CHORDATA 



137 



crabs, on which it feeds. The pearly nautilus dwells in 

 the Indian Ocean, crawling on the bottom at moderate 

 depths; and, while the shell is well known, only a few 

 specimens, comparatively, of the animal have ever been 

 obtained. 



Branch XII. CHORDATA 



This grand division includes the most perfect animals, 

 or such as have the most varied functions and the most 

 perfect and complex organs. Besides the unnumbered 

 host of extinct forms, 

 there are about twen- 

 ty-five thousand liv- 

 ing species, widely 

 differing among them- 

 selves in shape and 

 habits, yet closely al- 

 lied in the grand 

 features of their or- 

 ganization, the gen- 

 eral type being end- 

 lessly modified. 



The fundamental 

 distinctive character 

 of typical chordates 



is the Separation Of FIG. in. - Ideal plans of the branches. V. trans- 



, . ,. , verse section of vertebrate type; v, the same 



tne mam maSS OI tne inverted. M, transverse section of molluscous 



nprvniic; <;v<;t^m from type; and A/i/, of molluscoid. A and Ad, trans- 



sysieni yerse sections of articulate type> high and low . 



the general Cavity Of C > longitudinal section of ccelenterate type; a, 



alimentary canal ; c, body cavity. In the other 



the body. A tranS- figures, the alimentary canal is shaded, the heart 



, . , i_ is black, and the nervous cords are open rings. 



verse section of the 



body exhibits two cavities, or tubes the dorsal, con- 

 taining the cerebrospinal nervous system ; the ventral, 

 inclosing the alimentary canal, heart, lungs, and a 

 double chain of ganglia, or sympathetic system. This 



