The nervous system may be worked out in a specimen which has been immersed in 20 per cent nitric acid. The description of the nervous system of the toad (see pp. [12]-[13]) will suffice for a guide to the study of the nervous system of the snake. The special sense organs, as eyes and ears, should be examined and compared with those of the fish and toad.
Life-history and habits.—The garter snakes are more or less aquatic in habit and are good swimmers. They are often found far from water, but in greatest abundance where the cat-tails and rushes grow thickest. They feed on frogs, salamanders, and field-mice, which they swallow whole. All the garter snakes are ovoviviparous, i.e., hatch eggs within the body-cavity. The eggs, often as many as eighteen or twenty, are enclosed within widened portions of the oviducts during embryonic existence; when the young are born they are able to shift for themselves. During cold weather the garter snake hibernates, hiding then in some gopher-hole, or, in the warmer climates, under some log or stone, there to lie dormant until the warm days of spring come, when it resumes activity.
The garter snake sheds its skin at least once a year, sometimes oftener. This process may be observed in snakes kept in confinement. For some time before molting the animal remains torpid, the eyes become milky, and the skin loses its lustre. After a few days it conceals itself, the skin about the lips and snout pulls away and the animal slips out of its entire skin. The snake not only sheds the skin of the body but also the covering of the eyes. Snakes have no eyelids, as we have already noted, that which represents the eyelid being a transparent membrane which covers the eyeball.
No species of the garter snake group is poisonous. Sometimes a garter snake may appear to be vicious, but its teeth are very short and at best it can only make a small scratch scarcely piercing the skin.
Fig. 122.—Dissection of the garter snake, Thamnophis sp.
OTHER REPTILES.
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.