CHAPTER XXI. 

 CLASS II. AMPHIBIA (FROGS, TOADS, SALAMANDERS). 



387. The amphibians are especially interesting to the zoolo- 

 gist because they begin life as gill-breathers (tadpoles), and 

 later they replace the gills by lungs. The fact that the amphi- 

 bian in its individual life passes from a fish-like condition to 

 the form and habits of the higher air-breathing vertebrates is 

 taken as evidence that the higher vertebrates have sprung from 

 fish-like ancestors through forms similar to the amphibians. 

 The change from gills to lungs is not equally striking in all 

 the members of the group. The transition from water to air 

 involves important changes in the problem of physical sup- 

 port, of locomotion, and of respiration, and in consequence, of 

 the organs performing these functions, as well as correlated 

 changes in the integument and in the organs of circulation. 

 The amphibia were much more abundant in earlier geological 

 times than at present, and attained huge size, whereas the 

 modern forms, with a very few exceptions, are small. There 

 are about nine hundred living species. The tailless types 

 (frogs and toads) are much the more numerous, as well as 



more highly developed. 



/ 



388. General Characters. 



1. Amphibia are Vertebrata which possess gills during the 

 larval stage and lungs in the adult ; in some instances the gills 

 are retained throughout life. 



2. Paired appendages, when present, conform to the general 

 vertebrate type; i. e., limbs with digits (typically five), instead 

 of fins. 



3. Exoskeleton of scales and plates absent; skin glandular. 



4. Heart is three-chambered ; two auricles and one ventricle. 



5. A renal-portal and hepatic-portal circulation present. 

 Red corpuscles are nucleated. 



372 



