NERVOUS SYSTEM OF INVERTEBRATES. 



85 



..ns 

 r .. vert 



1-4 



while in vertebrates there is a separate cavity inclos- 

 ing and protecting the nervous centers (Fig. 56). (4) 

 In the vertebrates, as we have 

 seen, the nerve center con- 

 sists of a continuous tract of 

 gray matter lying along the 

 dorsal aspect of the body, above 

 the visceral canal. In the ar- 

 ticulata, on the contrary, the 

 nerve center is a continuous 

 chain lying along the ventral 

 aspect of the body, below the 

 visceral canal. (5) In the case 

 of vertebrates, as already seen, 

 the nerve system is subdivided 

 into two subsystems, the axial 

 and the ganglionic. In the case 

 of the articulata there is but 

 one'system, which probably per- 

 forms the functions of both, these subsystems not having 

 yet been differentiated. 



It is difficult, indeed impossible, to conceive how the 

 vertebrate nervous system could have been evolved out 

 of that of the articulates. If vertebrates came as a 

 branch from the articulates, as many think, they must 

 have come off so low down that the distinctive plan of 

 neither was yet declared. 



In treating of the plan of the nervous system we 

 shall take all the articulata together, as the plan is the 

 same in all, although most distinct in the arthropods. 



General Plan of Articulate Nervous System. — To bring 

 this out clearly it is best to take an example from about 

 the middle of the series — as, for instance, a leech, or a 

 crayfish. In Fig. 57 we give a side view of the nervous 

 system of one of the lower Crustacea, and in Figs. 58 and 



Fig. 56. — Cross section through 

 a fish : vs, visceral system ; 

 vert, vertebra ; 0, blocd sys- 

 tem ; ns, nervous system. 



