CLASS AMPHIBIA. 301 



have gradually changed from gills to lungs just as the 

 frog does in one generation. Indeed the belief is that 

 the amphibians have just been caught in crossing 

 the stream, so to speak, and have become adapted to 

 this as a permanent mode of life. 



310. Place in Nature. The group of amphibians is 

 not large, consisting of about 1,000 species. The tailless 

 forms are well distributed in the fresh waters of the 

 earth. None of the species have large individuals. The 

 " Giant Salamander " of Japan may attain a length of three 

 feet. They are of little economic importance. The 

 toads destroy a good many insects and thus help in hold- 

 ing this group in check. Toads are easily reared artifi- 

 cially, and might be made much more valuable in this 

 respect by increasing their numbers. The legs of some 

 of the larger species of frogs are prized for eating. They 

 are a delicate, light-colored meat. Frogs have been used 

 very widely in laboratories for dissection and for experi- 

 ments on the nervous system. A large amount of our 

 knowledge of the functions of the various portions of the 

 central nervous system, and of the way in which muscles 

 and nerves act, is derived from the study of this animal. 



311. Chief Types of Amphibians. There are two impor- 

 tant orders of these animals : the Urodela or tailed forms 

 (Fig. 119), and the Anura, or tailless forms (Figs. 120 and 

 1 2 1) . The latter are more common and better known to 

 the average student. 



Newts and salamanders are elongated and appear much 

 like the lizards. Their bodies are smooth, however, 

 instead of scaly as in the lizards. The type most likely 

 to be found by the student is probably the blunt-nosed 



