00 REPTILES. 



which form it, that arise most of the arteries which nourish the body, and 

 even those which conduct the blood to be respired in the lungs. 



In those species, however, which lose their branchiae, the arterial branches 

 distributed through them become obliterated, with the exception of two, 

 which, by their union, form 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 to their toes. 



The envelope of the eggs is simply membranous ; the female is im- 

 pelled by the male to lay them, and, in many species, they are only fe- 

 cundated at the moment of their expulsion. These eggs become greatly 

 enlarged in the water after being laid. 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 tail. 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. 



Inspiration is solely effected by the muscles of the throat, which, by di- 

 lating, receive air from the nostrils, and by contracting while the nostrils 

 are closed by the tongue, compel that air to enter the lungs. Expiration, 

 on the contrary, is produced by the muscles of the lower part of the ab- 

 domen: thus, if we open the belly of one of these animals while alive, the 

 lungs dilate without being able to contract, and if we force another to keep 

 its mouth open, asphyxia is the consequence, as it is no longer able to 

 renew the air in its lungs. 



The embraces of the male are long continued. His thumbs are fur- 

 nished with a spongy enlargement, which increases during the nuptial 

 season, and assists in attaching him to the female. He fecundates the 

 ovum at the moment of its expulsion. The little animal that is produced 

 from it is called a Tadpole; it is at first furnished with a long fleshy tail, 

 and a small horny beak, having no other apparent limbs than little fringes 

 on the sides of the neck. In a few days these disappear, and Swammer- 

 dam assures us that this is owing to their withdrawing under the skin, to 



* M. Schneider has proved that the Scaly Frog of Walbaum only appeared so from 

 accident, a few scales from some Lizards that were kept in the same jar having ad- 

 hered to its hack. Sehn. Hist. Amphib. Fasc. 1, p. lfiS. 



