THE VERTEBRATE BODY 145 



F. Distinctive 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 Inverte- 

 brates 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 Vertebrate skeleton 

 is primarily a living endoskeleton. It is an organic part of the 

 organism which, although it affords protection 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 

 importance of the notochord, the Vertebrates and their nearest 

 allies {e.g., the Tunica tes and Amphioxus) are technically known 

 as Chordates. (Fig. 67; Appendix I.) 



In the second place, it will be recalled that the central nervous 

 system of the Earthworm and Crayfish 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 a 

 dorsal brain. The position of the Vertebrate brain is similar, though 

 the spinal cord is not a 'cord' but a nerve tube which lies in the 

 vertebral canal embedded 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. 



The third fundamental characteristic is a series of perforations or 

 slits through the throat and body wall. In the lower forms the 

 gill slits provide an exit for the current of water entering by the 

 mouth and, being richly supplied with blood, afford the chief 

 means of respiratory interchange between the animal and the 

 surrounding medium. In the higher Vertebrates the gill slits are 

 present merely during a transient phase in the development of 

 the individual, since the function of aerating the blood is taken 

 over by the lungs. (Figs. 94, 117, 235.) 



