THE A¥ATOM\' 



OF 



VEETEBEATED ANIMALS. 



CHAPTER I. 



> GENERAL YIEW OF THE 0EGANIZATT0:N" OF THE VEKTE- 

 BRATA THE VERTEBRATE SKELETON. 



The Distiyictive Characters of the Yertebrata. — The Yerte^ 

 brata are distinguished 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-spmal 

 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 

 tlie 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 this rule may he found in the Ascid- 

 ians. The tails of the larvae of these animals exhibit an axial structure, which 

 has a certain resemblance to a vertebrate notochord ; and the walls of the 

 pharynx are perforated, much as in Ampliioxvs, 



