SECTION II 

 THE VERTEBRATE ORGAN SYSTEMS 



CHAPTER 4 



THE FROG: A REPRESENTATIVE VERTEBRATE 



To most people, the members of the subphylum Vertebrata of the 

 phykim Chordata are the most interesting of animals. The reason for 

 this is quite clear : first man is a chordate and a vertebrate, and sec- 

 ond his important domestic animals are vertebrates. 



The vertebrates are extremely numerous and diverse both in struc- 

 ture and habit ; thus the problem of selecting one as an example is 

 not at all simple. 



For many years, the common grass frog, Rana pipiens, has been a 

 very popular animal for study. Innumerable generations of students 

 have gained their first knowledge of the structure and function of verte- 

 brates from this small animal. Rana pipiens is an excellent species for 

 study for a number of reasons. First, the arrangement of its parts is 

 very similar to that of the land vertebrates. On the other hand, part 

 of the life of this animal is spent in water ; thus it shows many adapta- 

 tions which are specific for an aquatic environment. Finally, the frog 

 is an animal of convenient size and is easily obtainable for labora- 

 tory work. 



Classification.— The frogs are members of the class of vertebrates 

 known as the Amphibia. On the whole, these amphibians are compara- 

 tively small and inconspicuous forms. In former geologic tnues, how- 

 ever, the amphibians were much more numerous, and many attamed 

 a comparatively large size. 



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