336 SYSTEMATIC ZOOLOGY. 



supported by a peculiar skeleton, while the skull shows 

 many strange features. The lung-fishes present many 

 points of interest for the naturalist. By many they are 

 supposed to be nearest to the line from which the Amphibia 

 have sprung. 



CLASS II. AMPHIBIA (BATRACHIA). 



The frog may serve as a type of the Amphibia, which, 

 so far as living representatives are concerned, are marked 

 off from the fishes by a number of important characters. 

 With very few exceptions the Amphibia pass at least a 

 part of their life in the water, and many, in reaching the 

 adult condition, pass through great changes in structure 

 (all are familiar with the metamorphosis of the tadpole 

 into the frog), so that, in considering the group, the char- 

 acters of both larva and adult must be taken into account. 



In all the skin is very glandular and in all, except the 

 tropical group of blindworms, scales are lacking, and, ex- 

 cepting again these same limbless forms, fins have given 

 place to legs, much like the limbs of man, and like them 

 ending typically with five digits. In the larvae of all there 

 is a tail, and some (salamanders and newts) retain this 

 structure during life, while in others, as in the frog, 

 it is absorbed (not dropped off) during growth. The lar- 

 val tail bears a median fin, but this is never divided into 

 dorsal, caudal, and anal, and it differs further from the 

 fins of fishes in having no internal skeleton. 



Of internal features those most distinctive are the 

 skeleton of the limbs, unlike that occurring in any fish; 

 the union of the pelvic girdle with the back-bone; the 

 existence of an Eustachian tube in connection with the 

 ear; the connection of the nostrils with the caAdty of 

 the mouth; and the presence of two auricles in the heart. 



