Characters of A mphibians. 3 5 



toe-webs of some salamanders. They also undergo 

 regular metamorphoses, beginning life as little fish- 

 like creatures with large flat heads and external gills. 

 To this stage the name tadpole is commonly given. 

 Then, as development progresses, the air sacs (which 

 correspond to the swimming-bladder in fishes) grow, 

 become large, vascular, and capable of acting as 



FIG. 15. 



Skeleton of frog. 



*, skull ; Z>, vertebrae ; c, sacrum ; d, ilium ; e, urostyle ; /, suprascapula ; 

 g, humerus ; h, fore-arm bones ; /, wrist bones ; m, thigh bone ; n, leg 

 bones ; o, elongated first pair of ankle bones ; /, q> foot bones. 



breathing organs, which are then called lungs ; and 

 ultimately, in the adult state, a pulmonary or direct 

 air-breathing system supplants the gill or branchial 

 system of earlier life. The two-chambered larval 

 heart at the same time becomes three-chambered, 

 developing a special auricle in the left side for the 

 reception of the blood which has been purified in 



