CHAPTER XXIII. 

 CHORDATA (CONTINUED): CLASS AMPHIBIA. 



305. General Statement. There is no more interesting 

 group of animals than the amphibians, which include the 

 frogs, toads, salamanders, and newts. They stand mid- 

 way in habits between the fishes which use gills all their 

 lives and reptiles which use lungs only. The typical 

 amphibian starts life as a tadpole with gills. Gradually 

 the gills give place to lungs, the adult breathing air. There 

 are a few that never have anything but gills, and a few 

 that develop lungs at once, without any tadpole stage. 



The skin does not produce scales, but is usually glandu- 

 lar and slippery when moist, like that of the eel or cat- 

 fish. They are cold-blooded like the fishes. The heart 

 has two auricles and one ventricle. Their paired appen- 

 dages, when present, are leg-like and have toes. 



It will be seen that the amphibians are similar in 

 many respects to the fishes, especially to the dipnoi ; and 

 equally clearly they have passed beyond them in their 

 development. 



It is the smallest of the classes of vertebrates. 



Since the frog has already been studied in the labora- 

 tory as a type of the vertebrates (Ch. XXI), a detailed 

 account of the structures will not be given here. 



306. The Skeleton and Form of the Body. The sala- 

 manders and their allies have a long body with a tail, 

 somewhat like the fishes; but the frogs and toads lose 



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