THE ANATOM'Y 



VEETEBEATED AI^IMALS. 



CHAPTER L 



A GENEEAl VIEW 01" THE OEGAlTIZATIOMr OF THE VBltTE- 

 BEATA THE VEETEBEATE SKELETON. 



The Distinctive Characters of the Yertebrata. — The Verte- 

 brata are distinguished from all other animals by the circum- 

 etance 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 Mollusca. 

 And, in any case, the central parts of the cerebro-spinal ner- 

 vous 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 primitive epidermic covering of the germ, and only acquire 

 their ultimate position, in the interior of the dorsal tube, by 

 llie development and union of outgrowths of the blastoderm, 

 which are not formed in the Invertebrata* 



Again, in the partition between the cerebro-spinal and vis- 



* It is possible that an exception to tliis rule may be found in the Ascid- 

 «ns. The tails of the larva of these animals exhibit an axial structure, whioh 

 vis a certain resemblance to a vertebrate notochord ; and the walla of the 

 pharynx arc perforated, much as in Am/pMoxus, 



