EATRACHIA. 77 



shell ; a naked skin invests their body,(l) and, one genus ex- 

 cepted, they have no nails. 



The envelope of the ova is membranous, and, in many 

 species, they are only fecundated at the moment of their ex- 

 pulsion. 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 subse- 

 quently 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 largej 

 the tongue, in most of them, is soft, and not attached to the bot- 

 tom of the gullet, but to the edges of the jaw, and folds inwards. 

 There are but four toes to the anterior feetj 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 dilating, 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 abdomen: thus if we open the belly of one of 

 these animals while alive, the lungs dilate without being able to con- 

 tract, 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 

 furnished with a spongy enlargement which increases during the 

 nuptial season and assists in attaching him to the female. He fecun- 

 dates the ovum at the moment of its expulsion. The little animal 

 that is produced from it, called a Tadpole, 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 Swammerdam assures us that this is owing 



(1) 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 adhered to its back. Schn. Hist., Amphib. Fasc. I, p. 168. 



