CHAPTER XXI 



CLASS II. AMPHIBIA (FROGS, TOADS, SALAMANDERS) 



394. 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. This is the meaning of the name. 

 The fact that the amphibian 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 ver- 

 tebrates 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 support, 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 nearly two thousand living species. The tailless types 

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

 highly developed. 



395. General Characters. 



1. Amphibia are Vertebrata which possess gills during the 

 larval stage and usually 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. 



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