306 



COMPARATIVE ZOOLOGY. 



Snbkingdom VIII. — Vertebrata. 

 This grand division includes the most perfect animals, 

 or such as have the most varied functions and the most 

 numerous and complex organs. Besides the unnumbered 

 host of extinct forms, there are about twenty-five thousand 

 living species, widely differing among themselves in shape 

 and habits, yet closely allied in the grand features of their 

 organization, the general type being endlessly modified. 



The fundamental distinctive character of Vertebrates 

 is the separation of the main mass of the nervous system 



from the general cav- 

 ity of the body. A 

 transverse section of 

 the body exhibits two 

 cavities, or tubes — the 

 dorsal, containing the 

 cerebro- spinal nervous 

 system ; the ventral, in- 

 closing the alimentary 

 canal, heart, lungs, and 

 a double chain of gan- 

 glia, or sympathetic 

 system. This ven- 

 tral, or haemal, cavity 

 corresponds to the 

 whole body of an In- 

 vertebrate ; w T hile the 

 dorsal, or neural, is 

 added. 



Vertebrates are also 

 distinguished by an in- 

 ternal, jointed skeleton, endowed with vitality, and capa- 

 ble of growth and repair. During embryo-life it is rep- 

 resented by the notochord ; but this is afterwards replaced 



Fig. 280.— Ideal Plans of the Subkingdoms. F, 

 transverse section of vertebrate type ; v, the 

 same, inverted. M, transverse section of mol- 

 luscous type ; and Md, of molluscoid. A and 

 Ad, transverse sections of articulate type, high 

 and low. C, longitudinal section of ccelente- 

 rate type ; a, alimentary canal ; c, body-cavity. 

 In the other figures, the alimentary canal is 

 shaded, the heart is black, and the nervous 

 cords are open rings. 



