The class Reptilia includes the lizards, snakes, tortoises, turtles, crocodiles, and alligators. Although popularly associated in the common mind with the batrachians, the reptiles are really more nearly related to the birds than to the salamanders and frogs. In general shape they more nearly resemble the batrachians, but in the structural condition of the internal body organs they are more like the birds. They are cold-blooded, and breathe exclusively by means of lungs, the forms which live in water coming to the surface to breathe. They are covered with horny scales or plates, which with the entire absence of gills after hatching readily distinguish them from all the batrachians. While most reptiles live on land, some inhabit fresh water and some the ocean. As the young have the same habitat and general habits as the adult, there is no such metamorphosis in their life-history as is shown by the batrachians. The reptiles are widespread geographically, occurring, however, in greatest abundance in tropical regions, and being wholly absent from the Arctic zone. They are not capable of such migrations as are accomplished by the birds and many mammals, but withstand severely hot or cold seasons by passing into a state of suspended animation or seasonal sleep or torpor. [Illustration: FIG. 123.--A lizard in the grass. (Photograph from life by Cherry Kearton; permission of Cassell & Co.)] =Body form and organization.=--The chief variations in body form among the reptiles are manifest when a turtle, lizard, and snake are compared. In the turtles, the body is short, flattened, and heavy, and provided always with four limbs, each terminating in a five-toed foot; in the lizards the body is more elongate and with usually four legs, but sometimes with two only, or even none at all; while in the snakes the long, slender, cylindrical body is legless or at most has mere rudiments of the hinder limbs. With the reptiles locomotion is as often effected by the bending or serpentine movements of the trunk as by the use of legs. Among lizards and snakes the body is covered with horny epidermal scales or plates, while among the turtles and crocodiles there may be, in addition to the epidermal plates, a real deposit of bone in the skin whereby the effectiveness of the armor is increased. The epidermal covering of snakes and lizards is periodically molted, or, as we say, the skin is shed. The bright colors and patterns of snakes and of many lizards are due to the presence and arrangement of pigment-cells in the skin. Among some reptiles, notably the chameleons, the colors and markings can be quickly and radically changed by an automatic change in the tension of the skin.