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+*** START OF THE PROJECT GUTENBERG EBOOK 40362 ***
+
+[Transcriber's Notes
+
+Emphasis is denotes as _Italic_ and =Bold=. Most small caps text
+has been converted to ALL CAPS; but where some may have been
+rendered as Mixed Case letters where it seemed appropriate.]
+
+
+
+[Illustration: FIG. 1.--THE SCALLOP SHELL, _Pecten Opercularis_ (see
+page 107), SLIGHTLY REDUCED IN SIZE. THE LARGER SHELLS ARE FROM DOUGLAS,
+ISLE OF MAN; THE SMALLER SHELLS ARE YOUNG SPECIMENS FROM LLANDUDNO,
+NORTH WALES.]
+
+
+
+
+STORIES _of the_ UNIVERSE
+
+Animal Life
+
+
+By
+
+
+B. LINDSAY
+
+
+_WITH FORTY-SEVEN ILLUSTRATIONS_
+
+
+NEW YORK
+
+REVIEW _of_ REVIEWS COMPANY
+
+1909
+
+
+
+
+COPYRIGHT, 1902
+
+BY D. APPLETON AND COMPANY
+
+
+_All rights reserved_
+
+
+
+
+PREFACE
+
+
+Of the diagrams which illustrate this little volume, the majority were
+prepared by Miss E. C. Abbott (formerly Bathurst Scholar at Newnham
+College, Cambridge): the sketches were made from specimens in the South
+Kensington Museum of Natural History, which has kindly granted
+permission for their use. In addition to these, there are several
+figures that are taken from specimens in my possession, photographed by
+the publishers; two or three cuts are diagrammatic; and I owe to the
+kindness of Mr. J. Craggs, formerly president of the Northumberland
+Microscopical Association, the drawings of Polycystina and of the scales
+of the Sole.
+
+    B. L.
+
+
+
+
+CONTENTS
+
+
+    CHAPTER                                                     PAGE
+
+       I. THE STORY OF ANIMAL LIFE                                 9
+
+      II. HOW ANIMALS ADAPT THEMSELVES TO CIRCUMSTANCES           13
+
+     III. CLASSIFICATION: THE SORTING OF THE ANIMAL KINGDOM       30
+
+      IV. THE ONE-CELLED ANIMALS, OR PROTOZOA                     45
+
+       V. THE COELENTERATA                                        53
+
+      VI. THE SPONGES                                             63
+
+     VII. THE VERMES OR WORMS                                     68
+
+    VIII. THE ARTHROPODA: LOBSTERS, SPIDERS, AND INSECTS          76
+
+      IX. THE MOLLUSCA, OR SHELL-FISH                             98
+
+       X. THE BRACHIOPODA, OR LAMP-SHELLS                        117
+
+      XI. THE MOSS-CORALS, OR POLYZOA                            119
+
+     XII. THE ECHINODERMATA                                      122
+
+    XIII. THE CHORDATA                                           135
+
+     XIV. THE VERTEBRATA                                         138
+
+      XV. MAN                                                    167
+
+     XVI. HOW ZOOLOGISTS DO THEIR WORK                           180
+
+          INDEX                                                  193
+
+
+
+
+LIST OF ILLUSTRATIONS
+
+
+    FIGURE                                                      PAGE
+
+     1. The Scallop-Shell                             _Frontispiece_
+
+     2. Limpets and Periwinkles                                   19
+
+     3. Diagram of _Amoeba_                                       35
+
+     4. Section of _Hydra_                                        36
+
+     5. Diagrammatic Section of Earthworm                         38
+
+     6. Diagram of a Gastrula                                     41
+
+     7. Diagram of a Trochosphere                                 42
+
+     8. Shells of Radiolarians (Polycystina)                      47
+
+     9. A Coralline                                               58
+
+    10. Gorgonia                                                  59
+
+    11. Corals                                                    60
+
+    12. Marine Worms                                              73
+
+    13. A Centipede                                               77
+
+    14, 15. Shells of Barnacles                               79, 80
+
+    16. Hermit Crabs                                              81
+
+    17. A Land Crab                                               82
+
+    18. A Sand-hopper                                             83
+
+    19. A Spider                                                  84
+
+    20. Nest of Trap-door Spider                                  85
+
+    21. _Galeodes_                                                86
+
+    22. A Tick                                                    87
+
+    23. A Scorpion                                                88
+
+    24. Larvæ of Insects                                          90
+
+    25. Larva of the Bee                                          92
+
+    26. Ants                                                      92
+
+    27. White Ants                                                93
+
+    28. Cocoons of Moths                                          94
+
+    29. A Moth and its Larva                                      95
+
+    30. Nest of a Gregarious Caterpillar                          96
+
+    31. Development of an English Water-beetle (_Dytiscus_)       96
+
+    32. Insect Pests                                              97
+
+    33. Branchy Murex                                            102
+
+    34. Shell of the Common _Venus_                              104
+
+    35. Eggs of Molluscs                                         115
+
+    36. The Five-holed Sand-Cake                                 125
+
+    37. A Brittle-Star                                           129
+
+    38. A Sea-Cucumber                                           130
+
+    39. A Stone-Lily or Encrinite                                131
+
+    40. A Feather-Star                                           132
+
+    41. Sections showing Position of the Vertebrate Notochord    139
+
+    42. Scales of a Sole                                         143
+
+    43. Tadpoles                                                 153
+
+    44. Eggs of Reptiles                                         155
+
+    45. Skull of Kangaroo                                        162
+
+    46. Skull of Rodent                                          163
+
+    47. Slide with Rows of Sections for the Microscope           185
+
+
+
+
+THE STORY OF ANIMAL LIFE
+
+
+
+
+CHAPTER I
+
+THE STORY OF ANIMAL LIFE
+
+
+If the microscope had never been invented, the Story of Animal Life, as
+it is related by modern science, could never have been told. It is to
+the microscope that we owe our knowledge of innumerable little animals
+that are too small to be seen by the unassisted eye; and it is to the
+microscope that we owe the most important part of our knowledge about
+the bodies of larger animals, about the way in which they are built up,
+and the uses of their different parts. The earlier opticians who toiled,
+one after another, to bring the microscope to perfection, never dreamed,
+in their most ambitious moments, of the value of the gift that their
+labour was to confer upon mankind. For the microscope alone has made it
+possible for men of science to study the world of living things. This is
+the value of honest and thorough work in almost every department of
+intellectual labour; that it builds a firm and sure though perhaps
+hidden foundation for the loftier and more perfect work of after days.
+
+The microscope has shown us the intimate structure of every organ of
+the animal body; and thus, in most cases, the uses of the organ, and
+the steps by which it performs its tasks, have been made clear. The
+microscope has also shown the true nature of the sexual functions, and
+all the steps of the processes of growth in young animals. None of these
+things could ever have been rightly understood without the microscope,
+for all their most important details are invisible to the naked eye.
+To the microscope, too, we owe our knowledge of the essential kinship
+between plants and animals; to it, also, our understanding of the
+oneness, the "solidarity," as the French would say, of the animal
+kingdom, for it is in the structure of microscopic parts that
+resemblances are revealed under the most strikingly different
+circumstances of outward form.
+
+Let us inquire a little into the history of the animals that can only
+be seen by the aid of the microscope. Most of them live in water,
+especially dirty water, containing decaying remains of plants or
+animals. The naturalists who first discovered them studied them in
+"infusions" of hay, and so on, and hence these little creatures were
+named Infusoria--a name that has since been somewhat restricted in its
+application. By an "infusion" is meant that water is poured on some
+substance and allowed to stand; the more ancient and evil-smelling the
+infusion becomes, the more of these little animals do you find living in
+it. Nature provides dirty water ready made, in ditches and in ponds, and
+these are full of microscopic animals. And not only do they appear in
+dirty water, but kindred kinds appear in clean water also, and many in
+the waters of the sea.
+
+It will easily be understood that when the existence of microscopic
+animals was discovered, zoologists had greatly to modify their ideas of
+the animal world. Still more was this the case afterwards, when it was
+found that all animals were built up of minute parts much resembling
+these microscopic animals in their main features. To these unit parts,
+of which all animal bodies are composed, the term "cell" is applied.
+The name of cell is not very descriptive of these units in the animal
+body, but correctly describes the unit of plant structure. In certain
+important essential particulars both, however, are alike. Nowadays we
+are not content to describe the grouping and external features of cells;
+their minute structure also is made a subject of research and inquiry,
+and affords a field for most of the fashionable speculations of our own
+day.
+
+How great has been the progress made by the science of zoology since the
+eighteenth century may be estimated from the following quotation:--
+
+"I remember," says the late George J. Romanes (in his book called "The
+Scientific Evidences of Organic Evolution"), "once reading a very
+comical disquisition in one of Buffon's works on the question as to
+whether or not a crocodile was to be classified as an insect; and the
+instructive feature in the disquisition was this, that although a
+crocodile differs from an insect as regards every conceivable particular
+of its internal anatomy, no allusion at all is made to this fact, while
+the whole discussion is made to turn on the hardness of the external
+casing of a crocodile resembling the hardness of the external casing
+of a beetle; and when at last Buffon decides that, on the whole, a
+crocodile had better not be classified as an insect, the only reason
+given is, that as a crocodile is so very large an animal it would make
+'altogether too terrible an insect.'"
+
+How different is the state of knowledge now, when every part of a
+crocodile or a cockroach is described in print in the minutest detail,
+and set before even the beginner in zoology as a necessary lesson.
+
+But in spite of the labour necessary to master such detailed lessons,
+the study of the animal world is far from prosaic. The Story of Animal
+Life, indeed, bids fair to be the only element of romance left in the
+modern world for those who stay at home in their own land. The traveller
+of days of yore, when he ventured into the woods and fields, or upon
+the water, expected to meet with all sorts of strange things--fairies
+and elves and ugly gnomes; giants, ogres, and dragons; mermaids and
+water-witches. With the spread of education all these things have
+vanished now; it is quite certain that no Board-School-boy has ever met
+any of them: and one's walks abroad would be in these days as prosaic as
+they are safe, but for the world of animal life. If you have eyes for
+this, every field has its inhabitants, and every hedge its marvels.
+Instead of a fairy, you may be well contented to meet a dragon-fly with
+shining wings; instead of an ogre you will find the fierce spider, which
+not only makes away with every harmless fly that blunders into her net,
+but in many cases destroys her own kind also. Many a plant may be met
+with which has its own special caterpillar or other dependent insect,
+with ways of its own, which may amuse your idle hours. As for the change
+of a caterpillar or a tadpole into its adult form, it would be taken for
+a miracle if it were observed for the first time.
+
+The reader may have noticed that there are some unfortunate people who
+have no eyes for these things; from childhood upwards they have been so
+absorbed in money-making or in reading books--the one case is as bad as
+the other--that they have never learnt to observe the facts of nature.
+Some cannot even recognise the different kinds of plants that they see
+in the hedges, or in a country walk. Such natures are intellectually
+defective; they are much to be pitied, and require a special training to
+remedy their stupidity. I mention this, because the occurrence of this
+form of stupidity is one of the dangers resulting from town life and
+bookish education, which we have to guard against at the present time.
+
+But for all healthy people accustomed to the outdoor world, the study of
+animal life has always possessed an interest. Its interest has, however,
+been increased a hundred fold by the progress of modern discovery, which
+has taught us to see in the animal kingdom one large family, working its
+way upwards from humble beginnings, to more perfect structure of body,
+and more complete intelligence of mind.
+
+
+
+
+CHAPTER II
+
+HOW ANIMALS ADAPT THEMSELVES TO CIRCUMSTANCES
+
+
+We all know what it is to adapt ourselves to circumstances. Suppose two
+lads, fresh from school, go out into the world to earn their living;
+one becomes a navvy and one a clerk. In five years' time these two young
+men will probably be very different in appearance from one another.
+The navvy will have developed his muscles; he will be broad-built,
+broad-chested, and strong. The clerk, on the other hand, will probably
+be comparatively weak and slim, his chest will not be so broad, his
+muscles will not be so well developed. The navvy, too, will probably
+be of a fresh complexion, while the clerk will be pale. All these
+differences are due to the fact that their bodies have adapted
+themselves to circumstances. Both men may be equally healthy, and
+equally long-lived. Let us take another example. Let us compare two
+other youths, of whom one becomes a cobbler and one an Alpine guide.
+The latter, in five years' time will have become a perfect specimen
+of muscular humanity--active, agile, and hardy. The cobbler will be
+comparatively stiff in his limbs and unable to undertake any singular
+feat of muscular exertion, although he may be able to do a very hard
+day's work at his own trade. The mountaineer, too, will probably differ
+in disposition from the cobbler. He will be daring, resourceful, and not
+afraid of danger under circumstances which would terrify the cobbler.
+Now let us suppose that the sons and grandsons of the navvy are brought
+up to be navvies, and the sons and grandsons of the clerk are brought up
+to be clerks;--that the children and grandchildren of the Alpine guide
+follow his own calling, and the children and grandchildren of the
+cobbler do the same;--we shall probably have four families differing
+very much in type of physique from one another. Yet take one of the
+navvy's sturdy grandchildren and bring him up as a clerk, and he will
+lose much of his sturdiness. Let the mountaineer's grandsons be brought
+up as cobblers, and by the time they are thirty they will not be
+remarkable for their muscular capabilities.
+
+Just in a similar way the bodies of animals adapt themselves to
+circumstances. It is not always possible to trace the steps by which
+this has been done. But sometimes it is so; and we may find a whole
+series of varieties that are plainly due to adaptation. When we see an
+animal which is in some way especially fitted for its surroundings, we
+are therefore justified in concluding that it has become so by degrees.
+
+The way in which animals adapt themselves to their surroundings in the
+matter of colour would afford material for several volumes each as large
+as this one. Those who have not travelled in foreign countries may
+perhaps find it difficult to realise that brilliant colouring and showy
+patterns can ever enable an animal to hide itself successfully. But an
+instance may be taken from an animal common on our own shores which will
+illustrate how this principle works.
+
+In the spring there may be found in large numbers upon our rocky coasts
+a little oval shell-fish, about one-third of an inch long, sticking to
+the fronds of the tangle and other broad-leaved seaweeds. The animal is
+of a very pale brown colour; its shell brownish and semi-transparent,
+with several stripes of brilliant turquoise blue down the back. These
+stripes are not continuous, but interrupted at intervals so as to give
+them a beady look. Taken in the hand and looked at closely, the shell,
+with its contrast of blue stripes on a brown ground, is extremely
+conspicuous; brown being, in fact, the contrast-colour which shows blue
+in its greatest brilliancy. Yet, when perched upon the tangle, the
+creature is almost invisible, and might easily be mistaken for a natural
+irregularity of the surface of the seaweed. While the brown is the
+colour of the seaweed itself, the brilliant blue is indeed the exact
+colour of the spring sky at that season, everywhere reflected from the
+sea-water and from the wet surface of the seaweed. By matching that
+brilliant colour the animal therefore is rendered invisible. This
+little creature is the young of the Semi-transparent Limpet, _Patella
+pellucida_. This, at least, was the old-fashioned name for it, though
+it has received others. Its young and its adult form are so different
+in the appearance of the shell, that they have been described under
+different names. English readers who search for it in the spring will
+learn by experience that bright colouring may help to make a creature
+invisible. But this is not all that is to be said about the protective
+colouring of this little shell-fish. There are many creatures whose
+young live at the surface of the sea, and afterwards migrate to deeper
+water as they attain adult age. In early life they are transparent,
+because thus they best escape notice in the clear water of the surface,
+especially when seen from below, by the many enemies on the watch to
+devour them. But in their later life they become opaque, because thus
+they best escape notice from enemies watching from above, as they crawl
+along the bottom of the sea. Now this is the case with the little
+Patella. For this also migrates to the bottom--in this instance a
+comparatively short journey--when it is ready for adult life. Both
+shell and animal, therefore, are at first nearly transparent, but in
+older life both become more opaque; the blue stripes, too, are almost or
+quite obliterated in the after-growth of the shell, slight traces of
+them alone remaining at its apex. This change of colour fits the animal
+for the new home in which it settles, for it moves down from the leaf
+of the tangle to its root, and there finds a snug shelter among
+the coral-shaped branches of which the root is composed. Not many
+reflections of the blue sky are likely to reach the recesses of the
+tangle-root, so the creature has no longer any need of its protective
+colouring of blue.
+
+The adult shell, however, retains a certain degree of translucency,
+which matches very well with the colouring of the tangle-root; and thus
+presents a great contrast to the shell of the common Limpet, which is
+found on rocks. The rugged surface of the latter is usually more or less
+irregularly speckled in harmony with the surfaces on which it lives,
+though this shell also presents when young occasional touches of blue,
+which suggests a family likeness in colour tastes on the part of the two
+kinds of Limpet. The blue in this case, however, is of the dullest and
+dingiest shade. The _Patella pellucida_ is common on the more rocky
+portions of our coasts; in spring the young may be seen in thousands on
+the seaweeds of the Isle of Man; here its habits were first observed and
+described in detail by the Manx naturalist Forbes, who noticed its
+peculiar way of finding a hiding place among the roots of the tangle.
+The same shell-fish, in contrast with the commoner Limpet of the rocks,
+affords another instance of the way in which shells adapt their forms
+to their surroundings. In each case the shell is a plain conical cap,
+and the animal within keeps the shell firmly attached to the base on
+which it rests. The Limpet can move about at a very creditable snail's
+pace when it wishes to do so, and at low-water mark, when the tide is
+beginning to rise, you may easily find them moving about and off their
+guard; but during many hours of the day, when the tide is out, the main
+object of the Limpet is to keep its shell as firmly fixed to the rock as
+possible. It will at once be seen that if the margin of the shell were
+smooth like that of a tea-cup, and the surface of the rock to which it
+clung very irregular, many chinks would be left between the margin of
+the shell and the surface of the rock through which unwelcome visitors
+might find entrance. The loss of moisture through the crevices, too,
+would be a serious thing to the animal during the hours when the shell
+is uncovered by the tide and exposed to the rays of a hot sun. On the
+other hand, if the margin of the shell were irregular, and the surface
+on which it rested smooth, unprotected crevices would in the same way
+be left. So the Limpets adapt the shape of their shell to their
+surroundings; the _Patella pellucida_, which lives on the smooth
+branches of the tangle-root, has a shell with a smooth regular edge;
+while the _Patella vulgata_, which lives upon rocks, has a shell with an
+irregular, indented edge, whose irregularities fit into those of the
+rock on which it rests. (See Fig. 2.)
+
+[Illustration: FIG. 2.--Shells mentioned in Chap. II. 1, Common Limpet,
+old and young; 2, Semi-transparent Limpet, old and young (the remains
+of the young shell may be seen crowning the adult shell); 3, Common
+Yellow Periwinkle; 4, Common Edible Periwinkle; and 5, High-tide-mark
+Periwinkle, both with a sharp spire, for comparison. One specimen of
+the latter stands among group 3.]
+
+Probably every reader will be able to appreciate the above instances of
+creatures adapted to their surroundings. For there are few people who
+are not familiar with the common Limpet of the shore between tide-marks,
+and with the great seaweed called Tangle, which has its habitat a little
+lower down, and forms great sea-meadows, whose upper limits alone are
+ever laid bare by the tide. The _Patella pellucida_, too, is fairly
+common, and the dead shell may be found on most rocky parts of our coast
+all the year round. As for the blue-striped young shell, floating on the
+blades of the tangle, those who have leisure to visit the seaside during
+the months of spring and early summer, may have seen it as I have
+described it; and the mention of it will recall pleasant memories of
+clear spring skies, and fresh sea-winds, and fields of heavy tangle
+swaying gently on the swell that comes in from the open sea. It is
+interesting to know something of the habits of the creatures whose forms
+we study, and we have already spoken of the snug little hiding-place
+that the Semi-transparent Limpet finds for itself in the tangle-root.
+It is of interest to remember that the Common Limpet, too, is a
+home-loving creature, which knows and prefers the spot of rock on which
+it habitually rests; and can find its way back to it, aided by its two
+eyes and two smelling patches. This has been proved by Professor Lloyd
+Morgan, who has recorded the result of his observations, made on the
+coast of Dorsetshire. It is not easy to detach a Limpet from the rock
+without injuring or exhausting it, but these specimens were caught when
+moving of their own accord, and were therefore uninjured and brisk. They
+were removed to short distances, and the following table shows the
+result of the experiment, clearly proving that the Limpet prefers home,
+but regards a distance of two feet as a very long journey.
+
+    ----------+-------------+----------+---------+--------
+              |             |  Number  |         |
+      Number  | Distance in | Returned | In Four | Later.
+     Removed. |   Inches.   |  in Two  | Tides.  |
+              |             |  Tides.  |         |
+    ----------+-------------+----------+---------+--------
+        25    |      6      |    21    |    0    |   0
+        21    |     12      |    13    |    5    |   0
+        21    |     18      |    10    |    6    |   2
+        36    |     24      |     1    |    1    |   3
+    ----------+-------------+----------+---------+--------
+
+Similar observations were made at an earlier date, by Mr. George
+Roberts, at Lyme Regis.
+
+Let us now take an instance of adaptation in form. And this time we will
+take a shell so common that everybody will know it.
+
+Everyone who has spent a little time in naturalising on the shore, has
+noticed how often you may find univalve shells, such as those of the
+whelk and periwinkle, with the top of the shell knocked off. This is
+nearly always the case with the dead shells that you find strewn along
+the tide-line; and after a storm, on a rocky coast, you may find shells
+that still contain the living tenant, in the same sad condition. And you
+may also meet not infrequently with shells, dead or living, that bear
+evidence of the owners' efforts to repair them after an accident to the
+spire. A piece has been broken, and you find it cemented on again by a
+patch of shell, serviceable no doubt to the owner, but crooked and
+unsightly in appearance. Now there is a very common shell, the little
+yellow periwinkle, which has practically done away with its spire, the
+coils of the shell being so curved that the earlier part of the spire
+does not project beyond the later-formed coils, and the whole shell has
+a rounded outline. This little creature lives on the long seaweeds which
+grow at low-water mark or near it; and when the sea is rough it is
+obviously liable to be dashed from its foothold on the seaweed and flung
+violently down, as the huge seaweeds sway about in the shallow waves. We
+may easily satisfy ourselves that this is an accident that frequently
+happens, by examining the shore when the tide is going out, on some
+stormy spring or autumn day. Numbers of the yellow periwinkles are then
+to be found crawling on the sand, and striving to regain their place in
+the seaweedy rocks as soon as possible. On a calm day you will rarely
+see one crawling on sand above low-water mark, for it is a place they do
+not choose by preference; those that are to be found there on the stormy
+day have lost their foothold, and have been washed about by the tide.
+Had they, like some other kinds of periwinkle, a sharp spire, how many
+would be the casualties under these circumstances! But as it is, you do
+not see a single specimen with a broken top: the rounded spire is an
+adaptation to circumstances, required for the protection of the tenant
+of the shell. (See Fig. 2.)
+
+It may be added that the yellow Periwinkle is not only protected from
+mechanical sources of danger by its form, but is also in some degree
+protected from living enemies by its colour. This, at first sight, seems
+exceedingly conspicuous. We must remember, however, that the animal
+often lives in that part of the shore where the Bladder Seaweeds, or
+Fuci, are extremely abundant. The flowering ends of these are of a
+yellow colour, fairly bright. When seen from below, with the sunlight
+streaming through them, they no doubt appear much brighter than when
+seen, as we see them, from above, with the sunlight falling on them.
+Now protection from foes below is what the yellow periwinkle needs
+most: for fishes are quite ready to swallow it whole, and are not in
+any way deterred by the thickness of the shell, which is (by-the-way)
+in a measure a protection against birds when the tide is out; fishes
+habitually swallow shell-fish whole, although the inmate only is
+digested. The bright yellow, then, that seems to us so conspicuous, is
+probably a good means of hiding for the periwinkle when under water. Its
+common variations in colour, too, are probably protective in their use:
+some are a dull purplish brown, some drab. These are good colours in
+which to lie hidden, respectively, under darker tracts of seaweed, or
+upon the rock itself. This little shell is so abundant on rocky coasts
+that on some beaches the dead shells are as numerous as pebbles. No
+wonder, with all these adaptations for protection!
+
+Another instance of adaptation to circumstances is described in the
+sea-urchin shown on p. 125. This is one among many instances where
+animals that live on sand or mud acquire a flattened shape, so that
+their weight is distributed, and the danger lessened, of their sinking
+in a quick-sand. The flat-fish, such as soles and flounders, are a
+familiar example; and the same principle is illustrated by the flattened
+forms of many of the bivalve shell-fish, whose flat shell, when closed,
+can lie safely on the loosest sand. Equally is their form adapted for
+their circumstances, when, in their slow way, they begin to move. For
+the flat valves of the shell are placed to the right and left of the
+animal's body. So that when it stirs, or floats quietly in the current
+of the tide, the shells present their sharp edges to the resistance of
+the water, thus enabling the creature to move like a ship through the
+sea, or like a knife-blade through bread, with the least possible
+friction: and specially is this provision for the lessening of friction
+important, when we consider that many of these bivalve shell-fish have
+to move, not only through water, but also through sand and mud.
+
+It may be assumed that every reader is familiar with the common forms of
+the bivalve shell-fish. The frontispiece shows one of them, considerably
+flattened in shape.
+
+So far, however, we have not explained _how_ animals adapt themselves to
+circumstances; we have only pointed out the fact that they do so.
+
+Take the case of our little Limpet. It cannot say: "I will paint myself
+with blue and brown, so as to be mistaken for a bit of seaweed
+reflecting the blue sky"; nor can the periwinkle say: "I will paint
+myself with yellow, so as to pass unnoticed among the yellow ends of the
+_Fucus_; and I will build my spire low, so that it will not be broken."
+The bivalve shell-fish and the Sand-Cake sea-urchins do not say to one
+another, "Let us alter our shells, and build them a little flatter, so
+that we shall not sink in too deep when we lie upon the ooze and sand of
+the sea."
+
+How then do these adaptations take place? Darwin has explained this for
+us. Individuals often have some little peculiarity, in which they differ
+from the average of their kind. The establishment of such little marks
+of individuality is spoken of as Variation. If among these individual
+peculiarities there is one which is in any way disadvantageous, _e.g._
+one which tends to make the creature conspicuous in the sight of its
+foes, the owner will be quickly eaten, and of that peculiarity there
+will be an end. If, on the contrary, the peculiarity gives the owner
+some advantage over its fellows, that individual will survive, and
+probably transmit its peculiarity to some of its descendants.
+
+We have seen, for instance, that it is of advantage to our little
+periwinkle to be yellow, when it lives in certain situations; and that
+it sometimes presents other colours, likely to be favourable in other
+cases. If we gather together a large number of specimens, we shall find
+a surprising range of variation in colour. Some present a tint of bright
+orange, nearly red; some are a dull brown; the dark purple shade and the
+drab have been already referred to. The very young shell usually
+presents an unmistakable shade of pink; and we may find innumerable
+half-grown specimens in which we may trace the gradual establishment of
+the advantageous yellow colour, from an original shade of unmistakable
+pink, presented by the earlier whorls. Kindred varieties of the shell,
+too, may be found with stripes or speckles. Since this very common shell
+may be found in abundance on any rocky shore in the British Isles, the
+reader may easily study its colour-variations, both in the dead and the
+living shell. Study also the ground on which the creature lives, with
+its sharp colour-contrasts of rock and seaweed patches, and it will be
+easy to understand why the colours are thus varied, with a
+preponderance, on the whole, of the yellow shades. It is all a question
+of the survival of the fittest--the unfit being represented by colours
+too easily seen, and therefore quickly snapped up. As for the spire, it
+has already been shown how that is adapted to circumstances. It is
+worthy of remark that in the kindred Edible Periwinkle, _Littorina
+littorea_, which has a sharp spire, elderly specimens may be seen with
+the end of the spire damaged.
+
+Turn again for a moment to our first instance--the adaptation of men to
+a sedentary or an outdoor occupation. Here we dwelt upon the change
+produced by their mode of life; we left out of sight the "survival of
+the fittest." Yet here it is equally surely at work. How often does the
+young mountaineer, less agile than his fellows, come by a violent death?
+Only those who are equal to the necessities of the life survive--many
+are lost. How often does the clerk, tied to his desk, fail in health
+and die? How often, hating a sedentary life for which he is unfitted,
+does he throw his energies into athletics, lose interest in his office
+work, and get dismissed? Here again comes in "the survival of the
+fittest"--for a desk: alas! perhaps the only means of livelihood.
+
+But _why_ do variations occur? This is the question first asked by a
+child, when you try to explain the working of "natural selection." It is
+also the last question asked by scientists, who are still industriously
+engaged upon studying the problem.
+
+In the above instances from human life, we have considered the
+occurrence of changes brought about in the organism by the circumstances
+of life; or as scientists say, by the "environment." Scientific men are
+busily hunting for instances of variation of this sort. Take for
+example, an animal which lives sometimes in salt water, sometimes in
+water that is only brackish; there are cases in which small differences
+can be noticed, according to the difference in the habitat. Notice the
+marine shell-fish, for instance, near the estuary of a river: they are
+often less robust specimens than are found at a point free from the
+influence of fresh water.
+
+Not until the effect of known causes on the rise of variations has
+been studied much more fully than at present, will it be possible to
+judge regarding the nature of those variations which _appear_ to be
+spontaneous; for which, at present, no predisposing cause can be
+assigned.
+
+A very large number of variations, however, fall into the class of
+"Atavistic" variations; that is to say, those which show a return to an
+ancestral type. These are variations which are very rarely welcome. If,
+for instance, a boy has a pair of handsome black rabbits, he is not much
+pleased to find among their progeny, every now and then, one of the
+colour of the original wild Bunny. The probability, in this case, is
+that the atavistic variety will find its way into a pie, instead of
+being kept as a pet. Equally unsatisfactory to the owner, is the
+incorrigibly savage and intractable dog or horse--a reversion to the
+mental type of an ancestor which knew not the authority of a master.
+
+Atavistic variation often occurs when members of two well-marked
+varieties are mated; so that in some of the offspring produced, each
+parent seems to cancel out the more extreme characteristics of the
+other, leaving only the characteristics of the more generalized
+ancestral type, from which both parents have alike been derived.
+
+When the ancestral type is in some way inferior to the modern one,
+variation which consists in reverting to the former is often referred to
+as Degeneracy. There is reason to believe that discomfort and hardship
+of existence tend to produce variation of this kind--a fact of supreme
+importance, when the problem of Degeneracy is considered in connection
+with human life. When creatures begin to degenerate, it is, in fact, as
+if the species were saying to itself, "I have gone astray; let me
+retrace my steps along the road by which I came, and maybe I shall find
+comfort and safety; step by step I will try to go back to my ancestral
+form."
+
+Very rapid variation of any sort is indeed often a sign that the
+struggle for existence is too hard for the type in question. The
+palæontologist can tell us of types that present numerous variations
+before becoming extinct; while others, comfortably holding their own in
+the struggle for existence, remain practically unchanged during age
+after age of the geological record, and survive even up to the present
+day. We may borrow from commercial life a homely illustration that will
+explain this aspect of variation. When competition in trade is keen, the
+seller must have novelties; he will try all sorts, and find some good,
+some bad, some indifferent. If he now revives an out-of-date pattern of
+goods, for the sole sake of change, this is Degeneracy. But where, on
+the contrary, competition is dull, the same firm will turn out the
+same goods for a long period of time. There is an optimum in trade
+competition: a reasonable competition results in the production of
+sensible novelties, and consequent progress; but competition over-keen
+results in the production of rubbish, leading to eventual failure. So in
+the world of animal life; a certain degree of struggle for existence
+results in variation, establishment of new varieties, progress. A
+greater degree results in too rapid variation, new varieties that
+speedily perish, and finally, the extinction of the type.
+
+We have spoken of "varieties." Each of the domestic animals presents
+varieties, which are the cumulative result of the breeder's artificial
+selection of natural variations. Thus the Pug and the Collie for
+instance, are varieties of the Dog; the Bantam and the Dorking of the
+Fowl. Among wild animals, varieties are similarly produced by _natural_
+selection, resulting from the "survival of the fittest." By degrees,
+intermediate forms are lost; and new species are established by the
+greater and greater divergence of varieties originally derived from one
+ancestral type.
+
+TABLE SHOWING THE POSITION IN CLASSIFICATION OF THE ANIMALS NAMED IN THE
+FOREGOING CHAPTER
+
+    =Phylum=     =MOLLUSCA=, or Shell-fish.
+
+    =Class=      GASTEROPODA, or Snail-like Shell-fish.
+
+    =Sub-Class=  ANISOPLEURA, or Unequal-sided Gasteropods.
+
+    =Branch=     STREPTONEURA, or Unequal-sided Gasteropods with nerves
+                   twisted into the shape of a figure of 8.
+
+               /------------^-----------\  /--------------^--------------\
+    =Order=      ZYGOBRANCHIATA,             AZYGOBRANCHIATA,
+                 or Streptoneura with        or Streptoneura with
+                 a pair of gills.            only one gill.
+
+    =Genus=      _Patella_, the Limpet,      _Littorina_, the Periwinkle,
+                 with gills obliterated,     or Shore Shell.
+                 and only indirectly
+                 represented; breathing
+                 is performed by folds
+                 of the mantle.
+
+    =Species=    _Vulgata_, the Common       _Littoralis_, the (Yellow)
+                 Limpet.                     Periwinkle that lives
+                                             above low-tide-mark.
+
+
+
+CHAPTER III
+
+CLASSIFICATION, OR THE SORTING OF THE ANIMAL KINGDOM
+
+
+Give a child a few handfuls of shells. Probably the first thing he will
+do with them is to sort out the various kinds and separate them from
+one another. Each will go into a little heap by itself; and next, our
+young friend will find names for them. These are Cap-shells and those
+Sword-shells; these Saucers and those Plates; these Yellow-shells and
+those Pink-shells--according as some special character or form or colour
+strikes his fancy.
+
+Now this is what zoologists have been doing with the animal kingdom
+from the earliest days of science; trying to recognise each distinct
+kind of animal form, and to give it a name of its own. Unfortunately for
+the reader, zoologists have been obliged to choose names of Latin and
+Greek origin, and therefore in writing about animals we are often
+obliged to burden our pages with long words. This is a disadvantage, but
+it is a very slight one compared with the great advantage gained by
+using the learned tongues, which consists in this, that learned men from
+all countries of the globe can equally understand the names thus brought
+into use. One particular kind of creature may have one name in English,
+another in French, another in German, and so on; but the learned world
+does not trouble itself with this multiplicity of names--it gives the
+creature a couple of names in Latin, and these names stand good for
+learned readers in every part of the globe. The importance of this will
+be fully realised when, in a later page, we shall have to speak of the
+work done by zoologists, and the way in which they do it. Meantime we
+must ask our readers to have patience if now and then some long names
+must be used. These learned names sometimes convey a description of some
+important characteristic possessed by the animal, and sometimes they are
+merely fanciful names, such as the child we have spoken of gives to his
+zoological playthings. It does not greatly matter whether the name is
+descriptive or not; zoologists describe each animal kind in its most
+minute details, and the most commonplace or inappropriate name serves
+its purpose quite efficiently as a means of referring to published
+descriptions.
+
+We have spoken of sorting the animal kingdom into its various kinds.
+But how do we know when a number of animals are all of one kind? No two
+individual animals are ever exactly alike, any more than two persons are
+ever exactly alike. "It is a matter of common observation that no two
+individuals of a species are ever exactly alike; two tabby cats,
+for instance, however they may resemble one another in the general
+characters of their colour and markings, invariably present differences
+in detail by which they can be readily distinguished. _Individual
+variations_ of this kind are of universal occurrence" (T. J. Parker).
+
+Among a host of animals that present so many differences, how do we
+determine what shall be considered as belonging to one and the same
+kind? This is a point that nature usually settles thus. If two varieties
+when mated produce offspring which are perfectly fertile when mated
+again with another set of offspring similarly produced, then the two
+varieties, however differing in appearance, belong to one species. If on
+the other hand, the two belong to a different species, the offspring
+will be what is called a mule or hybrid, and will not produce offspring
+if mated with another mule. One of the most familiar examples of a mule
+is the animal, commonly so-called, which results from mating a horse and
+an ass, and partakes of the characteristics of both.
+
+Every animal receives two Latin or Latinised names, the first that of
+the genus, the second that of the species; this system of naming, often
+referred to as the "binary nomenclature," we owe to the industry of
+Linnaeus the great Swedish botanist and zoologist. Genera are groups
+consisting of a number of different species which closely resemble one
+another. Similarly genera, which are somewhat alike, are again formed
+into larger groups, and so on. The names of families, orders, and
+classes used to be given to these groups in ascending order; but it is
+now generally recognised that such names are arbitrary, and that the
+divisions into which animals may naturally be grouped are altogether
+irregular, and not comparable with one another. Those who know a little
+of botany will readily understand, from their knowledge of wild flowers,
+that natural groups cannot be arranged in a formal series.
+
+The main branches of the animal kingdom, the largest groups of all, used
+formerly to be called sub-kingdoms. Now the main divisions are often
+spoken of as phyla or races. Classifications, although they differ much
+in detail, according to the preferences of individual zoologists, yet
+agree as to the main branches of the animal kingdom, the chief of these
+are:--
+
+     1. The Protozoa, or One-celled Animals.
+     2. The Coelenterata or Two-layered Animals.
+     3. The Sponges or Porifera.
+     4. The Vermes or Worms.
+     5. The Arthropods or Jointed Animals, viz., Insects and Crustacea.
+     6. The Mollusca or Shell-fish.
+     7. The Brachiopoda or Lamp-Shells.
+     8. The Bryozoa or Moss-Corals.
+     9. The Echinodermata or Sea-Urchins.
+    10. The Chordata, including--(_a_) the Hemichordata;
+          (_b_) the Ascidians; (_c_) the Vertebrata.
+
+Within recent years an attempt has been made to express the relationship
+these groups bear to one another, by placing them in separate divisions
+or grades. The first grade includes only the Protozoa, or unicellular
+animals. The position of second grade has been assigned to the
+Coelenterata or diploblastic animals, whose bodies consist typically of
+two layers of cells. A third grade includes only a few groups of the
+lower worms, among which three body-layers may be distinguished, but no
+body-cavity is present. While the fourth grade, including practically
+the rest of the animal kingdom, have three body-layers (see p. 38), and
+a body-cavity surrounding the internal organs (see p. 38).
+
+This arrangement of groups is an extremely convenient one; all the more
+convenient because it easily admits of modification. Already, indeed, we
+might find room for a grade intermediate between I. and II., consisting
+of what might be termed monoblastic animals, namely, animals consisting
+of a single layer of cells. For the frequent occurrence of Larvæ of this
+kind, consisting of a hollow ball of cells, renders zoologists on the
+alert to find a grown-up organism built in the same way. It is doubtful
+whether any of the forms that have been supposed to answer to this
+description really do so. Certain forms of these often claimed as plants
+by the botanists are, however, in the meanwhile, invited in to fill the
+blank.
+
+There are also animals in which the internal layer of the body is very
+much reduced, consisting sometimes in fact of one cell only. Those are
+the Dicyemidæ and Orthonectidæ, both of them parasitic forms. They
+differ so completely from all other forms that it has been proposed to
+make for them a special group, the Mesozoa, or Midway animals, between
+the Protozoa and all the rest of the animal kingdom. It is, however,
+possible to group them under the head of Diploblastic animals; but
+nothing more different from the Coelenterata could well be imagined, and
+some regard them as a degraded form of worm.
+
+The animals which are higher in structure than the Protozoa, viz. our
+divisions 2 to 10, are often grouped under the name Metazoa. The Metazoa
+thus include Grades II., III., and IV.
+
+The meaning of the division of the animal kingdom into grades will be
+more apparent if we give an example of each.
+
+[Illustration: FIG. 3.--_Amoeba_, a typical unicellular animal: _n_,
+nucleus; _cv_, contractile vacuole; _ps_, pseudopodia; highly magnified.
+This represents Grade I. of animal existence.]
+
+GRADE I. _The One-Celled Animals._--_Amoeba_, the Mobile animal, is
+the typical example of these. It consists of a single microscopic cell.
+In this cell is seen a dark irregular speck, the nucleus, which is an
+essential character of cells, whether they are independent or form
+part of the body of a larger animal. There is often visible also a
+clear rounded space, called the "contractile vacuole," which squeezes
+out fluid, disappears, and reappears again, serving the purpose of
+excretion. The cell-substance, called protoplasm, is practically
+identical in this and in cells of all other kinds. It is jelly-like, and
+capable of a slow movement, which may be watched under the microscope.
+It suggests the flowing of treacle or thick gum. The movement may be
+traced by the change in outline of the cell and by the change in
+position of any granules that it may have taken in; for particles which
+touch the creature sink in and are surrounded; thus it obtains its food.
+These slow flowing movements of the protoplasm result in continual
+changes of shape; hence the name, Amoeba, the mobile animal. Sometimes
+the island of protoplasm, as it changes its shape, throws out, as it
+were, capes and headlands. These projections, which are presently drawn
+in again, are called pseudopodia or false feet. They are characteristic
+of the whole group of Amoeba-like animals, which are consequently called
+Rhizopoda, the root-footed. The production of new individuals is
+accomplished by the division of the old cell into two. Thus it may be
+said that there is always a bit of the old cell remaining, though
+divided into fragments; and for this reason the Amoeba-like Protozoans
+have been fancifully called "immortal."
+
+[Illustration: FIG. 4.--Section, highly magnified, of a two-layered
+animal, _Hydra_ (Grade II.). _Ec_, outer layer of Ectoderm; _En_, inner
+layer of Endoderm; _l_, lamella dividing the two, represented by a line;
+_n_, nuclei of the cells; _v_, thin vacuoles of small interstitial
+cells; _E_, the Enteron or digestive cavity.]
+
+GRADE II. _The Two-layered, or Diploblastic Animals._--The type of
+these usually chosen is _Hydra_, a two-layered animal, which is further
+described on p. 54. A section through Hydra (Fig. 4) shows (1) the outer
+or skin layer of cells, called the ectoderm, and (2) the inner or
+stomach layer of cells, called the endoderm (literally outer skin and
+inner skin). The clear recognition of the primary body-layers of the
+simpler invertebrates as identical with the primary body-layers of the
+embryo of higher forms, is largely owing to the teaching of Professor
+Huxley, the importance of whose work on this and in many other respects,
+is little guessed at by many readers who know his name merely as a
+popular exponent of scientific ideas. The two-layered body of Hydra
+encloses a hollow digestive space; from this the Coelenterata receive
+their name, which means "possessing a hollow space only, by way of
+intestines." The name of Acoelomata, animals without a body-cavity, has
+therefore been given to the Coelenterata and sponges. The meaning of the
+term body-cavity will be explained in the next paragraph but one. The
+Hydra, like all animals of its grade, and all those of the succeeding
+grades, reproduces itself by means of ova or egg-cells, and spermatozoa
+which fertilize them.
+
+GRADE III. _The Triploblastic Animals without Body-Cavity._--This is a
+small section including only some of the lowest worms, such as the
+forms called Planarians. Between the Ectoderm and Endoderm lies an
+intermediate layer the Mesoderm. There are the beginnings of this in
+the Coelenterata and Sponges, but here it is further established. It
+includes a very thick layer of muscles.
+
+[Illustration: FIG. 5.--Diagrammatic plan of section cut through an
+Earthworm to show the position of the three body-layers and the
+body-cavity (Grade IV.). _Sk_, skin; _al_, glandular lining of the
+alimentary canal; _w_, muscular wall of body; _w'_, muscular of
+intestine, both belonging to the third layer or mesoblast; _b.c._,
+body-cavity (shaded); _al.c._, cavity of alimentary canal (shaded); _n_,
+nerve.]
+
+GRADE IV. _The Coelomata or Triploblastic Animals with a
+Body-Cavity._--This grade includes all the remainder of the animal
+kingdom. As an example of it, we may take the Common Frog. If we open
+from the lower surface the dead body of a frog, we first cut through
+the skin, next the muscles; then we come to the viscera, lying neatly
+packed in a cavity from which we can dislodge them. This cavity is the
+Body-Cavity. The skin corresponds with the ectoderm of Hydra, although
+it is a vastly more complicated affair. The glandular lining of the
+alimentary canal corresponds with the endoderm of Hydra; although this,
+too, is a more complicated affair. The mass of the body, lying between
+these two layers, is considered to correspond somewhat with the mesoderm
+of Grade III., and has received the collective term of Mesoblast. This
+description applies equally to the earthworm, for the higher worms
+differ immensely from the lower worms, and stand on a level with more
+important members of the animal kingdom (see Fig. 41, p. 139). The
+body-cavity may be formed in different ways in different animal groups;
+but there is reason to believe that in certain cases it originates by a
+folding off of part of an original cavity corresponding with that of
+Hydra; so that part went to form the intestine, and part the cavity
+surrounding it.
+
+The above arrangement of the main great groups of animals into four
+grades is that given by Professor Arnold Lang.
+
+It should be added, that there are a few exceptional forms that present
+a departure from these broad rules of structure. They are, however, so
+few that they need only be named as curiosities. For instance, there are
+parasites in which the inner body-layer is practically done away with,
+because they are fitted to absorb food through the outer layer. And in
+one division of the Moss-Corals there is no body-cavity to be seen,
+although it is to be found in the other division.
+
+What is the outcome of all this sorting of the animal kingdom? This most
+important result: that a classification of the animal kingdom into the
+four grades we have named, presents, in serial order, the stages through
+which young animals of the higher forms pass in the course of their
+growth. Every creature begins as a unicellular organism--the fertilised
+egg-cell. A vast number of creatures belonging to the higher groups
+present, later on, a two-layered condition, comparable with that of
+Grade II. Later on they acquire a third layer, and therefore correspond
+with Grade III. By degrees the body-cavity is formed, and they then
+present the adult body-structure of Grade IV. The development of the
+chicken in the egg, for instance, presents these four stages.
+
+It will be sufficiently apparent that this coincidence is too striking
+to be without a meaning. Zoologists are all agreed in their
+interpretation of this meaning: it is, that the history of the
+individual presents a summary of the history of the race, and goes
+through the stages of structure which its ancestors presented in their
+adult forms. The story of the gradual upward struggle of the animal
+kingdom, from its humble beginnings to its present wonderful complexity,
+is written in the growing tissues of every young creature.
+
+The principle that ancestral traits betray themselves is accepted as a
+truism in common life. Do we see young people rude and stupid? We say,
+perhaps, "No wonder; their grandfather was a drunken, worthless lout."
+Do we see a family of the poorest class clever, and industrious, and
+refined? We say, "They come of a good stock." When we speak in this way,
+we reason from the common experience of mankind, that children resemble
+their ancestors. Similarly, when zoologists find an embryo starting its
+existence from one cell, they say, "No wonder; its ancestors were
+unicellular." And when they find it assuming a two-layered form, they
+say, "Its ancestors were two-layered creatures." So certain are
+zoologists of the existence of an ancestral two-layered form, the parent
+at once of the existing Coelenterata and of the higher forms, that
+Professor Hæckel has given it a special name--Gastræa. The two-layered
+young stage of higher creatures, when it has a free-swimming existence,
+is called a Gastrula (Fig. 6). Both names, meaning stomach-animal, refer
+to the structure, which is, in a still simpler form, that of _Hydra_--a
+two-layered bag of cells, of which the inner layer, lining the cavity,
+performs the work of digestion. The lowest of the Vertebrata, the
+Lancelet (see p. 140), has a larva of this kind. The same reasoning
+which suggests the existence of an ancestral Gastræa-animal, suggests
+that of an ancestral Planula-animal; for the two-layered animals, on
+their part, present us with a monoblastic larva of the form already
+described (p. 34), called a Planula. Hence it is that zoologists look
+with such eagerness for forms, of which it can be said that they consist
+of one layer of cells only. The name Planula signifies "wandering
+animal," because the Planula larva swims about by means of cilia.
+
+[Illustration: FIG. 6.--Diagrammatic representation of a typical
+Gastrula, or two-layered larval form, highly magnified; optical section,
+longitudinal. _Ec_, Ectoderm or skin layer; _En_, Endoderm or stomach
+layer; _m_, mouth leading into the enteric cavity. The dots are the
+nuclei of the cells.]
+
+[Illustration: FIG. 7.--Diagrammatic representation of a typical
+Trochosphere, or ciliated larva, considerably magnified. _M_ is
+the mouth; the stomach and intestine are seen showing through the
+transparent body.]
+
+Mention has been made above of larval forms. It is perhaps advisable to
+explain clearly what is meant by this term. It is a matter of every-day
+knowledge that in some animals the young form presents an appearance and
+structure very different from that of the grown-up form, and adapted for
+a different mode of life; the commonest instances are the caterpillar of
+the butterfly and the tadpole of the frog. We are apt to think of these
+creatures as somewhat exceptional in this respect. But the zoologist, in
+viewing the whole range of the animal kingdom, finds a vast number of
+animals with larvæ, differing much from the adult, and adapted for a
+different mode of life. It is, in fact, a very common arrangement; but
+often these larvæ are very minute, perhaps absolutely microscopic,
+therefore only known to the scientific observer. The two familiar
+instances we have named are fortunately big enough to be known to
+everyone. Now it is an axiom with modern zoologists (as has been
+explained above), that the history of the individual is a summary of the
+history of its ancestors; larval forms are therefore of special interest
+in this connection. A very wide-spread form of larva, more advanced in
+its structure than the little Gastrula that has been already named, has
+received the name of Trochosphere or Wheel-ball (Fig. 7), because it
+swims round and round, by means of cilia, usually distributed in bands.
+Its inner or stomach-layer, forms a definite alimentary canal, and is
+separated by a very simple mesoderm from the outside ciliated layer,
+which presents certain differences in form, according as the creature
+belongs to one group of animals or to another. The main characters of
+the Trochosphere are, however, the same in very widely differing groups.
+These little larvæ give rise to one of the most eagerly debated problems
+of zoology. Are we to suppose that animals which possess a Trochosphere
+larva are all descended from one common ancestor? Or are we to think
+that the Trochosphere is a form of body very convenient for the
+necessities of juvenile existence in the sea, and therefore
+independently evolved by animals which are not directly related to each
+other? Some authorities take the latter view; the former is perhaps more
+widely accepted, and has even been expressed by the application of the
+name Trochophora (Wheel-carriers), as a general term for those groups in
+which such larvæ are found. These include some of the higher worms,
+which present the typical Trochosphere, the Brachiopoda, and the
+Polyzoa; while variations of the Trochosphere type are shown by the
+earliest larvæ of Mollusca, the larvæ of the Echinoderms, and those of
+the Hemichordata (see p. 33), the latter bringing us, as it were, within
+eye-shot of the Vertebrata themselves. It will be seen, therefore, that
+the range of the Trochosphere larva covers a large portion of the ground
+occupied by our Grade IV. There is, however, one marked exception: the
+Arthropoda, which seem to have a prejudice against cilia in any form
+(since they include but one animal which possess any) have no example of
+a ciliated larva. Even their simplest larval forms belong to a higher
+type of structure, in which the shelly, jointed structure characteristic
+of the group is already indicated.
+
+When we speak, however, of the occurrence of the Trochosphere throughout
+a wide range of animal life, it must be understood that its presence is
+not necessarily uniform throughout a group in which it occurs. Larval
+forms are adaptations which conform with the conditions of life for the
+particular animal in question: and nearly related kinds of animal may be
+without a larva. The Trochosphere larva is, of course, only adapted for
+aquatic existence, and is necessarily absent in the case of terrestrial
+forms.
+
+TABLE OF THE CLASSIFICATION OF THE ANIMAL KINGDOM[A]
+
+    =Grade I.=--UNICELLULAR ANIMALS.    PROTOZOA.
+
+                          (Intermediate forms, see p. 34.)
+
+    =Grade II.=--TWO-LAYERED      { SPONGES.
+    ANIMALS.                      { COELENTERATA.
+
+    =Grade III.=--THREE-LAYERED   { PLATYHELMINTHES, OR FLAT-WORMS.
+    ANIMALS.                      { VERMES, THE HIGHER FORMS.
+
+                                  { ARTHROPODA.
+                                  { MOLLUSCA.
+    =Grade IV.=--COELOMATA, OR    { BRACHIOPODA.
+    THREE-LAYERED ANIMALS WITH A  { BRYOZOA.
+    BODY-CAVITY.                  { ECHINODERMATA.
+                                  { TUNICATA OR ASCIDIANS. } =Chordata.=
+                                  { VERTEBRATA.            }
+
+[A] In the subsequent tables which show the respective sub-divisions of
+these chief groups, the larger only of the sub-divisions are named.
+
+When an animal has no free larva, but quits the egg in a form
+practically identical with that of the adult, the development is said to
+be "direct." But changes equally startling with those displayed when a
+larva develops into the adult form, may take place while the young
+animal is enclosed within the egg itself. To these also zoologists apply
+the axiom referred to above, that the history of the individual
+summarises the history of the race. Thus, for example, the Amphibian
+larva, _e.g._ the tadpole of a frog (p. 153) has gills, which disappear
+in the adult form: the young reptile, bird, or mammal, which has no
+larval stage, has gills during a comparatively early stage; and loses
+them at a later period of its development. In each case zoologists
+conclude that the animal is descended from a fish-like ancestor, which
+possessed gills all its life, and that the more immediate ancestors in
+the family tree, have lost their gills by degrees.
+
+The study of the progressive changes of young forms, whether larval, or
+enclosed within the egg, is called Embryology, and constitutes, in these
+days, the major branch of zoological science. That it is of paramount
+importance to the student of classification, engaged upon the sorting of
+the animal kingdom, will be apparent from what has been stated above.
+
+
+
+
+CHAPTER IV
+
+THE ONE-CELLED ANIMALS OR PROTOZOA
+
+
+Some idea of the general characteristics of the Protozoa has already
+been given by the description of _Amoeba_. We may now say something
+about special groups of the Protozoa, which have minor characteristics
+of their own.
+
+Amoeba belongs to the class Rhizopoda, as has been already stated; but
+there are many of the Rhizopoda that greatly differ from Amoeba in
+appearance. The possession of a shell or skeleton gives a special
+importance to several groups. For, as the reader has no doubt already
+learnt from an earlier volume in this series, such skeletons or shells
+have played an important part in the history of the earth's surface,
+building up geological strata of vast extent, by the accumulation of the
+shells left after the decay of the owners' tiny bodies, during long
+periods of time. The chalk rocks that form the "white cliffs of Albion,"
+and that are so widely distributed in other parts of the globe, are
+formed in this manner; while the ooze of the Atlantic and other oceans,
+similarly composed of Protozoan _débris_, is now at the present time
+building up what will be the chalk rocks of future ages. Some of these
+Protozoans attain a remarkable size, instead of being microscopic, as is
+the case typically with the one-celled animals. Some forms of the
+Foraminifera found on the coast of North America measure as much as
+one-fifth of an inch across, while in warmer seas there are kinds which
+attain, as did the extinct Nummulite of Egypt, the size of a bean. Two
+inches across is mentioned as the maximum diameter, however, of either
+extinct or living forms. The Foraminifera are sometimes named
+Reticularia, because their pseudopodia interlace.
+
+[Illustration: FIG. 8.--Fossil Skeletons of Polycystina, from the
+so-called "Infusorial Earth" of Barbadoes, highly magnified.]
+
+The Foraminifera have shells composed of carbonate of lime, but there
+are other forms that build up geological deposits, in which the shell
+is flinty. The diagram (Fig. 8) shows some fossil shells of Protozoa
+from the marl of Barbadoes. These constitute a deposit which was named
+"Infusorial earth," in the earlier days of microscopic observation, when
+all Protozoans were spoken of as Infusoria. The name, Infusoria, it must
+be recollected, is now restricted to a special class, to which the forms
+in question do not belong. These fossil forms were named Polycystina,
+and are still often spoken of under that name, although the animals that
+present the peculiar feature of possessing "more than one cyst" now are
+called Radiolarians. The "cyst" consists of a basket-work supporting
+skeleton of flint; there may be several, one inside the other, and
+connected by radial bars. A living species named _Actinomma_ has three
+such layers of basket-work, one in the outer layer of protoplasm, one in
+the inner layer, and a central one. It will perhaps be remembered by the
+reader that the animals of this group, Radiolaria, are forms described
+in a previous volume of the series, as so curiously associated in
+Symbiosis with the algæ known as Yellow Cells.
+
+The famous polishing slate of Bilin in Bohemia consists of flinty
+Protozoan shells; it is 14 feet thick, and a cubic inch has been
+estimated to contain 41,000,000,000 of the shells.
+
+While the Radiolarians are marine, the Heliozoa, a group in which
+the skeleton is also present, but not usually so greatly developed,
+are predominantly fresh-water forms. Both classes take their name
+(Ray-animals, Sun-animals) from the stiff radiating rods of the
+skeleton.
+
+Strongly to be contrasted with the above groups belonging to the
+Rhizopoda are the Infusoria proper, which are characterized by the usual
+possession of cilia. Cilia (literally "eyelashes") are fine hair-like
+processes of the protoplasm of the cell, which fringe its exterior; by
+their constant movement they enable the animal to swim, and at the same
+time they create a current in the water, which washes up to the region
+of the mouth particles which may serve for food; for these creatures
+have this very great advantage over Amoeba, and the other forms above
+referred to, that they possess something which may be called a mouth.
+That is to say, there is one particular spot of the surface where
+particles are taken in. This may seem to be a restriction, when we
+compare the Infusorian with Amoeba, which is apparently able to take in
+food at any part of the surface. But it is a restriction which is
+associated with an advantage; the Infusorian cell, namely, has a firm
+exterior with a definite outline, instead of being soft and mobile all
+over. The firmer exterior layer of protoplasm, which is in turn covered
+by a thin cuticle or limiting membrane, is called the cortex or rind.
+For this reason the name Corticata is sometimes given to the group,
+_i.e._, Protozoa with a rind.
+
+_Vorticella_, the Bell Animalcule, is a stalked form living in ditches,
+which is usually selected as a typical form of the Infusoria. It
+receives its name, the Whirlpool Animal, from the current which its
+cilia create in the water. The purpose of this current is to wash food
+particles into the mouth. Associated with the Infusoria under the name
+of Corticata are the Gregarina and some other parasitic forms.
+
+It is interesting to note that the main types of the unicellular animals
+are repeated again in the cells of different parts of the bodies of
+multicellular animals. Amoeboid cells, so called because of their
+mobility and general resemblance to Amoeba, are found in various parts
+of the higher animals. The lymph corpuscles of vertebrata, and the white
+corpuscles of vertebrate blood, as well as the blood corpuscles of
+invertebrates, are among the instances of this. There are cells, on the
+contrary, such as those that line the mucous tracts, which are of a
+Vorticella type, so to speak; fixed to their bases, and presenting cilia
+on the free aspect.
+
+Two things must be noticed before we leave the subject of the Protozoa.
+One is, that some forms present the beginning of a multicellular
+condition. Several units sometimes join together, and in this way a
+complex object may be formed, in which there are several nuclei; or the
+original unit may keep on growing till it consists of many successive
+portions, and in some of them a fresh nucleus may arise. This occurs in
+some of the Foraminifera.
+
+The next thing to be noticed is, that there are a number of organisms
+which constitute a debateable ground, and are claimed now by the
+botanist, and now by the zoologist. While the latter insists on calling
+them Protozoa (Primitive Animals) the former would have them Protophyta
+(Primitive Plants). The fact is that in these organisms of the first
+grade, the distinction between "plant" and "animal" has not become a
+hard and fast line; and the disputed forms may be best described as
+links between the two. The chemistry of nutrition is probably more to be
+relied upon as a distinction, than the difference of structure. It is
+here that the two groups, plants and animals, start upon different
+roads, and many of the differences in structure must be regarded as the
+direct result of the fundamental difference in the mode of nutrition.
+The following very instructive remarks on the subject are taken from
+Professor Hertwig's valuable book "The Biological Problem of To-Day,"[B]
+pp. 111, 112.
+
+[B] "The Biological Problem of To-Day, Preformation or Epigenesis," by
+Professor O. Hertwig. Translated by P. C. Mitchell. Heinemann, 1896.
+
+"The different mode of nutrition of animals results in a totally
+different structural plan. Animal cells absorb material that is already
+organised, and that they may do so their cells are either quite naked,
+so affording an easy passage for solid particles, or they are clothed
+only by a thin membrane, through which solutions of slightly diffusible
+organic colloids may pass. Therefore, unlike plants, multicellular
+animals display a compact structure with internal organs adapted to the
+different conditions which result from the method of nutrition peculiar
+to animals. A unicellular animal takes organic particles bodily into its
+protoplasm, and forming around them temporary cavities known as food
+vacuoles, treats them chemically. The multicellular animal has become
+shaped so as to enclose a space within its body, into which solid
+organic food-particles are carried and digested thereafter in a state of
+solution, to be shared by the single cells lining the cavity. In this
+way the animal body does not require so close a relation with the medium
+surrounding it; its food, the first requirement of an organism, is
+distributed to it from inside outwards. In its further complication the
+animal organisation proceeds along the same lines. The system of
+internal hollows becomes more complicated by the specialisation of
+secreting surfaces, and by the formation of an alimentary canal, and of
+a body-cavity separate from the alimentary canal. In plants it is the
+external surface that is increased as much as possible. In animals, in
+obedience to their different requirements, increase takes place in the
+internal surface. The specialisation of plants displays itself in organs
+externally visible--in leaves, twigs, flowers, and tendrils. The
+specialisation of animals is concealed within the body, for the internal
+surface is the starting-point for the formation of the organs and
+tissues."
+
+    TABLE SHOWING THE CLASSIFICATION OF THE PROTOZOA
+
+    =Grade I.=                              { RHIZOPODA, OR
+                                            {   GYMNOMYXA.
+    UNICELLULAR ANIMALS.      =PROTOZOA=    {
+                                            { INFUSORIA, OR
+                                            {   CORTICATA.
+
+
+
+
+CHAPTER V
+
+THE COELENTERATA
+
+
+Next after the animals that consist of one cell only we have to consider
+the group of animals among which the lower kinds, at any rate, consist
+of a number of cells arranged in two layers. The representative of this
+group that the reader is most likely to meet with is the Sea-Anemone,
+the Coral animal probably he will be content to know from pictures.
+
+Everybody who has been accustomed to take a little interest in natural
+history, remembers the use of the old-fashioned term "Zoophyte." It was
+a name given to animals like those named above, which have a flower-like
+appearance, due to the possession of a set of petal-like arms or
+tentacles, placed all round the mouth; its literal meaning was animal
+plant, in allusion to the flower-like form. The great French zoologist,
+Cuvier, gave the group name Radiata to animals of this kind. This name
+is now not much used, because we have learnt to emphasize other
+peculiarities possessed by these animals, as well as that of radial
+symmetry, viz., their two-layered body-wall and simple digestive space
+(see p. 36). The group called Radiata by Cuvier, included, too, a number
+of animals which are widely separated from the "Zoophytes" in modern
+systems of classification.
+
+Sea-Anemones may be found on almost every rocky part of the English
+shores. Look for them in pools towards low-tide mark; if uncovered by
+the water, they will be found with the arms drawn in, so that the animal
+looks merely like a small round knob of shiny opaque coloured jelly; if
+covered by the water, they will usually be found open, that is to say,
+with the arms (often called Tentacles) spread out. In the middle of the
+circle of arms is the mouth; and the apparent "flower" possesses an
+excellent appetite, as will readily be seen if any unfortunate little
+shrimp or sea-snail should come within reach of the arms. The latter
+will then at once contract upon it, and draw it into the mouth. Touch
+any of the common Sea-Anemones, and you will find that it is firmly
+fixed to the rock; at an early period of life it becomes fixed, and
+practically it remains always in one place, although a slight movement
+of the base is sometimes possible. Hence the advantage of the "radial"
+structure, for the arms reach equally in all directions round that most
+important centre of activity, the mouth. The most common kind of
+Sea-Anemone is of a dull dark red colour, and small in size; but others
+are large and brilliant in colouring. No uncoloured drawing would convey
+much idea of their beauty: the reader should consult the works of the
+late P. Gosse, an authority on Sea-Anemones, in whose books many
+beautiful illustrations will be found.
+
+A much smaller animal than the Sea-Anemone is found in fresh water and
+is called _Hydra_. Its arms or tentacles are longer in proportion to its
+body, especially in one species, than is the case in the Sea-Anemones.
+Hence its name, fancifully derived from the seven-headed serpent of
+Greek Mythology, the Hydra killed by Hercules, which may be supposed to
+have presented a similar straggling appearance. The diagram on page 36
+represents a section through the middle of the body, only without the
+arms.
+
+Unlike the Sea-Anemone, the Hydra can walk about. This it does in a very
+awkward manner, much in the same way as the Caterpillar known as the
+"Looper," clinging first with the front and then with the back extremity
+of the body (for head and tail they can hardly be called in so simple an
+animal as the _Hydra_, although the Looper caterpillar boasts both head
+and tail).
+
+The _Hydra_ is so small an animal that it appears to the unaided eye
+merely as a tiny speck. It may be found anywhere in British ponds and
+ditches, standing on water-weeds. Like the Sea-Anemone it preys on
+animals smaller than itself. Nature has provided it with minute stinging
+cells, which benumb its prey; and in this all the animals of the
+Coelenterate group resemble it.
+
+One of the most curious things about the Hydra is, that it often throws
+out buds. It can, of course, produce eggs which are fertilized and
+hatched in the usual way of eggs; the buds are an additional way of
+multiplying itself.[C]
+
+[C] We may recall in comparison the way trees may be propagated by slips
+independently of flowers producing the seeds of the trees.
+
+These buds are at first merely swellings, in which both of the layers
+of the body join: they grow larger; become provided with tentacles and
+a mouth, like the parent, and finally are cast off as independent
+animals.
+
+For this reason the group to which _Hydra_ belongs has received the name
+of Eleutheroblasteæ, the animals with free buds. But Hydra has many near
+relations in which these buds are not so cast off, but remain attached
+to the parent; and they in turn may produce others which also remain
+attached.
+
+In this way, groups or colonies are formed, consisting of large numbers
+of individuals, and possessing a common stalk or stock which is formed
+by degrees as the process of multiplication goes on. The corals and the
+corallines are familiar examples of this.
+
+The matter is complicated by the fact that either the separate animals
+or the flesh of the stock, or both, may secrete within themselves a hard
+supporting structure forming what is known as Corals. This may be
+developed in such a complicated manner, that instead of the coral
+appearing to be the product of the animal, the animal seems to be
+inserted in the coral, into which indeed it can retract itself for
+shelter.
+
+The Corallines, on the contrary, secrete a leathery coating or sheath
+outside themselves and the stock. The leathery case is fairly
+transparent, so that on magnifying the creature the flesh of the common
+stock, as well as of the stalks of individual animals, may be seen
+inside. The "heads" of the animals poke out at the end of each branch
+(see Fig. 9).
+
+The _Hydra_, with which we started, had always the power of producing
+eggs; each animal could do so, besides producing buds. But in our
+Colonial Coralline this is not necessarily so. Some individuals lose the
+power of producing eggs. Others can do nothing else, and become greatly
+altered in structure, often losing the power of developing tentacles,
+and exhibiting other changes. So much are they altered sometimes that
+they seem to be mere buds, not separate animals at all.
+
+In other cases a still more surprising thing happens. The bud that is
+destined to produce eggs falls off, and becomes quite independent of the
+colony; more than this, it becomes quite different in appearance from
+the members of the colony: and instead of being a Hydra-like animal it
+becomes a jelly-fish. But the eggs of this jelly-fish do not produce
+jelly-fishes: they produce a more or less Hydra-like animal which gives
+rise by budding to a fresh colony. This is what is known to Zoologists
+as "alternation of generations."
+
+Now comes a puzzling question--Which part of this family group shall we
+select and call it an "animal"? Is each Hydroid of the colony an animal,
+and the jelly-fish another animal? Zoologists say "No": from the
+development of one egg, to the production of another, is the cycle that
+constitutes an individual animal. So we have the puzzling result in
+nomenclature, that an "individual" consists of a very large colony of
+creatures in one place, together with a perfect shoal of creatures quite
+unlike it, floating miles away from it on the ocean. What name must we
+give to the units, so curiously connected with one another? Zoologists
+call them "Zooids" (animal-like parts) or "persons."
+
+This is the story of the jelly-fish as originally told. But there are
+innumerable variations upon it. There are kinds of jelly-fish that
+produce jelly-fish and have no Hydroid stage at all. Sometimes the
+"persons" of the colony present many varieties, each taking up some
+different task for the community. Some may be "nutritive persons,"
+_i.e._ commonplace Zooids that have mouths and eat food; some
+"protective persons," reduced to mere folds or sheathing processes to
+guard the others; some are "stinging persons" armed with enormous
+quantities of thread cells. Then the whole colony may be like the
+jelly-fish, a floating affair, and not fixed at all.
+
+[Illustration: FIG. 9.--An example of the Hydrozoa. A, branch of a
+Coralline, _Sertularia Ellisii_, magnified. B, the same, more highly
+magnified.]
+
+We have several times above referred to the animals known as corallines.
+It may almost be assumed that the ordinary reader knows what these are;
+if not, a little search among the treasures of the sea-shore will almost
+certainly reveal some of them, living or dead. The texture and
+appearance of the dead stems remind one of soft horn or dried gelatine;
+the branching arrangement of the stems and the little cells disposed at
+the ends of the branches will easily be shown under slight
+magnification. Most people will remember the rage for dyed corallines,
+by which all the fancy shops and florists were possessed a few years
+ago. The corallines, dyed a bright emerald green, or a dull red, which
+were used for decorations at that time, were usually a variety of the
+Bottle-brush Coralline, found on English shores; but sometimes commoner
+kinds were employed.
+
+Fig. 9 shows an example of a coralline, slightly magnified in A, and in
+B much more highly magnified, so as to show the individual hydra-like
+zooids, each with its circle of tentacula.
+
+The Sea-Anemone and the Hydra respectively represent the two great
+groups of the Coelenterata, named after them, the Anthozoa
+(Flower-animals), and the Hydrozoa (Hydra-animals). The corals are
+forms of the Anthozoa, single or colonial, which possess a skeleton.
+
+[Illustration: FIG. 10.--_Gorgonia verrucosa_, from Guernsey, nearly
+one-third of the natural size.]
+
+[Illustration: FIG. 11.--Corals. _A_, _Acanthoporia horrida_.
+_B_, _Meandrina strigosa_. _C_, _Madrepora divaricata_. _D_, _Fungia
+papillosa_. _E_, Red Coral, _Corallium rubrum_. _F_, _Stylaster
+sanguineus_.]
+
+The above diagram shows examples of the Anthozoa. Fig. 10 is _Gorgonia_,
+the Sea-Fan; while Fig. 11 represents corals of six different kinds.
+
+Besides the two great groups we have named, the Hydra-like animals
+and the Sea-Anemone-like animals, the Coelenterata contain a third
+group, the Ctenophora, or Comb-bearers, so called on account of their
+possessing bands of cilia, fancifully compared to the teeth of a comb.
+At first sight most of them somewhat resemble jelly-fishes, being
+transparent forms swimming near the surface of the sea. They are
+carnivorous, and some of them highly phosphorescent at night. The
+gastric cavity is divided up into branches. The representatives of the
+Ctenophores, most often seen on our own coasts, are small rounded forms.
+
+Two remarks must be added before quitting the subject of the
+Coelenterata.
+
+Firstly, the description of them as two-layered Animals is one that only
+applies typically and to the simpler forms. In others, such as the
+jelly-fishes, there is an intermediate layer of jelly, which appears to
+acquire a cellular structure by the immigration of cells derived from
+the primary layers. Thus we see, within the group of the Coelenterata,
+the gradual establishment of that third body-layer, which is found in
+all animals of higher structure. Scarcely indicated in _Hydra_, as a
+faint trace of a boundary-line (lamella) between the ectoderm and
+endoderm, it attains a good thickness in the Jelly-fish and Ctenophora.
+In animals of higher structure the third body-layer, being now fully
+established, is cellular from its beginning in the embryo; in the
+Coelenterata its gradual formation is to be traced.
+
+Secondly, it must be remarked that the colonial structure and the
+arrangement sometimes concomitant with it of "alternation of
+generations," is by no means confined to the Coelenterata. Both are seen
+in other forms of life, in which the units, or zooids, differ greatly in
+structure from those of this group.
+
+TABLE SHOWING THE CLASSIFICATION OF THE COELENTERATA
+
+
+                                            { HYDROZOA, or
+                                            {   HYDRA-LIKE
+                                            {   ANIMALS.
+    =Grade II.=                             {
+                                            {
+    THE TWO-LAYERED      =COELENTERATA.=    { ACTINOZOA, or
+    ANIMALS.                                {   SEA-ANEMONE-LIKE
+                                            {   ANIMALS.
+                                            {
+                                            {
+                                            { CTENOPHORA.
+
+
+
+
+CHAPTER VI
+
+THE SPONGES
+
+
+Many who are familiar with the domestic sponge have never seen a sponge
+in a growing state, and would find it almost impossible to realise that
+a sponge may be a thing of beauty. And yet sponges are quite common on
+the rocky shores of our own country. It is true that they do not form
+large masses, like the sponges grown in warmer seas, which we import;
+but the smaller growths, massed together, often cover a considerable
+space of rock, and are conspicuous by their beautiful colouring. Some
+sponges are crimson, and some green; while one of the commonest is a
+brilliant orange-yellow. The latter may often be found near low-tide
+mark, on a shelf of rock under growing seaweed. If the explorer has
+any doubt what the object is, it may easily be identified by the
+touch, which though moist and firm in the growing state, is still the
+unmistakable "feel" of sponge. Where the receding tide exposes a large
+surface of steep rock, for instance in caves, sponges may be found
+covering the rocks as thickly as mosses do on land. Masses of dead
+sponge, consisting of branching parallel fingers a few inches long, may
+often be found in the dead state, washed up on the shore; these are the
+usual drab colour of a dead sponge.
+
+The encrusting sponges which grow on rocks present a mass, so to speak,
+of little hillocks: in kinds which attain a larger growth, these may
+almost be described as branches. Each little hillock or branch has
+a hole at the top; and on the exterior of the rounded mass of the
+bath-sponge may be found numbers of such holes. We should naturally
+suppose that these holes were the mouths of the various sponge branches,
+especially since they lead to the central cavity of the branch, and thus
+to that of the whole sponge; and indeed they are known by the Latin name
+of "oscula," little mouths. They are, however, nothing of the sort;
+the sponge once had a mouth, a single one, when it was young, but the
+adult sponge has lost it. For the young sponge is at first a little
+free-swimming, two-layered animal of the type which has been described
+above as the gastrula larva. When it gets old enough to settle down in
+life, it sinks upon some suitable surface, and becomes fixed to it,
+mouth downward: the mouth is thus lost. How, then, is the animal to be
+fed? As it grows, there is developed in its substance a system of hollow
+spaces, which communicate with the exterior by means of microscopic
+pores. Through the latter, water is drawn in, and passes, after devious
+wanderings, to the central cavity of the animal, whence it is expelled
+by the so-called osculum. At first, the young sponge has but one cavity
+and one osculum; but by degrees the sponge branches and spreads, the
+cavity of each new portion remaining in connection with the main cavity.
+If, as they grow in size, the branches touch one another, they sometimes
+coalesce--a fact which renders the growth of the sponge in some cases a
+very complicated matter.
+
+It will be seen from the above description that the sponge is a sort of
+living filter. As the water passes in through the pores, it deposits in
+the substance of the sponge all the little organisms that it contains;
+on these the sponge feeds.
+
+It will naturally be asked, how does this living filter work? Water will
+not pass through small holes to flow out again at large ones in an
+upward direction, unless helped by some mechanism. How is this supplied?
+By the industry of the cells of the sponge. Its canal-system includes a
+set of wide chambers, lined with cells which have long cilia, called
+flagella. These flagella, constantly moving in one direction (like the
+fan of a ventilator), create a current, which passes the water on with
+such force that it reaches the central cavity, whence it is expelled
+through the oscula. These chambers do not communicate directly with
+the exterior. They are closed, except at certain small holes, the
+"prosopyles," where they take in the water that enters from spaces
+connected with the pores. At the main end of the chamber is an aperture
+called the "apopyle," capable of being partly closed, and leading into
+an excurrent passage. This last communicates with the central cavity of
+the sponge.
+
+It will be seen that the topography of the sponge is a very complicated
+business. All its details have been studied by means of thin sections
+specially prepared and placed under the microscope (see p. 183); in
+these the labyrinth of canals and chambers is seen cut through at
+various points; the cells lining them and dividing them may be
+individually studied. The passage of water through the sponge was first
+observed by Robert Grant; many of the most recent discoveries regarding
+the structure of sponges we owe to Professor Sollas.
+
+We have not yet explained what our living filter does with its food
+when it gets it. The ciliated cells of the internal lining take in solid
+particles just as Amoeba does; and from these they may be passed on to
+the cells of the middle layer, amoeboid cells, which can move about.
+These cells are considered to be derived from the primary layers of the
+body, especially the inner one, and to have wandered into a cellless
+middle layer, comparable in nature with that of some Coelenterates.
+
+The sponge is full of firm or gritty particles, which form its skeleton,
+and remain when the sponge is dead, and the softer parts decayed. These,
+when magnified, often present beautiful and curious shapes. The use of
+them is not only to support the body, but also to prevent the sponge
+from being eaten by other animals.
+
+There is found in the English canals and rivers a small, fresh-water
+sponge, usually greenish in colour. This is named _Spongilla
+fluviatilis_, the River-sponge, and affords an exception to the usual
+marine distribution of sponges. In the winter it dies gradually away, at
+the same time forming asexual buds, or "gemmules," in the interior of
+its substance, which are liberated in the spring, and become young
+sponges.
+
+TABLE SHOWING THE CLASSIFICATION OF SPONGES
+
+                                       { CALCAREA, WITH CALCAREOUS
+                   {      =PORIFERA.=  {   SKELETON.
+    =Grade II.=    {                   {
+    Two-Layered    {                   {
+    Animals, or    {                   { NON-CALCAREA, WITH
+    Acoclomata.    {                   {   SKELETON ABSENT OR
+                   {                   {   FLINTY.
+                   { =COELENTERATA.=
+
+Some of the marine sponges are parasitic. Most people have doubtless
+found on the sea-shore now and then a dead oyster-shell, completely
+riddled with small round holes, very similar in appearance to those seen
+in "worm-eaten" wood. These are the work of _Clione_, a parasitic sponge
+which is very fatal to the oyster. At first sight it seems a puzzle how
+the sponge made its way into the hard shell; it has no mouth to bite or
+suck its way into the solid substance. The cells of the sponge, however,
+wear away the lime of the shell by means of some acid chemical action.
+Not only so, but they can attack stones as well, when these consist of
+limestone; and on some parts of the coast bits of sponge-eaten limestone
+washed up on the beach are quite common objects. They are pierced all
+through by holes, so that their appearance would suggest a sponge carved
+in stone, but for the fact that the holes are fairly uniform in size.
+Such stones, lying on the shore, often puzzle the finder, when they
+contain no apparent trace of the tenant that has worked its way through
+them.
+
+The sponges have received the name of Porifera, on account of the
+structure above described. They are often classed with the Coelenterata,
+because, among other reasons, they practically belong to the two-layered
+type of structure, and because they form a complex organism that may
+almost be called a colony. But some prefer to place them in a group by
+themselves, apart from the Coelenterata. The chief reason of this
+is that the sponges, as compared with a primitive two-layered type
+indicated by their own larvæ, are turned upside down, the mouth being,
+as above stated, originally situated at the fixed end.
+
+
+
+
+CHAPTER VII
+
+WORMS
+
+
+When the great naturalist, Linnæus, framed his classification of the
+animal kingdom, he included in the division Vermes or Worms, nearly
+everything except the vertebrates and insects.
+
+This assemblage would have been more correctly styled if instead of
+"Vermes" it had been described as "animals unsorted." Subsequent
+zoologists have by degrees picked out and separated from the Vermes
+first one group of animals and then another. But the process is still
+going on, and several of the groups which are still classed under the
+name of "worms" might, with very great justification, be separated from
+each other; it is custom, rather than family resemblance, that accounts
+for their being retained under one heading.
+
+Widely although the various "worms" may differ from one another, one
+thing may be stated regarding the most of them, and that is, that
+they "crawl"; that is to say, they move along by means of successive
+contractions of successive parts of the muscular wall of their elongated
+bodies. This "crawling" mode of progress is the chief thing involved
+in the popular idea of a worm; but the popular definition of a
+worm includes also the larvæ of insects, such as caterpillars and
+beetle-grubs. The latter, it must be noted, crawl with the assistance
+of legs, while the true worms crawl without any such assistance. Any
+adornments that they may possess, whatever else they may be, are not
+legs.
+
+The worms were formerly included along with the insects and lobsters, in
+a division called Annulosa, or, Ring-bodied animals, but it has now long
+been recognised that the latter are worthy of a division to themselves.
+It will easily be seen, however, that the term Ring-bodied animals is
+very appropriate to all of them. If we look at either an earthworm or
+a lobster, we can but recognise that the body consists of a number of
+successive parts very similar to each other; and since the body of
+each is, in section, more or less round, these successive parts may
+very aptly be termed rings. Modern writers, however, prefer to call
+these parts not rings, but Metameres, _i.e._ successive parts. The
+symmetrical arrangement of the body in a series of such parts is
+called "Metamerism"; and the animals which possess it are said to be
+"Metameric" in structure. Sometimes also the successive parts are spoken
+of as "segments." Compare Fig. 12; _A_ and _C_ show the successive
+body-rings of worms.
+
+The earthworm, with its many rings, is one of the higher forms among
+the worms. Among the lowest forms there are worms in which the ring
+structure cannot be detected. Between the limits thus marked out, there
+lies, so to speak, the battleground of modern zoology. For the origin of
+metamerism, and the pedigree of vertebrates, are among the questions
+that are being discussed in connection with various groups of the worms.
+
+Among the lowest forms of worms are the Planarian worms, already alluded
+to as examples of the third grade of animal existence. These belong
+to the class Turbellaria, which is represented by plenty of both
+fresh water and marine forms in our own country and on its coast. The
+Turbellaria are divided into groups called Acoela, Dendrocoela, and
+Rhabdocoela. These names allude to the intestine, which in the first
+group is wanting, in the second branched like a tree, and in the third
+straight. The Cestoda or tape-worms, which absorb nourishment through
+the skin, and therefore need no alimentary canal, and possess none; and
+the Trematodes, represented by the Liver-fluke, which infests sheep,
+together make up the group of flat-worms (Platyhelminthes), of which
+mention has already been made (p. 44). In all of them the body is more
+or less flat, and the digestive cavity, like that of Coelenterates, has
+but one opening, the mouth. The life-history of parasitic worms is
+described in a well-known volume by Leuckart, which forms the basis of
+our knowledge on the subject. Since its publication, discoveries
+regarding parasites have been constantly added by other observers.
+
+The history of the Liver-fluke is a most complicated example of
+alternation of generations. The adult form infests the sheep's liver.
+There it produces eggs, which afterwards find their way into water. Here
+they die unless they find their way into a certain water-snail, which
+many of them do. Within this snail--_Linnæa truncatula_--the egg
+develops into a sac-like body, called a sporocyst. This produces within
+itself numbers of a small creature which is called the Redia form. These
+in turn produce a tailed form, called a Cercaria, which gets out of the
+snail, swims in water, and finally settles down on some plant. Here it
+is eaten by an unfortunate sheep, within which it develops into the
+adult fluke.
+
+The other great divisions of the Vermes are as follows: The Nematodes
+or thread-worms, a group of parasites which includes the dreaded
+_Trichina_; the Nemertines, a group mostly carnivorous, possessing a
+curious proboscis, and often an armed skin; the Leeches or Hirudinea,
+and finally, the Chætopods (Bristle-footed Worms), the highest group
+of all, containing the forms often spoken of as Annelides--_i.e._
+Ring-shaped Worms.
+
+These last are again subdivided into the following: The Archiannelida or
+Primitive Annelids, some of which have a curious ciliated larva, already
+referred to (p. 42) as the typical Trochosphere or Wheel-ball; the
+Oligochæta (Few-Bristles), which include the familiar earthworms; and
+the Polychæta (Many-Bristles). Of the latter, some, the Tubicola, live
+in tubes which may or may not be fixed to some object; while others, the
+Errantia, or Wanderers, are free and very active. _Nereis_, the Rainbow
+Worm (p. 159) may be named as an example. Our illustration shows
+instances of each group. _A_ is the Sea-Mouse, a bristly creature so
+named by some very imaginative person. It has two kinds of bristles,
+long and short, the former being possessed of a peculiar lustre (see p.
+73). _C_ is _Syllis_, one of a very curious family of worms. In both _A_
+and _C_ are seen a row of paired appendages; these are not "legs," but
+expansions called "parapodia" which serve the purpose of legs, besides
+which they frequently act as breathing organs, a special part being
+appropriated to this purpose. Each of these animals is active and
+carnivorous, and has a head. The Syllidæ are remarkable for the very
+peculiar way in which they divide, new individuals being formed and
+cast off from the end of the body. There is, however, a deep-sea form
+of _Syllis_ which divides in a very odd manner, giving rise to new
+individuals placed transversely. The result is a most extraordinary
+looking creature, a network of worms with numerous heads, each branch
+being eventually provided with one of its own.
+
+[Illustration: FIG. 12.--Worms. _A_, a Sea-Mouse, _Aphrodite
+aculeata_; _B_, _Terebella littoralis_; _C_, _Syllis_; _D_,
+_Serpula vermicularis_; _E_, _Spirorbis nautiloides_, on a piece of
+seaweed.]
+
+The tube-dwelling worms are represented in our picture by _Terebella_,
+_Serpula_ and _Spirorbis_, all very common forms on the English coasts.
+The _Terebella_ glues around its body a number of grains of sand and
+bits of shell, thus forming a case; the projecting threads at the head
+end are the gill-filaments, borne by the anterior segments of the body.
+These are plumed; the thread-like structures which are seen to lie in
+front of them are the tentacles or feelers. _D_, _Serpula_, is common
+on shells and stones. The animal has a plumy bunch of gill-filaments,
+brilliantly coloured, and a stopper with which it can close the mouth
+of its tube. This precaution is necessary to keep out its predatory
+cousins belonging to the Errantia, who poke in their heads and eat the
+tube-dwelling worms. _E_ is _Spirorbis_, a minute form with a coiled
+tube, which looks at first sight like a small univalve shell. It is
+common everywhere, on shells and stones, and encrusting Fuci and other
+seaweeds, which it sometimes covers almost completely. Spirorbis also
+has plume-like gills and a stopper. In the latter is a cavity where the
+creature's eggs are incubated for a time.
+
+The reader will have no difficulty in finding and identifying both
+_Serpula_ and _Spirorbis_. _Terebella_ is frequently washed up on a
+sandy shore. On the Lancashire coast one may feel sure of finding this
+and many other sand-dwelling animals, after an east wind. The east wind,
+driving back the water at low tide, kills these creatures with cold,
+and presently they are washed up dead or dying by the high tide.
+_Pectinaria_, another worm with a tube of sand-grains, in which,
+however, the body lies loosely within the tube, may also be found in
+thousands under the same circumstances.
+
+TABLE SHOWING THE CLASSIFICATION OF VERMES OR WORMS
+
+    =Grade III.=    {                  {
+     WITH MESODERM, { PLATYHELMINTHES, { _A._ TURBELLARIA or PLANARIANS.
+      BUT WITHOUT   {   or FLAT-WORMS. { _B._ CESTODA or TAPE-WORMS.
+      BODY-CAVITY.  {                  { _C._ TREMATODA or FLUKE-WORMS.
+
+    =Grade IV.=     { NEMERTINES.
+     WITH MESODERM  { NEMATODA or THREAD-WORMS.
+      AND           { HIRUDINIA
+      BODY-CAVITY.  {   or LEECHES. { _A._ ARCHIANNELIDS.
+                    {               {
+                    { CHÆTOPODA.    { _B._ OLIGOCHÆTA.
+                                    {
+                                    { _C._ POLYCHÆTA. { (_a_) Tubieola.
+                                                      { (_b_) Errantia.
+
+We must not forget to say something regarding the most commonly known
+member of the Vermes, the familiar earthworm. The worms are the first of
+the great group of animal life in which we find true land animals. There
+are terrestrial forms among the lowest worms, at least forms that live
+in earth that is damp; but the earthworm is in the strictest sense a
+terrestrial animal. Darwin showed that it not only dwells in the soil,
+but is in a sense the manufacturer of soil, since the fertility of the
+earth depends greatly upon the work of earthworms. They pass the soil
+through their bodies, digesting the organic particles they find in it,
+and thereby loosen the soil, reduce it to a state of fine division, and
+render it more fit to support the growth of plants. The "worm-casts"
+formed by the soil that the earthworm has passed through its body may
+not have been noticed by everybody. More obvious are the worm-casts in
+sand left by the sand-dwelling marine annelids. These everyone must have
+seen who has walked on a sandy shore at low tide.
+
+The worms include many puzzling forms, which have not been alluded to
+here. Among these must not be forgotten the Rotifers, or wheel-bearing
+animals. These are of minute size, and when first discovered were
+therefore placed amongst the Infusoria. They are common in ponds.
+
+
+
+
+CHAPTER VIII
+
+ARTHROPODA, THE LOBSTERS, SPIDERS AND INSECTS
+
+
+The above is a very descriptive name for a division which includes the
+Crabs and Lobsters and the Insects. Formerly they were included, along
+with the worms, under the name Annulosa, the Ringed Animals. They
+resemble these as possessing what is termed metameric symmetry, but
+they are distinguished from them as the Leggy Animals, a fact which is
+explained in the name, Arthropoda, joint-footed. Worms, as we have seen,
+have no true legs, but the Arthropods, theoretically, have a pair of
+legs to every ring. In some of the lower members of the group this is
+literally the case, the Centipedes, or hundred-footed animals, for
+example (Fig. 13). In higher forms the number of legs is greatly
+reduced; several successive rings may become merged with one another,
+losing, along with their independence, their legs. The true Insects,
+thus, have only three pairs of legs and the Spiders four.
+
+[Illustration: FIG. 13.--A Centipede, _Lithobius elongatus_, from Tunis,
+slightly reduced in size.]
+
+What are theoretically regarded as legs, however, may practically be
+turned to many other uses, according to the position of the particular
+body-ring to which they are attached. Thus, in the case of a body-ring
+near the mouth, we find such things as "jaw-feet," maxillipedes--that is
+to say, legs used for jaws. It consequently results that zoologists are
+sometimes driven to speak of "walking legs," or, hiding the tautology
+under a Latin phrase, "ambulatory legs"; and absurd although this may
+seem, it is sometimes quite necessary for the sake of accuracy. It is
+therefore more convenient to speak of the "appendages" of a body-ring
+than of its legs. For this vague term can be applied equally to all the
+row, whatever their uses. Among the different forms taken by the
+"appendages" are those of "antennæ," long, hair-like feelers attached to
+the head; "chelæ," or claws, such as the large claws of the lobster;
+"cheliceræ," or "claw-horns," tearing appendages attached to the head;
+"mandibles," mouth appendages used for biting, etc., etc. The reader who
+wishes to attain a clear idea of the structure of a segmented animal,
+and of the ways in which its parts are modified, should consult Huxley's
+classical study of "The Crayfish" (International Science Series).
+
+The Arthropoda include two main groups--the Crustacea, or Jointed
+Animals of the water, which breathe by gills; the Insects, or Jointed
+Animals of the land, which breathe through tubes in their sides, called
+tracheæ.
+
+The Crustacea include the familiar Crabs and Lobsters. These are among
+their highest forms as well as their largest, and if we begin at the
+beginning we must seek much smaller forms. The group called Entomostraca
+include the so-called Freshwater Flea, a very active little thing found
+in English ditches, and a great many other freshwater forms: also the
+little Cypris, which has a shield forming a sort of bivalve-shell, and
+is interesting from its wide occurrence as a fossil form. Most of the
+Entomostraca have a larval form called a Nauplius; but this larva
+refuses to tell us anything about the past history of the Arthropods. It
+is itself already a jointed animal with legs. So we see that the
+Arthropods, unlike the worms and the Chordata, have obliterated all
+record of their poor relations. The parasitic "fish-lice," so-called,
+are entomostracous Crustacea, often greatly degenerated in consequence
+of their habit of life. Some live in the gill-chambers of a fish, some
+on, or even embedded in the skin.
+
+[Illustration: FIG. 14.--Shell of the Bell Barnacle, _Balanus
+tintinnabulum_, one-half the natural size. The figure shows several
+successive generations, perched one upon another.]
+
+Among the most curiously modified forms of the Crustacea are the
+Barnacles or Cirripedia. These creatures, like the sponges, have a
+free-swimming larvæ, which eventually fixes itself by its anterior end,
+so that the adult animal passes its existence upside down. The young is
+an ordinary little creature with jointed legs, but the adult protects
+itself by a strange armour of shell. An intermediate stage exists in
+which the creature eats no food; it has therefore been compared with the
+chrysalis of insects. At the top of the adult shell two little valves
+open and shut, allowing the legs to dart out and seize upon prey. These
+legs, gathered into a bunch, and extended and retracted together, remind
+one of the fingers of a hand opening and closing. They are clothed with
+a fringe of "cirrhi" or small processes; hence the name of the group.
+The Common Barnacle of our own shores, sometimes called the Acorn-Shell,
+is found on shells and stones, and often on those that are left
+uncovered between tides. In places where the rocks of the coast are
+very steep, a belt of white, several feet or yards deep, may often be
+seen above low-water mark. This white zone, when examined more nearly,
+is found to consist of barnacles, so crowded together that they obscure
+the natural colour of the rock. The Common Barnacle is one of the
+smaller species of the genus: in warmer seas barnacles attain to a much
+greater size (Figs. 14 and 15).
+
+[Illustration: FIG. 15.--Shells of a Barnacle, _Balanus hameri_, found
+in European and North American Seas, natural size.]
+
+The higher Crustacea, Malacostraca, include the familiar Crabs and
+Lobsters, Decapoda. The lobsters receive the name of Macrura or
+Big-tails; associated with them are the Shrimps and the Hermit-Crabs
+(Fig. 16). The latter are therefore not crabs at all, but somewhat
+divergent lobsters. Their tails are soft, and they thus require
+protection: they choose the dried shell of some univalve mollusc and
+live in it (Fig. 16). How far the case is that they need a house because
+their tails are soft, and how far the contrary is true that their tails
+are soft because they live in a house, it would be difficult to say.
+Readers of another volume in this series, Professor Hickson's "Story of
+Animal Life in the Sea," will remember that the hermit-crab often offers
+a curious instance of "commensalism" or partnership with other animals.
+The hermit-crab was, in fact, one of the earliest instances in which
+such a partnership was observed, the companion being in this case a
+sea-anemone perched on the shell in which the crab lives.
+
+[Illustration: FIG. 16.--Hermit Crabs. _A_, _Aniculus typicus_, from
+the Indo-Pacific Seas, one-half of the natural size. _B_, _Caternus
+tibicen_, from the Indo Pacific Seas, slightly enlarged.]
+
+The true Crabs are called Brachyura, or Short-tails; for obvious
+reasons, the tail of a crab being very curiously modified and tucked in
+under the carapace or "shell." A form exceptional in the fact that
+frequents the land is the Land-Crab of the West Indies (Fig. 17).
+Another land crustacean, _Birgus latro_, the Robber Crab, belongs to the
+previous group.
+
+[Illustration: FIG. 17.--Land Crab, _Gecarcinus ruricola_, from the West
+Indies, one-half of the natural size.]
+
+In addition to the above the Malacostraca include the Arthrostraca, or
+crustaceans which have the front of the body jointed as well as the
+tail, so that there is no large shield formed by the fused armour of
+several segments (cephalo-thoracic shield, _cf._ Figs. 16 and 17), as in
+crabs and lobsters. The Amphipoda, or Sand-hoppers, sometimes called
+Sand-fleas, are familiar examples of these. There are several common
+kinds found on our English shores, and sometimes they appear in such
+numbers, hopping above sand or seaweed left by the tide, that they seem
+to form a sort of cloud, every unit of which, however, is but in the air
+an instant, falling and giving place to some other, while it prepares
+for a fresh hop. The so-called Freshwater Shrimp, _Gammarus_, is another
+common member of the Amphipoda. Fig. 18 shows the general form of a
+Sand-hopper. Nearly allied are the Isopoda or Wood-lice, interesting
+because they are among the few terrestrial forms of the crustacea; they
+live, however, in damp places, and are but too well-known in gardens,
+where the gardener often mis-names them "insects."
+
+[Illustration: FIG. 18.--A Sand-hopper, _Pallasea Cancellus_, from
+Siberia, natural size.]
+
+[Illustration: FIG. 19.--A South American Spider, _Ctenus ferus_, from
+the Amazon region, natural size.]
+
+The mention of terrestrial forms would naturally bring us to the
+discussion of the true Insects. In the Arthropoda we for the first time
+meet with terrestrial animals except in scattered instances, and the
+true Insects are the largest and most important group of these. There
+are, however, various creatures belonging to the Arthropoda which are
+neither Crustacea nor yet Insects. Among these is the familiar spider,
+an "insect" in popular language, but not so described by the zoologist.
+Among other differences, the true spiders have eight legs, whereas the
+true insects have only six. Fig. 19 shows a typical spider; the eight
+jointed legs are attached to the thorax ("breastplate"); with the latter
+the head is united. The abdomen, as in insects, is formed by the fusion
+of several segments, and has no legs, but it has, however, out of sight,
+the spinning legs or "spinnerets," out of which the thread of the
+spider's web is spun. The venom of the spider is not a fable; spiders
+have poison-glands with ducts which open on the tops of the cheliceræ.
+They dispose of their prey by sucking it; they do not swallow solid
+food. The habits and webs of spiders are familiar to every one: their
+nests, as a rule, are only noticed by close observers. The nest is made
+of spun threads closely felted together to form a round hollow ball.
+This the house-spiders hang on a wall or among the rafters of a roof.
+There are, however, spiders which build their nests under ground; and in
+this case the nest may be conveniently furnished with a lid, which can
+be pushed up when the animal wishes to come out. Fig. 20 shows the nest
+of the Trap-door Spider, so called from the construction of its nest.
+
+[Illustration: FIG. 20.--Nest of the Trap-door Spider, from the South of
+France, three-quarters of the natural size.]
+
+Fig. 21 shows a spider-like animal which, at first sight, seems to have
+five pairs of legs. In fact, however, it has only three pairs, thus
+approaching the insects in structure. These three pairs of legs are
+attached to the thorax, while the head, which is separate from the
+thorax, unlike that of the true spider, bears two pairs of leg-like
+appendages. This is the chief of a group which are sometimes placed in a
+class by themselves, on account of their great differences from real
+spiders. Their head is separated from the thorax; and the thorax is
+divided into three segments; these, however, do not come out clearly in
+the diagram. The head bears, posteriorly, a pair of appendages which
+are practically legs; in front of these a pair of long "pedipalps" or
+"foot-feelers"; and quite in front the comparatively short "cheliceræ."
+These creatures are very venomous; they move about by night to seek
+their prey.
+
+[Illustration: FIG. 21.--A venomous spider-like animal, _Galeodes
+araneoides_, from North Africa, natural size (Diagrammatic).]
+
+Another kind of spider-like animal is familiar in English fields and
+waysides--the long-legged spiders, called Harvestmen or Phalangidæ,
+which spin no web, but jump upon their prey. Unlike the last group, the
+body differs from that of true spiders, in being more, not less,
+compact: for not only is the head joined to the thorax, but also the
+thorax is joined to the abdomen, the outline of the body being therefore
+almost globular. They receive the name Phalangidæ, Joint-Spiders, from
+the sharp joints in their long legs.
+
+Allied also to the spiders are the Mites, Acarina, so destructive to
+cheese, flour, and other eatables; and the Ticks, which infest the skins
+of various animals Fig. 22 shows a specimen of the latter. They are
+practically blood-sucking Mites. It is the female which attacks animals,
+while the males live among vegetation.
+
+[Illustration: FIG. 22.--The Tick which infests the Hippopotamus, from
+South Africa, twice natural size.]
+
+The Scorpions, also, are relatives of the Spiders. They are inhabitants
+of hot countries, and highly venomous. They possess a jointed tail,
+instead of an abdomen with fused segments, and a lobster-like pair of
+appendages in front; these are the second pair of appendages, the
+"pedipalps," while the short "cheliceræ" lie in front. In the living
+animal the tail is often carried curled up over the back. The Mites,
+Ticks, and Scorpions all agree with the true spiders in possessing eight
+legs. The King-Crab, Limulus, has not hitherto been named, because,
+though living in the sea, it is not a crab at all. It has been shown by
+Professor Ray Lankester to be related to the spiders. It is a large
+crab-like creature, which may be seen in museums and aquaria, and is
+brought from the tropical seas.
+
+[Illustration: FIG. 23. A Scorpion, _Buthus Kochii_, from India.]
+
+Before passing to consider the true Insects, or Hexapoda, something must
+be said about the discovery of _Peripatus_, a creature which comes from
+Cape Colony. It has been called caterpillar-like in appearance, but its
+structure is in many respects so peculiar, that it has been described as
+a link between insects and the higher worms. Its legs, for instance,
+although jointed, and much resembling those of insects in appearance,
+are hollow, like the "parapodia" of worms.
+
+The Centipedes have been already referred to. These, with the
+Millipedes, form the group Myriapoda. In outward form, at any rate,
+these suggest an intermediate position between Peripatus and Insects.
+
+The true Insects have a definite head, separated from the thorax, and a
+constriction between the thorax and the abdomen; this is why they are
+called insects, "cut in two." The thorax bears three pairs of legs, the
+mouth has typically three pairs of appendages, which may be altered and
+modified in many different ways, according to the nature of the animal's
+way of feeding. While the Crustacea are typically adapted for breathing
+in water by means of their gills, the Insects are adapted for breathing
+air. This they do by means of their air-tubes or tracheæ, the inlets of
+which open on their sides. These are divided into fine branches, which
+diffuse air through the body of the Insect. Two interesting points must
+be noticed about insects. The first is that they were the first group in
+which zoologists were able to study the nature of larval forms, long
+before the microscope had revealed the larval forms of marine animals.
+The changes undergone by insects are known as metamorphosis, or change
+of form; and are typically represented by the life-history of a
+caterpillar, which assumes during the winter a resting form called a
+Chrysalis or Pupa, and finally emerges as a Butterfly. Insects have
+sometimes been classified according to the greater or less completeness
+of the metamorphosis they undergo, which in some cases is comparatively
+slight. It has been mentioned elsewhere that larval forms usually exist
+where the young animal is placed under very different conditions from
+the adult. Fig. 24 shows two well-known instances of insect larvæ in
+which this is strikingly the case, the larval form being a
+water-dweller, and the adult a winged fly. Of these, one, the larvæ of
+the Dragon-fly, crawls about free; while the other, the so-called
+caddis-"worm," builds itself a case of grains of stone and shell
+cemented together.
+
+[Illustration: FIG. 24.--Larvæ of insects. _A_, of a Dragon-Fly,
+enlarged; _B_, House of the larva of the Caddis Fly, natural size; _C_,
+the Caddis Larva itself, enlarged.]
+
+The second point of interest is the wonderful part which has been played
+by insects in modifying the world we live in. We owe the bright colours
+and the sweet honey of flowers to the selection exercised by insects;
+they carry the pollen of flowers from one plant to its neighbouring
+kindred, thus securing cross-fertilization for the advantage of the
+plant, and thereby perpetuating any quality, such as colour or
+sweetness, which has originally attracted the insect to the flower.
+While a few plants only are fertilised by means of the wind, a vast
+majority depend entirely upon insects for the cross-fertilisation which
+is so necessary for the production of healthy seeds. We have already
+alluded to the part played by the earthworm in preparing the soil. If
+the earthworm has been the ploughman the insect has been the more
+intelligent gardener, who has filled the world with bright flowers.
+The earlier forms of plant life had green and inconspicuous flowers
+(Cryptogamia); the Phanerogamia, or showy-flowered plants, including all
+those that bear what are popularly termed flowers, have been produced by
+the artificial selection exercised by insects long before man was here
+to admire the result, and to carry on the same work in his gardens. The
+insect owes its food to the plant world; the plant world owes health and
+beauty to the constant ministration of the insect; so marvellous is the
+inter-connexion of one form of life with another.
+
+The number of different kinds of insect is enormous; the number of named
+species has been estimated at nearly a quarter of a million. It is
+therefore no wonder that entomology, the study of insects, has claimed
+the rank of a special science. We cannot here do more than refer in
+passing to a few of the more familiar types. First of all, by right of
+its work in fertilising flowers, let us take the Bee. Fig. 25 shows its
+honeycomb and its larvæ. The bee-grub differs from the caterpillar in
+its comparative helplessness. It is fed like a child by the worker bees,
+which are undeveloped females; and it does not leave the cell in which
+the egg is originally placed until it is ready to take on the adult
+form. The metamorphosis is complete; that is to say there is a grub
+stage and a pupa stage before the adult stage. There are three kinds of
+bees--the workers, which are sexless; the drones, which are males, and
+the queen, who is the sole female of the hive. The bee-grub may develop
+into a worker or a queen, according to the food it receives as a grub,
+the grubs that are intended to become queens being placed in a larger
+cell. The bee-grub differs from the caterpillar in having no feet.
+
+[Illustration: FIG. 25.--_A_, Larva of the Bee, _Apis mellifica_; _B_,
+Section of Honeycomb.]
+
+[Illustration: FIG. 26.--Ants, _Formica rufa_, English, enlarged. _A_,
+Female; _B_, Neuter, or Worker.]
+
+The ants are nearly allied to the bees, and also have a complete
+metamorphosis. Fig. 26 shows the English red ant, female and neuter. The
+wings of the female drop off after the pairing season, a fact which has
+given a name, Hymenoptera, to the whole group to which the ant belongs,
+although the name is often quite inapplicable. A recent discovery in
+entomology is the fact that ants have a voice. Dr. D. Sharp of Cambridge
+has described their "stridulating," _i.e._ noise-producing, organs.
+These consist of parallel ridges present on the sides of certain
+segments. By working the body up and down, the insect scrapes these
+ridges with the edge of the preceding segment, so that a musical note is
+produced, intelligible to other ants. The question has also been
+investigated by French observers. The principle involved will readily be
+recognised by those who in childhood were guilty of trying to extract
+music from a comb.
+
+[Illustration: FIG. 27.--White Ants, _Eutermes morio_, from Pernambuco,
+twice the natural size. _A_, Soldier; _B_, Worker; _C_, Young male; _D_,
+Female.]
+
+The white ants, so destructive in tropical climates, are not true ants,
+but belong to a different order. These also live in colonies; like the
+bees, they have an egg-laying queen. She has a partner, the king. There
+are neuter soldiers and neuter workers, both wingless, while the male
+and female have wings, afterwards lost.
+
+[Illustration: FIG. 28.--Cocoons of Moths. _A_, Compound Cocoon of
+_Coenodomuc hockingi_, from India, one-half natural size; _B_, of a
+Silkworm, _Bombyx Japonica_, one-half natural size; _C_, of Green-shaded
+Honey Moth; _D_, of Death's Head Moth, one-quarter natural size; _E_, of
+_Metura Savendersii_, from New South Wales, natural size; _F_, of
+_Castnia Endesmia_, from Chili, one-sixth of the natural size; _G_,
+of _Attacus attas_ from Bombay, one-fourth of the natural size.]
+
+The Lepidoptera or butterflies and moths receive their name,
+Scaly-winged, from the beautiful microscopic scales with which their
+wings are covered. Fig. 28 shows the cocoons which the larvæ of some of
+the moths make for themselves in which to pass their pupa stage. Some
+are made wholly of silk, others of dried leaves woven together. Fig. 29
+shows a Moth with its caterpillar, cocoon, and chrysalis. The threads of
+which a caterpillar weaves its cocoon are familiarly exemplified in the
+silk of commerce. The caterpillar, in some cases, is gregarious, and
+builds a common nest (Fig. 30).
+
+[Illustration: FIG. 29.--A Moth, _Saturnia pyri_ (S. Europe), with its
+Caterpillar, _A_; its Cocoon, _B_; Cocoon cut open to show Chrysalis,
+_C_; Adult insect, _D_.]
+
+The beetles, Coleoptera, are, like the butterflies, endlessly numerous.
+They are characterised by the striking difference in their two pairs of
+wings, of which the anterior pair is strong and horny, and forms, when
+at rest, a sheath which covers the thinner posterior pair of wings. The
+metamorphosis is complete in this group also. Fig. 31 shows an example
+which is typical except in one respect--the adult form, namely, is one
+of the comparatively few instances of adult insects that live in water.
+
+[Illustration: FIG. 30.--Nest of gregarious Caterpillar of a Moth,
+_Hypsoides_.]
+
+[Illustration: FIG. 31.--Development of an English Water-Beetle,
+_Dytiscus_. Grub; Pupa; Adult insect.]
+
+Much has been said above in praise of insects and their wonderful work
+in selecting flowers. There is, however, another side to this, as the
+gardener and farmer know too well. While the winged honey seekers help
+the plants, their larvæ devour them, and so do many other forms of
+insect. Fig. 32 gives us in miniature some of the most notorious insect
+pests. The work of the locust has been dreaded since the days of the
+Pharaohs and before: the Colorado beetle which infests the potato, is a
+plague as terrible, if more modern. The weevils and caterpillars that
+destroy trees, though not directly dangerous to our food supply, are
+sufficiently destructive. The terror of insect pests lies in their vast
+numbers, which may render an otherwise harmless creature dangerous. I
+read last year of a curious railway mishap in the United States. A train
+was brought to a standstill by the wheels sliding on something greasy
+that covered the track. It proved to be a flock of the so-called "Army
+worm," a variety of caterpillar which travels long distances in crowds,
+when its numbers have become too many for the supply of food, or when it
+is about to enter into the pupa stage. These covered the railway track,
+and the whole country for a long distance; and the "greasiness" of
+the rails was produced by the crushed bodies of the unfortunate
+caterpillars. The train was delayed for hours, while a gang of men
+with brooms cleared the way in front of it.
+
+[Illustration: FIG. 32. Insect pests. _A_, Locust, _Acridium
+peregrinum_, one-fourth natural size; _B_, Caterpillar of Wood Leopard
+Moth, _Zeutzera Æsculi_, boring in wood, about one-thirtieth of natural
+size; _C_, Colorado Beetle, one-fourth natural size; _D_, Leaf-rolling
+Weevil of the Oak.]
+
+
+
+
+CHAPTER IX
+
+MOLLUSCA, THE SHELL-FISH
+
+
+The shell-fish are called Mollusca, the soft-bodied animals. It will
+easily be seen that this name was intended to point out the distinction
+between them and the Arthropoda, as regards the way in which the skin is
+protected. In the latter, as we have seen, the skin itself is hardened.
+In the shell-fish, the skin secretes a covering which lies outside it.
+Just as our skins pass out superfluous moisture to the outside, in the
+form of perspiration, so the skin of the mollusc continually passes to
+the outside the solid substances which the body has taken in from the
+sea-water; and by the continual accumulation of these, the shell is
+formed. This, at least, is the view taken by modern authorities of the
+formation of the shell in most instances.
+
+The juvenile shell-collector usually begins his knowledge of the
+classification of the Mollusca, by learning that shells are classified
+as Univalves and Bivalves. This distinction is useful as a beginning.
+Univalves, that is to say shells which consist of one piece, are those
+of the snail-like animals, Gasteropoda, or Gastropoda, as some prefer to
+spell it. Bivalves, or shells which consist of two flaps, are those of
+the Lamellibranchiata or animals with plate-like gills, such as the
+mussel or oyster.
+
+Let us begin with the former. Everybody knows the snail. The snail
+proper bears a typical univalve shell: though in its relatives (the
+slugs), the shell is more or less suppressed. The name, Gasteropoda
+(stomach-footed animals), is supposed to be descriptive of the way in
+which a snail crawls. Half getting out of its shell, so to speak, it
+does its best to lay its body to the ground, and its so-called "foot" is
+an extensive muscular expansion underlying its body, not just a muscular
+organ thrust out of the shell, as in some other groups. The shell, the
+mode of crawling, and the "horns," tipped with eye-specks, and directed,
+intelligently and inquisitively, towards things of interest--these make
+up, for most people, the idea of Snail. But the most distinctive feature
+of the class is a less obvious feature, namely, the structure of the
+tongue. We may see, on any damp day or dewy evening, the snail working
+away with its tongue at some tender leaf. Its tongue is practically a
+file with which it files away the substance of the leaf, the resulting
+green mash being thus made ready in minute quantities for the snail to
+swallow. Thus are made the too familiar holes which disfigure the leaves
+of plants in our garden. When seen under the microscope, the file-like
+structure of the tongue is visible; indeed, in large tongues, it may, to
+some extent, be made out with the naked eye. Across the tongue, which is
+a flat ribbon-like structure, there runs a pattern of small teeth,
+bilaterally symmetrical, and this pattern is repeated over and over
+again throughout the whole length of the tongue. It might be thought
+that snails' tongues, being so much alike in their mode of use, would
+not need to be very various in pattern: but far from this, they vary in
+appearance as much as the shell. Not only is there a different pattern
+for every different order of the class, but a different pattern for
+every genus; nay, there are even distinctions between the tongues of
+different species in the same genus. Consequently some authorities on
+shell-fish prefer to classify them by their tongues, a classification
+which for the most part holds good. So characteristic is the tongue
+of the Gasteropod, that when new animals have turned up which were
+difficult to classify by means of the structure of the body, they have
+been finally recognised as Molluscs, somewhat related to the snails, by
+the tongue. This file-like tongue-ribbon of the snails is often called
+the Odontophore or Tooth-Carrier; sometimes the part which actually
+bears the teeth receives the name of the radula.
+
+The snail and its relative, the slugs, belong to the Pulmonate (_i.e._
+air-breathing) division of the Gasteropoda. The sea-slugs, in which,
+like the land slugs, the shell is absent or reduced, are relatives of
+the land snails. Some of those found on our own shores are handsome
+creatures, brilliantly coloured. Both groups fall under the division
+Euthyneura, while the majority of the marine univalves belong to the
+division Streptoneura (_i.e._ Gasteropods with twisted nerves). The
+Gasteropods, in the course of the evolution of their shell, have had
+the body thrown crooked by the burden of carrying it; the Streptoneura
+are the forms in which this crookedness is most pronounced; in the
+Euthyneura it is less so. There are degrees of crookedness even among
+the Streptoneura; and the limpet is less crooked than the periwinkle
+(see Table, p. 30).
+
+The older classifications of the Gasteropoda were largely founded on the
+characters of the shell; but these, though in the main they hold good,
+have required some modifications in recent times. Conchology, the study
+of shells, was at one time the hobby of many collectors whose knowledge
+of the animals possessing the shells was not of a very extensive kind;
+and consequently the very name of conchology is often enough to ruffle
+the feelings of the zoologist of the present day. Yet many interesting
+problems of variation may be studied from shells alone, by those whose
+circumstances forbid them to study the living animal. Nor is there any
+branch of zoology which is more useful to the teacher who wishes to
+catch the eye and the attention of the beginner in the study of natural
+history, especially if the beginner is young, as beginners ought to
+be. Therefore we must by no means undervalue the past labours of
+conchologists, or the valuable collections which their industry has
+brought together and set in order for the benefit of the world.
+
+For example of the most crooked, or Azygobranchiate division of the
+Streptoneura, turn now to Fig. 33, in which we see a typical Gasteropod
+shell, _Murex ramosus_, the Branchy Murex, aptly enough named from the
+many prickly branches which beset it. These rough points are probably
+assumed for protective purposes; any animal that might wish to dine upon
+the _Murex ramosus_ would think twice before trying to swallow it--the
+morsel of shell-fish is so small, its shelly case so large and so
+prickly. If we look for its nearest English relative, that is _Murex
+erinaceus_, the Hedgehog Murex, or Sting-winkle. This, though a
+comparatively plain shell, has still enough rough ridges upon it to have
+secured it a comparison to the prickly hedgehog. Perhaps the most
+prickly member of the genus, however, is _Murex tenuispina_, sometimes
+called Venus' Comb, because the crowded parallel spines which decorate
+the elongated front of the shell somewhat resemble the parallel teeth of
+a comb.
+
+[Illustration: FIG. 33.--The Branchy Murex, _M. ramosus_, a typical
+specimen of the shell of the Carnivorous Gasteropods. _Sp._, spire or
+posterior end of the shell; _S_, siphon or anterior end of the shell.
+Fig. A, shows the mouth of the shell; Fig. B, the exterior only. Less
+than one-half the natural size.]
+
+How does the _Murex_ get its living? Let us notice the shape of the
+shell, drawn out to a point, at the end opposite to the spire. According
+to the older classification of the Mollusca, now somewhat fallen out of
+use, this point marks the shell as belonging to one of the
+Siphonostomata (shell-fish with a siphon at the mouth of the shell,
+_i.e._). These shell-fish are, with few exceptions, carnivorous; not
+that the siphon shape of the shell has any direct connection with the
+animal's way of feeding. Just as the snail files among soft vegetable
+substances, so the Murex and many of its relations file away much
+harder things. A Sting-winkle, or a Dog-whelk, can sit down over a
+helpless bivalve shell-fish, and patiently file away, until it has
+worked a neat round hole in the protecting shell of the latter. You may
+find, among the dead shells on any sandy part of the English coast, any
+number of bivalve half-shells with a neat little round hole in them,
+indicating unmistakeably how the tenant came to its death. There is some
+controversy as to the spot chosen by the assailant for its attack. Some
+authorities have stated that the predatory mollusc is so wise that it
+knows where to find a weak spot, and makes a hole just over some vital
+organ of the bivalve, or else above its adductor muscles, so that, when
+these are cut, the half-shells cannot be drawn tightly together and kept
+shut. Recently this has been denied, and statistics of the attacks of
+_Purpura_, the common small whelk, a relation of the _Murex_, on
+_Mytilus edulis_, the Common Mussel, have shown that the perforation
+occurs in every part of the shell. It is possible, however, that the
+Mussel, from the peculiar shape of its shell, offers an exceptional
+case; and I am inclined to think that in the case of bivalves of a more
+flattened shape, the earlier statement holds true. At South Shields,
+England, perforated half-shells of the Common _Venus_ (Fig. 34) are so
+abundant that the children string them for necklaces; yet I have never
+been able, by the most industrious search, to find more than one or two
+specimens in which the hole is at all near the lip of the shell. It is
+possible that these exceptional instances were the work of a young and
+inexperienced univalve mollusc, or a stupid one. It is possible, also,
+that the mode of attack differs somewhat according to the species of
+the assailant. (It should perhaps be explained, for the benefit of those
+who have no experience in the ways of children or of shell necklaces,
+that the hole must be moderately near the beak of the shell, to enable
+the shell to "sit" properly on a string. Every unit in the necklace may
+therefore be counted as one in favour of the older theory.) Many of the
+Siphonostomatous molluscs are surprisingly active and strong, so that
+they are well fitted for a predatory existence. In fact, they not only
+eat bivalves, but occasionally attack the vegetable-feeding univalves
+when nothing better is to be got, so that occasionally the shells of
+these also may be found displaying the deadly little round hole we have
+described.
+
+[Illustration: FIG. 34.--Half Shells of the Common Venus, several of
+them perforated by carnivorous molluscs. From South Shields, England.]
+
+Let us contrast with the _Murex_ one of the shells which are
+"holostomatous," _i.e._ possessing an unindented shell-mouth--that is to
+say, one without a "siphon." The common edible periwinkle, _Littorina
+littorea_, may be taken as an example. No shell is more familiar; even
+the town-dweller, who has never found it on the sea-shore, has seen it
+often on stalls in the slums. The mouth of the shell is quite round and
+unindented, and in this case the character holds good as the mark of a
+vegetable-feeder--a non-predatory sea-snail. It is hardly necessary to
+remind the reader that its name (the shore-shell) is given it because it
+lives where the tide leaves the rocks exposed during part of the day.
+Another common species of _Littorina_, which frequently lives a little
+lower down, where the large sea-weeds grow, has been described in
+Chapter II.; and another, _L. rudis_, lives a little higher up, so that
+it spends most of its time in a dry state, and is fast on its way to
+become a land-shell. At most of the familiar English seaside resorts one
+may see dozens of it baking in a hot July sun on rocks where only the
+highest tides can reach them: and yet under these conditions they
+continue to live and flourish. The periwinkles are remarkable for the
+great length of the tooth-ribbon, in comparison with the size of the
+animal. The number of separate teeth upon it has been estimated at 3500.
+
+A familiar feature of the common periwinkle is the lid or stopper
+(Operculum), with which the animal can close the mouth of the shell.
+This is developed and carried by the outside of the animal's foot. In
+the periwinkle and other English molluscs it is comparatively soft and
+semi-transparent, and reminds one of a thin slice of horn. In many
+tropical molluscs, however, it is hard and shelly. The large tropical
+shells named _Turbo_ have massive lids of considerable weight. These
+shells, which are nearly allied to the pearly Top-shells (_Trochus_) of
+the English shores, are sold as ornaments, the outer coat of the shell
+being partly scraped off to show the inner coat of pearl: it is rarely,
+however, that the purchaser obtains a lid, or even knows that the
+creature had one. The reverse is the case with some of the smaller
+kinds, the lids of which, being brightly coloured, are imported without
+the shell, and sometimes set as articles of jewellery. Some of these are
+of a bright green hue.
+
+While the lids of the Holostomata are rounded in shape, those that
+belong to the Siphonostomatous shells are necessarily more or less
+modified so as to fit the mouth of the shell, and are consequently oval
+or even claw-like in shape. The Sting-winkle already spoken of, the
+common small whelk, _Purpura lapillus_, and the large whelk, _Buccinum
+undatum_, are common shell-fish in which the elongated lid may be
+studied. The lid is not, however, like the tongue-ribbon, an essential
+feature of the structure of every univalve mollusc.[D] Not only are
+there special instances in which it is greatly modified, but also
+there are whole groups of univalve molluscs in which it is absent.
+
+[D] There are one or two exceptional cases of gasteropod molluscs that
+have no tongue-ribbon. The majority of these are parasitic forms, which
+can get their food without the trouble of filing it down.
+
+A curious suggestion has been made with regard to the lids of univalve
+shell-fish; namely, that the snapping to of the lid is capable of
+producing a sound, which may perhaps be audible at a distance under the
+water. Various molluscs have been credited with producing sounds, either
+by muscular movements or by the grating of the shell as the animal
+walks. The common Tortoise-shell Snail, _Helix aspersa_, sometimes
+makes a most alarming noise when crawling over a window. It has been
+disputed whether the sounds thus made are produced by the grating of the
+creature's tongue-ribbon on the glass, as it files off small particles
+of algae and vegetable moulds, which are invisible to our eyes: or
+whether they are sounds due to suction of the muscular surfaces, such as
+may be produced by drawing a wet finger across glass. The noise, however
+produced, is, as I can testify from experience, sufficiently loud and
+weird to be very startling, if heard in the dead of night.
+
+Turn now to the Bivalves or Lamellibranchiate molluscs, which include
+the familiar oyster, cockle, and mussel. These are also known as the
+Pelecypoda, and as the Aglossa, or molluscs without a tongue-ribbon. The
+name Lamellibranchiate refers to the shape of the gills--"plate-like,"
+or flat; the name Pelecypoda to the shape of the foot, "hatchet-foot."
+
+The animal usually chosen as a type of these, the fresh-water mussel, is
+rather a dull sort of creature, so we have chosen a prettier and more
+lively specimen as a representative of the class; namely, one of the
+Scallops, _Pecten opercularis_, sometimes called the Quin, the shell of
+which is shown in the frontispiece of the book. This is one of the most
+beautiful, perhaps the most beautiful, of the English shells. The
+generic name, _Pecten_, the Comb-shell, probably refers, not to the
+shape of the gills, which is somewhat peculiar, but to the marking of
+the shell, which presents raised ridges, side by side. Anyone familiar
+with shells will see at once that this is an unusual pattern. There are
+plenty of bivalve shells with concentric ridge markings, comparatively
+few with radiating ridges. We shall see presently that there is a good
+reason for this. The specific name "opercularis," lid-like, refers to
+the neat round shape of the shell. Each half of the shell has a pair of
+"ears," so-called. The person who first gave this name to these flaps of
+shell, three of which are three-cornered and the fourth nondescript,
+must have been familiar in his youth with books afflicted with the
+"dog's-ear" disfigurement; for certainly there is no other kind of ear
+which greatly resembles these. The notch beneath the irregularly shaped
+ear is called the "byssal notch": many Pectens spin a byssus or thread,
+like that spun by the common Sea-Mussel, and thus anchor themselves to
+fixed objects for a time; this notch is the place where threads of this
+kind leave the shell.
+
+The two valves of the shell differ in depth, one being flatter than the
+other; and the "ears" of the two valves differ in shape. The inside of
+the shell shows muscular impressions, but these cannot be seen in a
+photograph. The picture, however, shows the strong hinge-ligament which
+joins the halves of the shell, and the difference in depth and shape of
+the two valves. The valve on which the animal usually lies is the
+lighter in colour of the two, and has one ear much longer than the
+other.
+
+The creature swims by means of the "mantle," or muscular margin of the
+body. It contracts this suddenly, after first opening the shell and
+taking in as much water as possible. Thus the water is squeezed out
+again, and the effect of this is to propel the animal in an opposite
+direction.
+
+Now we are in a position to understand a little more about the shape of
+the shell. These curious "ears" possessed by the two valves, together
+form a straight, strong edge, which cuts the water as the animal flies
+along. It reminds us of a ship's prow, and not without reason, for the
+use of each is the same. A boat's sharp prow, compared with the rounded
+front of a "tub," makes all the difference in the possibilities of
+straight steering, and favours the putting on of speed: the ears of the
+shell are not less useful to our Scallop. The following account of the
+swimming powers of this species of Scallop, quoted by Woodward, was
+given by the Rev. D. Landsborough, who observed young specimens, about
+the size of the small ones in our picture, swimming about in a pool of
+sea-water, left by the ebbing tide. "Their motion was rapid and zigzag;
+they seemed, by the sudden opening and shutting of their valves, to have
+the power of darting like an arrow through the water. One jerk carried
+them some yards, and then by another sudden jerk they were off in a
+moment on a different tack." To the sharp prow, the _Pecten_ owes this
+capability of arrow-like flight. Its eyes are situated on the fringe of
+its mantle, and consequently near the wide end of the shell; its
+peculiar mode of progression, therefore, enables it to back away
+instantly from any enemy it sees.
+
+Something must be said regarding the interior of the shell. The majority
+of bivalve shells have a complicated system of so-called "teeth," or
+interlocking projections, at the hinges of the shell: these exhibit
+great variety in different kinds of shell, and are therefore often a
+ready means of distinguishing one shell from another. The Scallop,
+however, is very deficient in this respect, as are also some of its near
+relations, for instance the oyster and its family group. The Fresh-water
+Mussel also gains its name, _Anodon_, or _Anodonta_, the Toothless One,
+from the same circumstance. The name often puzzles the beginner, who
+asks, bewildered, "But do Bivalves ever have any teeth?" True teeth, of
+course, they have none--it is the shell-hinge that has teeth, not the
+animal inside it. Not only have the bivalve shell-fish no teeth indeed,
+or tongue-ribbon, but furthermore they have no head. For this reason the
+group has not only received the name already mentioned, of Aglossa, the
+Tongue-less Ones, but also that of Lipocephala, _i.e._ Molluscs in which
+the head is not developed. The reason of its absence is not far to
+seek--a head would be no use inside such a shell. The snail-shell, so
+differently built, allows freedom for the head; the bivalve mollusc,
+squeezed in between its valves, has room only for a mouth.
+
+We have referred above to the ridges on the outside of the shell. Now
+that we have learnt that the _Pecten_ is a very active animal, and moves
+in the manner described, we see that these ridges run parallel to the
+direction in which it moves as it darts away ears foremost. Let us try
+to realise what is the effect of this.
+
+Take a mat with parallel stripes and move it along the floor or table
+in the direction of the stripes; then try moving it in an opposite
+direction across the stripes. It is easy to perceive that in the former
+case one's eye does not detect the movement nearly so soon as in the
+latter case. To explain this would necessitate a lengthy digression on
+the subject of optical illusions: that the fact is so everyone may
+easily ascertain by experiment. The ridges, therefore, converging in the
+direction towards which the shell is going, are a protective decoration,
+enabling it to slip away more easily from under the eyes of its foes.
+The reader will readily recall a parallel instance in the common Cockle.
+This also is a very active creature; it takes leaps by means of a strong
+muscular foot; and the ridges on the shell, like those of the Scallop,
+converge towards the hinges, that is to say, in the direction in which
+the shell moves. Another instance of a very active shell-fish with
+similar markings is afforded by certain kinds of Lima, a near relative
+of the Scallops. It may be added that all Scallops are not equally
+active, nor all Limas; and various modifications of their form and
+colour might be pointed out which lead us to suspect that in the less
+active kinds the pattern of ridges is often somewhat obscured by means
+of these differences.
+
+Now, take up a comb and draw it over your fingers, firstly _along_ the
+teeth, and secondly _across_ them, and you will be able to estimate the
+gain in speed and comfort to the comb-shell, Pecten, and to the common
+Cockle, from having its ridges set in the direction in which it is
+going. Were the ridges concentric, as is so often the case in bivalve
+shell-fish of a more sluggish disposition, the friction caused by the
+ridges would seriously delay the progress of the shell.
+
+Something must be added regarding the colouring of the shell, which is
+vivid, corresponding with that of the animal within. It is capable of
+great variety, though perhaps not so great as in some of the smaller
+_Pectens_. The predominant shades are pink, crimson and yellow, either
+separately or mixed; that is to say, some shells are pure pink, some
+almost pure yellow, some almost pure crimson, while others present every
+imaginable shade of pinkish yellow, reddish brown and brownish crimson.
+Local variation of colour is so marked that we may suspect the
+variations in tint to be in some degree protective. The shell also
+varies considerably in size and strength according to the neighbourhood
+in which it has grown.
+
+This scallop-shell is but one of many: a number of other species are
+found on our own shores, and many others again in foreign seas.
+
+One shell of the English coast is very annoying to the juvenile
+shell-collector who gathers specimens on the shore. This is _Pecten
+pusio_, a very small and delicate kind, with a raised pattern of fine
+markings upon the ridges, which are very narrow. A good specimen of the
+deeper valve is common enough, but the shallow valve, if of any size, is
+distorted into all manner of shapes, as if it had been squeezed and
+crumpled. The disappointing character of these specimens, from an
+æsthetic point of view, is explained when we learn that it not only lies
+on its shallow valve, but becomes fixed in this position, instead of
+hopping about freely like the _P. opercularis_. It therefore has
+frequently to adapt its shape to the nature of the ground where it has
+happened to fix itself. Thus arises the disfigurement of the shell.
+
+So far we have only considered two great groups of the Mollusca, two
+which are represented by common shells, familiar to everybody. We must
+not leave the subject of the Mollusca without referring to their most
+aristocratic group, the Cephalopoda. These are represented in museums by
+the shells of the Pearly Nautilus, and of its not very near relative,
+the Paper Nautilus; and they are represented on English shores by the
+cuttle-fishes. All these agree with the Gasteropoda in the possession of
+a tongue-ribbon, and in classification are therefore treated with them
+under the name Glossophora.
+
+With the Pteropods, transparent forms found swimming over the surface
+of the deep sea, the reader is not likely to have much to do. In
+classification they are now placed near the Sea-Slugs.
+
+The Placophora, or Polyplacophora, wholly different from our usual idea
+of a shell-fish, may be named as creatures which the reader is quite
+likely to meet with. Though not very common, they are widely distributed
+over our coasts, and may be found near low-tide mark clinging to stones.
+Imagine a wood-louse without any apparent head which has taken to
+clinging to the rock like a limpet, so that it cannot be removed without
+injury, and you have a rough idea of their general appearance. _Chiton_
+is the name of these animals, which have received the group name of
+Polyplacophora, carriers of many plates, because their external covering
+consists of an armour of successive shelly plates. These also belong to
+the Glossophora or Tongue-ribbon Carriers, of which they present a
+comparatively primitive form.
+
+Reference has already been made to the labours of the earthworm and of
+the insects, and to their important effects upon the vegetable world.
+Although the Mollusca include but one terrestrial group, the Snails,
+they, too, have played an appreciable part in modifying plant life. If
+we owe our flowers to the insects, we have probably to thank the snail
+for our medicines. For the snail dislikes bitter-tasting leaves, and
+lets them alone, thus exercising an artificial selection in favour of
+the survival of medicinal plants. In the same way the snail has favoured
+the survival of hairy and thorny plants, upon which it cannot easily
+crawl.
+
+The larval forms of the Mollusca differ considerably from the adult.
+That of _Anodon_, the fresh-water mussel, at first received, in
+consequence, a different name, that of Glochidium, by which it is still
+known, although it has now been long identified as a larval form. It is
+exceptional in the fact that it is parasitic on fish.
+
+The usual Molluscan larva is a ciliated creature which has been compared
+to a modified trochosphere. It is preceded by a gastrula stage, and it
+develops later on into what is called a "Veliger," or "veil-carrying"
+larva, so called because it has in front a broad two-lobed ciliated
+expansion, the velum. This larva is adapted for swimming, which is
+accomplished by means of the velum. In terrestrial molluscs, the
+development is necessarily much more direct. It is worthy of note that
+the periwinkle mentioned above, which lives high and dry (_L. rudis_)
+has no larval form, while its relatives that live under water develop
+in the usual way.
+
+The eggs of Mollusca are often enclosed in tough cases, calculated to
+resist waves and weather. Some of these are shown in miniature, in the
+group of eggs of various kinds, Fig. 35.
+
+[Illustration: FIG. 35.--Eggs (reduced to half the natural size).
+_A_, Egg-Capsules of _Murex_. _B_, Frog's Eggs. _C_, Eggs of large
+Land-snail. _D_, Eggs of Snail placed on a leaf. _E_, Cockchafer's Eggs.
+_F_, Egg-case of Cockroach. _G_, Egg-cases of Locust. _H_, _I_, _J_,
+Eggs of Gasteropod Molluscs. _H_, _Sycotypus_ (_Pyrula_). _J_,
+_Fusus_.]
+
+TABLE SHOWING THE CLASSIFICATION OF THE MOLLUSCA
+
+                   { AGLOSSA: the LAMELLIBRANCHIATA, also called
+                   {   CONCHIFERA, and PELECYPODA.
+    =MOLLUSCA.=    {
+                   {
+                   { GLOSSOPHORA { GASTEROPODA.
+                                 { CEPHALOPODA.
+
+
+
+
+CHAPTER X
+
+THE BRACHIOPODA OR LAMP-SHELLS
+
+
+These were at one time included under the Mollusca, on account of
+their possession of a bivalve shell. This shell, however, is placed
+practically back and front of the animal, not to the right and left of
+it, as is the case with the shells of the bivalve Mollusca.
+
+The name, arm-footed, was given them in reference to a pair of special
+structures called the arms, bearing a large number of tentacles; it is
+now more frequently spoken of as the lophophore (see p. 122), and
+regarded as comparable to the lophophore of the Polyzoa, spread out into
+two portions. With the latter group the Brachiopods were formerly united
+by Huxley, under the name of Molluscoidea. This name is now obsolete,
+because it is understood that all these creatures are widely different
+from Molluscs; but the theory of relationship of the Brachiopoda to the
+Polyzoa, implied in it, still holds good.
+
+The chief importance of this group lies in its fossil forms, which are
+exceedingly numerous, particularly in the Mountain Limestone of the
+Carboniferous Period; it is crowded with their shells, especially a form
+named, from its elongated shape, Productus. The shells of Brachiopods
+are equal-sided; that is to say, the right and left valves match; but
+they are inequivalve, the ventral valve being much the biggest. It often
+contains a foramen, or hole, at the beak, for the passage of the
+pedicle, or stalk, by which the animal is attached to the ground (_e.g._
+_Terebratula_, _Rhynchonella_). Sometimes, however, the pedicle passes
+out between the valves (_Lingula_, _e.g._), in which case there is no
+foramen; or it may be arranged in other ways. Sometimes the shell is
+merely attached to the ground by its side, like an oyster. Some forms
+are enormously widened in a lateral direction, _e.g._ _Spirifera_, and
+the _Productus_ above named. _Lingula_, among others, is remarkable as
+being a form that has survived from the earliest geological period to
+the present day.
+
+TABLE SHOWING THE CLASSIFICATION OF THE BRACHIOPODA
+
+                      { TESTICARDINIS.--Shell Calcareous, with hinges.
+                      {   Skeleton present in the arms.
+    =BRACHIOPODA.=    {
+                      { ECARDINES.--Shell comparatively soft, composed
+                      {   of Chitin, only strengthened by deposits
+                      {   of lime, without hinges. No skeleton in
+                      {   the arms.
+
+The larva, in its best known forms, passes through the typical larval
+stages of the animal kingdom. It is first a one-layered larva, then a
+two-layered form, and then becomes a ciliated animal. In this three
+regions may be distinguished, representing respectively the head, body,
+and pedicle.
+
+The shells of the Brachiopoda, including the kinds above named, may be
+seen by the reader in any geological museum.
+
+
+
+
+CHAPTER XI
+
+THE POLYZOA; MOSS-CORALS AND SEA-MATS
+
+
+We have already described the creatures which are popularly known
+as Corallines. Modern zoologists have long separated off from the
+Corallines of the older writers, a group of animals known as the
+Sea-Mats, which also are colonies made up of unit individuals. The
+common Sea-Mat, _Flustra foliacea_, may be picked up on almost any part
+of the English coast, being often torn up "by the roots" and washed in
+by the tide. When fresh it has a pleasant scent, which has been compared
+to that of Lemon Verbena, and a pinkish colour, due to the presence of
+the little inhabitants in their cells. When dry it has no odour,
+the cells are empty, and the colour a pale drab like that of a dead
+Coralline. Its texture is, however, much more crisp and brittle, and
+less horny, than that of a dead Coralline: it grows in flat, forked
+expansions, much resembling in outline the fronds of several common
+seaweeds; and each side of these is covered with a diamond pattern of
+little cells. This crowded arrangement of the cells, with a tendency to
+assume a geometrical pattern, is the readiest feature by which the
+beginner may distinguish a Sea-Mat from a Coralline. The latter arrange
+their cells in a free-growing, tree-like or fernlike form, without any
+crowding of the units into a geometrical pattern. The division of the
+flat leaf-like colony by two, resulting in bifurcated branches, is
+another obvious feature of the Sea-Mat.
+
+Covering--and to the botanist's eye disfiguring--the branches of many
+sea-weeds, and growing upon oyster-shells, tangle-roots, and other fixed
+objects, we may find many little incrustations which remind us of the
+lichens of the land: the diamond pattern of little cells shows us,
+however, that these things are relations of the Sea-Mats. The name of
+Bryozoa, Moss-Corals, was formerly given to these growths. Many of
+them bear long hair-like processes at regular intervals; these, which
+are large enough to be plainly seen with the naked eye, afford a ready
+means of recognising these creatures.
+
+TABLE SHOWING THE CLASSIFICATION OF THE POLYZOA
+
+                  { ECTOPROCTA, with excretory aperture outside
+                  {   the ring of tentacles, _e.g._, _Flustra_.
+    =POLYZOA.=    {
+                  { ENDOPROCTA with excretory aperture inside
+                  {   the ring of tentacles.
+
+The Polyzoa include freshwater as well as marine forms. They have a
+free-swimming larva, which becomes fixed after a time, and gives rise
+to the adult Colonial forms. The zooids of the latter have each an
+independent head with a crown of tentacles, called the Lophophore
+(Crest-carrier); but the fixed ends of their bodies communicate with one
+another. The hard covering of the colony, which retains its form after
+the animal is dead, is a kind of hardened skin: the apparent "cells" are
+the openings through which the individual zooids protrude themselves.
+Sometimes certain of the zooids undergo modification for special
+purposes: in this way are formed the "avicularia," snapping appendages,
+probably defensive in purpose, so called because they open and shut like
+a bird's beak. There are two divisions of the Polyzoa, the Ectoprocta
+and the Endoprocta. Among the latter there is found a form which is not
+colonial.
+
+_Phoronis_, a curious worm-like animal, which has a larval form called
+_Actinotrocha_ is sometimes placed in classification near the Polyzoa,
+which it resembles in possessing a crown of tentacles (Lophophore).
+
+
+
+
+CHAPTER XII
+
+THE ECHINODERMATA
+
+
+Everybody knows the Star-fish and many people know the Sea-Urchin.
+An "urchin" is not a name for a naughty little boy, but the French
+(_oursin_) for a hedgehog. A Sea-Urchin is therefore a "Sea-Hedgehog,"
+a name very appropriate for a creature armed with prickles. The Greek
+word _echinos_ also means a hedgehog, so that the long name given to the
+group means simply hedgehog-skinned. The prickles attain their maximum
+in the Sea-Urchin, but they are well represented in the Star-fish, while
+in the Sea-cucumber the general tendency to "prickliness" is much
+reduced, and represented only by "spicules" (needles) of shelly stuff
+underneath the skin of the animal.
+
+The largest and the most beautiful of the Sea-Urchins of the English
+coast is known as the Purple-tipped Sea-Urchin, on account of the
+beautiful colour of the spines. It lives on rocky coasts, and during
+very low tides may be seen at home, although it usually takes care not
+to stray above the water-line. It is a shelly ball with a flat base; its
+surface is covered with long spines. Its mouth, which is in the centre
+of the base, shows five wicked-looking teeth peeping out. The shell is
+pierced by what look like hundreds of minute pin-holes, arranged in a
+complicated pattern; these are the holes through which it pokes its
+feet, which greatly resemble those of a Star-fish, being white suckers
+with a disc at the end. When thrown out to their full length they are,
+however, much longer than those of the Starfish, for they are naturally
+obliged to be thrown out to a distance longer than the length of the
+animal's own prickles. When moored by all its feet, extended from all
+sides of the shelly ball, the animal presents a curious and pretty
+sight. Large specimens are almost as big as a child's head, but smaller
+ones are more common. There is a considerable range of variation
+in colour; not only are various shades of purple found, but also
+purplish-red and red. The spines are mounted on something resembling a
+ball and socket joint, with a ring-shaped pad, so that they have a wide
+range of movement; if any of the spines are touched they are immediately
+set back over a considerable part of the neighbouring surface.
+
+Other kinds may be found upon a more sandy shore. These are heart-shaped
+and much lighter in colour. The shell is thinner and of less weight.
+These adaptations for lessening the animal's weight enable it to move
+over sand: the species above described has no occasion for such
+precautions. When it crawls over rocks and the strong seaweeds that grow
+on them, there is no fear of its sinking in. The sand-dweller, on the
+contrary, must take care that it is not swallowed up.
+
+[Illustration: FIG. 36.--The Five-holed Sand Cake, _Mellita pentapora_,
+a flat sea-urchin from the east coast of tropical North America. _A_,
+upper surface; =B=, lower surface; _C_, side view.]
+
+There are Sea-Urchins that carry their precautions against sinking to an
+extreme degree. These are the Shield-Urchins or Clypeastridæ, so-called
+from their flat shape; they include the American forms popularly known
+as "sand-cakes." The diagram (Fig. 36) shows one of the most curious of
+these flattened forms adapted for moving over fine sand and ooze, and
+literally "as flat as a pancake." The mouth is approximately in the
+centre of the lower surface, _B_; the upper surface, _A_, shows a
+rosette pattern on the top of the shell. This is formed by the rows of
+holes for the very minute tube feet. In the English Sea-Urchin above
+described, which is one of the group called (for that reason) Regulares,
+the rows of holes are uniformly continued all along the rounded sides of
+the body down to the neighbourhood of the mouth. Here they are much
+restricted, forming merely a rosette at the top of the shell: hence
+they are described as circumscript or "petaloid." The excretory aperture
+is shown in the photograph as a smaller dot on one side of the mouth,
+while in the Echinus, on the contrary, it is at the top of the shell.
+The five odd-looking, elongated holes are a curious individual
+peculiarity of this Sea-Urchin. It has already been explained that the
+Shield-Urchins are flattened in order to distribute their weight; these
+holes are a contrivance for still further reducing the weight in
+comparison with the area. This is when the animal is lying quiet at the
+bottom of the water, but when it moves about what effect will the
+presence of the holes produce? Flattened animals are usually supposed
+to derive an advantage from the fact that they sink more slowly
+through depths of water; as in lying upon the ground, their weight is
+distributed, and they float, as it were, in the same stratum of water
+without sinking further down. This creature, on the contrary, has
+apparently feared lest it should move too slowly when it moves in a
+vertical direction, and it presents us with an arrangement by means of
+which its sinking through water is facilitated. Water will pass readily
+through the five holes as the animal goes either up or down, and the
+resistance of the whole flat area to the water is thus reduced and
+vertical movement rendered more easy. Thus, by one and the same
+contrivance, the animal has lessened its weight when lying quiet, and
+diminished the resistance it meets with when it moves. The distribution
+of the holes, moreover, is such as to regulate the animal's position in
+sinking, and to prevent it from falling "headlong." For although the
+creature has, strictly speaking, no "head," yet the end nearest the
+mouth is the thickest and heaviest part of the "cake," and would
+naturally tend downwards. This tendency is counteracted by the fact that
+the thicker end is unperforated, while the thinner and lighter end has a
+large central hole to diminish its resistance and enable it to sink more
+rapidly.
+
+Adapted for living in sand rather than on rocks, but not so extreme
+in the peculiarity of their form as the Shield-Urchins, are the
+Heart-Urchins, already referred to, shaggy-looking creatures whose fine
+yellowish-white spines give them almost the appearance of being clothed
+with fur. The excretory aperture is at the narrow end of the "heart,"
+and the mouth at one side of the lower surface towards the wide end. The
+complicated apparatus of teeth found in other Sea-Urchins is absent in
+these. They are abundant on sandy shores. During the severe winter of
+1894-5, when the Mersey at Liverpool was frozen nearly for one memorable
+day, and filled with floating ice for many more, I saw the shore beyond
+New Brighton heaped all along with a bank, often two feet across, of the
+common Heart-Urchin. These, which afforded a fine feast for the hungry
+sea-gulls, had been killed by the intense cold, and afterwards washed
+ashore by the tide. The vast numbers of this creature which exist on
+that coast were thus unexpectedly brought to light.
+
+These animals are sometimes described as "burrowing" creatures, because
+they live covered in sand. The term is rather misleading. Far from
+wishing to burrow, they spend their lives in a constant struggle with
+sand that closes over them only too readily; and their whole structure
+is adapted to prevent their sinking in a quick-sand.
+
+We began our chapter with the Sea-Urchins, because they are the most
+important members of the group to which they give their name; but there
+are forms belonging to the Echinodermata that are more familiar to the
+ordinary observer--the Starfishes. Those who take an interest in the
+cultivation of the oyster find them far too familiar--for the starfish
+is the oyster's deadliest foe, not even excepting man.
+
+The common Starfish, _Asterias rubens_, may constantly be found among
+stones, about low-tide mark. Its manner of walking is peculiar and
+characteristic. On the under surface of each ray are rows of white
+sucker-like tube-feet, which can either be drawn in or pushed out. By
+doing each alternately the animal walks. First the feet are extended to
+their full length; then the terminal sucking disc of each catches hold
+of the ground. Then the feet are again retracted, while their discs
+still cling; the effect of this is, naturally, to pull the ray onwards.
+This process is repeated again and again, until some appreciable degree
+of movement is effected. The tube-feet are in connection with a system
+of vessels filled with fluid, known as the Water-vascular System of the
+Starfish. The fluid is driven on by muscular contractions until the feet
+are fully extended, and again driven back when the feet are retracted.
+The Water-vascular System is a structure common to all Echinoderms; and
+vessels of a comparable character are found in some worms.
+
+How does the Starfish know where it is going? Underneath each ray, near
+the tip, is a little feeler (or tentacle) and a little eye spot. By
+means of these it gets an idea where each ray is going to; and, since it
+often moves but one ray at a time, this is sufficient for it. When
+necessary, however, the several rays can act in concert with one
+another.
+
+The rayed form of the Starfishes led to their being at first included
+in the group of Radiate Animals, along with the tentacle-bearing
+Coelenterata; but it has long been recognised that they are animals of
+much higher structure. Their very larvæ can barely be brought into
+comparison with animals so simple as the true "radiates."
+
+The Snake-Stars, or Ophiuroidea, are closely allied to the Starfishes.
+In these the arms are thin and sharply defined from the little central
+disc, instead of sloping gently out of it, as in the Starfishes. The
+rapid wriggling movements of the arms have gained for them their very
+appropriate name. They are also called Brittle Stars, because the arms
+break off easily, sometimes at the will of the animal. Several kinds of
+them are common on our shores, although they are not so common as the
+ordinary Starfishes. Fig. 37 shows the general form of a Brittle Star.
+
+[Illustration: FIG. 37.--A Brittle-Star, _Ophiopteris antipodum_.]
+
+[Illustration: FIG. 38. A Sea-Cucumber, _Cucumaria Planci_, from Naples,
+natural size.]
+
+The Sea-Cucumbers, Holothuroidea, are another group of Echinodermata
+that are represented on our own coasts; by small specimens, however,
+while the Pacific Ocean furnishes instances of larger size--the
+Trepangs--which are used by the Chinese as articles of food. The name
+Sea-Cucumber is given in fanciful comparison to a small Gherkin;
+presumably one that has been very badly pickled--for the colour of the
+animal is brownish and by no means green. The mouth of a Sea-Cucumber is
+surrounded by a circlet of tentacles (partially indicated in the
+diagram, Fig. 38). The body is elongated and crawls along: the "star"
+shape, so characteristic of the Echinoderms, is scarcely to be
+recognised except in cross section, where the longitudinal rows
+of tube-feet are seen to outline a pentagon. The skeleton of the
+Sea-Cucumber is of a very meagre description. Instead of forming a
+rounded case, as in the Sea-Urchin, it consists only of loose pieces
+of very small size, situated below the skin. The Starfishes are
+intermediate in this respect. Their "skeleton" consists of a vast number
+of pieces or "ossicles," which are of fair size, but are not closely
+united, as in the Sea-Urchin. They are, however, so numerous and so well
+knit, that the skeleton of a dead Starfish presents the complete outward
+form of the animal. It must be noted that the ordinary skeleton of the
+Sea-Urchin is only _apparently_ exterior. As is the case with the
+ossicles of the Starfish and Sea-Cucumber, the skin lies outside, and
+the hard particles belong to the middle layer, or mesoderm. In this the
+skeleton of Echinoderms differs from the "shell" of a crab or lobster,
+which is formed by a hardening of the skin itself.
+
+[Illustration: FIG. 39.--_A_, Head of a Stone Lily or Encrinite,
+_Encrinus liliformis_, a fossil from the Muschelkalk of Brunswick,
+natural size. _B_, Rock with stalks of encrinites. _C_, Section of a
+stalk.]
+
+The Crinoidea, Encrinites or Stone-Lilies, form another group of the
+Echinodermata. Though still represented by living forms, they attained
+their maximum development in past ages. The English "Mountain Limestone"
+of the Carboniferous period is full of their fossilized remains, which
+form a marble often used for ornamental purposes. The so-called "Stone
+Lily" consists of a "head" comparable with the body of a Star-fish or
+other Echinoderm, which is borne at the end of a long fixed stalk. The
+marble above named owes its ornamental appearance to the presence of
+these stalks, often very long, and cut through at every possible angle.
+The Crinoids have their living representative in English Seas,
+_Antedon_, the Feather-Star (Fig. 40). On the side opposite the mouth,
+where, in the Encrinite, the stalk would be, there are a group of
+elongated processes called cirrhi, by means of which the animal can
+attach itself to stones or seaweeds. When not thus fixed, it swims
+about, by moving its fringed arms, each of which is forked. It will be
+seen that when the animal is fixed by its cirrhi, it stands mouth
+upwards, so that its position compared with that of the Starfish or
+Sea-Urchin is upside down. The young of the Feather-Stars have stalks by
+which they are fixed, like the Encrinites; but afterwards the stalk is
+lost.
+
+[Illustration: FIG. 40.--A Feather-Star, _Antedon bifida_, British Seas,
+three-quarters of the natural size. The short threads in the middle are
+the cirrhi.]
+
+Among fossil Echinoderms there are two groups of stalked forms which
+have no living representatives. These are the Cystoidea and the
+Blastoidea. In both of these the stalk bears, as in Encrinites, a calyx
+or head, which is comparable, with the body of the free Echinoderms.
+
+The Sea-Urchins possess a swimming larval stage, which goes through
+remarkable changes after passing out of the two-layered (Gastrula)
+form. It becomes provided with cilia, which are arranged in bands,
+and outgrowths of peculiar form are established in the case of the
+Sea-Urchins, while the larvæ of the other groups also present
+characteristic shapes. Within the larva the adult form develops, the
+outside of the larva being finally thrown off.
+
+In the young Feather-Star, a subsequent stage of the young animal has a
+stalk, by which, like the Encrinite, it is fixed. This animal therefore
+is at first free-swimming, afterwards fixed, and again free in its final
+stage--a remarkable series of changes.
+
+These queer-shaped things, the Sea-Urchins and their allies, are perhaps
+the last creatures amongst which we should think of looking for
+relations of the Worms. Yet the earliest stages of the larva are
+considered to present a certain amount of resemblance to the Wheel-ball
+larva, which has been referred to elsewhere (pp. 42 and 72). Still more
+startling fact, these larvæ have been compared to that of
+_Balanoglossus_, the lowest member of the Chordata, and a relation of
+the Vertebrates themselves (see p. 143).
+
+TABLE SHOWING THE CLASSIFICATION OF THE ECHINODERMATA
+
+                        { ECHINOIDEA, OR SEA-URCHINS.
+                        { ASTEROIDEA, OR STAR-FISHES.
+    =ECHINODERMATA.=    { OPHIUROIDEA, OR BRITTLE-STARS.
+                        { CRINOIDEA, OR FEATHER-STARS AND
+                        {   STONE-LILIES.
+                        { HOLOTHUROIDEA, OR SEA-CUCUMBERS.
+
+
+
+
+CHAPTER XIII
+
+THE CHORDATA
+
+
+The older zoologists used to speak of Vertebrata and Invertebrata as
+animals with a back-bone and animals without one, and everyone thought
+it a very natural way of dividing up the animal kingdom. It never
+occurred to anyone that it was possible to bridge the interval between
+them and find a link between the two. But now the Vertebrata have been
+compelled to give up their aristocratic pretensions, and own that they
+have risen from the ranks of the common people of the animal world; in
+other words, that they are descended from the Invertebrates. Their
+family secrets have been published to the world, and now everybody knows
+that they have poor relations. But how many, and how nearly related?
+This we do not accurately know; consequently the whole zoological world
+for many years has concentrated all its energies on attempts to find out
+the truth about the matter.
+
+A great sensation was caused by the first discovery of a poor relation
+of the vertebrates among the Ascidians, or Leather-bottle animals. These
+are named from their shape and texture, for they have a leathery skin.
+Now some of these Ascidians have larvæ with a tail; and in the tail
+there is a long cord-like structure, which in many essential particulars
+resembles the cord which precedes the back-bone in the vertebrate
+embryo. This structure is called the Notochord (a string down the back).
+The credit of this great discovery belongs to Russia; for the presence
+of the Notochord in the Ascidian larva was discovered by A. Kowalevsky,
+in 1866.
+
+To the present generation of zoological students, the Chordate
+affinities of Ascidians are part of the ABC of knowledge; and it is
+hardly possible for them to realise that it is only thirty years ago
+since the idea was so new that Huxley, in his "Text-book of the
+Vertebrata," only alluded to it in a footnote. Would-be zoological
+critics, at a somewhat later period, met the theory with ridicule, for
+want of better argument. For critics include not only "those who have
+failed in literature and art," but also those who have failed in
+science.
+
+The majority of the Ascidians are sessile animals, which fix themselves,
+like Sea-Anemones, to some object when they have passed their earliest
+stages of growth; and although there are many forms that swim freely,
+most authorities are inclined to believe that these have arisen by
+adaptation, and that the kinds that are fixed when adult are the
+original type of the group.
+
+Anything more unlike what we should expect to find as a relative of the
+vertebrates could not possibly be imagined. What has been written about
+these little animals by various observers would make a whole series of
+volumes of the size of this one, so many are the puzzles afforded by
+their internal structure. The arrangement of their organs is in many
+respects very unsymmetrical. Their most striking peculiarity, perhaps,
+is the nature of the gills. These form a kind of basket-work, consisting
+of minute holes with intermediate supports; and they are associated with
+a special cavity outside them called the Atrial chamber. Into this the
+gills pass the sea-water which they have breathed.
+
+The group, as a whole, is sometimes considered to present evidence of
+having degenerated from a higher type; but whatever else may be doubtful
+or obscure in its history, the nature of the larval notochord is quite
+clear and certain; zoologists have never had any doubt about its nature
+since the first few years after its discovery.
+
+Ascidians are not at all uncommon animals on the English coast. Some of
+them may be met with on stones near low-water mark, and I have often
+seen them on the shells of oysters sold in the shops--for there the
+town-dwelling naturalist may often find a good many interesting things
+without much trouble. They are like little lumps of tough jelly; of
+various colours, according to the kind, red being the most common, and
+of very indefinite shape. You may see some of the colonial kinds forming
+pretty star-shaped patterns, attached to various objects, such as stones
+and the larger seaweeds.
+
+The place of Ascidians in classification was a puzzle, until their
+relationship with Vertebrates was discovered. At one time they were
+placed with the Mollusca. Now they are grouped, together with the
+Vertebrata and some other creatures that remain to be spoken of, under
+the name of Chordata, or animals possessing a Notochord.
+
+Some of the Ascidians present what has been already described in other
+types (p. 57) as "alternation of generations." The discovery of this
+fact was made by the poet Adelbert von Chamisso. Some of his verses are
+known to English readers, for whom they were translated by Mary Howitt,
+a poetess whose writings were popular with our grandmothers, and
+deserved to be so. This is not the only case in which a poet has been
+also a zoologist: Goethe studied the science, and framed a theory
+regarding the vertebrate skull, which he regarded as consisting of a
+series of vertebræ. In this he was less fortunate than the Italian
+poet; for while Chamisso's observations were correct, and were confirmed
+by subsequent writers, Goethe's theory of the skull is anything but
+correct. It was made worse, too, by the speculations of subsequent
+writers, who attempted to follow it into detail, with the result of
+demonstrating its absurdity.
+
+
+
+
+CHAPTER XIV
+
+THE VERTEBRATA
+
+
+We have spoken of the Notochord as a structure which precedes the
+formation of the spinal column in Vertebrates. This needs a little more
+definite explanation. We all know that the spinal column of vertebrates
+is formed to protect the spinal cord. This protection is, however, an
+afterthought, so to speak, of the vertebrate structure; the lowest of
+all vertebrates is quite without it; and in the lower groups of fishes
+we may trace various steps of its formation. But in these cases where
+the spinal column is absent or incomplete, there is a large and
+well-developed notochord; and in the embryo of higher vertebrates, when
+the spinal column has not yet begun to be formed, the notochord is
+equally a conspicuous feature. It runs from the region known as the
+mid-brain, to the end of the tail, and lies throughout just beneath the
+spinal cord. Whatever its original use in the animal body may have
+been, it undoubtedly acts now as a support to the spinal cord, and
+indeed to the whole body. Bones, we must explain, do not exist either in
+the lower vertebrate, or in the early embryo. In the latter they are
+formed by degrees. The spinal cord and the notochord each begin to be
+surrounded by rings of cartilage or gristle, which by degrees is changed
+into bone. The rings surrounding the notochord, however, gradually
+encroach upon it and obliterate it. The place where it has been becomes
+the Centrum, or most solid part of each vertebra. The notochord at first
+is continuous, and has no division into successive parts; but when the
+bony spinal column is developed, it consists of a series of successive
+vertebræ. Each of them is made up of several parts, which by degrees
+become consolidated into the vertebræ.
+
+[Illustration: FIG. 41.--_A_, The Notochord of Vertebrates. Section,
+considerably magnified, through the middle of an embryo one inch long,
+of Acanthias, one of the Spiny Dog-fishes allied to the sharks. 1,
+Section through Spinal Cord; 2, Section through Notochord; below it lies
+a bean-shaped space, which is a section through a large blood-vessel;
+_sk_, epiblast or skin; _me_, mesoblast or middle layer of the body; the
+dots represent the nuclei of its transparent cells. The intestine, _i_,
+lined with hypoblast, is traversed by a spiral valve, and surrounded by
+the horse-shoe shaped body-cavity. _B_, Diagram indicating the position
+of the Notochord in the vertebra of an adult Common Dog-fish (_Scyllium
+canicula_). 1, "Neural arch" of the vertebra, consisting of processes of
+bone enclosing the central nervous system, or spinal cord; 2, bony
+centrum of the vertebra, hollowed out into a cup, in which lies a soft
+pad, the remains of the notochord.]
+
+The lowest member of the vertebrate group, separated in fact from the
+true vertebrates and placed in a lower division all by itself, is the
+little animal called the Lancelet or Amphioxus. It is often spoken of as
+a "fish"; but it is only by a stretch of our courtesy that it can
+receive that name, being an animal of a much lower form than the fishes.
+It was discovered in 1834, in the Mediterranean, and described as a
+fish; but it had previously been discovered in 1778, by a German
+naturalist who described it as a slug. The latter was misled by its
+external shape. He had not the advantage of the modern methods of
+preparing animals for examination under the microscope; in these days,
+Amphioxus is cut into successive slices along its whole length, and each
+of these carefully magnified, so that no detail of structure is lost.
+The Amphioxus burrows in the sea-sand; it lies buried in it, with its
+mouth just uncovered. Its food consists of microscopic vegetable
+organisms. Its distribution is very wide; it is found in both the
+Atlantic and Pacific waters. It occurs most abundantly in the salt-water
+lakes of Sicily, and in the Gulf of Naples. The specimen first seen, in
+1778, came from the coast of Cornwall. There are eight species; the one
+which is found in the English Channel is the _Amphioxus lanceolatum_,
+also found in the Mediterranean and on the shores of North America.
+
+The classes of the Vertebrata are Fishes, Amphibia, Reptiles, Birds and
+Mammals. We used to learn that of these, fishes had gills, and Amphibia
+gills for a time; but, to be strictly accurate, we must say that fishes
+have gills, and _all_ the rest of the Vertebrata have gills for a
+time. There is no exception to this rule, not even among the highest
+vertebrates of all. But in those vertebrates which stand higher in the
+scale of life than Amphibia, viz., Reptiles, Birds, and Mammals, these
+gills are never brought into use. They only exist in the early embryo,
+and afterwards disappear, giving rise by their modification to other
+structures.
+
+Strange to say, one of these structures is the ear. This takes its
+origin from one of the gill-"clefts" or spaces. The Eustachian tube,
+which communicates between the ear and the nose, is part of this cleft;
+and the little bones which are inside the ear represent the bones
+of that gill-cleft. For, in fishes, bones support each gill, and
+are connected together to form a complex arrangement. In the higher
+vertebrates, which possess gills only in the embryo, this gill-skeleton
+is much modified, and persists as a bone, the hyoid bone supporting the
+tongue.
+
+The gills of vertebrates, arranged in successive pairs along the throat,
+are "perforating gills"; that is to say, they consist essentially of
+holes or spaces which pass right through the wall of the throat.
+
+If we were to seek for a general character of the vertebrates, besides
+those mentioned above, that they all possess a notochord and gills, we
+might also find it in the character of the skin. Fishes, Reptiles, Birds
+and Mammals, all agree in this, that they have a special clothing of the
+skin--scales, feathers and fur, respectively. These three kinds of
+structure, although so widely differing in appearance, are practically
+formed all in the same way, viz., by alternate ingrowths and outgrowths
+of the skin; the ingrowth forming the root of the scale, hair or
+feather, and the outgrowth its projecting part. If these infoldings and
+outgrowths of the skin could be straightened out into a plane surface,
+the skin of a small vertebrate would cover an enormous area. The above
+list excludes the Amphibia: in this class, it should be mentioned, the
+scales have been lost, and are only found in one group.
+
+The scales of Fishes were at one time proposed as a basis of
+classification: large groups being characterized respectively by the
+possession of plain rounded scales (cycloids), scales fringed at the
+posterior end (ctenoid, or comb-like); placoid scales, consisting of
+bony plates, and ganoid scales, large plates covered with shiny enamel.
+These distinctions, however, were not found useful as a guide in
+classification. The diagram shows the elaborate scales of the common
+sole.
+
+[Illustration: FIG. 42.--Scales of the Common Sole, highly magnified.]
+
+Let us now consider some other creatures that resemble vertebrates in
+some ways, and help to form the group of Chordata. Balanoglossus is one
+of them, the Acorn-tongue Animal. This odd name is given to it on
+account of a structure which is called (like the elephant's trunk) a
+Proboscis; this may be compared with a tongue, so far as its use goes,
+for it is thrust out to catch prey and again drawn in. It is oval in
+shape, and therefore fancifully compared to an acorn. It is highly
+sensitive, being richly supplied with nerves. The creature is to all
+intents and purposes a kind of worm; and, like many of the higher
+worms, it has a larva with bands of cilia. This larva, which is better
+represented in some species than in others, was originally described
+under the name of _Tornaeria_. It is considered to resemble, in some
+degree, the larva of Echinoderms; on this hint, some zoologists have
+sought to establish a connection between Vertebrates and Echinoderms,
+and have been able to find other points of comparison besides the one
+named. It remains to be seen whether this suggestion will lead to
+further results. It may be added that the larva of Balanoglossus has
+also been compared with that of Phoronis (p. 122), thus assuming a
+relationship with the Polyzoa, and through them with the Brachiopoda. It
+appears, therefore, that the subject of the possible relationships of
+the Vertebrata is one of the greatest complexity. The last named theory,
+however, has been adversely criticised by very high authority.
+
+We have not, however, explained yet what is the claim of Balanoglossus
+to be grouped with the Chordata. This consists in the fact that a
+certain part associated with the interior of the proboscis has been
+identified, from its structure, mode of origin, and relations with
+the nerves, as a notochord. Balanoglossus also agrees with the true
+vertebrates in possessing successive pairs of perforating gills (see p.
+142), which are especially noticeable in the young animal. The presence
+of this feature is important, in view of the fact that some authorities
+have sought to throw doubt on the genuineness of the notochord of
+Balanoglossus.
+
+Balanoglossus is not without relations, some of which have been
+recently discovered, while others have been known for some time,
+although their affinities were not at first recognised. Among these
+the most remarkable are sessile forms which have received the names
+respectively of _Cephalodiscus_ and _Rhabdopleura_. Both produce buds
+and form a colony, and in both a notochord has been distinguished. The
+former was procured from the Straits of Magellan, while the latter makes
+its dwelling-place in a nearer region, having been found off the
+Shetland Islands, and off the Lofoden Islands. Cephalodiscus, which is a
+very curious creature, receives its name from a disc placed at the head
+end. The use of this structure is believed to be as follows. The units
+of the colony live inside a common system of tubes, which they secrete;
+each unit, when adult, is independent, and can move about inside the
+tubes; the disc is used as a means of attachment to successive spots of
+the tube-wall, as the animal wanders from place to place. Above the disc
+are twelve plume-like tentacles covered with cilia, which create a
+current in the water surrounding the head, and wash food particles into
+the mouth.
+
+That these creatures are but distant relations of the true vertebrates
+is a fact expressed by the names under which they are grouped in
+classification. Those forms which we have just described have received
+the name of Hemichordata--that is to say, Chordata which have but half a
+notochord, since the notochord is very restricted in extent; while the
+Ascidians are grouped under the name of Urochordata, or Chordata which
+only possess a notochord in the tail. The name of Adelochorda, "with an
+obscure chord," is sometimes applied to the Hemichordata.
+
+TABLE SHOWING THE CLASSIFICATION OF THE CHORDATA
+
+                   { HEMICHORDATA, BALANOGLOSSUS, &c.
+    =CHORDATA.=    { UROCHORDATA, THE ASCIDIANS.
+                   { VERTEBRATA, THE BACK-BONED ANIMALS.
+
+Let us return now to the Vertebrate. A character common to all the
+groups of the Vertebrata is the possession of teeth. Readers of the
+previous volumes of this series will recollect that, even among birds,
+instances of the possession of teeth may be found among fossil forms,
+although they are absent in the birds of the present day. In all the
+other divisions of the Vertebrata, the presence of teeth is the rule,
+their absence an exception so rare that we may easily note the chief
+instances of it. Among Amphibia, there are Toads that have no teeth;
+among Reptiles, the Tortoises and Turtles have none; among Mammals,
+teeth are wanting in _Echidna_, the Spiny Ant-eater; and in the
+Ant-eaters and the Whalebone Whales they are absent in the adult,
+although present in early embryonic life.
+
+The majority of people, if asked to give a definition of the meaning of
+teeth, would reply that they are hard structures that grow in the jaw.
+But this is an idea that requires very considerable modification from a
+scientific point of view. In the first place, they are found in other
+places besides the jaws; and in the second place, they are by rights
+structures originally belonging to the skin. Both these important facts
+must be illustrated by reference to the Fishes, which exhibit the
+primitive types of teeth.
+
+In fishes, not only are teeth found on the jawbone, but sometimes also
+on other bones which border upon the cavity of the mouth; they are found
+on the palatine bone, or roof-plate of the mouth, and, still more
+strange, upon bones which belong to the "hyoid apparatus," or skeleton
+of the gills (see above). The latter may form a set of throat-teeth,
+which are used for grinders, while the jaw-teeth are used for biting.
+Among the Carps, the jaw-teeth are reduced, and the fish depends upon
+its throat-teeth only. In the Wrasses, one pair of the bones that bear
+throat-teeth (the inferior pharyngeal bones) are fused, so as to form a
+stronger apparatus: and from this circumstance, the group of Fishes to
+which they belong has been given the name of Pharyngognathi, fishes
+possessing throat-jaws. They have, however, biting teeth as well, in the
+true jaws. The grinding teeth are apparently used for consuming the food
+in a leisurely manner when once it has been taken into the mouth.
+
+A curious circumstance in connection with these "throat-jaws" is, that
+they produce musical sounds. Fishes have other means, however, of
+producing a voice--usually by means of the swimming-bladder and muscles
+in connection with it. Probably they are able, to some extent, to effect
+communication with each other in this way.
+
+It has already been stated that teeth, in their primitive form, are to
+be regarded as skin-structures. Certain fish, which are looked upon as
+ancestral types, have, dispersed throughout the skin, a number of bony
+plates, or granules (placoid scales), more or less formidable, and
+tipped with a hard enamel-like substance. Teeth are regarded as but a
+special form of these. But if they are skin-structures, how come they in
+the mouth and throat? Because the mouth and throat are lined by an
+ingrowth from the external skin; the origin and growth of this is seen
+in the embryo.
+
+In the Mammalia the teeth, though restricted in number, attain the
+greatest possible variety of form, so that the jaws of different but
+allied species may be distinguished by their teeth.
+
+Let us now return to the lowest vertebrate of all, which has a large
+notochord and no bones. This is the _Amphioxus_, the Lancelet. Amphioxus
+has no bones whatever, and no head, in the sense in which we usually
+employ that term; that is to say, most of the structures which we see in
+the vertebrate head are undeveloped. The peculiarities of the structure
+of Amphioxus are many. Among them may be named the curious gills: these
+form a sort of basket-work along the sides of the throat, which at first
+sight bears little resemblance to the gills of fishes, and reminds us of
+those of Ascidians. The gills lead also, as in Ascidians, to another
+cavity, the Atrial chamber. This basket-work is formed, however, by the
+subdivision of the primary pairs of gills. These are very numerous,
+ninety pairs being sometimes named as the number. They cut up the wall
+of the throat to such an extent, that additional supporting bars are
+needed to strengthen it; and, by the formation of these, both in
+parallel and in transverse directions to the primary partitions, the
+"basket-work" is produced, as the growth of the animal proceeds.
+
+The primitive nature of the notochord is, however, perhaps the most
+striking feature of Amphioxus. The chord passes to the front of the
+animal's snout--head it can hardly be called--instead of ending in the
+middle of the brain, as in true vertebrates, for there is, indeed, no
+"brain" of any extent to lie in front of it; and the notochord, together
+with the spinal cord itself, have no other protection than a fibrous
+sheath. The spinal column is thus entirely absent, except so far as it
+may be regarded as represented by this thin sheath.
+
+The Lancelet also differs from the true vertebrates, in that it has no
+limbs. There is a fringing fin along the body, but it is not comparable
+with the fins of fishes. It differs also in possessing no teeth.
+
+In one respect, however, the Lancelet reminds us of a fish: and that is
+in the arrangement of its muscles; these form a successive series of
+overlapping masses on each side of the body, as in a fish.
+
+The development of the Lancelet presents us with an instance of
+the two-layered larva, or Gastrula. This shows that Amphioxus is a
+comparatively primitive type. But it has been suspected that it is less
+primitive than it looks, and that it has degenerated from some higher
+form, owing to its preferring a dull mode of existence, half-buried in
+sand or mud.
+
+There is a huge gap between the Lancelet and the true vertebrates. The
+lowest form of the latter is _Ammocoetes_, the larva of the Lamprey
+(_Petromyzon_). The latter, even in the adult form, has no true limbs,
+though there are fringing fins. The notochord sheath is supplemented,
+however, by cartilage bars which are equivalent to the beginnings of the
+vertebræ of the back-bone. The gills are very different from those of
+other true vertebrates, and it has no jaws. Teeth it has, however, on
+the tongue and the lining of the mouth. Probably this creature is
+greatly altered by adaptation to its peculiar mode of life, so that no
+certain conclusions can be drawn from it regarding the structure of
+primitive fishes. It has a sucking mouth, by means of which it hangs on
+to fishes, while it rasps away their flesh with its rough tongue. When
+not thus engaged, it hangs on to a stone by means of its suctional
+mouth, thus fixing itself at rest. The Hag-fish, _Myxine_, in many
+respects similar, devours dead fishes chiefly. The Hag-fish is found on
+English coasts: so is the Marine Lamprey; while two freshwater forms are
+found in streams.
+
+Leaving the Cyclostomata, as the above fishes are called, we reach the
+true fishes, which have limbs and scales. Something has already been
+said regarding their teeth and gills. The Cartilaginous fishes, in
+which most part of the skeleton remains gristle and does not become
+transformed into bone, include the Sharks, Rays, and Dog-fishes, all
+savage animals with strong teeth. The common spotted Dog-fish of our own
+shores is familiar to everybody: fishermen regard it with disgust, as it
+is not eatable. The Rays are flattened fishes, which live at the bottom
+of rather deep water, and attain enormous size even on our own coasts.
+The Thornback Skate is covered with prickles (placoid scales). All these
+fishes are grouped under the name of Elasmobranchii, the Strap-gilled,
+so called from the structure of the gill-arches.
+
+The majority of familiar fishes, such as the herring, mackerel, cod and
+sole, belong to the group of _Teleostei_, or Bony Fishes, in which, by
+contradistinction from the last group, as much of the skeleton as
+possible becomes bone. Nevertheless, traces of the notochord persist in
+the back-bone of these fishes. Break the back-bone across, of a cod or a
+sole, and you will find between adjacent sides of the centra, or middle
+parts of the vertebræ, a pad of gristly substance. This is the remaining
+substance of the notochord, which finds room between the cup-shaped
+sides of the centra. When the centrum, instead of being biconcave, is
+solid, as in the higher Vertebrata, the notochord is obliterated by its
+encroachment.
+
+The Amphibia, familiarly represented by Frogs and Toads, receive their
+name, "adapted for both lives," from the fact that they usually divide
+their lives between land and water. They are, from one point of view,
+the most interesting of the classes of the Vertebrata, for they form a
+dividing line between the lower and upper Chordata. Below we have
+Hemichordata, Ascidians, Amphioxus, Fishes; all water-dwellers,
+breathing by gills. Above, we have Reptiles, Birds, Mammals,
+air-breathers, never possessing gills, except for a short time, as
+rudiments in the embryo, not brought into use. They are linked by the
+Amphibia, in which we see the larva a water-dweller, breathing by gills;
+the adult, an air-breather, adapted for life on land, and obliged to
+come to the surface to breathe, even when it passes its time in the
+water. The individual Amphibian tells us the past history of the higher
+groups; once they had gills--but growing older, they lost them.
+
+Fig. 43 shows us an outline sketch of Amphibian larvæ; we should require
+an enlarged diagram of an earlier stage, to show the gills, which are
+external and projecting at first, but afterwards are overgrown by the
+skin with the exception of an orifice on each side. The diagram shows
+the gradual change of form. The tails in these tadpoles will presently
+be lost, for they belong to the Anura, or tail-less order of Amphibia
+(Frogs and Toads). The tailed Amphibians, Urodela, are represented in
+Great Britain by the Newts, _Triton_, popularly called Efts. Belonging
+to the Tailed Amphibians also, is the Axolotl, a creature found in the
+lakes of Mexico, and in those of the Rocky Mountains. It may or may not
+retain its gills; and forms with gills, and forms without, may be found
+in the same lake, each capable of laying eggs. The two forms were at
+first described under two different generic names: but when specimens of
+the gill-bearing _Siredon_, kept in confinement, lost their gills, it
+was seen that they became _Amblystoma_. There are other cases of larval
+forms that produce young, and this curious occurrence is known as
+"pædogenesis."
+
+[Illustration: FIG. 43.--Tadpoles, three-quarters of their natural size.
+_A_ to _D_, different stages of the Tadpole of the Common Toad, from
+Epping Forest, England. _E_, Tadpole of _Pelodytes punctatus_, dorsal
+view.]
+
+The Amphibia include the curious creatures called Cæciliæ (blind
+animals), or Gymnophiona. They are snake-like in form, and are without
+limbs; they burrow underground. Their real place in classification was
+not found out at first, but they were classed, by a wrong shot, with the
+Reptiles. They are interesting as being the only Amphibians that have
+scales. These are very minute, embedded in the skin, and arranged in
+transverse rings. The name Gymnophiona, naked serpents, is therefore
+doubly inapplicable: for they are not serpents, and not scaleless.
+
+The Reptiles and Birds at first sight seem to be widely different. The
+latter are the warmest blooded of all vertebrates, the former are
+coldblooded. The one wear feathers, the other scales. Nevertheless,
+there is an intimate connection between them; the reader has doubtless
+already learned from other sources the facts about their relationship,
+so we will not here do more than recall a few of these facts. One is,
+that the birds of earlier times had teeth in their beaks, and possessed
+jointed tails. Another, that the Reptiles of earlier times included
+forms that were able to fly. A third notable fact is the presence of
+claws on the wings of some birds, showing that the wing of the bird was
+not always wholly specialised for use in flight.
+
+We owe to Professor Huxley, the recognition of the close relationship of
+Birds and Reptiles, and the name Sauropsida (Reptile-like animals),
+under which both are included. They agree in being air-breathers and
+never having gills, except the rudiments present in the early embryo:
+this distinguishes them from Amphibia. They agree in having the skull
+set on to the back-bone by a single articulating surface or condyle; and
+thus differ alike from Amphibia and from Vertebrata. They agree in
+having the red corpuscles of the blood nucleated; and in this differ
+from the Mammalia, in which the red corpuscles are non-nucleated discs.
+From a popular point of view, we may say that the striking distinction
+between birds and reptiles lies in beauty and ugliness. Even in their
+eggs, the reptiles display no love for adornment, no colouring or
+pattern. Fig. 44 shows the eggs of some reptiles.
+
+[Illustration: FIG. 44.--Eggs of Reptiles, half the natural size.
+_A_, of African Cobra. _B_, of Common English Snake. _C_, of Common
+English Lizard, _Lacerta agilis_. _D_, of Elephantine Tortoise. _E_,
+of Crocodile.]
+
+The five chief groups of existing reptiles are the Chelonia (Tortoises
+and Turtles); the Rhyncocephala, represented only by _Hatteria_, a
+lizard found in New Zealand; the Lacertilia or Lizards; the Ophidia, or
+Snakes and Serpents; and the Crocodilia.
+
+Perhaps the most interesting point regarding the reptiles that can be
+mentioned in brief space, is the fact that they present traces of a
+median third eye, which have been described by Baldwin Spencer, in the
+New Zealand Hatteria, and in other reptiles. It is situated on the roof
+of the brain. While the structure in Hatteria shows it to be an eye, its
+position corresponds with that of the pineal gland of vertebrates
+generally; so that we find, in fact, the trace of a third eye in all
+vertebrates, including ourselves. It is, however, a trace only. In the
+Lamprey fishes as well as in _Hatteria_, it reaches a further degree of
+development. This pineal eye has been compared in structure to the eye
+of Ascidians.
+
+The Birds, excluding the extinct form with teeth and a jointed tail,
+to which the group name of Archæornithes is given, fall into two
+groups. These are the Ratitæ, or Birds with Raft-like, _i.e._ flat,
+breast-bones, and the Carinatæ, or Birds with keeled breast-bones. The
+former include the African Ostrich (_Struthio_), the American Ostrich
+(_Rhea_), the Australian Emu, the Cassowary of New Guinea, and the Kiwi,
+or Apteryx of New Zealand; all of them birds that cannot fly. The
+vast majority of birds belong to the Carinatæ, characterised by the
+projecting keel (Carina) in the middle of the breast-bone. The presence
+of this, which affords a safe attachment for strong muscles, is
+associated with the power of flight. It is impossible to treat the birds
+more fully in the space allotted to this little story, but a few words
+about feathers, however, may find a place here.
+
+The colour of feathers is a subject of much interest. Everyone is
+familiar with the brilliant tints often presented by the feathers of
+birds, and everyone who is a close observer of natural objects knows
+that there are some feathers which are iridescent, changing colour
+according to the direction in which light falls on them. It has been
+shown by Dr. Gadow that this variation of the colour of a feather is due
+to its structure; this may be described as prismatic, for the small
+divisions of the feather present acute angular edges, which reflect the
+light like the edges of a prism. These are symmetrically repeated all
+along the feathers, so as to reflect the same colour throughout. Thus in
+the plumage of the common red and green parrot, we see feathers that are
+red when held in one position, and yellow when shifted to another
+position; while there are also feathers that are blue when seen in one
+position, and green when seen in another; the alternative colour being
+the one next in order in the rainbow.
+
+Another point regarding the colours of feathers has no doubt puzzled
+many of our readers; and that is, the metallic quality of the colouring
+in some exceptional feathers, and in these only. The feathers of the
+parrot just referred to, are, for instance, simply red and yellow, or
+blue and green; but the feathers of the peacock, though displaying the
+same colours, show a metallic lustre which is wanting in the other case.
+The feathers of the starling, the blackbird, and the black hen of the
+farmyard, though not so brilliant as those of the peacock, are the same
+as regards the quality of the light they reflect. The secret of the
+difference lies in the greater opacity of the feathers named; they are
+_black_ feathers, while those of the parrot are light-coloured. Now
+after the metals themselves, there are few objects in nature so opaque
+as the black pigment of a black feather. If a thin section through
+the roots of young black feathers is cut for examination under the
+microscope, the pigmented parts, although cut so very thin, appear
+completely opaque. And just as a glass gives a better reflection when
+backed by something opaque, so does the reflecting surface of the
+feather. Hence it is that the quality of the colours reflected by these
+feathers is what we call "metallic." If we ask for a definition of this
+metallic brightness, other than the accepted fact that it resembles the
+light reflected from metals, the artist will reply that it consists in
+two things--(1) the greater brilliancy of the light reflected, that is
+to say the greater completeness of the reflection; and (2) the entire
+absence of those gradations of light which are afforded by the
+reflections from any object, however dark, that possesses a surface
+translucent, even in the smallest degree. "Metallic" reflections, in
+fact, may be defined as those in which the greatest amount of light is
+reflected, and the reflected sunlight receives from the reflecting
+surface the least possible degree of modification. While the actual tint
+of the colour reflected by a black feather, then, is determined by the
+form and position of its angular ridges, the quality of the reflection
+is determined by the opacity of the substance itself. It is interesting
+to note that the opacity necessary for reflecting a "metallic" lustre,
+may be produced by means of pigment, in the vegetable as well as in the
+animal organism; for instance, in the dark centres of _Coreopsis_ (the
+Beetle Flower), and several other fashionable garden plants belonging to
+the Compositæ or Daisy family. Within the animal kingdom, we may note
+that the metallic lustre is almost entirely confined to land animals;
+their dry skins have more chance to develop opaque parts, than the moist
+tissues of creatures that live in the water. The most familiar exception
+to this rule is the Sea-Mouse, an Annelid worm found on English coasts
+(p. 73), which receives its odd name because it is a fat oval creature,
+covered with bristles, thus greatly differing in appearance from most
+worms. The larger bristles, which are of a dark purplish-black colour,
+have a bronze or golden metallic lustre. Various other annelids exhibit
+brilliant rainbow colours; for example, _Nereis_, the Rainbow Worm, also
+found on English shores; but without the underlying black opaque
+pigment, the reflections from the surface fall short of absolutely
+metallic brightness. On land, we see among the insects innumerable forms
+which present a metallic lustre, the beetles being the most notable in
+this respect. To return to the vertebrates, from which we started,
+everybody must have noticed that the fur of a clean well-kept black cat,
+when lit up by the bright sunlight in which the animal loves to bask,
+shows little rainbow reflections of red and green. These are due to the
+presence of little grooves and irregularities on the surface of the
+hairs, which play the same part in breaking up the light which they
+reflect, as do the sharp angles of iridescent feathers. Like the
+iridescence of the Rainbow Worm, they fall short of absolutely metallic
+brightness; the fault in this case being due not to the nature of the
+underlying stratum, so much as to the incomplete development of the
+light-reflecting grooves. Yet this instance serves to show the part
+taken by the dark pigment; for while the play of colours is perfectly
+obvious in the fur of a black cat, it is almost impossible to
+distinguish it in the case of cats with fur of lighter shades.
+
+The Mammalia, or animals that suckle their young and produce them by
+birth, were formerly considered to be sharply defined from animals that
+lay eggs, such as the birds and reptiles. But in 1884 Mr. Caldwell
+confirmed the statement which had been made previously, yet hardly
+credited by the scientific world, to the effect that the lowest form of
+mammals lays eggs. This, the Duck-Mole or _Ornithorhyncus anatinus_
+(Bird-billed animal much like a goose), is a native of Australia and
+Tasmania. It lives on the banks of rivers, and burrows in the bank. It
+has webbed feet, and therefore sometimes receives the name of Platypus
+(flat-foot). It lays eggs two at a time, in its burrow; and these eggs,
+like those of other egg-laying vertebrates, have a yolk.
+
+A kindred form, _Echidna hystrix_ or Spiny Ant-eater, is found in
+Australia, Tasmania, and New Guinea. The _Echidna_ hatches its young
+in a temporary pocket, which appears in the neighbourhood of the
+breasts, and disappears after the young are old enough to take care of
+themselves. The _Ornithorhyncus_ has fur, the _Echidna_ has spines, with
+hairs between them. Neither bears the slightest resemblance to a bird;
+the comparison suggested in the name of _Ornithorhyncus_ is fanciful,
+and depends chiefly on the flat beak-like mouth; these egg-laying
+quadrupeds may, however, be reasonably brought into comparison with
+Reptiles. Neither of them has any teeth; the _Echidna_ has no teeth at
+all; the _Ornithorhyncus_ loses them at an early stage of growth, and
+develops instead hard horny patches in each jaw. With these it crushes
+its food, which consists of small insects, worms, etc. The _Echidna_, on
+the contrary, lives in rocky places, and feeds on ants, which it
+searches for with its long-pointed snout. These two genera are grouped
+under the name of Prototheria or Primitive Mammals.
+
+The pocket in which _Echidna_ hatches its young, suggests a relationship
+with the next group, the Metatheria or Marsupialia, which are the
+characteristic mammals of Australasia. These are distinguished by the
+possession of a permanent nursery-pocket, the "marsupium." In this they
+put their young, which are born, like those of other mammals, not
+hatched from eggs like those of the last group. They are, however, born
+in a very backward condition, and therefore require to go through a
+further period of incubation, so to speak, in the marsupium. Here each
+one attaches itself to a teat, to which it remains fixed. But it cannot
+suck as a new-born kitten or puppy does; and the milk is forced down its
+throat by the muscles of the teat.
+
+[Illustration: FIG. 45.--Skull and Lower Jaw of Great Kangaroo,
+_Macropus giganteus_, much reduced.]
+
+The Marsupialia are not entirely confined to Australasia; a few occur
+in South America, and in North America they are represented by the
+"'possum," _i.e._ Opossum, of American stories. The Marsupials seem
+almost to mimic the forms of ordinary quadrupeds. Thus _Notoryctes_, a
+form discovered a few years ago, mimics a mole. The fact is that, just
+as among the Eutheria, or higher mammals, special types have become
+established, possessed of certain habits, and especially of certain
+habits with regard to food, and modified in accordance with those
+habits. Thus there are among them savage carnivora, harmless herbivora,
+and rodents; and these respectively share certain characteristics in
+common with the carnivora, herbivora, and rodents, belonging to the
+Eutheria. One of the herbivorous marsupials is the Great Kangaroo,
+_Macropus_. It gets its name, Large-foot, from the size of its
+hind-paws; on these it stands, and by their aid it takes remarkably long
+leaps. Its skull is shown in Fig. 45; this, however, has not the full
+set of teeth, some of which are soon shed. It crops the herbage with its
+front teeth, and grinds it with its back teeth, like other herbivora.
+
+[Illustration: FIG. 46.--Skull and lower jaw of Rodent; _i_, _i_,
+incisor teeth, separated by a long interval from the molars. About
+one-half the natural size.]
+
+The study of the teeth is of great help in the classification of the
+Mammalia. Of the eight orders of the Eutheria, two alone, the Sloth
+order and the Whale order, show a tendency to the suppression of the
+teeth. Those of the herbivora and carnivora may easily be compared by
+anyone, in the sheep and the dog respectively. Fig. 46 shows the skull
+of a Rodent, with elongated front teeth, adapted for that persistent
+gnawing which makes the animals of the order, such as the Rat and
+Rabbit, so terribly destructive.
+
+TABLE SHOWING THE CLASSIFICATION OF THE MAMMALIA
+
+
+                   { 1. PROTOTHERIA, or EGG-LAYING MAMMALS.
+                   {      One order, the MONOTREMATA.
+    =MAMMALIA.=    {
+                   { 2. METATHERIA, or MARSUPIAL MAMMALS.
+                   {
+                   { 3. EUTHERIA, or HIGHER MAMMALS.
+
+The Mammalia are a terrestrial group. Exceptions are the Cetacea
+(Whales), Sirenia (Dugongs), and Seals or Sea-Carnivora, but all of
+these are air-breathers; even the Whale can only stay under water for a
+limited period of time. Hence we see that none of them are really
+animals belonging to the water; they are land animals adapted for life
+in the water.
+
+This brings us very near to the last chapter in the Story of Animal
+Life. We have seen that our story began with the One-celled Animals, and
+went on with the tale of the Two-layered Animals, in which each layer
+was built up by cells in partnership. From Two-layered Animals we passed
+to Three-layered Animals, and from them to Three-layered Animals with a
+"body-cavity." When we reached the latter, we found amongst them traces
+of the ancestry of the vertebrates. From the lowest of the Vertebrata,
+the Lancelet, we passed on to the Lamprey, and from that to the true
+fishes. In the latter we found the parent type of all the other
+Vertebrata, possessing gills in the adult, while the latter only possess
+them, or traces of them, in early stages of growth. The Amphibia formed
+a group to themselves, in which we traced the loss of gills in the
+adult. In the Reptiles, four-legged egg-laying animals, we found not
+only a close relationship with birds, but also, through the four-legged
+egg-laying _Ornithorhyncus_, a relationship with the Mammalia. The last
+group comprises all the furry animals, and culminates in the order
+Primates, in which the great Cuvier included Man.
+
+TABLE SHOWING THE DISTRIBUTION OF ANIMAL LIFE BETWEEN LAND AND WATER
+
+                                                          KEY TO TABLE:
+
+                                                          A ARTHROPODA.
+                                                          B VERTEBRATA.
+                                                          C CHORDATA.
+
+               LAND                         WATER
+
+    Except a few forms living  --All the PROTOZOA.
+      living in damp
+      places, or as
+      parasites
+                               --All the SPONGES.
+                               --All the COELENTERATA.
+    Except a few forms         --VERMES.
+      terrestrial, and
+      many parasitic
+
+    Insects, except            --A very few adult forms and }
+    Except Wood-lice and           a few larvæ.             }
+      a very few others        --CRUSTACEA.                 } A
+    Spider-like animals,                                    }
+      except                   --_Limulus._                 }
+                               --All the BRACHIOPODA.
+                               --All the POLYZOA.
+                               --All the ECHINODERMATA.
+    Except the Land-snails     --MOLLUSCA.
+                               --HEMICHORDATA.                      }
+                               --UROCHORDATA, or Ascidians.         }
+                               --All the Fishes: (some few can      }
+                                    exist in damp places)      }    }
+                      Amphibia belong to both.                 }    }
+    All the Reptiles           --Except swimming forms,        }    }
+                                   which are nevertheless      }    }
+                                   air-breathers, only         }    }
+                                   partially adapted for       }    }
+                                   water life: Tortoises       }    }
+                                   and Turtles, Crocodiles     }    }
+                                   and Water-Snakes, _e.g._    }    } C
+                                                               }    }
+    All the Birds: swimming and                                } B  }
+     diving forms are only adapted                             }    }
+     for temporary visits to the                               }    }
+     water                                                     }    }
+    All the Mammals            --Except Whales, Sirenia,       }    }
+                                   and Seals, which are        }    }
+                                   nevertheless                }    }
+                                   air-breathers, only         }    }
+                                   partially adapted           }    }
+                                   for water life.             }    }
+
+Another volume of this series, "The Story of the Earth," has already
+dealt with the distribution of animal life in time; while "The Story of
+Animal Life in the Sea" tells about the present inhabitants of the
+ocean. It is therefore unnecessary to say much in this volume regarding
+the distribution of animal life. A table is, however, appended, which is
+not without interest. It shows how the chief great groups of animals are
+divided between land life and water life, whether in fresh water or
+salt. It will be seen that the terrestrial animals are much in a
+minority, and that they belong, for the most part, to the higher types.
+They are, in fact, stragglers, bold emigrants from the early home of
+animal life, which lies in the more shallow parts of the waters of the
+sea.
+
+
+
+
+CHAPTER XV
+
+MAN
+
+
+If we are to accept the opinion of Dr. Isaac Watts, man, as a moral
+being, is distinctly inferior to the "birds in their little nests," who
+live in harmony with one another; and, again, if we are to believe
+Solomon, he is by no means always the equal in intelligence of the Ant.
+Yet somehow it came as a shock to many who had been accustomed to revere
+both these authors, when they were asked, early in the latter half of
+the nineteenth century, to regard man, from a zoological point of view,
+as just a little superior to the Apes.
+
+Then arose a great agitation as to the possibility of finding the
+Missing Link. We shall see later on in this chapter, that if Research
+had been content, like Charity, to begin at home, its industry would
+have been duly rewarded.
+
+But inquiry, carried far afield in time and place, has not been without
+result. For it is generally believed that the remains found in 1894 in
+Java by Dr. Eugène Dubois, are veritably those of the Missing Link.
+These remains, which consist of the top of a skull, two teeth and a
+thigh bone, belong either to the oldest Pleistocene age, or to the upper
+Pliocene; they are found in association with the remains of other
+animals, among which are included some forms now extinct, or absent from
+that region. These ape-like remains have been carefully compared with
+those of the lowest races of man which have hitherto been found in a
+fossil state, and the result of the comparison is as follows: Of twelve
+experts present at the Zoological Congress held at Leyden, "three held
+that the fossil remains belonged to a low race of man, three declared
+them to be those of a man-like ape of great size; the rest maintained
+that they belonged to an intermediate form, which directly connected
+primitive man with the anthropoid apes" (Haeckel). To the creature
+represented by these bones has been assigned the name of
+_Pithecanthropus erectus_, the Upright Ape-Man.
+
+Let us now return from the subject of the Java fossil to those inquiries
+which, as we have above suggested, begin at home. We have already
+referred to the great principle of modern zoology, that the history of
+the development of the individual sums up the history of the development
+of the race. Of late years it has occurred to scientific men to apply
+this principle in the case of human beings, and to ask, "What can the
+baby teach us?"
+
+The Baby, for one thing, has a very small nose, insignificant compared
+with the size of its jaw. At least scientists find that this is the case
+with their babies--if would, of course, be invidious to make such a
+remark regarding their friends' children; and still more so to add, that
+in this the Baby differs from the human adult, and somewhat resembles
+the Ape, in which the nose is still less prominent, and the jaw still
+more so. Observations have been made, too, regarding the Baby's
+remarkable power of "holding on" with its hands. While a Baby is, in
+most respects, a very weak creature, yet its powers of grip have been
+favourably compared with those of adult human beings. No one who has
+ever tried to rescue his watch or his hair from the clutches of a
+friend's Baby, will feel inclined to doubt the conclusions of scientific
+observers regarding the point in question.
+
+The observations above referred to were made by Dr. Louis Robinson. He
+drew his conclusions from the study of sixty cases, all of them infants
+less than a month old; and of these at least half were tested within an
+hour of their birth.
+
+In every instance except two, says Dr. Robinson, the child was able to
+hang on by its hands to the finger, or to a small stick three quarters
+of an inch in diameter, and to sustain the whole weight of its body for
+at least ten seconds. "In twelve cases, in infants under an hour old,
+half a minute passed before the grasp relaxed, and in three or four
+cases nearly a minute." In infants of about four days old, increased
+strength was shown, and "nearly all, when tried at this age, could
+sustain their weight for half a minute. At about a fortnight or three
+weeks after birth the faculty appeared to have attained its maximum,
+for several at this period succeeded in hanging for over a minute and a
+half, two for over two minutes, and one infant of three weeks old for
+_two minutes thirty-five seconds_!" "Thus," says Dr. Robinson, "a
+three-weeks-old baby can perform a feat of muscular strength that would
+tax the powers of many a healthy adult. If any of my readers doubt
+this," he adds, "let them try hanging by their hands from a horizontal
+bar for three minutes."
+
+In these facts Dr. Robinson finds something to remind us of the
+ape-babies that owe their safety to their capability of holding on to a
+tree-climbing mother; and also something to suggest connection with an
+ancestor which, although well accustomed to the use of its hands, had
+yet to learn the use of its feet for walking on flat ground.
+
+The same author, in discussing the "Meaning of a Baby's Footprint," has
+shown that the foot of a young child bears traces of adaptation to a
+state of existence in which it was used for purposes other than that of
+walking.
+
+"The toes of infants," says Dr. Robinson, "are much more mobile than
+those of adults. The great toe is shorter than the second and third,
+and is often separated from the second by a considerable interval.
+The four outer toes can be, and frequently are, bent downwards so as
+to show a distinct knuckle on the upper aspect of the foot at the
+metatarso-phalangeal joint, and when at the same time the great toe is
+flexed and turned inwards towards the sole, the front part of the foot
+makes a very respectable fist. The great and little toes are often made
+to approach one another beneath the rest, and I have seen one child who
+could almost make them touch, and who habitually would endeavour to make
+the great toe oppose the others when any graspable object was brought
+into contact with the front part of the sole."[E]
+
+[E] _Nineteenth Century_ for May, 1892.
+
+Regarding the lines in the sole of the foot, Dr. Robinson says: "The
+sole is covered with lines of a character exactly similar to those on
+the hand; and when the toes are bent downwards these become deep
+creases, showing that they are, like the palmar lines, the natural
+folding-places of the integument to facilitate the action of
+grasping.... The lines are scarcely visible at fourteen months old, and
+are only present in a few cases after the age of two years. In adults no
+trace of them can be seen when the foot is at rest, and only the
+faintest indication at one or two spots when the toes are flexed to the
+utmost. The obliteration is doubtless owing to the foot being used as an
+organ for progression rather than prehension, and it will be seen that
+the most distinct line crosses the sole at the spot where the epidermis
+is always dense and callous, and the subcutaneous tissues thickened into
+a cushion-like pad by the pressure and friction consequent on walking.
+This line undoubtedly marks the place where the chief fold in the skin
+was situated, when the toes were habitually clasped round some object
+such as the branch of a tree." It has been pointed out by other writers
+that the lines of the sole of the foot can plainly be seen in the adult
+foot of some savage races. It must be added, however, that the survival
+of the lines in the adult civilised foot is by no means so rare as Dr.
+Robinson's remarks would lead one to suppose. I have seen instances in
+which they were quite clearly marked. It must be added that anyone who
+wishes to confirm my observations in this respect must be careful not to
+mistake lines of disfigurement, caused by the pressure of boots, which
+are sufficiently common, for the primitive lines of the foot.
+
+The child, as it grows, ceases to remind us of the ape. Its nose gets
+bigger as its toes cease to wriggle and learn to stand. But, for years
+of its life, it is only too apt to remind us of the savage. How greedy
+it often is! How readily it snatches that which does not belong to it!
+How quick it is to quarrel with its playmates, and to fight! How noisy
+when at play! How cross when it meets with disappointment! How fond of
+tawdry things! In all these qualities we see the history of the race,
+repeating itself in the life of the individual. The savage has preceded
+the civilised family--the child shows us the faults of a lower race.
+With the elapse of years they disappear, and are replaced by the more
+amiable and gracious manners of the adult human being.
+
+Nor do we need to go into the nursery to find links with our inferiors.
+Much, indeed far too much, has been written of late years about
+"atavistic degeneracy"; that is to say, degeneracy which imitates the
+characteristics of our forefathers. Many things which are classed as
+diseases, whether of the body, mind, or moral nature, may be explained
+in this way. Take the gills, which, as we have stated, exist in all
+vertebrates, but not in the adult of the highest groups. In a sickly
+individual, even among the highest vertebrates, traces of these are
+sometimes seen existing in the adult, as a gap or open space in the
+neck, called by the medical man "cervical fistula": this is an instance
+of degeneracy in the body. Take, for another instance, the kleptomaniac,
+who snatches up everything he takes a fancy to, although he is not in
+want. This is degeneracy of the mind, a relic of savage nature out of
+place in civilised man. Yet the gill-space is an ancestral feature which
+has its right time to appear, though it is out of place in the adult;
+and the "want-to-snatch" stage, as we have already seen, is quite
+natural in the young child. A parallel instance to the last is that of
+the hysterical girl who invents all sorts of tales about her harrowing
+adventures, weaving in stories she has heard of other people, with an
+account of her own life. She is an impostor; but her instinct for
+weaving yarns is that of the savage, who is the more admired by his
+fellows the more he can show himself a liar. Even the dangerous
+criminal, such as the Anarchist assassin, is comparable with the
+treacherous savage, who stabs his guest, and with the fierce animal that
+bites the hand that feeds it.
+
+The causes of degeneracy may seem obscure. But if we turn to our
+gardens, how easily is the process understood! Leave a cultivated plant
+to look after itself; neither watered, nor manured, nor weeded; and how
+long will it be before the plant resembles its wild ancestors? The
+flower will be less fine, the leaves more weedy; the whole aspect of the
+plant is changed. The causes: insufficient food and water, and the
+struggle for root space, standing-room, and light, with the weeds
+around it. Just in like manner the human being, when unfed, unwashed,
+and untaught, begins to degenerate. The want of fresh air and light
+associated with slum life, and even in the country, associated with the
+homes of the poor, are factors in the case that are not to be forgotten.
+Add to these drink, and the other sins of the fathers which are visited
+on the children. All these are among the causes of degeneracy.
+
+Nay more, the very virtues of the parents, as we account them, may lead
+to the degeneracy of the offspring. Overwork, either physical or mental,
+causes the deterioration of the family, and in our days nearly every man
+successful in any career, either commercial or intellectual, is guilty
+of overwork. The "haste to be rich," equally with the haste to be
+famous, tells on the next generation. Those who are fond of moralising
+at the expense of their neighbours, enjoy pointing out the
+unsatisfactory careers of the sons of men who have become rich. Almost
+invariably such a one is idle, we are told, and fond of pleasure.
+Good cause has he to be so. He comes into the world with weakened
+constitution, owing to his father's strenuous career; and if he were to
+work as hard as his father, he would probably soon be dead; or at least
+his children, in their turn, would be miserable and diseased. Nature
+guides his inclinations, and whispers "Do not work too hard," "Do not
+deny yourself too much"; and thus, so long as his father's money
+maintains him, his life is preserved.
+
+What is the kind of degeneracy that overtakes the family of the
+brain-worker? The modern world is full of it. We owe to the unamiable
+genius of Max Nordau a criticism of the intellectual world of the
+present day, which attributes well-nigh all the follies of intellectual
+cliques to degeneracy. Poetry, which is "full of sound and fury,
+signifying nothing," rich in rhyme and alliteration, but wanting in
+sense; art which seeks effect by loud and inharmonious colours; music
+which rejects "mere melody": in these the critic sees the taste of the
+savage, fond of a jingle of words, fond of bright colours, and ignorant
+of middle tints; and fond of noise without a tune.
+
+The so-called æsthetic movement which, a few years ago, wrought such
+marvels in decoration and in dress, comes in for a share of the critic's
+analysis. The dull senses of the degenerate cannot appreciate the soft
+colours which ordinary persons like to look at; to attract his attention
+and to please his fancy, he must have staring red, or staring blue. Or,
+if he possesses an object which is of special interest, he must bring
+this into contrast with a very sombre background, lest by chance it
+should miss being seen.
+
+I met with an amusing instance the other day which is much to the point.
+In a remote part of the British Isles, two friends, immigrants from the
+world of "culture," had been criticising the landscape. It was a pity,
+they agreed, that everything was so grey and dull; otherwise the
+neighbourhood might have been pretty. If only the cottagers could be got
+to grow something in their gardens that would give a touch of _colour_
+to the scene! These poor creatures had before their purblind sight all
+Nature's rich harmony of colour, which affords such pleasure to persons
+of true taste. Green fields, brown rocks, blue sea, and blue sky, all
+were dull to them. Wild flowers of a score of kinds, and bright with
+every colour--these were too insignificant to be visible. They wanted
+some big patch of vivid colour, perfectly inappropriate to the climate
+and surroundings. Some exotic plant was needed, in their opinion, to
+give a touch of brightness. The harmony of colour and beauty of form in
+our native plants, and in the common flowers of cottage gardens, were
+imperceptible to their unobservant eyes. Their intelligence was on a
+level with that of the savage, who is impressed by new and striking
+objects, and delighted by gaudy colours, but finds no beauty in wild
+nature or in accustomed things. These people were typical specimens of
+the degenerate of the book-reading classes; dull of understanding and
+wanting in taste, as the result of mental overwork in several successive
+generations; immeasurably inferior in æsthetic capabilities to the
+untaught peasants and fishermen of the district they would fain
+enlighten--for these appreciate the beauty of their country, and love
+its flowers.
+
+Much might be added regarding atavistic degeneracy, as an explanation of
+the mental and moral defects of human beings. Its most frequent form,
+perhaps, is that of mere laziness. The Ape does not work; nor does the
+savage, if he can possibly help it. Civilised man, if thoroughly sound
+in mind and body, likes activity, and activity with a purpose. The poor
+man takes a pride in his labour; the rich man takes a pride in his skill
+in games, his learning, or his efforts to benefit others. The idler,
+disinclined for either hearty work or hearty play, is a Degenerate. Of
+late there has been much discussion of a plan for treating the confirmed
+idler as a criminal. It will be seen from the remarks made above, that
+there are equally good reasons for treating him as an invalid. In
+criticising the plans of would-be reformers, this fact should not be
+forgotten. He was a wise man who said "You cannot, by passing an Act of
+Parliament, make a Vice into a Crime."
+
+It must, however, be remarked that the doctrine of degeneracy has lost
+both in force and in usefulness, by the treatment it has received at the
+hands of those who have constituted themselves its popular exponents.
+Some of these writers have made it but too evident that their criticisms
+are often captious, and that their definition of degeneracy includes all
+human failings--except their own. The reader who devotes a little
+attention to the subject will, however, readily find an explanation
+of this: for he will easily recognise, in the popular writers on
+Degeneracy, the characteristics of the Degenerate, as described by
+themselves.
+
+First, the choice of a disagreeable subject, when the whole field of
+science lay open to them: for the Degenerate prefers a disagreeable
+subject. Secondly, the almost universal discovery of causes of
+dissatisfaction, in every possible direction: for the Degenerate is
+always vexed with everybody--except himself. Again, the want of
+principle shown in appealing to the morbid tastes of the public,
+by laying before it information on disagreeable subjects: for the
+Degenerate is lacking in principle; what does it matter to him how much
+harm is done to weak minds by his writings, so long as he sees in such
+writings a safe means of securing eager readers and liberal pay? Again,
+the Degenerate seeks notoriety; and this is easily secured by writing
+books that discuss the morbid side of life.
+
+Above all, the habit of carrying the war of criticism into regions of
+art and culture with which the writer is obviously unfamiliar: this
+also marks the tendency of the writer's mind. To criticise the doing of
+that which he can by no means do; to destroy that which he can by no
+means make; to leave no margin of leniency in his judgment, for the
+imperfections which disfigure all human work: these are the familiar
+failings of youth, of the unripe mind. They are also those of the
+type of mind that never attains ripeness--of the Degenerate: we are
+forbidden, on high authority, to apply to our brethren a shorter and
+less modern term.
+
+But although the doctrine of Degeneracy has thus found its way to the
+general reader in a form which is often much to be regretted, it is
+nevertheless a doctrine which, if wisely used, may lead to the most
+beneficial results. Already it is widely recognised, by the thinkers of
+all nations, that the theory of degeneracy, when thoroughly understood,
+must revolutionise our treatment of the criminal classes. Instead of the
+attempt to punish, civilised legislation must eventually, in many cases,
+substitute a system of restraint.
+
+It is useless to try to reform the idler or the thief, whose instinct
+for idling or thieving is as imperative as a cat's instinct for catching
+mice. So long as he goes free, so long will the instinct reassert itself
+at every renewal of opportunity. Repeated punishment of the offender,
+who is powerless against his own impulses, is frequently a mere cruelty;
+while his repeated release, at the termination of every punitive
+sentence, is, on the other hand, still more certainly, a cruelty to the
+community at large, which he afflicts by his presence. Public opinion is
+gradually becoming awake to the necessity for fresh methods of dealing
+with these problems; it is by the patient investigations of scientific
+men that it has been enlightened.
+
+TABLE SHOWING THE PLACE OF MAN IN CLASSIFICATIONS
+
+    =Grade IV.=  TRIPLOBLASTIC Animals with a BODY-CAVITY.
+
+    =Group.=     CHORDATA; Animals with a Notochord.
+
+    =Phylum.=    VERTEBRATA; Animals with a Back-bone.
+
+    =Class.=     MAMMALIA; Animals that suckle the young.
+
+    =Order.=     Primates.
+
+    =Genus.=     Homo, i.e. man.
+
+    _Species._   Sapiens (possessed of sense).
+
+Meanwhile, it must not be forgotten that the theory of degeneracy
+has its cheerful aspect. It enables us to look at the offending
+fellow-creature who belongs to the criminal classes, as an incomplete
+development rather than as a hardened sinner. It reminds us, too, that
+the criminal and the idler of to-day are now, what in the times of
+savagery and animalism, every man once was. The degenerate criminal, in
+fact, stands as a landmark, to point out the progress which has been
+made by the human race. This was the starting-point, where now he
+stands. How great the progress that is measured by the distance between
+him, and the orderly, kindly-hearted citizen of the present age!
+
+
+
+
+CHAPTER XVI
+
+HOW ZOOLOGISTS DO THEIR WORK
+
+
+It is one of the most well-worn of commonplace sayings, that "one half
+the world does not know how the other half lives." It is equally true
+that one half the world does not know how the other half works; and
+especially is this the case when one of the world's halves is its
+learned, and the other its unlearned, half. The average business man
+probably has an idea that the man of learning has a pretty easy time of
+it, and that his most arduous occupation is to enlighten an attentive
+world by reading papers at the meetings of the British Association
+and the Royal Society. He has a vague idea that the man of learning
+sometimes uses midnight oil, but it would surprise him to be informed
+that the man of learning often sets to work at five o'clock in the
+morning--as is actually the case. And well he may, considering the
+magnitude of the task he has in hand, and the variety of the odds and
+ends of labour that it includes.
+
+_Firstly_, how does he obtain the raw material for his work? The
+scientist, like the cook, must "first catch his hare" before any further
+details of work can be arranged. He does not, as a rule, do this in
+person, except when an animal of unusual interest is concerned. An army
+of collectors, all the world over, are constantly busy in searching for
+material for the zoologists, on land and sea. They look for employment
+and pay to the museums and laboratories of the learned world. When the
+specimens arrive, what is to be done with them? Some arrive alive, and
+may be dismissed from present consideration. The dead specimens give
+employment to a number of workers who are under the command of the man
+of learning. There are skins to be mounted and stuffed, bones to be
+articulated and set up, each practically the work of a different trade.
+There are drawings to be made of all important specimens, a task which
+affords employment for the artist and the photographer. There are
+carcases large and small, to be immersed in preservative fluids until
+they can be thoroughly examined in detail. And woe betide the zoologist
+who allows any of these tasks to be performed without his own personal
+supervision. He will realise, as all careless masters do, that blunders
+may be made in an hour, which cannot be repaired in a day. But when all
+is done that servants and helpers can accomplish, the real business
+remains to be done. Is there among the specimens one which has not been
+thoroughly overhauled by other writers, one whose every detail of
+structure is not already to be found printed in a book? That one must
+be examined with the utmost accuracy. If it is big enough, it must
+be dissected, and every part recorded and figured in diagrams. But
+suppose it is a small creature, whose parts can only be seen under the
+microscope, a long series of processes are necessary before it is ready
+for use. In its fresh state, it contains a quantity of water, and if
+left to itself would shortly decompose. Even if already immersed during
+carriage in various preservative fluids, it still contains much water,
+and, if so, neither will it keep for an indefinite length of time, nor
+could it be satisfactorily examined under the microscope. It must be
+soaked in one of various chemical solutions, to harden and preserve
+it. If very small indeed, a mere speck, it perhaps only needs to be
+transferred to a fluid in which it can be "mounted" and placed under the
+microscope. But with the vast majority of specimens, an immense amount
+of labour is needed before they are ready for inspection under the
+microscope.
+
+This will easily be understood if we reflect for a moment on the way in
+which objects are examined under the microscope. For purposes of
+scientific investigation, they are rarely looked at under light that
+falls upon their surfaces, that is to say, by reflected light; for this
+method can show nothing but details which are external and comparatively
+unimportant. They are seen by light placed behind them so as to shine
+through them, _i.e._ by transmitted light. If the object is not
+extremely thin, it will shut out too much light, and thus it cannot be
+clearly seen, therefore all objects, except the most minute, must be
+divided into thin slices, technically known as "sections."
+
+If we want to know not only the microscopic structure of organs, but
+also their shape and position in the body, and their relations with
+other parts, we must have every successive section carefully preserved,
+and the whole row arranged in correct successive order; the physiologist
+may often content himself with single sections; the zoologist must have
+rows and rows of them. What a task this was, a quarter of a century ago,
+for scientists who cut their sections by hand!
+
+Let us, however, describe first the way in which objects are prepared
+for section-cutting--whether by hand or by machine. It has already
+been noticed that animal substances contain a quantity of water, and
+therefore will not keep. The same circumstance renders them soft and
+squashy, so that the sharpest razor in the world, in cutting a section,
+must necessarily do more or less damage to the structure of the delicate
+tissues. The water is held in the meshes of the tissues just as it is
+held, for example, in the meshes of a sponge. Now, if we were dealing
+with the sponge, we could get it to absorb any other fluid substance
+besides water; we might choose one that would prevent decomposition; we
+might choose one that would go harder by cooling; so as to change the
+sponge into a strong solid block that could be knocked about without
+sustaining any damage. This is exactly what we must do with our animal
+tissue to prepare it for section-cutting; and the most convenient fluid
+for the purpose is melted wax. But whereas we might take our sponge
+out of water, squeeze it dry, and dip it straight into melted wax, we
+can by no means do so with our animal tissues. For one thing they
+usually cannot be squeezed, and where they can, they would of course be
+irretrievably ruined by such a rough process. Even the transference of
+the specimen from one fluid to another of very different qualities and
+density, would deface the tissues. Cells would burst, or be squeezed out
+of shape, and organs would be loosed from their right position by the
+currents set up in all parts of the specimen, under such circumstances.
+We must, therefore, try to get rid of the water by degrees. This may be
+done by gradually adding alcohol, a fluid which may be diluted with
+water in any proportion. We begin with a comparatively weak solution of
+alcohol, say about fifty per cent., and immerse the specimen in this for
+some little time. The time required depends somewhat upon the size of
+the specimen; if a large one, a new fluid will take longer to filter
+through it. Then we must change this solution of alcohol for stronger
+ones, say seventy per cent. and ninety per cent. successively, and
+finally to absolute alcohol. By this time the alcohol will have removed
+almost nearly all trace of water from the specimen. The latter is now
+nearly but not quite ready to be imbedded in melted wax; but first we
+must soak it for a while in a fluid intermediate in thickness between
+the alcohol and the wax, and capable of mixing in a friendly manner with
+both. Then it goes into a bath of melted wax, and is kept for hours at a
+stated temperature until the wax permeates it thoroughly. Then the
+melted wax and the specimen along with it is poured into a little mould
+and left to cool. The block of wax containing the specimen is cut down
+to a quadrangular shape, and is now ready for section cutting. In old
+days the block was placed in a stand, and successive sections were cut
+from it by hand with a razor. But this process is much too slow for
+modern days. Machines called microtomes (_i.e._ cutters of small parts)
+have been invented, and of these there are several kinds--in all,
+however, the razor is worked by machine and not by hand, so as to secure
+steadiness and a uniform thinness of the sections. The old microtomes
+threw off each section separately; but now matters are so arranged that
+the wax of each section adheres to that of the next, and the whole
+series of sections forms a continuous ribbon of thin wax. A large
+specimen, affording a number of sections, thus results in a ribbon of
+considerable length. Further processes are now required to fit the
+sections for the microscope. The ribbon must be divided into successive
+pieces of a length determined by that of the slides to be used. These
+are mounted in order on the slides, steps are taken to melt away the
+wax from the sections, the latter are covered with Canada Balsam
+surmounted by a glass cover slip, and left for some time to dry. After
+this they are ready for examination, and it is only now that the work
+really begins. All that has gone before is mere handicraft; it is time
+now for science to be called into play.
+
+[Illustration: FIG. 47.--Sections of Embryo Chick, eight days old. A
+slide mounted for microscopic examination, showing sections arranged in
+ribbons.]
+
+The sections must be compared with others of the same kind which have
+been cut before. Do they entirely resemble these, or is there a
+difference somewhere? Happy the man who finds that his sections
+represent a fresh stage, perhaps older or younger than any that has been
+seen before in the history of the particular animal which is under
+investigation. Happier still the man who has succeeded in getting hold
+of an animal which has not been described before. He will make haste to
+write a full description of it, illustrated by drawings; to found a new
+theory on it, if that can possibly be done; and to publish it to the
+world. It will go all over the globe. To every country in Europe; to the
+centres of learning in the United States; to universities in New Zealand
+and Australia, and our other colonies; and perhaps even to "Far Japan."
+
+When in his turn he receives publications from all countries, written in
+all languages, he is in a position to realise the very great advantage
+(referred to in an earlier page, p. 31) that results from the use of the
+learned tongues, in the terminology of zoological science. For the
+educated classes in all countries are equally acquainted with these; and
+when half of a sentence consists of words of Greek or Latin derivation,
+the labour of translation from a foreign tongue is necessarily greatly
+lightened. To no writer is this advantage of so great importance as to
+the Englishman, who is usually less familiar with the tongues of other
+nations than his colleagues abroad. It will easily be understood that in
+the world of zoology, there is no "predominance of the English-speaking
+races." Far from it. German is the language which supplies the fullest
+literature of every scientific subject; and in England even our
+text-books are, for the most part, translated from the German. German,
+in short, is to the seeker after Knowledge, what English is to the
+seeker after Money.
+
+Let us now pause a moment to consider how large a number of different
+industries profit by the labour of the zoologist. First there is the
+shipping trade; for, of course, all specimens from foreign lands are
+brought by sea. The chemist supplies preservative substances, and
+reagents used in the preparation of objects for the microscope. The
+construction of microscopes is a profession in itself, and one which
+employs many industries; for the making of a microscope includes not
+only the work of the optician, but also that of the artificer in brass,
+and of many other handicraftsmen. The glass-worker supplies "slides,"
+that is to say, the thin pieces of glass upon which objects for the
+microscope are placed, and "cover-slips," the little sheets of thinner
+glass which are laid over them; and, besides these, the bottles in which
+specimens are placed. Then comes the microtome, already spoken of, by
+means of which sections for the microscope are cut; how many skilled
+workmen have been engaged in the construction of its parts! Sheffield,
+perhaps, has supplied the razor which it holds, as well as the
+instruments for the dissection of the larger zoological specimens. We
+have already spoken of the laboratory servants, and the
+bone-articulators and skin-stuffers, who are personally and directly
+employed by the zoologist; and of the artists and photographers who
+depict his specimens, or perhaps copy his drawings. We must add to the
+list of the zoologist's helpers, last, but not least, the printer who
+"sets" the learned treatise in which the final result of his work is
+usually embodied; and attendant on the work of the printer is that of
+the bookbinder. With the bookseller the zoologist has but little to do;
+the general public, even the reading public, has no knowledge whatever
+of the writings of the zoological specialist. They are addressed to his
+equals and co-workers, not to critics and reviewers. Their publication
+is provided for, not by the law of supply and demand, but by the funds
+of the learned societies and the universities. It is only occasionally
+that a writer arises who is able and willing, like Huxley or Darwin, to
+express himself in a book that the general public can read; and it is
+only after a lifetime of detailed work, such as is understood only by
+the specialist, that writers like these think it fitting to lay the
+results of their labour before outsiders.
+
+The librarian, finally, must not be forgotten, in making up our list of
+the zoologist's helpers. The preservation and cataloguing of zoological
+literature is obviously a task all the more important, because, as we
+have already stated, zoological writings are not regulated by the law of
+supply and demand. A very little paper, read to a very small meeting of
+a learned society, and wholly ignored by the world at large, may
+contain facts priceless to the world of science. It is on the accurate
+and painstaking work of the librarian, who preserves and catalogues
+small things as conscientiously as large ones, that we rely for the
+completeness of our record of zoological knowledge. Such work has at all
+times been carried on in the libraries of our universities; but at the
+present time there are in existence libraries specially devoted to
+zoological literature alone.
+
+The museum, again, must not be forgotten, in which our man of learning
+stores his specimens, duly labelled and arranged. Here, again, is a
+staff of curators and sub-curators; and, under their direction, work for
+various workmen, and for perhaps even a humble charwoman to dust the
+shelves.
+
+Turn now to another aspect of the zoologist's work--that of teaching. We
+should think it very wrong to turn men loose on the world to practise in
+the professions of law or medicine without a long and careful training
+to fit them for their task. No less impossible is it for anyone to
+become a man of science without a similar training; for the profession
+of the man of science, whether zoologist, chemist, botanist, or expert
+in whatever branch, if defined in plain English, is the profession of
+seeking after knowledge of the order of things in which we live; and
+what profession can be more important to the world than this? To attain
+a scientific degree of any value, years of study are therefore required,
+and a series of examinations tests--or is supposed to test--the success
+of the student. Both the work of teaching and the work of examining must
+be the tasks of the scientist who has attained a position of eminence
+in the world of learning. The preparation of lectures, with their
+accompanying illustrations of diagrams and lantern slides: the guiding
+of classes engaged in the actual work of making acquaintance with animal
+specimens--these are the labours of the great man who is at the head of
+things. His task is carried out with the aid of junior helpers of his
+own profession--the demonstrators, who "point out" detail after detail
+of the work described in the lectures. Another helper, more esteemed by
+the students than by the professor who teaches them, is the "coach" who
+prepares them directly for their examinations. His aid, in the shape of
+extra teaching, given at the last moment, will often secure for the
+careless and inattentive pupil, better success than is the lot of the
+painstaking and industrious one, who cannot afford to pay extra fees.
+
+Few, however, of all the many pupils who crowd the lecture room of the
+zoologist, will ever become zoologists themselves. A vast proportion of
+them are students of medicine, of whom some knowledge of the subject is
+required. Others are preparing to be schoolmasters or schoolmistresses,
+and seek just such an amount of knowledge as they expect to find useful
+in teaching pupils of their own. To the students who are preparing to be
+doctors or teachers, circumstances often assign a limit--"thus far and
+no farther"--when they would fain bring their knowledge to a higher
+standard. But the time they have spent already has not been wasted. How
+keen an observer of animal life is the country doctor! How often,
+isolated from the world of learning, and ill-provided with books, he
+finds in this his chief recreation! As for the schoolmaster, how is the
+routine of school-work relaxed, and labour changed into pleasure, when
+he lets his boys exchange grammar and Euclid for zoology, and the
+lessons of the schoolroom for lessons in the fields!
+
+The most important part, however, of a zoologist's work is not the
+giving of instruction, but the labour of original research, to which we
+have already alluded; not the mere communication of information, but the
+task of adding to the general store of knowledge; not teaching, but
+discovery. The work of the man of science is, in fact, within the limits
+of his own department, the work of seeking after truth.
+
+
+
+
+INDEX
+
+
+    A.
+
+    Acoelomata, 37.
+    Adaptation, 13.
+    Alternation of Generations, 57, 137.
+    Amoeba, 35, 45.
+    Amphibia, 152-154.
+    Ancestors, 40, 42.
+    Animalcule (minute animal), 49.
+    Anisopleura, 29.
+    Annelids, 72.
+    Annulosa, 69.
+    Ants, 92.
+    Appendages, 77.
+    Arachnida (spiders), 84.
+    Arthropoda, 33, 76.
+    Ascidians, 33, 44, 135.
+    Asexual reproduction, 55.
+    Atavistic variation, 27.
+    Azygo-branchiata, 29.
+
+
+    B.
+
+    Balanoglossus, 133, 143.
+    Barnacles, 79, 80.
+    Bees, 91.
+    Beetles, 95.
+    Bell Animalcule, 49.
+    Birds, 156.
+    Bivalve shell-fish, 23, 27, 107.
+    Body-cavity, 34, 37, 38.
+    Body-cavity (diagrams), 38, 139.
+    Body-rings, or "segments," 69.
+    Brachiopoda, 33, 43, 44, 117.
+    Bryozoa, 33, 44, 119.
+    Buds, 55.
+    Butterflies, 89, 93.
+
+
+    C.
+
+    Cat, fur of black, 160.
+    Cell, 11.
+    Cell-types, 49.
+    Cephalodiscus, 145.
+    Cephalopoda, 113.
+    Centipedes, 77.
+    Chætopoda, 71.
+    Chalk, 46.
+    Chordata, 33, 44, 135, 143-146.
+    Cilia, 42, 43, 48, 65.
+    Classes, 33.
+    Classification, 30.
+    Classification, tables of, 30, 44, 52, 62, 67, 75, 116, 118,
+      121, 134, 146, 164, 179.
+    Coelenterata, 33, 44, 53.
+    Coelomata, 37, 44.
+    Cockle, 111.
+    Colony, 57.
+    Corals, 59.
+    Corallines, 56, 58.
+    Corticata (or Infusoria), 47.
+    Crabs, 81.
+    Crocodile, 11.
+    Crustacea, 78-83.
+    Ctenophora, 60, 62.
+
+
+    D.
+
+    Degeneracy, 28, 172-180.
+    Development by metamorphosis (change of form), 41, 89.
+    Development, direct, 45.
+    Dicyemidæ, 34.
+    Diploblastic (two-layered), 34, 36.
+    Diploblastic larva, 41.
+    Duck-mole, 160.
+
+
+    E.
+
+    Earthworm, 74.
+    Earthworm, diagrammatic section of, to show position of body-cavity, 38.
+    Echinodermata, 33, 43, 44, 122.
+    Ectoderm, outer or skin-layer of adult animals and larvæ
+      (corresponding with the epiblast of embryos in the egg),
+      34, 37, 41, 139.
+    Eleutheroblasteæ (hydroid animals which throw off "free buds"), 56.
+    Embryology, 45.
+    Encrinites, 131.
+    Endoderm, inner or digestive layer of adult animals and larvæ
+      (corresponding with the hypoblast of embryos in the egg),
+      37, 41, 139.
+    Enteron, 36.
+    Environment, 26.
+    Errantia, or Wandering Annelids, 72.
+    Euthyneura, 100.
+
+
+    F.
+
+    Families, 33.
+    Fertility, 32.
+    Feathers, 157.
+    Feather-stars, 132.
+    Fishes, 150-152.
+    Flagella, 65.
+    Flat-fish, 23.
+    Foraminifera, 46.
+    Frogs, 38, 152.
+
+
+    G.
+
+    Galeodes, a spider-like animal, 85, 86.
+    Gasteropoda, 29, 98-107.
+    Gasteropoda, classification of, 29.
+    Gastræa, 40.
+    Gastrula, larva, 41, 150.
+    Genus, 32.
+    Gills, 45, 141, 149.
+    Grades, 34, 35-38.
+    Gregarina, 49.
+
+
+    H.
+
+    Heliozoa, 48.
+    Hemichordata (or Adelochorda), 33, 43, 145.
+    Hermit Crabs, 80.
+    Holostomata, 105.
+    Hybrid, 32.
+    Hydra, 36, 41, 54, 59.
+
+
+    I.
+
+    Infusoria 10, 47.
+    "Infusorial earth," 47.
+    Insects, true, 88-97.
+
+
+    J.
+
+    Jelly-fish, 57, 58.
+
+
+    K.
+
+    Kangaroo, 163.
+
+
+    L.
+
+    Lamellibranchiata, 107.
+    Lamp-shells, 119.
+    Land Animals, 166.
+    Larvæ, larval forms, 40, 41.
+    Larvæ of Brachiopods, 119.
+    Larvæ of Insects, 90.
+    Larvæ of Molluscs, 115.
+    Lancelet (Amphioxus), 41, 140, 149.
+    Leeches, 71.
+    Limpet, Common, 17, 19, 20, 29, 30.
+    Limpet, Semi-transparent, 15-20.
+    Liver-fluke, 71.
+    Lobsters, 80.
+    Lophophore, 117, 122.
+    Lustre, metallic, of feathers, 157.
+
+
+    M.
+
+    Mammalia, 160.
+    Man, 13, 26, 167-180.
+    Mantle (of bivalve molluscs), 108.
+    Marsupialia (or Metatheria), 161.
+    Marsupium or nursery-pocket, 161.
+    Mesoblast, 38.
+    Mesoderm or middle body-layer, 37, 61.
+    Metameric symmetry, 70.
+    Mesozoa, 35.
+    Metazoa, 35.
+    Microscope, 9, 10, 182.
+    Microscope, Sections for the, 182.
+    Microtome, 185.
+    Mites, 87.
+    Mollusca, 29.
+    Mollusca (classification of Gasteropod), 29.
+    Moths, 93.
+    Monoblastic, 34.
+    Moss-Corals, 33, 39, 119.
+    Mule, 32.
+    Mussel, Common, 103.
+
+
+    N.
+
+    Nematodes, 71.
+    Notochord, 135, 139, 145, 149, 151.
+    Nucleus, 35.
+    Nummulite, 46.
+
+
+    O.
+
+    Odontophore, 100.
+    Operculum (of univalve molluscs), 105.
+    Opossum, 161.
+    Orders, 33.
+    Orthonectidae, 34.
+
+
+    P.
+
+    Pelecypoda, 107.
+    Perforating gills (of vertebrates and other chordata), 142, 144.
+    Peripatus, 88.
+    Periwinkle, Common or Edible, 19, 26, 105.
+    Periwinkle, High-tide-mark (_L. rudis_) 19, 105, 114.
+    Periwinkle, Yellow, 19, 21, 23, 25, 30, 105.
+    "Persons" of a colony, 58.
+    Phoronis, 122.
+    Phylum, pl. phyla, 33.
+    Placophora, 113.
+    Planarian Worms, 37, 70.
+    Planula Larva, 41.
+    Platyhelminthes, 44, 71.
+    Polycystina, 47.
+    Polyzoa, 119.
+    Porifera, 33, 63, 68.
+    Protective Coloration, 15, 25.
+    Protophyta, 50.
+    Protoplasm, 35.
+    Prototheria, 161.
+    Protozoa, 33, 44, 45.
+    Pseudopodia, 36.
+
+
+    R.
+
+    Radial Symmetry, 53.
+    Radiata, 53.
+    Radiolarians, 47.
+    Rainbow Worm, 72, 159.
+    Reptiles, 154-156.
+    Rhabdopleura, 145.
+    Rhizopoda, 36, 46, 48.
+    Rodent, Teeth of, 163.
+    Rotifers, 76.
+
+
+    S.
+
+    Sand-hoppers, 83.
+    Sauropsida, 154.
+    Scales of fish, 142, 143.
+    Scallop, 107-112.
+    Scorpion, 87, 88.
+    Sex, 10.
+    Sea-Anemone, 54, 59.
+    Sea-Cucumbers, 129, 130.
+    Sea-Fan, 59.
+    Sea-Mats, 119.
+    Sea-Mouse, 72, 159.
+    Sea-Urchins, 23, 33, 122.
+    Shell-fish, 33.
+    Siphonostomata, 102, 106.
+    Skin of Vertebrates, 142.
+    Snail, 98, 114.
+    Snake-Stars, or Brittle-Stars, 128.
+    Species, 30.
+    Spiders, 84.
+    Spiny Ant-eater, 160.
+    Sponges, 33, 44, 63, 68.
+    Sponges, Parasitic, 68.
+    Starfishes, 127.
+    Streptoneura, 29, 100.
+    Symbiosis, 48.
+
+
+    T.
+
+    Teeth, 147, 163.
+    Tentacles (arms or feelers), 54.
+    Ticks, 87.
+    Trichina, 71.
+    Triploblastic (three-layered), 37.
+    Trochophora, 43.
+    Trochosphere larva, 42, 43, 72.
+    Tubicolous (tube-dwelling) Annelids, 72, 74.
+    Tunicata, 33, 44, 135.
+    Turbellaria, 70.
+    Two-layered animals, 34, 36.
+
+
+    U.
+
+    Unicellular animals, 11, 34, 35, 39, 44.
+    Univalve shell-fish, 98.
+    Urochordata, 145.
+
+
+    V.
+
+    Vacuole, contractile, 35.
+    Variation, 24, 26, 28, 32.
+    Varieties, 29.
+    Vermes, 33, 44, 68.
+    Vertebrae (joints of the backbone), 138, 139.
+    Vertebrata, 33, 44, 138.
+
+
+    W.
+
+    Water Animals, 166.
+    Wheel-ball larva, 42, 43.
+    White ants, 93.
+    Wood-lice, 83.
+    Worms, 33, 44, 68.
+
+
+    Z.
+
+    Zooids, 58.
+    Zoologists (_see below_).
+    Zoophyte, 53.
+    Zygobranchiata, 30.
+    Zoologists, names of--
+      Buffon, 11.
+      Caldwell, 160.
+      Chamisso, 137.
+      Cuvier, 54.
+      Darwin, 24, 76.
+      Dubois, Eugène, 168.
+      Forbes, 17.
+      Gadow, 157.
+      Gosse, P., 54.
+      Grant, Robert, 65.
+      Hæckel, 40, 168.
+      Hertwig, O., 50.
+      Huxley, 37, 78, 136, 154.
+      Kowalevsky, 136.
+      Landsborough, W., 109.
+      Lang, A., 39.
+      Linnaeus, 32, 68.
+      Leuckart, 71.
+      Morgan, Lloyd, 20.
+      Parker, T. J., 32.
+      Roberts, G., 21.
+      Romanes, G. J., 11.
+      Sharp, D., 92.
+      Sollas, 65.
+      Woodward, 109.
+
+
+
+
+Transcriber's Note:
+
+Obvious typographical errors have been corrected. Original published
+spelling and hyphenation have been retained as they appear in the
+original publication, including "debateable" and "cellless". A possible
+missing "wall" has not been added to the caption for Fig. 5 ("--muscular
+of intestine"; and reference to "the Italian poet" on Page 138 of the
+original publication has been preserved. Where figures or tables cut
+paragraphs, they were moved above or below the paragraph.
+
+
+
+
+
+
+
+End of Project Gutenberg's Stories of the Universe: Animal Life, by B. Lindsay
+
+*** END OF THE PROJECT GUTENBERG EBOOK 40362 ***