STORIES of the UNIVERSE
Animal Life

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

LIST OF ILLUSTRATIONS

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].)

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.

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, or Streptoneura with a pair of gills.	Azygobranchiata, or Streptoneura a pair of gills.
Genus	Patella, the Limpet, with gills obliterated, a pair of gills. and only indirectly represented; breathing is performed by folds of the mantle.	Littorina, the Periwinkle, or Shore Shell.
Species	Vulgata, the Common Limpet.	Littoralis, the (Yellow) 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 Cœlenterata 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 Cœlenterata 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 Cœlenterata 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.

Fig. 3.—Amœba, 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.—Amœba, 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, Amœba, 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 Amœba-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 Amœba-like Protozoans have been fancifully called "immortal."

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 Cœlenterata receive their name, which means "possessing a hollow space only, by way of intestines." The name of Acœlomata, animals without a body-cavity, has therefore been given to the Cœlenterata 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 Cœlenterata and Sponges, but here it is further established. It includes a very thick layer of muscles.

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 wall 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 Cœlomata 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 Cœlenterata 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.

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.

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 Animals.		SPONGES. CŒLENTERATA.
Grade III.—Three-Layered Animals.		PLATYHELMINTHES, or FLAT-WORMS.
Grade IV.—Cœlomata, or Three-Layered Animals with a Body-Cavity.		VERMES, the Higher Forms. ARTHROPODA. MOLLUSCA. BRACHIOPODA. BRYOZOA. ECHINODERMATA. TUNICATA OR ASCIDIANS. VERTEBRATA.		Chordata.

[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 Amœba. We may now say something about special groups of the Protozoa, which have minor characteristics of their own.

Amœba belongs to the class Rhizopoda, as has been already stated; but there are many of the Rhizopoda that greatly differ from Amœba 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.

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 Amœba, 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 Amœba, 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. Amœboid cells, so called because of their mobility and general resemblance to Amœba, 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

CHAPTER V
THE CŒLENTERATA

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 Cœlenterate 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.

Fig. 9.—An example of the Hydrozoa. A, branch of a Coralline, Sertularia Ellisii, magnified. B, the same, more highly magnified.

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.

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 Cœlenterata, 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.

Fig. 10.—Gorgonia verrucosa, from Guernsey, nearly one-third of the natural size.

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 Cœlenterata 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 Cœlenterata.

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 Cœlenterata, 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 Cœlenterata 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 Cœlenterata. 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 CŒLENTERATA

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 Amœba does; and from these they may be passed on to the cells of the middle layer, amœboid 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 Cœlenterates.

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

Grade II. Two-Layered Animals, or Acoclomata.		PORIFERA.		CALCAREA, with Calcareous Skeleton. NON-CALCAREA, with Skeleton absent or flinty.
		CŒLENTERATA.

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 Cœlenterata, 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 Cœlenterata. 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 Acœla, Dendrocœla, and Rhabdocœla. 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 Cœlenterates, 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.

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, but without Body-Cavity.		PLATYHELMINTHES, or FLAT-WORMS.		A. Turbellaria or Planarians. B. Cestoda or Tape-Worms. C.Trematoda or Fluke-Worms.
Grade III. With Mesoderm, and Body-Cavity.		NEMERTINES. NEMATODA or THREAD-WORMS. HIRUDINIA or LEECHES. CHÆTOPODA.		A. Archiannelids. 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.

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.

Fig. 14.—Shell of the Bell Barnacle, Balanus tintinnabulum, one-half the natural size. The figure shows several successive generations, perched one upon another.

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.

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]).

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.

Fig. 15.—Shells of a Barnacle, Balanus hameri, found in European and North American Seas, natural size.

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.

Fig. 17.—Land Crab, Gecarcinus ruricola, from the West Indies, one-half of the natural size.

Fig. 18.—A Sand-hopper, Pallasea Cancellus, from Siberia, 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."

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.

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.

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.

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.

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.

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.

Fig. 25.—A, Larva of the Bee, Apis mellifica; B, Section of Honeycomb.

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.

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.

Fig. 26.—Ants, Formica rufa, English, enlarged. A, Female; B, Neuter, or Worker.

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.

Fig. 28.—Cocoons of Moths. A, Compound Cocoon of Cœnodomuc 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]).