REPTILES belong to the great division of vertebral animals which have coup iilmoo. They differ in ap pearance from quadrupeds in the absence of hair, and from birds in being destitute of feathers, and from both in the heart not being quadrilocular, nor the blood warm. They agree with fishes in having their body either na ked, or covered with scales—in the temperature of their body corresponding with that of the medium in which they reside—in the brain not filling the cavity of the cranium—and in the cerebellum being destitute of the arbor vitm. They differ from fishes in the nature of their circulation. In fishes, the whole circulating fluid is poured into a pulmonic heart, consisting of a single auricle and ventricle, through which it is conveyed to the gills. The whole blood, thus xrated, is taken up in the gills by vessels which serve the place of the pul monary veins in quadrupeds, and which carry the red blood directly to all parts of the body, without the inter vention of a systemic auricle or ventricle. In reptiles, on the other hand, the blood collected from different parts of the body is again transmitted through a syste mic auricle and ventricle, and a part only of this fluid is conveyed to the lungs or gills. This xrated blood, in returning from the gills or lungs, is either conveyed di rectly to the branches of the aorta, to mix with the cir :ulating mass, or, after being collected into a separate auricle, enters the common ventricle. The partitions
which occur in the common ventricle probably prevent the blood, already xrated, from being again, even in the smallest quantity, returned along with the limited portion to the lungs or gills.
The differences which prevail between reptiles and fishes are not greater than those which characterize the different groups into which the former may be di vided. In one group, including the frogs, the heart is of the most simple kind, and possesses only one auricle. In the other, the heart possesses two auricles. Among these the turtles are enveloped in an osseous shield, which is wanting in the lizards and serpents. The lizards aye furnished with two or four feet, with which they exe cute progressive motion, while the serpents are truly apodal.
In the article HERPETOLOGY, the characters of three of these groups (viz. the Chelonian, including turtles— the Sautian, including lizards—and the 13atracian, in cluding frogs) have been given in considerable detail. The fourth group, containing the serpents, was there omitted, and a reference made to the present article, in which their peculiar structure and classification fall to be illustrated.