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diff --git a/40362-0.txt b/40362-0.txt new file mode 100644 index 0000000..1bcc2d2 --- /dev/null +++ b/40362-0.txt @@ -0,0 +1,4970 @@ +*** 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 *** |