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VERTEBRATE LIFE AND ORGANIZATION 



Figure 21.1. The leopard frog, Rana pipiens. 



amphibians, the toads, are particularly active at night when the hu- 

 midity is relatively high. 



Contemporary members of the class are grouped into three orders. 

 The frogs and toads are placed in the order Anura. The other orders 

 consist of the lizard-shaped, scaleless salamanders (order Urodela), and 

 the legless, wormlike caecilians of tropical continents (order Apoda). The 

 several orders of extinct amphibians are discussed in a subsequent 

 chapter (23). 



Anurans differ from the others in having powerful hind legs for 

 jumping on land and swimming in the water. Their short trunk, the 

 absence of a tail, and the enlarged hind legs with webbed feet are among 

 the many features which adapt them for their mode of life. 



Approximately 100 species of frogs and toads occur in the United 

 States and Canada. The most widespread is the leopai-d frog, Rana 

 pipiens (Fig. 21.1). This species is found throughout North America 

 except for the more northern parts and the west coast of the continent. 

 The following description applies specifically to Rana pipiens, but most 

 of what follows applies to other anurans as well. 



180. External Features 



The body of most terrestrial vertebrates can be divided into four 

 regions: a head containing the mouth, brain and organs of special sense; 

 a somewhat narrower neck connecting the head with the trunk; and a 

 tail located posterior to the anus, or termination of the digestive tract. Of 



