds,</span> sweat, <a
+    href="#page448">448</a></p>
+    <p><span class="hid">Gla</span>"<span class="hid">ds,</span> thymus, <a
+    href="#page425">425</a></p>
+    <p><span class="hid">Gla</span>"<span class="hid">ds,</span> thyroid, of Ammoc&#x0153;tes, <a
+    href="#page193">193</a>, <b><a href="#page194">194</a></b>, <b><a href="#page196">196</a></b>,
+    <b><a href="#page201">201</a></b>, <a href="#page205">205</a>, <a href="#page429">429</a></p>
+    <p><span class="hid">Gla</span>"<span class="hid">ds,</span> tissue round brain of
+    Ammoc&#x0153;tes, <a href="#page209">209</a>, <a href="#page379">379</a></p>
+    <p><span class="hid">Gla</span>"<span class="hid">ds,</span> uterine, of Scorpion, <a
+    href="#page202">202</a>, <b><a href="#page203">203</a></b>, <b><a href="#page204">204</a></b>,
+    <b><a href="#page205">205</a></b></p>
+    <p>Gnathostomata, <a href="#page60">60</a>, <a href="#page343">343</a></p>
+    <p>Goblet, <a href="#page359">359</a>, <b><a href="#page360">360</a></b>, <b><a
+    href="#page373">373</a></b></p>
+    <p>Goitre, <a href="#page215">215</a></p>
+    <p>Gonad, <a href="#page475">475</a>, <a href="#page479">479</a></p>
+    <p>Gonoc&#x0153;le, <a href="#page475">475</a>, <a href="#page481">481</a></p>
+    <p>Grooves, ciliated, <a href="#page188">188</a>, <a href="#page197">197</a>, <a
+    href="#page212">212</a></p>
+    <p><span class="hid">Gro</span>"<span class="hid">ves,</span> hyper-pharyngeal of Amphioxus, <a
+    href="#page410">410</a></p>
+    <p><span class="hid">Gro</span>"<span class="hid">ves,</span> ventral, of apus and trilobites,
+    <a href="#page436">436</a></p>
+    <p>Gymnophiona, <a href="#page393">393</a></p>
+    <p class="stanza">Hæmocytes, <a href="#page472">472</a></p>
+    <p>Head of embryo dogfish, <b><a href="#page121">121</a></b>, <b><a
+    href="#page123">123</a></b></p>
+    <p>Head-shield, dorsal, of Ammoc&#x0153;tes, <b><a href="#page330">330</a></b>, <a
+    href="#page331">331</a>, <a href="#page338">338</a></p>
+    <p><span class="hid">Head-s</span>"<span class="hid">ield, dor</span>"<span class="hid">al,
+    of</span> Auchenaspis, <a href="#page29">29</a>, <b><a href="#page31">31</a></b>, <b><a
+    href="#page338">338</a></b></p>
+    <p><span class="hid">Head-s</span>"<span class="hid">ield, dor</span>"<span class="hid">al,
+    of</span> Cephalaspis, <a href="#page327">327</a>, <b><a href="#page328">328</a></b>, <b><a
+    href="#page330">330</a></b>, <b><a href="#page338">338</a></b>, <a href="#page348">348</a></p>
+    <p><span class="hid">Head-s</span>"<span class="hid">ield, dor</span>"<span class="hid">al,
+    of</span> Cyathaspis, <b><a href="#page340">340</a></b></p>
+    <p><span class="hid">Head-s</span>"<span class="hid">ield, dor</span>"<span class="hid">al,
+    of</span> Didymaspis, <b><a href="#page338">338</a></b></p>
+    <p><span class="hid">Head-s</span>"<span class="hid">ield, dor</span>"<span class="hid">al,
+    of</span> evidence of segmentation, <a href="#page339">339</a></p>
+    <p><span class="hid">Head-s</span>"<span class="hid">ield, dor</span>"<span class="hid">al,
+    of</span> Keraspis, <b><a href="#page328">328</a></b></p>
+    <p><span class="hid">Head-s</span>"<span class="hid">ield, dor</span>"<span class="hid">al,
+    of</span> Ostreostraci, <a href="#page327">327</a>, <a href="#page348">348</a></p>
+    <p><span class="hid">Head-s</span>"<span class="hid">ield, dor</span>"<span class="hid">al,
+    of</span> Palæostracan, <a href="#page348">348</a></p>
+    <p><span class="hid">Head-s</span>"<span class="hid">ield, dor</span>"<span class="hid">al,
+    of</span> Pteraspis, <a href="#page29">29</a></p>
+    <p><span class="hid">Head-s</span>"<span class="hid">ield, dor</span>"<span class="hid">al,
+    of</span> Thyestes, <a href="#page29">29</a>, <b><a href="#page31">31</a></b>, <a
+    href="#page327">327</a>, <a href="#page332">332</a>, <a href="#page338">338</a>, <a
+    href="#page340">340</a>, <b><a href="#page341">341</a></b>, <a href="#page348">348</a></p>
+    <p><span class="hid">Head-s</span>"<span class="hid">ield,</span> ventral, Scaphaspis, <a
+    href="#page349">349</a></p>
+    <p>Heart, nerves, <a href="#page2">2</a>, <a href="#page447">447</a></p>
+    <p><span class="hid">Her</span>"<span class="hid">t,</span> origin of vertebrate, <a
+    href="#page179">179</a>, <a href="#page451">451</a>, <a href="#page459">459</a></p>
+    <p><span class="hid">Her</span>"<span class="hid">t,</span> relative position in vertebrate and
+    invertebrate, <a href="#page175">175</a></p>
+    <p><span class="hid">Her</span>"<span class="hid">t,</span> veins forming vertebrate, <b><a
+    href="#page180">180</a></b></p>
+    <p>Hemiaspis, <a href="#page24">24</a>, <b><a href="#page25">25</a></b>, <a
+    href="#page249">249</a>, <b><a href="#page250">250</a></b>, <a href="#page351">351</a>, <a
+    href="#page414">414</a></p>
+    <p>Hemispheres, cerebral, <a href="#page47">47</a></p>
+    <p>Hepatopancreas of Ammoc&#x0153;tes, <a href="#page452">452</a></p>
+    <p><span class="hid">Hepatop</span>"<span class="hid">ncreas of</span> Limulus, <a
+    href="#page211">211</a></p>
+    <p><span class="pagenum" id="page527">{527}</span>Heterostraci, <a href="#page29">29</a>, <a
+    href="#page275">275</a>, <a href="#page326">326</a>, <a href="#page343">343</a></p>
+    <p>Hirudinea, <a href="#page478">478</a></p>
+    <p>Histolysis in transformation of the lamprey, <a href="#page59">59</a></p>
+    <p>Homology of branchial region of vertebrate and invertebrate, <a href="#page149">149</a></p>
+    <p><span class="hid">Homol</span>"<span class="hid">gy of</span> ductless glands and nephridial
+    organs, <a href="#page418">418</a></p>
+    <p><span class="hid">Homol</span>"<span class="hid">gy of</span> external genital ducts of
+    arthropods and nephridia of annelids, <a href="#page429">429</a></p>
+    <p><span class="hid">Homol</span>"<span class="hid">gy of</span> germinal layers in all <span
+    class="correction" title="Original reads 'Metozoa'">Metazoa</span>, <a
+    href="#page459">459</a></p>
+    <p><span class="hid">Homol</span>"<span class="hid">gy of</span> pituitary body of
+    Ammoc&#x0153;tes and coxal glands of Limulus, <a href="#page319">319</a></p>
+    <p><span class="hid">Homol</span>"<span class="hid">gy of</span> tubular muscles of
+    Ammoc&#x0153;tes and veno-pericardial muscles of Limulus, <a href="#page309">309</a></p>
+    <p><span class="hid">Homol</span>"<span class="hid">gy of</span> ventral aorta of vertebrate and
+    longitudinal venous sinuses of Limulus, <a href="#page178">178</a></p>
+    <p>Hydra, <a href="#page441">441</a>, <a href="#page465">465</a>, <a href="#page472">472</a>, <a
+    href="#page476">476</a></p>
+    <p>Hydrophilus larva, eye, <b><a href="#page84">84</a></b></p>
+    <p>Hyoid segment in Ammoc&#x0153;tes, <a href="#page186">186</a>, <a href="#page267">267</a></p>
+    <p>Hypoblast, <a href="#page434">434</a>, <a href="#page438">438</a>, <a
+    href="#page444">444</a>, <a href="#page445">445</a>, <a href="#page459">459</a></p>
+    <p>Hypoctonus, cartilage cells in entosternite, <b><a href="#page133">133</a></b></p>
+    <p><span class="hid">Hypo</span>"<span class="hid">tonus,</span> operculum, <a
+    href="#page189">189</a>, <a href="#page207">207</a></p>
+    <p>Hypogastric plexus, <a href="#page3">3</a></p>
+    <p>Hypogeophis, <a href="#page393">393</a></p>
+    <p>Hypophysis, <a href="#page229">229</a>, <a href="#page244">244</a>, <a
+    href="#page317">317</a>, <b><a href="#page318">318</a></b>, <b><a
+    href="#page340">340</a></b></p>
+    <p class="stanza">Infundibulum, position, <a href="#page122">122</a>,<a
+    href="#page132">132</a></p>
+    <p><span class="hid">Infun</span>"<span class="hid">ibulum,</span> tube, the ancestral
+    &#x0153;sophagus, <a href="#page4">4</a>, <a href="#page37">37</a>, <a href="#page244">244</a>,
+    <a href="#page318">318</a></p>
+    <p><span class="hid">Infun</span>"<span class="hid">ibulum, tu</span>"<span
+    class="hid">e,</span> relation to neural canal, <a href="#page14">14</a>, <a
+    href="#page36">36</a>, <b><a href="#page318">318</a></b>, <b><a href="#page440">440</a></b>, <a
+    href="#page457">457</a></p>
+    <p><span class="hid">Infun</span>"<span class="hid">ibulum, tu</span>"<span class="hid">e,
+    relati</span>"<span class="hid">n to</span> notochord, <b><a href="#page318">318</a></b>, <a
+    href="#page435">435</a>,<a href="#page440">440</a></p>
+    <p><span class="hid">Infun</span>"<span class="hid">ibulum, tu</span>"<span class="hid">e,
+    relati</span>"<span class="hid">n to</span> olfactory tube, <a href="#page220">220</a>, <a
+    href="#page228">228</a>, <b><a href="#page318">318</a></b>, <a href="#page340">340</a></p>
+    <p>Insects, chordotonal organs, <a href="#page364">364</a>, <b><a
+    href="#page370">370</a></b></p>
+    <p>Invertebrate, heart, <a href="#page175">175</a>, <a href="#page179">179</a></p>
+    <p><span class="hid">Invert</span>"<span class="hid">brate,</span> excretory organs, <a
+    href="#page418">418</a></p>
+    <p><span class="hid">Invert</span>"<span class="hid">brate,</span> nervous system, <b><a
+    href="#page13">13</a></b>, <b><a href="#page54">54</a></b></p>
+    <p><span class="hid">Invert</span>"<span class="hid">brate,</span> segmental nerves, <a
+    href="#page152">152</a></p>
+    <p class="stanza">Keraspis, <a href="#page75">75</a>, <b><a href="#page328">328</a></b>, <a
+    href="#page338">338</a></p>
+    <p>Kidney, <a href="#page420">420</a>, <a href="#page459">459</a>, <a
+    href="#page476">476</a></p>
+    <p><span class="hid">Kid</span>"<span class="hid">ey,</span> nerves, <a
+    href="#page477">477</a></p>
+    <p>King-crab, <i>v.</i> Limulus</p>
+    <p class="stanza">Labyrinthodont, <a href="#page21">21</a>, <a href="#page28">28</a></p>
+    <p>Lamina terminalis, <a href="#page49">49</a></p>
+    <p>Lamprey, <i>v.</i> Ammoc&#x0153;tes and Petromyzon</p>
+    <p>Larva, <i>v.</i> Transformation of the Lamprey</p>
+    <p>Lateral line system, <a href="#page261">261</a>, <a href="#page355">355</a>, <a
+    href="#page411">411</a>, <a href="#page470">470</a></p>
+    <p>Law of Progress, <a href="#page19">19</a></p>
+    <p><span class="hid">Lw</span>"<span class="hid"> of</span> Recapitulation, <a
+    href="#page434">434</a>, <a href="#page456">456</a>, <a href="#page498">498</a></p>
+    <p>Layer, germinal, <a href="#page459">459</a></p>
+    <p><span class="hid">Lay</span>"<span class="hid">r,</span> laminated, <a
+    href="#page347">347</a>, <b><a href="#page348">348</a></b></p>
+    <p>Leech, <a href="#page421">421</a></p>
+    <p>Lens, formation, <a href="#page83">83</a>, <a href="#page115">115</a></p>
+    <p>Lepidosiren, <a href="#page148">148</a>, <a href="#page461">461</a>, <a
+    href="#page466">466</a></p>
+    <p>Limulus or king-crab, <b><a href="#page25">25</a></b>, <b><a href="#page140">140</a></b>,
+    <b><a href="#page236">236</a></b>, <b><a href="#page240">240</a></b></p>
+    <p><span class="hid">Liml</span>"<span class="hid">us</span> appendages, branchial, <b><a
+    href="#page138">138</a></b>, <b><a href="#page164">164</a></b>, <b><a
+    href="#page175">175</a></b></p>
+    <p><span class="pagenum" id="page528">{528}</span><span class="hid">Liml</span>"<span
+    class="hid">us</span> appendages, prosomatic, <b><a href="#page381">381</a></b></p>
+    <p><span class="hid">Liml</span>"<span class="hid">us</span> brain, <b><a
+    href="#page54">54</a></b></p>
+    <p><span class="hid">Liml</span>"<span class="hid">us</span> circulation, <a
+    href="#page174">174</a>, <a href="#page176">176</a></p>
+    <p><span class="hid">Liml</span>"<span class="hid">us</span> classification, <a
+    href="#page26">26</a>, <a href="#page249">249</a></p>
+    <p><span class="hid">Liml</span>"<span class="hid">us</span> c&#x0153;lomic cavities, <a
+    href="#page252">252</a>, <a href="#page328">328</a></p>
+    <p><span class="hid">Liml</span>"<span class="hid">us</span> coxal glands, <a
+    href="#page321">321</a>, <a href="#page389">389</a>, <a href="#page397">397</a>, <a
+    href="#page403">403</a>, <a href="#page429">429</a></p>
+    <p><span class="hid">Liml</span>"<span class="hid">us</span> eyes, median, <a
+    href="#page62">62</a>, <a href="#page74">74</a>, <a href="#page81">81</a></p>
+    <p><span class="hid">Liml</span>"<span class="hid">us</span> entosternite or plastron, <a
+    href="#page142">142</a>, <b><a href="#page143">143</a></b></p>
+    <p><span class="hid">Liml</span>"<span class="hid">us</span> flabellum, <b><a
+    href="#page360">360</a></b>, <b><a href="#page362">362</a></b>, <b><a
+    href="#page363">363</a></b>, <a href="#page380">380</a>, <b><a href="#page381">381</a></b></p>
+    <p><span class="hid">Liml</span>"<span class="hid">us</span> generative organs and ducts, <b><a
+    href="#page189">189</a></b>, <a href="#page202">202</a>, <a href="#page208">208</a>, <a
+    href="#page209">209</a>, <a href="#page380">380</a></p>
+    <p><span class="hid">Liml</span>"<span class="hid">us</span> heart, <b><a
+    href="#page180">180</a></b></p>
+    <p><span class="hid">Liml</span>"<span class="hid">us</span> musculature, branchial, <a
+    href="#page170">170</a></p>
+    <p><span class="hid">Liml</span>"<span class="hid">us muscl</span>"<span
+    class="hid">ature,</span> prosomatic, <a href="#page247">247</a></p>
+    <p><span class="hid">Liml</span>"<span class="hid">us muscl</span>"<span
+    class="hid">ature,</span> veno-pericardial, <a href="#page177">177</a>, <a
+    href="#page297">297</a>, <a href="#page309">309</a>, <a href="#page313">313</a></p>
+    <p><span class="hid">Liml</span>"<span class="hid">us</span> nerves, appendage, <b><a
+    href="#page140">140</a></b>, <a href="#page157">157</a></p>
+    <p><span class="hid">Liml</span>"<span class="hid">us ner</span>"<span class="hid">es,</span>
+    cardiac, <a href="#page314">314</a></p>
+    <p><span class="hid">Liml</span>"<span class="hid">us ner</span>"<span class="hid">es,</span>
+    segmental, tripartite division of, <a href="#page157">157</a>, <a href="#page235">235</a>, <a
+    href="#page267">267</a>, <a href="#page355">355</a></p>
+    <p><span class="hid">Liml</span>"<span class="hid">us</span> segments, branchial, <a
+    href="#page152">152</a></p>
+    <p><span class="hid">Liml</span>"<span class="hid">us segm</span>"<span class="hid">nts,</span>
+    first mesosomatic, <a href="#page188">188</a></p>
+    <p><span class="hid">Liml</span>"<span class="hid">us segm</span>"<span class="hid">nts,</span>
+    prosomatic, <a href="#page233">233</a></p>
+    <p><span class="hid">Liml</span>"<span class="hid">us</span> operculum, <b><a
+    href="#page189">189</a></b>, <a href="#page202">202</a>, <a href="#page235">235</a>, <a
+    href="#page295">295</a></p>
+    <p><span class="hid">Liml</span>"<span class="hid">us</span> sense-organs, poriferous, of
+    appendages, <a href="#page359">359</a></p>
+    <p>Lip, lower, of Ammoc&#x0153;tes, <a href="#page246">246</a>, <a href="#page289">289</a>, <a
+    href="#page297">297</a>, <a href="#page458">458</a></p>
+    <p><span class="hid">Li</span>"<span class="hid">,</span> upper, <span class="hid">of
+    Am</span>"<span class="hid">oc&#x0153;tes,</span> <a href="#page228">228</a>, <a
+    href="#page243">243</a>, <a href="#page303">303</a>, <a href="#page336">336</a></p>
+    <p>Liver, Ammoc&#x0153;tes, <a href="#page452">452</a></p>
+    <p><span class="hid">Liv</span>"<span class="hid">r,</span> Limulus, <a href="#page209">209</a>,
+    <a href="#page211">211</a></p>
+    <p>Lizard, pineal eye, <a href="#page80">80</a></p>
+    <p><span class="hid">Liz</span>"<span class="hid">rd,</span> suprarenals, <a
+    href="#page424">424</a></p>
+    <p><span class="hid">Liz</span>"<span class="hid">rd,</span> tail, <a href="#page50">50</a></p>
+    <p>Lobes, optic, <a href="#page101">101</a></p>
+    <p>Lobster, <a href="#page489">489</a></p>
+    <p>Lungs, <a href="#page148">148</a></p>
+    <p>Lung-books of scorpions, <a href="#page150">150</a></p>
+    <p>Lymph, <a href="#page474">474</a></p>
+    <p>Lymph-corpuscles, <a href="#page463">463</a>, <a href="#page490">490</a></p>
+    <p>Lymphocytes, <a href="#page472">472</a></p>
+    <p class="stanza">Malapterurus, <a href="#page470">470</a></p>
+    <p>Mammal, dominance of, <a href="#page21">21</a></p>
+    <p>Man, dominance of, <a href="#page17">17</a></p>
+    <p>Marsipobranchs, <a href="#page23">23</a>, <a href="#page35">35</a></p>
+    <p>Medullation of nerve-fibres, <a href="#page20">20</a>, <a href="#page267">267</a>, <a
+    href="#page467">467</a>, <a href="#page477">477</a></p>
+    <p>Membranes, basement, <a href="#page436">436</a></p>
+    <p>Meroblastic egg, <a href="#page485">485</a></p>
+    <p>Merostomata, <a href="#page25">25</a>, <a href="#page249">249</a>, <a
+    href="#page321">321</a></p>
+    <p>Mesencepalon, <a href="#page48">48</a></p>
+    <p>Mesoblast, <a href="#page444">444</a>, <a href="#page455">455</a>, <a
+    href="#page459">459</a></p>
+    <p>Mesogl&#x0153;a, <a href="#page474">474</a></p>
+    <p>Mesonephros, <a href="#page389">389</a>, <a href="#page400">400</a>, <a
+    href="#page424">424</a>, <a href="#page429">429</a></p>
+    <p>Mesosoma, <a href="#page52">52</a></p>
+    <p>Mesothelium, <a href="#page472">472</a>, <a href="#page477">477</a></p>
+    <p>Metanephros, <a href="#page389">389</a></p>
+    <p><span class="pagenum" id="page529">{529}</span>Metasoma, <a href="#page52">52</a>, <a
+    href="#page387">387</a>, <a href="#page411">411</a></p>
+    <p>Metastoma, <a href="#page239">239</a>, <a href="#page246">246</a>, <a
+    href="#page272">272</a>, <a href="#page289">289</a>, <a href="#page342">342</a>, <a
+    href="#page458">458</a></p>
+    <p>Metazoa, <a href="#page444">444</a>, <a href="#page459">459</a>, <a href="#page471">471</a>,
+    <a href="#page472">472</a></p>
+    <p>Meynert's bundle, <a href="#page48">48</a>, <a href="#page77">77</a></p>
+    <p>Mollusca, dominance of, <a href="#page23">23</a></p>
+    <p>Mouth, old, or palæostoma, <a href="#page14">14</a>, <a href="#page317">317</a>, <a
+    href="#page322">322</a>, <a href="#page440">440</a>, <a href="#page458">458</a></p>
+    <p><span class="hid">Mo</span>"<span class="hid">th,</span> vertebrate, <a
+    href="#page317">317</a></p>
+    <p>Muco-cartilage, <i>v.</i> Cartilage</p>
+    <p>Muscles, antagonistic, <a href="#page447">447</a></p>
+    <p><span class="hid">Mus</span>"<span class="hid">les,</span> branchial, <a
+    href="#page170">170</a></p>
+    <p><span class="hid">Mus</span>"<span class="hid">les,</span> connection of, with central
+    nervous system, <a href="#page464">464</a></p>
+    <p><span class="hid">Mus</span>"<span class="hid">les,</span> eye, and their nerves, <a
+    href="#page263">263</a></p>
+    <p><span class="hid">Mus</span>"<span class="hid">les,</span> prosomatic, <a
+    href="#page243">243</a>, <a href="#page247">247</a></p>
+    <p><span class="hid">Mus</span>"<span class="hid">les,</span> phylogeny of origin of skeletal,
+    <a href="#page478">478</a></p>
+    <p><span class="hid">Mus</span>"<span class="hid">les,</span> rudimentary, in Ammoc&#x0153;tes,
+    <a href="#page289">289</a></p>
+    <p><span class="hid">Mus</span>"<span class="hid">les,</span> somatic trunk, origin of, <a
+    href="#page406">406</a></p>
+    <p><span class="hid">Mus</span>"<span class="hid">les,</span> striated, <a
+    href="#page20">20</a>, <a href="#page155">155</a></p>
+    <p><span class="hid">Mus</span>"<span class="hid">les,</span> tubular, of Ammoc&#x0153;tes, <a
+    href="#page309">309</a></p>
+    <p><span class="hid">Mus</span>"<span class="hid">les,</span> unstriped, <a
+    href="#page20">20</a>, <a href="#page447">447</a>, <a href="#page491">491</a></p>
+    <p><span class="hid">Mus</span>"<span class="hid">les,</span> visceral and parietal, <a
+    href="#page155">155</a>, <a href="#page172">172</a></p>
+    <p><span class="hid">Mus</span>"<span class="hid">les,</span> veno-pericardial of Limulus and
+    Scorpion, <a href="#page177">177</a>, <a href="#page297">297</a>, <a href="#page309">309</a></p>
+    <p>Muscle-spindles, <a href="#page267">267</a></p>
+    <p>Mygalidæ, stomach, <a href="#page109">109</a></p>
+    <p><span class="hid">Mygl</span>"<span class="hid">idæ,</span> segmentation, <a
+    href="#page249">249</a>, <a href="#page306">306</a></p>
+    <p>Myomeres, <a href="#page262">262</a>, <a href="#page337">337</a>, <a href="#page414">414</a>,
+    <a href="#page479">479</a></p>
+    <p>Myotomes, <a href="#page332">332</a>, <a href="#page337">337</a>, <a href="#page338">338</a>,
+    <b><a href="#page391">391</a></b>, <a href="#page407">407</a>, <b><a
+    href="#page408">408</a></b></p>
+    <p>Mysis, eyes, <a href="#page100">100</a></p>
+    <p><span class="hid">My</span>"<span class="hid">is,</span> ductless glands, <a
+    href="#page422">422</a></p>
+    <p>Myxine, <a href="#page220">220</a>, <a href="#page392">392</a>, <a href="#page402">402</a>,
+    <a href="#page419">419</a></p>
+    <p class="stanza">Nebalia, <a href="#page144">144</a>, <a href="#page422">422</a></p>
+    <p>Nemertina, <a href="#page475">475</a></p>
+    <p>Nephridia, <a href="#page395">395</a>, <a href="#page421">421</a>, <a
+    href="#page429">429</a></p>
+    <p>Nephroc&#x0153;le, <a href="#page430">430</a></p>
+    <p>Nephrotome, <a href="#page393">393</a></p>
+    <p>Nerves, abducens, <a href="#page155">155</a>, <a href="#page263">263</a>, <a
+    href="#page266">266</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es,</span> auditory, <a
+    href="#page356">356</a>, <a href="#page376">376</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es,</span> autonomic, <a
+    href="#page3">3</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es,</span> facial, <a href="#page155">155</a>,
+    <a href="#page156">156</a>, <a href="#page186">186</a>, <a href="#page188">188</a>, <a
+    href="#page192">192</a>, <b><a href="#page311">311</a></b>, <a href="#page356">356</a>, <a
+    href="#page378">378</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es, fa</span>"<span class="hid">ial,</span>
+    ramus branchialis profundus, <a href="#page311">311</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es,</span> to flabellum, in Limulus, <a
+    href="#page361">361</a>, <a href="#page375">375</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es,</span> glossopharyngeal, <a
+    href="#page155">155</a>, <a href="#page156">156</a>, <a href="#page186">186</a>, <a
+    href="#page356">356</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es,</span> hypoglossal, <a
+    href="#page156">156</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es,</span> inhibitory, <a
+    href="#page447">447</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es,</span> inedullation of, <a
+    href="#page20">20</a>, <a href="#page267">267</a>, <a href="#page467">467</a>, <a
+    href="#page477">477</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es,</span> occulomotor, <a
+    href="#page155">155</a>, <a href="#page234">234</a>, <a href="#page263">263</a>, <a
+    href="#page274">274</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es,</span> olfactory, <a
+    href="#page229">229</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es,</span> optic, <a href="#page101">101</a>,
+    <a href="#page104">104</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es, ot</span>"<span class="hid">ic,</span> of
+    pineal eye, <a href="#page79">79</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es,</span> origin of ganglia of cranial and
+    spinal, <a href="#page281">281</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es,</span> to pecten of Scorpion, <a
+    href="#page375">375</a>, <a href="#page376">376</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es,</span> preganglionic, <a
+    href="#page2">2</a></p>
+    <p><span class="pagenum" id="page530">{530}</span><span class="hid">Ner</span>"<span
+    class="hid">es,</span> of prosoma in Limulus, <a href="#page235">235</a>, <a
+    href="#page355">355</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es,</span> regeneration of, <a
+    href="#page469">469</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es,</span> roots, of Limulus, <a
+    href="#page157">157</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es,</span> sacral, <a
+    href="#page448">448</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es,</span> segmental, <a
+    href="#page152">152</a>, <a href="#page156">156</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es,</span> segmental nature of cranial, <a
+    href="#page259">259</a>, <a href="#page411">411</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es,</span> spinal, absence of lateral roots
+    in, <a href="#page388">388</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es,</span> spinal accessory, <a
+    href="#page154">154</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es,</span> trigeminal, <a
+    href="#page151">151</a>, <a href="#page155">155</a>, <a href="#page156">156</a>, <a
+    href="#page234">234</a>, <a href="#page243">243</a>, <a href="#page257">257</a>, <a
+    href="#page279">279</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es, trigm</span>"<span
+    class="hid">inal,</span> motor nucleus of, <a href="#page280">280</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es, trigm</span>"<span
+    class="hid">inal,</span> of Ammoc&#x0153;tes, <b><a href="#page288">288</a></b></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es,</span> tripartite arrangement of cranial
+    nerves, <a href="#page154">154</a>, <a href="#page157">157</a>, <a href="#page235">235</a>, <a
+    href="#page267">267</a>, <a href="#page355">355</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es,</span> trochlear, <a
+    href="#page48">48</a>, <a href="#page155">155</a>, <a href="#page234">234</a>, <a
+    href="#page263">263</a>, <a href="#page276">276</a></p>
+    <p><span class="hid">Ner</span>"<span class="hid">es,</span> vagus, <a href="#page151">151</a>,
+    <a href="#page154">154</a>, <a href="#page156">156</a>, <a href="#page173">173</a>, <a
+    href="#page186">186</a>, <a href="#page356">356</a>, <a href="#page447">447</a>, <a
+    href="#page449">449</a></p>
+    <p>Nervous system, central, comparison of Vertebrate and Arthropod, <a href="#page36">36</a>, <a
+    href="#page457">457</a></p>
+    <p><span class="hid">Nervous </span>"<span class="hid">ystem, ce</span>"<span
+    class="hid">tral,</span> connection of, with muscular and epithelial tissues, <a
+    href="#page464">464</a></p>
+    <p><span class="hid">Nervous </span>"<span class="hid">ystem, ce</span>"<span class="hid">tral,
+    conne</span>"<span class="hid">tion of,</span> with retina, <a href="#page71">71</a></p>
+    <p><span class="hid">Nervous </span>"<span class="hid">ystem, ce</span>"<span
+    class="hid">tral,</span> disease of, <a href="#page50">50</a></p>
+    <p><span class="hid">Nervous </span>"<span class="hid">ystem, ce</span>"<span
+    class="hid">tral,</span> evidence of, <a href="#page8">8</a></p>
+    <p><span class="hid">Nervous </span>"<span class="hid">ystem, ce</span>"<span
+    class="hid">tral,</span> evolution of, <a href="#page34">34</a></p>
+    <p><span class="hid">Nervous </span>"<span class="hid">ystem, ce</span>"<span
+    class="hid">tral,</span> importance of, <a href="#page16">16</a>, <a href="#page463">463</a>, <a
+    href="#page482">482</a>, <a href="#page498">498</a></p>
+    <p><span class="hid">Nervous </span>"<span class="hid">ystem, ce</span>"<span
+    class="hid">tral,</span> invertebrate, <b><a href="#page10">10</a></b>, <b><a
+    href="#page13">13</a></b>, <b><a href="#page54">54</a></b></p>
+    <p><span class="hid">Nervous </span>"<span class="hid">ystem, ce</span>"<span
+    class="hid">tral,</span> origin of, <a href="#page480">480</a></p>
+    <p><span class="hid">Nervous </span>"<span class="hid">ystem, ce</span>"<span
+    class="hid">tral,</span> relation of germ-band to, <a href="#page483">483</a></p>
+    <p><span class="hid">Nervous </span>"<span class="hid">ystem, ce</span>"<span
+    class="hid">tral,</span> segmentation of vertebrate, <a href="#page51">51</a></p>
+    <p><span class="hid">Nervous </span>"<span class="hid">ystem, ce</span>"<span
+    class="hid">tral,</span> tube of, <a href="#page36">36</a>-<a href="#page51">51</a>, <a
+    href="#page102">102</a>, <a href="#page211">211</a>, <a href="#page433">433</a>, <a
+    href="#page455">455</a>, <a href="#page457">457</a></p>
+    <p><span class="hid">Nervous </span>"<span class="hid">ystem, ce</span>"<span
+    class="hid">tral,</span> vertebrate, <b><a href="#page10">10</a></b>, <b><a
+    href="#page13">13</a></b>, <b><a href="#page40">40</a></b>, <a href="#page41">41</a>, <a
+    href="#page152">152</a></p>
+    <p><span class="hid">Nervous </span>"<span class="hid">ystem,</span> enteric, <a
+    href="#page447">447</a></p>
+    <p><span class="hid">Nervous </span>"<span class="hid">ystem,</span> sympathetic, <a
+    href="#page2">2</a>, <a href="#page424">424</a>, <a href="#page428">428</a>, <a
+    href="#page448">448</a>, <a href="#page491">491</a></p>
+    <p>Neurenteric canal, <a href="#page37">37</a></p>
+    <p>Neuroblast, <a href="#page465">465</a></p>
+    <p>Neuromeres, <a href="#page55">55</a>, <a href="#page247">247</a>, <a href="#page262">262</a>,
+    <a href="#page312">312</a>, <a href="#page316">316</a></p>
+    <p>Neurones, <a href="#page72">72</a>, <a href="#page92">92</a>, <a href="#page465">465</a></p>
+    <p>Neuropil, <a href="#page71">71</a>, <a href="#page91">91</a></p>
+    <p>Neuropore, <a href="#page220">220</a>, <a href="#page457">457</a></p>
+    <p>Nose, <a href="#page219">219</a></p>
+    <p><span class="hid">No</span>"<span class="hid">e,</span> of Osteostraci, <a
+    href="#page329">329</a>, <a href="#page352">352</a>, <a href="#page458">458</a></p>
+    <p>Notochord, <a href="#page120">120</a>, <a href="#page122">122</a>, <a
+    href="#page180">180</a>, <b><a href="#page181">181</a></b>, <a href="#page220">220</a>, <a
+    href="#page244">244</a>, <a href="#page295">295</a>, <a href="#page318">318</a>, <a
+    href="#page405">405</a>, <a href="#page417">417</a>, <a href="#page433">433</a>, <b><a
+    href="#page436">436</a></b>, <a href="#page494">494</a></p>
+    <p class="stanza">Ocelli, <a href="#page70">70</a></p>
+    <p>&#x0152;sophagus of Ammoc&#x0153;tes, <a href="#page405">405</a></p>
+    <p><span class="hid">&#x0152;sop</span>"<span class="hid">agus of</span> Arthropod, compared to
+    tube of infundibulum, <a href="#page4">4</a>, <a href="#page244">244</a>, <a
+    href="#page440">440</a></p>
+    <p>Olfactory apparatus, evidence of the, <a href="#page218">218</a></p>
+    <p><span class="hid">Olfa</span>"<span class="hid">tory</span> organs of the Scorpion group, <a
+    href="#page220">220</a></p>
+    <p><span class="hid">Olfa</span>"<span class="hid">tory</span> tube of Ammoc&#x0153;tes, <a
+    href="#page219">219</a>, <a href="#page225">225</a>, <a href="#page244">244</a>, <a
+    href="#page317">317</a></p>
+    <p>Oligochæta, <a href="#page421">421</a>, <a href="#page478">478</a></p>
+    <p>Operculum of Eurypterus, <b><a href="#page191">191</a></b>, <a href="#page212">212</a>, <a
+    href="#page291">291</a></p>
+    <p><span class="hid">Operc</span>"<span class="hid">lum of</span> Limulus, <b><a
+    href="#page189">189</a></b>, <a href="#page202">202</a>, <a href="#page235">235</a>, <a
+    href="#page295">295</a></p>
+    <p><span class="hid">Operc</span>"<span class="hid">lum of</span> Phrynus, <a
+    href="#page191">191</a></p>
+    <p><span class="hid">Operc</span>"<span class="hid">lum of</span> Scorpion, <b><a
+    href="#page189">189</a></b>, <a href="#page206">206</a>, <a href="#page212">212</a>, <b><a
+    href="#page372">372</a></b></p>
+    <p><span class="hid">Operc</span>"<span class="hid">lum of</span> Thelyphonus, <a
+    href="#page189">189</a>, <b><a href="#page190">190</a></b>, <a href="#page206">206</a></p>
+    <p>Organs, arrangement of, <a href="#page10">10</a></p>
+    <p><span class="pagenum" id="page531">{531}</span><span class="hid">Org</span>"<span
+    class="hid">ns,</span> auditory, of arachnids and Insects, <a href="#page368">368</a></p>
+    <p><span class="hid">Org</span>"<span class="hid">ns,</span> branchial, innervation of
+    vertebrate, <a href="#page151">151</a></p>
+    <p><span class="hid">Org</span>"<span class="hid">ns, bran</span>"<span class="hid">hial,</span>
+    sense-organs of embryo vertebrate, <a href="#page261">261</a>, <a href="#page281">281</a></p>
+    <p><span class="hid">Org</span>"<span class="hid">ns,</span> chordotonal, of insects, <a
+    href="#page364">364</a>, <a href="#page369">369</a>, <b><a href="#page370">370</a></b></p>
+    <p><span class="hid">Org</span>"<span class="hid">ns,</span> electric, <a
+    href="#page470">470</a></p>
+    <p><span class="hid">Org</span>"<span class="hid">ns,</span> generative, of Limulus, <a
+    href="#page208">208</a>, <a href="#page209">209</a></p>
+    <p><span class="hid">Org</span>"<span class="hid">ns, gener</span>"<span
+    class="hid">tive,</span> connection between Thyroid gland and, <a href="#page215">215</a></p>
+    <p><span class="hid">Org</span>"<span class="hid">ns,</span> genital, of sea-scorpions, <a
+    href="#page206">206</a></p>
+    <p><span class="hid">Org</span>"<span class="hid">ns,</span> lateral line, <a
+    href="#page355">355</a>, <a href="#page411">411</a></p>
+    <p><span class="hid">Org</span>"<span class="hid">ns,</span> lyriform, of arachnids, <a
+    href="#page364">364</a>, <a href="#page369">369</a></p>
+    <p><span class="hid">Org</span>"<span class="hid">ns,</span> olfactory, of Scorpion group, <a
+    href="#page220">220</a></p>
+    <p><span class="hid">Org</span>"<span class="hid">ns,</span> phagocytic, <a
+    href="#page420">420</a></p>
+    <p><span class="hid">Org</span>"<span class="hid">ns,</span> racquet, of Galeodes, <a
+    href="#page369">369</a>, <a href="#page375">375</a></p>
+    <p><span class="hid">Org</span>"<span class="hid">ns,</span> segmental excretory, <a
+    href="#page389">389</a>, <b><a href="#page391">391</a></b>, <b><a href="#page408">408</a></b>,
+    <a href="#page418">418</a>, <a href="#page459">459</a>, <a href="#page477">477</a></p>
+    <p><span class="hid">Org</span>"<span class="hid">ns,</span> sense, of appendages of Limulus, <a
+    href="#page358">358</a></p>
+    <p><span class="hid">Org</span>"<span class="hid">ns,</span> vestigial, <a
+    href="#page456">456</a></p>
+    <p><span class="hid">Org</span>"<span class="hid">ns,</span> of vision, evidence of, <a
+    href="#page68">68</a></p>
+    <p><span class="hid">Org</span>"<span class="hid">ns,</span> vital, <a href="#page57">57</a></p>
+    <p>Origin of alimentary canal, <a href="#page444">444</a></p>
+    <p><span class="hid">Ori</span>"<span class="hid">in of</span> arthropods from annelids, <a
+    href="#page395">395</a></p>
+    <p><span class="hid">Ori</span>"<span class="hid">in of</span> atrial cavity, <a
+    href="#page409">409</a></p>
+    <p><span class="hid">Ori</span>"<span class="hid">in of</span> auditory capsules and
+    parachordals, <a href="#page377">377</a></p>
+    <p><span class="hid">Ori</span>"<span class="hid">in of</span> c&#x0153;lom, <a
+    href="#page475">475</a>, <a href="#page481">481</a></p>
+    <p><span class="hid">Ori</span>"<span class="hid">in of</span> ductless glands, <a
+    href="#page428">428</a></p>
+    <p><span class="hid">Ori</span>"<span class="hid">in of</span> free cells, <a
+    href="#page472">472</a></p>
+    <p><span class="hid">Ori</span>"<span class="hid">in of</span> heart of vertebrate, <a
+    href="#page179">179</a></p>
+    <p><span class="hid">Ori</span>"<span class="hid">in of</span> lateral line organs, <a
+    href="#page356">356</a></p>
+    <p><span class="hid">Ori</span>"<span class="hid">in of</span> muscles, <a
+    href="#page478">478</a></p>
+    <p><span class="hid">Ori</span>"<span class="hid">in of</span> musculature, branchial, <a
+    href="#page170">170</a></p>
+    <p><span class="hid">Ori</span>"<span class="hid">in of muscul</span>"<span
+    class="hid">ture,</span> somatic trunk, <a href="#page406">406</a></p>
+    <p><span class="hid">Ori</span>"<span class="hid">in of</span> nervous system, central, <a
+    href="#page480">480</a></p>
+    <p><span class="hid">Ori</span>"<span class="hid">in of</span> notochord, <a
+    href="#page434">434</a></p>
+    <p><span class="hid">Ori</span>"<span class="hid">in of</span> segmental excretory organs, <a
+    href="#page389">389</a></p>
+    <p><span class="hid">Ori</span>"<span class="hid">in of</span> skeleton of vertebrates, <a
+    href="#page119">119</a></p>
+    <p><span class="hid">Ori</span>"<span class="hid">in of</span> vertebrates, <a
+    href="#page9">9</a>, <a href="#page36">36</a>, <a href="#page351">351</a>, <a
+    href="#page433">433</a>, <a href="#page493">493</a></p>
+    <p>Ostracodermata, <a href="#page326">326</a>, <a href="#page343">343</a></p>
+    <p>Osteostraci, <a href="#page29">29</a>, <a href="#page75">75</a>, <a href="#page275">275</a>,
+    <a href="#page326">326</a>, <a href="#page343">343</a></p>
+    <p>Otoliths, <a href="#page378">378</a></p>
+    <p>Ovum, <a href="#page473">473</a></p>
+    <p class="stanza">Pacinian bodies, <a href="#page470">470</a>, <a href="#page477">477</a></p>
+    <p>Palæmon, <a href="#page20">20</a>, <a href="#page422">422</a></p>
+    <p>Palæontology, evidence of, <a href="#page20">20</a>, <a href="#page497">497</a></p>
+    <p>Palæostoma, <a href="#page317">317</a></p>
+    <p>Palæostraca, <a href="#page27">27</a>, <a href="#page396">396</a></p>
+    <p><span class="hid">Palæo</span>"<span class="hid">traca,</span> median eyes, <a
+    href="#page74">74</a></p>
+    <p><span class="hid">Palæo</span>"<span class="hid">traca,</span> mesosomatic appendages, <a
+    href="#page188">188</a></p>
+    <p><span class="hid">Palæo</span>"<span class="hid">traca,</span> olfactory organs, <a
+    href="#page221">221</a></p>
+    <p><span class="hid">Palæo</span>"<span class="hid">traca,</span> segments, compared to
+    Ammoc&#x0153;tes, <a href="#page308">308</a></p>
+    <p>Pantopoda, glands, <a href="#page423">423</a></p>
+    <p>Parachordals, <a href="#page121">121</a>, <a href="#page132">132</a>, <a
+    href="#page377">377</a></p>
+    <p>Parapodia, <a href="#page357">357</a></p>
+    <p><span class="pagenum" id="page532">{532}</span>Parapodopsis, foot glands, <a
+    href="#page422">422</a></p>
+    <p>Parathymus, <a href="#page427">427</a></p>
+    <p>Parathyroids, <a href="#page427">427</a></p>
+    <p>Parietal organ, <a href="#page76">76</a></p>
+    <p>Pecten of scorpion, <a href="#page114">114</a>, <a href="#page359">359</a>, <a
+    href="#page366">366</a>, <a href="#page371">371</a>, <b><a href="#page372">372</a></b>, <b><a
+    href="#page373">373</a></b>, <b><a href="#page374">374</a></b></p>
+    <p>Pedipalpi, <a href="#page190">190</a></p>
+    <p>Periblast, <a href="#page471">471</a></p>
+    <p>Peripatus, <a href="#page396">396</a>, <b><a href="#page399">399</a></b>, <a
+    href="#page400">400</a>, <a href="#page411">411</a>, <a href="#page421">421</a>, <a
+    href="#page429">429</a></p>
+    <p>Petromyzon, alimentary canal, <a href="#page405">405</a>, <a href="#page445">445</a></p>
+    <p><span class="hid">Petro</span>"<span class="hid">mzon,</span> auditory organ, <a
+    href="#page378">378</a></p>
+    <p><span class="hid">Petro</span>"<span class="hid">mzon,</span> branchial segments, <b><a
+    href="#page169">169</a></b></p>
+    <p><span class="hid">Petro</span>"<span class="hid">mzon,</span> life-history, <a
+    href="#page59">59</a></p>
+    <p><span class="hid">Petro</span>"<span class="hid">mzon,</span> olfactory tube, <a
+    href="#page219">219</a>, <b><a href="#page226">226</a></b></p>
+    <p><span class="hid">Petro</span>"<span class="hid">mzon,</span> pronephric duct, <a
+    href="#page402">402</a></p>
+    <p><span class="hid">Petro</span>"<span class="hid">mzon,</span> retina and optic nerve, <b><a
+    href="#page95">95</a></b></p>
+    <p><span class="hid">Petro</span>"<span class="hid">mzon,</span> skeleton, <b><a
+    href="#page125">125</a></b></p>
+    <p><span class="hid">Petro</span>"<span class="hid">mzon,</span> suctorial apparatus, <a
+    href="#page287">287</a>, <a href="#page304">304</a></p>
+    <p><span class="hid">Petro</span>"<span class="hid">mzon,</span> transformation, <i>v.</i>
+    Transformation of the Lamprey</p>
+    <p>Phagocytes, <a href="#page420">420</a>, <a href="#page471">471</a></p>
+    <p>Pharynx of Amphioxus, <a href="#page410">410</a></p>
+    <p><span class="hid">Phar</span>"<span class="hid">nx of</span> Vertebrate, <a
+    href="#page440">440</a></p>
+    <p>Phoronis, <a href="#page439">439</a></p>
+    <p>Phrynus, brain, <a href="#page53">53</a></p>
+    <p><span class="hid">Phry</span>"<span class="hid">us,</span> caudal brain, <a
+    href="#page450">450</a></p>
+    <p><span class="hid">Phry</span>"<span class="hid">us,</span> carapace and carapace removed,
+    <b><a href="#page250">250</a></b></p>
+    <p><span class="hid">Phry</span>"<span class="hid">us,</span> c&#x0153;cal diverticula, <a
+    href="#page109">109</a></p>
+    <p><span class="hid">Phry</span>"<span class="hid">us,</span> evidence of segmentation of
+    carapace, <a href="#page249">249</a>, <b><a href="#page250">250</a></b>, <a
+    href="#page341">341</a></p>
+    <p><span class="hid">Phry</span>"<span class="hid">us,</span> operculum, <a
+    href="#page191">191</a></p>
+    <p><span class="hid">Phry</span>"<span class="hid">us,</span> prosomatic appendages, <a
+    href="#page306">306</a></p>
+    <p><span class="hid">Phry</span>"<span class="hid">us,</span> crossing of dorso-ventral muscles,
+    <a href="#page271">271</a>, <a href="#page277">277</a></p>
+    <p><span class="hid">Phry</span>"<span class="hid">us,</span> stridulating apparatus, <a
+    href="#page368">368</a></p>
+    <p>Phyllodoce, <a href="#page395">395</a></p>
+    <p>Phyllopoda, <a href="#page321">321</a></p>
+    <p>Pigment, in Ammoc&#x0153;tes, in position of atrial cavity, <a href="#page412">412</a></p>
+    <p><span class="hid">Pigm</span>"<span class="hid">nt, in</span> epithelial lining of central
+    nervous system, <a href="#page43">43</a>, <a href="#page457">457</a></p>
+    <p><span class="hid">Pigm</span>"<span class="hid">nt, in</span> choroid of vertebrate eye, <a
+    href="#page104">104</a>, <a href="#page107">107</a></p>
+    <p><span class="hid">Pigm</span>"<span class="hid">nt, in</span> between glandular cells round
+    brain of Ammoc&#x0153;tes, <a href="#page211">211</a>, <a href="#page379">379</a></p>
+    <p><span class="hid">Pigm</span>"<span class="hid">nt, in</span> tapetal layer of retina, <a
+    href="#page70">70</a></p>
+    <p><span class="hid">Pigm</span>"<span class="hid">nt, in</span> white, of right pineal eye of
+    Lamprey, <a href="#page76">76</a>, <a href="#page80">80</a></p>
+    <p>Pineal body, <a href="#page14">14</a>, <a href="#page15">15</a></p>
+    <p><span class="hid">Pin</span>"<span class="hid">al</span> eyes, <a href="#page74">74</a>, <a
+    href="#page233">233</a>, <a href="#page244">244</a></p>
+    <p><span class="hid">Pin</span>"<span class="hid">al ey</span>"<span class="hid">s,</span> of
+    Ammoc&#x0153;tes, <a href="#page80">80</a>, <b><a href="#page78">78</a></b>, <b><a
+    href="#page85">85</a></b></p>
+    <p><span class="hid">Pin</span>"<span class="hid">al</span> gland, <a href="#page63">63</a>, <a
+    href="#page75">75</a>, <a href="#page456">456</a></p>
+    <p>Pits, epithelial, of diaphragms in Ammoc&#x0153;tes, <a href="#page164">164</a></p>
+    <p><span class="hid">Pi</span>"<span class="hid">t, epith</span>"<span class="hid">lial,
+    of</span> skin in Ammoc&#x0153;tes, <a href="#page173">173</a>, <b><a
+    href="#page200">200</a></b></p>
+    <p>Pituitary body, <a href="#page244">244</a>, <a href="#page246">246</a>, <a
+    href="#page319">319</a>, <b><a href="#page321">321</a></b>, <a href="#page425">425</a>, <a
+    href="#page430">430</a></p>
+    <p>Plasma-cells, <a href="#page471">471</a></p>
+    <p>Plakodes, <a href="#page283">283</a></p>
+    <p>Planarians, <a href="#page475">475</a></p>
+    <p>Plastron, formation of cranial walls from the, <a href="#page86">86</a>, <a
+    href="#page322">322</a>, <a href="#page341">341</a></p>
+    <p><span class="hid">Plat</span>"<span class="hid">ron,</span> of Limulus, <a
+    href="#page136">136</a>, <a href="#page142">142</a>, <b><a href="#page143">143</a></b></p>
+    <p><span class="hid">Plat</span>"<span class="hid">ron,</span> Palæostracan, compared to
+    trabeculæ of Ammoc&#x0153;tes, <a href="#page145">145</a>, <a href="#page377">377</a></p>
+    <p><span class="pagenum" id="page533">{533}</span><span class="hid">Plat</span>"<span
+    class="hid">ron,</span> muscles attached to the, <a href="#page270">270</a></p>
+    <p><span class="hid">Plat</span>"<span class="hid">ron,</span> of Thelyphonus, <b><a
+    href="#page143">143</a></b></p>
+    <p>Platyhelmia, <a href="#page475">475</a></p>
+    <p>Pleuron, <a href="#page410">410</a>, <a href="#page415">415</a></p>
+    <p>Plexus, of Auerbach, <a href="#page447">447</a></p>
+    <p><span class="hid">Plex</span>"<span class="hid">s, of</span> choroid, <a
+    href="#page38">38</a>, <a href="#page45">45</a>, <a href="#page49">49</a>, <a
+    href="#page103">103</a></p>
+    <p><span class="hid">Plex</span>"<span class="hid">s, of</span> hypogastric, <a
+    href="#page3">3</a></p>
+    <p>Polychæta, <a href="#page357">357</a>, <a href="#page395">395</a></p>
+    <p>Pores, abdominal, <a href="#page430">430</a></p>
+    <p>Porifera, <a href="#page473">473</a></p>
+    <p>Pouch, formation of gill, <a href="#page165">165</a>, <b><a href="#page166">166</a></b></p>
+    <p>Prestwichia, <a href="#page24">24</a>, <b><a href="#page25">25</a></b>, <a
+    href="#page249">249</a>, <a href="#page351">351</a></p>
+    <p>Principle of concentration and cephalization, <a href="#page51">51</a></p>
+    <p><span class="hid">Prini</span>"<span class="hid">ple of</span> embryology, <a
+    href="#page455">455</a></p>
+    <p>Pristiurus, <a href="#page424">424</a></p>
+    <p>Progress, law of, <a href="#page19">19</a></p>
+    <p><span class="hid">Progr</span>"<span class="hid">ss,</span> result of, <a
+    href="#page56">56</a></p>
+    <p>Pronephros, <a href="#page389">389</a>, <a href="#page397">397</a>, <a
+    href="#page419">419</a>, <a href="#page424">424</a>, <a href="#page449">449</a></p>
+    <p>Prosencephalon, <a href="#page48">48</a></p>
+    <p>Prosoma, <a href="#page52">52</a></p>
+    <p>Protopterus, <a href="#page148">148</a></p>
+    <p>Protostraca, <a href="#page27">27</a>, <a href="#page396">396</a>, <a
+    href="#page417">417</a></p>
+    <p><span class="hid">Proto</span>"<span class="hid">traca,</span> dominance of, <a
+    href="#page28">28</a></p>
+    <p>Protozoa, <a href="#page166">166</a>, <a href="#page479">479</a></p>
+    <p>Pseudoniscus, <b><a href="#page25">25</a></b>, <a href="#page249">249</a></p>
+    <p>Pteraspis, <a href="#page29">29</a>, <b><a href="#page30">30</a></b>, <a
+    href="#page275">275</a>, <a href="#page326">326</a>, <a href="#page343">343</a>, <a
+    href="#page344">344</a>, <b><a href="#page350">350</a></b></p>
+    <p>Pterichthys, <a href="#page29">29</a>, <b><a href="#page31">31</a></b>, <a
+    href="#page239">239</a>, <a href="#page326">326</a>, <a href="#page351">351</a></p>
+    <p>Pterygoid, pedicle of, <a href="#page295">295</a></p>
+    <p>Pterygotus, <a href="#page25">25</a>, <b><a href="#page27">27</a></b>, <a
+    href="#page56">56</a>, <a href="#page170">170</a>, <a href="#page191">191</a>, <a
+    href="#page221">221</a>, <a href="#page235">235</a>, <a href="#page238">238</a>, <a
+    href="#page249">249</a>, <a href="#page276">276</a></p>
+    <p>Ptychodera, <a href="#page494">494</a>, <b><a href="#page495">495</a></b></p>
+    <p class="stanza">Ramus branchialis profundus of facial nerve, <a href="#page311">311</a></p>
+    <p><span class="hid">Ra</span>"<span class="hid">ms</span> communicans, <a href="#page2">2</a>,
+    <a href="#page3">3</a></p>
+    <p>Raphe, <a href="#page46">46</a></p>
+    <p>Recapitulation, law of, <a href="#page434">434</a>, <a href="#page456">456</a>, <a
+    href="#page498">498</a></p>
+    <p>Regeneration of nerves, <a href="#page469">469</a></p>
+    <p>Reptiles, dominance of, <a href="#page21">21</a></p>
+    <p>Retina, compound, <a href="#page71">71</a></p>
+    <p><span class="hid">Ret</span>"<span class="hid">ia,</span> development of, <a
+    href="#page101">101</a></p>
+    <p><span class="hid">Ret</span>"<span class="hid">ia,</span> inversion of, in Vertebrates, <a
+    href="#page114">114</a></p>
+    <p><span class="hid">Ret</span>"<span class="hid">ia,</span> inverted, <b><a
+    href="#page70">70</a></b></p>
+    <p><span class="hid">Ret</span>"<span class="hid">ia,</span> layers of compound, <a
+    href="#page73">73</a></p>
+    <p><span class="hid">Ret</span>"<span class="hid">ia, lay</span>"<span class="hid">rs of</span>
+    in Crustacean eye, <b><a href="#page100">100</a></b></p>
+    <p><span class="hid">Ret</span>"<span class="hid">ia,</span> of lateral eye of Ammoc&#x0153;tes,
+    <a href="#page93">93</a>, <b><a href="#page95">95</a></b>, <b><a href="#page111">111</a></b></p>
+    <p><span class="hid">Ret</span>"<span class="hid">ia,</span> Musca, <b><a
+    href="#page89">89</a></b></p>
+    <p><span class="hid">Ret</span>"<span class="hid">ia,</span> Pecten and Spondylus, <a
+    href="#page114">114</a></p>
+    <p><span class="hid">Ret</span>"<span class="hid">ia,</span> upright compound, <b><a
+    href="#page72">72</a></b></p>
+    <p><span class="hid">Ret</span>"<span class="hid">ia, upri</span>"<span class="hid">ht</span>
+    simple, <b><a href="#page69">69</a></b></p>
+    <p>Rhabdites, <a href="#page69">69</a>, <a href="#page81">81</a></p>
+    <p class="stanza">Saccus vasculosus, <a href="#page244">244</a>, <a href="#page322">322</a></p>
+    <p>Scales, <a href="#page345">345</a></p>
+    <p><span class="pagenum" id="page534">{534}</span>Scaphaspis, <a href="#page349">349</a></p>
+    <p>Schwann, sheath of, <a href="#page469">469</a></p>
+    <p>Sclerotomes, <a href="#page388">388</a></p>
+    <p>Scorpion, brain, <b><a href="#page54">54</a></b></p>
+    <p><span class="hid">Scor</span>"<span class="hid">ion,</span> branchial lamellæ, <b><a
+    href="#page175">175</a></b></p>
+    <p><span class="hid">Scor</span>"<span class="hid">ion,</span> development, <a
+    href="#page482">482</a></p>
+    <p><span class="hid">Scor</span>"<span class="hid">ion,</span> entochondrites, <a
+    href="#page377">377</a></p>
+    <p><span class="hid">Scor</span>"<span class="hid">ion,</span> excretory organs, <a
+    href="#page397">397</a></p>
+    <p><span class="hid">Scor</span>"<span class="hid">ion,</span> eyes, <a
+    href="#page75">75</a></p>
+    <p><span class="hid">Scor</span>"<span class="hid">ion,</span> lung-books, <a
+    href="#page150">150</a>, <a href="#page170">170</a></p>
+    <p><span class="hid">Scor</span>"<span class="hid">ion,</span> lymphatic glands, <a
+    href="#page423">423</a></p>
+    <p><span class="hid">Scor</span>"<span class="hid">ion,</span> muscles, oblique, <b><a
+    href="#page278">278</a></b></p>
+    <p><span class="hid">Scor</span>"<span class="hid">ion, mus</span>"<span class="hid">les,</span>
+    recti, <a href="#page271">271</a></p>
+    <p><span class="hid">Scor</span>"<span class="hid">ion, mus</span>"<span class="hid">les,</span>
+    respiration, <a href="#page171">171</a></p>
+    <p><span class="hid">Scor</span>"<span class="hid">ion, mus</span>"<span class="hid">les,</span>
+    veno-pericardial, <a href="#page177">177</a></p>
+    <p><span class="hid">Scor</span>"<span class="hid">ion,</span> muscular system, <a
+    href="#page247">247</a>, <a href="#page268">268</a>, <b><a href="#page269">269</a></b></p>
+    <p><span class="hid">Scor</span>"<span class="hid">ion,</span> nerves to Cheliceræ, <a
+    href="#page237">237</a></p>
+    <p><span class="hid">Scor</span>"<span class="hid">ion,</span> olfactory organs, <a
+    href="#page220">220</a></p>
+    <p><span class="hid">Scor</span>"<span class="hid">ion,</span> operculum of male, <b><a
+    href="#page189">189</a></b>, <a href="#page206">206</a>, <a href="#page212">212</a></p>
+    <p><span class="hid">Scor</span>"<span class="hid">ion,</span> pecten, <a
+    href="#page359">359</a>, <a href="#page366">366</a>, <a href="#page371">371</a>, <a
+    href="#page373">373</a>, <a href="#page374">374</a>, <a href="#page377">377</a></p>
+    <p><span class="hid">Scor</span>"<span class="hid">ion,</span> under surface, <b><a
+    href="#page372">372</a></b></p>
+    <p><span class="hid">Scor</span>"<span class="hid">ion,</span> uterus, <b><a
+    href="#page189">189</a></b>, <a href="#page202">202</a>, <b><a href="#page203">203</a></b>,
+    <b><a href="#page204">204</a></b>, <b><a href="#page205">205</a></b>, <a
+    href="#page212">212</a></p>
+    <p>Sea-scorpions, <a href="#page25">25</a>, <b><a href="#page26">26</a></b>, <b><a
+    href="#page27">27</a></b>, <a href="#page56">56</a>, <a href="#page150">150</a>, <a
+    href="#page170">170</a>, <a href="#page191">191</a>, <a href="#page208">208</a>, <a
+    href="#page221">221</a>, <a href="#page232">232</a>, <a href="#page235">235</a>, <a
+    href="#page241">241</a>, <a href="#page349">349</a>, <a href="#page359">359</a></p>
+    <p>Segmentation, branchiomeric, <a href="#page124">124</a></p>
+    <p><span class="hid">Segmet</span>"<span class="hid">ation,</span> body-muscles in vertebrate,
+    <a href="#page388">388</a></p>
+    <p><span class="hid">Segmet</span>"<span class="hid">ation,</span> eye-muscles, <a
+    href="#page248">248</a></p>
+    <p><span class="hid">Segmet</span>"<span class="hid">ation,</span> of head, double, <a
+    href="#page155">155</a>, <a href="#page157">157</a>, <a href="#page173">173</a>, <a
+    href="#page234">234</a>, <a href="#page258">258</a>, <a href="#page411">411</a>, <a
+    href="#page459">459</a></p>
+    <p><span class="hid">Segmet</span>"<span class="hid">ation,</span> of head-shield, <a
+    href="#page339">339</a></p>
+    <p><span class="hid">Segmet</span>"<span class="hid">ation,</span> history of cranial, <a
+    href="#page258">258</a></p>
+    <p>Segments, branchial of Ammoc&#x0153;tes, <a href="#page161">161</a>, <b><a
+    href="#page178">178</a></b>, <a href="#page186">186</a></p>
+    <p><span class="hid">Segm</span>"<span class="hid">nts,</span> hyoid, in Ammoc&#x0153;tes,
+    double, <a href="#page186">186</a>, <b><a href="#page201">201</a></b>, <a
+    href="#page267">267</a>, <a href="#page300">300</a></p>
+    <p><span class="hid">Segm</span>"<span class="hid">nts,</span> innervation of branchial, <a
+    href="#page151">151</a></p>
+    <p><span class="hid">Segm</span>"<span class="hid">nts,</span> first mesosomatic, in Limulus and
+    its allies, <a href="#page188">188</a></p>
+    <p><span class="hid">Segm</span>"<span class="hid">nts,</span> mesosomatic, of Eurypterus, <b><a
+    href="#page192">192</a></b></p>
+    <p><span class="hid">Segm</span>"<span class="hid">nts,</span> prosomatic of Limulus and its
+    allies, <a href="#page233">233</a>, <a href="#page249">249</a></p>
+    <p><span class="hid">Segm</span>"<span class="hid">nts, prosm</span>"<span class="hid">atic
+    of</span> Ammoc&#x0153;tes, <a href="#page286">286</a></p>
+    <p><span class="hid">Segm</span>"<span class="hid">nts,</span> of spinal region of Vertebrates,
+    <a href="#page388">388</a></p>
+    <p><span class="hid">Segm</span>"<span class="hid">nts,</span> of trigeminal nerve-group, <a
+    href="#page257">257</a>, <a href="#page279">279</a></p>
+    <p><span class="hid">Segm</span>"<span class="hid">nts,</span> tubular muscles of hyoid, <a
+    href="#page299">299</a></p>
+    <p>Sense-organs of Amphioxus, <a href="#page34">34</a></p>
+    <p><span class="hid">Sense-o</span>"<span class="hid">rans of</span> branchial, of Limulus, <a
+    href="#page359">359</a>, <b><a href="#page360">360</a></b></p>
+    <p><span class="hid">Sense-o</span>"<span class="hid">rans of</span> lateral, of Annelids, <a
+    href="#page357">357</a>, <a href="#page367">367</a></p>
+    <p><span class="hid">Sense-o</span>"<span class="hid">rans of</span> lateral-line system, <a
+    href="#page356">356</a>, <a href="#page411">411</a>, <a href="#page470">470</a></p>
+    <p>Serum, <a href="#page492">492</a></p>
+    <p>Significance of the optic diverticula, <a href="#page102">102</a></p>
+    <p>Silurus, <a href="#page488">488</a></p>
+    <p>Sinus, longitudinal venous, of Limulus, <a href="#page176">176</a>, <a
+    href="#page312">312</a>, <a href="#page451">451</a></p>
+    <p><span class="hid">Sin</span>"<span class="hid">s,</span> rhomboidal of bird, <b><a
+    href="#page46">46</a></b></p>
+    <p>Skeleton, Ammoc&#x0153;tes, <b><a href="#page126">126</a></b>, <b><a
+    href="#page296">296</a></b>, <b><a href="#page335">335</a></b></p>
+    <p><span class="hid">Skel</span>"<span class="hid">ton, Amm</span>"<span
+    class="hid">c&#x0153;tes,</span> branchial, <a href="#page126">126</a>, <b><a
+    href="#page126">126</a></b></p>
+    <p><span class="hid">Skel</span>"<span class="hid">ton, Amm</span>"<span
+    class="hid">c&#x0153;tes,</span> basi-cranial, <a href="#page132">132</a></p>
+    <p><span class="hid">Skel</span>"<span class="hid">ton, Amm</span>"<span
+    class="hid">c&#x0153;tes,</span> muco-cartilaginous, <a href="#page291">291</a>, <b><a
+    href="#page296">296</a></b>, <a href="#page330">330</a>, <a href="#page331">331</a></p>
+    <p><span class="pagenum" id="page535">{535}</span><span class="hid">Skel</span>"<span
+    class="hid">ton,</span> aponeurotic, <a href="#page414">414</a></p>
+    <p><span class="hid">Skel</span>"<span class="hid">ton,</span> Cephalaspis, <a
+    href="#page414">414</a>, <b><a href="#page415">415</a></b></p>
+    <p><span class="hid">Skel</span>"<span class="hid">ton,</span> evidence of the, <a
+    href="#page119">119</a></p>
+    <p><span class="hid">Skel</span>"<span class="hid">ton,</span> Limulus, cartilaginous, <b><a
+    href="#page126">126</a></b>, <a href="#page136">136</a></p>
+    <p><span class="hid">Skel</span>"<span class="hid">ton, Lim</span>"<span class="hid">lus,</span>
+    mesosomatic, <a href="#page137">137</a></p>
+    <p><span class="hid">Skel</span>"<span class="hid">ton, Lim</span>"<span class="hid">lus,</span>
+    prosomatic, <a href="#page142">142</a></p>
+    <p><span class="hid">Skel</span>"<span class="hid">ton,</span> Petromyzon, <b><a
+    href="#page125">125</a></b></p>
+    <p><span class="hid">Skel</span>"<span class="hid">ton,</span> Vertebrate, commencement of bony,
+    <a href="#page120">120</a>, <a href="#page121">121</a></p>
+    <p>Skin, digestive power of cells of, in Ammoc&#x0153;tes, <a href="#page58">58</a>, <a
+    href="#page442">442</a></p>
+    <p><span class="hid">Si</span>"<span class="hid">n,</span> of Ammoc&#x0153;tes, <a
+    href="#page346">346</a></p>
+    <p><span class="hid">Si</span>"<span class="hid">n,</span> nerves of, <a
+    href="#page448">448</a></p>
+    <p>Skull of dogfish, <b><a href="#page123">123</a></b></p>
+    <p><span class="hid">Skl</span>"<span class="hid">l of</span> pig-embryo, <b><a
+    href="#page121">121</a></b></p>
+    <p>Slimonia, <b><a href="#page27">27</a>,</b> <a href="#page56">56</a>, <a
+    href="#page170">170</a>, <a href="#page235">235</a>, <a href="#page238">238</a>, <a
+    href="#page249">249</a>, <a href="#page276">276</a>, <a href="#page303">303</a></p>
+    <p>Solenocytes, <a href="#page395">395</a>, <a href="#page477">477</a></p>
+    <p>Solpugidæ, <a href="#page109">109</a></p>
+    <p>Sphæroma serratum, brain, <b><a href="#page62">62</a></b>, <b><a href="#page90">90</a></b>,
+    <a href="#page101">101</a>, <a href="#page225">225</a></p>
+    <p>Spiders, eyes, <a href="#page75">75</a></p>
+    <p><span class="hid">Spi</span>"<span class="hid">ers,</span> stomach, <a
+    href="#page109">109</a></p>
+    <p>Spina bifida, <a href="#page50">50</a></p>
+    <p>Spinal cord, difference between brain and, <a href="#page45">45</a></p>
+    <p><span class="hid">Spi</span>"<span class="hid">al cr</span>"<span class="hid">d,</span>
+    region of, <a href="#page385">385</a></p>
+    <p><span class="hid">Spi</span>"<span class="hid">al cr</span>"<span class="hid">d,</span>
+    termination in bird-embryo, <a href="#page51">51</a></p>
+    <p>Spondylus, retina of, <a href="#page114">114</a></p>
+    <p>Squilla, eyes, <a href="#page100">100</a></p>
+    <p><span class="hid">Sqi</span>"<span class="hid">lla,</span> glands, <a
+    href="#page422">422</a></p>
+    <p>Stomach, cephalic, <a href="#page4">4</a>, <a href="#page43">43</a>, <a
+    href="#page102">102</a>, <a href="#page244">244</a></p>
+    <p>Stylonurus Lagani, <b><a href="#page27">27</a></b>, <a href="#page235">235</a>, <a
+    href="#page239">239</a>, <a href="#page249">249</a></p>
+    <p>Substantia gelatinosa Rolandi, <a href="#page44">44</a></p>
+    <p>Suprarenal body, <a href="#page423">423</a></p>
+    <p>Surfaces, dorsal and ventral, <a href="#page11">11</a></p>
+    <p><span class="hid">Surf</span>"<span class="hid">ces,</span> reversal of, <a
+    href="#page15">15</a>, <a href="#page29">29</a>, <a href="#page36">36</a>, <a
+    href="#page87">87</a>, <a href="#page175">175</a>, <a href="#page352">352</a>, <a
+    href="#page433">433</a>, <a href="#page484">484</a></p>
+    <p>Synapse, <a href="#page72">72</a></p>
+    <p>Syncytium, <a href="#page464">464</a>, <a href="#page471">471</a>, <a
+    href="#page479">479</a></p>
+    <p class="stanza">Tail of lizards, <a href="#page50">50</a></p>
+    <p>Tapetum, <a href="#page69">69</a></p>
+    <p>Teleosteans, <a href="#page23">23</a>, <a href="#page345">345</a>, <a
+    href="#page420">420</a>, <a href="#page424">424</a></p>
+    <p>Tendon-organs, <a href="#page470">470</a></p>
+    <p>Tentacles of Ammoc&#x0153;tes, <a href="#page246">246</a>, <a href="#page289">289</a>, <a
+    href="#page303">303</a></p>
+    <p>Tergo-coxal muscles, <a href="#page247">247</a></p>
+    <p>Test, biological, of relationship of animals, <a href="#page492">492</a></p>
+    <p>Thalainencephalon, <a href="#page48">48</a></p>
+    <p>Thelodus, <a href="#page344">344</a></p>
+    <p>Thelyphonus, <b><a href="#page231">231</a></b></p>
+    <p><span class="hid">Thely</span>"<span class="hid">honus,</span> brain, <a
+    href="#page53">53</a>, <b><a href="#page54">54</a></b>, <b><a href="#page56">56</a></b>, <b><a
+    href="#page224">224</a></b></p>
+    <p><span class="hid">Thely</span>"<span class="hid">honus, br</span>"<span
+    class="hid">in,</span> caudal, <a href="#page450">450</a></p>
+    <p><span class="hid">Thely</span>"<span class="hid">honus,</span> c&#x0153;cal diverticula, <a
+    href="#page109">109</a></p>
+    <p><span class="hid">Thely</span>"<span class="hid">honus,</span> entosternite, <b><a
+    href="#page143">143</a></b></p>
+    <p><span class="hid">Thely</span>"<span class="hid">honus,</span> genital organs, <a
+    href="#page206">206</a></p>
+    <p><span class="hid">Thely</span>"<span class="hid">honus,</span> lyriform organs, <a
+    href="#page368">368</a></p>
+    <p><span class="hid">Thely</span>"<span class="hid">honus,</span> olfactory passage, <b><a
+    href="#page226">226</a></b>, <a href="#page306">306</a></p>
+    <p><span class="pagenum" id="page536">{536}</span><span class="hid">Thely</span>"<span
+    class="hid">honus,</span> operculum, <a href="#page189">189</a>, <b><a
+    href="#page190">190</a></b>, <a href="#page206">206</a>, <b><a href="#page207">207</a></b></p>
+    <p>Theory, gastræa, <a href="#page444">444</a>, <a href="#page461">461</a></p>
+    <p>Theories of the origin of vertebrates, <a href="#page9">9</a>, <a href="#page411">411</a>, <a
+    href="#page433">433</a>, <a href="#page457">457</a></p>
+    <p>Thionin reaction, <a href="#page131">131</a>, <a href="#page139">139</a>, <a
+    href="#page213">213</a>, <a href="#page330">330</a>, <a href="#page336">336</a></p>
+    <p>Throat, formation of, <a href="#page179">179</a></p>
+    <p>Thyestes, <b><a href="#page30">30</a></b>, <b><a href="#page31">31</a></b>, <a
+    href="#page275">275</a>, <a href="#page326">326</a>, <b><a href="#page328">328</a></b>, <b><a
+    href="#page329">329</a></b>, <a href="#page339">339</a>, <b><a href="#page340">340</a></b>,
+    <b><a href="#page341">341</a></b></p>
+    <p>Thymus, <a href="#page425">425</a>, <a href="#page430">430</a></p>
+    <p>Thyroid gland of Ammoc&#x0153;tes, <a href="#page61">61</a>, <a href="#page127">127</a>, <a
+    href="#page192">192</a>, <b><a href="#page194">194</a></b>, <b><a href="#page196">196</a></b>,
+    <a href="#page429">429</a>, <a href="#page459">459</a></p>
+    <p><span class="hid">Thyr</span>"<span class="hid">id gl</span>"<span class="hid">nd</span>
+    evidence of the, <a href="#page185">185</a></p>
+    <p><span class="hid">Thyr</span>"<span class="hid">id gl</span>"<span class="hid">nd</span>
+    function of, in Ammoc&#x0153;tes, <a href="#page213">213</a></p>
+    <p>Tissues, connective, <a href="#page471">471</a>, <a href="#page474">474</a>, <a
+    href="#page481">481</a></p>
+    <p><span class="hid">Tiss</span>"<span class="hid">es,</span> evolution of, <a
+    href="#page19">19</a></p>
+    <p><span class="hid">Tiss</span>"<span class="hid">es,</span> notochordal, <a
+    href="#page435">435</a></p>
+    <p><span class="hid">Tiss</span>"<span class="hid">es,</span> two groups of, <a
+    href="#page463">463</a></p>
+    <p>Tongue of Ammoc&#x0153;tes, <a href="#page246">246</a>, <a href="#page303">303</a></p>
+    <p>Tonsils, <a href="#page427">427</a>, <a href="#page430">430</a></p>
+    <p>Torpedo, <a href="#page262">262</a>, <a href="#page392">392</a>, <a
+    href="#page470">470</a></p>
+    <p>Trabeculæ, <a href="#page121">121</a>, <a href="#page132">132</a>, <a
+    href="#page133">133</a>, <a href="#page145">145</a>, <a href="#page277">277</a>, <a
+    href="#page295">295</a>, <a href="#page377">377</a></p>
+    <p>Transformation of the Lamprey, <a href="#page18">18</a>, <a href="#page35">35</a>, <a
+    href="#page59">59</a>, <a href="#page61">61</a>, <a href="#page125">125</a>, <a
+    href="#page168">168</a>, <a href="#page193">193</a>, <a href="#page199">199</a>, <a
+    href="#page200">200</a>, <a href="#page220">220</a>, <a href="#page227">227</a>, <a
+    href="#page228">228</a>, <a href="#page287">287</a>, <a href="#page291">291</a>, <a
+    href="#page304">304</a>, <a href="#page307">307</a>, <a href="#page309">309</a>, <a
+    href="#page331">331</a>, <a href="#page336">336</a>, <a href="#page347">347</a>, <a
+    href="#page349">349</a>, <a href="#page389">389</a>, <a href="#page445">445</a></p>
+    <p>Tremataspis, <b><a href="#page32">32</a></b>, <a href="#page75">75</a>, <b><a
+    href="#page275">275</a></b>, <a href="#page326">326</a>, <a href="#page351">351</a>, <b><a
+    href="#page352">352</a></b></p>
+    <p>Trilobites, <a href="#page24">24</a>, <b><a href="#page25">25</a></b>, <b><a
+    href="#page26">26</a></b>, <b><a href="#page437">437</a></b></p>
+    <p><span class="hid">Trilo</span>"<span class="hid">ites,</span> appendages, <a
+    href="#page351">351</a>, <b><a href="#page437">437</a></b></p>
+    <p><span class="hid">Trilo</span>"<span class="hid">ites,</span> diagram of section through a
+    trilobite-like animal, <b><a href="#page413">413</a></b></p>
+    <p><span class="hid">Trilo</span>"<span class="hid">ites,</span> dominance of, <a
+    href="#page26">26</a></p>
+    <p><span class="hid">Trilo</span>"<span class="hid">ites,</span> excretory organs, <a
+    href="#page396">396</a></p>
+    <p><span class="hid">Trilo</span>"<span class="hid">ites,</span> eyes, <a href="#page74">74</a>,
+    <a href="#page88">88</a></p>
+    <p><span class="hid">Trilo</span>"<span class="hid">ites,</span> glabellum, <a
+    href="#page339">339</a></p>
+    <p><span class="hid">Trilo</span>"<span class="hid">ites,</span> relations of, <a
+    href="#page249">249</a>, <a href="#page283">283</a></p>
+    <p><span class="hid">Trilo</span>"<span class="hid">ites,</span> respiratory apparatus, <a
+    href="#page170">170</a></p>
+    <p><span class="hid">Trilo</span>"<span class="hid">ites,</span> ventral surface, <b><a
+    href="#page437">437</a></b></p>
+    <p>Tube of central nervous system, <a href="#page37">37</a>, <a href="#page38">38</a>, <a
+    href="#page42">42</a>, <a href="#page102">102</a>, <a href="#page211">211</a>, <a
+    href="#page433">433</a>, <a href="#page455">455</a>, <a href="#page457">457</a></p>
+    <p><span class="hid">Tu</span>"<span class="hid">e</span> from IVth ventricle to surface of
+    brain in Ammoc&#x0153;tes, <a href="#page209">209</a></p>
+    <p><span class="hid">Tu</span>"<span class="hid">e</span> Fallopian, <a
+    href="#page431">431</a></p>
+    <p><span class="hid">Tu</span>"<span class="hid">e</span> hypophysial, <a
+    href="#page229">229</a>, <a href="#page244">244</a>, <a href="#page317">317</a>, <a
+    href="#page440">440</a></p>
+    <p><span class="hid">Tu</span>"<span class="hid">e</span> meeting of four tubes in vertebrate,
+    <b><a href="#page318">318</a></b>, <b><a href="#page440">440</a></b></p>
+    <p><span class="hid">Tu</span>"<span class="hid">e</span> notochord originally a, <a
+    href="#page436">436</a>, <a href="#page440">440</a></p>
+    <p><span class="hid">Tu</span>"<span class="hid">e</span> olfactory, of Ammoc&#x0153;tes, <a
+    href="#page219">219</a>, <a href="#page225">225</a>, <a href="#page317">317</a>, <a
+    href="#page440">440</a></p>
+    <p><span class="hid">Tu</span>"<span class="hid">e</span> unsegmented, in segmented animal, <a
+    href="#page439">439</a></p>
+    <p>Tunicata, <a href="#page16">16</a></p>
+    <p><span class="hid">Tuni</span>"<span class="hid">ata,</span> budding of, <a
+    href="#page441">441</a></p>
+    <p><span class="hid">Tuni</span>"<span class="hid">ata,</span> degeneration, <a
+    href="#page12">12</a>, <a href="#page17">17</a>, <a href="#page19">19</a>, <a
+    href="#page60">60</a></p>
+    <p><span class="hid">Tuni</span>"<span class="hid">ata,</span> endostyle, <a
+    href="#page198">198</a>, <a href="#page212">212</a></p>
+    <p><span class="hid">Tuni</span>"<span class="hid">ata,</span> hypophysis, <a
+    href="#page425">425</a></p>
+    <p><span class="hid">Tuni</span>"<span class="hid">ata,</span> notochord, <a
+    href="#page438">438</a></p>
+    <p><span class="hid">Tuni</span>"<span class="hid">ata,</span> position of, <a
+    href="#page494">494</a></p>
+    <p class="stanza">Unit, appendage, in non-branchial segments, <a href="#page185">185</a></p>
+    <p><span class="hid">Ui</span>"<span class="hid">t,</span> branchial, <a
+    href="#page161">161</a>, <a href="#page165">165</a>, <a href="#page168">168</a>, <a
+    href="#page185">185</a></p>
+    <p>Ureters, nerves of, <a href="#page448">448</a></p>
+    <p>Uterus of Scorpion group, <a href="#page189">189</a>, <a href="#page202">202</a>, <b><a
+    href="#page203">203</a></b>, <b><a href="#page204">204</a></b>, <b><a
+    href="#page205">205</a></b>, <a href="#page214">214</a></p>
+    <p><span class="hid">Utr</span>"<span class="hid">us</span> vertebrate, nerves of, <a
+    href="#page448">448</a></p>
+    <p class="stanza"><span class="pagenum" id="page537">{537}</span>Valve, ileo-colic, <a
+    href="#page449">449</a></p>
+    <p><span class="hid">Val</span>"<span class="hid">e,</span> of Vieussens, <a
+    href="#page48">48</a></p>
+    <p>Variation in dominant races, <a href="#page21">21</a>, <a href="#page88">88</a></p>
+    <p><span class="hid">Vari</span>"<span class="hid">tion</span> meristic, in spinal nerves, <a
+    href="#page154">154</a>, <a href="#page387">387</a></p>
+    <p>Veins, forming vertebrate heart, <b><a href="#page180">180</a></b></p>
+    <p>Velum, <a href="#page228">228</a>, <a href="#page289">289</a>, <a href="#page298">298</a>,
+    <b><a href="#page302">302</a></b></p>
+    <p>Vertebrates, alimentary canal, innervation of, <a href="#page446">446</a></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates,</span> atrial cavity, <a
+    href="#page410">410</a></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates,</span> auditory apparatus and
+    lateral-line system, <a href="#page356">356</a></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates,</span> body-cavity, <a
+    href="#page401">401</a>, <a href="#page430">430</a></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates,</span> brains, <b><a
+    href="#page40">40</a></b></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates,</span> branchial organs, <a
+    href="#page151">151</a></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates,</span> c&#x0153;lomic cavities in
+    head region, <a href="#page251">251</a>, <a href="#page266">266</a></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates,</span> cranium, evolution of, <a
+    href="#page342">342</a></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates,</span> egg of, <a
+    href="#page483">483</a></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates,</span> evolution of, <a
+    href="#page11">11</a></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates,</span> excretory organs, <a
+    href="#page389">389</a>, <b><a href="#page391">391</a></b>, <b><a
+    href="#page408">408</a></b></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates,</span> glands, ductless, <a
+    href="#page418">418</a></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates, gla</span>"<span
+    class="hid">ds,</span> internal secretion of, <a href="#page215">215</a></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates,</span> heart, <a
+    href="#page175">175</a>, <a href="#page179">179</a>, <b><a href="#page180">180</a></b></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates,</span> muscles, evidence of
+    segmentation of eye, <a href="#page248">248</a></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates, mus</span>"<span
+    class="hid">les,</span> oblique, <b><a href="#page278">278</a></b></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates, mus</span>"<span
+    class="hid">les,</span> origin of somatic trunk, <a href="#page406">406</a></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates,</span> nervous system, central, <b><a
+    href="#page13">13</a></b></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates,</span> nerves, segmental, <a
+    href="#page152">152</a></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates,</span> notochord and gut, <a
+    href="#page434">434</a></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates,</span> organs of, <b><a
+    href="#page10">10</a></b></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates,</span> origin of, <a
+    href="#page9">9</a>, <a href="#page411">411</a>, <a href="#page433">433</a>, <a
+    href="#page457">457</a></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates,</span> segments, prosomatic, <a
+    href="#page257">257</a></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates,</span> skeleton, commencement of
+    bony, <a href="#page120">120</a>, <a href="#page458">458</a></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates,</span> spinal cord and medulla
+    oblongata, <b><a href="#page44">44</a></b></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates,</span> spinal region, <a
+    href="#page385">385</a></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates,</span> thyroid, connection between
+    generative organs and, <a href="#page215">215</a></p>
+    <p><span class="hid">Verte</span>"<span class="hid">rates,</span> tubes, meeting of four, <b><a
+    href="#page318">318</a></b>, <b><a href="#page440">440</a></b></p>
+    <p>Vesicles, cerebral, formation of, <a href="#page48">48</a>, <a href="#page458">458</a></p>
+    <p>Vitellophags, <a href="#page471">471</a>, <a href="#page483">483</a></p>
+    <p>Volvox, <a href="#page479">479</a></p>
+    <p class="stanza">Wolffian body, <a href="#page390">390</a></p>
+    <p class="stanza">Xiphosura, <a href="#page24">24</a>, <b><a href="#page26">26</a></b>, <a
+    href="#page249">249</a></p>
+    <p class="stanza">Yolk, <a href="#page482">482</a></p>
+  </div>
+
+  <p class="sp4 ac">THE END</p>
+
+  <div><span class="pagenum" id="page538">{538}</span></div>
+
+  <p class="ac" style="margin-bottom:1.5ex;"><span class="x-larger">Notes.</span></p>
+
+  <div class="foot">
+    <a class="fnote" id="Nt_1" href="#NtA_1">[1]</a>
+    <p>N.B.&mdash;In addition to the nerves mentioned, C. Bell included, in his respiratory system
+    of nerves, the fourth nerve or trochlearis, the phrenic and the external respiratory of
+    Bell.</p>
+  </div>
+
+  <div class="foot">
+    <a class="fnote" id="Nt_2" href="#NtA_2">[2]</a>
+    <p>"The Origin of Vertebrates, deduced from the Study of Ammoc&#x0153;tes." Part X., "The Origin
+    of the Auditory Organ: the Meaning of the VIIIth Cranial Nerve." <i>Journ. Anat. and
+    Physiol.</i>, vol. 36, 1902.</p>
+  </div>
+
+<div>*** END OF THE PROJECT GUTENBERG EBOOK 44000 ***</div>
+</body>
+</html>
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