Part III 



VERTEBRATE LIFE 

 AND ORGANIZATION 



CHAPTER 21 



The Frog — A Representative Vertebrate 



The vertebrates will be considered more fully than any other gioup of 

 animals because a knowledge of their biology is particularly important 

 for an appreciation of human form and function. The frog is selected as 

 an example of the vertebrates because of its availability, ease of study 

 and importance in zoological research. It is not the most representative 

 of vertebrates; indeed no single type can be truly representative of so 

 diverse a subphylum. As a member of the class Amphibia, it occupies an 

 evolutionary position between the primitive, ancestral fishes and the 

 advanced, terrestrial mammals. A frog retains certain of the primitive 

 features of fishes, yet it has also evolved certain of the features charac- 

 teristic of the more advanced terrestrial vertebrates. 



1 79. Frogs and Other Amphibians 



Amphibians live both in water and in moist places on land. The 

 eggs and immature individuals are normally aquatic, and the adults 

 never get far from the water, for their ability to prevent excessive loss 

 of body water in a terrestrial environment is rather rudimentary. The 

 adults are found on the land close to ponds, streams and other bodies 

 of fresh water to which they can retreat, or in other moist places such as 

 beneath stones and logs in damp woods. The most terrestrial of the 



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