CHAPTER V 



AMPHIBIA 



The Class Amphibia consists of those four-footed animals which 

 have a double life, one adapted for water living in the larval 

 stage, and an adult phase when they take on the characteristics 

 of land animals. The newts, salamanders, frogs and toads are 

 representative of the class. The living examples make a very 

 homogeneous group, although the earliest forms resembled the 

 Crossopterygii on the one hand, and the reptiles on the other. 

 They are in every respect more closely allied to the fish than to 

 the reptiles. The most important difference from other land 

 animals lies in their embryology, the eggs always being laid in 

 water or moist places, where they hatch as gilled larvae. 



The first amphibia varied from their fish ancestors in several 

 respects, most largely in the development of four walking ap- 

 pendages with toes. The larva retained the external gills and an 

 internal anatomy so like their ancestors that the tadpole is es- 

 sentially a fish in structure and mode of life. At metamorphosis 

 the early amphibia evidently lost their gills and breathed with 

 lungs, but the fossil evidence indicates that they remained mud- 

 living animals. The bones of the skull and the dermal covering 

 of the body were essentially similar to those of the fossil Cross- 

 opterygii. The spiracle, which is found in the ancestral fish, is 

 modified in the amphibia into the middle ear. As can be seen 

 in a frog, the middle ear is a slightly enlarged cavity lying next 

 the internal ear, and covered on the outside by a skin membrane 

 (the tympanum or ear drum). The wide inner part opens directly 

 into the pharynx. 



The recent amphibia can be identified by their smooth skin, 

 usually well supplied with mucous glands; and, except in forms 

 with degenerate legs, by four front toes and five on the hind legs. 



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