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Something remains to be said regarding the specialisation of tissues in this group. We have already mentioned the stinging cells, and the beginnings of muscular tissue, in Hydra. The former are a constant feature of the Coelenterates, while the latter reaches a very considerable development in the higher forms, as may be judged from the surprising rapidity with which the Medusa can swim, or from the strength with which the sea-anemone can retract its tentacles and draw itself together. Important, further, is the nerve tissue. This consists of cells whose business it is to receive and transmit stimuli. They have long fibrous projections connecting them with each other, so that there is a network of communication throughout the whole animal. In the Medusa, where co-ordinated movements of various portions is necessary, there is a concentration of nerve cells into a double ring near the edge. Here also there are special organs, probably of sight and of the sense of balance; but as these cannot be regarded as the forerunners of the analogous organs in higher animals, we need not pause to describe them. The anatomy of the Coelenterates will be better understood if the reader will study the diagrams in Figs. 35 and 36, while some idea of the beauty and variety met with in the group may be obtained from Fig. 37.

Regarding the interrelationships of the various types that we have described, and their respective importance with reference to the descent of man, opinions are somewhat divided. Some believe the Ctenophora to have been derived from the Medusa form, but the more probable view seems to be that they have evolved separately from some earlier and more primitive type than any existing Coelenterate, and that their ancestors have all been free-swimming and ciliated. Now the Ctenophora are considered, on good grounds, to be somewhat nearly akin to the lowest worms, and thus to stand fairly close to the main line of Evolution. If this view be correct, the whole group of existing Coelenterates forms a side branch of the Evolution tree. This fact, however, does not take away the importance of the group in relation to the theory of the descent of the higher animals, for the Coelenterates have certainly retained many of the characters which were possessed by the direct ancestors of man, such, for instance, as the simple digestive cavity, the primitive type of body, consisting of two cell layers, the diffuse and elementary nervous system, and the radial arrangement of parts. Moreover, the course of Evolution in the group, leading from the Hydra to the sea-anemone and the Medusa, has probably been in many respects parallel to that which started from some primitive extinct form, and led up to the Ctenophora. Therefore the study of the group has thrown much light on the earlier history of the animal world. Regarding the age of the group, it may be mentioned that fossil corals, etc., are found, along with Crustaceans and Molluscs, in the earliest known fossil-bearing beds, belonging to the Cambrian age.

THE WORMS AND SOME OF THEIR POSTERITY

The worms comprise many greatly divergent groups, and the difference between the lowest and the highest of these has been produced by many important steps in Evolution. Of these groups but few immediately concern us; the first and lowest of those which do, is that of the Turbellarians, a section of the Platodes or flat-worms. The Turbellarians are small or microscopic tongue-shaped organisms, of which the majority of species live on the sea-floor, others however being found in fresh water. The surface of the body is covered uniformly with cilia, which serve, in the smaller forms, as organs of propulsion, while in the larger they appear to have the function of maintaining a flow of fresh water over the surface, and thus of assisting respiration. In some respects there has been little advance from the condition of the Coelenterate. The digestive cavity is a simple or more or less divided sac, communicating with the exterior only by means of the mouth. Unlike the condition of affairs in the Coelenterates and Ctenophora, however, the sex glands do not discharge the reproductive bodies into the digestive cavity, but directly to the exterior by means of a special opening. Each individual has a pair of male and a pair of female reproductive glands, but the eggs are not self-fertilised; nor is the fertilisation of the eggs trusted to chance and the sea-water, as in the lower groups. Instead there is a definite exchange of sperms between two individuals, and the eggs are fertilised before they leave the body. They are also frequently supplied with a store of nutritive material by a pair of special yolk glands. A distinct step of progress can thus be recognised in the arrangements for reproduction. Between the outer skin and the inner digestive layer is developed a considerable mass of cells, forming muscular and connective tissue, etc. It will readily be understood that the development of such thick tissue masses occasions two distinct new difficulties in the animal economy; for where cells are in direct contact neither with the digestive layer nor with the exterior, their nutrition and the removal of their waste products can no longer be efficiently carried on without special devices. Hence on the one hand a circulatory system, for the transport of food materials, and on the other an excretory system, become necessary. The first of these new departures was not destined to be made until the next stage of progress; the Turbellarians seem to have temporarily got over the difficulty, like the Ctenophora, by developing a complex and ramifying digestive cavity. An excretory system, however, makes its appearance here. Indeed, the beginnings of such a system can be seen in the Ctenophora, in which there are small excretory organs opening into the digestive cavity. The corresponding organs in the worms, as in all subsequent types, open directly to the outside. In the Turbellarians these organs, which are termed nephridia, are two in number, and consist of long tubes which branch and ramify throughout the body, the small branches terminating in special excreting cells, and the whole constituting a complete and thorough drainage system. The nervous system consists of one or two small masses of nerve cells termed ganglia in the front region, with a somewhat complex network of nerves connecting them with various parts of the body. There are frequently two pairs of sense organs, probably rudimentary eyes and ears respectively. The main features of the digestive, reproductive, excretory, and nervous system are shown in the figures in Fig. 40.

The next class of worms with which we have to deal is that of the Rotifera. In their general structure, and in their excretory and sensory-nervous systems, the Rotifers do not differ essentially from the Turbellarians. They do differ, however, in that the digestive cavity has a second opening to the exterior, at the end opposite to the mouth. The advantage of this arrangement, which was retained in the subsequent stages of Evolution, is obvious, for it renders possible a much more regular and thorough digestive process. Instead of the food passing in, and the undigested remains passing out, by the same opening, and instead of the contents of the digestive cavity being a general mixture of food material in all stages of digestion, there is now a regular stream of food passing through the cavity in one direction, and being digested as it goes. A near relative of the Rotifers is shown in Fig. 42.

Thirdly, we must briefly allude to the Nemertines. These are a group of flattened thread-like worms of very variable size, found both in fresh and salt water. The most notable advance in this group is to be seen in the occurrence of a special circulatory system. It has already been indicated that the gastric cavity of the lower forms has the double function of digestion and of the transport of nutritive substances to the various parts of the body. In the Nemertines the second of these functions is carried out by the blood system, which consists of two or three vessels that run parallel throughout the length of the body and anastomose at either end. There is no indication of any enlarged or specially contractile portion of any of these, no indication, that is to say, of a heart. The blood conveys not only nutritive substances, but also, as in the higher animals, oxygen. Some Nemertines have indeed red blood, containing true haemoglobin, which is well known as the oxygen-carrying material in the vertebrates. A typical Nemertine is shown in Fig. 44, and a diagram showing some features of the anatomy in Fig. 45. It will be seen that the nervous system is of the same type as in the worms already described. There are two pairs of sense organs, one pair being eyes, and the other probably having the function of gauging the chemical nature of the water. The Nemertines possess a peculiar organ in a snout or proboscis, which they can protrude or withdraw into a special sac. The snout is armed with a sharp sting, and forms an effective weapon whether against the creature's enemies or its prey.

About this stage of Evolution, the exact point being somewhat difficult to fix, there appears the body cavity. This, which is altogether distinct from the digestive cavity, is a familiar feature of the anatomy of the higher animals. In it are suspended the heart and lungs and the whole of the digestive organs and glands. The question of the origin of the body cavity and the blood system is a very difficult one, and a thorough theoretical discussion would take us too far.

Before proceeding to the question of the origin of the vertebrates, we may pause briefly to consider the other groups to which the worms appear to have given rise. First of these we may take the Echinoderms, which include the well-known star-fishes and sea-urchins, and the very beautiful feather stars. As already indicated, it is believed that the radial symmetry, which is so characteristic of this group, is not a primitive feature, but that, in fact, the Echinoderms are descended from bilaterally symmetrical ancestors. One reason for this view is that the larval or immature form is always markedly bilaterally symmetrical. In an ordinary star-fish, which we may take as typical of the group, the mouth is in the middle of the lower aspect, and the excretory opening of the digestive cavity in the upper side just opposite. There is no blood system, or excretory organs, and no concentration of nerve cells into any form of brain. Eyes, however, are present, and sensitiveness to light may be easily demonstrated. The most remarkable feature of the group is the water-vascular system, consisting of a series of radial canals, one in each ray, which join a circular one situated in the central portion of the body. The system of canals communicates with the exterior by means of a sieve-like plate on the upper surface, and it is kept full of water by the continual pumping action of cilia on the walls of the tube which leads down from the sieve plate.

The ordinary star-fish is carnivorous, and lives largely on ordinary mussels, which it bridges over with its arms, and opens by a steady and long-continued pulling, the soft parts being then sucked up by the partially protruded stomach. A few types of Echinoderms are shown in Figs. 46, 47, 48.

The group of the Mollusca includes such common forms as cuttle-fishes, whelks, slugs, snails, mussels, and oysters. These, it will be observed, comprise marine, freshwater, and land forms. The molluscs, like the next two groups with which we have to deal, have made a conquest of the land, though in the present instance it cannot be regarded as very complete. The anatomy of the group shows much variation, and only a few of the leading features can be alluded to. The digestive system is highly developed. The mouth is provided with a jaw or jaws, and with a tongue-like ribbon, which is covered with rows of teeth, like a file, and by whose action the food is torn and disintegrated. A gullet leads from the mouth to a stomach, which is followed by an intestine. Salivary glands and a large hepatic gland or liver are present. Respiration occurs partly through the skin, but special organs also exist for this function, gills in the water forms, and a lung cavity in those which breathe air. There is a well-developed blood system, and generally a heart; the blood is pumped direct from the heart to the general body tissues, and returns to it by way of the kidneys or nephridia, which purify it of waste materials, and the respiratory organs, where it is freed of carbon dioxide and supplied with oxygen. The nervous system varies greatly, but a pair of cerebral ganglia--a brain--is usually present. There is a particularly keen sense of smell, and taste and hearing may also easily be shown to exist. Some forms are blind, from which condition there is a regular series of stages of development of the eye, up to forms in which it becomes a highly perfected organ, with cornea, iris, lens, and retina. The close similarity between this and the ordinary vertebrate eye, which must have evolved quite separately, is one of the strangest coincidences of Evolution. Thus in many ways the molluscs are to be regarded as highly specialised types. But in two important directions, in intelligence and in their arrangements for locomotion, they stand as a group on a low plane of development. Figs. 49, 50, and 51 illustrate some of the forms met with in the group. The origin of the molluscs, as well as that of the Echinoderms, is wrapped in obscurity. That each group is derived from some form of worm is probable, yet some zoologists hold even such a general statement as this to be lacking in support.

Our third great group is that of the Arthropods , including the Crustaceans , spiders and mites, centipedes and insects. The Arthropods are sometimes classed together with their ancestors, the ringed worms , as Articulata, a name which refers to a very obvious feature, the repetition of similar segments in a regular series from front to rear. This is perhaps most apparent in the ringed worms and centipedes, but it is to be seen in all members of the group. This same tendency to reduplication of parts in a regular series may be observed in the vertebrates, as we shall see. Slight indications of it are also to be found in the Nemertines. Numerous theories have been proposed which derive the vertebrates from some of the Articulata--from the ringed worms or the Crustaceans, and even from the air-breathing members; and at first sight such theories seem attractive, for in some of their more obvious characters there is a certain resemblance between the two groups. But there are also many and fundamental differences, and few zoologists have accepted any hypothesis of this type. We may briefly allude to some of these differences.

The Arthropods are an extraordinarily successful group. A multitude of forms of Crustaceans populate the waters, and they are excelled in numbers and variety only by the insects upon land. While the individual size appears to be somewhat strictly limited, probably by the nature of the respiratory and blood systems, many types show exceedingly high development in various directions--in intelligence, in social and parental instincts, etc. The insects are of course to be regarded as the highest Articulata, and have, like the highest vertebrates, the mammals and birds, almost completely forsaken the water for the dry land and the air. An interesting member of the Articulata, from the standpoint of the Evolution theory, is the Peripatus, shown with a ringed worm on Fig. 52 . It gains its interest for us from the fact that, while classed as an Arthropod, it stands very nearly half-way between the ringed worms ) and the true Arthropoda, and thus forms a solitary link between the two types. In Fig. 52 and Figs. 53 to 58 are shown a number of types of Arthropods.

We must now go back and take up the main thread of our story. The next stage that falls to be described is that of a highly interesting group of worms known as Enteropneusta, a name signifying 'gut breathers.' This group contains a very small number of worm forms, which are to be found burrowing in the sand of the sea floor. A typical example is the Balanoglossus, a worm of some four inches in length, whose general appearance is illustrated in Fig. 59. The creature has, as will be observed, a large muscular snout or proboscis, behind which follows a small portion called the collar, and behind this again the long body. The most noteworthy feature of the group, as the name implies, is the respiratory system. The mouth, which is situated in the region of the 'collar,' leads into a gullet, which is partially divided into an upper and a lower canal by means of two inwardly projecting longitudinal folds, one on either side. Only the lower of these canals is used as a food passage; the upper communicates with the outside by means of a large number of transverse slits on its sides. Water is continually being taken in by the mouth and passed along this upper canal, to reach the outside by way of the gill slits, and in doing so it passes over the gills, where the blood is circulating in fine capillary vessels. Here the blood is supplied with oxygen from the water, and is at the same time relieved of carbonic oxide. This, it will be observed, is the same method of respiration as that of the fishes. Behind the last gill slit the digestive canal becomes a simple tube, with two digestive glands or liver sacs. There are two main blood vessels, the larger running along above the digestive canal, and the smaller below it, the two being connected by means of numerous branches. There is a swelling of the dorsal vessel--a heart--at its forward extremity, in the base of the proboscis. The nervous system is peculiar; it consists of two nerve cords, the smaller below the gut and the larger above it--the latter therefore occupying a position similar to that of the spinal cord in the vertebrates. Thus in two respects, in its respiratory and nervous systems, Balanoglossus must be regarded as a highly extraordinary member of the worm group, and in both its peculiarities it shows an approach to the vertebrate. There can be little doubt as to the position of this group as an important connecting link between the ordinary worms and the vertebrates. Fig. 60 illustrates the main features of the anatomy.

As to the origin of the Enteropneusta, opinions are somewhat divided. Their blood system and their development would seem to suggest a descent from the ringed worms. On the other hand, their possession of a snout, and their very slight indication of division into segments, would seem to separate them from the group mentioned and to connect them rather with the Nemertines. The latter view is perhaps the more probable.

THE EARLY VERTEBRATES AND THE FISHES

The lowest of the vertebrates--if indeed it can be called a vertebrate at all, seeing that it has no vertebrae--is the lancelet, Amphioxus. The common species of this animal occurs in the sea off our own coasts, and is usually to be found half buried in the sand or mud of the sea floor. It is some two inches in length and has the shape of a laterally flattened cigar, and one of its very obvious features is the arrangement of the muscles in regular layers from front to back, in the same manner as those of a fish.

To describe some of its features in detail, the alimentary canal bears a somewhat striking similarity to that of Balanoglossus. There is a round, simple mouth, unprovided with jaws, and surrounded by a number of projecting bristles. This leads into a large pharynx, through the walls of which, on either side, pass a large number of gill slits. The pharynx is not divided into an upper and a lower canal, but there is a shallow groove along the bottom which serves the same purpose as the food canal in Balanoglossus. The remaining, digestive, part of the gut is practically a simple tube, with a blind sac attached, representing the liver. The gill slits do not open directly to the exterior, but into the so-called peribranchial chamber, formed by the junction below the body of two flap-like outgrowths, one from the upper part of either side. This chamber opens to the outside by a single pore.

The lancelet forms a most important link between the lower and the higher animals. It is in all probability derived from some form similar to Balanoglossus, and it certainly leads up to the round-mouths, which form the next step in the ladder.

Before describing these latter, however, we must briefly allude to the highly remarkable group of the tunicates or sea squirts, one of which is shown in Fig. 64. They are sedentary creatures found attached to rocks or weeds on the sea floor, and in appearance they remind one rather of misshapen potatoes than of higher animals. They are in fact regarded by the fishermen who bring them to the surface as plants, and they were for long looked upon by zoologists as akin to the molluscs. The only definite external features of the tunicates are two apertures at the upper end, one in the centre and one somewhat on one side. The absence of any other definite external characters is due to the fact that the creature is enclosed in a mantle of cellulose. The central opening is the mouth, which leads into a large pharynx, the walls of which are perforated by numerous gill slits. This is surrounded by the mantle cavity, which connects with the outer water by means of the second pore. The gut is continued into a simple stomach and intestine, the latter bending back upon itself and opening into the bottom of the mantle cavity, as shown in the diagram in Fig. 63. In the adult animal there is no trace of the notochord, and only a remnant of the nerve cord; and there are either no special sense organs or only traces of these. On the other hand, the tunicates possess a centralised heart. They are hermaphrodite, and, very curiously, a number of forms multiply like corals, by a simple process of budding.

Now the remarkable fact has been made out that the young tunicate bears a most striking resemblance to the immature lancelet. It is a free-swimming, tadpole-like creature, and possesses a notochord and nerve cord and in general the same characters that we described for Amphioxus. It is only later that the creature settles down and assumes its final degenerate sedentary form. There can be no doubt that the tunicates have been derived from some lancelet-like form, but the course of their evolution has been unique. The type is the lost brother of the vertebrate family, who has chosen a distinctly downward path in life; yet who has come to no miserable end, but lives on, more or less successfully, in his lower social sphere.

The round-mouths, including the lampreys and the hag-fish, stand midway between the lancelet and the fishes, and therefore constitute for us an important group.

There remain, even after the most thorough investigation of Balanoglossus, the lancelet, and the round-mouths, some questions with regard to the origin of the vertebrates that are still unanswered. It must, however, be regarded as an extremely fortunate circumstance that representatives have come down to us of the three ancient groups to which these three types respectively belong. This is the more fortunate in that the groups in question are known only from their few living members, a circumstance which is of course easily accounted for by the absence of any hard parts capable of being preserved as fossils. From this point onwards there is a skeleton, and we are consequently enabled to draw valuable information from fossils. Partly in consequence of this, the Evolution chain from this point onwards is much more complete than the portion that we have dealt with thus far.

We have already observed that the true fishes, to which we must now direct our attention, differ from the round-mouths in several important characters. They possess two pairs of extremities, the pectoral and pelvic fins; they have a pair of nostrils; there is also a well-developed skull, which includes a series of cartilaginous or bony arches situated in the wall of the gut and between the successive gill clefts. These branchial arches bear a certain resemblance to the basket-work arrangement of cartilage in the round-mouths, but for various reasons are not regarded as having been derived from the latter. It is from the first pair of these arches that the jaws are formed, organs which make their first appearance in the lower fishes. The skeleton shows great development in other directions. The notochord is present in its primitive condition during the earlier stages of development, but it becomes surrounded, and in many cases largely suppressed, by the portions of the vertebrae. Each vertebra consists of an upper and a lower portion, the upper forming an arch round the nerve cord and the lower bearing lateral processes or ribs. In the higher forms the two portions become united round the notochord, and the resulting vertebra may encroach inwards until it becomes solid, the notochord then remaining only as a series of small pieces of cartilage between the successive units of the vertebral column. There is also, of course, a skeleton in connection with the limbs, but this does not yet correspond in detail to that of the other classes of vertebrates. The brain is much more highly developed than in the round-mouths; in many forms, particularly, there is a considerable development of the cerebral hemispheres, a portion of the fore brain, and the seat of the higher intelligence. The eyes and ears show the same main features as in the higher groups. The ear has three semicircular canals, the same number as in man, as against two in the lamprey and one in the hag. Fishes are possessed of a peculiar 'sixth sense,' the organs for which are situated in two lines running along the sides of the body, the latter forming a familiar feature of a cod or whiting. The nature of this sense is not definitely known, but it appears to be of the nature of a very refined appreciation of wave motions in the water. It is probably by means of these 'lateral line' sense organs, for instance, that fishes are so easily able to avoid obstacles when swimming in the dark.

The heart has one auricle and one ventricle, except in a single group which we shall afterwards mention. The heart is situated immediately behind the gills, to which the blood is pumped directly by the ventricle. From the gills, the blood is collected and distributed throughout the body, is re-collected and returned to the auricle. The circulatory system is provided with a set of blood-glands, essentially similar to those in man himself. There is a spleen, a thymus and a thyroid gland, and a pair of suprarenal bodies. The several functions of these glands form an extremely difficult chapter of physiology, but, broadly speaking, they are concerned in the formation of the white blood corpuscles, the removal of worn-out red corpuscles, and in certain obscure but important chemical changes in the composition of the blood. The blood itself consists of a fluid plasma in which float white and red blood corpuscles, the latter being flat and oval, and containing the same oxygen-carrying substance, haemoglobin, as is found in mammals.

The alimentary canal is simple. The mouth cavity is succeeded by the pharynx, the walls of which are perforated by the gill clefts. Next follow the gullet, the stomach, and the intestine, the division into the three portions being apparent often only after close examination. There are generally gastric glands, of simple form, a large liver, and almost always a pancreas. The kidneys and the reproductive organs open to the exterior by a common duct. A further characteristic feature of the fishes is their external covering of scales. True teeth, comparable to those of the higher vertebrates, appear first in this group. Some of the main features that we have mentioned are illustrated in Fig. 70.

Careful study of the fishes makes it evident that they have very much in common with the higher groups of vertebrates. It is not too much to say, with Haeckel, that there is far more difference between Amphioxus and the fishes than between the fishes and man.

There are four main divisions of the fish group. The first, that of the Elasmobranchs, comprises the sharks and dog-fishes, the skates and the rays. The second group, the Ganoids, includes the sturgeon and a few less well-known forms. The third, the so-called bony or food fishes, includes the vast majority of ordinary species, such as the salmon and trout, the cod, herring, eel, and all our ordinary freshwater species. The fourth, the 'lung fishes,' consists of three very remarkable species, which we shall later describe in detail. The mutual relationships of these groups is well understood, and it is possible to make fairly definite statements regarding their evolution.

The Elasmobranchs are at once the most primitive and, so far as is known, the oldest of the four. From these evolved the lower Ganoids, which then divided into two main branches, the first of which led up to the higher Ganoids and through them, at a comparatively late date, to the bony fishes. The second led to the lung fishes and, either through them or along a somewhat parallel line, to the amphibians and the land vertebrates generally. It is with the second line, therefore, that we shall be mainly concerned.

The Elasmobranchs are characterised by the fact that the gill slits open individually to the exterior, there being no gill cover, such as is found in the other groups. Their scales are simple, tooth-like projections, and in fact there is no essential difference between them and the teeth. The skull is more primitive than in the other groups, but a discussion of its details would necessarily be very involved. The living members of the group show a fairly high stage of development of the vertebrae--considerably higher, in fact, than that found in the lung fishes--but some extinct members showed a very primitive condition with regard to this point. In the fossil skeleton shown in Fig. 71, for instance, it is apparent that the notochord was present as a simple continuous rod. The skeleton in question is from the Permian and belongs to what is regarded as the most primitive type of fish known. Two specimens of Elasmobranchs are shown in Figs. 72 and 73, and the teeth of a shark in Fig. 74.

In the Ganoids and bony fishes there is a gill cover, and in all but a few Ganoids there is some formation of true bone, whereas in the Elasmobranchs the skeleton is wholly cartilaginous. One of the most striking anatomical features of these groups, and one which distinguishes them from the Elasmobranchs, is the presence of a swim bladder, a large sac-like outgrowth from the upper part of the gut. The function of the swim bladder is that of regulating the specific gravity of the fish, which becomes greater or less according as air is expelled or taken in. The Ganoids and bony fishes are illustrated in Figs. 75 to 79.

The lung fishes or Dipnoi present a curious mixture of primitive and of highly advanced characters. In their persistent notochord and their inconsiderable formation of bone, they are much more primitive than the food fishes. On the other hand, an extremely important departure is seen in the adaption of the swim bladder as a respiratory organ. In one of the three existing species this organ is single, in the others it is double. The wall of the swim bladder is thick, and contains considerable muscle tissue. Its inner surface is covered with a complex system of pits and blind sacs, the walls of which contain numerous capillary vessels.

There are three living species of lung fishes, one of which is found in Australia, another in Tropical Africa, and a third in the tributaries of the Amazon. All live under conditions which make ordinary respiration by gills difficult. The Australian species inhabits rivers which become reduced, in the dry season, to stagnant pools of foul water, in which ordinary fish frequently fail to survive. Under such circumstances the creature comes periodically to the surface to breathe. The other two species live in rivers which actually dry up in summer, and the fishes bury themselves in the mud, and remain in a torpid condition, breathing air by their lungs, until the rainy season comes round, perhaps four or even six months later. Correlated with the special method of respiration is a special type of blood system, whereby part of the blood is pumped direct to the lungs, and returns direct to the heart. There are two auricles, to receive the blood from the lungs and the general circulation respectively, but only one ventricle, in which the two streams become mixed. Figs. 80 to 82 illustrate the three existing Dipnoi, and the structure of the lung is shown in Fig. 83.

It is obvious, from the distribution of the lung fishes, and also from geological evidence, that the group was once very plentifully represented, and has only been preserved from total extinction by peculiar circumstances.

Regarding the position of the group, some zoologists regard them as the direct ancestors of the Amphibians. Others believe that the group had a common origin with the bony fishes and the Amphibia in some early form of Ganoid. In any case, the Dipnoi possess an extraordinary interest as showing the beginnings of adaption to a life out of the water.

THE CONQUEST OF THE LAND

The Amphibia are the oldest and the lowest group of vertebrates that are able to lead an active existence on land, and the characters which distinguish them most definitely from the fishes are all to be interpreted as adaptions to the new mode of life. One of the most obvious external differences between the two groups is in the structure of the extremities, the fish having fins, while the amphibian has limbs constructed on the same general lines as our own arms and legs. The fish's fin is to be regarded as an extremity with a very great number of fingers or toes. It has the function of a paddle, and is obviously useless whether for supporting or propelling the body on land. The first obvious necessity for a land existence is some mechanism by which the limb can be alternately pushed forward and, being fixed to some solid object, drawn upon, so as to pull the body after it. A different arrangement of bones and muscles, so as to give a much more complex lever system than that of a fin, and some kind of clawing arrangement at the end, were thus necessary. The similarity in the limbs of all the land vertebrates is very striking, as is indicated by the comparison of human and a frog's limbs on Fig. 84. In each case there is a single bone in the upper arm or thigh, which is attached to a bony girdle in the trunk. There are two elements in the forearm and in the lower leg respectively, below which, in either case, is a group of small bones constituting a complex joint at the wrist or ankle. Then follows the set of five bones in the foot or hand, to each of which is attached a jointed finger or toe. We have no reason to believe that this particular arrangement, and the particular number of digits, was arrived at except by accident. Once arrived at, however, the arrangement was adhered to with considerable strictness. For although the number of digits is in some groups--in the birds especially--reduced, the primary design is almost always readily recognisable. The second function of the limb, that of supporting the body, was developed very slowly. In the amphibians and reptiles, and even in the lower mammals, the legs are comparatively weak and sprawling, and the creature crawls on the belly.

The second great change which required to be made was of course in the method of breathing. An ordinary fish, when taken out of the water, dies of suffocation, because its gills become inefficient for respiration as soon as they become dry. An entirely new type of organ had therefore to be evolved, and this occurred on the same lines as in the Dipnoi, by the development of a pair of sacs from the upper part of the digestive canal, in which the blood is made to circulate, and which are kept filled with air taken direct from the atmosphere. It is of course very well known that an ordinary amphibian is not a lung breather throughout its whole life. The metamorphosis of a gill-breathing tadpole into a lung-breathing frog, illustrated in Fig. 85, is a phenomenon with which everyone is familiar. And this condition, in which a change in the mode of life is made by each individual in the course of its development, is the typical one. But the modern amphibians include types ranging from completely water to perfect land forms. Some, like the Austrian Olm , are gill breathers throughout their whole life. One which is normally of this type, the Axolotyl, illustrated in Fig. 87, can be made to acquire lungs and assume a land mode of life. Others, which normally make the metamorphosis, can be prevented from doing so by being confined to the water, and complete their life-histories in the condition of gill breathers. In still other forms the change is made before the young creature leaves the egg, and the independent life is commenced in the condition of a land animal.

Correlated with the development of the lungs is a change in the structure of the nostrils, from the condition of blind sacs, as they occur in the fishes, to that of air passages, communicating with the upper part of the alimentary canal, and thence with the lungs.

The living forms of the Amphibia differ considerably from the types which constituted the group in those long-distant ages when it was in the heyday of its prosperity. The latter forms were characterised especially by a system of armour-plating over the head, which frequently extended under the breast and even covered the greater part of the lower surface, and which appears to have formed a protection against the multitude of sharks which populated the waters in which the amphibians partly lived. The armour-plated type has long ago become extinct, but it is in it, rather than in the modern forms of Amphibia, that we must look for the direct ancestor of man. A fossil of this earlier group is shown on Fig. 90.

Our next group in the order of Evolution is that of the reptiles, the main differences between which and the Amphibia are of the nature of more complete adaptions to a life on land. The reptiles have in fact completed the conquest of the land which was undertaken by the previous group, and many of them, living as they do in dry and hot deserts, are as independent of the water as any form of animal life. Thus whereas the amphibian has a thin skin, which is kept moist by the secretions of numerous skin glands, the reptile has a body-covering of scales, which form an effective protection against a too rapid loss of moisture. Evidently with the same object, the reptile egg is enclosed in a hard and resistant shell. Correlated with the change in the skin is a much more perfect development of the lungs, for while the amphibian breathes to a considerable extent through its thin moist skin, this method of assisting respiration is not available to the reptile.

Again connected with the improvement of the respiratory process, there is a partial development of a septum dividing the ventricle or pumping chamber of the heart. The value of this division of course lies in the fact that the purified and oxygenated blood from the lungs is prevented from mixing with the venous blood from the body. The course of the blood is from the body to the right auricle, thence to the right ventricle, and thence to the lungs. The pure blood from the lungs returns to the left auricle, passes thence to the ventricle on the same side to be pumped to the general circulation. The disadvantage of a single ventricle, such as occurs in the Amphibia, and the advantage of the regular double circulation, such as that in man, are sufficiently obvious. The division of the ventricle into two chambers is less complete in the lizards and snakes, very nearly perfect in the crocodiles.

The reptiles, like the Amphibia, are 'cold blooded,' by which is meant, not that their blood is necessarily cold, but that its temperature varies with that of the surroundings, while that of the blood of the mammals and birds is practically constant.

A very important feature of the reptiles, which they possess in common with the mammals and the birds, is that the embryo produces two membranous outgrowths called respectively the amnion and the allantois, which completely envelop it, and which have important functions in connection with nutrition, respiration, and excretion during the period when the young creature is enclosed in the egg. It is, of course, not until we reach the higher mammals that these membranes assume their greatest importance.

For a considerable time in the world's history the reptiles were the dominant vertebrate class, and in the chalk period especially they were represented by a great variety of forms, and by a number of species of colossal stature, one at least of which was over a hundred feet long. In those times the reptiles were by no means all condemned to crawl on their bellies, for they included a large number of marine forms, comparable to the porpoises and whales among the mammals, and flying forms whose aspect must have resembled, and been equally terrifying with, that of our mythical dragons. A few reconstructions of these are shown in Fig. 91.

All living reptiles, with a single exception, belong to the four comparatively modern types of the lizards, snakes, tortoises, and crocodiles, of which examples are illustrated in Figs. 92 to 95. None of these are closely related to the mammals or birds. For the common ancestor of all these types we must go back to some primitive reptile form. Fortunately such a type is represented at the present day in a single peculiar species found in New Zealand, which bears the Maori name of the Tuatara. It was formerly found commonly on the mainland, but is now confined to a few small islands in the Bay of Plenty, North Island, where it enjoys Government protection. It is, as the illustration in Fig. 96 shows, a lizard-like creature, and reaches a length of about two feet. It lives in burrows near the shore, and feeds on small animals that are left behind by the tide. The Sphenodon, as zoologists have named it, has apparently been preserved owing to the absence of competition by the mammals, and by adopting the rather curious mode of life just described. In all its features, but especially in the primitive condition of its vertebrae, it is very much lower than any other living reptile, and it connects the higher groups with the Amphibia. Many closely related fossil species are known, one of which is shown in Fig. 97.

To the lay mind the distinctions between the Amphibia and the reptiles are not very obvious, and indeed in the older classifications the former group was not separated from the latter. The differences between a reptile and a bird, on the other hand, are very striking. It might therefore be regarded as a matter for surprise that zoologists now make the greater distinction between the Amphibia and the reptiles, grouping the former in one great class with the fishes, the latter in a second great section with the birds. But in fact there are many fundamental points of agreement between reptiles and birds, and it is impossible to doubt that the latter have sprung from a reptilian stock. Indeed, a most interesting connecting link is known, in the fossil Archiopteryx shown in Fig. 98, of which only two specimens have been found, and which is the only creature of its type of which we have any record. In all its skeletal features, the Archiopteryx is reptilian, and it would undoubtedly have been classed as a new type of reptile but for the obvious and unmistakable traces of feathers. From what particular class of reptiles the birds have sprung is not known.

The birds have assumed the position of almost unquestioned masters of the air, but like other great groups they show possibilities of evolution in other directions wherever opportunity offers, and types like the kiwi and the penguin shown in Figs. 100 and 101 have forsaken their native element--the one for the land, the other for the water.

The birds agree with the mammals in the development of a four-chambered heart, in their warm blood, in their external covering for the skin, and in the development of arrangements and instincts for the parental care of the young. Their line of Evolution has thus been to some extent parallel to that of the mammals. On the other hand, they differ obviously in the structure and function of their fore limbs, in the absence of a diaphragm, and in their special methods for the care of the young, and there can be no doubt that the two groups have had quite different origins in the reptile class.

THE MAMMALS AND MAN

The subject of our discussion is now narrowed down to the group of the mammals. The mammals are characterised by two very obvious features: a body-covering of hair, and a set of special glands in the female which secrete milk for the nourishment of the young. These are constant characters, and neither is ever found in any other group. As to how the hair originated, nothing definite is known; but it is on the whole reasonable to suppose that the mammalian hair arose, as the bird's feather undoubtedly did, as a modification of the reptile's scale. The mammary gland appears to represent a modification of other skin glands, either of sweat glands or more probably of the oil glands which exist in connection with the hairs.

Very characteristic of the mammals are, further, their teeth, for whereas the teeth of the reptiles are indefinite in number, and generally very numerous, those of the mammal are relatively few, and each species has a definite normal number. Moreover, the teeth of the reptile are all of a kind, and they may be replaced almost any number of times during the animal's life, whereas those of the mammal show differentiation according to their respective functions, and are only once changed, that is, when the milk teeth are replaced by the permanent set. The teeth of the mammal are of four kinds: incisors, or chisel-like cutting teeth, canines, which are especially well developed in carnivorous species, and which are used in the tearing up of flesh, etc., and two groups of grinders--premolars, which are replaced during the animal's life, and molars, which occur only as permanent teeth.

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