STRUCTURE OF TURTLES AND LIZARDS.

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formed by an expansion of the sternum or breast-bone, which is spread out sideways, instead of being raised into a projecting keel as in Birds. The carapace and plastron are covered with large horny plates, variously arranged in the dif ferent species, and constituting what is commonly called tortoise-shell. These

plates are often very beautifully disposed, forming a kind of tesselated pavement; as in the common Tortoise (fig. 32), which is often preserved alive in our gardens.

Fig. 32.-TORTOISE.

84. In the tribe of Lizards, the body has no such covering; but these animals, having more activity than the tortoises (which are proverbially slow), are enabled to make their escape from danger, whilst the latter are obliged to trust to their bony casing for protection from it. In their general form, Lizards approach Mammals, being four-footed, and living for the most part on land; but they differ from them not only in their essential reptilian characters, but also in several others of less consequence. Their bodies are usually covered with scales, which lap over one another like the tiles of a roof; but in the Crocodile tribe, many parts of

Fig. 33.-CROCODILE.

the surface are covered with large knotted horny plates, that meet at their edges like the scales of tortoise-shell, and afford an almost impenetrable covering. Although some of the Lizard tribe spend a large part of their time in water, they all breathe air; but, as their respiration is very inactive, they can remain for long periods beneath the surface, without being obliged to come up to breathe.

85. The tribe of Serpents may be regarded as lizards without feet; their spinal column is immensely prolonged; and

96

STRUCTURE OF SERPENTS.

their ribs are also very numerous; and they are able to crawl upon the points of these, using them almost as Centipedes do their legs (fig.42). But in general the movement of their

bodies is accomplished by their being drawn-up into folds. and then straightened so as to project the head. The prolonged form of the body in Serpents occasions a curious. variation in the arrangement of the principal organs, which is shown in the accompanying figure. The oesophagus or

STRUCTURE OF SERPENTS AND BATRACHIA.

97

gullet, œ, which leads from the mouth to the stomach, is a long and very wide canal, being even larger than the stomach at its commencement; a portion of it is removed at œ', in order to show the heart, &c., which would otherwise be concealed by it. The stomach, i, is long and narrow; and the intestinal tube, i, after making a few turns or convolutions, passes backwards in a straight line, to terminate in the cloaca, cl, which opens externally by the orifice, an. The liver, f, is also much lengthened. From the mouth also proceeds the long windpipe, tt, which conveys air to the lungs, or rather to the single lung; for the lung on the left side, p', is scarcely at all developed, whilst that on the right, p, extends along a great part of the body. At o is seen the ovarium, in which the eggs, o'o', are produced; and this also is very much lengthened, extending from the cloaca a good way up the body, so as nearly to meet the lung. The other references are to the parts of the heart, and the principal vessels; the structure and arrangement of which will be explained hereafter (§ 284).

86. The Batrachia, or animals of the Frog tribe, are readily distinguished from all the preceding, by their soft naked skins; even when the form of the body, as in the common Salamander or Water-Newt, resembles that of the lizards. They are also remarkable for the metamorphosis which they undergo in the early part of their lives; for they come forth from the egg in a condition which is, in all essential particulars, that of a fish, and undergo a gradual series of changes, by which their form and structure become assimilated to those of the true reptiles. This change is most complete in the Frogs and Toads; the early form of which is known as the tadpole. The principal stages of this change are represented in figs. 35 to 39; in which, however, the relative sizes are not preserved, the tadpoles being much larger in proportion (for the sake of displaying their form and the gradual development of their legs) than the complete frog. Soon after the young tadpole has come forth from the egg, it presents the form which is shown in fig. 35; its head and trunk are large, and the latter is prolonged into a flattened tail, by which the little animal swims freely through the water. There is not the least appearance of limbs or members. It breathes by gills, which are long fringes, hanging

H

98

METAMORPHOSIS OF BATRACHIA.

loosely in the water on either side of the head. At a later period, however, these gills, which are merely temporary, disappear; and the breathing is carried on by another set, which are situated behind the head, and are covered in by a fold of skin; the water gains access to these by passing through the mouth, exactly as in Fishes. The form is then that which is represented in fig. 36. In a short time afterwards, the animal still breathing by its gills, the hind-legs begin to sprout forth, as it were, at the base of the tail;

this

stage is shown in fig. 37. At a still later period, the forelegs begin to be developed, as seen in fig. 38; and from that time they are nourished at the expense of the tail, which gradually disappears, as seen in fig. 39, a, b. During this period, other important changes are taking place in the interior of the body; the chief of which are the development of the lungs and the gradual disuse of the gills, so that the animal becomes fitted to live on land and breathe air, and is no longer capable of remaining long under water without coming to the surface to respire.

87. The metamorphosis in other members of the group is

PERENNIBRANCHIATE BATRACHIA.

99

less complete than in the Frog, being checked at a less advanced stage. Thus in the common Water-Newt, the tail is retained during the whole of life, and the animal continues to be an inhabitant of the water, though breathing air alone. There are some very curious animals, however, in which the change is stopped, as it were, at a much earlier period, so that the gills also are retained; and in these, the lungs are sufficiently developed to enable the animals to breathe air, so that they can live either on land or in water. Such Batrachia are scientifically known as perennibranchiate, this term expressing the persistency of their gills. In fig. 40 is represented

an animal of this kind, the Axolotl, which inhabits some of the lakes of Mexico. And in fig. 41 is shown the form of a still more remarkable animal, the Lepidosiren, or mud-fish, recently

brought from the rivers of Africa, the metamorphosis of which appears to be checked at a still earlier period, so that it is very difficult to decide whether it should be regarded as