194 REPTILIA. 



which nourish the body, and even those which conduct the blood to 

 be oxygenated in the lungs. 



In those species, however, which lose their branchiae, the attend- 

 ant arteries are obliterated, with the exception of two, which unite 

 in a dorsal artery, giving, each, a small branch to the lungs. It is 

 the circulation of a Fish metamorphosed into that of a Reptile. 

 Batrachians have neither scales nor shell; a naked skin invests their 

 body, and, one genus excepted, they have no nails. 



The envelope of the ova is membranous. These eggs become 

 greatly enlarged in the water. The young do not only differ from 

 the adult in the presence of the branchiae; their feet are developed 

 by degrees, and in several species there are a beak and tail, which 

 they subsequently lose, and intestines of a different form. 



Some species are viviparous. 



Rana, Lin. 



Frogs have four legs in their perfect state, but no taU. Their head is flat, 

 muzzle rounded, and the opening' of their jaws large; the tongue, in most of 

 them, is soft, and not attached to the bottom of the gullet, but to the edges 

 of the jaw, and folds inwards. There are but four toes to the anterior feet; 

 the hind ones frequently exhibit the rudiment of a sixth. 



There are no ribs to their skeleton, and a prominent cartilaginous plate 

 supplies the place of a tympanum, and renders the ear visible externally. 

 The eye is furnished with two fleshy lids, and a third, which is transparent 

 and horizontal, concealed under the lower one. 



The hind feet of the Tadpole are very gradually and visibly developed; 

 the fore feet are also developed, but under the skin, through which they 

 subsequently penetrate. The tad is gradually absorbed. The beak faUs 

 and discloses the true jaws, which at first were soft and concealed beneath 

 the skin; and the branchiae are annihilated, leaving to the lungs alone the 

 function of respiration in which they participated. The eyes which at first 

 could only be discerned through a transparent spot in the skin of the Tad- 

 pole, are now visible with their three lids. Tadpoles reproduce their limbs 

 almost Tike Salamanders. 



The period at which each of these changes takes place varies with the 

 species. 



In cold and temperate cUmates, the perfect animal passes the winter un- 

 der ground, or in the mud under water, without eating or breathing, though 

 if we prevent it from respiring during the summer for a few minutes by 

 keeping its mouth open, it dies. 



HxLA, Laur. 

 Tree-Frogs only differ from Frogs in the extremities of their toes, each of 

 which is expanded into around, viscous peUet, that enables them to adhere 



