B. Lindsay

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.

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.

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

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."

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.

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.

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.

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

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

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.