ART OF | USEFUL STORIES THE STORY OF L LIFE B, LIN DSAV LIBRARY OF THE UNIVERSITY OF CALIFORNIA. BIOLOGY LIBRARY G THE LIBRARY OF USEFUL STORIES Cfte Elbrary of Useful Stories. A series of little books dealing with various branches of useful knowledge, and treating each subject in dear, concise language, as free as possible from technical words and phrases, by writers of authority in their various spheres. 1 ach book complete in itself. Illustrated. 16mo. Cloth, 35 cents net per volume ; postage, U cents per volume additional. THE STORY OF EXTINCT CIVILIZATIONS OF THE WEST. By ROBERT E.ANDERSON, M \. P. A. 8. THE STORY OF ALCHEMY. By tf. M. PATTISON Mum. THE STORY OF ANIMAL LIFE - . - - By B. LINDSAY THE STORY OF THE ART OF MUSIC By F. J. CLOWEST. THE STORY OF THE ART OF BUILDING. By P. L WATERHOISE THE STORY OF BOOKS - - By GERTRUDE B. RA\VLI> GS. THE STORY OF KING ALFRED - - By Sir WALTER BESANT. THE STORY OF THE ALPHABET - - - By EDWARD Ci ODD. THE STORY OF ECLIPSES - By G. F. CHAMBERS, F.R. A. S. THE STORY OF THE LIVING MACHINE - - By H. W. CONN THE STORY OF THE BRITISH RACE By JOHN MUNFO, C. E. THE STORY OF GEOGRAPHICAL DISCOVERY. Fy JOSEPH JACOBS THE STORY OF THE COTTON PLANT. By F. WILKJNSON, F .G.S. THE STORY OF THE MIND - - By Prof. J. MARK BALDWIN. THE STORY OF PHOTOGRAPHY - By ALFRED T. STORY. THE STORY OF LIFE IN THE SEAS. By SYDNEY J. EICKSON THE STORY OF GERM LIFE - - By H. W. CONN THE STORY OF THE EARTH'S ATMOSPHERE. By D. ARCHIBALD THE STORY OF EXTINCT CIVILIZATIONS OF THE EAST. By ROBERT ANDERSON. M. A., F. A.S. THE STORY OF ELECTRICITY - - By JOHN MUNRO, C. E. THE STORY OF A PIECE OF COAL. By E. A. MARTIN, F. G. S. THE STORY OF THE SOLAR SYSTEM. By G F. CHAMBERS, F. R. A. S. THE STORY OF THE EARTH - - - By H. G. SEELEY, F.R.S. THE STORY OF THE PLANTS - - . - By GRANT ALLEN. THE STORY OF "PRIMITIVE" MAN - - By EDWARD CLODD. THE STORY OF THE STARS. By G F. CHAMBERS, F R.A.S. OTHERS IN PREPARATION. D. APPLETON AND COMPANY, NEW YORK. OP THE UNIVERSITY OF FIG. i. THE SCALLOP SHELL, Pecten Opercularis (see page ID/), SLIGHTLY REDUCED IN SIZE. THE LARGER SHELLS ARE FROM DOUGLAS, ISLE OF MAN ; THE SMALLER SHELLS ARE YOUNG SPECIMENS FROM LLANDUDNO, NORTH WALES. THE STORY OF ANIMAL LIFE BY B. LINDSAY WITH FORTY-SEVEN ILLUSTRATIONS NEW YORK D. APPLETON AND COMPANY 1907 BIOLOGY LIBRARY G 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 Newn- ham 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 diagram- matic; and I owe to the kindness of Mr. J. Craggs, formerly president of the Northumber- land Microscopical Association, the drawings of Polycystina and of the scales of the Sole. B. L. 193719 CONTENTS CHAPTER PAGE I. THE STORY OF ANIMAL LIFE g II. How ANIMALS ADAPT THEMSELVES TO CIR- CUMSTANCES 13 III. CLASSIFICATION : THE SORTING OF THE ANIMAL KINGDOM 30 IV. THE ONE-CELLED ANIMALS, OR PROTOZOA . 45 V. THE CCELENTERATA 53 VI. THE SPONGES . . . . . . .63 VII. THE VERMES OR WORMS ..... 68 VIII. THE ARTHROPODA : LOBSTERS, SPIDERS, AND INSECTS . 76 IX. THE MOLLUSCA, OR SHELL-FISH ... 98 X. THE BRACHIOPODA, OR LAMP-SHELLS . .117 XL THE MOSS-CORALS, OR POLYZOA . . .119 XII. THE ECHINODERMATA 122 XIII. THE CHORDATA 135 XIV. THE VERTEBRATA 138 XV. MAN 167 XVI. How ZOOLOGISTS DO THEIR WORK . . . 180 INDEX 193 6 LIST OF ILLUSTRATIONS FIGURE PAGE 1. The Scallop-Shell .... Frontispiece 2. Limpets and Periwinkles ...... 19 3. Diagram of A mceba ....... 35 4. Section of Hydra ....... 36 5. Diagrammatic Section of Earthworm ... 38 6. Diagram of a Gastrula ...... 41 7. Diagram of a Trochosphere ..... 42 8. Shells of Radiolarians (Polycystina) .... 47 9. A Coralline ........ 58 10. Gorgonia ......... 59 11. Corals 60 12. Marine Worms ........ 73 13. A Centipede 77 14. 15. Shells of Barnacles 79, 80 16. Hermit Crabs 81 17. A Land Crab ........ 82 18. A Sand-hopper ........ 83 19. A Spider ......... 84 20. Nest of Trap-door Spider 85 21. Galeodes 86 22. A Tick 87 23. A Scorpion ........ 88 24. Larvae of Insects ....... 90 25. Larva of the Bee 92 26. Ants .......... 92 7 8 THE STORY OF ANIMAL LIFE FIGURE PAGE 27. White Ants 93 28. Cocoons of Moths ....... 94 29. A Moth and its Larva ...... 95 30. Nest of a Gregarious Caterpillar .... 96 31. Development of an English Water-beetle (Dytiscus) 96 32. Insect Pests . . . . . . . -97 33. Branchy Murex 102 34. Shell of the Common Venus . . . . . 104 35. Eggs of Molluscs 115 36. The Five-holed Sand-Cake . . . . .125 37. A Brittle-Star 129 38. A Sea-Cucumber 130 39. A Stone-Lily or Encrinite ..... 131 40. A Feather-Star 132 41. Sections showing Position of the Vertebrate Noto- choid 139 42. Scales of a Sole 143 43. Tadpoles 153 44. Eggs of Reptiles 155 45. Skull of Kangaroo ....... 162 46. Skull of Rodent .163 47. Slide with Rows of Sections for the Microscope . 185 THE STORY OF ANIMAL LIFE CHAPTER I THE STORY OF ANIMAL LIFE IF the microscope had never been invented, the Story of Animal Life, as it is related by modern science, could never have been told. It is to the microscope that we owe our knowledge of in- numerable little animals that are too small to be seen by the unassisted eye; and it is to the mi- croscope that we owe the most important part of our knowledge about the bodies of larger ani- mals, 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 in- tellectual 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 10 THE STORY OF ANIMAL LIFE 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 be- tween plants and animals; to it, also, our under- standing of the oneness, the " solidarity," as the French would say, of the animal kingdom, for it is in the structure of microscopic parts that resem- blances 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 ani- mals. 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, THE STORY OF ANIMAL LIFE II zoologists had greatly to modify their ideas of the animal world. Still more was this the case after- wards, 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 u 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 quota- tion : '"I remember," says the late George J. Ro- manes (in his book called "The Scientific Evi- dences 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 croc- 12 THE STORY OF ANIMAL LIFE odile 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 neces- sary 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 maybe 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. HOW ANIMALS ADAPT THEMSELVES 13 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 differ- ent kinds of plants that they see in the hedges, or in a country walk. Such natures are intel- lectually 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 in- telligence 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; 14 THE STORY OF ANIMAL LIFE 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 de- veloped. 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 be- comes a cobbler and one an Alpine guide. The latter, in five years' time will have become a per- fect 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 cob- bler. Now let us suppose that the sons and grand- sons 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 HOW ANIMALS ADAPT THEMSELVES 15 up as a clerk, and he will lose much of his sturdi- ness. 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 plain- ly due to adaptation. When we see an animal which is in some way especially fitted for its sur- roundings, we are therefore justified in conclud- ing 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 trav- elled in foreign countries may perhaps find it dif- ficult 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 continu- ous, 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 conspic- 1 6 THE STORY OF ANIMAL LIFE uous ; 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 sea- weed. While the brown is the colour of the sea- weed itself, the brilliant blue is indeed the exact colour of the spring sky at that season, ever) where reflected from the sea-water and from tl.c 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 ap- pearance of the shell, that they have been de- scribed under different names. English readers who search for it in the spring will learn by ex- perience 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 \\hose young live at the surface of the sea, and after- wards 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 de- vour 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 HOW ANIMALS ADAPT THEMSELVES 17 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 reflec- tions 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 de- gree of translucency, which matches very well with the colouring of the tangle-root; and thus presents a great contrast to the shell of the com- mon Limpet, which is found on rocks. The rug- ged surface of the latter is usually more or less irregularly speckled in harmony with the sur- faces on which it lives, though this shell also presents when young occasional touches of blue, which suggests a family likeness in colour tastes o i 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 pe- culiar way of finding a hiding place among the roots of the tangle. The same shell-fish, in con- trast with the commoner Limpet of the rocks, affords another instance of the way in which 1 8 THE STORY OF ANIMAL LIFE 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 irregu- lar, many chinks \vould be left between the mar- gin of the shell and the surface of the rock through which unwelcome visitors might find en- trance. The loss of moisture through the crev- ices, 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; \hzPatella pdlu- cida, which lives on the smooth branches of the tan- gle-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.) Probably every reader will be able to appre- ciate the above instances of creatures adapted to their surroundings. For there are few people who are not familiar with the common Limpet of 20 THE STORY OF ANIMAL LIFE 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 in- terest 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 re- moved to short distances, and the following table shows the result of the experiment, clearly prov- ing that the Limpet prefers home, but regards a distance of two feet as a very long journey. HOW ANIMALS ADAPT THEMSELVES Number Number Removed. Distance in Inches. Returned in Two In Four Tides. Later. Tides. 25 6 21 21 12 13 5 o 21 18 10 6 2 36 24 i i 3 Similar observations were made at an earlier date, by Mr. George Roberts, at Lyme Regis. Let us now take an instance of adaptation in form. And this time we will take a shell so com- mon that everybody will know it. Everyone who has spent a little time in nat- uralising 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 ap- pearance. 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 22 THE STORY OF ANIMAL LIFE 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 vio- lently down, as the huge seaweeds sway about in the shallow waves. We may easily satisfy our- selves 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 circum- stances! 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 HOW ANIMALS ADAPT THEMSELVES 23 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, al- though the inmate only is digested. The bright yellow, then, that seems to us so conspicuous, is probably a good means of hiding for the peri- winkle when under water. Its common varia- tions 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 sea- weed, 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 circum- stances, when, in their slow way, they begin to 24 THE STORY OF ANIMAL LIFE 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 cur- rent 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 reflect- ing 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 Fucits ; 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 estab- lishment of such little marks of individuality HOW ANIMALS ADAPT THEMSELVES 25 is spoken of as Variation. If among these in- dividual 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 ad- vantage 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 26 THE STORY OF ANIMAL LIFE 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 in- stance 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 w r ork. 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 study- ing the problem. In the above instances from human life, we have considered the occurrence of changes brought about in the organism by the circum- stances of life; or as scientists say, by the "en- HOW ANIMALS ADAPT THEMSELVES 27 vironment." 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 habi- tat. 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 no*t 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 un- satisfactory 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 character- istics of the other, leaving only the character- 28 THE STORY OF ANIMAL LIFE istics of the more generalized ancestral type, from which both parents have alike been derived. When the ancestral type is in some way in- ferior to the modern one, variation which con- sists 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 De- generacy 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 palaeontolo- gist 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 dur- ing age after age of the geological record, and survive even up to the present day. We may borrow from commercial life a homely illus- tration that will explain this aspect of varia- tion. When competition in trade is keen, the seller must have novelties; he will try all sorts, and find some good, some bad, some indif- ferent. If he now revives an out-of-date pat- tern of goods, for the sole sake of change, this is Degeneracy. But where, on the contrary, com- petition 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 HOW ANIMALS ADAPT THEMSELVES 29 competition results in the production of sensible novelties, and consequent progress ; but com- petition 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 selec- tion 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 origi- nally derived from one ancestral type. TABLE SHOWING THE POSITION IN CLASSIFICA- TION OF THE ANIMALS NAMED IN THE FORE- GOING CHAPTER Phylum MOLLTJSCA, or Shell-fish. Class GASTEROPODA, or Snail-like Shell- fish. Sub-Class ANISOPLEURA, or Unequal-sided Gas- teropods. Branch STREPTONEURA, or Unequal-sided Gas- teropods with nerves twisted into the shape of a figure of 8. THE STORY OF ANIMAL LIFE Order ZYGOBRANCHIATA, AZYGOBRANCHIATA, or Streptoneura or Streptoneura, with a pair of with only one gills. gill. Genus Patella, the Lim- Littorina, the Peri- pet, with gills winkle, or Shore obliterated, and Shell, only indirectly represented; breathing is per- formed by folds of the mantle. Species Vulgata, the Com- Littoralis, the (Yel- mon Limpet. low) 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. Prob- ably 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 CLASSIFICATION 31 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 con- sists 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 real- ised 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 impor- tant characteristic possessed by the animal, and sometimes they are merely fanciful names, such as the child we have spoken of gives to his zoo- logical playthings. It does not greatly matter whether the name is descriptive or not ; zoologists describe each animal kind in its most minute de- tails, 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 3 32 THE STORY OF ANIMAL LIFE 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 dif- ferences 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 off- spring which are perfectly fertile when mated again with another set of offspring similarly pro- duced, 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 hy- brid, 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 re- ferred to as the " binary nomenclature," we owe to the industry of Linnaeus the great Swedish botanist and zoologist. Genera are groups con- sisting of a number of different species which CLASSIFICATION 33 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 gener- ally 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 under- stand, 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 Ccelenterata or Two -layered Ani- mals. 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 Hemi- chordata ; (b) the Ascidians ; (c) the Ver- tebrata. 34 THE STORY OF ANIMAL LIFE 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 Pro- tozoa, or unicellular animals. The position of second grade has been assigned to the Ccelente- rata 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 distin- guished, 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 con- sisting of a single layer of cells. For the fre- quent occurrence of Larvae of this kind, consist- ing 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, how- ever, 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, consist- ing sometimes in fact of one cell only. Those are the Dicyemidae and Orthonectidae, both of them parasitic forms. They differ so completely from all other forms that it has been proposed CLASSIFICATION 35 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 Diplo- blastic animals; but nothing more different from the Coelenterata could well be imagined, and some regard them as a degraded form of worm. The animals which are higher in structure than the Protozoa, viz. our divisions 2 to 10, are often grouped under the name Metazoa. The Metazoa thus include Grades II., III., and IV. The meaning of the division of the animal kingdom into grades will be more apparent . r i FIG. 3. Amoeba, a typical uni- if we give an example cellular animal: , nucleus; of each. cv, contractile vacuole ; ps, OR4DF T The One- pseudopodia ; highly magni- fied. This represents Grade Celled Animals. Am and the body-cavity (Grade IV.). Sk, skin ; #/, glandular lining of the ali- mentary canal ; zt>, muscular wall of body ; w' , muscular of intestine, both belonging to the third layer or meso- blast ; b.c., body - cavity (shaded); al.c., cavity of alimentary canal (sha- ded) ; , nerve. complicated affair. The mass of the body, lying be- tween these two layers, is consid- ered 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 mem- bers 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 CLASSIFICATION 39 reason to believe that in certain cases it orig- inates 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 ex- ceptional forms that present a departure from these broad rules of structure. They are, how- ever, so few that they need only be named as cu- riosities. 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 di- vision. 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 num- ber of creatures belonging to the higher groups present, later on, a two-layered condition, com- parable 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. 40 THE STORY OF ANIMAL LIFE It will be sufficiently apparent that this co- incidence 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 in- dividual 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 strug- gle of the animal kingdom, from its humble be- ginnings 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 industri- ous, 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 ex- istence 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 cer- tain are zoologists of the existence of an ances- tral two-layered form, the parent at once of the existing Coelenterata and of the higher forms, that Professor Haeckel has given it a special name Gastrsea. 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 CLASSIFICATION 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 sug- gests the existence of an ances- tral Gastraea-animal, suggests that of an ancestral Planula- animal ; for the two-layered ani- mals, on their part, present us with a monoblastic larva of the ^ form already described (p. 34), FlG . 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 " wander- ing animal," because the Planula larva swims about by means of cilia. 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 knowl- edge that in some animals the young form pre- sents an appearance and structure very differ- ent from that of the grown-up form, and adapted for a different mode of life; the commonest in- stances 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 larvae, differing much from the 6. Diagram matic representation of a typical Gastru la, or two -layered larval form, highly magnified ; optical section, longitudi- nal. JSc 1 Ectoderm or skin layer ; En^ Endoderm or stom- ach layer ; m, mouth leading into the en- teric cavity. The dots are the nuclei of the cells. 42 THE STORY OF ANIMAL LIFE adult, and adapted for a different mode of life. It is, in fact, a very common arrangement ; but often these larvae are very minute, perhaps abso- lutely 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 his- tory of the individual is a sum- mary of the history of its ances- tors ; larval forms are therefore of special interest in this con- nection. A very wide-spread form of larva, more advanced To' r 7 e^e D str a of in ** structure than the little a typical Trocho- Gastrula that has been already sphere, or ciliated named, has received the name mag^ifie^ nS J/Ts\he f Trochosphere or Wheel-ball mouth; the stomach (Fig. 7), because it swims round a "d round, by means of cilia, transparent body. 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 cili- ated layer, which presents certain differences in form, according as the creature belongs to one group of animals or to another. The main char- acters of the Trochosphere are, however, the same in very widely differing groups. These little larvae give rise to one of the most eagerly debated prob- lems of zoology. Are we to suppose that animals which possess a Trochosphere larva are all de- scended from one common ancestor ? Or are we to think that the Trochosphere is a form of body very convenient for the necessities of juvenile CLASSIFICATION 43 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 ac- cepted, and has even been expressed by the appli- cation of the name Trochophora (Wheel-carriers), as a general term for those groups in which such larvae are found. These include some of the higher worms, which present the typical Trocho- sphere, the Brachiopoda, and the Polyzoa; while variations of the Trochosphere type are shown by the earliest larvae of Mollusca, the larvae 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 Trocho- sphere 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 pres- ence 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. 44 THE STORY OF ANIMAL LIFE * * s CO Ul, ! O r O g o ft i^-t 2 I o SPONGES. CCELENTERATA. PLATYHELMINTH] WORMS. VERMES, THE HIGH ARTHROPODA. MOLLUSCA. BRACHIOPODA. BRYOZOA. ECHINODERMATA. TUNICATA OR ASC VERTEBRATA. respective sub-divisions of t o '^ -Y"^ H 1 , ^~ o rt 00 g w ^ E 00 < < & 1 -s 00 g s -M z ft w H S P4 ^ : s rt CJ < c HH Q p< w o ^ 1 W p< w oo - tJ 2 S H 5 > hi H g -2 1 h j ^ N s 5 s s 'S -5 Hjl pa X s lr D O 6 P4 >j j O" nJ HH 5 Q > rO ^ V V4-I HH H B ^ " <5 O G 'c J> CD o S O> t 1 P q t ( o s ONE-CELLED ANIMALS OR PROTOZOA 45 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 ani- mal 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 Am- phibian 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 con- clude 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 sci- ence. That it is of paramount importance to the student of classification, engaged upon the sort- ing 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 descrip- tion of Amoeba. We may now say something 46 THE STORY OF ANIMAL LIFE about special groups of the Protozoa, which have minor characteristics of their own. Amoeba belongs to the class Rhizopoda, as has been already stated ; but there are many of the Rhizopoda that greatly differ from Amoeba in appearance. The possession of a shell or skele- ton gives a special importance to several groups. For, as the reader has no doubt already learnt from an earlier volume in this series, such skele- tons or shells have played an important part in the history of the earth's surface, building up geological strata of vast extent, by the accu- mulation 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 distribu- ted in other parts of the globe, are formed in this manner; while the ooze of the Atlantic and other oceans, similarly composed of Protozoan debris, 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 Forami- nifera are sometimes named Reticularia, because their pseudopodia interlace. The Foraminifera have shells composed of carbonate of lime, but there are other forms that build up geological deposits, in which the shell is ONE-CELLED ANIMALS OR PROTOZOA 47 flinty. The diagram (Fig. 8) shows some fossil shells of Protozoa from the marl of Barbadoes. These constitute a deposit which was named " In- fusorial earth," in the earlier days of microscopic observation, when all Protozoans were spoken of as Infusoria. The name, Infusoria, it must be FIG. 8. Fossil Skeletons of Polycystina, from the so-called " Infusorial Earth " of Barbadoes, highly magnified. 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, al- though the animals that present the peculiar fea- ture of possessing " more than one cyst " now are 4 48 THE STORY OF ANIMAL LIFE called Radiolarians. The " cyst " consists of a basket-work supporting skeleton of flint; there may be several, one inside the other, and con- nected 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 as- sociated in Symbiosis with the algae known as Yellow Cells. The famous polishing slate of Bilin in Bo- hemia 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 Heli- ozoa, a group in which the skeleton is also pres- ent, 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 Rhizoppda are the In- fusoria proper, which are characterized by the usual possession of cilia. Cilia (literally "eye- lashes") are fine hair-like processes of the proto- plasm of the cell, which fringe its exterior; by their constant movement they enable the anima' to swim, and at the same time they create a cur- rent in the water, which washes up to the region of the mouth particles which may serve for food ; for these creatures have this very great advantage over Amoeba, and the other forms above referred to, that they possess something which may be called a mouth. That is to say, there is one par- ONE CELLED ANIMALS OR PROTOZOA 49 ticular spot of the surface where particles are taken in. This may seem to be a restriction, when we compare the Infusorian with Amoeba, which is apparently able to take in food at any part of the surface. But it is a restriction which is associated with an advantage; the Infusorian cell, namely, has a firm exterior with a definite outline, instead of being soft and mobile all over. The firmer exterior layer of protoplasm, which is in turn covered by a thin cuticle or limiting mem- brane, is called the cortex or rind. For this rea- son 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 multi- cellular animals. Amoeboid cells, so called be- cause of their mobility and general resemblance to Amoeba, are found in various parts of the higher animals. The lymph corpuscles of verte- brata, and the white corpuscles of vertebrate blood, as well as the blood corpuscles of inverte- brates, 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. 50 THE STORY OF ANIMAL LIFE 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 debate- able ground, and are claimed now by the botanist, and now by the zoologist. While the latter insists on calling them Protozoa (Primitive Ani- mals) the former would have them Protophyta (Primitive Plants). The fact is that in these organisms of the first grade, the distinction be- tween " plant " and u 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 follow- ing very instructive remarks on the subject are taken from Professor Hertwig's valuable book " The Biological Problem of To-Day,"* pp. in, I 12. " The different mode of nutrition of animals * " The Biological Problem of To-Day, Preformation or Epigenesis," by Professor O. Hertwig. Translated by P. C. Mitchell. Heinemann, 1896. ONE-CELLED ANIMALS OR PROTOZOA 51 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 com- plicated 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 special- isation of plants displays itself in organs exter- nally visible in leaves, twigs, flowers, and ten- drils. The specialisation of animals is concealed 52 THE STORY OF ANIMAL LIFE j5 28 *S >Hffi, Serpula^is com- mon 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. Tere- bella is frequently washed up on a sandy shore. On the Lancashire coast one may feel sure of finding this and many other sand-dwelling ani- mals, 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, how- ever, the body lies loosely within the tube, may also be found in thousands under the same cir- cumstances. We must not forget to say something regard- ing 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 WORMS 75 OR WORMS or PLANARIANS. APE-WORMS. r FLUKE-WORMS. ELIDS. TA. f (^ Tubieola. oj *4-> a Gj IH IH w -- S -^-^i <* H En H VERMES RBELLARIj STODA or ' EMATODA 1 ARCHIAN OLIGOCH POLYCH^E to P H H U H o H ^' U O ^ V -v ^ ^ *5 U in O HH rfo w m H < H w t \ q > "> cv^ P P M M ^5 , of Death's Head Moth, one-quarter natural size \ E, of Metura Savendersii, from New South Wales, natural size; T 7 , of Castnia Endesmia, from Chili, one-sixth of the natural size ; G, of Attacus attas from Bom- bay, one-fourth of the natural size. together. Fig. 29 shows a Moth with its cater- pillar, cocoon, and chrysalis. The threads of which a caterpillar weaves its cocoon are famil- ARTHROPODA 95 iarly exemplified in the silk of commerce. The caterpillar, in some cases, is gregarious, and builds a common nest (Fig. 30). The beetles, Coleoptera, are, like the butter- flies, endlessly numerous. They are characterised by the striking difference in their two pairs of wings, of which the anterior pair is strong and horny, and forms, when at rest, a sheath which FlG. 29. A Moth, Saturnia pyri (S. Europe), with its Caterpillar, A ; its Cocoon, B ; Cocoon cut open to show Chrysalis, C ; Adult insect, D. covers the thinner posterior pair of wings. The metamorphosis is complete in this group also. Fig. 31 shows an example which is typical except in one respect the adult form, namely, is one of 7 9 6 THE STORY OF ANIMAL LIFE the comparatively few instances of adult insects that live in water. Much has been said above in praise of insects and their wonderful work in selecting flowers. FIG. 30. Nest of gregarious Caterpillar of a Moth, Hypsoides. There is, however, another side to this, as the gardener and farmer know too well. While the winged honey seekers help the plants, their larvae FIG. 31. Development of an English Water-Beetle, Dytiscus. Grub ; Pupa ; Adult insect. devour them, and so do many other forms of in- sect. Fig. 32 gives us in miniature some of the ARTHROPODA 97 most notorious insect pests. The work of the locust has been dreaded since the days of the Pharaohs and before: the Colorado beetle which infests the potato, is a plague as terrible, if more modern. The weevils and caterpillars that de- stroy trees, though not directly dangerous to our food supply, are sufficiently destructive. The terror of insect pests lies in their vast numbers, which may render an otherwise harmless crea- ture dangerous. I read last year of a curious railway mishap in the United States. A train was brought to a standstill by the wheels slid- FIG. 32.-insect pests. A, ing on something greasy that covered the track. It proved to be a flock of the so-called " Army worm," a variety of caterpillar which travels long distances in crowds, when its numbers have become too many for the supply of food, or when it is about to enter into the pupa stage. These covered the railway track, and the whole country for a long distance; and the " greasiness " of the rails was produced by the crushed bodies of the unfortunate caterpillars. The train was delayed for hours, while a gang of men with brooms cleared the way in front of it. . . . , Locust, Acridium pere- grinum, one-fourth nat- ural size ; B, Caterpillar of Wood Leopard Moth, Zeutzera j&sculi, bor- ing in wood, about one- thirtieth of natural size ; C, Colorado Beetle, one- fourth natural size ; D, Leaf-rolling Weevil of the Oak. 98 THE STORY OF ANIMAL LIFE CHAPTER IX MOLLUSCA, THE SHELL-FISH THE shell-fish are called Mollusca, the soft- bodied animals. It will easily be seen that this name was intended to point out the distinction between them and the Arthropoda, as regards the way in which the skin is protected. In the latter, as we have seen, the skin itself is hard- ened. In the shell-fish, the skin secretes a cov- ering which lies outside it. Just as our skins pass out superfluous moisture to the outside, in the form of perspiration, so the skin of the mol- lusc continually passes to the outside the solid substances which the body has taken in from the sea-water; and by the continual accumulation of these, the shell is formed. This, at least, is the view taken by modern authorities of the forma- tion of the shell in most instances. The juvenile shell-collector usually begins his knowledge of the classification of the Mollusca, by learning that shells are classified as Univalves and Bivalves. This distinction is useful as a be- ginning. Univalves, that is to say shells which consist of one piece, are those of the snail-like animals, Gasteropoda, or Gastropoda, as some prefer to spell it. Bivalves, or shells which con- sist of two flaps, are those of the Lamellibranchiata or animals with plate-like gills, such as the mussel or oyster. Let us begin with the former. Everybody knows the snail. The snail proper bears a typical univalve shell : though in its relatives (the slugs), the shell is more or less suppressed. The name, MOLLUSCA, THE SHELL-FISH 99 Gasteropoda (stomach-footed animals), is sup- posed to be descriptive of the way in which a snail crawls. Half getting out of its shell, so to speak, it does its best to lay its body to the ground, and its so-called "foot" is an extensive muscular expansion underlying its body, not just a muscular organ thrust out of the shell, as in some other groups. The shell, the mode of crawl- ing, and the " horns," tipped with eye-specks, and directed, intelligently and inquisitively, towards things of interest these make up, for most peo- ple, the idea of Snail. But the most distinctive feature of the class is a less obvious feature, namely, the structure of the tongue. We may see, on any damp day or dewy evening, the snail working away with its tongue at seme tender leaf. Its tongue is practically a file with which it files away the substance of the leaf, the result- ing green mash being thus made ready in minute quantities for the snail to swallow. Thus are made the too familiar holes which disfigure the leaves of plants in our garden, \\hen seen under the microscope, the file-like structure of the tongue is visible; indeed, in large tongues, it may, to some extent, be made out with the naked eye. Across the tongue, which is a flat ribbon- like structure, there runs a pattern of small teeth, bilaterally symmetrical, and this pattern is re- peated over and over again throughout the whole length of the tongue, it might be thought that snails' tongues, being so much alike in their mode of use, would not need to be very various in pat- tern : but far from this, they vary in appearance as much as the shell. Not only is there a differ- ent pattern for every different order of the class, but a different pattern for every genus; nay, 100 THE STORY OF ANIMAL LIFE there are even distinctions between the tongues of different species in the same genus. Conse- quently some authorities on shell-fish prefer to classify them by their tongues, a classification which for the most part holds good. So char- acteristic is the tongue of the Gasteropod, that when new animals have turned up which were difficult to classify by means of the structure of the body, they have been finally recognised as Molluscs, somewhat related to the snails, by the tongue. This file-like tongue-ribbon of the snails is often called the Odontophore or Tooth-Carrier; sometimes the part which actually bears the teeth receives the name of the radula. The snail and its relative, the slugs, belong to the Pulmonate (i.e. air-breathing) division of the Gasteropoda. The sea-slugs, in which, like the land slugs, the shell is absent or reduced, are relatives of the land snails. Some of those found on our own shores are handsome creatures, bril- liantly coloured. Both groups fall under the divi- sion Euthyneura, while the majority of the marine univalves belong to the division Streptoneura (i.e. Gasterop3ds with twisted nerves). The Gastero- pods, in the course of the evolution of their shell, have had the body thrown crooked by the burden of carrying it; the Streptoneura are the forms in which this crookedness is most pronounced; in the Euthyneura it is less so. There are degrees of crookedness even among the Streptoneura ; and the limpet is less crooked than the periwinkle (see Table, p. 30). The older classifications of the Gasteropoda were largely founded on the characters of the shell; but these, though in the mam they hold good, have required some modifications in recent MOLLUSCA, THE SHELL-FISH 101 times. Conchology, the study of shells, was at one time the hobby of many collectors whose knowledge of the animals possessing the shells was not of a very extensive kind ; and conse- quently the very name of conchology is often enough to ruffle the feelings of the zoologist of the present day. Yet many interesting problems of variation may be studied from shells alone, by those whose circumstances forbid them to study the living animal. Nor is there any branch of zoology which is more useful to the teacher who wishes to catch the eye and the attention of the beginner in the study of natural history, espe- cially if the beginner is young, as beginners ought to be. Therefore we must by no means under- value the past labours of conchologists, or the valuable collections which their industry has brought together and set in order for the benefit of the world. For example of the most crooked, or Azygo- branchiate division of the Streptoneura, turn now to Fig. 33, in which we see a typical Gas- teropod shell, Murex ramosus, the Branchy Murex, aptly enough named from the many prickly branches which beset it. These rough points are probably assumed for protective pur- poses; any animal that might wish to dine upon the Murex ramosus would think twice before trying to swallow it the morsel of shell-fish is so small, its shelly case so large and so prickly. If we look for its nearest English relative, that is Murex erinaceus, the Hedgehog Murex, or Sting- winkle. This, though a comparatively plain shell, has still enough rough ridges upon it to have secured it a comparison to the prickly hedgehog. Perhaps the most prickly member of the genus, 102 THE STORY OF ANIMAL LIFE however, is Murex tenuispina, sometimes called Venus' Comb, because the crowded parallel spines which decorate the elongated front of the shell somewhat resemble the parallel teeth of a comb. How does the Mu- rex get its living \ Let us notice the shape of the shell, drawn out to a point, at the end opposite to the spire. Ac- s cording to the older classification of the Mollusca, now some- what fallen out of use, this point marks the shell as belong- ing to one of the Si- phonostomata (shell- fish with a siphon at the mouth of the shell, /. , Eggs of Snail placed on a leaf. ", Cockchafer's Eggs. F\ Egg-case of Cockroach. G, Egg-cases of Locust. H, /, y, Eggs of Gasteropod Molluscs. H, Sycotypus (Pyr- ula). y, FUSHS. weather. Some of these are shown in miniature, in the group of eggs of various kinds, Fig. 35. n6 THE STORY OF ANIMAL LIFE CO J K H to O -3 CJ 05 - 1> 135 - ^ H i 1 c/3 *7 D c/ < S Q, U Q a to P^ PC jz OQ cy: "^H C 6 w *w as tes, the larva of the Lamprey (Petromyzon). The latter, even in the adult form, has no true limbs, though there are fringing fins. The notochord sheath issupplemented, however, by cartilage bars which are equivalent to the begin- nings of the vertebrae of the back-bone. The gills are very different from those of other true verte- brates r and it has no jaws. Teeth it has, however, on the tongue and the lining of the mouth. Prob- ably this creature is greatly altered by adaptation to its peculiar mode of life, so that no certain conclusions can be drawn from it regarding the structure of primitive fishes. It has a sucking mouth, by means of which it hangs on to fishes, while it rasps away their flesh with its rough tongue. When not thus engaged, it hangs on to THE VERTEBRATA 151 a stone by means of its suctional mouth, thus fixing itself at rest. The Hag-fish, Myxine, in many respects similar, devours dead fishes chiefly. The Hag-fish is found on English coasts: so is the Marine Lamprey ; while two freshwater forms are found in streams. Leaving the Cyclostomata, as the above fishes are called, we reach the true fishes, which have limbs and scales. Something has already been said regarding their teeth and gills. The Car- tilaginous fishes, in which most part of the skele- ton remains gristle and does not become trans- formed into bone, include the Sharks, Rays, and Dog-fishes, all savage animals with strong teeth. The common spotted Dog-fish of our own shores is familiar to everybody : fishermen regard it with disgust, as it is not eatable. The Rays are flat- tened fishes, which live at the bottom of rather deep water, and attain enormous size even on our own coasts. The Thornback Skate is covered with prickles (placoid scales). All these fishes are grouped under the name of Elasmobranchii, the Strap-gilled, so called from the structure of the gill-arches. The majority of familiar fishes, such as the herring, mackerel, cod and sole, belong to the group of Teleostei, or Bony Fishes, in which, by contradistinction from the last group, as much of the skeleton as possible becomes bone. Never- theless, traces of the notochord persist in the back-bone of these fishes. Break the back-bone across, of a cod or a sole, and you will find be- tween adjacent sides of the centra, or middle parts of the vertebrae, a pad of gristly substance. This is the remaining substance of the notochord, which finds room between the cup-shaped sides of 152 THE STORY OF ANIMAL LIFE the centra. When the centrum, instead of being biconcave, is solid, as in the higher Vertebrata, the notochord is obliterated by its encroachment. The Amphibia, familiarly represented by Frogs and Toads, receive their name, "adapted for both lives," from the fact that they usually divide their lives between land and water. They are, from one point of view, the most interesting of the classes of the Vertebrata, for they form a divid- ing line between the lower and upper Chordata. Below we have Hemichordata, Ascidians, Amphi- oxus, Fishes ; all water-dwellers, breathing by gills. Above, we have Reptiles, Birds, Mammals, air-breathers, never possessing gills, except for a short time, as rudiments in the embryo, not brought into use. They are linked by the Am- phibia, in which we see the larva a water-dweller, breathing by gills; the adult, an air-breather, adapted for life on land, and obliged to come to the surface to breathe, even when it passes its time in the water. The individual Amphibian tells us the past history of the higher groups ; once they had gills but growing older, they lost them. Fig. 43 shows us an outline sketch of Am- phibian larvae; we should require an enlarged diagram of an earlier stage, to show the gills, which are external and projecting at first, but afterwards are overgrown by the skin with the exception of an orifice on each side. The dia- gram shows the gradual change of form. The tails in these tadpoles will presently be lost, for they belong to the Anura, or tail-less order of Amphibia (Frogs and Toads). The tailed Amphibians, Urodela, are represented in Great Britain by the Newts, Triton, popularly called THE VERTEBRATA 153 Efts. Belonging to the Tailed Amphibians also, is the Axolotl, a creature found in the lakes of Mexico, and in those of the Rocky Mountains. FlG. 43. Tadpoles, three-quarters of their natural size. A to Z>, different stages of the Tadpole of the Common Toad, from Epping Forest, England. E, Tadpole of Pelodytes punctatus, dorsal view. It may or may not retain its gills; and forms with gills, and forms without, may be found in the same lake, each capable of laying eggs. The two forms were at first described under two different generic names: but when speci- mens of the gill-bearing Siredon, kept in con- finement, lost their gills, it was seen that they became Amblystoma. There are other cases of larval forms that produce young, and this curi- ous occurrence is known as " paedogenesis." The Amphibia include the curious creatures called Caeciliae (blind animals), or Gymnophiona. They are snake-like in form, and are without limbs; they burrow underground. Their real place in classification was not found out at first, but they were classed, by a wrong shot, with the Reptiles. They are interesting as being the only 154 THE STORY OF ANIMAL LIFE Amphibians that have scales. These are very minute, embedded in the skin, and arranged in transverse rings. The name Gymnophiona, naked serpents, is therefore doubly inapplicable : for they are not serpents, and not scaleless. The Reptiles and Birds at first sight seem to be widely different. The latter are the warmest blooded of all vertebrates, the former are cold- blooded. The one wear feathers, the other scales. Nevertheless, there is an intimate connection be- tween them; the reader has doubtless already learned from other sources the facts about their relationship, so we will not here do more than re- call a few of these facts. One is, that the birds of earlier times had teeth in their beaks, and pos- sessed jointed tails. Another, that the Reptiles of earlier times included forms that were able to fly. A third notable fact is the presence of claws on the wings of some birds, showing that the wing of the bird was not always wholly spe- cialised for use in flight. We owe to Professor Huxley, the recognition of the close relationship of Birds and Reptiles, and the name Sauropsida (Reptile-like animals), under which both are included. They agree in being air-breathers and never having gills, except the rudiments present in the early embryo : this distinguishes them from Amphibia. They agree in having the skull set on to the back-bone by a single articulating surface or condyle ; and thus differ alike from Amphibia and from Vertebrata. They agree in having the red corpuscles of the blood nucleated ; and in this differ from the Mam- malia, in which the red corpuscles are non- nucleated discs. From a popular point of view, we may say that the striking distinction between THE VERTEBRATA birds and reptiles lies in beauty and ugliness. Even in their eggs, the reptiles display no love for adornment, no colouring or pattern. Fig. 44 shows the eggs of some reptiles. FIG. 44. Eggs of Reptiles, half the natural size. A^ of African Cobra. B, of Common English Snake. <7, of Common English Lizard, Lacerta agilis. D, of Elephantine Tor- toise. E, of Crocodile. The five chief groups of existing reptiles are the Chelonia (Tortoises and Turtles) ; the Rhyn- cocephala, represented only by Hatteria, a lizard 156 THE STORY OF ANIMAL LIFE found in New Zealand ; the Lacertilia or Lizards; the Ophidia, or Snakes and Serpents; and the Crocodilia. Perhaps the most interesting point regarding the reptiles that can be mentioned in brief space, is the fact that they present traces of a median third eye, which have been described by Baldwin Spencer, in the New Zealand Hatteria, and in other reptiles. It is situated on the roof of the brain. While the structure in Hatteria shows it to be an eye, its position corresponds with that of the pineal gland of vertebrates gen- erally; so that we find, in fact, the trace of a third eye in all vertebrates, including ourselves. It is, however, a trace only. In the Lamprey fishes as well as in Hatteria, it reaches a further degree of development. This pineal eye has been compared in structure to the eye of As- cidians. The Birds, excluding the extinct form with teeth and a jointed tail, to which the group name of Archaeornithes is given, fall into two groups. These are the Ratitae, or Birds with Raft-like, i.e. flat, breast-bones, and the Carinatae, or Birds with keeled breast-bones. The former include the African Ostrich (Struthid), the Ameri- can Ostrich (Rhea), the Australian Emu, the Cassowary of New Guinea, and the Kiwi, or Apteryx of New Zealand ; all of them birds that cannot fly. The vast majority of birds belong to the Carinatae, characterised by the projecting keel (Carina) in the middle of the breast-bone. The presence of this, which affords a safe at- tachment for strong muscles, is associated with the power of flight. It is impossible to treat the birds more fully in the space allotted to this little THE VERTEBRATA I $j story, but a few words about feathers, however, ^ay find a place here. The colour of feathers is a subject of murh interest. Everyone is familiar with the brilliant tints often presented by the feathers of birds, and everyone who is a close observer of natural ob- jects knows that there are some feathers which are iridescent, changing colour according to the direction in which light falls on them. It has been shown by Dr. Gadow that this variation of the colour of a feather is due to its structure; this may be described as prismatic, for the small divisions of the feather present acute angular edges, which reflect the light like the edges of a prism. These are symmetrically repeated all along the feathers, so as to reflect the same colour throughout. Thus in the plumage of the common red and green parrot, we see feathers that are red when held in one position, and yellow when shifted to another position ; while there are also feathers that are blue when seen in one position, and green when seen in another ; the alternative colour being the one next in order in the rainbow. Another point regarding the colours of feathers has no doubt puzzled many of our readers; and that is, the metallic quality of the colouring in some exceptional feathers, and in these only. The feathers of the parrot just referred to, are, for instance, simply red and yellow, or blue and green ; but the feathers of the peacock, though displaying the same colours, show a metallic lustre which is wanting in the other case. The feathers of the starling, the blackbird, and the black hen of the farmyard, though not so brilliant as those of the peacock, are the same 158 THE STORY OF ANIMAL LIFE as regards the quality of the light they reflect. The secret of the difference lies in the greater opacity of the feathers named ; they are black feathers, while those of the parrot are light- coloured. Now after the metals themselves, there are few objects in nature so opaque as the black pigment of a black feather. If a thin section through the roots of young black feathers is cut for examination under the microscope, the pigmented parts, although cut so very thin, appear completely opaque. And just as a glass gives a better reflection when backed by some- thing opaque, so does the reflecting surface of the feather. Hence it is that the quality of the colours reflected by these feathers is what we call "metallic." If we ask for a definition of this metallic brightness, other than the accepted fact that it resembles the light reflected from metals, the artist will reply that it consists in two things (i) the greater brilliancy of the light reflected, that is to say the greater completeness of the reflection ; and (2) the entire absence of those gradations of light which are afforded by the reflections from any object, however dark, that possesses a surface translucent, even in the smallest degree. " Metallic " reflections, in fact, may be defined as those in which the greatest amount of light is reflected, and the reflected sunlight receives from the reflecting surface the least possible degree of modification. While the actual tint of the colour reflected by a black feather, then, is determined by the form and position of its angular ridges, the quality of the reflection is determined by the opacity of the substance itself. It is interesting to note that the opacity necessary for reflecting a " me- THE VERTEBRATA 159 tallic " lustre, may be produced by means of pigment, in the vegetable as well as in the animal organism ; for instance, in the dark centres of Coreopsis (the Beetle Flower), and several other fashionable garden plants be- longing to the Composite or Daisy family. Within the animal kingdom, we may note that the metallic lustre is almost entirely confined to land animals ; their dry skins have more chance to develop opaque parts, than the moist tissues of creatures that live in the water. The most familiar exception to this rule is the Sea- Mouse, an Annelid worm found on English coasts (p- 73)) which receives its odd name because it is a fat oval creature, covered with bristles, thus greatly differing in appearance from most worms. The larger bristles, which are of a dark purplish- black colour, have a bronze or golden metallic lustre. Various other annelids exhibit brilliant rainbow colours ; for example, Nereis^ the Rain- bow Worm, also found on English shores; but without the underlying black opaque pigment, the reflections from the surface fall short of absolutely metallic brightness. On land, we see among the insects innumerable forms which pre- sent a metallic lustre, the beetles being the most notable in this respect. To return to the verte- brates, from which we started, everybody must have noticed that the fur of a clean well-kept black cat, when lit up by the bright sunlight in which the animal loves to bask, shows little rainbow reflections of red and green. These are due to the presence of little grooves and irregularities on the surface of the hairs, which play the same part in breaking up the' light which they reflect, as do the sharp angles of 160 THE STORY OF ANIMAL LIFE iridescent feathers. Like the iridescence of the Rainbow Worm, they fall short of absolutely metallic brightness ; the fault in this case being due not to the nature of the underlying stratum, so much as to the incomplete development of the light-reflecting grooves. Yet this instance serves to show the part taken by the dark pigment ; for while the play of colours is perfectly obvious in the fur of a black cat, it is almost impossible to distinguish it in the case of cats with fur of lighter shades. The Mammalia, or animals that suckle their young and produce them by birth, were formerly considered to be sharply defined from animals that lay eggs, such as the birds and reptiles. But in 1884 Mr. Caldwell confirmed the state- ment which had been made previously, yet hardly credited by the scientific world, to the effect that the lowest form of mammals lays eggs. This, the Duck-Mole or Ornithorhyncus anatinus (Bird- billed animal much like a goose), is a native of Australia and Tasmania. It lives on the banks of rivers, and burrows in the bank. It has webbed feet, and therefore sometimes receives the name of Platypus (flat-foot). It lays eggs two at a time, in its burrow ; and these eggs, like those of other egg-laying vertebrates, have a yolk. A kindred form, Echidna hystrix or Spiny Ant-eater, is found in Australia, Tasmania, and New 'Guinea. The Echidna hatches its young in a temporary pocket, which appears in the neigh- bourhood of the breasts, and disappears after the young are old enough to take care of themselves. The Ornithorhyncus has fur, the Echidna has spines, with hairs between them. Neither bears the slight- THE VERTEBRATA l6l est resemblance to a bird ; the comparison sug- gested in the name of Ornithorhyncus is fanciful, and depends chiefly on the flat beak-like mouth ; these egg-laying quadrupeds may, however, be reasonably brought into comparison with Reptiles. Neither of them has any teeth ; the Echidna has no teeth at all ; the Ornithorhyncus loses them at an early stage of growth, and develops instead hard horny patches in each jaw. With these it crushes its food, which consists of small insects, worms, etc. The Echidna^ on the contrary, lives in rocky places, and feeds on ants, which it searches for with its long-pointed snout. These two genera are grouped under the name of Proto- theria or Primitive Mammals. The pocket in which Echidna hatches its young, suggests a relationship with the next group, the Metatheria or Marsupialia, which are the charac- teristic mammals of Australasia. These are dis- tinguished by the possession of a permanent nur- sery-pocket, the "marsupium." In this they put their young, which are born, like those of other mammals, not hatched from eggs like those of the last group. They are, however, born in a very backward condition, and therefore require to go through a further period of incubation, so to speak, in the marsupium. Here each one at- taches itself to a teat, to which it remains fixed. But it cannot suck as a new-born kitten or puppy does; and the milk is forced down its throat by the muscles of the teat. The Marsupialia are not entirely confined to Australasia; a few occur in South America, and in North America they are represented by the "'possum," i.e. Opossum, of American stories. The Marsupials seem almost to mimic the forms 162 THE STORY OF ANIMAL LIFE of ordinary quadrupeds. Thus Notoryctes, a form discovered a few years ago, mimics a mole. The fact is that, just as among the Eutheria, or higher FIG. 45. Skull and Lower Jaw of Great Kangaroo, Macropus giganteus, much reduced. mammals, special types have become established, possessed of certain habits, and especially of cer- tain habits with regard to food, and modified in accordance with those habits. Thus there are among them savage carnivora, harmless herbiv- ora, and rodents; and these respectively share THE VERTEBRATA certain characteristics in common with the car- nivora, herbivora, and rodents, belonging to the Eutheria. One of the herbivorous marsupials is the Great Kangaroo, Macropus. It gets its name, Large-foot, from the size of its hind-paws; on these it stands, and by their aid it takes remarka- bly long leaps. Its skull is shown in Fig. 45 ; this, however, has not the full set of teeth, some of which are soon shed. It crops the herbage with its front teeth, and grinds it with its back teeth, like other herbivora. Thestudyofthe teeth is of great help in the classifi- cation of the Mam- malia. Oftheeight orders of the Eu- theria, two alone, the Sloth order and the Whale or- der, show a tend- ency to the sup- pression of the teeth. Those of the herbivora and carnivora may eas- ily be compared by anyone, in the sheep and the dog respectively. Fig. 46 shows the skull of a Rodent, with elongated front teeth, adapted for that persistent .gnawing which makes the ani- mals of the order, such as the Rat and Rabbit, so terribly destructive. -i FIG. 46. Skull and lower jaw of Ro- dent ; *', t\ incisor teeth, separated by a long interval from the molars. About one-half the natural size. 164 THE STORY OF ANIMAL LIFE s s en ,J en s s w ^H ^ 1. i H 5 ^ O < ^ S d H S O H- 1 o o m O 05 ^ & tf O <1 Ex] S w o P s o a E k II 40 Harold's Quests. Book III 50 Harold's Explorations. Book IV 60 Harold's Discussions. Book V 72 Uncle Robert's Geography. FRANCIS W. PARKER and NBLLIE L. HELM. Playtime and Seedtime. Book I . . . .32 On the Farm. Book II 42 Uncle Robert's Visit. 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