60 VERTEBRATA. REPTILIA, 



ORDER IV. BATRACHIA.* 



IN these the heart has but two cavities, distinguish- 

 ing them from the other Reptilia, in which it has 

 three : the lungs are equal. They are covered with 

 a naked skin, destitute of all scales or shell. But 

 they are particularly remarkable for the import- 

 ant alteration which in most cases takes place 

 in their structure at a certain period of their life. 

 The eggs are invariably committed to the water, 

 where the young animals are hatched under the 

 semblance of fishes, and that not merely in outward 

 form, but in the essential condition of their exist- 

 ence. Unlike the adult from which it is descended 

 the tadpole breathes not by lungs but by gills, re- 

 spiring water instead of air. It possesses a long 

 and muscular tail, by the flexure of which it swims 

 rapidly as fishes do : it has no limbs. It feeds 

 chiefly on vegetable matters in a state of decom- 

 position. After living in this fish-like state for 

 some weeks, the gills become gradually absorbed, 

 the lungs begin to appear, the hind legs form, and 

 soon the fore ones, the tail (in some instances) be- 

 comes shorter and finally vanishes, and thus the 

 fish becomes a reptile, capable of breathing the air 

 and of capturing living prey. Some of the lowest 



Ji latrwlios, a frog. 



