90 CLASSES OF VERTEBRATA :-MAMMALS. with one exception, they have all red blood (§ 226); and that they possess a complex apparatus for circulating this through the body. 76. The four principal modifications under which the Vertebrated type presents itself, constituting the classes of MAMMALS, BIRDS, REPTILES, and FISHES, are respectively characterised by the mode in which the principal functions of life are performed in each.* Thus there are some Vertebrated animals which produce their young alive, and which nourish them afterwards by suckling; while the greater part rear them from eggs which contain a store of nutritive matter, and do not afford them any further nourishment from their own bodies. Again, some breathe air; whilst others live constantly in water, and have no direct communication with the atmosphere. Some, moreover, have the power of keeping up a high temperature, so that their bodies always feel warm to the touch; whilst the temperature of others varies with that of the atmosphere, so that their bodies give a feeling of coldness the former are termed warm-blooded-the latter cold-blooded. There is a like difference in their mode of life; some of them being destined to live on the surface of the earth, whilst others are chiefly inhabitants of the air, and others again are the tenants of the ocean. 77. MAMMALS are distinguished from all other Vertebrata by the first of the characters just adverted to; being the only animals that produce their young alive, and nourish them afterwards by suckling. Like Birds and Reptiles, they breathe air by means of lungs; and, in common with Birds, they are warm-blooded and have a complete double circulation of their blood, carried on by a heart with four cavities. They are for the most part quadruped (that is, four-footed), and are destined to live upon the surface of the earth; but Man, and the Apes that approach nearest to him, are biped, having the power of walking on two limbs, and of using the others for different purposes; whilst the Bat tribe have the two arms converted into wings, which enable them to fly through the air like birds (for which the older naturalists mistook Many Zoologists range the Frogs and their allies in a separate class, under the name of AMPHIBIA; but when looked at from a physiological point of view, the author does not see that they require to be separated from the true Reptiles. GENERAL STRUCTURE OF MAMMALS. 91 them); and the Whale tribe are adapted in their general form to lead the life of fishes (among which they are still commonly ranked by persons ignorant of natural history). Notwithstanding these marked differences in external form, there is a great correspondence as to internal structure; for bats and whales, as well as ordinary quadrupeds, produce their young alive, and suckle them afterwards; they are also warm-blooded, breathing air, and having an active circulation. The bodies of Mammals are, for the most part, more or less completely covered with hair, which serves to keep in their warmth; and this is seldom absent, except in such as inhabit warm climates and do not require this provision. In the Whales, the same end is answered by the thick layer of oil in the substance of the skin, constituting the blubber; and Man is left to form a protective covering for his body by the exercise of his own ingenuity. The general arrangement of the internal organs of Mammals will be seen from the accompanying figure of the body of a Monkey, laid open in such 92 GENERAL STRUCTURE OF BIRDS. a manner as to exhibit the chief of them. The cavity of the trunk is completely divided, by the muscular partition termed the diaphragm, into two portions-the thorax, containing the heart and lungs; and the abdomen, containing the digestive apparatus. It is chiefly by the alternate contraction and relaxation of this muscle, that the act of breathing is performed in Mammals, as will be explained hereafter (§ 331). 78. In BIRDS there is a much closer conformity to one general plan than we find among Mammals. The covering of feathers, by which we ordinarily distinguish the members of this class, prevails universally; and there is no wide departure from the typical form. This class belongs to the oviparous division of the Vertebrata; the young being reared from eggs. But it is distinguished from Reptiles, which are also oviparous and air-breathing, by being warm-blooded; and by having a very energetic instead of a very slow circulation. The warmth of the maternal body, moreover, is imparted to the egg in the act of incubation; and without the heat thus communicated (unless it be supplied from some other source) the embryo cannot be developed. The covering of feathers is given, not only to keep in the heat of the body, which is even greater than that of Mammals, but also to afford the required surface for the wings, on which the Bird is supported and propelled through the air. The feathered portion of the wings is stretched out upon the bones which answer to those of our arm, and is moved by its muscles. The wings are very small, or are entirely absent, in the Ostrich and a few other birds, which present the nearest approach to the Mammalia in their internal structure; and these cannot rise from the ground, but run swiftly along it, by means of their powerful legs. In the Penguin, also, the wings are small; and they are used as fins, by the assistance of which this bird, which can neither walk nor fly with rapidity, can swim very quickly through the water. 79. Generally speaking, Birds are characterized by the extraordinary power of motion which they possess, and by the great acuteness of the sense of sight, by which their movements are chiefly directed. They are also remarkable for their instinctive actions, which are chiefly related to their care of their young, for whom they usually construct a protective GENERAL STRUCTURE OF BIRDS. 93 nest. The nutritive functions are performed with extraordinary activity in Birds, that the means may be supplied for the maintenance of their locomotive activity. Their blood is particularly rich in red particles, and its heat is usually considerably above that of Mammals. Its circulation is very energetically carried on; and although the lungs themselves are constructed upon a type inferior to that of Mammals, and the mechanism of respiration is less complete, yet, by an extension of the respiratory organs through the whole fabric, the aeration of the blood is carried on with unequalled energy (§ 326). 80. The arrangement of the organs contained in the cavity of the trunk of Birds differs from that which has been described in Mammals, chiefly in this, that there is usually Fig. 31.-LUNGS OF THE OSTRICH. the heart; b, the stomach; cc, the intestines; d, the trachea; e, the lungs; fff, air-cells, in which are also seen the tubes by which these aircells communicate with the lungs. the lungs and with each other, and which even occupy a large part of the cavity of the abdomen, as seen in the figure. 81. In the class of REPTILES we find a variety of form so remarkable, that, if we were influenced by this alone, we should scarcely regard the animals it contains as belonging to 94 GENERAL STRUCTURE OF REPTILES. the same group; yet the structure of the internal organs, on which classification is founded, is essentially alike in all; and their physiological condition presents no important dissimilarity. Four obviously different tribes, Turtles, Lizards, Serpents, and Frogs, are brought together by the following characters. They are all oviparous, in this respect agreeing with Birds and Fishes; but they are cold-blooded, and have not a complete apparatus for the double circulation of the blood, in which respect they differ from Birds; and they breathe air by means of lungs, instead of breathing water by gills, in which respect they differ from Fishes. But by the lowest group, that of Frogs and their allies, this class is united to that of Fishes in a most remarkable manner; for these animals in their young state breathe by gills, and lead the life of a fish; and some of them retain their gills during the whole of life, even after the lungs are developed (§ 87). The first three of the tribes just mentioned undergo no such change and they further agree in this, that they breathe air during the whole of their lives, coming forth from the egg in the same condition as that in which they are subsequently to live, and also in having their bodies covered with horny scales or plates, whilst the skin of the Frog tribe is soft and unprotected. 82. The class of Reptiles presents a marked contrast to that of Birds, in the comparative slowness and feebleness of its movements, the dulness of its sensibility, and the inactivity of its organic functions. As there is no fixed temperature to be maintained, one important source of demand for food is withdrawn; and when not excited to activity by external warmth, these animals may pass long periods without fresh supplies of food. Their blood is very poor in red corpuscles, and its circulation is comparatively languid. A reduction of the temperature of their bodies to within a few degrees of freezing point, induces complete torpidity, which continues until they are roused by a renewal of warmth. 83. The Turtle tribe is peculiarly distinguished by the inclosure of the body in a bony covering; of which the upper arched portion (termed the carapace) is formed by the coalescence of the ribs with a set of bony plates developed in the substance of the skin; whilst the lower flat plate (termed the plastron), which is often incomplete, is |