CHAPTER X 

 THE VERTEBRATE BODY 



If we contemplate the method of Nature, we see that everywhere 

 vast results are brought about by accumulating minute actions. 



— Spencer. 



As we know from our survey of the Animal Kingdom, the 

 Vertebrates form one of the most clearly defined divisions and in- 

 clude all the larger and more familiar forms — Fishes, Amphib- 

 ians, Reptiles, Birds, and Mammals. There is, in fact, less 

 diversity in structure among the Vertebrates as a whole than is 

 present, for example, in the one subdivision of the Arthropods, 

 the Crustacea, of which the Crayfish is a member. Accordingly 

 we shall confine our attention largely to a description of the 

 structure and physiology of an 'ideal' Vertebrate, and mention 

 incidentally some of the chief modifications of general signifi- 

 cance which appear in the different groups, and specifically in 

 Man. 



A. Body Plan 



The ideal Vertebrate body is more or less cylindrical in form, 

 and is bilaterally symmetrical with respect to a plane passed ver- 

 tically through the main axis which extends from the anterior to 

 the posterior end. Three regions of the body may be distinguished, 

 head, trunk, and tail. Frequently there is a narrow neck be- 

 tween the head and trunk. (Figs. 67, 94, 95.) 



The head forms the anterior end and contains the brain, eyes, 

 ears, and nostrils, or anterior nares, as well as the mouth and 

 throat, or pharynx. On either side of the head, behind the mouth, 

 is a series of openings, or gill slits, leading into the pharynx 

 which, however, in air-breathing Vertebrates disappear before the 

 adult condition is attained. 



The trunk forms the body proper and contains the coelom, and 

 the major part of the alimentary canal leading posteriorly to the 

 exterior by the anus, as well as the chief circulatory, excretory, and 

 reproductive organs. 



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