146 FOUNDATIONS OF BIOLOGY 



the axial skeleton by groups of bones which form respectively 

 the PECTORAL and PELVIC GIRDLES. (Figs. 80, 81, 185, 186.) 



F. DIAGNOSTIC VERTEBRATE CHARACTERS 



As a summary of this general outline of the structure of the 

 Vertebrate body, we may emphasize three characters which 

 are of prime diagnostic importance. 



In the first place, whereas the skeletal structures of Inver- 

 tebrates typically consist, as in the Crayfish, of an exoskeleton 

 of hard non-living materials deposited on the surface of the 

 body, the chief function of which is protection, the Verte- 

 brate skeleton is primarily a living endoskeleton. It is an 

 organic part of the organism which, although it affords pro- 

 tection for delicate parts, provides adequately for support 

 and supplies muscle levers, and thus makes practicable the 

 relatively large bodies of the higher animals. The notochord 

 is at once the foundation and axis of the Vertebrate internal 

 skeleton and either persists throughout life as such, or simply 

 long enough to function as a scaffolding about which the 

 vertebral column is built. In recognition of the prime im- 

 portance of the notochord, the Vertebrates and their nearest 

 allies (e.g., the Tunicates and Amphioxus) are technically 

 known as CHORDATES (cf. pp. 415, 416).. 



Glancing back at the Earthworm and Crayfish, it will be 

 recalled that the central nervous system consists of a ventral 

 nerve cord running along in the coelom below the digestive 

 tract, except at the anterior end where it encircles the 

 pharynx to form the brain above. The position of the Verte- 

 brate brain is similar, though the spinal cord is not a 'cord' 

 but a nerve tube, which lies in the neural canal imbedded in 

 the muscles of the body wall above the digestive tract and, 

 of course, outside of the coelom. Thus the spinal cord itself 

 and its location are highly characteristic. 



