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diff --git a/44000-0.txt b/44000-0.txt new file mode 100644 index 0000000..74db6a0 --- /dev/null +++ b/44000-0.txt @@ -0,0 +1,20700 @@ +*** 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 *** |