THE ANATOMY 



OF 



VEKTEBRATED ANIMALS. 



CHAPTER I. 



A GENERAL VIEW OF THE ORGANIZATION OF THE VEETE- 

 BRATA — THE VERTEBRATE SKELETON. 



The Distinctive Characters of the Vertebrata. — The Verte- 

 brata are distinguislied from all other animals by the circum- 

 stance that a transverse and vertical section of the body 

 exhibits two cavities, completely separated from one another 

 by a partition. The dorsal cavity contains the cerebro-spinal 

 nervous system; the ventral, the alimentary canal, the 

 heart, and, usually, a double chain of ganglia, which passes 

 under the name of the " sympathetic." It is probable 

 that this sympathetic nervous system represents, wholly 

 or partially, the principal nervous system of the Annulosa 

 and MoUusca. And, in any case, the central parts of 

 the cerebro-spinal nervous system, viz. the brain and the 

 spinal cord, would appear to be unrepresented among 

 invertebrated animals. For these structures are the results 

 of the metamorphosis of a part of the pi-imitive epidermic 

 covering of the germ, and only acquire their ultimate 

 position, in the interior of the dorsal tube, by the develop- 

 ment and union of outgrowths of the blastoderm, which 

 are not formed in the Invertebrata* 



* It is possible that an excep- tain resemblance to a vertebrate 



tion to this rule may be found in notochord; and the walls of the 



the Ascidians. The tails of the pharynx are perlbrated, much as 



larvae of these animals exhibit an in Amphioxus. 

 axial structure, which has a cer- 



