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diff --git a/40362-8.txt b/40362-8.txt deleted file mode 100644 index e6b1887..0000000 --- a/40362-8.txt +++ /dev/null @@ -1,5360 +0,0 @@ -Project Gutenberg's Stories of the Universe: Animal Life, by B. Lindsay - -This eBook is for the use of anyone anywhere at no cost and with -almost no restrictions whatsoever. You may copy it, give it away or -re-use it under the terms of the Project Gutenberg License included -with this eBook or online at www.gutenberg.org/license - - -Title: Stories of the Universe: Animal Life - -Author: B. Lindsay - -Release Date: July 28, 2012 [EBook #40362] - -Language: English - -Character set encoding: ISO-8859-1 - -*** START OF THIS PROJECT GUTENBERG EBOOK STORIES OF THE UNIVERSE: *** - - - - -Produced by Robert Baruch, Tom Cosmas and the Online -Distributed Proofreading Team at http://www.pgdp.net - - - - - - - -[Transcriber's Notes - -Emphasis is denotes as _Italic_ and =Bold=. Most small caps text -has been converted to ALL CAPS; but where some may have been -rendered as Mixed Case letters where it seemed appropriate.] - - - -[Illustration: FIG. 1.--THE SCALLOP SHELL, _Pecten Opercularis_ (see -page 107), SLIGHTLY REDUCED IN SIZE. THE LARGER SHELLS ARE FROM DOUGLAS, -ISLE OF MAN; THE SMALLER SHELLS ARE YOUNG SPECIMENS FROM LLANDUDNO, -NORTH WALES.] - - - - -STORIES _of the_ UNIVERSE - -Animal Life - - -By - - -B. LINDSAY - - -_WITH FORTY-SEVEN ILLUSTRATIONS_ - - -NEW YORK - -REVIEW _of_ REVIEWS COMPANY - -1909 - - - - -COPYRIGHT, 1902 - -BY D. APPLETON AND COMPANY - - -_All rights reserved_ - - - - -PREFACE - - -Of the diagrams which illustrate this little volume, the majority were -prepared by Miss E. C. Abbott (formerly Bathurst Scholar at Newnham -College, Cambridge): the sketches were made from specimens in the South -Kensington Museum of Natural History, which has kindly granted -permission for their use. In addition to these, there are several -figures that are taken from specimens in my possession, photographed by -the publishers; two or three cuts are diagrammatic; and I owe to the -kindness of Mr. J. Craggs, formerly president of the Northumberland -Microscopical Association, the drawings of Polycystina and of the scales -of the Sole. - - B. L. - - - - -CONTENTS - - - CHAPTER PAGE - - I. THE STORY OF ANIMAL LIFE 9 - - II. HOW ANIMALS ADAPT THEMSELVES TO CIRCUMSTANCES 13 - - III. CLASSIFICATION: THE SORTING OF THE ANIMAL KINGDOM 30 - - IV. THE ONE-CELLED ANIMALS, OR PROTOZOA 45 - - V. THE COELENTERATA 53 - - VI. THE SPONGES 63 - - VII. THE VERMES OR WORMS 68 - - VIII. THE ARTHROPODA: LOBSTERS, SPIDERS, AND INSECTS 76 - - IX. THE MOLLUSCA, OR SHELL-FISH 98 - - X. THE BRACHIOPODA, OR LAMP-SHELLS 117 - - XI. THE MOSS-CORALS, OR POLYZOA 119 - - XII. THE ECHINODERMATA 122 - - XIII. THE CHORDATA 135 - - XIV. THE VERTEBRATA 138 - - XV. MAN 167 - - XVI. HOW ZOOLOGISTS DO THEIR WORK 180 - - INDEX 193 - - - - -LIST OF ILLUSTRATIONS - - - FIGURE PAGE - - 1. The Scallop-Shell _Frontispiece_ - - 2. Limpets and Periwinkles 19 - - 3. Diagram of _Amoeba_ 35 - - 4. Section of _Hydra_ 36 - - 5. Diagrammatic Section of Earthworm 38 - - 6. Diagram of a Gastrula 41 - - 7. Diagram of a Trochosphere 42 - - 8. Shells of Radiolarians (Polycystina) 47 - - 9. A Coralline 58 - - 10. Gorgonia 59 - - 11. Corals 60 - - 12. Marine Worms 73 - - 13. A Centipede 77 - - 14, 15. Shells of Barnacles 79, 80 - - 16. Hermit Crabs 81 - - 17. A Land Crab 82 - - 18. A Sand-hopper 83 - - 19. A Spider 84 - - 20. Nest of Trap-door Spider 85 - - 21. _Galeodes_ 86 - - 22. A Tick 87 - - 23. A Scorpion 88 - - 24. Larvæ of Insects 90 - - 25. Larva of the Bee 92 - - 26. Ants 92 - - 27. White Ants 93 - - 28. Cocoons of Moths 94 - - 29. A Moth and its Larva 95 - - 30. Nest of a Gregarious Caterpillar 96 - - 31. Development of an English Water-beetle (_Dytiscus_) 96 - - 32. Insect Pests 97 - - 33. Branchy Murex 102 - - 34. Shell of the Common _Venus_ 104 - - 35. Eggs of Molluscs 115 - - 36. The Five-holed Sand-Cake 125 - - 37. A Brittle-Star 129 - - 38. A Sea-Cucumber 130 - - 39. A Stone-Lily or Encrinite 131 - - 40. A Feather-Star 132 - - 41. Sections showing Position of the Vertebrate Notochord 139 - - 42. Scales of a Sole 143 - - 43. Tadpoles 153 - - 44. Eggs of Reptiles 155 - - 45. Skull of Kangaroo 162 - - 46. Skull of Rodent 163 - - 47. Slide with Rows of Sections for the Microscope 185 - - - - -THE STORY OF ANIMAL LIFE - - - - -CHAPTER I - -THE STORY OF ANIMAL LIFE - - -If the microscope had never been invented, the Story of Animal Life, as -it is related by modern science, could never have been told. It is to -the microscope that we owe our knowledge of innumerable little animals -that are too small to be seen by the unassisted eye; and it is to the -microscope that we owe the most important part of our knowledge about -the bodies of larger animals, about the way in which they are built up, -and the uses of their different parts. The earlier opticians who toiled, -one after another, to bring the microscope to perfection, never dreamed, -in their most ambitious moments, of the value of the gift that their -labour was to confer upon mankind. For the microscope alone has made it -possible for men of science to study the world of living things. This is -the value of honest and thorough work in almost every department of -intellectual labour; that it builds a firm and sure though perhaps -hidden foundation for the loftier and more perfect work of after days. - -The microscope has shown us the intimate structure of every organ of -the animal body; and thus, in most cases, the uses of the organ, and -the steps by which it performs its tasks, have been made clear. The -microscope has also shown the true nature of the sexual functions, and -all the steps of the processes of growth in young animals. None of these -things could ever have been rightly understood without the microscope, -for all their most important details are invisible to the naked eye. -To the microscope, too, we owe our knowledge of the essential kinship -between plants and animals; to it, also, our understanding of the -oneness, the "solidarity," as the French would say, of the animal -kingdom, for it is in the structure of microscopic parts that -resemblances are revealed under the most strikingly different -circumstances of outward form. - -Let us inquire a little into the history of the animals that can only -be seen by the aid of the microscope. Most of them live in water, -especially dirty water, containing decaying remains of plants or -animals. The naturalists who first discovered them studied them in -"infusions" of hay, and so on, and hence these little creatures were -named Infusoria--a name that has since been somewhat restricted in its -application. By an "infusion" is meant that water is poured on some -substance and allowed to stand; the more ancient and evil-smelling the -infusion becomes, the more of these little animals do you find living in -it. Nature provides dirty water ready made, in ditches and in ponds, and -these are full of microscopic animals. And not only do they appear in -dirty water, but kindred kinds appear in clean water also, and many in -the waters of the sea. - -It will easily be understood that when the existence of microscopic -animals was discovered, zoologists had greatly to modify their ideas of -the animal world. Still more was this the case afterwards, when it was -found that all animals were built up of minute parts much resembling -these microscopic animals in their main features. To these unit parts, -of which all animal bodies are composed, the term "cell" is applied. -The name of cell is not very descriptive of these units in the animal -body, but correctly describes the unit of plant structure. In certain -important essential particulars both, however, are alike. Nowadays we -are not content to describe the grouping and external features of cells; -their minute structure also is made a subject of research and inquiry, -and affords a field for most of the fashionable speculations of our own -day. - -How great has been the progress made by the science of zoology since the -eighteenth century may be estimated from the following quotation:-- - -"I remember," says the late George J. Romanes (in his book called "The -Scientific Evidences of Organic Evolution"), "once reading a very -comical disquisition in one of Buffon's works on the question as to -whether or not a crocodile was to be classified as an insect; and the -instructive feature in the disquisition was this, that although a -crocodile differs from an insect as regards every conceivable particular -of its internal anatomy, no allusion at all is made to this fact, while -the whole discussion is made to turn on the hardness of the external -casing of a crocodile resembling the hardness of the external casing -of a beetle; and when at last Buffon decides that, on the whole, a -crocodile had better not be classified as an insect, the only reason -given is, that as a crocodile is so very large an animal it would make -'altogether too terrible an insect.'" - -How different is the state of knowledge now, when every part of a -crocodile or a cockroach is described in print in the minutest detail, -and set before even the beginner in zoology as a necessary lesson. - -But in spite of the labour necessary to master such detailed lessons, -the study of the animal world is far from prosaic. The Story of Animal -Life, indeed, bids fair to be the only element of romance left in the -modern world for those who stay at home in their own land. The traveller -of days of yore, when he ventured into the woods and fields, or upon -the water, expected to meet with all sorts of strange things--fairies -and elves and ugly gnomes; giants, ogres, and dragons; mermaids and -water-witches. With the spread of education all these things have -vanished now; it is quite certain that no Board-School-boy has ever met -any of them: and one's walks abroad would be in these days as prosaic as -they are safe, but for the world of animal life. If you have eyes for -this, every field has its inhabitants, and every hedge its marvels. -Instead of a fairy, you may be well contented to meet a dragon-fly with -shining wings; instead of an ogre you will find the fierce spider, which -not only makes away with every harmless fly that blunders into her net, -but in many cases destroys her own kind also. Many a plant may be met -with which has its own special caterpillar or other dependent insect, -with ways of its own, which may amuse your idle hours. As for the change -of a caterpillar or a tadpole into its adult form, it would be taken for -a miracle if it were observed for the first time. - -The reader may have noticed that there are some unfortunate people who -have no eyes for these things; from childhood upwards they have been so -absorbed in money-making or in reading books--the one case is as bad as -the other--that they have never learnt to observe the facts of nature. -Some cannot even recognise the different kinds of plants that they see -in the hedges, or in a country walk. Such natures are intellectually -defective; they are much to be pitied, and require a special training to -remedy their stupidity. I mention this, because the occurrence of this -form of stupidity is one of the dangers resulting from town life and -bookish education, which we have to guard against at the present time. - -But for all healthy people accustomed to the outdoor world, the study of -animal life has always possessed an interest. Its interest has, however, -been increased a hundred fold by the progress of modern discovery, which -has taught us to see in the animal kingdom one large family, working its -way upwards from humble beginnings, to more perfect structure of body, -and more complete intelligence of mind. - - - - -CHAPTER II - -HOW ANIMALS ADAPT THEMSELVES TO CIRCUMSTANCES - - -We all know what it is to adapt ourselves to circumstances. Suppose two -lads, fresh from school, go out into the world to earn their living; -one becomes a navvy and one a clerk. In five years' time these two young -men will probably be very different in appearance from one another. -The navvy will have developed his muscles; he will be broad-built, -broad-chested, and strong. The clerk, on the other hand, will probably -be comparatively weak and slim, his chest will not be so broad, his -muscles will not be so well developed. The navvy, too, will probably -be of a fresh complexion, while the clerk will be pale. All these -differences are due to the fact that their bodies have adapted -themselves to circumstances. Both men may be equally healthy, and -equally long-lived. Let us take another example. Let us compare two -other youths, of whom one becomes a cobbler and one an Alpine guide. -The latter, in five years' time will have become a perfect specimen -of muscular humanity--active, agile, and hardy. The cobbler will be -comparatively stiff in his limbs and unable to undertake any singular -feat of muscular exertion, although he may be able to do a very hard -day's work at his own trade. The mountaineer, too, will probably differ -in disposition from the cobbler. He will be daring, resourceful, and not -afraid of danger under circumstances which would terrify the cobbler. -Now let us suppose that the sons and grandsons of the navvy are brought -up to be navvies, and the sons and grandsons of the clerk are brought up -to be clerks;--that the children and grandchildren of the Alpine guide -follow his own calling, and the children and grandchildren of the -cobbler do the same;--we shall probably have four families differing -very much in type of physique from one another. Yet take one of the -navvy's sturdy grandchildren and bring him up as a clerk, and he will -lose much of his sturdiness. Let the mountaineer's grandsons be brought -up as cobblers, and by the time they are thirty they will not be -remarkable for their muscular capabilities. - -Just in a similar way the bodies of animals adapt themselves to -circumstances. It is not always possible to trace the steps by which -this has been done. But sometimes it is so; and we may find a whole -series of varieties that are plainly due to adaptation. When we see an -animal which is in some way especially fitted for its surroundings, we -are therefore justified in concluding that it has become so by degrees. - -The way in which animals adapt themselves to their surroundings in the -matter of colour would afford material for several volumes each as large -as this one. Those who have not travelled in foreign countries may -perhaps find it difficult to realise that brilliant colouring and showy -patterns can ever enable an animal to hide itself successfully. But an -instance may be taken from an animal common on our own shores which will -illustrate how this principle works. - -In the spring there may be found in large numbers upon our rocky coasts -a little oval shell-fish, about one-third of an inch long, sticking to -the fronds of the tangle and other broad-leaved seaweeds. The animal is -of a very pale brown colour; its shell brownish and semi-transparent, -with several stripes of brilliant turquoise blue down the back. These -stripes are not continuous, but interrupted at intervals so as to give -them a beady look. Taken in the hand and looked at closely, the shell, -with its contrast of blue stripes on a brown ground, is extremely -conspicuous; brown being, in fact, the contrast-colour which shows blue -in its greatest brilliancy. Yet, when perched upon the tangle, the -creature is almost invisible, and might easily be mistaken for a natural -irregularity of the surface of the seaweed. While the brown is the -colour of the seaweed itself, the brilliant blue is indeed the exact -colour of the spring sky at that season, everywhere reflected from the -sea-water and from the wet surface of the seaweed. By matching that -brilliant colour the animal therefore is rendered invisible. This -little creature is the young of the Semi-transparent Limpet, _Patella -pellucida_. This, at least, was the old-fashioned name for it, though -it has received others. Its young and its adult form are so different -in the appearance of the shell, that they have been described under -different names. English readers who search for it in the spring will -learn by experience that bright colouring may help to make a creature -invisible. But this is not all that is to be said about the protective -colouring of this little shell-fish. There are many creatures whose -young live at the surface of the sea, and afterwards migrate to deeper -water as they attain adult age. In early life they are transparent, -because thus they best escape notice in the clear water of the surface, -especially when seen from below, by the many enemies on the watch to -devour them. But in their later life they become opaque, because thus -they best escape notice from enemies watching from above, as they crawl -along the bottom of the sea. Now this is the case with the little -Patella. For this also migrates to the bottom--in this instance a -comparatively short journey--when it is ready for adult life. Both -shell and animal, therefore, are at first nearly transparent, but in -older life both become more opaque; the blue stripes, too, are almost or -quite obliterated in the after-growth of the shell, slight traces of -them alone remaining at its apex. This change of colour fits the animal -for the new home in which it settles, for it moves down from the leaf -of the tangle to its root, and there finds a snug shelter among -the coral-shaped branches of which the root is composed. Not many -reflections of the blue sky are likely to reach the recesses of the -tangle-root, so the creature has no longer any need of its protective -colouring of blue. - -The adult shell, however, retains a certain degree of translucency, -which matches very well with the colouring of the tangle-root; and thus -presents a great contrast to the shell of the common Limpet, which is -found on rocks. The rugged surface of the latter is usually more or less -irregularly speckled in harmony with the surfaces on which it lives, -though this shell also presents when young occasional touches of blue, -which suggests a family likeness in colour tastes on the part of the two -kinds of Limpet. The blue in this case, however, is of the dullest and -dingiest shade. The _Patella pellucida_ is common on the more rocky -portions of our coasts; in spring the young may be seen in thousands on -the seaweeds of the Isle of Man; here its habits were first observed and -described in detail by the Manx naturalist Forbes, who noticed its -peculiar way of finding a hiding place among the roots of the tangle. -The same shell-fish, in contrast with the commoner Limpet of the rocks, -affords another instance of the way in which shells adapt their forms -to their surroundings. In each case the shell is a plain conical cap, -and the animal within keeps the shell firmly attached to the base on -which it rests. The Limpet can move about at a very creditable snail's -pace when it wishes to do so, and at low-water mark, when the tide is -beginning to rise, you may easily find them moving about and off their -guard; but during many hours of the day, when the tide is out, the main -object of the Limpet is to keep its shell as firmly fixed to the rock as -possible. It will at once be seen that if the margin of the shell were -smooth like that of a tea-cup, and the surface of the rock to which it -clung very irregular, many chinks would be left between the margin of -the shell and the surface of the rock through which unwelcome visitors -might find entrance. The loss of moisture through the crevices, too, -would be a serious thing to the animal during the hours when the shell -is uncovered by the tide and exposed to the rays of a hot sun. On the -other hand, if the margin of the shell were irregular, and the surface -on which it rested smooth, unprotected crevices would in the same way -be left. So the Limpets adapt the shape of their shell to their -surroundings; the _Patella pellucida_, which lives on the smooth -branches of the tangle-root, has a shell with a smooth regular edge; -while the _Patella vulgata_, which lives upon rocks, has a shell with an -irregular, indented edge, whose irregularities fit into those of the -rock on which it rests. (See Fig. 2.) - -[Illustration: FIG. 2.--Shells mentioned in Chap. II. 1, Common Limpet, -old and young; 2, Semi-transparent Limpet, old and young (the remains -of the young shell may be seen crowning the adult shell); 3, Common -Yellow Periwinkle; 4, Common Edible Periwinkle; and 5, High-tide-mark -Periwinkle, both with a sharp spire, for comparison. One specimen of -the latter stands among group 3.] - -Probably every reader will be able to appreciate the above instances of -creatures adapted to their surroundings. For there are few people who -are not familiar with the common Limpet of the shore between tide-marks, -and with the great seaweed called Tangle, which has its habitat a little -lower down, and forms great sea-meadows, whose upper limits alone are -ever laid bare by the tide. The _Patella pellucida_, too, is fairly -common, and the dead shell may be found on most rocky parts of our coast -all the year round. As for the blue-striped young shell, floating on the -blades of the tangle, those who have leisure to visit the seaside during -the months of spring and early summer, may have seen it as I have -described it; and the mention of it will recall pleasant memories of -clear spring skies, and fresh sea-winds, and fields of heavy tangle -swaying gently on the swell that comes in from the open sea. It is -interesting to know something of the habits of the creatures whose forms -we study, and we have already spoken of the snug little hiding-place -that the Semi-transparent Limpet finds for itself in the tangle-root. -It is of interest to remember that the Common Limpet, too, is a -home-loving creature, which knows and prefers the spot of rock on which -it habitually rests; and can find its way back to it, aided by its two -eyes and two smelling patches. This has been proved by Professor Lloyd -Morgan, who has recorded the result of his observations, made on the -coast of Dorsetshire. It is not easy to detach a Limpet from the rock -without injuring or exhausting it, but these specimens were caught when -moving of their own accord, and were therefore uninjured and brisk. They -were removed to short distances, and the following table shows the -result of the experiment, clearly proving that the Limpet prefers home, -but regards a distance of two feet as a very long journey. - - ----------+-------------+----------+---------+-------- - | | Number | | - Number | Distance in | Returned | In Four | Later. - Removed. | Inches. | in Two | Tides. | - | | Tides. | | - ----------+-------------+----------+---------+-------- - 25 | 6 | 21 | 0 | 0 - 21 | 12 | 13 | 5 | 0 - 21 | 18 | 10 | 6 | 2 - 36 | 24 | 1 | 1 | 3 - ----------+-------------+----------+---------+-------- - -Similar observations were made at an earlier date, by Mr. George -Roberts, at Lyme Regis. - -Let us now take an instance of adaptation in form. And this time we will -take a shell so common that everybody will know it. - -Everyone who has spent a little time in naturalising on the shore, has -noticed how often you may find univalve shells, such as those of the -whelk and periwinkle, with the top of the shell knocked off. This is -nearly always the case with the dead shells that you find strewn along -the tide-line; and after a storm, on a rocky coast, you may find shells -that still contain the living tenant, in the same sad condition. And you -may also meet not infrequently with shells, dead or living, that bear -evidence of the owners' efforts to repair them after an accident to the -spire. A piece has been broken, and you find it cemented on again by a -patch of shell, serviceable no doubt to the owner, but crooked and -unsightly in appearance. Now there is a very common shell, the little -yellow periwinkle, which has practically done away with its spire, the -coils of the shell being so curved that the earlier part of the spire -does not project beyond the later-formed coils, and the whole shell has -a rounded outline. This little creature lives on the long seaweeds which -grow at low-water mark or near it; and when the sea is rough it is -obviously liable to be dashed from its foothold on the seaweed and flung -violently down, as the huge seaweeds sway about in the shallow waves. We -may easily satisfy ourselves that this is an accident that frequently -happens, by examining the shore when the tide is going out, on some -stormy spring or autumn day. Numbers of the yellow periwinkles are then -to be found crawling on the sand, and striving to regain their place in -the seaweedy rocks as soon as possible. On a calm day you will rarely -see one crawling on sand above low-water mark, for it is a place they do -not choose by preference; those that are to be found there on the stormy -day have lost their foothold, and have been washed about by the tide. -Had they, like some other kinds of periwinkle, a sharp spire, how many -would be the casualties under these circumstances! But as it is, you do -not see a single specimen with a broken top: the rounded spire is an -adaptation to circumstances, required for the protection of the tenant -of the shell. (See Fig. 2.) - -It may be added that the yellow Periwinkle is not only protected from -mechanical sources of danger by its form, but is also in some degree -protected from living enemies by its colour. This, at first sight, seems -exceedingly conspicuous. We must remember, however, that the animal -often lives in that part of the shore where the Bladder Seaweeds, or -Fuci, are extremely abundant. The flowering ends of these are of a -yellow colour, fairly bright. When seen from below, with the sunlight -streaming through them, they no doubt appear much brighter than when -seen, as we see them, from above, with the sunlight falling on them. -Now protection from foes below is what the yellow periwinkle needs -most: for fishes are quite ready to swallow it whole, and are not in -any way deterred by the thickness of the shell, which is (by-the-way) -in a measure a protection against birds when the tide is out; fishes -habitually swallow shell-fish whole, although the inmate only is -digested. The bright yellow, then, that seems to us so conspicuous, is -probably a good means of hiding for the periwinkle when under water. Its -common variations in colour, too, are probably protective in their use: -some are a dull purplish brown, some drab. These are good colours in -which to lie hidden, respectively, under darker tracts of seaweed, or -upon the rock itself. This little shell is so abundant on rocky coasts -that on some beaches the dead shells are as numerous as pebbles. No -wonder, with all these adaptations for protection! - -Another instance of adaptation to circumstances is described in the -sea-urchin shown on p. 125. This is one among many instances where -animals that live on sand or mud acquire a flattened shape, so that -their weight is distributed, and the danger lessened, of their sinking -in a quick-sand. The flat-fish, such as soles and flounders, are a -familiar example; and the same principle is illustrated by the flattened -forms of many of the bivalve shell-fish, whose flat shell, when closed, -can lie safely on the loosest sand. Equally is their form adapted for -their circumstances, when, in their slow way, they begin to move. For -the flat valves of the shell are placed to the right and left of the -animal's body. So that when it stirs, or floats quietly in the current -of the tide, the shells present their sharp edges to the resistance of -the water, thus enabling the creature to move like a ship through the -sea, or like a knife-blade through bread, with the least possible -friction: and specially is this provision for the lessening of friction -important, when we consider that many of these bivalve shell-fish have -to move, not only through water, but also through sand and mud. - -It may be assumed that every reader is familiar with the common forms of -the bivalve shell-fish. The frontispiece shows one of them, considerably -flattened in shape. - -So far, however, we have not explained _how_ animals adapt themselves to -circumstances; we have only pointed out the fact that they do so. - -Take the case of our little Limpet. It cannot say: "I will paint myself -with blue and brown, so as to be mistaken for a bit of seaweed -reflecting the blue sky"; nor can the periwinkle say: "I will paint -myself with yellow, so as to pass unnoticed among the yellow ends of the -_Fucus_; and I will build my spire low, so that it will not be broken." -The bivalve shell-fish and the Sand-Cake sea-urchins do not say to one -another, "Let us alter our shells, and build them a little flatter, so -that we shall not sink in too deep when we lie upon the ooze and sand of -the sea." - -How then do these adaptations take place? Darwin has explained this for -us. Individuals often have some little peculiarity, in which they differ -from the average of their kind. The establishment of such little marks -of individuality is spoken of as Variation. If among these individual -peculiarities there is one which is in any way disadvantageous, _e.g._ -one which tends to make the creature conspicuous in the sight of its -foes, the owner will be quickly eaten, and of that peculiarity there -will be an end. If, on the contrary, the peculiarity gives the owner -some advantage over its fellows, that individual will survive, and -probably transmit its peculiarity to some of its descendants. - -We have seen, for instance, that it is of advantage to our little -periwinkle to be yellow, when it lives in certain situations; and that -it sometimes presents other colours, likely to be favourable in other -cases. If we gather together a large number of specimens, we shall find -a surprising range of variation in colour. Some present a tint of bright -orange, nearly red; some are a dull brown; the dark purple shade and the -drab have been already referred to. The very young shell usually -presents an unmistakable shade of pink; and we may find innumerable -half-grown specimens in which we may trace the gradual establishment of -the advantageous yellow colour, from an original shade of unmistakable -pink, presented by the earlier whorls. Kindred varieties of the shell, -too, may be found with stripes or speckles. Since this very common shell -may be found in abundance on any rocky shore in the British Isles, the -reader may easily study its colour-variations, both in the dead and the -living shell. Study also the ground on which the creature lives, with -its sharp colour-contrasts of rock and seaweed patches, and it will be -easy to understand why the colours are thus varied, with a -preponderance, on the whole, of the yellow shades. It is all a question -of the survival of the fittest--the unfit being represented by colours -too easily seen, and therefore quickly snapped up. As for the spire, it -has already been shown how that is adapted to circumstances. It is -worthy of remark that in the kindred Edible Periwinkle, _Littorina -littorea_, which has a sharp spire, elderly specimens may be seen with -the end of the spire damaged. - -Turn again for a moment to our first instance--the adaptation of men to -a sedentary or an outdoor occupation. Here we dwelt upon the change -produced by their mode of life; we left out of sight the "survival of -the fittest." Yet here it is equally surely at work. How often does the -young mountaineer, less agile than his fellows, come by a violent death? -Only those who are equal to the necessities of the life survive--many -are lost. How often does the clerk, tied to his desk, fail in health -and die? How often, hating a sedentary life for which he is unfitted, -does he throw his energies into athletics, lose interest in his office -work, and get dismissed? Here again comes in "the survival of the -fittest"--for a desk: alas! perhaps the only means of livelihood. - -But _why_ do variations occur? This is the question first asked by a -child, when you try to explain the working of "natural selection." It is -also the last question asked by scientists, who are still industriously -engaged upon studying the problem. - -In the above instances from human life, we have considered the -occurrence of changes brought about in the organism by the circumstances -of life; or as scientists say, by the "environment." Scientific men are -busily hunting for instances of variation of this sort. Take for -example, an animal which lives sometimes in salt water, sometimes in -water that is only brackish; there are cases in which small differences -can be noticed, according to the difference in the habitat. Notice the -marine shell-fish, for instance, near the estuary of a river: they are -often less robust specimens than are found at a point free from the -influence of fresh water. - -Not until the effect of known causes on the rise of variations has -been studied much more fully than at present, will it be possible to -judge regarding the nature of those variations which _appear_ to be -spontaneous; for which, at present, no predisposing cause can be -assigned. - -A very large number of variations, however, fall into the class of -"Atavistic" variations; that is to say, those which show a return to an -ancestral type. These are variations which are very rarely welcome. If, -for instance, a boy has a pair of handsome black rabbits, he is not much -pleased to find among their progeny, every now and then, one of the -colour of the original wild Bunny. The probability, in this case, is -that the atavistic variety will find its way into a pie, instead of -being kept as a pet. Equally unsatisfactory to the owner, is the -incorrigibly savage and intractable dog or horse--a reversion to the -mental type of an ancestor which knew not the authority of a master. - -Atavistic variation often occurs when members of two well-marked -varieties are mated; so that in some of the offspring produced, each -parent seems to cancel out the more extreme characteristics of the -other, leaving only the characteristics of the more generalized -ancestral type, from which both parents have alike been derived. - -When the ancestral type is in some way inferior to the modern one, -variation which consists in reverting to the former is often referred to -as Degeneracy. There is reason to believe that discomfort and hardship -of existence tend to produce variation of this kind--a fact of supreme -importance, when the problem of Degeneracy is considered in connection -with human life. When creatures begin to degenerate, it is, in fact, as -if the species were saying to itself, "I have gone astray; let me -retrace my steps along the road by which I came, and maybe I shall find -comfort and safety; step by step I will try to go back to my ancestral -form." - -Very rapid variation of any sort is indeed often a sign that the -struggle for existence is too hard for the type in question. The -palæontologist can tell us of types that present numerous variations -before becoming extinct; while others, comfortably holding their own in -the struggle for existence, remain practically unchanged during age -after age of the geological record, and survive even up to the present -day. We may borrow from commercial life a homely illustration that will -explain this aspect of variation. When competition in trade is keen, the -seller must have novelties; he will try all sorts, and find some good, -some bad, some indifferent. If he now revives an out-of-date pattern of -goods, for the sole sake of change, this is Degeneracy. But where, on -the contrary, competition is dull, the same firm will turn out the -same goods for a long period of time. There is an optimum in trade -competition: a reasonable competition results in the production of -sensible novelties, and consequent progress; but competition over-keen -results in the production of rubbish, leading to eventual failure. So in -the world of animal life; a certain degree of struggle for existence -results in variation, establishment of new varieties, progress. A -greater degree results in too rapid variation, new varieties that -speedily perish, and finally, the extinction of the type. - -We have spoken of "varieties." Each of the domestic animals presents -varieties, which are the cumulative result of the breeder's artificial -selection of natural variations. Thus the Pug and the Collie for -instance, are varieties of the Dog; the Bantam and the Dorking of the -Fowl. Among wild animals, varieties are similarly produced by _natural_ -selection, resulting from the "survival of the fittest." By degrees, -intermediate forms are lost; and new species are established by the -greater and greater divergence of varieties originally derived from one -ancestral type. - -TABLE SHOWING THE POSITION IN CLASSIFICATION OF THE ANIMALS NAMED IN THE -FOREGOING CHAPTER - - =Phylum= =MOLLUSCA=, or Shell-fish. - - =Class= GASTEROPODA, or Snail-like Shell-fish. - - =Sub-Class= ANISOPLEURA, or Unequal-sided Gasteropods. - - =Branch= STREPTONEURA, or Unequal-sided Gasteropods with nerves - twisted into the shape of a figure of 8. - - /------------^-----------\ /--------------^--------------\ - =Order= ZYGOBRANCHIATA, AZYGOBRANCHIATA, - or Streptoneura with or Streptoneura with - a pair of gills. only one gill. - - =Genus= _Patella_, the Limpet, _Littorina_, the Periwinkle, - with gills obliterated, or Shore Shell. - and only indirectly - represented; breathing - is performed by folds - of the mantle. - - =Species= _Vulgata_, the Common _Littoralis_, the (Yellow) - Limpet. Periwinkle that lives - above low-tide-mark. - - - -CHAPTER III - -CLASSIFICATION, OR THE SORTING OF THE ANIMAL KINGDOM - - -Give a child a few handfuls of shells. Probably the first thing he will -do with them is to sort out the various kinds and separate them from -one another. Each will go into a little heap by itself; and next, our -young friend will find names for them. These are Cap-shells and those -Sword-shells; these Saucers and those Plates; these Yellow-shells and -those Pink-shells--according as some special character or form or colour -strikes his fancy. - -Now this is what zoologists have been doing with the animal kingdom -from the earliest days of science; trying to recognise each distinct -kind of animal form, and to give it a name of its own. Unfortunately for -the reader, zoologists have been obliged to choose names of Latin and -Greek origin, and therefore in writing about animals we are often -obliged to burden our pages with long words. This is a disadvantage, but -it is a very slight one compared with the great advantage gained by -using the learned tongues, which consists in this, that learned men from -all countries of the globe can equally understand the names thus brought -into use. One particular kind of creature may have one name in English, -another in French, another in German, and so on; but the learned world -does not trouble itself with this multiplicity of names--it gives the -creature a couple of names in Latin, and these names stand good for -learned readers in every part of the globe. The importance of this will -be fully realised when, in a later page, we shall have to speak of the -work done by zoologists, and the way in which they do it. Meantime we -must ask our readers to have patience if now and then some long names -must be used. These learned names sometimes convey a description of some -important characteristic possessed by the animal, and sometimes they are -merely fanciful names, such as the child we have spoken of gives to his -zoological playthings. It does not greatly matter whether the name is -descriptive or not; zoologists describe each animal kind in its most -minute details, and the most commonplace or inappropriate name serves -its purpose quite efficiently as a means of referring to published -descriptions. - -We have spoken of sorting the animal kingdom into its various kinds. -But how do we know when a number of animals are all of one kind? No two -individual animals are ever exactly alike, any more than two persons are -ever exactly alike. "It is a matter of common observation that no two -individuals of a species are ever exactly alike; two tabby cats, -for instance, however they may resemble one another in the general -characters of their colour and markings, invariably present differences -in detail by which they can be readily distinguished. _Individual -variations_ of this kind are of universal occurrence" (T. J. Parker). - -Among a host of animals that present so many differences, how do we -determine what shall be considered as belonging to one and the same -kind? This is a point that nature usually settles thus. If two varieties -when mated produce offspring which are perfectly fertile when mated -again with another set of offspring similarly produced, then the two -varieties, however differing in appearance, belong to one species. If on -the other hand, the two belong to a different species, the offspring -will be what is called a mule or hybrid, and will not produce offspring -if mated with another mule. One of the most familiar examples of a mule -is the animal, commonly so-called, which results from mating a horse and -an ass, and partakes of the characteristics of both. - -Every animal receives two Latin or Latinised names, the first that of -the genus, the second that of the species; this system of naming, often -referred to as the "binary nomenclature," we owe to the industry of -Linnaeus the great Swedish botanist and zoologist. Genera are groups -consisting of a number of different species which closely resemble one -another. Similarly genera, which are somewhat alike, are again formed -into larger groups, and so on. The names of families, orders, and -classes used to be given to these groups in ascending order; but it is -now generally recognised that such names are arbitrary, and that the -divisions into which animals may naturally be grouped are altogether -irregular, and not comparable with one another. Those who know a little -of botany will readily understand, from their knowledge of wild flowers, -that natural groups cannot be arranged in a formal series. - -The main branches of the animal kingdom, the largest groups of all, used -formerly to be called sub-kingdoms. Now the main divisions are often -spoken of as phyla or races. Classifications, although they differ much -in detail, according to the preferences of individual zoologists, yet -agree as to the main branches of the animal kingdom, the chief of these -are:-- - - 1. The Protozoa, or One-celled Animals. - 2. The Coelenterata or Two-layered Animals. - 3. The Sponges or Porifera. - 4. The Vermes or Worms. - 5. The Arthropods or Jointed Animals, viz., Insects and Crustacea. - 6. The Mollusca or Shell-fish. - 7. The Brachiopoda or Lamp-Shells. - 8. The Bryozoa or Moss-Corals. - 9. The Echinodermata or Sea-Urchins. - 10. The Chordata, including--(_a_) the Hemichordata; - (_b_) the Ascidians; (_c_) the Vertebrata. - -Within recent years an attempt has been made to express the relationship -these groups bear to one another, by placing them in separate divisions -or grades. The first grade includes only the Protozoa, or unicellular -animals. The position of second grade has been assigned to the -Coelenterata or diploblastic animals, whose bodies consist typically of -two layers of cells. A third grade includes only a few groups of the -lower worms, among which three body-layers may be distinguished, but no -body-cavity is present. While the fourth grade, including practically -the rest of the animal kingdom, have three body-layers (see p. 38), and -a body-cavity surrounding the internal organs (see p. 38). - -This arrangement of groups is an extremely convenient one; all the more -convenient because it easily admits of modification. Already, indeed, we -might find room for a grade intermediate between I. and II., consisting -of what might be termed monoblastic animals, namely, animals consisting -of a single layer of cells. For the frequent occurrence of Larvæ of this -kind, consisting of a hollow ball of cells, renders zoologists on the -alert to find a grown-up organism built in the same way. It is doubtful -whether any of the forms that have been supposed to answer to this -description really do so. Certain forms of these often claimed as plants -by the botanists are, however, in the meanwhile, invited in to fill the -blank. - -There are also animals in which the internal layer of the body is very -much reduced, consisting sometimes in fact of one cell only. Those are -the Dicyemidæ and Orthonectidæ, both of them parasitic forms. They -differ so completely from all other forms that it has been proposed to -make for them a special group, the Mesozoa, or Midway animals, between -the Protozoa and all the rest of the animal kingdom. It is, however, -possible to group them under the head of Diploblastic animals; but -nothing more different from the Coelenterata could well be imagined, and -some regard them as a degraded form of worm. - -The animals which are higher in structure than the Protozoa, viz. our -divisions 2 to 10, are often grouped under the name Metazoa. The Metazoa -thus include Grades II., III., and IV. - -The meaning of the division of the animal kingdom into grades will be -more apparent if we give an example of each. - -[Illustration: FIG. 3.--_Amoeba_, a typical unicellular animal: _n_, -nucleus; _cv_, contractile vacuole; _ps_, pseudopodia; highly magnified. -This represents Grade I. of animal existence.] - -GRADE I. _The One-Celled Animals._--_Amoeba_, the Mobile animal, is -the typical example of these. It consists of a single microscopic cell. -In this cell is seen a dark irregular speck, the nucleus, which is an -essential character of cells, whether they are independent or form -part of the body of a larger animal. There is often visible also a -clear rounded space, called the "contractile vacuole," which squeezes -out fluid, disappears, and reappears again, serving the purpose of -excretion. The cell-substance, called protoplasm, is practically -identical in this and in cells of all other kinds. It is jelly-like, and -capable of a slow movement, which may be watched under the microscope. -It suggests the flowing of treacle or thick gum. The movement may be -traced by the change in outline of the cell and by the change in -position of any granules that it may have taken in; for particles which -touch the creature sink in and are surrounded; thus it obtains its food. -These slow flowing movements of the protoplasm result in continual -changes of shape; hence the name, Amoeba, the mobile animal. Sometimes -the island of protoplasm, as it changes its shape, throws out, as it -were, capes and headlands. These projections, which are presently drawn -in again, are called pseudopodia or false feet. They are characteristic -of the whole group of Amoeba-like animals, which are consequently called -Rhizopoda, the root-footed. The production of new individuals is -accomplished by the division of the old cell into two. Thus it may be -said that there is always a bit of the old cell remaining, though -divided into fragments; and for this reason the Amoeba-like Protozoans -have been fancifully called "immortal." - -[Illustration: FIG. 4.--Section, highly magnified, of a two-layered -animal, _Hydra_ (Grade II.). _Ec_, outer layer of Ectoderm; _En_, inner -layer of Endoderm; _l_, lamella dividing the two, represented by a line; -_n_, nuclei of the cells; _v_, thin vacuoles of small interstitial -cells; _E_, the Enteron or digestive cavity.] - -GRADE II. _The Two-layered, or Diploblastic Animals._--The type of -these usually chosen is _Hydra_, a two-layered animal, which is further -described on p. 54. A section through Hydra (Fig. 4) shows (1) the outer -or skin layer of cells, called the ectoderm, and (2) the inner or -stomach layer of cells, called the endoderm (literally outer skin and -inner skin). The clear recognition of the primary body-layers of the -simpler invertebrates as identical with the primary body-layers of the -embryo of higher forms, is largely owing to the teaching of Professor -Huxley, the importance of whose work on this and in many other respects, -is little guessed at by many readers who know his name merely as a -popular exponent of scientific ideas. The two-layered body of Hydra -encloses a hollow digestive space; from this the Coelenterata receive -their name, which means "possessing a hollow space only, by way of -intestines." The name of Acoelomata, animals without a body-cavity, has -therefore been given to the Coelenterata and sponges. The meaning of the -term body-cavity will be explained in the next paragraph but one. The -Hydra, like all animals of its grade, and all those of the succeeding -grades, reproduces itself by means of ova or egg-cells, and spermatozoa -which fertilize them. - -GRADE III. _The Triploblastic Animals without Body-Cavity._--This is a -small section including only some of the lowest worms, such as the -forms called Planarians. Between the Ectoderm and Endoderm lies an -intermediate layer the Mesoderm. There are the beginnings of this in -the Coelenterata and Sponges, but here it is further established. It -includes a very thick layer of muscles. - -[Illustration: FIG. 5.--Diagrammatic plan of section cut through an -Earthworm to show the position of the three body-layers and the -body-cavity (Grade IV.). _Sk_, skin; _al_, glandular lining of the -alimentary canal; _w_, muscular wall of body; _w'_, muscular of -intestine, both belonging to the third layer or mesoblast; _b.c._, -body-cavity (shaded); _al.c._, cavity of alimentary canal (shaded); _n_, -nerve.] - -GRADE IV. _The Coelomata or Triploblastic Animals with a -Body-Cavity._--This grade includes all the remainder of the animal -kingdom. As an example of it, we may take the Common Frog. If we open -from the lower surface the dead body of a frog, we first cut through -the skin, next the muscles; then we come to the viscera, lying neatly -packed in a cavity from which we can dislodge them. This cavity is the -Body-Cavity. The skin corresponds with the ectoderm of Hydra, although -it is a vastly more complicated affair. The glandular lining of the -alimentary canal corresponds with the endoderm of Hydra; although this, -too, is a more complicated affair. The mass of the body, lying between -these two layers, is considered to correspond somewhat with the mesoderm -of Grade III., and has received the collective term of Mesoblast. This -description applies equally to the earthworm, for the higher worms -differ immensely from the lower worms, and stand on a level with more -important members of the animal kingdom (see Fig. 41, p. 139). The -body-cavity may be formed in different ways in different animal groups; -but there is reason to believe that in certain cases it originates by a -folding off of part of an original cavity corresponding with that of -Hydra; so that part went to form the intestine, and part the cavity -surrounding it. - -The above arrangement of the main great groups of animals into four -grades is that given by Professor Arnold Lang. - -It should be added, that there are a few exceptional forms that present -a departure from these broad rules of structure. They are, however, so -few that they need only be named as curiosities. For instance, there are -parasites in which the inner body-layer is practically done away with, -because they are fitted to absorb food through the outer layer. And in -one division of the Moss-Corals there is no body-cavity to be seen, -although it is to be found in the other division. - -What is the outcome of all this sorting of the animal kingdom? This most -important result: that a classification of the animal kingdom into the -four grades we have named, presents, in serial order, the stages through -which young animals of the higher forms pass in the course of their -growth. Every creature begins as a unicellular organism--the fertilised -egg-cell. A vast number of creatures belonging to the higher groups -present, later on, a two-layered condition, comparable with that of -Grade II. Later on they acquire a third layer, and therefore correspond -with Grade III. By degrees the body-cavity is formed, and they then -present the adult body-structure of Grade IV. The development of the -chicken in the egg, for instance, presents these four stages. - -It will be sufficiently apparent that this coincidence is too striking -to be without a meaning. Zoologists are all agreed in their -interpretation of this meaning: it is, that the history of the -individual presents a summary of the history of the race, and goes -through the stages of structure which its ancestors presented in their -adult forms. The story of the gradual upward struggle of the animal -kingdom, from its humble beginnings to its present wonderful complexity, -is written in the growing tissues of every young creature. - -The principle that ancestral traits betray themselves is accepted as a -truism in common life. Do we see young people rude and stupid? We say, -perhaps, "No wonder; their grandfather was a drunken, worthless lout." -Do we see a family of the poorest class clever, and industrious, and -refined? We say, "They come of a good stock." When we speak in this way, -we reason from the common experience of mankind, that children resemble -their ancestors. Similarly, when zoologists find an embryo starting its -existence from one cell, they say, "No wonder; its ancestors were -unicellular." And when they find it assuming a two-layered form, they -say, "Its ancestors were two-layered creatures." So certain are -zoologists of the existence of an ancestral two-layered form, the parent -at once of the existing Coelenterata and of the higher forms, that -Professor Hæckel has given it a special name--Gastræa. The two-layered -young stage of higher creatures, when it has a free-swimming existence, -is called a Gastrula (Fig. 6). Both names, meaning stomach-animal, refer -to the structure, which is, in a still simpler form, that of _Hydra_--a -two-layered bag of cells, of which the inner layer, lining the cavity, -performs the work of digestion. The lowest of the Vertebrata, the -Lancelet (see p. 140), has a larva of this kind. The same reasoning -which suggests the existence of an ancestral Gastræa-animal, suggests -that of an ancestral Planula-animal; for the two-layered animals, on -their part, present us with a monoblastic larva of the form already -described (p. 34), called a Planula. Hence it is that zoologists look -with such eagerness for forms, of which it can be said that they consist -of one layer of cells only. The name Planula signifies "wandering -animal," because the Planula larva swims about by means of cilia. - -[Illustration: FIG. 6.--Diagrammatic representation of a typical -Gastrula, or two-layered larval form, highly magnified; optical section, -longitudinal. _Ec_, Ectoderm or skin layer; _En_, Endoderm or stomach -layer; _m_, mouth leading into the enteric cavity. The dots are the -nuclei of the cells.] - -[Illustration: FIG. 7.--Diagrammatic representation of a typical -Trochosphere, or ciliated larva, considerably magnified. _M_ is -the mouth; the stomach and intestine are seen showing through the -transparent body.] - -Mention has been made above of larval forms. It is perhaps advisable to -explain clearly what is meant by this term. It is a matter of every-day -knowledge that in some animals the young form presents an appearance and -structure very different from that of the grown-up form, and adapted for -a different mode of life; the commonest instances are the caterpillar of -the butterfly and the tadpole of the frog. We are apt to think of these -creatures as somewhat exceptional in this respect. But the zoologist, in -viewing the whole range of the animal kingdom, finds a vast number of -animals with larvæ, differing much from the adult, and adapted for a -different mode of life. It is, in fact, a very common arrangement; but -often these larvæ are very minute, perhaps absolutely microscopic, -therefore only known to the scientific observer. The two familiar -instances we have named are fortunately big enough to be known to -everyone. Now it is an axiom with modern zoologists (as has been -explained above), that the history of the individual is a summary of the -history of its ancestors; larval forms are therefore of special interest -in this connection. A very wide-spread form of larva, more advanced in -its structure than the little Gastrula that has been already named, has -received the name of Trochosphere or Wheel-ball (Fig. 7), because it -swims round and round, by means of cilia, usually distributed in bands. -Its inner or stomach-layer, forms a definite alimentary canal, and is -separated by a very simple mesoderm from the outside ciliated layer, -which presents certain differences in form, according as the creature -belongs to one group of animals or to another. The main characters of -the Trochosphere are, however, the same in very widely differing groups. -These little larvæ give rise to one of the most eagerly debated problems -of zoology. Are we to suppose that animals which possess a Trochosphere -larva are all descended from one common ancestor? Or are we to think -that the Trochosphere is a form of body very convenient for the -necessities of juvenile existence in the sea, and therefore -independently evolved by animals which are not directly related to each -other? Some authorities take the latter view; the former is perhaps more -widely accepted, and has even been expressed by the application of the -name Trochophora (Wheel-carriers), as a general term for those groups in -which such larvæ are found. These include some of the higher worms, -which present the typical Trochosphere, the Brachiopoda, and the -Polyzoa; while variations of the Trochosphere type are shown by the -earliest larvæ of Mollusca, the larvæ of the Echinoderms, and those of -the Hemichordata (see p. 33), the latter bringing us, as it were, within -eye-shot of the Vertebrata themselves. It will be seen, therefore, that -the range of the Trochosphere larva covers a large portion of the ground -occupied by our Grade IV. There is, however, one marked exception: the -Arthropoda, which seem to have a prejudice against cilia in any form -(since they include but one animal which possess any) have no example of -a ciliated larva. Even their simplest larval forms belong to a higher -type of structure, in which the shelly, jointed structure characteristic -of the group is already indicated. - -When we speak, however, of the occurrence of the Trochosphere throughout -a wide range of animal life, it must be understood that its presence is -not necessarily uniform throughout a group in which it occurs. Larval -forms are adaptations which conform with the conditions of life for the -particular animal in question: and nearly related kinds of animal may be -without a larva. The Trochosphere larva is, of course, only adapted for -aquatic existence, and is necessarily absent in the case of terrestrial -forms. - -TABLE OF THE CLASSIFICATION OF THE ANIMAL KINGDOM[A] - - =Grade I.=--UNICELLULAR ANIMALS. PROTOZOA. - - (Intermediate forms, see p. 34.) - - =Grade II.=--TWO-LAYERED { SPONGES. - ANIMALS. { COELENTERATA. - - =Grade III.=--THREE-LAYERED { PLATYHELMINTHES, OR FLAT-WORMS. - ANIMALS. { VERMES, THE HIGHER FORMS. - - { ARTHROPODA. - { MOLLUSCA. - =Grade IV.=--COELOMATA, OR { BRACHIOPODA. - THREE-LAYERED ANIMALS WITH A { BRYOZOA. - BODY-CAVITY. { ECHINODERMATA. - { TUNICATA OR ASCIDIANS. } =Chordata.= - { VERTEBRATA. } - -[A] In the subsequent tables which show the respective sub-divisions of -these chief groups, the larger only of the sub-divisions are named. - -When an animal has no free larva, but quits the egg in a form -practically identical with that of the adult, the development is said to -be "direct." But changes equally startling with those displayed when a -larva develops into the adult form, may take place while the young -animal is enclosed within the egg itself. To these also zoologists apply -the axiom referred to above, that the history of the individual -summarises the history of the race. Thus, for example, the Amphibian -larva, _e.g._ the tadpole of a frog (p. 153) has gills, which disappear -in the adult form: the young reptile, bird, or mammal, which has no -larval stage, has gills during a comparatively early stage; and loses -them at a later period of its development. In each case zoologists -conclude that the animal is descended from a fish-like ancestor, which -possessed gills all its life, and that the more immediate ancestors in -the family tree, have lost their gills by degrees. - -The study of the progressive changes of young forms, whether larval, or -enclosed within the egg, is called Embryology, and constitutes, in these -days, the major branch of zoological science. That it is of paramount -importance to the student of classification, engaged upon the sorting of -the animal kingdom, will be apparent from what has been stated above. - - - - -CHAPTER IV - -THE ONE-CELLED ANIMALS OR PROTOZOA - - -Some idea of the general characteristics of the Protozoa has already -been given by the description of _Amoeba_. We may now say something -about special groups of the Protozoa, which have minor characteristics -of their own. - -Amoeba belongs to the class Rhizopoda, as has been already stated; but -there are many of the Rhizopoda that greatly differ from Amoeba in -appearance. The possession of a shell or skeleton gives a special -importance to several groups. For, as the reader has no doubt already -learnt from an earlier volume in this series, such skeletons or shells -have played an important part in the history of the earth's surface, -building up geological strata of vast extent, by the accumulation of the -shells left after the decay of the owners' tiny bodies, during long -periods of time. The chalk rocks that form the "white cliffs of Albion," -and that are so widely distributed in other parts of the globe, are -formed in this manner; while the ooze of the Atlantic and other oceans, -similarly composed of Protozoan _débris_, is now at the present time -building up what will be the chalk rocks of future ages. Some of these -Protozoans attain a remarkable size, instead of being microscopic, as is -the case typically with the one-celled animals. Some forms of the -Foraminifera found on the coast of North America measure as much as -one-fifth of an inch across, while in warmer seas there are kinds which -attain, as did the extinct Nummulite of Egypt, the size of a bean. Two -inches across is mentioned as the maximum diameter, however, of either -extinct or living forms. The Foraminifera are sometimes named -Reticularia, because their pseudopodia interlace. - -[Illustration: FIG. 8.--Fossil Skeletons of Polycystina, from the -so-called "Infusorial Earth" of Barbadoes, highly magnified.] - -The Foraminifera have shells composed of carbonate of lime, but there -are other forms that build up geological deposits, in which the shell -is flinty. The diagram (Fig. 8) shows some fossil shells of Protozoa -from the marl of Barbadoes. These constitute a deposit which was named -"Infusorial earth," in the earlier days of microscopic observation, when -all Protozoans were spoken of as Infusoria. The name, Infusoria, it must -be recollected, is now restricted to a special class, to which the forms -in question do not belong. These fossil forms were named Polycystina, -and are still often spoken of under that name, although the animals that -present the peculiar feature of possessing "more than one cyst" now are -called Radiolarians. The "cyst" consists of a basket-work supporting -skeleton of flint; there may be several, one inside the other, and -connected by radial bars. A living species named _Actinomma_ has three -such layers of basket-work, one in the outer layer of protoplasm, one in -the inner layer, and a central one. It will perhaps be remembered by the -reader that the animals of this group, Radiolaria, are forms described -in a previous volume of the series, as so curiously associated in -Symbiosis with the algæ known as Yellow Cells. - -The famous polishing slate of Bilin in Bohemia consists of flinty -Protozoan shells; it is 14 feet thick, and a cubic inch has been -estimated to contain 41,000,000,000 of the shells. - -While the Radiolarians are marine, the Heliozoa, a group in which -the skeleton is also present, but not usually so greatly developed, -are predominantly fresh-water forms. Both classes take their name -(Ray-animals, Sun-animals) from the stiff radiating rods of the -skeleton. - -Strongly to be contrasted with the above groups belonging to the -Rhizopoda are the Infusoria proper, which are characterized by the usual -possession of cilia. Cilia (literally "eyelashes") are fine hair-like -processes of the protoplasm of the cell, which fringe its exterior; by -their constant movement they enable the animal to swim, and at the same -time they create a current in the water, which washes up to the region -of the mouth particles which may serve for food; for these creatures -have this very great advantage over Amoeba, and the other forms above -referred to, that they possess something which may be called a mouth. -That is to say, there is one particular spot of the surface where -particles are taken in. This may seem to be a restriction, when we -compare the Infusorian with Amoeba, which is apparently able to take in -food at any part of the surface. But it is a restriction which is -associated with an advantage; the Infusorian cell, namely, has a firm -exterior with a definite outline, instead of being soft and mobile all -over. The firmer exterior layer of protoplasm, which is in turn covered -by a thin cuticle or limiting membrane, is called the cortex or rind. -For this reason the name Corticata is sometimes given to the group, -_i.e._, Protozoa with a rind. - -_Vorticella_, the Bell Animalcule, is a stalked form living in ditches, -which is usually selected as a typical form of the Infusoria. It -receives its name, the Whirlpool Animal, from the current which its -cilia create in the water. The purpose of this current is to wash food -particles into the mouth. Associated with the Infusoria under the name -of Corticata are the Gregarina and some other parasitic forms. - -It is interesting to note that the main types of the unicellular animals -are repeated again in the cells of different parts of the bodies of -multicellular animals. Amoeboid cells, so called because of their -mobility and general resemblance to Amoeba, are found in various parts -of the higher animals. The lymph corpuscles of vertebrata, and the white -corpuscles of vertebrate blood, as well as the blood corpuscles of -invertebrates, are among the instances of this. There are cells, on the -contrary, such as those that line the mucous tracts, which are of a -Vorticella type, so to speak; fixed to their bases, and presenting cilia -on the free aspect. - -Two things must be noticed before we leave the subject of the Protozoa. -One is, that some forms present the beginning of a multicellular -condition. Several units sometimes join together, and in this way a -complex object may be formed, in which there are several nuclei; or the -original unit may keep on growing till it consists of many successive -portions, and in some of them a fresh nucleus may arise. This occurs in -some of the Foraminifera. - -The next thing to be noticed is, that there are a number of organisms -which constitute a debateable ground, and are claimed now by the -botanist, and now by the zoologist. While the latter insists on calling -them Protozoa (Primitive Animals) the former would have them Protophyta -(Primitive Plants). The fact is that in these organisms of the first -grade, the distinction between "plant" and "animal" has not become a -hard and fast line; and the disputed forms may be best described as -links between the two. The chemistry of nutrition is probably more to be -relied upon as a distinction, than the difference of structure. It is -here that the two groups, plants and animals, start upon different -roads, and many of the differences in structure must be regarded as the -direct result of the fundamental difference in the mode of nutrition. -The following very instructive remarks on the subject are taken from -Professor Hertwig's valuable book "The Biological Problem of To-Day,"[B] -pp. 111, 112. - -[B] "The Biological Problem of To-Day, Preformation or Epigenesis," by -Professor O. Hertwig. Translated by P. C. Mitchell. Heinemann, 1896. - -"The different mode of nutrition of animals results in a totally -different structural plan. Animal cells absorb material that is already -organised, and that they may do so their cells are either quite naked, -so affording an easy passage for solid particles, or they are clothed -only by a thin membrane, through which solutions of slightly diffusible -organic colloids may pass. Therefore, unlike plants, multicellular -animals display a compact structure with internal organs adapted to the -different conditions which result from the method of nutrition peculiar -to animals. A unicellular animal takes organic particles bodily into its -protoplasm, and forming around them temporary cavities known as food -vacuoles, treats them chemically. The multicellular animal has become -shaped so as to enclose a space within its body, into which solid -organic food-particles are carried and digested thereafter in a state of -solution, to be shared by the single cells lining the cavity. In this -way the animal body does not require so close a relation with the medium -surrounding it; its food, the first requirement of an organism, is -distributed to it from inside outwards. In its further complication the -animal organisation proceeds along the same lines. The system of -internal hollows becomes more complicated by the specialisation of -secreting surfaces, and by the formation of an alimentary canal, and of -a body-cavity separate from the alimentary canal. In plants it is the -external surface that is increased as much as possible. In animals, in -obedience to their different requirements, increase takes place in the -internal surface. The specialisation of plants displays itself in organs -externally visible--in leaves, twigs, flowers, and tendrils. The -specialisation of animals is concealed within the body, for the internal -surface is the starting-point for the formation of the organs and -tissues." - - TABLE SHOWING THE CLASSIFICATION OF THE PROTOZOA - - =Grade I.= { RHIZOPODA, OR - { GYMNOMYXA. - UNICELLULAR ANIMALS. =PROTOZOA= { - { INFUSORIA, OR - { CORTICATA. - - - - -CHAPTER V - -THE COELENTERATA - - -Next after the animals that consist of one cell only we have to consider -the group of animals among which the lower kinds, at any rate, consist -of a number of cells arranged in two layers. The representative of this -group that the reader is most likely to meet with is the Sea-Anemone, -the Coral animal probably he will be content to know from pictures. - -Everybody who has been accustomed to take a little interest in natural -history, remembers the use of the old-fashioned term "Zoophyte." It was -a name given to animals like those named above, which have a flower-like -appearance, due to the possession of a set of petal-like arms or -tentacles, placed all round the mouth; its literal meaning was animal -plant, in allusion to the flower-like form. The great French zoologist, -Cuvier, gave the group name Radiata to animals of this kind. This name -is now not much used, because we have learnt to emphasize other -peculiarities possessed by these animals, as well as that of radial -symmetry, viz., their two-layered body-wall and simple digestive space -(see p. 36). The group called Radiata by Cuvier, included, too, a number -of animals which are widely separated from the "Zoophytes" in modern -systems of classification. - -Sea-Anemones may be found on almost every rocky part of the English -shores. Look for them in pools towards low-tide mark; if uncovered by -the water, they will be found with the arms drawn in, so that the animal -looks merely like a small round knob of shiny opaque coloured jelly; if -covered by the water, they will usually be found open, that is to say, -with the arms (often called Tentacles) spread out. In the middle of the -circle of arms is the mouth; and the apparent "flower" possesses an -excellent appetite, as will readily be seen if any unfortunate little -shrimp or sea-snail should come within reach of the arms. The latter -will then at once contract upon it, and draw it into the mouth. Touch -any of the common Sea-Anemones, and you will find that it is firmly -fixed to the rock; at an early period of life it becomes fixed, and -practically it remains always in one place, although a slight movement -of the base is sometimes possible. Hence the advantage of the "radial" -structure, for the arms reach equally in all directions round that most -important centre of activity, the mouth. The most common kind of -Sea-Anemone is of a dull dark red colour, and small in size; but others -are large and brilliant in colouring. No uncoloured drawing would convey -much idea of their beauty: the reader should consult the works of the -late P. Gosse, an authority on Sea-Anemones, in whose books many -beautiful illustrations will be found. - -A much smaller animal than the Sea-Anemone is found in fresh water and -is called _Hydra_. Its arms or tentacles are longer in proportion to its -body, especially in one species, than is the case in the Sea-Anemones. -Hence its name, fancifully derived from the seven-headed serpent of -Greek Mythology, the Hydra killed by Hercules, which may be supposed to -have presented a similar straggling appearance. The diagram on page 36 -represents a section through the middle of the body, only without the -arms. - -Unlike the Sea-Anemone, the Hydra can walk about. This it does in a very -awkward manner, much in the same way as the Caterpillar known as the -"Looper," clinging first with the front and then with the back extremity -of the body (for head and tail they can hardly be called in so simple an -animal as the _Hydra_, although the Looper caterpillar boasts both head -and tail). - -The _Hydra_ is so small an animal that it appears to the unaided eye -merely as a tiny speck. It may be found anywhere in British ponds and -ditches, standing on water-weeds. Like the Sea-Anemone it preys on -animals smaller than itself. Nature has provided it with minute stinging -cells, which benumb its prey; and in this all the animals of the -Coelenterate group resemble it. - -One of the most curious things about the Hydra is, that it often throws -out buds. It can, of course, produce eggs which are fertilized and -hatched in the usual way of eggs; the buds are an additional way of -multiplying itself.[C] - -[C] We may recall in comparison the way trees may be propagated by slips -independently of flowers producing the seeds of the trees. - -These buds are at first merely swellings, in which both of the layers -of the body join: they grow larger; become provided with tentacles and -a mouth, like the parent, and finally are cast off as independent -animals. - -For this reason the group to which _Hydra_ belongs has received the name -of Eleutheroblasteæ, the animals with free buds. But Hydra has many near -relations in which these buds are not so cast off, but remain attached -to the parent; and they in turn may produce others which also remain -attached. - -In this way, groups or colonies are formed, consisting of large numbers -of individuals, and possessing a common stalk or stock which is formed -by degrees as the process of multiplication goes on. The corals and the -corallines are familiar examples of this. - -The matter is complicated by the fact that either the separate animals -or the flesh of the stock, or both, may secrete within themselves a hard -supporting structure forming what is known as Corals. This may be -developed in such a complicated manner, that instead of the coral -appearing to be the product of the animal, the animal seems to be -inserted in the coral, into which indeed it can retract itself for -shelter. - -The Corallines, on the contrary, secrete a leathery coating or sheath -outside themselves and the stock. The leathery case is fairly -transparent, so that on magnifying the creature the flesh of the common -stock, as well as of the stalks of individual animals, may be seen -inside. The "heads" of the animals poke out at the end of each branch -(see Fig. 9). - -The _Hydra_, with which we started, had always the power of producing -eggs; each animal could do so, besides producing buds. But in our -Colonial Coralline this is not necessarily so. Some individuals lose the -power of producing eggs. Others can do nothing else, and become greatly -altered in structure, often losing the power of developing tentacles, -and exhibiting other changes. So much are they altered sometimes that -they seem to be mere buds, not separate animals at all. - -In other cases a still more surprising thing happens. The bud that is -destined to produce eggs falls off, and becomes quite independent of the -colony; more than this, it becomes quite different in appearance from -the members of the colony: and instead of being a Hydra-like animal it -becomes a jelly-fish. But the eggs of this jelly-fish do not produce -jelly-fishes: they produce a more or less Hydra-like animal which gives -rise by budding to a fresh colony. This is what is known to Zoologists -as "alternation of generations." - -Now comes a puzzling question--Which part of this family group shall we -select and call it an "animal"? Is each Hydroid of the colony an animal, -and the jelly-fish another animal? Zoologists say "No": from the -development of one egg, to the production of another, is the cycle that -constitutes an individual animal. So we have the puzzling result in -nomenclature, that an "individual" consists of a very large colony of -creatures in one place, together with a perfect shoal of creatures quite -unlike it, floating miles away from it on the ocean. What name must we -give to the units, so curiously connected with one another? Zoologists -call them "Zooids" (animal-like parts) or "persons." - -This is the story of the jelly-fish as originally told. But there are -innumerable variations upon it. There are kinds of jelly-fish that -produce jelly-fish and have no Hydroid stage at all. Sometimes the -"persons" of the colony present many varieties, each taking up some -different task for the community. Some may be "nutritive persons," -_i.e._ commonplace Zooids that have mouths and eat food; some -"protective persons," reduced to mere folds or sheathing processes to -guard the others; some are "stinging persons" armed with enormous -quantities of thread cells. Then the whole colony may be like the -jelly-fish, a floating affair, and not fixed at all. - -[Illustration: FIG. 9.--An example of the Hydrozoa. A, branch of a -Coralline, _Sertularia Ellisii_, magnified. B, the same, more highly -magnified.] - -We have several times above referred to the animals known as corallines. -It may almost be assumed that the ordinary reader knows what these are; -if not, a little search among the treasures of the sea-shore will almost -certainly reveal some of them, living or dead. The texture and -appearance of the dead stems remind one of soft horn or dried gelatine; -the branching arrangement of the stems and the little cells disposed at -the ends of the branches will easily be shown under slight -magnification. Most people will remember the rage for dyed corallines, -by which all the fancy shops and florists were possessed a few years -ago. The corallines, dyed a bright emerald green, or a dull red, which -were used for decorations at that time, were usually a variety of the -Bottle-brush Coralline, found on English shores; but sometimes commoner -kinds were employed. - -Fig. 9 shows an example of a coralline, slightly magnified in A, and in -B much more highly magnified, so as to show the individual hydra-like -zooids, each with its circle of tentacula. - -The Sea-Anemone and the Hydra respectively represent the two great -groups of the Coelenterata, named after them, the Anthozoa -(Flower-animals), and the Hydrozoa (Hydra-animals). The corals are -forms of the Anthozoa, single or colonial, which possess a skeleton. - -[Illustration: FIG. 10.--_Gorgonia verrucosa_, from Guernsey, nearly -one-third of the natural size.] - -[Illustration: FIG. 11.--Corals. _A_, _Acanthoporia horrida_. -_B_, _Meandrina strigosa_. _C_, _Madrepora divaricata_. _D_, _Fungia -papillosa_. _E_, Red Coral, _Corallium rubrum_. _F_, _Stylaster -sanguineus_.] - -The above diagram shows examples of the Anthozoa. Fig. 10 is _Gorgonia_, -the Sea-Fan; while Fig. 11 represents corals of six different kinds. - -Besides the two great groups we have named, the Hydra-like animals -and the Sea-Anemone-like animals, the Coelenterata contain a third -group, the Ctenophora, or Comb-bearers, so called on account of their -possessing bands of cilia, fancifully compared to the teeth of a comb. -At first sight most of them somewhat resemble jelly-fishes, being -transparent forms swimming near the surface of the sea. They are -carnivorous, and some of them highly phosphorescent at night. The -gastric cavity is divided up into branches. The representatives of the -Ctenophores, most often seen on our own coasts, are small rounded forms. - -Two remarks must be added before quitting the subject of the -Coelenterata. - -Firstly, the description of them as two-layered Animals is one that only -applies typically and to the simpler forms. In others, such as the -jelly-fishes, there is an intermediate layer of jelly, which appears to -acquire a cellular structure by the immigration of cells derived from -the primary layers. Thus we see, within the group of the Coelenterata, -the gradual establishment of that third body-layer, which is found in -all animals of higher structure. Scarcely indicated in _Hydra_, as a -faint trace of a boundary-line (lamella) between the ectoderm and -endoderm, it attains a good thickness in the Jelly-fish and Ctenophora. -In animals of higher structure the third body-layer, being now fully -established, is cellular from its beginning in the embryo; in the -Coelenterata its gradual formation is to be traced. - -Secondly, it must be remarked that the colonial structure and the -arrangement sometimes concomitant with it of "alternation of -generations," is by no means confined to the Coelenterata. Both are seen -in other forms of life, in which the units, or zooids, differ greatly in -structure from those of this group. - -TABLE SHOWING THE CLASSIFICATION OF THE COELENTERATA - - - { HYDROZOA, or - { HYDRA-LIKE - { ANIMALS. - =Grade II.= { - { - THE TWO-LAYERED =COELENTERATA.= { ACTINOZOA, or - ANIMALS. { SEA-ANEMONE-LIKE - { ANIMALS. - { - { - { CTENOPHORA. - - - - -CHAPTER VI - -THE SPONGES - - -Many who are familiar with the domestic sponge have never seen a sponge -in a growing state, and would find it almost impossible to realise that -a sponge may be a thing of beauty. And yet sponges are quite common on -the rocky shores of our own country. It is true that they do not form -large masses, like the sponges grown in warmer seas, which we import; -but the smaller growths, massed together, often cover a considerable -space of rock, and are conspicuous by their beautiful colouring. Some -sponges are crimson, and some green; while one of the commonest is a -brilliant orange-yellow. The latter may often be found near low-tide -mark, on a shelf of rock under growing seaweed. If the explorer has -any doubt what the object is, it may easily be identified by the -touch, which though moist and firm in the growing state, is still the -unmistakable "feel" of sponge. Where the receding tide exposes a large -surface of steep rock, for instance in caves, sponges may be found -covering the rocks as thickly as mosses do on land. Masses of dead -sponge, consisting of branching parallel fingers a few inches long, may -often be found in the dead state, washed up on the shore; these are the -usual drab colour of a dead sponge. - -The encrusting sponges which grow on rocks present a mass, so to speak, -of little hillocks: in kinds which attain a larger growth, these may -almost be described as branches. Each little hillock or branch has -a hole at the top; and on the exterior of the rounded mass of the -bath-sponge may be found numbers of such holes. We should naturally -suppose that these holes were the mouths of the various sponge branches, -especially since they lead to the central cavity of the branch, and thus -to that of the whole sponge; and indeed they are known by the Latin name -of "oscula," little mouths. They are, however, nothing of the sort; -the sponge once had a mouth, a single one, when it was young, but the -adult sponge has lost it. For the young sponge is at first a little -free-swimming, two-layered animal of the type which has been described -above as the gastrula larva. When it gets old enough to settle down in -life, it sinks upon some suitable surface, and becomes fixed to it, -mouth downward: the mouth is thus lost. How, then, is the animal to be -fed? As it grows, there is developed in its substance a system of hollow -spaces, which communicate with the exterior by means of microscopic -pores. Through the latter, water is drawn in, and passes, after devious -wanderings, to the central cavity of the animal, whence it is expelled -by the so-called osculum. At first, the young sponge has but one cavity -and one osculum; but by degrees the sponge branches and spreads, the -cavity of each new portion remaining in connection with the main cavity. -If, as they grow in size, the branches touch one another, they sometimes -coalesce--a fact which renders the growth of the sponge in some cases a -very complicated matter. - -It will be seen from the above description that the sponge is a sort of -living filter. As the water passes in through the pores, it deposits in -the substance of the sponge all the little organisms that it contains; -on these the sponge feeds. - -It will naturally be asked, how does this living filter work? Water will -not pass through small holes to flow out again at large ones in an -upward direction, unless helped by some mechanism. How is this supplied? -By the industry of the cells of the sponge. Its canal-system includes a -set of wide chambers, lined with cells which have long cilia, called -flagella. These flagella, constantly moving in one direction (like the -fan of a ventilator), create a current, which passes the water on with -such force that it reaches the central cavity, whence it is expelled -through the oscula. These chambers do not communicate directly with -the exterior. They are closed, except at certain small holes, the -"prosopyles," where they take in the water that enters from spaces -connected with the pores. At the main end of the chamber is an aperture -called the "apopyle," capable of being partly closed, and leading into -an excurrent passage. This last communicates with the central cavity of -the sponge. - -It will be seen that the topography of the sponge is a very complicated -business. All its details have been studied by means of thin sections -specially prepared and placed under the microscope (see p. 183); in -these the labyrinth of canals and chambers is seen cut through at -various points; the cells lining them and dividing them may be -individually studied. The passage of water through the sponge was first -observed by Robert Grant; many of the most recent discoveries regarding -the structure of sponges we owe to Professor Sollas. - -We have not yet explained what our living filter does with its food -when it gets it. The ciliated cells of the internal lining take in solid -particles just as Amoeba does; and from these they may be passed on to -the cells of the middle layer, amoeboid cells, which can move about. -These cells are considered to be derived from the primary layers of the -body, especially the inner one, and to have wandered into a cellless -middle layer, comparable in nature with that of some Coelenterates. - -The sponge is full of firm or gritty particles, which form its skeleton, -and remain when the sponge is dead, and the softer parts decayed. These, -when magnified, often present beautiful and curious shapes. The use of -them is not only to support the body, but also to prevent the sponge -from being eaten by other animals. - -There is found in the English canals and rivers a small, fresh-water -sponge, usually greenish in colour. This is named _Spongilla -fluviatilis_, the River-sponge, and affords an exception to the usual -marine distribution of sponges. In the winter it dies gradually away, at -the same time forming asexual buds, or "gemmules," in the interior of -its substance, which are liberated in the spring, and become young -sponges. - -TABLE SHOWING THE CLASSIFICATION OF SPONGES - - { CALCAREA, WITH CALCAREOUS - { =PORIFERA.= { SKELETON. - =Grade II.= { { - Two-Layered { { - Animals, or { { NON-CALCAREA, WITH - Acoclomata. { { SKELETON ABSENT OR - { { FLINTY. - { =COELENTERATA.= - -Some of the marine sponges are parasitic. Most people have doubtless -found on the sea-shore now and then a dead oyster-shell, completely -riddled with small round holes, very similar in appearance to those seen -in "worm-eaten" wood. These are the work of _Clione_, a parasitic sponge -which is very fatal to the oyster. At first sight it seems a puzzle how -the sponge made its way into the hard shell; it has no mouth to bite or -suck its way into the solid substance. The cells of the sponge, however, -wear away the lime of the shell by means of some acid chemical action. -Not only so, but they can attack stones as well, when these consist of -limestone; and on some parts of the coast bits of sponge-eaten limestone -washed up on the beach are quite common objects. They are pierced all -through by holes, so that their appearance would suggest a sponge carved -in stone, but for the fact that the holes are fairly uniform in size. -Such stones, lying on the shore, often puzzle the finder, when they -contain no apparent trace of the tenant that has worked its way through -them. - -The sponges have received the name of Porifera, on account of the -structure above described. They are often classed with the Coelenterata, -because, among other reasons, they practically belong to the two-layered -type of structure, and because they form a complex organism that may -almost be called a colony. But some prefer to place them in a group by -themselves, apart from the Coelenterata. The chief reason of this -is that the sponges, as compared with a primitive two-layered type -indicated by their own larvæ, are turned upside down, the mouth being, -as above stated, originally situated at the fixed end. - - - - -CHAPTER VII - -WORMS - - -When the great naturalist, Linnæus, framed his classification of the -animal kingdom, he included in the division Vermes or Worms, nearly -everything except the vertebrates and insects. - -This assemblage would have been more correctly styled if instead of -"Vermes" it had been described as "animals unsorted." Subsequent -zoologists have by degrees picked out and separated from the Vermes -first one group of animals and then another. But the process is still -going on, and several of the groups which are still classed under the -name of "worms" might, with very great justification, be separated from -each other; it is custom, rather than family resemblance, that accounts -for their being retained under one heading. - -Widely although the various "worms" may differ from one another, one -thing may be stated regarding the most of them, and that is, that -they "crawl"; that is to say, they move along by means of successive -contractions of successive parts of the muscular wall of their elongated -bodies. This "crawling" mode of progress is the chief thing involved -in the popular idea of a worm; but the popular definition of a -worm includes also the larvæ of insects, such as caterpillars and -beetle-grubs. The latter, it must be noted, crawl with the assistance -of legs, while the true worms crawl without any such assistance. Any -adornments that they may possess, whatever else they may be, are not -legs. - -The worms were formerly included along with the insects and lobsters, in -a division called Annulosa, or, Ring-bodied animals, but it has now long -been recognised that the latter are worthy of a division to themselves. -It will easily be seen, however, that the term Ring-bodied animals is -very appropriate to all of them. If we look at either an earthworm or -a lobster, we can but recognise that the body consists of a number of -successive parts very similar to each other; and since the body of -each is, in section, more or less round, these successive parts may -very aptly be termed rings. Modern writers, however, prefer to call -these parts not rings, but Metameres, _i.e._ successive parts. The -symmetrical arrangement of the body in a series of such parts is -called "Metamerism"; and the animals which possess it are said to be -"Metameric" in structure. Sometimes also the successive parts are spoken -of as "segments." Compare Fig. 12; _A_ and _C_ show the successive -body-rings of worms. - -The earthworm, with its many rings, is one of the higher forms among -the worms. Among the lowest forms there are worms in which the ring -structure cannot be detected. Between the limits thus marked out, there -lies, so to speak, the battleground of modern zoology. For the origin of -metamerism, and the pedigree of vertebrates, are among the questions -that are being discussed in connection with various groups of the worms. - -Among the lowest forms of worms are the Planarian worms, already alluded -to as examples of the third grade of animal existence. These belong -to the class Turbellaria, which is represented by plenty of both -fresh water and marine forms in our own country and on its coast. The -Turbellaria are divided into groups called Acoela, Dendrocoela, and -Rhabdocoela. These names allude to the intestine, which in the first -group is wanting, in the second branched like a tree, and in the third -straight. The Cestoda or tape-worms, which absorb nourishment through -the skin, and therefore need no alimentary canal, and possess none; and -the Trematodes, represented by the Liver-fluke, which infests sheep, -together make up the group of flat-worms (Platyhelminthes), of which -mention has already been made (p. 44). In all of them the body is more -or less flat, and the digestive cavity, like that of Coelenterates, has -but one opening, the mouth. The life-history of parasitic worms is -described in a well-known volume by Leuckart, which forms the basis of -our knowledge on the subject. Since its publication, discoveries -regarding parasites have been constantly added by other observers. - -The history of the Liver-fluke is a most complicated example of -alternation of generations. The adult form infests the sheep's liver. -There it produces eggs, which afterwards find their way into water. Here -they die unless they find their way into a certain water-snail, which -many of them do. Within this snail--_Linnæa truncatula_--the egg -develops into a sac-like body, called a sporocyst. This produces within -itself numbers of a small creature which is called the Redia form. These -in turn produce a tailed form, called a Cercaria, which gets out of the -snail, swims in water, and finally settles down on some plant. Here it -is eaten by an unfortunate sheep, within which it develops into the -adult fluke. - -The other great divisions of the Vermes are as follows: The Nematodes -or thread-worms, a group of parasites which includes the dreaded -_Trichina_; the Nemertines, a group mostly carnivorous, possessing a -curious proboscis, and often an armed skin; the Leeches or Hirudinea, -and finally, the Chætopods (Bristle-footed Worms), the highest group -of all, containing the forms often spoken of as Annelides--_i.e._ -Ring-shaped Worms. - -These last are again subdivided into the following: The Archiannelida or -Primitive Annelids, some of which have a curious ciliated larva, already -referred to (p. 42) as the typical Trochosphere or Wheel-ball; the -Oligochæta (Few-Bristles), which include the familiar earthworms; and -the Polychæta (Many-Bristles). Of the latter, some, the Tubicola, live -in tubes which may or may not be fixed to some object; while others, the -Errantia, or Wanderers, are free and very active. _Nereis_, the Rainbow -Worm (p. 159) may be named as an example. Our illustration shows -instances of each group. _A_ is the Sea-Mouse, a bristly creature so -named by some very imaginative person. It has two kinds of bristles, -long and short, the former being possessed of a peculiar lustre (see p. -73). _C_ is _Syllis_, one of a very curious family of worms. In both _A_ -and _C_ are seen a row of paired appendages; these are not "legs," but -expansions called "parapodia" which serve the purpose of legs, besides -which they frequently act as breathing organs, a special part being -appropriated to this purpose. Each of these animals is active and -carnivorous, and has a head. The Syllidæ are remarkable for the very -peculiar way in which they divide, new individuals being formed and -cast off from the end of the body. There is, however, a deep-sea form -of _Syllis_ which divides in a very odd manner, giving rise to new -individuals placed transversely. The result is a most extraordinary -looking creature, a network of worms with numerous heads, each branch -being eventually provided with one of its own. - -[Illustration: FIG. 12.--Worms. _A_, a Sea-Mouse, _Aphrodite -aculeata_; _B_, _Terebella littoralis_; _C_, _Syllis_; _D_, -_Serpula vermicularis_; _E_, _Spirorbis nautiloides_, on a piece of -seaweed.] - -The tube-dwelling worms are represented in our picture by _Terebella_, -_Serpula_ and _Spirorbis_, all very common forms on the English coasts. -The _Terebella_ glues around its body a number of grains of sand and -bits of shell, thus forming a case; the projecting threads at the head -end are the gill-filaments, borne by the anterior segments of the body. -These are plumed; the thread-like structures which are seen to lie in -front of them are the tentacles or feelers. _D_, _Serpula_, is common -on shells and stones. The animal has a plumy bunch of gill-filaments, -brilliantly coloured, and a stopper with which it can close the mouth -of its tube. This precaution is necessary to keep out its predatory -cousins belonging to the Errantia, who poke in their heads and eat the -tube-dwelling worms. _E_ is _Spirorbis_, a minute form with a coiled -tube, which looks at first sight like a small univalve shell. It is -common everywhere, on shells and stones, and encrusting Fuci and other -seaweeds, which it sometimes covers almost completely. Spirorbis also -has plume-like gills and a stopper. In the latter is a cavity where the -creature's eggs are incubated for a time. - -The reader will have no difficulty in finding and identifying both -_Serpula_ and _Spirorbis_. _Terebella_ is frequently washed up on a -sandy shore. On the Lancashire coast one may feel sure of finding this -and many other sand-dwelling animals, after an east wind. The east wind, -driving back the water at low tide, kills these creatures with cold, -and presently they are washed up dead or dying by the high tide. -_Pectinaria_, another worm with a tube of sand-grains, in which, -however, the body lies loosely within the tube, may also be found in -thousands under the same circumstances. - -TABLE SHOWING THE CLASSIFICATION OF VERMES OR WORMS - - =Grade III.= { { - WITH MESODERM, { PLATYHELMINTHES, { _A._ TURBELLARIA or PLANARIANS. - BUT WITHOUT { or FLAT-WORMS. { _B._ CESTODA or TAPE-WORMS. - BODY-CAVITY. { { _C._ TREMATODA or FLUKE-WORMS. - - =Grade IV.= { NEMERTINES. - WITH MESODERM { NEMATODA or THREAD-WORMS. - AND { HIRUDINIA - BODY-CAVITY. { or LEECHES. { _A._ ARCHIANNELIDS. - { { - { CHÆTOPODA. { _B._ OLIGOCHÆTA. - { - { _C._ POLYCHÆTA. { (_a_) Tubieola. - { (_b_) Errantia. - -We must not forget to say something regarding the most commonly known -member of the Vermes, the familiar earthworm. The worms are the first of -the great group of animal life in which we find true land animals. There -are terrestrial forms among the lowest worms, at least forms that live -in earth that is damp; but the earthworm is in the strictest sense a -terrestrial animal. Darwin showed that it not only dwells in the soil, -but is in a sense the manufacturer of soil, since the fertility of the -earth depends greatly upon the work of earthworms. They pass the soil -through their bodies, digesting the organic particles they find in it, -and thereby loosen the soil, reduce it to a state of fine division, and -render it more fit to support the growth of plants. The "worm-casts" -formed by the soil that the earthworm has passed through its body may -not have been noticed by everybody. More obvious are the worm-casts in -sand left by the sand-dwelling marine annelids. These everyone must have -seen who has walked on a sandy shore at low tide. - -The worms include many puzzling forms, which have not been alluded to -here. Among these must not be forgotten the Rotifers, or wheel-bearing -animals. These are of minute size, and when first discovered were -therefore placed amongst the Infusoria. They are common in ponds. - - - - -CHAPTER VIII - -ARTHROPODA, THE LOBSTERS, SPIDERS AND INSECTS - - -The above is a very descriptive name for a division which includes the -Crabs and Lobsters and the Insects. Formerly they were included, along -with the worms, under the name Annulosa, the Ringed Animals. They -resemble these as possessing what is termed metameric symmetry, but -they are distinguished from them as the Leggy Animals, a fact which is -explained in the name, Arthropoda, joint-footed. Worms, as we have seen, -have no true legs, but the Arthropods, theoretically, have a pair of -legs to every ring. In some of the lower members of the group this is -literally the case, the Centipedes, or hundred-footed animals, for -example (Fig. 13). In higher forms the number of legs is greatly -reduced; several successive rings may become merged with one another, -losing, along with their independence, their legs. The true Insects, -thus, have only three pairs of legs and the Spiders four. - -[Illustration: FIG. 13.--A Centipede, _Lithobius elongatus_, from Tunis, -slightly reduced in size.] - -What are theoretically regarded as legs, however, may practically be -turned to many other uses, according to the position of the particular -body-ring to which they are attached. Thus, in the case of a body-ring -near the mouth, we find such things as "jaw-feet," maxillipedes--that is -to say, legs used for jaws. It consequently results that zoologists are -sometimes driven to speak of "walking legs," or, hiding the tautology -under a Latin phrase, "ambulatory legs"; and absurd although this may -seem, it is sometimes quite necessary for the sake of accuracy. It is -therefore more convenient to speak of the "appendages" of a body-ring -than of its legs. For this vague term can be applied equally to all the -row, whatever their uses. Among the different forms taken by the -"appendages" are those of "antennæ," long, hair-like feelers attached to -the head; "chelæ," or claws, such as the large claws of the lobster; -"cheliceræ," or "claw-horns," tearing appendages attached to the head; -"mandibles," mouth appendages used for biting, etc., etc. The reader who -wishes to attain a clear idea of the structure of a segmented animal, -and of the ways in which its parts are modified, should consult Huxley's -classical study of "The Crayfish" (International Science Series). - -The Arthropoda include two main groups--the Crustacea, or Jointed -Animals of the water, which breathe by gills; the Insects, or Jointed -Animals of the land, which breathe through tubes in their sides, called -tracheæ. - -The Crustacea include the familiar Crabs and Lobsters. These are among -their highest forms as well as their largest, and if we begin at the -beginning we must seek much smaller forms. The group called Entomostraca -include the so-called Freshwater Flea, a very active little thing found -in English ditches, and a great many other freshwater forms: also the -little Cypris, which has a shield forming a sort of bivalve-shell, and -is interesting from its wide occurrence as a fossil form. Most of the -Entomostraca have a larval form called a Nauplius; but this larva -refuses to tell us anything about the past history of the Arthropods. It -is itself already a jointed animal with legs. So we see that the -Arthropods, unlike the worms and the Chordata, have obliterated all -record of their poor relations. The parasitic "fish-lice," so-called, -are entomostracous Crustacea, often greatly degenerated in consequence -of their habit of life. Some live in the gill-chambers of a fish, some -on, or even embedded in the skin. - -[Illustration: FIG. 14.--Shell of the Bell Barnacle, _Balanus -tintinnabulum_, one-half the natural size. The figure shows several -successive generations, perched one upon another.] - -Among the most curiously modified forms of the Crustacea are the -Barnacles or Cirripedia. These creatures, like the sponges, have a -free-swimming larvæ, which eventually fixes itself by its anterior end, -so that the adult animal passes its existence upside down. The young is -an ordinary little creature with jointed legs, but the adult protects -itself by a strange armour of shell. An intermediate stage exists in -which the creature eats no food; it has therefore been compared with the -chrysalis of insects. At the top of the adult shell two little valves -open and shut, allowing the legs to dart out and seize upon prey. These -legs, gathered into a bunch, and extended and retracted together, remind -one of the fingers of a hand opening and closing. They are clothed with -a fringe of "cirrhi" or small processes; hence the name of the group. -The Common Barnacle of our own shores, sometimes called the Acorn-Shell, -is found on shells and stones, and often on those that are left -uncovered between tides. In places where the rocks of the coast are -very steep, a belt of white, several feet or yards deep, may often be -seen above low-water mark. This white zone, when examined more nearly, -is found to consist of barnacles, so crowded together that they obscure -the natural colour of the rock. The Common Barnacle is one of the -smaller species of the genus: in warmer seas barnacles attain to a much -greater size (Figs. 14 and 15). - -[Illustration: FIG. 15.--Shells of a Barnacle, _Balanus hameri_, found -in European and North American Seas, natural size.] - -The higher Crustacea, Malacostraca, include the familiar Crabs and -Lobsters, Decapoda. The lobsters receive the name of Macrura or -Big-tails; associated with them are the Shrimps and the Hermit-Crabs -(Fig. 16). The latter are therefore not crabs at all, but somewhat -divergent lobsters. Their tails are soft, and they thus require -protection: they choose the dried shell of some univalve mollusc and -live in it (Fig. 16). How far the case is that they need a house because -their tails are soft, and how far the contrary is true that their tails -are soft because they live in a house, it would be difficult to say. -Readers of another volume in this series, Professor Hickson's "Story of -Animal Life in the Sea," will remember that the hermit-crab often offers -a curious instance of "commensalism" or partnership with other animals. -The hermit-crab was, in fact, one of the earliest instances in which -such a partnership was observed, the companion being in this case a -sea-anemone perched on the shell in which the crab lives. - -[Illustration: FIG. 16.--Hermit Crabs. _A_, _Aniculus typicus_, from -the Indo-Pacific Seas, one-half of the natural size. _B_, _Caternus -tibicen_, from the Indo Pacific Seas, slightly enlarged.] - -The true Crabs are called Brachyura, or Short-tails; for obvious -reasons, the tail of a crab being very curiously modified and tucked in -under the carapace or "shell." A form exceptional in the fact that -frequents the land is the Land-Crab of the West Indies (Fig. 17). -Another land crustacean, _Birgus latro_, the Robber Crab, belongs to the -previous group. - -[Illustration: FIG. 17.--Land Crab, _Gecarcinus ruricola_, from the West -Indies, one-half of the natural size.] - -In addition to the above the Malacostraca include the Arthrostraca, or -crustaceans which have the front of the body jointed as well as the -tail, so that there is no large shield formed by the fused armour of -several segments (cephalo-thoracic shield, _cf._ Figs. 16 and 17), as in -crabs and lobsters. The Amphipoda, or Sand-hoppers, sometimes called -Sand-fleas, are familiar examples of these. There are several common -kinds found on our English shores, and sometimes they appear in such -numbers, hopping above sand or seaweed left by the tide, that they seem -to form a sort of cloud, every unit of which, however, is but in the air -an instant, falling and giving place to some other, while it prepares -for a fresh hop. The so-called Freshwater Shrimp, _Gammarus_, is another -common member of the Amphipoda. Fig. 18 shows the general form of a -Sand-hopper. Nearly allied are the Isopoda or Wood-lice, interesting -because they are among the few terrestrial forms of the crustacea; they -live, however, in damp places, and are but too well-known in gardens, -where the gardener often mis-names them "insects." - -[Illustration: FIG. 18.--A Sand-hopper, _Pallasea Cancellus_, from -Siberia, natural size.] - -[Illustration: FIG. 19.--A South American Spider, _Ctenus ferus_, from -the Amazon region, natural size.] - -The mention of terrestrial forms would naturally bring us to the -discussion of the true Insects. In the Arthropoda we for the first time -meet with terrestrial animals except in scattered instances, and the -true Insects are the largest and most important group of these. There -are, however, various creatures belonging to the Arthropoda which are -neither Crustacea nor yet Insects. Among these is the familiar spider, -an "insect" in popular language, but not so described by the zoologist. -Among other differences, the true spiders have eight legs, whereas the -true insects have only six. Fig. 19 shows a typical spider; the eight -jointed legs are attached to the thorax ("breastplate"); with the latter -the head is united. The abdomen, as in insects, is formed by the fusion -of several segments, and has no legs, but it has, however, out of sight, -the spinning legs or "spinnerets," out of which the thread of the -spider's web is spun. The venom of the spider is not a fable; spiders -have poison-glands with ducts which open on the tops of the cheliceræ. -They dispose of their prey by sucking it; they do not swallow solid -food. The habits and webs of spiders are familiar to every one: their -nests, as a rule, are only noticed by close observers. The nest is made -of spun threads closely felted together to form a round hollow ball. -This the house-spiders hang on a wall or among the rafters of a roof. -There are, however, spiders which build their nests under ground; and in -this case the nest may be conveniently furnished with a lid, which can -be pushed up when the animal wishes to come out. Fig. 20 shows the nest -of the Trap-door Spider, so called from the construction of its nest. - -[Illustration: FIG. 20.--Nest of the Trap-door Spider, from the South of -France, three-quarters of the natural size.] - -Fig. 21 shows a spider-like animal which, at first sight, seems to have -five pairs of legs. In fact, however, it has only three pairs, thus -approaching the insects in structure. These three pairs of legs are -attached to the thorax, while the head, which is separate from the -thorax, unlike that of the true spider, bears two pairs of leg-like -appendages. This is the chief of a group which are sometimes placed in a -class by themselves, on account of their great differences from real -spiders. Their head is separated from the thorax; and the thorax is -divided into three segments; these, however, do not come out clearly in -the diagram. The head bears, posteriorly, a pair of appendages which -are practically legs; in front of these a pair of long "pedipalps" or -"foot-feelers"; and quite in front the comparatively short "cheliceræ." -These creatures are very venomous; they move about by night to seek -their prey. - -[Illustration: FIG. 21.--A venomous spider-like animal, _Galeodes -araneoides_, from North Africa, natural size (Diagrammatic).] - -Another kind of spider-like animal is familiar in English fields and -waysides--the long-legged spiders, called Harvestmen or Phalangidæ, -which spin no web, but jump upon their prey. Unlike the last group, the -body differs from that of true spiders, in being more, not less, -compact: for not only is the head joined to the thorax, but also the -thorax is joined to the abdomen, the outline of the body being therefore -almost globular. They receive the name Phalangidæ, Joint-Spiders, from -the sharp joints in their long legs. - -Allied also to the spiders are the Mites, Acarina, so destructive to -cheese, flour, and other eatables; and the Ticks, which infest the skins -of various animals Fig. 22 shows a specimen of the latter. They are -practically blood-sucking Mites. It is the female which attacks animals, -while the males live among vegetation. - -[Illustration: FIG. 22.--The Tick which infests the Hippopotamus, from -South Africa, twice natural size.] - -The Scorpions, also, are relatives of the Spiders. They are inhabitants -of hot countries, and highly venomous. They possess a jointed tail, -instead of an abdomen with fused segments, and a lobster-like pair of -appendages in front; these are the second pair of appendages, the -"pedipalps," while the short "cheliceræ" lie in front. In the living -animal the tail is often carried curled up over the back. The Mites, -Ticks, and Scorpions all agree with the true spiders in possessing eight -legs. The King-Crab, Limulus, has not hitherto been named, because, -though living in the sea, it is not a crab at all. It has been shown by -Professor Ray Lankester to be related to the spiders. It is a large -crab-like creature, which may be seen in museums and aquaria, and is -brought from the tropical seas. - -[Illustration: FIG. 23. A Scorpion, _Buthus Kochii_, from India.] - -Before passing to consider the true Insects, or Hexapoda, something must -be said about the discovery of _Peripatus_, a creature which comes from -Cape Colony. It has been called caterpillar-like in appearance, but its -structure is in many respects so peculiar, that it has been described as -a link between insects and the higher worms. Its legs, for instance, -although jointed, and much resembling those of insects in appearance, -are hollow, like the "parapodia" of worms. - -The Centipedes have been already referred to. These, with the -Millipedes, form the group Myriapoda. In outward form, at any rate, -these suggest an intermediate position between Peripatus and Insects. - -The true Insects have a definite head, separated from the thorax, and a -constriction between the thorax and the abdomen; this is why they are -called insects, "cut in two." The thorax bears three pairs of legs, the -mouth has typically three pairs of appendages, which may be altered and -modified in many different ways, according to the nature of the animal's -way of feeding. While the Crustacea are typically adapted for breathing -in water by means of their gills, the Insects are adapted for breathing -air. This they do by means of their air-tubes or tracheæ, the inlets of -which open on their sides. These are divided into fine branches, which -diffuse air through the body of the Insect. Two interesting points must -be noticed about insects. The first is that they were the first group in -which zoologists were able to study the nature of larval forms, long -before the microscope had revealed the larval forms of marine animals. -The changes undergone by insects are known as metamorphosis, or change -of form; and are typically represented by the life-history of a -caterpillar, which assumes during the winter a resting form called a -Chrysalis or Pupa, and finally emerges as a Butterfly. Insects have -sometimes been classified according to the greater or less completeness -of the metamorphosis they undergo, which in some cases is comparatively -slight. It has been mentioned elsewhere that larval forms usually exist -where the young animal is placed under very different conditions from -the adult. Fig. 24 shows two well-known instances of insect larvæ in -which this is strikingly the case, the larval form being a -water-dweller, and the adult a winged fly. Of these, one, the larvæ of -the Dragon-fly, crawls about free; while the other, the so-called -caddis-"worm," builds itself a case of grains of stone and shell -cemented together. - -[Illustration: FIG. 24.--Larvæ of insects. _A_, of a Dragon-Fly, -enlarged; _B_, House of the larva of the Caddis Fly, natural size; _C_, -the Caddis Larva itself, enlarged.] - -The second point of interest is the wonderful part which has been played -by insects in modifying the world we live in. We owe the bright colours -and the sweet honey of flowers to the selection exercised by insects; -they carry the pollen of flowers from one plant to its neighbouring -kindred, thus securing cross-fertilization for the advantage of the -plant, and thereby perpetuating any quality, such as colour or -sweetness, which has originally attracted the insect to the flower. -While a few plants only are fertilised by means of the wind, a vast -majority depend entirely upon insects for the cross-fertilisation which -is so necessary for the production of healthy seeds. We have already -alluded to the part played by the earthworm in preparing the soil. If -the earthworm has been the ploughman the insect has been the more -intelligent gardener, who has filled the world with bright flowers. -The earlier forms of plant life had green and inconspicuous flowers -(Cryptogamia); the Phanerogamia, or showy-flowered plants, including all -those that bear what are popularly termed flowers, have been produced by -the artificial selection exercised by insects long before man was here -to admire the result, and to carry on the same work in his gardens. The -insect owes its food to the plant world; the plant world owes health and -beauty to the constant ministration of the insect; so marvellous is the -inter-connexion of one form of life with another. - -The number of different kinds of insect is enormous; the number of named -species has been estimated at nearly a quarter of a million. It is -therefore no wonder that entomology, the study of insects, has claimed -the rank of a special science. We cannot here do more than refer in -passing to a few of the more familiar types. First of all, by right of -its work in fertilising flowers, let us take the Bee. Fig. 25 shows its -honeycomb and its larvæ. The bee-grub differs from the caterpillar in -its comparative helplessness. It is fed like a child by the worker bees, -which are undeveloped females; and it does not leave the cell in which -the egg is originally placed until it is ready to take on the adult -form. The metamorphosis is complete; that is to say there is a grub -stage and a pupa stage before the adult stage. There are three kinds of -bees--the workers, which are sexless; the drones, which are males, and -the queen, who is the sole female of the hive. The bee-grub may develop -into a worker or a queen, according to the food it receives as a grub, -the grubs that are intended to become queens being placed in a larger -cell. The bee-grub differs from the caterpillar in having no feet. - -[Illustration: FIG. 25.--_A_, Larva of the Bee, _Apis mellifica_; _B_, -Section of Honeycomb.] - -[Illustration: FIG. 26.--Ants, _Formica rufa_, English, enlarged. _A_, -Female; _B_, Neuter, or Worker.] - -The ants are nearly allied to the bees, and also have a complete -metamorphosis. Fig. 26 shows the English red ant, female and neuter. The -wings of the female drop off after the pairing season, a fact which has -given a name, Hymenoptera, to the whole group to which the ant belongs, -although the name is often quite inapplicable. A recent discovery in -entomology is the fact that ants have a voice. Dr. D. Sharp of Cambridge -has described their "stridulating," _i.e._ noise-producing, organs. -These consist of parallel ridges present on the sides of certain -segments. By working the body up and down, the insect scrapes these -ridges with the edge of the preceding segment, so that a musical note is -produced, intelligible to other ants. The question has also been -investigated by French observers. The principle involved will readily be -recognised by those who in childhood were guilty of trying to extract -music from a comb. - -[Illustration: FIG. 27.--White Ants, _Eutermes morio_, from Pernambuco, -twice the natural size. _A_, Soldier; _B_, Worker; _C_, Young male; _D_, -Female.] - -The white ants, so destructive in tropical climates, are not true ants, -but belong to a different order. These also live in colonies; like the -bees, they have an egg-laying queen. She has a partner, the king. There -are neuter soldiers and neuter workers, both wingless, while the male -and female have wings, afterwards lost. - -[Illustration: FIG. 28.--Cocoons of Moths. _A_, Compound Cocoon of -_Coenodomuc hockingi_, from India, one-half natural size; _B_, of a -Silkworm, _Bombyx Japonica_, one-half natural size; _C_, of Green-shaded -Honey Moth; _D_, of Death's Head Moth, one-quarter natural size; _E_, of -_Metura Savendersii_, from New South Wales, natural size; _F_, of -_Castnia Endesmia_, from Chili, one-sixth of the natural size; _G_, -of _Attacus attas_ from Bombay, one-fourth of the natural size.] - -The Lepidoptera or butterflies and moths receive their name, -Scaly-winged, from the beautiful microscopic scales with which their -wings are covered. Fig. 28 shows the cocoons which the larvæ of some of -the moths make for themselves in which to pass their pupa stage. Some -are made wholly of silk, others of dried leaves woven together. Fig. 29 -shows a Moth with its caterpillar, cocoon, and chrysalis. The threads of -which a caterpillar weaves its cocoon are familiarly exemplified in the -silk of commerce. The caterpillar, in some cases, is gregarious, and -builds a common nest (Fig. 30). - -[Illustration: FIG. 29.--A Moth, _Saturnia pyri_ (S. Europe), with its -Caterpillar, _A_; its Cocoon, _B_; Cocoon cut open to show Chrysalis, -_C_; Adult insect, _D_.] - -The beetles, Coleoptera, are, like the butterflies, endlessly numerous. -They are characterised by the striking difference in their two pairs of -wings, of which the anterior pair is strong and horny, and forms, when -at rest, a sheath which covers the thinner posterior pair of wings. The -metamorphosis is complete in this group also. Fig. 31 shows an example -which is typical except in one respect--the adult form, namely, is one -of the comparatively few instances of adult insects that live in water. - -[Illustration: FIG. 30.--Nest of gregarious Caterpillar of a Moth, -_Hypsoides_.] - -[Illustration: FIG. 31.--Development of an English Water-Beetle, -_Dytiscus_. Grub; Pupa; Adult insect.] - -Much has been said above in praise of insects and their wonderful work -in selecting flowers. There is, however, another side to this, as the -gardener and farmer know too well. While the winged honey seekers help -the plants, their larvæ devour them, and so do many other forms of -insect. Fig. 32 gives us in miniature some of the most notorious insect -pests. The work of the locust has been dreaded since the days of the -Pharaohs and before: the Colorado beetle which infests the potato, is a -plague as terrible, if more modern. The weevils and caterpillars that -destroy trees, though not directly dangerous to our food supply, are -sufficiently destructive. The terror of insect pests lies in their vast -numbers, which may render an otherwise harmless creature dangerous. I -read last year of a curious railway mishap in the United States. A train -was brought to a standstill by the wheels sliding on something greasy -that covered the track. It proved to be a flock of the so-called "Army -worm," a variety of caterpillar which travels long distances in crowds, -when its numbers have become too many for the supply of food, or when it -is about to enter into the pupa stage. These covered the railway track, -and the whole country for a long distance; and the "greasiness" of -the rails was produced by the crushed bodies of the unfortunate -caterpillars. The train was delayed for hours, while a gang of men -with brooms cleared the way in front of it. - -[Illustration: FIG. 32. Insect pests. _A_, Locust, _Acridium -peregrinum_, one-fourth natural size; _B_, Caterpillar of Wood Leopard -Moth, _Zeutzera Æsculi_, boring in wood, about one-thirtieth of natural -size; _C_, Colorado Beetle, one-fourth natural size; _D_, Leaf-rolling -Weevil of the Oak.] - - - - -CHAPTER IX - -MOLLUSCA, THE SHELL-FISH - - -The shell-fish are called Mollusca, the soft-bodied animals. It will -easily be seen that this name was intended to point out the distinction -between them and the Arthropoda, as regards the way in which the skin is -protected. In the latter, as we have seen, the skin itself is hardened. -In the shell-fish, the skin secretes a covering which lies outside it. -Just as our skins pass out superfluous moisture to the outside, in the -form of perspiration, so the skin of the mollusc continually passes to -the outside the solid substances which the body has taken in from the -sea-water; and by the continual accumulation of these, the shell is -formed. This, at least, is the view taken by modern authorities of the -formation of the shell in most instances. - -The juvenile shell-collector usually begins his knowledge of the -classification of the Mollusca, by learning that shells are classified -as Univalves and Bivalves. This distinction is useful as a beginning. -Univalves, that is to say shells which consist of one piece, are those -of the snail-like animals, Gasteropoda, or Gastropoda, as some prefer to -spell it. Bivalves, or shells which consist of two flaps, are those of -the Lamellibranchiata or animals with plate-like gills, such as the -mussel or oyster. - -Let us begin with the former. Everybody knows the snail. The snail -proper bears a typical univalve shell: though in its relatives (the -slugs), the shell is more or less suppressed. The name, Gasteropoda -(stomach-footed animals), is supposed to be descriptive of the way in -which a snail crawls. Half getting out of its shell, so to speak, it -does its best to lay its body to the ground, and its so-called "foot" is -an extensive muscular expansion underlying its body, not just a muscular -organ thrust out of the shell, as in some other groups. The shell, the -mode of crawling, and the "horns," tipped with eye-specks, and directed, -intelligently and inquisitively, towards things of interest--these make -up, for most people, the idea of Snail. But the most distinctive feature -of the class is a less obvious feature, namely, the structure of the -tongue. We may see, on any damp day or dewy evening, the snail working -away with its tongue at some tender leaf. Its tongue is practically a -file with which it files away the substance of the leaf, the resulting -green mash being thus made ready in minute quantities for the snail to -swallow. Thus are made the too familiar holes which disfigure the leaves -of plants in our garden. When seen under the microscope, the file-like -structure of the tongue is visible; indeed, in large tongues, it may, to -some extent, be made out with the naked eye. Across the tongue, which is -a flat ribbon-like structure, there runs a pattern of small teeth, -bilaterally symmetrical, and this pattern is repeated over and over -again throughout the whole length of the tongue. It might be thought -that snails' tongues, being so much alike in their mode of use, would -not need to be very various in pattern: but far from this, they vary in -appearance as much as the shell. Not only is there a different pattern -for every different order of the class, but a different pattern for -every genus; nay, there are even distinctions between the tongues of -different species in the same genus. Consequently some authorities on -shell-fish prefer to classify them by their tongues, a classification -which for the most part holds good. So characteristic is the tongue -of the Gasteropod, that when new animals have turned up which were -difficult to classify by means of the structure of the body, they have -been finally recognised as Molluscs, somewhat related to the snails, by -the tongue. This file-like tongue-ribbon of the snails is often called -the Odontophore or Tooth-Carrier; sometimes the part which actually -bears the teeth receives the name of the radula. - -The snail and its relative, the slugs, belong to the Pulmonate (_i.e._ -air-breathing) division of the Gasteropoda. The sea-slugs, in which, -like the land slugs, the shell is absent or reduced, are relatives of -the land snails. Some of those found on our own shores are handsome -creatures, brilliantly coloured. Both groups fall under the division -Euthyneura, while the majority of the marine univalves belong to the -division Streptoneura (_i.e._ Gasteropods with twisted nerves). The -Gasteropods, in the course of the evolution of their shell, have had -the body thrown crooked by the burden of carrying it; the Streptoneura -are the forms in which this crookedness is most pronounced; in the -Euthyneura it is less so. There are degrees of crookedness even among -the Streptoneura; and the limpet is less crooked than the periwinkle -(see Table, p. 30). - -The older classifications of the Gasteropoda were largely founded on the -characters of the shell; but these, though in the main they hold good, -have required some modifications in recent times. Conchology, the study -of shells, was at one time the hobby of many collectors whose knowledge -of the animals possessing the shells was not of a very extensive kind; -and consequently the very name of conchology is often enough to ruffle -the feelings of the zoologist of the present day. Yet many interesting -problems of variation may be studied from shells alone, by those whose -circumstances forbid them to study the living animal. Nor is there any -branch of zoology which is more useful to the teacher who wishes to -catch the eye and the attention of the beginner in the study of natural -history, especially if the beginner is young, as beginners ought to -be. Therefore we must by no means undervalue the past labours of -conchologists, or the valuable collections which their industry has -brought together and set in order for the benefit of the world. - -For example of the most crooked, or Azygobranchiate division of the -Streptoneura, turn now to Fig. 33, in which we see a typical Gasteropod -shell, _Murex ramosus_, the Branchy Murex, aptly enough named from the -many prickly branches which beset it. These rough points are probably -assumed for protective purposes; any animal that might wish to dine upon -the _Murex ramosus_ would think twice before trying to swallow it--the -morsel of shell-fish is so small, its shelly case so large and so -prickly. If we look for its nearest English relative, that is _Murex -erinaceus_, the Hedgehog Murex, or Sting-winkle. This, though a -comparatively plain shell, has still enough rough ridges upon it to have -secured it a comparison to the prickly hedgehog. Perhaps the most -prickly member of the genus, however, is _Murex tenuispina_, sometimes -called Venus' Comb, because the crowded parallel spines which decorate -the elongated front of the shell somewhat resemble the parallel teeth of -a comb. - -[Illustration: FIG. 33.--The Branchy Murex, _M. ramosus_, a typical -specimen of the shell of the Carnivorous Gasteropods. _Sp._, spire or -posterior end of the shell; _S_, siphon or anterior end of the shell. -Fig. A, shows the mouth of the shell; Fig. B, the exterior only. Less -than one-half the natural size.] - -How does the _Murex_ get its living? Let us notice the shape of the -shell, drawn out to a point, at the end opposite to the spire. According -to the older classification of the Mollusca, now somewhat fallen out of -use, this point marks the shell as belonging to one of the -Siphonostomata (shell-fish with a siphon at the mouth of the shell, -_i.e._). These shell-fish are, with few exceptions, carnivorous; not -that the siphon shape of the shell has any direct connection with the -animal's way of feeding. Just as the snail files among soft vegetable -substances, so the Murex and many of its relations file away much -harder things. A Sting-winkle, or a Dog-whelk, can sit down over a -helpless bivalve shell-fish, and patiently file away, until it has -worked a neat round hole in the protecting shell of the latter. You may -find, among the dead shells on any sandy part of the English coast, any -number of bivalve half-shells with a neat little round hole in them, -indicating unmistakeably how the tenant came to its death. There is some -controversy as to the spot chosen by the assailant for its attack. Some -authorities have stated that the predatory mollusc is so wise that it -knows where to find a weak spot, and makes a hole just over some vital -organ of the bivalve, or else above its adductor muscles, so that, when -these are cut, the half-shells cannot be drawn tightly together and kept -shut. Recently this has been denied, and statistics of the attacks of -_Purpura_, the common small whelk, a relation of the _Murex_, on -_Mytilus edulis_, the Common Mussel, have shown that the perforation -occurs in every part of the shell. It is possible, however, that the -Mussel, from the peculiar shape of its shell, offers an exceptional -case; and I am inclined to think that in the case of bivalves of a more -flattened shape, the earlier statement holds true. At South Shields, -England, perforated half-shells of the Common _Venus_ (Fig. 34) are so -abundant that the children string them for necklaces; yet I have never -been able, by the most industrious search, to find more than one or two -specimens in which the hole is at all near the lip of the shell. It is -possible that these exceptional instances were the work of a young and -inexperienced univalve mollusc, or a stupid one. It is possible, also, -that the mode of attack differs somewhat according to the species of -the assailant. (It should perhaps be explained, for the benefit of those -who have no experience in the ways of children or of shell necklaces, -that the hole must be moderately near the beak of the shell, to enable -the shell to "sit" properly on a string. Every unit in the necklace may -therefore be counted as one in favour of the older theory.) Many of the -Siphonostomatous molluscs are surprisingly active and strong, so that -they are well fitted for a predatory existence. In fact, they not only -eat bivalves, but occasionally attack the vegetable-feeding univalves -when nothing better is to be got, so that occasionally the shells of -these also may be found displaying the deadly little round hole we have -described. - -[Illustration: FIG. 34.--Half Shells of the Common Venus, several of -them perforated by carnivorous molluscs. From South Shields, England.] - -Let us contrast with the _Murex_ one of the shells which are -"holostomatous," _i.e._ possessing an unindented shell-mouth--that is to -say, one without a "siphon." The common edible periwinkle, _Littorina -littorea_, may be taken as an example. No shell is more familiar; even -the town-dweller, who has never found it on the sea-shore, has seen it -often on stalls in the slums. The mouth of the shell is quite round and -unindented, and in this case the character holds good as the mark of a -vegetable-feeder--a non-predatory sea-snail. It is hardly necessary to -remind the reader that its name (the shore-shell) is given it because it -lives where the tide leaves the rocks exposed during part of the day. -Another common species of _Littorina_, which frequently lives a little -lower down, where the large sea-weeds grow, has been described in -Chapter II.; and another, _L. rudis_, lives a little higher up, so that -it spends most of its time in a dry state, and is fast on its way to -become a land-shell. At most of the familiar English seaside resorts one -may see dozens of it baking in a hot July sun on rocks where only the -highest tides can reach them: and yet under these conditions they -continue to live and flourish. The periwinkles are remarkable for the -great length of the tooth-ribbon, in comparison with the size of the -animal. The number of separate teeth upon it has been estimated at 3500. - -A familiar feature of the common periwinkle is the lid or stopper -(Operculum), with which the animal can close the mouth of the shell. -This is developed and carried by the outside of the animal's foot. In -the periwinkle and other English molluscs it is comparatively soft and -semi-transparent, and reminds one of a thin slice of horn. In many -tropical molluscs, however, it is hard and shelly. The large tropical -shells named _Turbo_ have massive lids of considerable weight. These -shells, which are nearly allied to the pearly Top-shells (_Trochus_) of -the English shores, are sold as ornaments, the outer coat of the shell -being partly scraped off to show the inner coat of pearl: it is rarely, -however, that the purchaser obtains a lid, or even knows that the -creature had one. The reverse is the case with some of the smaller -kinds, the lids of which, being brightly coloured, are imported without -the shell, and sometimes set as articles of jewellery. Some of these are -of a bright green hue. - -While the lids of the Holostomata are rounded in shape, those that -belong to the Siphonostomatous shells are necessarily more or less -modified so as to fit the mouth of the shell, and are consequently oval -or even claw-like in shape. The Sting-winkle already spoken of, the -common small whelk, _Purpura lapillus_, and the large whelk, _Buccinum -undatum_, are common shell-fish in which the elongated lid may be -studied. The lid is not, however, like the tongue-ribbon, an essential -feature of the structure of every univalve mollusc.[D] Not only are -there special instances in which it is greatly modified, but also -there are whole groups of univalve molluscs in which it is absent. - -[D] There are one or two exceptional cases of gasteropod molluscs that -have no tongue-ribbon. The majority of these are parasitic forms, which -can get their food without the trouble of filing it down. - -A curious suggestion has been made with regard to the lids of univalve -shell-fish; namely, that the snapping to of the lid is capable of -producing a sound, which may perhaps be audible at a distance under the -water. Various molluscs have been credited with producing sounds, either -by muscular movements or by the grating of the shell as the animal -walks. The common Tortoise-shell Snail, _Helix aspersa_, sometimes -makes a most alarming noise when crawling over a window. It has been -disputed whether the sounds thus made are produced by the grating of the -creature's tongue-ribbon on the glass, as it files off small particles -of algae and vegetable moulds, which are invisible to our eyes: or -whether they are sounds due to suction of the muscular surfaces, such as -may be produced by drawing a wet finger across glass. The noise, however -produced, is, as I can testify from experience, sufficiently loud and -weird to be very startling, if heard in the dead of night. - -Turn now to the Bivalves or Lamellibranchiate molluscs, which include -the familiar oyster, cockle, and mussel. These are also known as the -Pelecypoda, and as the Aglossa, or molluscs without a tongue-ribbon. The -name Lamellibranchiate refers to the shape of the gills--"plate-like," -or flat; the name Pelecypoda to the shape of the foot, "hatchet-foot." - -The animal usually chosen as a type of these, the fresh-water mussel, is -rather a dull sort of creature, so we have chosen a prettier and more -lively specimen as a representative of the class; namely, one of the -Scallops, _Pecten opercularis_, sometimes called the Quin, the shell of -which is shown in the frontispiece of the book. This is one of the most -beautiful, perhaps the most beautiful, of the English shells. The -generic name, _Pecten_, the Comb-shell, probably refers, not to the -shape of the gills, which is somewhat peculiar, but to the marking of -the shell, which presents raised ridges, side by side. Anyone familiar -with shells will see at once that this is an unusual pattern. There are -plenty of bivalve shells with concentric ridge markings, comparatively -few with radiating ridges. We shall see presently that there is a good -reason for this. The specific name "opercularis," lid-like, refers to -the neat round shape of the shell. Each half of the shell has a pair of -"ears," so-called. The person who first gave this name to these flaps of -shell, three of which are three-cornered and the fourth nondescript, -must have been familiar in his youth with books afflicted with the -"dog's-ear" disfigurement; for certainly there is no other kind of ear -which greatly resembles these. The notch beneath the irregularly shaped -ear is called the "byssal notch": many Pectens spin a byssus or thread, -like that spun by the common Sea-Mussel, and thus anchor themselves to -fixed objects for a time; this notch is the place where threads of this -kind leave the shell. - -The two valves of the shell differ in depth, one being flatter than the -other; and the "ears" of the two valves differ in shape. The inside of -the shell shows muscular impressions, but these cannot be seen in a -photograph. The picture, however, shows the strong hinge-ligament which -joins the halves of the shell, and the difference in depth and shape of -the two valves. The valve on which the animal usually lies is the -lighter in colour of the two, and has one ear much longer than the -other. - -The creature swims by means of the "mantle," or muscular margin of the -body. It contracts this suddenly, after first opening the shell and -taking in as much water as possible. Thus the water is squeezed out -again, and the effect of this is to propel the animal in an opposite -direction. - -Now we are in a position to understand a little more about the shape of -the shell. These curious "ears" possessed by the two valves, together -form a straight, strong edge, which cuts the water as the animal flies -along. It reminds us of a ship's prow, and not without reason, for the -use of each is the same. A boat's sharp prow, compared with the rounded -front of a "tub," makes all the difference in the possibilities of -straight steering, and favours the putting on of speed: the ears of the -shell are not less useful to our Scallop. The following account of the -swimming powers of this species of Scallop, quoted by Woodward, was -given by the Rev. D. Landsborough, who observed young specimens, about -the size of the small ones in our picture, swimming about in a pool of -sea-water, left by the ebbing tide. "Their motion was rapid and zigzag; -they seemed, by the sudden opening and shutting of their valves, to have -the power of darting like an arrow through the water. One jerk carried -them some yards, and then by another sudden jerk they were off in a -moment on a different tack." To the sharp prow, the _Pecten_ owes this -capability of arrow-like flight. Its eyes are situated on the fringe of -its mantle, and consequently near the wide end of the shell; its -peculiar mode of progression, therefore, enables it to back away -instantly from any enemy it sees. - -Something must be said regarding the interior of the shell. The majority -of bivalve shells have a complicated system of so-called "teeth," or -interlocking projections, at the hinges of the shell: these exhibit -great variety in different kinds of shell, and are therefore often a -ready means of distinguishing one shell from another. The Scallop, -however, is very deficient in this respect, as are also some of its near -relations, for instance the oyster and its family group. The Fresh-water -Mussel also gains its name, _Anodon_, or _Anodonta_, the Toothless One, -from the same circumstance. The name often puzzles the beginner, who -asks, bewildered, "But do Bivalves ever have any teeth?" True teeth, of -course, they have none--it is the shell-hinge that has teeth, not the -animal inside it. Not only have the bivalve shell-fish no teeth indeed, -or tongue-ribbon, but furthermore they have no head. For this reason the -group has not only received the name already mentioned, of Aglossa, the -Tongue-less Ones, but also that of Lipocephala, _i.e._ Molluscs in which -the head is not developed. The reason of its absence is not far to -seek--a head would be no use inside such a shell. The snail-shell, so -differently built, allows freedom for the head; the bivalve mollusc, -squeezed in between its valves, has room only for a mouth. - -We have referred above to the ridges on the outside of the shell. Now -that we have learnt that the _Pecten_ is a very active animal, and moves -in the manner described, we see that these ridges run parallel to the -direction in which it moves as it darts away ears foremost. Let us try -to realise what is the effect of this. - -Take a mat with parallel stripes and move it along the floor or table -in the direction of the stripes; then try moving it in an opposite -direction across the stripes. It is easy to perceive that in the former -case one's eye does not detect the movement nearly so soon as in the -latter case. To explain this would necessitate a lengthy digression on -the subject of optical illusions: that the fact is so everyone may -easily ascertain by experiment. The ridges, therefore, converging in the -direction towards which the shell is going, are a protective decoration, -enabling it to slip away more easily from under the eyes of its foes. -The reader will readily recall a parallel instance in the common Cockle. -This also is a very active creature; it takes leaps by means of a strong -muscular foot; and the ridges on the shell, like those of the Scallop, -converge towards the hinges, that is to say, in the direction in which -the shell moves. Another instance of a very active shell-fish with -similar markings is afforded by certain kinds of Lima, a near relative -of the Scallops. It may be added that all Scallops are not equally -active, nor all Limas; and various modifications of their form and -colour might be pointed out which lead us to suspect that in the less -active kinds the pattern of ridges is often somewhat obscured by means -of these differences. - -Now, take up a comb and draw it over your fingers, firstly _along_ the -teeth, and secondly _across_ them, and you will be able to estimate the -gain in speed and comfort to the comb-shell, Pecten, and to the common -Cockle, from having its ridges set in the direction in which it is -going. Were the ridges concentric, as is so often the case in bivalve -shell-fish of a more sluggish disposition, the friction caused by the -ridges would seriously delay the progress of the shell. - -Something must be added regarding the colouring of the shell, which is -vivid, corresponding with that of the animal within. It is capable of -great variety, though perhaps not so great as in some of the smaller -_Pectens_. The predominant shades are pink, crimson and yellow, either -separately or mixed; that is to say, some shells are pure pink, some -almost pure yellow, some almost pure crimson, while others present every -imaginable shade of pinkish yellow, reddish brown and brownish crimson. -Local variation of colour is so marked that we may suspect the -variations in tint to be in some degree protective. The shell also -varies considerably in size and strength according to the neighbourhood -in which it has grown. - -This scallop-shell is but one of many: a number of other species are -found on our own shores, and many others again in foreign seas. - -One shell of the English coast is very annoying to the juvenile -shell-collector who gathers specimens on the shore. This is _Pecten -pusio_, a very small and delicate kind, with a raised pattern of fine -markings upon the ridges, which are very narrow. A good specimen of the -deeper valve is common enough, but the shallow valve, if of any size, is -distorted into all manner of shapes, as if it had been squeezed and -crumpled. The disappointing character of these specimens, from an -æsthetic point of view, is explained when we learn that it not only lies -on its shallow valve, but becomes fixed in this position, instead of -hopping about freely like the _P. opercularis_. It therefore has -frequently to adapt its shape to the nature of the ground where it has -happened to fix itself. Thus arises the disfigurement of the shell. - -So far we have only considered two great groups of the Mollusca, two -which are represented by common shells, familiar to everybody. We must -not leave the subject of the Mollusca without referring to their most -aristocratic group, the Cephalopoda. These are represented in museums by -the shells of the Pearly Nautilus, and of its not very near relative, -the Paper Nautilus; and they are represented on English shores by the -cuttle-fishes. All these agree with the Gasteropoda in the possession of -a tongue-ribbon, and in classification are therefore treated with them -under the name Glossophora. - -With the Pteropods, transparent forms found swimming over the surface -of the deep sea, the reader is not likely to have much to do. In -classification they are now placed near the Sea-Slugs. - -The Placophora, or Polyplacophora, wholly different from our usual idea -of a shell-fish, may be named as creatures which the reader is quite -likely to meet with. Though not very common, they are widely distributed -over our coasts, and may be found near low-tide mark clinging to stones. -Imagine a wood-louse without any apparent head which has taken to -clinging to the rock like a limpet, so that it cannot be removed without -injury, and you have a rough idea of their general appearance. _Chiton_ -is the name of these animals, which have received the group name of -Polyplacophora, carriers of many plates, because their external covering -consists of an armour of successive shelly plates. These also belong to -the Glossophora or Tongue-ribbon Carriers, of which they present a -comparatively primitive form. - -Reference has already been made to the labours of the earthworm and of -the insects, and to their important effects upon the vegetable world. -Although the Mollusca include but one terrestrial group, the Snails, -they, too, have played an appreciable part in modifying plant life. If -we owe our flowers to the insects, we have probably to thank the snail -for our medicines. For the snail dislikes bitter-tasting leaves, and -lets them alone, thus exercising an artificial selection in favour of -the survival of medicinal plants. In the same way the snail has favoured -the survival of hairy and thorny plants, upon which it cannot easily -crawl. - -The larval forms of the Mollusca differ considerably from the adult. -That of _Anodon_, the fresh-water mussel, at first received, in -consequence, a different name, that of Glochidium, by which it is still -known, although it has now been long identified as a larval form. It is -exceptional in the fact that it is parasitic on fish. - -The usual Molluscan larva is a ciliated creature which has been compared -to a modified trochosphere. It is preceded by a gastrula stage, and it -develops later on into what is called a "Veliger," or "veil-carrying" -larva, so called because it has in front a broad two-lobed ciliated -expansion, the velum. This larva is adapted for swimming, which is -accomplished by means of the velum. In terrestrial molluscs, the -development is necessarily much more direct. It is worthy of note that -the periwinkle mentioned above, which lives high and dry (_L. rudis_) -has no larval form, while its relatives that live under water develop -in the usual way. - -The eggs of Mollusca are often enclosed in tough cases, calculated to -resist waves and weather. Some of these are shown in miniature, in the -group of eggs of various kinds, Fig. 35. - -[Illustration: FIG. 35.--Eggs (reduced to half the natural size). -_A_, Egg-Capsules of _Murex_. _B_, Frog's Eggs. _C_, Eggs of large -Land-snail. _D_, Eggs of Snail placed on a leaf. _E_, Cockchafer's Eggs. -_F_, Egg-case of Cockroach. _G_, Egg-cases of Locust. _H_, _I_, _J_, -Eggs of Gasteropod Molluscs. _H_, _Sycotypus_ (_Pyrula_). _J_, -_Fusus_.] - -TABLE SHOWING THE CLASSIFICATION OF THE MOLLUSCA - - { AGLOSSA: the LAMELLIBRANCHIATA, also called - { CONCHIFERA, and PELECYPODA. - =MOLLUSCA.= { - { - { GLOSSOPHORA { GASTEROPODA. - { CEPHALOPODA. - - - - -CHAPTER X - -THE BRACHIOPODA OR LAMP-SHELLS - - -These were at one time included under the Mollusca, on account of -their possession of a bivalve shell. This shell, however, is placed -practically back and front of the animal, not to the right and left of -it, as is the case with the shells of the bivalve Mollusca. - -The name, arm-footed, was given them in reference to a pair of special -structures called the arms, bearing a large number of tentacles; it is -now more frequently spoken of as the lophophore (see p. 122), and -regarded as comparable to the lophophore of the Polyzoa, spread out into -two portions. With the latter group the Brachiopods were formerly united -by Huxley, under the name of Molluscoidea. This name is now obsolete, -because it is understood that all these creatures are widely different -from Molluscs; but the theory of relationship of the Brachiopoda to the -Polyzoa, implied in it, still holds good. - -The chief importance of this group lies in its fossil forms, which are -exceedingly numerous, particularly in the Mountain Limestone of the -Carboniferous Period; it is crowded with their shells, especially a form -named, from its elongated shape, Productus. The shells of Brachiopods -are equal-sided; that is to say, the right and left valves match; but -they are inequivalve, the ventral valve being much the biggest. It often -contains a foramen, or hole, at the beak, for the passage of the -pedicle, or stalk, by which the animal is attached to the ground (_e.g._ -_Terebratula_, _Rhynchonella_). Sometimes, however, the pedicle passes -out between the valves (_Lingula_, _e.g._), in which case there is no -foramen; or it may be arranged in other ways. Sometimes the shell is -merely attached to the ground by its side, like an oyster. Some forms -are enormously widened in a lateral direction, _e.g._ _Spirifera_, and -the _Productus_ above named. _Lingula_, among others, is remarkable as -being a form that has survived from the earliest geological period to -the present day. - -TABLE SHOWING THE CLASSIFICATION OF THE BRACHIOPODA - - { TESTICARDINIS.--Shell Calcareous, with hinges. - { Skeleton present in the arms. - =BRACHIOPODA.= { - { ECARDINES.--Shell comparatively soft, composed - { of Chitin, only strengthened by deposits - { of lime, without hinges. No skeleton in - { the arms. - -The larva, in its best known forms, passes through the typical larval -stages of the animal kingdom. It is first a one-layered larva, then a -two-layered form, and then becomes a ciliated animal. In this three -regions may be distinguished, representing respectively the head, body, -and pedicle. - -The shells of the Brachiopoda, including the kinds above named, may be -seen by the reader in any geological museum. - - - - -CHAPTER XI - -THE POLYZOA; MOSS-CORALS AND SEA-MATS - - -We have already described the creatures which are popularly known -as Corallines. Modern zoologists have long separated off from the -Corallines of the older writers, a group of animals known as the -Sea-Mats, which also are colonies made up of unit individuals. The -common Sea-Mat, _Flustra foliacea_, may be picked up on almost any part -of the English coast, being often torn up "by the roots" and washed in -by the tide. When fresh it has a pleasant scent, which has been compared -to that of Lemon Verbena, and a pinkish colour, due to the presence of -the little inhabitants in their cells. When dry it has no odour, -the cells are empty, and the colour a pale drab like that of a dead -Coralline. Its texture is, however, much more crisp and brittle, and -less horny, than that of a dead Coralline: it grows in flat, forked -expansions, much resembling in outline the fronds of several common -seaweeds; and each side of these is covered with a diamond pattern of -little cells. This crowded arrangement of the cells, with a tendency to -assume a geometrical pattern, is the readiest feature by which the -beginner may distinguish a Sea-Mat from a Coralline. The latter arrange -their cells in a free-growing, tree-like or fernlike form, without any -crowding of the units into a geometrical pattern. The division of the -flat leaf-like colony by two, resulting in bifurcated branches, is -another obvious feature of the Sea-Mat. - -Covering--and to the botanist's eye disfiguring--the branches of many -sea-weeds, and growing upon oyster-shells, tangle-roots, and other fixed -objects, we may find many little incrustations which remind us of the -lichens of the land: the diamond pattern of little cells shows us, -however, that these things are relations of the Sea-Mats. The name of -Bryozoa, Moss-Corals, was formerly given to these growths. Many of -them bear long hair-like processes at regular intervals; these, which -are large enough to be plainly seen with the naked eye, afford a ready -means of recognising these creatures. - -TABLE SHOWING THE CLASSIFICATION OF THE POLYZOA - - { ECTOPROCTA, with excretory aperture outside - { the ring of tentacles, _e.g._, _Flustra_. - =POLYZOA.= { - { ENDOPROCTA with excretory aperture inside - { the ring of tentacles. - -The Polyzoa include freshwater as well as marine forms. They have a -free-swimming larva, which becomes fixed after a time, and gives rise -to the adult Colonial forms. The zooids of the latter have each an -independent head with a crown of tentacles, called the Lophophore -(Crest-carrier); but the fixed ends of their bodies communicate with one -another. The hard covering of the colony, which retains its form after -the animal is dead, is a kind of hardened skin: the apparent "cells" are -the openings through which the individual zooids protrude themselves. -Sometimes certain of the zooids undergo modification for special -purposes: in this way are formed the "avicularia," snapping appendages, -probably defensive in purpose, so called because they open and shut like -a bird's beak. There are two divisions of the Polyzoa, the Ectoprocta -and the Endoprocta. Among the latter there is found a form which is not -colonial. - -_Phoronis_, a curious worm-like animal, which has a larval form called -_Actinotrocha_ is sometimes placed in classification near the Polyzoa, -which it resembles in possessing a crown of tentacles (Lophophore). - - - - -CHAPTER XII - -THE ECHINODERMATA - - -Everybody knows the Star-fish and many people know the Sea-Urchin. -An "urchin" is not a name for a naughty little boy, but the French -(_oursin_) for a hedgehog. A Sea-Urchin is therefore a "Sea-Hedgehog," -a name very appropriate for a creature armed with prickles. The Greek -word _echinos_ also means a hedgehog, so that the long name given to the -group means simply hedgehog-skinned. The prickles attain their maximum -in the Sea-Urchin, but they are well represented in the Star-fish, while -in the Sea-cucumber the general tendency to "prickliness" is much -reduced, and represented only by "spicules" (needles) of shelly stuff -underneath the skin of the animal. - -The largest and the most beautiful of the Sea-Urchins of the English -coast is known as the Purple-tipped Sea-Urchin, on account of the -beautiful colour of the spines. It lives on rocky coasts, and during -very low tides may be seen at home, although it usually takes care not -to stray above the water-line. It is a shelly ball with a flat base; its -surface is covered with long spines. Its mouth, which is in the centre -of the base, shows five wicked-looking teeth peeping out. The shell is -pierced by what look like hundreds of minute pin-holes, arranged in a -complicated pattern; these are the holes through which it pokes its -feet, which greatly resemble those of a Star-fish, being white suckers -with a disc at the end. When thrown out to their full length they are, -however, much longer than those of the Starfish, for they are naturally -obliged to be thrown out to a distance longer than the length of the -animal's own prickles. When moored by all its feet, extended from all -sides of the shelly ball, the animal presents a curious and pretty -sight. Large specimens are almost as big as a child's head, but smaller -ones are more common. There is a considerable range of variation -in colour; not only are various shades of purple found, but also -purplish-red and red. The spines are mounted on something resembling a -ball and socket joint, with a ring-shaped pad, so that they have a wide -range of movement; if any of the spines are touched they are immediately -set back over a considerable part of the neighbouring surface. - -Other kinds may be found upon a more sandy shore. These are heart-shaped -and much lighter in colour. The shell is thinner and of less weight. -These adaptations for lessening the animal's weight enable it to move -over sand: the species above described has no occasion for such -precautions. When it crawls over rocks and the strong seaweeds that grow -on them, there is no fear of its sinking in. The sand-dweller, on the -contrary, must take care that it is not swallowed up. - -[Illustration: FIG. 36.--The Five-holed Sand Cake, _Mellita pentapora_, -a flat sea-urchin from the east coast of tropical North America. _A_, -upper surface; =B=, lower surface; _C_, side view.] - -There are Sea-Urchins that carry their precautions against sinking to an -extreme degree. These are the Shield-Urchins or Clypeastridæ, so-called -from their flat shape; they include the American forms popularly known -as "sand-cakes." The diagram (Fig. 36) shows one of the most curious of -these flattened forms adapted for moving over fine sand and ooze, and -literally "as flat as a pancake." The mouth is approximately in the -centre of the lower surface, _B_; the upper surface, _A_, shows a -rosette pattern on the top of the shell. This is formed by the rows of -holes for the very minute tube feet. In the English Sea-Urchin above -described, which is one of the group called (for that reason) Regulares, -the rows of holes are uniformly continued all along the rounded sides of -the body down to the neighbourhood of the mouth. Here they are much -restricted, forming merely a rosette at the top of the shell: hence -they are described as circumscript or "petaloid." The excretory aperture -is shown in the photograph as a smaller dot on one side of the mouth, -while in the Echinus, on the contrary, it is at the top of the shell. -The five odd-looking, elongated holes are a curious individual -peculiarity of this Sea-Urchin. It has already been explained that the -Shield-Urchins are flattened in order to distribute their weight; these -holes are a contrivance for still further reducing the weight in -comparison with the area. This is when the animal is lying quiet at the -bottom of the water, but when it moves about what effect will the -presence of the holes produce? Flattened animals are usually supposed -to derive an advantage from the fact that they sink more slowly -through depths of water; as in lying upon the ground, their weight is -distributed, and they float, as it were, in the same stratum of water -without sinking further down. This creature, on the contrary, has -apparently feared lest it should move too slowly when it moves in a -vertical direction, and it presents us with an arrangement by means of -which its sinking through water is facilitated. Water will pass readily -through the five holes as the animal goes either up or down, and the -resistance of the whole flat area to the water is thus reduced and -vertical movement rendered more easy. Thus, by one and the same -contrivance, the animal has lessened its weight when lying quiet, and -diminished the resistance it meets with when it moves. The distribution -of the holes, moreover, is such as to regulate the animal's position in -sinking, and to prevent it from falling "headlong." For although the -creature has, strictly speaking, no "head," yet the end nearest the -mouth is the thickest and heaviest part of the "cake," and would -naturally tend downwards. This tendency is counteracted by the fact that -the thicker end is unperforated, while the thinner and lighter end has a -large central hole to diminish its resistance and enable it to sink more -rapidly. - -Adapted for living in sand rather than on rocks, but not so extreme -in the peculiarity of their form as the Shield-Urchins, are the -Heart-Urchins, already referred to, shaggy-looking creatures whose fine -yellowish-white spines give them almost the appearance of being clothed -with fur. The excretory aperture is at the narrow end of the "heart," -and the mouth at one side of the lower surface towards the wide end. The -complicated apparatus of teeth found in other Sea-Urchins is absent in -these. They are abundant on sandy shores. During the severe winter of -1894-5, when the Mersey at Liverpool was frozen nearly for one memorable -day, and filled with floating ice for many more, I saw the shore beyond -New Brighton heaped all along with a bank, often two feet across, of the -common Heart-Urchin. These, which afforded a fine feast for the hungry -sea-gulls, had been killed by the intense cold, and afterwards washed -ashore by the tide. The vast numbers of this creature which exist on -that coast were thus unexpectedly brought to light. - -These animals are sometimes described as "burrowing" creatures, because -they live covered in sand. The term is rather misleading. Far from -wishing to burrow, they spend their lives in a constant struggle with -sand that closes over them only too readily; and their whole structure -is adapted to prevent their sinking in a quick-sand. - -We began our chapter with the Sea-Urchins, because they are the most -important members of the group to which they give their name; but there -are forms belonging to the Echinodermata that are more familiar to the -ordinary observer--the Starfishes. Those who take an interest in the -cultivation of the oyster find them far too familiar--for the starfish -is the oyster's deadliest foe, not even excepting man. - -The common Starfish, _Asterias rubens_, may constantly be found among -stones, about low-tide mark. Its manner of walking is peculiar and -characteristic. On the under surface of each ray are rows of white -sucker-like tube-feet, which can either be drawn in or pushed out. By -doing each alternately the animal walks. First the feet are extended to -their full length; then the terminal sucking disc of each catches hold -of the ground. Then the feet are again retracted, while their discs -still cling; the effect of this is, naturally, to pull the ray onwards. -This process is repeated again and again, until some appreciable degree -of movement is effected. The tube-feet are in connection with a system -of vessels filled with fluid, known as the Water-vascular System of the -Starfish. The fluid is driven on by muscular contractions until the feet -are fully extended, and again driven back when the feet are retracted. -The Water-vascular System is a structure common to all Echinoderms; and -vessels of a comparable character are found in some worms. - -How does the Starfish know where it is going? Underneath each ray, near -the tip, is a little feeler (or tentacle) and a little eye spot. By -means of these it gets an idea where each ray is going to; and, since it -often moves but one ray at a time, this is sufficient for it. When -necessary, however, the several rays can act in concert with one -another. - -The rayed form of the Starfishes led to their being at first included -in the group of Radiate Animals, along with the tentacle-bearing -Coelenterata; but it has long been recognised that they are animals of -much higher structure. Their very larvæ can barely be brought into -comparison with animals so simple as the true "radiates." - -The Snake-Stars, or Ophiuroidea, are closely allied to the Starfishes. -In these the arms are thin and sharply defined from the little central -disc, instead of sloping gently out of it, as in the Starfishes. The -rapid wriggling movements of the arms have gained for them their very -appropriate name. They are also called Brittle Stars, because the arms -break off easily, sometimes at the will of the animal. Several kinds of -them are common on our shores, although they are not so common as the -ordinary Starfishes. Fig. 37 shows the general form of a Brittle Star. - -[Illustration: FIG. 37.--A Brittle-Star, _Ophiopteris antipodum_.] - -[Illustration: FIG. 38. A Sea-Cucumber, _Cucumaria Planci_, from Naples, -natural size.] - -The Sea-Cucumbers, Holothuroidea, are another group of Echinodermata -that are represented on our own coasts; by small specimens, however, -while the Pacific Ocean furnishes instances of larger size--the -Trepangs--which are used by the Chinese as articles of food. The name -Sea-Cucumber is given in fanciful comparison to a small Gherkin; -presumably one that has been very badly pickled--for the colour of the -animal is brownish and by no means green. The mouth of a Sea-Cucumber is -surrounded by a circlet of tentacles (partially indicated in the -diagram, Fig. 38). The body is elongated and crawls along: the "star" -shape, so characteristic of the Echinoderms, is scarcely to be -recognised except in cross section, where the longitudinal rows -of tube-feet are seen to outline a pentagon. The skeleton of the -Sea-Cucumber is of a very meagre description. Instead of forming a -rounded case, as in the Sea-Urchin, it consists only of loose pieces -of very small size, situated below the skin. The Starfishes are -intermediate in this respect. Their "skeleton" consists of a vast number -of pieces or "ossicles," which are of fair size, but are not closely -united, as in the Sea-Urchin. They are, however, so numerous and so well -knit, that the skeleton of a dead Starfish presents the complete outward -form of the animal. It must be noted that the ordinary skeleton of the -Sea-Urchin is only _apparently_ exterior. As is the case with the -ossicles of the Starfish and Sea-Cucumber, the skin lies outside, and -the hard particles belong to the middle layer, or mesoderm. In this the -skeleton of Echinoderms differs from the "shell" of a crab or lobster, -which is formed by a hardening of the skin itself. - -[Illustration: FIG. 39.--_A_, Head of a Stone Lily or Encrinite, -_Encrinus liliformis_, a fossil from the Muschelkalk of Brunswick, -natural size. _B_, Rock with stalks of encrinites. _C_, Section of a -stalk.] - -The Crinoidea, Encrinites or Stone-Lilies, form another group of the -Echinodermata. Though still represented by living forms, they attained -their maximum development in past ages. The English "Mountain Limestone" -of the Carboniferous period is full of their fossilized remains, which -form a marble often used for ornamental purposes. The so-called "Stone -Lily" consists of a "head" comparable with the body of a Star-fish or -other Echinoderm, which is borne at the end of a long fixed stalk. The -marble above named owes its ornamental appearance to the presence of -these stalks, often very long, and cut through at every possible angle. -The Crinoids have their living representative in English Seas, -_Antedon_, the Feather-Star (Fig. 40). On the side opposite the mouth, -where, in the Encrinite, the stalk would be, there are a group of -elongated processes called cirrhi, by means of which the animal can -attach itself to stones or seaweeds. When not thus fixed, it swims -about, by moving its fringed arms, each of which is forked. It will be -seen that when the animal is fixed by its cirrhi, it stands mouth -upwards, so that its position compared with that of the Starfish or -Sea-Urchin is upside down. The young of the Feather-Stars have stalks by -which they are fixed, like the Encrinites; but afterwards the stalk is -lost. - -[Illustration: FIG. 40.--A Feather-Star, _Antedon bifida_, British Seas, -three-quarters of the natural size. The short threads in the middle are -the cirrhi.] - -Among fossil Echinoderms there are two groups of stalked forms which -have no living representatives. These are the Cystoidea and the -Blastoidea. In both of these the stalk bears, as in Encrinites, a calyx -or head, which is comparable, with the body of the free Echinoderms. - -The Sea-Urchins possess a swimming larval stage, which goes through -remarkable changes after passing out of the two-layered (Gastrula) -form. It becomes provided with cilia, which are arranged in bands, -and outgrowths of peculiar form are established in the case of the -Sea-Urchins, while the larvæ of the other groups also present -characteristic shapes. Within the larva the adult form develops, the -outside of the larva being finally thrown off. - -In the young Feather-Star, a subsequent stage of the young animal has a -stalk, by which, like the Encrinite, it is fixed. This animal therefore -is at first free-swimming, afterwards fixed, and again free in its final -stage--a remarkable series of changes. - -These queer-shaped things, the Sea-Urchins and their allies, are perhaps -the last creatures amongst which we should think of looking for -relations of the Worms. Yet the earliest stages of the larva are -considered to present a certain amount of resemblance to the Wheel-ball -larva, which has been referred to elsewhere (pp. 42 and 72). Still more -startling fact, these larvæ have been compared to that of -_Balanoglossus_, the lowest member of the Chordata, and a relation of -the Vertebrates themselves (see p. 143). - -TABLE SHOWING THE CLASSIFICATION OF THE ECHINODERMATA - - { ECHINOIDEA, OR SEA-URCHINS. - { ASTEROIDEA, OR STAR-FISHES. - =ECHINODERMATA.= { OPHIUROIDEA, OR BRITTLE-STARS. - { CRINOIDEA, OR FEATHER-STARS AND - { STONE-LILIES. - { HOLOTHUROIDEA, OR SEA-CUCUMBERS. - - - - -CHAPTER XIII - -THE CHORDATA - - -The older zoologists used to speak of Vertebrata and Invertebrata as -animals with a back-bone and animals without one, and everyone thought -it a very natural way of dividing up the animal kingdom. It never -occurred to anyone that it was possible to bridge the interval between -them and find a link between the two. But now the Vertebrata have been -compelled to give up their aristocratic pretensions, and own that they -have risen from the ranks of the common people of the animal world; in -other words, that they are descended from the Invertebrates. Their -family secrets have been published to the world, and now everybody knows -that they have poor relations. But how many, and how nearly related? -This we do not accurately know; consequently the whole zoological world -for many years has concentrated all its energies on attempts to find out -the truth about the matter. - -A great sensation was caused by the first discovery of a poor relation -of the vertebrates among the Ascidians, or Leather-bottle animals. These -are named from their shape and texture, for they have a leathery skin. -Now some of these Ascidians have larvæ with a tail; and in the tail -there is a long cord-like structure, which in many essential particulars -resembles the cord which precedes the back-bone in the vertebrate -embryo. This structure is called the Notochord (a string down the back). -The credit of this great discovery belongs to Russia; for the presence -of the Notochord in the Ascidian larva was discovered by A. Kowalevsky, -in 1866. - -To the present generation of zoological students, the Chordate -affinities of Ascidians are part of the ABC of knowledge; and it is -hardly possible for them to realise that it is only thirty years ago -since the idea was so new that Huxley, in his "Text-book of the -Vertebrata," only alluded to it in a footnote. Would-be zoological -critics, at a somewhat later period, met the theory with ridicule, for -want of better argument. For critics include not only "those who have -failed in literature and art," but also those who have failed in -science. - -The majority of the Ascidians are sessile animals, which fix themselves, -like Sea-Anemones, to some object when they have passed their earliest -stages of growth; and although there are many forms that swim freely, -most authorities are inclined to believe that these have arisen by -adaptation, and that the kinds that are fixed when adult are the -original type of the group. - -Anything more unlike what we should expect to find as a relative of the -vertebrates could not possibly be imagined. What has been written about -these little animals by various observers would make a whole series of -volumes of the size of this one, so many are the puzzles afforded by -their internal structure. The arrangement of their organs is in many -respects very unsymmetrical. Their most striking peculiarity, perhaps, -is the nature of the gills. These form a kind of basket-work, consisting -of minute holes with intermediate supports; and they are associated with -a special cavity outside them called the Atrial chamber. Into this the -gills pass the sea-water which they have breathed. - -The group, as a whole, is sometimes considered to present evidence of -having degenerated from a higher type; but whatever else may be doubtful -or obscure in its history, the nature of the larval notochord is quite -clear and certain; zoologists have never had any doubt about its nature -since the first few years after its discovery. - -Ascidians are not at all uncommon animals on the English coast. Some of -them may be met with on stones near low-water mark, and I have often -seen them on the shells of oysters sold in the shops--for there the -town-dwelling naturalist may often find a good many interesting things -without much trouble. They are like little lumps of tough jelly; of -various colours, according to the kind, red being the most common, and -of very indefinite shape. You may see some of the colonial kinds forming -pretty star-shaped patterns, attached to various objects, such as stones -and the larger seaweeds. - -The place of Ascidians in classification was a puzzle, until their -relationship with Vertebrates was discovered. At one time they were -placed with the Mollusca. Now they are grouped, together with the -Vertebrata and some other creatures that remain to be spoken of, under -the name of Chordata, or animals possessing a Notochord. - -Some of the Ascidians present what has been already described in other -types (p. 57) as "alternation of generations." The discovery of this -fact was made by the poet Adelbert von Chamisso. Some of his verses are -known to English readers, for whom they were translated by Mary Howitt, -a poetess whose writings were popular with our grandmothers, and -deserved to be so. This is not the only case in which a poet has been -also a zoologist: Goethe studied the science, and framed a theory -regarding the vertebrate skull, which he regarded as consisting of a -series of vertebræ. In this he was less fortunate than the Italian -poet; for while Chamisso's observations were correct, and were confirmed -by subsequent writers, Goethe's theory of the skull is anything but -correct. It was made worse, too, by the speculations of subsequent -writers, who attempted to follow it into detail, with the result of -demonstrating its absurdity. - - - - -CHAPTER XIV - -THE VERTEBRATA - - -We have spoken of the Notochord as a structure which precedes the -formation of the spinal column in Vertebrates. This needs a little more -definite explanation. We all know that the spinal column of vertebrates -is formed to protect the spinal cord. This protection is, however, an -afterthought, so to speak, of the vertebrate structure; the lowest of -all vertebrates is quite without it; and in the lower groups of fishes -we may trace various steps of its formation. But in these cases where -the spinal column is absent or incomplete, there is a large and -well-developed notochord; and in the embryo of higher vertebrates, when -the spinal column has not yet begun to be formed, the notochord is -equally a conspicuous feature. It runs from the region known as the -mid-brain, to the end of the tail, and lies throughout just beneath the -spinal cord. Whatever its original use in the animal body may have -been, it undoubtedly acts now as a support to the spinal cord, and -indeed to the whole body. Bones, we must explain, do not exist either in -the lower vertebrate, or in the early embryo. In the latter they are -formed by degrees. The spinal cord and the notochord each begin to be -surrounded by rings of cartilage or gristle, which by degrees is changed -into bone. The rings surrounding the notochord, however, gradually -encroach upon it and obliterate it. The place where it has been becomes -the Centrum, or most solid part of each vertebra. The notochord at first -is continuous, and has no division into successive parts; but when the -bony spinal column is developed, it consists of a series of successive -vertebræ. Each of them is made up of several parts, which by degrees -become consolidated into the vertebræ. - -[Illustration: FIG. 41.--_A_, The Notochord of Vertebrates. Section, -considerably magnified, through the middle of an embryo one inch long, -of Acanthias, one of the Spiny Dog-fishes allied to the sharks. 1, -Section through Spinal Cord; 2, Section through Notochord; below it lies -a bean-shaped space, which is a section through a large blood-vessel; -_sk_, epiblast or skin; _me_, mesoblast or middle layer of the body; the -dots represent the nuclei of its transparent cells. The intestine, _i_, -lined with hypoblast, is traversed by a spiral valve, and surrounded by -the horse-shoe shaped body-cavity. _B_, Diagram indicating the position -of the Notochord in the vertebra of an adult Common Dog-fish (_Scyllium -canicula_). 1, "Neural arch" of the vertebra, consisting of processes of -bone enclosing the central nervous system, or spinal cord; 2, bony -centrum of the vertebra, hollowed out into a cup, in which lies a soft -pad, the remains of the notochord.] - -The lowest member of the vertebrate group, separated in fact from the -true vertebrates and placed in a lower division all by itself, is the -little animal called the Lancelet or Amphioxus. It is often spoken of as -a "fish"; but it is only by a stretch of our courtesy that it can -receive that name, being an animal of a much lower form than the fishes. -It was discovered in 1834, in the Mediterranean, and described as a -fish; but it had previously been discovered in 1778, by a German -naturalist who described it as a slug. The latter was misled by its -external shape. He had not the advantage of the modern methods of -preparing animals for examination under the microscope; in these days, -Amphioxus is cut into successive slices along its whole length, and each -of these carefully magnified, so that no detail of structure is lost. -The Amphioxus burrows in the sea-sand; it lies buried in it, with its -mouth just uncovered. Its food consists of microscopic vegetable -organisms. Its distribution is very wide; it is found in both the -Atlantic and Pacific waters. It occurs most abundantly in the salt-water -lakes of Sicily, and in the Gulf of Naples. The specimen first seen, in -1778, came from the coast of Cornwall. There are eight species; the one -which is found in the English Channel is the _Amphioxus lanceolatum_, -also found in the Mediterranean and on the shores of North America. - -The classes of the Vertebrata are Fishes, Amphibia, Reptiles, Birds and -Mammals. We used to learn that of these, fishes had gills, and Amphibia -gills for a time; but, to be strictly accurate, we must say that fishes -have gills, and _all_ the rest of the Vertebrata have gills for a -time. There is no exception to this rule, not even among the highest -vertebrates of all. But in those vertebrates which stand higher in the -scale of life than Amphibia, viz., Reptiles, Birds, and Mammals, these -gills are never brought into use. They only exist in the early embryo, -and afterwards disappear, giving rise by their modification to other -structures. - -Strange to say, one of these structures is the ear. This takes its -origin from one of the gill-"clefts" or spaces. The Eustachian tube, -which communicates between the ear and the nose, is part of this cleft; -and the little bones which are inside the ear represent the bones -of that gill-cleft. For, in fishes, bones support each gill, and -are connected together to form a complex arrangement. In the higher -vertebrates, which possess gills only in the embryo, this gill-skeleton -is much modified, and persists as a bone, the hyoid bone supporting the -tongue. - -The gills of vertebrates, arranged in successive pairs along the throat, -are "perforating gills"; that is to say, they consist essentially of -holes or spaces which pass right through the wall of the throat. - -If we were to seek for a general character of the vertebrates, besides -those mentioned above, that they all possess a notochord and gills, we -might also find it in the character of the skin. Fishes, Reptiles, Birds -and Mammals, all agree in this, that they have a special clothing of the -skin--scales, feathers and fur, respectively. These three kinds of -structure, although so widely differing in appearance, are practically -formed all in the same way, viz., by alternate ingrowths and outgrowths -of the skin; the ingrowth forming the root of the scale, hair or -feather, and the outgrowth its projecting part. If these infoldings and -outgrowths of the skin could be straightened out into a plane surface, -the skin of a small vertebrate would cover an enormous area. The above -list excludes the Amphibia: in this class, it should be mentioned, the -scales have been lost, and are only found in one group. - -The scales of Fishes were at one time proposed as a basis of -classification: large groups being characterized respectively by the -possession of plain rounded scales (cycloids), scales fringed at the -posterior end (ctenoid, or comb-like); placoid scales, consisting of -bony plates, and ganoid scales, large plates covered with shiny enamel. -These distinctions, however, were not found useful as a guide in -classification. The diagram shows the elaborate scales of the common -sole. - -[Illustration: FIG. 42.--Scales of the Common Sole, highly magnified.] - -Let us now consider some other creatures that resemble vertebrates in -some ways, and help to form the group of Chordata. Balanoglossus is one -of them, the Acorn-tongue Animal. This odd name is given to it on -account of a structure which is called (like the elephant's trunk) a -Proboscis; this may be compared with a tongue, so far as its use goes, -for it is thrust out to catch prey and again drawn in. It is oval in -shape, and therefore fancifully compared to an acorn. It is highly -sensitive, being richly supplied with nerves. The creature is to all -intents and purposes a kind of worm; and, like many of the higher -worms, it has a larva with bands of cilia. This larva, which is better -represented in some species than in others, was originally described -under the name of _Tornaeria_. It is considered to resemble, in some -degree, the larva of Echinoderms; on this hint, some zoologists have -sought to establish a connection between Vertebrates and Echinoderms, -and have been able to find other points of comparison besides the one -named. It remains to be seen whether this suggestion will lead to -further results. It may be added that the larva of Balanoglossus has -also been compared with that of Phoronis (p. 122), thus assuming a -relationship with the Polyzoa, and through them with the Brachiopoda. It -appears, therefore, that the subject of the possible relationships of -the Vertebrata is one of the greatest complexity. The last named theory, -however, has been adversely criticised by very high authority. - -We have not, however, explained yet what is the claim of Balanoglossus -to be grouped with the Chordata. This consists in the fact that a -certain part associated with the interior of the proboscis has been -identified, from its structure, mode of origin, and relations with -the nerves, as a notochord. Balanoglossus also agrees with the true -vertebrates in possessing successive pairs of perforating gills (see p. -142), which are especially noticeable in the young animal. The presence -of this feature is important, in view of the fact that some authorities -have sought to throw doubt on the genuineness of the notochord of -Balanoglossus. - -Balanoglossus is not without relations, some of which have been -recently discovered, while others have been known for some time, -although their affinities were not at first recognised. Among these -the most remarkable are sessile forms which have received the names -respectively of _Cephalodiscus_ and _Rhabdopleura_. Both produce buds -and form a colony, and in both a notochord has been distinguished. The -former was procured from the Straits of Magellan, while the latter makes -its dwelling-place in a nearer region, having been found off the -Shetland Islands, and off the Lofoden Islands. Cephalodiscus, which is a -very curious creature, receives its name from a disc placed at the head -end. The use of this structure is believed to be as follows. The units -of the colony live inside a common system of tubes, which they secrete; -each unit, when adult, is independent, and can move about inside the -tubes; the disc is used as a means of attachment to successive spots of -the tube-wall, as the animal wanders from place to place. Above the disc -are twelve plume-like tentacles covered with cilia, which create a -current in the water surrounding the head, and wash food particles into -the mouth. - -That these creatures are but distant relations of the true vertebrates -is a fact expressed by the names under which they are grouped in -classification. Those forms which we have just described have received -the name of Hemichordata--that is to say, Chordata which have but half a -notochord, since the notochord is very restricted in extent; while the -Ascidians are grouped under the name of Urochordata, or Chordata which -only possess a notochord in the tail. The name of Adelochorda, "with an -obscure chord," is sometimes applied to the Hemichordata. - -TABLE SHOWING THE CLASSIFICATION OF THE CHORDATA - - { HEMICHORDATA, BALANOGLOSSUS, &c. - =CHORDATA.= { UROCHORDATA, THE ASCIDIANS. - { VERTEBRATA, THE BACK-BONED ANIMALS. - -Let us return now to the Vertebrate. A character common to all the -groups of the Vertebrata is the possession of teeth. Readers of the -previous volumes of this series will recollect that, even among birds, -instances of the possession of teeth may be found among fossil forms, -although they are absent in the birds of the present day. In all the -other divisions of the Vertebrata, the presence of teeth is the rule, -their absence an exception so rare that we may easily note the chief -instances of it. Among Amphibia, there are Toads that have no teeth; -among Reptiles, the Tortoises and Turtles have none; among Mammals, -teeth are wanting in _Echidna_, the Spiny Ant-eater; and in the -Ant-eaters and the Whalebone Whales they are absent in the adult, -although present in early embryonic life. - -The majority of people, if asked to give a definition of the meaning of -teeth, would reply that they are hard structures that grow in the jaw. -But this is an idea that requires very considerable modification from a -scientific point of view. In the first place, they are found in other -places besides the jaws; and in the second place, they are by rights -structures originally belonging to the skin. Both these important facts -must be illustrated by reference to the Fishes, which exhibit the -primitive types of teeth. - -In fishes, not only are teeth found on the jawbone, but sometimes also -on other bones which border upon the cavity of the mouth; they are found -on the palatine bone, or roof-plate of the mouth, and, still more -strange, upon bones which belong to the "hyoid apparatus," or skeleton -of the gills (see above). The latter may form a set of throat-teeth, -which are used for grinders, while the jaw-teeth are used for biting. -Among the Carps, the jaw-teeth are reduced, and the fish depends upon -its throat-teeth only. In the Wrasses, one pair of the bones that bear -throat-teeth (the inferior pharyngeal bones) are fused, so as to form a -stronger apparatus: and from this circumstance, the group of Fishes to -which they belong has been given the name of Pharyngognathi, fishes -possessing throat-jaws. They have, however, biting teeth as well, in the -true jaws. The grinding teeth are apparently used for consuming the food -in a leisurely manner when once it has been taken into the mouth. - -A curious circumstance in connection with these "throat-jaws" is, that -they produce musical sounds. Fishes have other means, however, of -producing a voice--usually by means of the swimming-bladder and muscles -in connection with it. Probably they are able, to some extent, to effect -communication with each other in this way. - -It has already been stated that teeth, in their primitive form, are to -be regarded as skin-structures. Certain fish, which are looked upon as -ancestral types, have, dispersed throughout the skin, a number of bony -plates, or granules (placoid scales), more or less formidable, and -tipped with a hard enamel-like substance. Teeth are regarded as but a -special form of these. But if they are skin-structures, how come they in -the mouth and throat? Because the mouth and throat are lined by an -ingrowth from the external skin; the origin and growth of this is seen -in the embryo. - -In the Mammalia the teeth, though restricted in number, attain the -greatest possible variety of form, so that the jaws of different but -allied species may be distinguished by their teeth. - -Let us now return to the lowest vertebrate of all, which has a large -notochord and no bones. This is the _Amphioxus_, the Lancelet. Amphioxus -has no bones whatever, and no head, in the sense in which we usually -employ that term; that is to say, most of the structures which we see in -the vertebrate head are undeveloped. The peculiarities of the structure -of Amphioxus are many. Among them may be named the curious gills: these -form a sort of basket-work along the sides of the throat, which at first -sight bears little resemblance to the gills of fishes, and reminds us of -those of Ascidians. The gills lead also, as in Ascidians, to another -cavity, the Atrial chamber. This basket-work is formed, however, by the -subdivision of the primary pairs of gills. These are very numerous, -ninety pairs being sometimes named as the number. They cut up the wall -of the throat to such an extent, that additional supporting bars are -needed to strengthen it; and, by the formation of these, both in -parallel and in transverse directions to the primary partitions, the -"basket-work" is produced, as the growth of the animal proceeds. - -The primitive nature of the notochord is, however, perhaps the most -striking feature of Amphioxus. The chord passes to the front of the -animal's snout--head it can hardly be called--instead of ending in the -middle of the brain, as in true vertebrates, for there is, indeed, no -"brain" of any extent to lie in front of it; and the notochord, together -with the spinal cord itself, have no other protection than a fibrous -sheath. The spinal column is thus entirely absent, except so far as it -may be regarded as represented by this thin sheath. - -The Lancelet also differs from the true vertebrates, in that it has no -limbs. There is a fringing fin along the body, but it is not comparable -with the fins of fishes. It differs also in possessing no teeth. - -In one respect, however, the Lancelet reminds us of a fish: and that is -in the arrangement of its muscles; these form a successive series of -overlapping masses on each side of the body, as in a fish. - -The development of the Lancelet presents us with an instance of -the two-layered larva, or Gastrula. This shows that Amphioxus is a -comparatively primitive type. But it has been suspected that it is less -primitive than it looks, and that it has degenerated from some higher -form, owing to its preferring a dull mode of existence, half-buried in -sand or mud. - -There is a huge gap between the Lancelet and the true vertebrates. The -lowest form of the latter is _Ammocoetes_, the larva of the Lamprey -(_Petromyzon_). The latter, even in the adult form, has no true limbs, -though there are fringing fins. The notochord sheath is supplemented, -however, by cartilage bars which are equivalent to the beginnings of the -vertebræ of the back-bone. The gills are very different from those of -other true vertebrates, and it has no jaws. Teeth it has, however, on -the tongue and the lining of the mouth. Probably this creature is -greatly altered by adaptation to its peculiar mode of life, so that no -certain conclusions can be drawn from it regarding the structure of -primitive fishes. It has a sucking mouth, by means of which it hangs on -to fishes, while it rasps away their flesh with its rough tongue. When -not thus engaged, it hangs on to a stone by means of its suctional -mouth, thus fixing itself at rest. The Hag-fish, _Myxine_, in many -respects similar, devours dead fishes chiefly. The Hag-fish is found on -English coasts: so is the Marine Lamprey; while two freshwater forms are -found in streams. - -Leaving the Cyclostomata, as the above fishes are called, we reach the -true fishes, which have limbs and scales. Something has already been -said regarding their teeth and gills. The Cartilaginous fishes, in -which most part of the skeleton remains gristle and does not become -transformed into bone, include the Sharks, Rays, and Dog-fishes, all -savage animals with strong teeth. The common spotted Dog-fish of our own -shores is familiar to everybody: fishermen regard it with disgust, as it -is not eatable. The Rays are flattened fishes, which live at the bottom -of rather deep water, and attain enormous size even on our own coasts. -The Thornback Skate is covered with prickles (placoid scales). All these -fishes are grouped under the name of Elasmobranchii, the Strap-gilled, -so called from the structure of the gill-arches. - -The majority of familiar fishes, such as the herring, mackerel, cod and -sole, belong to the group of _Teleostei_, or Bony Fishes, in which, by -contradistinction from the last group, as much of the skeleton as -possible becomes bone. Nevertheless, traces of the notochord persist in -the back-bone of these fishes. Break the back-bone across, of a cod or a -sole, and you will find between adjacent sides of the centra, or middle -parts of the vertebræ, a pad of gristly substance. This is the remaining -substance of the notochord, which finds room between the cup-shaped -sides of the centra. When the centrum, instead of being biconcave, is -solid, as in the higher Vertebrata, the notochord is obliterated by its -encroachment. - -The Amphibia, familiarly represented by Frogs and Toads, receive their -name, "adapted for both lives," from the fact that they usually divide -their lives between land and water. They are, from one point of view, -the most interesting of the classes of the Vertebrata, for they form a -dividing line between the lower and upper Chordata. Below we have -Hemichordata, Ascidians, Amphioxus, Fishes; all water-dwellers, -breathing by gills. Above, we have Reptiles, Birds, Mammals, -air-breathers, never possessing gills, except for a short time, as -rudiments in the embryo, not brought into use. They are linked by the -Amphibia, in which we see the larva a water-dweller, breathing by gills; -the adult, an air-breather, adapted for life on land, and obliged to -come to the surface to breathe, even when it passes its time in the -water. The individual Amphibian tells us the past history of the higher -groups; once they had gills--but growing older, they lost them. - -Fig. 43 shows us an outline sketch of Amphibian larvæ; we should require -an enlarged diagram of an earlier stage, to show the gills, which are -external and projecting at first, but afterwards are overgrown by the -skin with the exception of an orifice on each side. The diagram shows -the gradual change of form. The tails in these tadpoles will presently -be lost, for they belong to the Anura, or tail-less order of Amphibia -(Frogs and Toads). The tailed Amphibians, Urodela, are represented in -Great Britain by the Newts, _Triton_, popularly called Efts. Belonging -to the Tailed Amphibians also, is the Axolotl, a creature found in the -lakes of Mexico, and in those of the Rocky Mountains. It may or may not -retain its gills; and forms with gills, and forms without, may be found -in the same lake, each capable of laying eggs. The two forms were at -first described under two different generic names: but when specimens of -the gill-bearing _Siredon_, kept in confinement, lost their gills, it -was seen that they became _Amblystoma_. There are other cases of larval -forms that produce young, and this curious occurrence is known as -"pædogenesis." - -[Illustration: FIG. 43.--Tadpoles, three-quarters of their natural size. -_A_ to _D_, different stages of the Tadpole of the Common Toad, from -Epping Forest, England. _E_, Tadpole of _Pelodytes punctatus_, dorsal -view.] - -The Amphibia include the curious creatures called Cæciliæ (blind -animals), or Gymnophiona. They are snake-like in form, and are without -limbs; they burrow underground. Their real place in classification was -not found out at first, but they were classed, by a wrong shot, with the -Reptiles. They are interesting as being the only Amphibians that have -scales. These are very minute, embedded in the skin, and arranged in -transverse rings. The name Gymnophiona, naked serpents, is therefore -doubly inapplicable: for they are not serpents, and not scaleless. - -The Reptiles and Birds at first sight seem to be widely different. The -latter are the warmest blooded of all vertebrates, the former are -coldblooded. The one wear feathers, the other scales. Nevertheless, -there is an intimate connection between them; the reader has doubtless -already learned from other sources the facts about their relationship, -so we will not here do more than recall a few of these facts. One is, -that the birds of earlier times had teeth in their beaks, and possessed -jointed tails. Another, that the Reptiles of earlier times included -forms that were able to fly. A third notable fact is the presence of -claws on the wings of some birds, showing that the wing of the bird was -not always wholly specialised for use in flight. - -We owe to Professor Huxley, the recognition of the close relationship of -Birds and Reptiles, and the name Sauropsida (Reptile-like animals), -under which both are included. They agree in being air-breathers and -never having gills, except the rudiments present in the early embryo: -this distinguishes them from Amphibia. They agree in having the skull -set on to the back-bone by a single articulating surface or condyle; and -thus differ alike from Amphibia and from Vertebrata. They agree in -having the red corpuscles of the blood nucleated; and in this differ -from the Mammalia, in which the red corpuscles are non-nucleated discs. -From a popular point of view, we may say that the striking distinction -between birds and reptiles lies in beauty and ugliness. Even in their -eggs, the reptiles display no love for adornment, no colouring or -pattern. Fig. 44 shows the eggs of some reptiles. - -[Illustration: FIG. 44.--Eggs of Reptiles, half the natural size. -_A_, of African Cobra. _B_, of Common English Snake. _C_, of Common -English Lizard, _Lacerta agilis_. _D_, of Elephantine Tortoise. _E_, -of Crocodile.] - -The five chief groups of existing reptiles are the Chelonia (Tortoises -and Turtles); the Rhyncocephala, represented only by _Hatteria_, a -lizard found in New Zealand; the Lacertilia or Lizards; the Ophidia, or -Snakes and Serpents; and the Crocodilia. - -Perhaps the most interesting point regarding the reptiles that can be -mentioned in brief space, is the fact that they present traces of a -median third eye, which have been described by Baldwin Spencer, in the -New Zealand Hatteria, and in other reptiles. It is situated on the roof -of the brain. While the structure in Hatteria shows it to be an eye, its -position corresponds with that of the pineal gland of vertebrates -generally; so that we find, in fact, the trace of a third eye in all -vertebrates, including ourselves. It is, however, a trace only. In the -Lamprey fishes as well as in _Hatteria_, it reaches a further degree of -development. This pineal eye has been compared in structure to the eye -of Ascidians. - -The Birds, excluding the extinct form with teeth and a jointed tail, -to which the group name of Archæornithes is given, fall into two -groups. These are the Ratitæ, or Birds with Raft-like, _i.e._ flat, -breast-bones, and the Carinatæ, or Birds with keeled breast-bones. The -former include the African Ostrich (_Struthio_), the American Ostrich -(_Rhea_), the Australian Emu, the Cassowary of New Guinea, and the Kiwi, -or Apteryx of New Zealand; all of them birds that cannot fly. The -vast majority of birds belong to the Carinatæ, characterised by the -projecting keel (Carina) in the middle of the breast-bone. The presence -of this, which affords a safe attachment for strong muscles, is -associated with the power of flight. It is impossible to treat the birds -more fully in the space allotted to this little story, but a few words -about feathers, however, may find a place here. - -The colour of feathers is a subject of much interest. Everyone is -familiar with the brilliant tints often presented by the feathers of -birds, and everyone who is a close observer of natural objects knows -that there are some feathers which are iridescent, changing colour -according to the direction in which light falls on them. It has been -shown by Dr. Gadow that this variation of the colour of a feather is due -to its structure; this may be described as prismatic, for the small -divisions of the feather present acute angular edges, which reflect the -light like the edges of a prism. These are symmetrically repeated all -along the feathers, so as to reflect the same colour throughout. Thus in -the plumage of the common red and green parrot, we see feathers that are -red when held in one position, and yellow when shifted to another -position; while there are also feathers that are blue when seen in one -position, and green when seen in another; the alternative colour being -the one next in order in the rainbow. - -Another point regarding the colours of feathers has no doubt puzzled -many of our readers; and that is, the metallic quality of the colouring -in some exceptional feathers, and in these only. The feathers of the -parrot just referred to, are, for instance, simply red and yellow, or -blue and green; but the feathers of the peacock, though displaying the -same colours, show a metallic lustre which is wanting in the other case. -The feathers of the starling, the blackbird, and the black hen of the -farmyard, though not so brilliant as those of the peacock, are the same -as regards the quality of the light they reflect. The secret of the -difference lies in the greater opacity of the feathers named; they are -_black_ feathers, while those of the parrot are light-coloured. Now -after the metals themselves, there are few objects in nature so opaque -as the black pigment of a black feather. If a thin section through -the roots of young black feathers is cut for examination under the -microscope, the pigmented parts, although cut so very thin, appear -completely opaque. And just as a glass gives a better reflection when -backed by something opaque, so does the reflecting surface of the -feather. Hence it is that the quality of the colours reflected by these -feathers is what we call "metallic." If we ask for a definition of this -metallic brightness, other than the accepted fact that it resembles the -light reflected from metals, the artist will reply that it consists in -two things--(1) the greater brilliancy of the light reflected, that is -to say the greater completeness of the reflection; and (2) the entire -absence of those gradations of light which are afforded by the -reflections from any object, however dark, that possesses a surface -translucent, even in the smallest degree. "Metallic" reflections, in -fact, may be defined as those in which the greatest amount of light is -reflected, and the reflected sunlight receives from the reflecting -surface the least possible degree of modification. While the actual tint -of the colour reflected by a black feather, then, is determined by the -form and position of its angular ridges, the quality of the reflection -is determined by the opacity of the substance itself. It is interesting -to note that the opacity necessary for reflecting a "metallic" lustre, -may be produced by means of pigment, in the vegetable as well as in the -animal organism; for instance, in the dark centres of _Coreopsis_ (the -Beetle Flower), and several other fashionable garden plants belonging to -the Compositæ or Daisy family. Within the animal kingdom, we may note -that the metallic lustre is almost entirely confined to land animals; -their dry skins have more chance to develop opaque parts, than the moist -tissues of creatures that live in the water. The most familiar exception -to this rule is the Sea-Mouse, an Annelid worm found on English coasts -(p. 73), which receives its odd name because it is a fat oval creature, -covered with bristles, thus greatly differing in appearance from most -worms. The larger bristles, which are of a dark purplish-black colour, -have a bronze or golden metallic lustre. Various other annelids exhibit -brilliant rainbow colours; for example, _Nereis_, the Rainbow Worm, also -found on English shores; but without the underlying black opaque -pigment, the reflections from the surface fall short of absolutely -metallic brightness. On land, we see among the insects innumerable forms -which present a metallic lustre, the beetles being the most notable in -this respect. To return to the vertebrates, from which we started, -everybody must have noticed that the fur of a clean well-kept black cat, -when lit up by the bright sunlight in which the animal loves to bask, -shows little rainbow reflections of red and green. These are due to the -presence of little grooves and irregularities on the surface of the -hairs, which play the same part in breaking up the light which they -reflect, as do the sharp angles of iridescent feathers. Like the -iridescence of the Rainbow Worm, they fall short of absolutely metallic -brightness; the fault in this case being due not to the nature of the -underlying stratum, so much as to the incomplete development of the -light-reflecting grooves. Yet this instance serves to show the part -taken by the dark pigment; for while the play of colours is perfectly -obvious in the fur of a black cat, it is almost impossible to -distinguish it in the case of cats with fur of lighter shades. - -The Mammalia, or animals that suckle their young and produce them by -birth, were formerly considered to be sharply defined from animals that -lay eggs, such as the birds and reptiles. But in 1884 Mr. Caldwell -confirmed the statement which had been made previously, yet hardly -credited by the scientific world, to the effect that the lowest form of -mammals lays eggs. This, the Duck-Mole or _Ornithorhyncus anatinus_ -(Bird-billed animal much like a goose), is a native of Australia and -Tasmania. It lives on the banks of rivers, and burrows in the bank. It -has webbed feet, and therefore sometimes receives the name of Platypus -(flat-foot). It lays eggs two at a time, in its burrow; and these eggs, -like those of other egg-laying vertebrates, have a yolk. - -A kindred form, _Echidna hystrix_ or Spiny Ant-eater, is found in -Australia, Tasmania, and New Guinea. The _Echidna_ hatches its young -in a temporary pocket, which appears in the neighbourhood of the -breasts, and disappears after the young are old enough to take care of -themselves. The _Ornithorhyncus_ has fur, the _Echidna_ has spines, with -hairs between them. Neither bears the slightest resemblance to a bird; -the comparison suggested in the name of _Ornithorhyncus_ is fanciful, -and depends chiefly on the flat beak-like mouth; these egg-laying -quadrupeds may, however, be reasonably brought into comparison with -Reptiles. Neither of them has any teeth; the _Echidna_ has no teeth at -all; the _Ornithorhyncus_ loses them at an early stage of growth, and -develops instead hard horny patches in each jaw. With these it crushes -its food, which consists of small insects, worms, etc. The _Echidna_, on -the contrary, lives in rocky places, and feeds on ants, which it -searches for with its long-pointed snout. These two genera are grouped -under the name of Prototheria or Primitive Mammals. - -The pocket in which _Echidna_ hatches its young, suggests a relationship -with the next group, the Metatheria or Marsupialia, which are the -characteristic mammals of Australasia. These are distinguished by the -possession of a permanent nursery-pocket, the "marsupium." In this they -put their young, which are born, like those of other mammals, not -hatched from eggs like those of the last group. They are, however, born -in a very backward condition, and therefore require to go through a -further period of incubation, so to speak, in the marsupium. Here each -one attaches itself to a teat, to which it remains fixed. But it cannot -suck as a new-born kitten or puppy does; and the milk is forced down its -throat by the muscles of the teat. - -[Illustration: FIG. 45.--Skull and Lower Jaw of Great Kangaroo, -_Macropus giganteus_, much reduced.] - -The Marsupialia are not entirely confined to Australasia; a few occur -in South America, and in North America they are represented by the -"'possum," _i.e._ Opossum, of American stories. The Marsupials seem -almost to mimic the forms of ordinary quadrupeds. Thus _Notoryctes_, a -form discovered a few years ago, mimics a mole. The fact is that, just -as among the Eutheria, or higher mammals, special types have become -established, possessed of certain habits, and especially of certain -habits with regard to food, and modified in accordance with those -habits. Thus there are among them savage carnivora, harmless herbivora, -and rodents; and these respectively share certain characteristics in -common with the carnivora, herbivora, and rodents, belonging to the -Eutheria. One of the herbivorous marsupials is the Great Kangaroo, -_Macropus_. It gets its name, Large-foot, from the size of its -hind-paws; on these it stands, and by their aid it takes remarkably long -leaps. Its skull is shown in Fig. 45; this, however, has not the full -set of teeth, some of which are soon shed. It crops the herbage with its -front teeth, and grinds it with its back teeth, like other herbivora. - -[Illustration: FIG. 46.--Skull and lower jaw of Rodent; _i_, _i_, -incisor teeth, separated by a long interval from the molars. About -one-half the natural size.] - -The study of the teeth is of great help in the classification of the -Mammalia. Of the eight orders of the Eutheria, two alone, the Sloth -order and the Whale order, show a tendency to the suppression of the -teeth. Those of the herbivora and carnivora may easily be compared by -anyone, in the sheep and the dog respectively. Fig. 46 shows the skull -of a Rodent, with elongated front teeth, adapted for that persistent -gnawing which makes the animals of the order, such as the Rat and -Rabbit, so terribly destructive. - -TABLE SHOWING THE CLASSIFICATION OF THE MAMMALIA - - - { 1. PROTOTHERIA, or EGG-LAYING MAMMALS. - { One order, the MONOTREMATA. - =MAMMALIA.= { - { 2. METATHERIA, or MARSUPIAL MAMMALS. - { - { 3. EUTHERIA, or HIGHER MAMMALS. - -The Mammalia are a terrestrial group. Exceptions are the Cetacea -(Whales), Sirenia (Dugongs), and Seals or Sea-Carnivora, but all of -these are air-breathers; even the Whale can only stay under water for a -limited period of time. Hence we see that none of them are really -animals belonging to the water; they are land animals adapted for life -in the water. - -This brings us very near to the last chapter in the Story of Animal -Life. We have seen that our story began with the One-celled Animals, and -went on with the tale of the Two-layered Animals, in which each layer -was built up by cells in partnership. From Two-layered Animals we passed -to Three-layered Animals, and from them to Three-layered Animals with a -"body-cavity." When we reached the latter, we found amongst them traces -of the ancestry of the vertebrates. From the lowest of the Vertebrata, -the Lancelet, we passed on to the Lamprey, and from that to the true -fishes. In the latter we found the parent type of all the other -Vertebrata, possessing gills in the adult, while the latter only possess -them, or traces of them, in early stages of growth. The Amphibia formed -a group to themselves, in which we traced the loss of gills in the -adult. In the Reptiles, four-legged egg-laying animals, we found not -only a close relationship with birds, but also, through the four-legged -egg-laying _Ornithorhyncus_, a relationship with the Mammalia. The last -group comprises all the furry animals, and culminates in the order -Primates, in which the great Cuvier included Man. - -TABLE SHOWING THE DISTRIBUTION OF ANIMAL LIFE BETWEEN LAND AND WATER - - KEY TO TABLE: - - A ARTHROPODA. - B VERTEBRATA. - C CHORDATA. - - LAND WATER - - Except a few forms living --All the PROTOZOA. - living in damp - places, or as - parasites - --All the SPONGES. - --All the COELENTERATA. - Except a few forms --VERMES. - terrestrial, and - many parasitic - - Insects, except --A very few adult forms and } - Except Wood-lice and a few larvæ. } - a very few others --CRUSTACEA. } A - Spider-like animals, } - except --_Limulus._ } - --All the BRACHIOPODA. - --All the POLYZOA. - --All the ECHINODERMATA. - Except the Land-snails --MOLLUSCA. - --HEMICHORDATA. } - --UROCHORDATA, or Ascidians. } - --All the Fishes: (some few can } - exist in damp places) } } - Amphibia belong to both. } } - All the Reptiles --Except swimming forms, } } - which are nevertheless } } - air-breathers, only } } - partially adapted for } } - water life: Tortoises } } - and Turtles, Crocodiles } } - and Water-Snakes, _e.g._ } } C - } } - All the Birds: swimming and } B } - diving forms are only adapted } } - for temporary visits to the } } - water } } - All the Mammals --Except Whales, Sirenia, } } - and Seals, which are } } - nevertheless } } - air-breathers, only } } - partially adapted } } - for water life. } } - -Another volume of this series, "The Story of the Earth," has already -dealt with the distribution of animal life in time; while "The Story of -Animal Life in the Sea" tells about the present inhabitants of the -ocean. It is therefore unnecessary to say much in this volume regarding -the distribution of animal life. A table is, however, appended, which is -not without interest. It shows how the chief great groups of animals are -divided between land life and water life, whether in fresh water or -salt. It will be seen that the terrestrial animals are much in a -minority, and that they belong, for the most part, to the higher types. -They are, in fact, stragglers, bold emigrants from the early home of -animal life, which lies in the more shallow parts of the waters of the -sea. - - - - -CHAPTER XV - -MAN - - -If we are to accept the opinion of Dr. Isaac Watts, man, as a moral -being, is distinctly inferior to the "birds in their little nests," who -live in harmony with one another; and, again, if we are to believe -Solomon, he is by no means always the equal in intelligence of the Ant. -Yet somehow it came as a shock to many who had been accustomed to revere -both these authors, when they were asked, early in the latter half of -the nineteenth century, to regard man, from a zoological point of view, -as just a little superior to the Apes. - -Then arose a great agitation as to the possibility of finding the -Missing Link. We shall see later on in this chapter, that if Research -had been content, like Charity, to begin at home, its industry would -have been duly rewarded. - -But inquiry, carried far afield in time and place, has not been without -result. For it is generally believed that the remains found in 1894 in -Java by Dr. Eugène Dubois, are veritably those of the Missing Link. -These remains, which consist of the top of a skull, two teeth and a -thigh bone, belong either to the oldest Pleistocene age, or to the upper -Pliocene; they are found in association with the remains of other -animals, among which are included some forms now extinct, or absent from -that region. These ape-like remains have been carefully compared with -those of the lowest races of man which have hitherto been found in a -fossil state, and the result of the comparison is as follows: Of twelve -experts present at the Zoological Congress held at Leyden, "three held -that the fossil remains belonged to a low race of man, three declared -them to be those of a man-like ape of great size; the rest maintained -that they belonged to an intermediate form, which directly connected -primitive man with the anthropoid apes" (Haeckel). To the creature -represented by these bones has been assigned the name of -_Pithecanthropus erectus_, the Upright Ape-Man. - -Let us now return from the subject of the Java fossil to those inquiries -which, as we have above suggested, begin at home. We have already -referred to the great principle of modern zoology, that the history of -the development of the individual sums up the history of the development -of the race. Of late years it has occurred to scientific men to apply -this principle in the case of human beings, and to ask, "What can the -baby teach us?" - -The Baby, for one thing, has a very small nose, insignificant compared -with the size of its jaw. At least scientists find that this is the case -with their babies--if would, of course, be invidious to make such a -remark regarding their friends' children; and still more so to add, that -in this the Baby differs from the human adult, and somewhat resembles -the Ape, in which the nose is still less prominent, and the jaw still -more so. Observations have been made, too, regarding the Baby's -remarkable power of "holding on" with its hands. While a Baby is, in -most respects, a very weak creature, yet its powers of grip have been -favourably compared with those of adult human beings. No one who has -ever tried to rescue his watch or his hair from the clutches of a -friend's Baby, will feel inclined to doubt the conclusions of scientific -observers regarding the point in question. - -The observations above referred to were made by Dr. Louis Robinson. He -drew his conclusions from the study of sixty cases, all of them infants -less than a month old; and of these at least half were tested within an -hour of their birth. - -In every instance except two, says Dr. Robinson, the child was able to -hang on by its hands to the finger, or to a small stick three quarters -of an inch in diameter, and to sustain the whole weight of its body for -at least ten seconds. "In twelve cases, in infants under an hour old, -half a minute passed before the grasp relaxed, and in three or four -cases nearly a minute." In infants of about four days old, increased -strength was shown, and "nearly all, when tried at this age, could -sustain their weight for half a minute. At about a fortnight or three -weeks after birth the faculty appeared to have attained its maximum, -for several at this period succeeded in hanging for over a minute and a -half, two for over two minutes, and one infant of three weeks old for -_two minutes thirty-five seconds_!" "Thus," says Dr. Robinson, "a -three-weeks-old baby can perform a feat of muscular strength that would -tax the powers of many a healthy adult. If any of my readers doubt -this," he adds, "let them try hanging by their hands from a horizontal -bar for three minutes." - -In these facts Dr. Robinson finds something to remind us of the -ape-babies that owe their safety to their capability of holding on to a -tree-climbing mother; and also something to suggest connection with an -ancestor which, although well accustomed to the use of its hands, had -yet to learn the use of its feet for walking on flat ground. - -The same author, in discussing the "Meaning of a Baby's Footprint," has -shown that the foot of a young child bears traces of adaptation to a -state of existence in which it was used for purposes other than that of -walking. - -"The toes of infants," says Dr. Robinson, "are much more mobile than -those of adults. The great toe is shorter than the second and third, -and is often separated from the second by a considerable interval. -The four outer toes can be, and frequently are, bent downwards so as -to show a distinct knuckle on the upper aspect of the foot at the -metatarso-phalangeal joint, and when at the same time the great toe is -flexed and turned inwards towards the sole, the front part of the foot -makes a very respectable fist. The great and little toes are often made -to approach one another beneath the rest, and I have seen one child who -could almost make them touch, and who habitually would endeavour to make -the great toe oppose the others when any graspable object was brought -into contact with the front part of the sole."[E] - -[E] _Nineteenth Century_ for May, 1892. - -Regarding the lines in the sole of the foot, Dr. Robinson says: "The -sole is covered with lines of a character exactly similar to those on -the hand; and when the toes are bent downwards these become deep -creases, showing that they are, like the palmar lines, the natural -folding-places of the integument to facilitate the action of -grasping.... The lines are scarcely visible at fourteen months old, and -are only present in a few cases after the age of two years. In adults no -trace of them can be seen when the foot is at rest, and only the -faintest indication at one or two spots when the toes are flexed to the -utmost. The obliteration is doubtless owing to the foot being used as an -organ for progression rather than prehension, and it will be seen that -the most distinct line crosses the sole at the spot where the epidermis -is always dense and callous, and the subcutaneous tissues thickened into -a cushion-like pad by the pressure and friction consequent on walking. -This line undoubtedly marks the place where the chief fold in the skin -was situated, when the toes were habitually clasped round some object -such as the branch of a tree." It has been pointed out by other writers -that the lines of the sole of the foot can plainly be seen in the adult -foot of some savage races. It must be added, however, that the survival -of the lines in the adult civilised foot is by no means so rare as Dr. -Robinson's remarks would lead one to suppose. I have seen instances in -which they were quite clearly marked. It must be added that anyone who -wishes to confirm my observations in this respect must be careful not to -mistake lines of disfigurement, caused by the pressure of boots, which -are sufficiently common, for the primitive lines of the foot. - -The child, as it grows, ceases to remind us of the ape. Its nose gets -bigger as its toes cease to wriggle and learn to stand. But, for years -of its life, it is only too apt to remind us of the savage. How greedy -it often is! How readily it snatches that which does not belong to it! -How quick it is to quarrel with its playmates, and to fight! How noisy -when at play! How cross when it meets with disappointment! How fond of -tawdry things! In all these qualities we see the history of the race, -repeating itself in the life of the individual. The savage has preceded -the civilised family--the child shows us the faults of a lower race. -With the elapse of years they disappear, and are replaced by the more -amiable and gracious manners of the adult human being. - -Nor do we need to go into the nursery to find links with our inferiors. -Much, indeed far too much, has been written of late years about -"atavistic degeneracy"; that is to say, degeneracy which imitates the -characteristics of our forefathers. Many things which are classed as -diseases, whether of the body, mind, or moral nature, may be explained -in this way. Take the gills, which, as we have stated, exist in all -vertebrates, but not in the adult of the highest groups. In a sickly -individual, even among the highest vertebrates, traces of these are -sometimes seen existing in the adult, as a gap or open space in the -neck, called by the medical man "cervical fistula": this is an instance -of degeneracy in the body. Take, for another instance, the kleptomaniac, -who snatches up everything he takes a fancy to, although he is not in -want. This is degeneracy of the mind, a relic of savage nature out of -place in civilised man. Yet the gill-space is an ancestral feature which -has its right time to appear, though it is out of place in the adult; -and the "want-to-snatch" stage, as we have already seen, is quite -natural in the young child. A parallel instance to the last is that of -the hysterical girl who invents all sorts of tales about her harrowing -adventures, weaving in stories she has heard of other people, with an -account of her own life. She is an impostor; but her instinct for -weaving yarns is that of the savage, who is the more admired by his -fellows the more he can show himself a liar. Even the dangerous -criminal, such as the Anarchist assassin, is comparable with the -treacherous savage, who stabs his guest, and with the fierce animal that -bites the hand that feeds it. - -The causes of degeneracy may seem obscure. But if we turn to our -gardens, how easily is the process understood! Leave a cultivated plant -to look after itself; neither watered, nor manured, nor weeded; and how -long will it be before the plant resembles its wild ancestors? The -flower will be less fine, the leaves more weedy; the whole aspect of the -plant is changed. The causes: insufficient food and water, and the -struggle for root space, standing-room, and light, with the weeds -around it. Just in like manner the human being, when unfed, unwashed, -and untaught, begins to degenerate. The want of fresh air and light -associated with slum life, and even in the country, associated with the -homes of the poor, are factors in the case that are not to be forgotten. -Add to these drink, and the other sins of the fathers which are visited -on the children. All these are among the causes of degeneracy. - -Nay more, the very virtues of the parents, as we account them, may lead -to the degeneracy of the offspring. Overwork, either physical or mental, -causes the deterioration of the family, and in our days nearly every man -successful in any career, either commercial or intellectual, is guilty -of overwork. The "haste to be rich," equally with the haste to be -famous, tells on the next generation. Those who are fond of moralising -at the expense of their neighbours, enjoy pointing out the -unsatisfactory careers of the sons of men who have become rich. Almost -invariably such a one is idle, we are told, and fond of pleasure. -Good cause has he to be so. He comes into the world with weakened -constitution, owing to his father's strenuous career; and if he were to -work as hard as his father, he would probably soon be dead; or at least -his children, in their turn, would be miserable and diseased. Nature -guides his inclinations, and whispers "Do not work too hard," "Do not -deny yourself too much"; and thus, so long as his father's money -maintains him, his life is preserved. - -What is the kind of degeneracy that overtakes the family of the -brain-worker? The modern world is full of it. We owe to the unamiable -genius of Max Nordau a criticism of the intellectual world of the -present day, which attributes well-nigh all the follies of intellectual -cliques to degeneracy. Poetry, which is "full of sound and fury, -signifying nothing," rich in rhyme and alliteration, but wanting in -sense; art which seeks effect by loud and inharmonious colours; music -which rejects "mere melody": in these the critic sees the taste of the -savage, fond of a jingle of words, fond of bright colours, and ignorant -of middle tints; and fond of noise without a tune. - -The so-called æsthetic movement which, a few years ago, wrought such -marvels in decoration and in dress, comes in for a share of the critic's -analysis. The dull senses of the degenerate cannot appreciate the soft -colours which ordinary persons like to look at; to attract his attention -and to please his fancy, he must have staring red, or staring blue. Or, -if he possesses an object which is of special interest, he must bring -this into contrast with a very sombre background, lest by chance it -should miss being seen. - -I met with an amusing instance the other day which is much to the point. -In a remote part of the British Isles, two friends, immigrants from the -world of "culture," had been criticising the landscape. It was a pity, -they agreed, that everything was so grey and dull; otherwise the -neighbourhood might have been pretty. If only the cottagers could be got -to grow something in their gardens that would give a touch of _colour_ -to the scene! These poor creatures had before their purblind sight all -Nature's rich harmony of colour, which affords such pleasure to persons -of true taste. Green fields, brown rocks, blue sea, and blue sky, all -were dull to them. Wild flowers of a score of kinds, and bright with -every colour--these were too insignificant to be visible. They wanted -some big patch of vivid colour, perfectly inappropriate to the climate -and surroundings. Some exotic plant was needed, in their opinion, to -give a touch of brightness. The harmony of colour and beauty of form in -our native plants, and in the common flowers of cottage gardens, were -imperceptible to their unobservant eyes. Their intelligence was on a -level with that of the savage, who is impressed by new and striking -objects, and delighted by gaudy colours, but finds no beauty in wild -nature or in accustomed things. These people were typical specimens of -the degenerate of the book-reading classes; dull of understanding and -wanting in taste, as the result of mental overwork in several successive -generations; immeasurably inferior in æsthetic capabilities to the -untaught peasants and fishermen of the district they would fain -enlighten--for these appreciate the beauty of their country, and love -its flowers. - -Much might be added regarding atavistic degeneracy, as an explanation of -the mental and moral defects of human beings. Its most frequent form, -perhaps, is that of mere laziness. The Ape does not work; nor does the -savage, if he can possibly help it. Civilised man, if thoroughly sound -in mind and body, likes activity, and activity with a purpose. The poor -man takes a pride in his labour; the rich man takes a pride in his skill -in games, his learning, or his efforts to benefit others. The idler, -disinclined for either hearty work or hearty play, is a Degenerate. Of -late there has been much discussion of a plan for treating the confirmed -idler as a criminal. It will be seen from the remarks made above, that -there are equally good reasons for treating him as an invalid. In -criticising the plans of would-be reformers, this fact should not be -forgotten. He was a wise man who said "You cannot, by passing an Act of -Parliament, make a Vice into a Crime." - -It must, however, be remarked that the doctrine of degeneracy has lost -both in force and in usefulness, by the treatment it has received at the -hands of those who have constituted themselves its popular exponents. -Some of these writers have made it but too evident that their criticisms -are often captious, and that their definition of degeneracy includes all -human failings--except their own. The reader who devotes a little -attention to the subject will, however, readily find an explanation -of this: for he will easily recognise, in the popular writers on -Degeneracy, the characteristics of the Degenerate, as described by -themselves. - -First, the choice of a disagreeable subject, when the whole field of -science lay open to them: for the Degenerate prefers a disagreeable -subject. Secondly, the almost universal discovery of causes of -dissatisfaction, in every possible direction: for the Degenerate is -always vexed with everybody--except himself. Again, the want of -principle shown in appealing to the morbid tastes of the public, -by laying before it information on disagreeable subjects: for the -Degenerate is lacking in principle; what does it matter to him how much -harm is done to weak minds by his writings, so long as he sees in such -writings a safe means of securing eager readers and liberal pay? Again, -the Degenerate seeks notoriety; and this is easily secured by writing -books that discuss the morbid side of life. - -Above all, the habit of carrying the war of criticism into regions of -art and culture with which the writer is obviously unfamiliar: this -also marks the tendency of the writer's mind. To criticise the doing of -that which he can by no means do; to destroy that which he can by no -means make; to leave no margin of leniency in his judgment, for the -imperfections which disfigure all human work: these are the familiar -failings of youth, of the unripe mind. They are also those of the -type of mind that never attains ripeness--of the Degenerate: we are -forbidden, on high authority, to apply to our brethren a shorter and -less modern term. - -But although the doctrine of Degeneracy has thus found its way to the -general reader in a form which is often much to be regretted, it is -nevertheless a doctrine which, if wisely used, may lead to the most -beneficial results. Already it is widely recognised, by the thinkers of -all nations, that the theory of degeneracy, when thoroughly understood, -must revolutionise our treatment of the criminal classes. Instead of the -attempt to punish, civilised legislation must eventually, in many cases, -substitute a system of restraint. - -It is useless to try to reform the idler or the thief, whose instinct -for idling or thieving is as imperative as a cat's instinct for catching -mice. So long as he goes free, so long will the instinct reassert itself -at every renewal of opportunity. Repeated punishment of the offender, -who is powerless against his own impulses, is frequently a mere cruelty; -while his repeated release, at the termination of every punitive -sentence, is, on the other hand, still more certainly, a cruelty to the -community at large, which he afflicts by his presence. Public opinion is -gradually becoming awake to the necessity for fresh methods of dealing -with these problems; it is by the patient investigations of scientific -men that it has been enlightened. - -TABLE SHOWING THE PLACE OF MAN IN CLASSIFICATIONS - - =Grade IV.= TRIPLOBLASTIC Animals with a BODY-CAVITY. - - =Group.= CHORDATA; Animals with a Notochord. - - =Phylum.= VERTEBRATA; Animals with a Back-bone. - - =Class.= MAMMALIA; Animals that suckle the young. - - =Order.= Primates. - - =Genus.= Homo, i.e. man. - - _Species._ Sapiens (possessed of sense). - -Meanwhile, it must not be forgotten that the theory of degeneracy -has its cheerful aspect. It enables us to look at the offending -fellow-creature who belongs to the criminal classes, as an incomplete -development rather than as a hardened sinner. It reminds us, too, that -the criminal and the idler of to-day are now, what in the times of -savagery and animalism, every man once was. The degenerate criminal, in -fact, stands as a landmark, to point out the progress which has been -made by the human race. This was the starting-point, where now he -stands. How great the progress that is measured by the distance between -him, and the orderly, kindly-hearted citizen of the present age! - - - - -CHAPTER XVI - -HOW ZOOLOGISTS DO THEIR WORK - - -It is one of the most well-worn of commonplace sayings, that "one half -the world does not know how the other half lives." It is equally true -that one half the world does not know how the other half works; and -especially is this the case when one of the world's halves is its -learned, and the other its unlearned, half. The average business man -probably has an idea that the man of learning has a pretty easy time of -it, and that his most arduous occupation is to enlighten an attentive -world by reading papers at the meetings of the British Association -and the Royal Society. He has a vague idea that the man of learning -sometimes uses midnight oil, but it would surprise him to be informed -that the man of learning often sets to work at five o'clock in the -morning--as is actually the case. And well he may, considering the -magnitude of the task he has in hand, and the variety of the odds and -ends of labour that it includes. - -_Firstly_, how does he obtain the raw material for his work? The -scientist, like the cook, must "first catch his hare" before any further -details of work can be arranged. He does not, as a rule, do this in -person, except when an animal of unusual interest is concerned. An army -of collectors, all the world over, are constantly busy in searching for -material for the zoologists, on land and sea. They look for employment -and pay to the museums and laboratories of the learned world. When the -specimens arrive, what is to be done with them? Some arrive alive, and -may be dismissed from present consideration. The dead specimens give -employment to a number of workers who are under the command of the man -of learning. There are skins to be mounted and stuffed, bones to be -articulated and set up, each practically the work of a different trade. -There are drawings to be made of all important specimens, a task which -affords employment for the artist and the photographer. There are -carcases large and small, to be immersed in preservative fluids until -they can be thoroughly examined in detail. And woe betide the zoologist -who allows any of these tasks to be performed without his own personal -supervision. He will realise, as all careless masters do, that blunders -may be made in an hour, which cannot be repaired in a day. But when all -is done that servants and helpers can accomplish, the real business -remains to be done. Is there among the specimens one which has not been -thoroughly overhauled by other writers, one whose every detail of -structure is not already to be found printed in a book? That one must -be examined with the utmost accuracy. If it is big enough, it must -be dissected, and every part recorded and figured in diagrams. But -suppose it is a small creature, whose parts can only be seen under the -microscope, a long series of processes are necessary before it is ready -for use. In its fresh state, it contains a quantity of water, and if -left to itself would shortly decompose. Even if already immersed during -carriage in various preservative fluids, it still contains much water, -and, if so, neither will it keep for an indefinite length of time, nor -could it be satisfactorily examined under the microscope. It must be -soaked in one of various chemical solutions, to harden and preserve -it. If very small indeed, a mere speck, it perhaps only needs to be -transferred to a fluid in which it can be "mounted" and placed under the -microscope. But with the vast majority of specimens, an immense amount -of labour is needed before they are ready for inspection under the -microscope. - -This will easily be understood if we reflect for a moment on the way in -which objects are examined under the microscope. For purposes of -scientific investigation, they are rarely looked at under light that -falls upon their surfaces, that is to say, by reflected light; for this -method can show nothing but details which are external and comparatively -unimportant. They are seen by light placed behind them so as to shine -through them, _i.e._ by transmitted light. If the object is not -extremely thin, it will shut out too much light, and thus it cannot be -clearly seen, therefore all objects, except the most minute, must be -divided into thin slices, technically known as "sections." - -If we want to know not only the microscopic structure of organs, but -also their shape and position in the body, and their relations with -other parts, we must have every successive section carefully preserved, -and the whole row arranged in correct successive order; the physiologist -may often content himself with single sections; the zoologist must have -rows and rows of them. What a task this was, a quarter of a century ago, -for scientists who cut their sections by hand! - -Let us, however, describe first the way in which objects are prepared -for section-cutting--whether by hand or by machine. It has already -been noticed that animal substances contain a quantity of water, and -therefore will not keep. The same circumstance renders them soft and -squashy, so that the sharpest razor in the world, in cutting a section, -must necessarily do more or less damage to the structure of the delicate -tissues. The water is held in the meshes of the tissues just as it is -held, for example, in the meshes of a sponge. Now, if we were dealing -with the sponge, we could get it to absorb any other fluid substance -besides water; we might choose one that would prevent decomposition; we -might choose one that would go harder by cooling; so as to change the -sponge into a strong solid block that could be knocked about without -sustaining any damage. This is exactly what we must do with our animal -tissue to prepare it for section-cutting; and the most convenient fluid -for the purpose is melted wax. But whereas we might take our sponge -out of water, squeeze it dry, and dip it straight into melted wax, we -can by no means do so with our animal tissues. For one thing they -usually cannot be squeezed, and where they can, they would of course be -irretrievably ruined by such a rough process. Even the transference of -the specimen from one fluid to another of very different qualities and -density, would deface the tissues. Cells would burst, or be squeezed out -of shape, and organs would be loosed from their right position by the -currents set up in all parts of the specimen, under such circumstances. -We must, therefore, try to get rid of the water by degrees. This may be -done by gradually adding alcohol, a fluid which may be diluted with -water in any proportion. We begin with a comparatively weak solution of -alcohol, say about fifty per cent., and immerse the specimen in this for -some little time. The time required depends somewhat upon the size of -the specimen; if a large one, a new fluid will take longer to filter -through it. Then we must change this solution of alcohol for stronger -ones, say seventy per cent. and ninety per cent. successively, and -finally to absolute alcohol. By this time the alcohol will have removed -almost nearly all trace of water from the specimen. The latter is now -nearly but not quite ready to be imbedded in melted wax; but first we -must soak it for a while in a fluid intermediate in thickness between -the alcohol and the wax, and capable of mixing in a friendly manner with -both. Then it goes into a bath of melted wax, and is kept for hours at a -stated temperature until the wax permeates it thoroughly. Then the -melted wax and the specimen along with it is poured into a little mould -and left to cool. The block of wax containing the specimen is cut down -to a quadrangular shape, and is now ready for section cutting. In old -days the block was placed in a stand, and successive sections were cut -from it by hand with a razor. But this process is much too slow for -modern days. Machines called microtomes (_i.e._ cutters of small parts) -have been invented, and of these there are several kinds--in all, -however, the razor is worked by machine and not by hand, so as to secure -steadiness and a uniform thinness of the sections. The old microtomes -threw off each section separately; but now matters are so arranged that -the wax of each section adheres to that of the next, and the whole -series of sections forms a continuous ribbon of thin wax. A large -specimen, affording a number of sections, thus results in a ribbon of -considerable length. Further processes are now required to fit the -sections for the microscope. The ribbon must be divided into successive -pieces of a length determined by that of the slides to be used. These -are mounted in order on the slides, steps are taken to melt away the -wax from the sections, the latter are covered with Canada Balsam -surmounted by a glass cover slip, and left for some time to dry. After -this they are ready for examination, and it is only now that the work -really begins. All that has gone before is mere handicraft; it is time -now for science to be called into play. - -[Illustration: FIG. 47.--Sections of Embryo Chick, eight days old. A -slide mounted for microscopic examination, showing sections arranged in -ribbons.] - -The sections must be compared with others of the same kind which have -been cut before. Do they entirely resemble these, or is there a -difference somewhere? Happy the man who finds that his sections -represent a fresh stage, perhaps older or younger than any that has been -seen before in the history of the particular animal which is under -investigation. Happier still the man who has succeeded in getting hold -of an animal which has not been described before. He will make haste to -write a full description of it, illustrated by drawings; to found a new -theory on it, if that can possibly be done; and to publish it to the -world. It will go all over the globe. To every country in Europe; to the -centres of learning in the United States; to universities in New Zealand -and Australia, and our other colonies; and perhaps even to "Far Japan." - -When in his turn he receives publications from all countries, written in -all languages, he is in a position to realise the very great advantage -(referred to in an earlier page, p. 31) that results from the use of the -learned tongues, in the terminology of zoological science. For the -educated classes in all countries are equally acquainted with these; and -when half of a sentence consists of words of Greek or Latin derivation, -the labour of translation from a foreign tongue is necessarily greatly -lightened. To no writer is this advantage of so great importance as to -the Englishman, who is usually less familiar with the tongues of other -nations than his colleagues abroad. It will easily be understood that in -the world of zoology, there is no "predominance of the English-speaking -races." Far from it. German is the language which supplies the fullest -literature of every scientific subject; and in England even our -text-books are, for the most part, translated from the German. German, -in short, is to the seeker after Knowledge, what English is to the -seeker after Money. - -Let us now pause a moment to consider how large a number of different -industries profit by the labour of the zoologist. First there is the -shipping trade; for, of course, all specimens from foreign lands are -brought by sea. The chemist supplies preservative substances, and -reagents used in the preparation of objects for the microscope. The -construction of microscopes is a profession in itself, and one which -employs many industries; for the making of a microscope includes not -only the work of the optician, but also that of the artificer in brass, -and of many other handicraftsmen. The glass-worker supplies "slides," -that is to say, the thin pieces of glass upon which objects for the -microscope are placed, and "cover-slips," the little sheets of thinner -glass which are laid over them; and, besides these, the bottles in which -specimens are placed. Then comes the microtome, already spoken of, by -means of which sections for the microscope are cut; how many skilled -workmen have been engaged in the construction of its parts! Sheffield, -perhaps, has supplied the razor which it holds, as well as the -instruments for the dissection of the larger zoological specimens. We -have already spoken of the laboratory servants, and the -bone-articulators and skin-stuffers, who are personally and directly -employed by the zoologist; and of the artists and photographers who -depict his specimens, or perhaps copy his drawings. We must add to the -list of the zoologist's helpers, last, but not least, the printer who -"sets" the learned treatise in which the final result of his work is -usually embodied; and attendant on the work of the printer is that of -the bookbinder. With the bookseller the zoologist has but little to do; -the general public, even the reading public, has no knowledge whatever -of the writings of the zoological specialist. They are addressed to his -equals and co-workers, not to critics and reviewers. Their publication -is provided for, not by the law of supply and demand, but by the funds -of the learned societies and the universities. It is only occasionally -that a writer arises who is able and willing, like Huxley or Darwin, to -express himself in a book that the general public can read; and it is -only after a lifetime of detailed work, such as is understood only by -the specialist, that writers like these think it fitting to lay the -results of their labour before outsiders. - -The librarian, finally, must not be forgotten, in making up our list of -the zoologist's helpers. The preservation and cataloguing of zoological -literature is obviously a task all the more important, because, as we -have already stated, zoological writings are not regulated by the law of -supply and demand. A very little paper, read to a very small meeting of -a learned society, and wholly ignored by the world at large, may -contain facts priceless to the world of science. It is on the accurate -and painstaking work of the librarian, who preserves and catalogues -small things as conscientiously as large ones, that we rely for the -completeness of our record of zoological knowledge. Such work has at all -times been carried on in the libraries of our universities; but at the -present time there are in existence libraries specially devoted to -zoological literature alone. - -The museum, again, must not be forgotten, in which our man of learning -stores his specimens, duly labelled and arranged. Here, again, is a -staff of curators and sub-curators; and, under their direction, work for -various workmen, and for perhaps even a humble charwoman to dust the -shelves. - -Turn now to another aspect of the zoologist's work--that of teaching. We -should think it very wrong to turn men loose on the world to practise in -the professions of law or medicine without a long and careful training -to fit them for their task. No less impossible is it for anyone to -become a man of science without a similar training; for the profession -of the man of science, whether zoologist, chemist, botanist, or expert -in whatever branch, if defined in plain English, is the profession of -seeking after knowledge of the order of things in which we live; and -what profession can be more important to the world than this? To attain -a scientific degree of any value, years of study are therefore required, -and a series of examinations tests--or is supposed to test--the success -of the student. Both the work of teaching and the work of examining must -be the tasks of the scientist who has attained a position of eminence -in the world of learning. The preparation of lectures, with their -accompanying illustrations of diagrams and lantern slides: the guiding -of classes engaged in the actual work of making acquaintance with animal -specimens--these are the labours of the great man who is at the head of -things. His task is carried out with the aid of junior helpers of his -own profession--the demonstrators, who "point out" detail after detail -of the work described in the lectures. Another helper, more esteemed by -the students than by the professor who teaches them, is the "coach" who -prepares them directly for their examinations. His aid, in the shape of -extra teaching, given at the last moment, will often secure for the -careless and inattentive pupil, better success than is the lot of the -painstaking and industrious one, who cannot afford to pay extra fees. - -Few, however, of all the many pupils who crowd the lecture room of the -zoologist, will ever become zoologists themselves. A vast proportion of -them are students of medicine, of whom some knowledge of the subject is -required. Others are preparing to be schoolmasters or schoolmistresses, -and seek just such an amount of knowledge as they expect to find useful -in teaching pupils of their own. To the students who are preparing to be -doctors or teachers, circumstances often assign a limit--"thus far and -no farther"--when they would fain bring their knowledge to a higher -standard. But the time they have spent already has not been wasted. How -keen an observer of animal life is the country doctor! How often, -isolated from the world of learning, and ill-provided with books, he -finds in this his chief recreation! As for the schoolmaster, how is the -routine of school-work relaxed, and labour changed into pleasure, when -he lets his boys exchange grammar and Euclid for zoology, and the -lessons of the schoolroom for lessons in the fields! - -The most important part, however, of a zoologist's work is not the -giving of instruction, but the labour of original research, to which we -have already alluded; not the mere communication of information, but the -task of adding to the general store of knowledge; not teaching, but -discovery. The work of the man of science is, in fact, within the limits -of his own department, the work of seeking after truth. - - - - -INDEX - - - A. - - Acoelomata, 37. - Adaptation, 13. - Alternation of Generations, 57, 137. - Amoeba, 35, 45. - Amphibia, 152-154. - Ancestors, 40, 42. - Animalcule (minute animal), 49. - Anisopleura, 29. - Annelids, 72. - Annulosa, 69. - Ants, 92. - Appendages, 77. - Arachnida (spiders), 84. - Arthropoda, 33, 76. - Ascidians, 33, 44, 135. - Asexual reproduction, 55. - Atavistic variation, 27. - Azygo-branchiata, 29. - - - B. - - Balanoglossus, 133, 143. - Barnacles, 79, 80. - Bees, 91. - Beetles, 95. - Bell Animalcule, 49. - Birds, 156. - Bivalve shell-fish, 23, 27, 107. - Body-cavity, 34, 37, 38. - Body-cavity (diagrams), 38, 139. - Body-rings, or "segments," 69. - Brachiopoda, 33, 43, 44, 117. - Bryozoa, 33, 44, 119. - Buds, 55. - Butterflies, 89, 93. - - - C. - - Cat, fur of black, 160. - Cell, 11. - Cell-types, 49. - Cephalodiscus, 145. - Cephalopoda, 113. - Centipedes, 77. - Chætopoda, 71. - Chalk, 46. - Chordata, 33, 44, 135, 143-146. - Cilia, 42, 43, 48, 65. - Classes, 33. - Classification, 30. - Classification, tables of, 30, 44, 52, 62, 67, 75, 116, 118, - 121, 134, 146, 164, 179. - Coelenterata, 33, 44, 53. - Coelomata, 37, 44. - Cockle, 111. - Colony, 57. - Corals, 59. - Corallines, 56, 58. - Corticata (or Infusoria), 47. - Crabs, 81. - Crocodile, 11. - Crustacea, 78-83. - Ctenophora, 60, 62. - - - D. - - Degeneracy, 28, 172-180. - Development by metamorphosis (change of form), 41, 89. - Development, direct, 45. - Dicyemidæ, 34. - Diploblastic (two-layered), 34, 36. - Diploblastic larva, 41. - Duck-mole, 160. - - - E. - - Earthworm, 74. - Earthworm, diagrammatic section of, to show position of body-cavity, 38. - Echinodermata, 33, 43, 44, 122. - Ectoderm, outer or skin-layer of adult animals and larvæ - (corresponding with the epiblast of embryos in the egg), - 34, 37, 41, 139. - Eleutheroblasteæ (hydroid animals which throw off "free buds"), 56. - Embryology, 45. - Encrinites, 131. - Endoderm, inner or digestive layer of adult animals and larvæ - (corresponding with the hypoblast of embryos in the egg), - 37, 41, 139. - Enteron, 36. - Environment, 26. - Errantia, or Wandering Annelids, 72. - Euthyneura, 100. - - - F. - - Families, 33. - Fertility, 32. - Feathers, 157. - Feather-stars, 132. - Fishes, 150-152. - Flagella, 65. - Flat-fish, 23. - Foraminifera, 46. - Frogs, 38, 152. - - - G. - - Galeodes, a spider-like animal, 85, 86. - Gasteropoda, 29, 98-107. - Gasteropoda, classification of, 29. - Gastræa, 40. - Gastrula, larva, 41, 150. - Genus, 32. - Gills, 45, 141, 149. - Grades, 34, 35-38. - Gregarina, 49. - - - H. - - Heliozoa, 48. - Hemichordata (or Adelochorda), 33, 43, 145. - Hermit Crabs, 80. - Holostomata, 105. - Hybrid, 32. - Hydra, 36, 41, 54, 59. - - - I. - - Infusoria 10, 47. - "Infusorial earth," 47. - Insects, true, 88-97. - - - J. - - Jelly-fish, 57, 58. - - - K. - - Kangaroo, 163. - - - L. - - Lamellibranchiata, 107. - Lamp-shells, 119. - Land Animals, 166. - Larvæ, larval forms, 40, 41. - Larvæ of Brachiopods, 119. - Larvæ of Insects, 90. - Larvæ of Molluscs, 115. - Lancelet (Amphioxus), 41, 140, 149. - Leeches, 71. - Limpet, Common, 17, 19, 20, 29, 30. - Limpet, Semi-transparent, 15-20. - Liver-fluke, 71. - Lobsters, 80. - Lophophore, 117, 122. - Lustre, metallic, of feathers, 157. - - - M. - - Mammalia, 160. - Man, 13, 26, 167-180. - Mantle (of bivalve molluscs), 108. - Marsupialia (or Metatheria), 161. - Marsupium or nursery-pocket, 161. - Mesoblast, 38. - Mesoderm or middle body-layer, 37, 61. - Metameric symmetry, 70. - Mesozoa, 35. - Metazoa, 35. - Microscope, 9, 10, 182. - Microscope, Sections for the, 182. - Microtome, 185. - Mites, 87. - Mollusca, 29. - Mollusca (classification of Gasteropod), 29. - Moths, 93. - Monoblastic, 34. - Moss-Corals, 33, 39, 119. - Mule, 32. - Mussel, Common, 103. - - - N. - - Nematodes, 71. - Notochord, 135, 139, 145, 149, 151. - Nucleus, 35. - Nummulite, 46. - - - O. - - Odontophore, 100. - Operculum (of univalve molluscs), 105. - Opossum, 161. - Orders, 33. - Orthonectidae, 34. - - - P. - - Pelecypoda, 107. - Perforating gills (of vertebrates and other chordata), 142, 144. - Peripatus, 88. - Periwinkle, Common or Edible, 19, 26, 105. - Periwinkle, High-tide-mark (_L. rudis_) 19, 105, 114. - Periwinkle, Yellow, 19, 21, 23, 25, 30, 105. - "Persons" of a colony, 58. - Phoronis, 122. - Phylum, pl. phyla, 33. - Placophora, 113. - Planarian Worms, 37, 70. - Planula Larva, 41. - Platyhelminthes, 44, 71. - Polycystina, 47. - Polyzoa, 119. - Porifera, 33, 63, 68. - Protective Coloration, 15, 25. - Protophyta, 50. - Protoplasm, 35. - Prototheria, 161. - Protozoa, 33, 44, 45. - Pseudopodia, 36. - - - R. - - Radial Symmetry, 53. - Radiata, 53. - Radiolarians, 47. - Rainbow Worm, 72, 159. - Reptiles, 154-156. - Rhabdopleura, 145. - Rhizopoda, 36, 46, 48. - Rodent, Teeth of, 163. - Rotifers, 76. - - - S. - - Sand-hoppers, 83. - Sauropsida, 154. - Scales of fish, 142, 143. - Scallop, 107-112. - Scorpion, 87, 88. - Sex, 10. - Sea-Anemone, 54, 59. - Sea-Cucumbers, 129, 130. - Sea-Fan, 59. - Sea-Mats, 119. - Sea-Mouse, 72, 159. - Sea-Urchins, 23, 33, 122. - Shell-fish, 33. - Siphonostomata, 102, 106. - Skin of Vertebrates, 142. - Snail, 98, 114. - Snake-Stars, or Brittle-Stars, 128. - Species, 30. - Spiders, 84. - Spiny Ant-eater, 160. - Sponges, 33, 44, 63, 68. - Sponges, Parasitic, 68. - Starfishes, 127. - Streptoneura, 29, 100. - Symbiosis, 48. - - - T. - - Teeth, 147, 163. - Tentacles (arms or feelers), 54. - Ticks, 87. - Trichina, 71. - Triploblastic (three-layered), 37. - Trochophora, 43. - Trochosphere larva, 42, 43, 72. - Tubicolous (tube-dwelling) Annelids, 72, 74. - Tunicata, 33, 44, 135. - Turbellaria, 70. - Two-layered animals, 34, 36. - - - U. - - Unicellular animals, 11, 34, 35, 39, 44. - Univalve shell-fish, 98. - Urochordata, 145. - - - V. - - Vacuole, contractile, 35. - Variation, 24, 26, 28, 32. - Varieties, 29. - Vermes, 33, 44, 68. - Vertebrae (joints of the backbone), 138, 139. - Vertebrata, 33, 44, 138. - - - W. - - Water Animals, 166. - Wheel-ball larva, 42, 43. - White ants, 93. - Wood-lice, 83. - Worms, 33, 44, 68. - - - Z. - - Zooids, 58. - Zoologists (_see below_). - Zoophyte, 53. - Zygobranchiata, 30. - Zoologists, names of-- - Buffon, 11. - Caldwell, 160. - Chamisso, 137. - Cuvier, 54. - Darwin, 24, 76. - Dubois, Eugène, 168. - Forbes, 17. - Gadow, 157. - Gosse, P., 54. - Grant, Robert, 65. - Hæckel, 40, 168. - Hertwig, O., 50. - Huxley, 37, 78, 136, 154. - Kowalevsky, 136. - Landsborough, W., 109. - Lang, A., 39. - Linnaeus, 32, 68. - Leuckart, 71. - Morgan, Lloyd, 20. - Parker, T. J., 32. - Roberts, G., 21. - Romanes, G. J., 11. - Sharp, D., 92. - Sollas, 65. - Woodward, 109. - - - - -Transcriber's Note: - -Obvious typographical errors have been corrected. Original published -spelling and hyphenation have been retained as they appear in the -original publication, including "debateable" and "cellless". A possible -missing "wall" has not been added to the caption for Fig. 5 ("--muscular -of intestine"; and reference to "the Italian poet" on Page 138 of the -original publication has been preserved. Where figures or tables cut -paragraphs, they were moved above or below the paragraph. - - - - - - - -End of Project Gutenberg's Stories of the Universe: Animal Life, by B. 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