CHAPTER XXI 



CLASS AMPHIBIA (Progs, Toads, Salamanders) 



394. The amphibians are especially interesting to the zoolo- 

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

 they usually 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 vertebrates have sprung from fish-like ancestors through 

 forms similar to the amphibians. The change from giUs to 

 lungs is not equally complete or 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 

 df 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. Vertebrata possessing gills during the larval stage and 

 usually lungs in the adult; in some instances gills retained 

 throughout life. 



2. Paired appendages when present conforming to the gen- 

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

 instead of fins. 



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



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



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

 Red corpuscles. nucleated. 



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