404 



CEUSTACEA. 



ramifying largely upon the coats of the stomach, joins that of the oppo- 

 site side, and, assuming a ganglionic structure, is ultimately lost upon 

 the intestine. 



(1032.) The nerves of the extremities, derived from the central ab- 

 dominal ganglion, are represented in the annexed figure (fig. 206), 

 which requires no explanation*. 



Fig. 206. 



Nervous system of the Crab : from the dorsal aspect. 



(1033.) We have already ( 858), when describing the nervous 

 system of insects, hinted at the probable existence in the HOMOGAN- 

 GLIATA of distinct tracts of nervous matter in the composition of the 

 central chain of ganglia, and in the filaments whereby they are con- 

 nected with each other : reasoning therefore from analogy, it seems 

 fair to presume that, if this be the case, such tracts correspond with the 

 sensitive and motor columns which have been distinctly proved to exist 

 in the spinal axis of vertebrate animals. It is to Mr. Newport that we 

 are indebted for the first indication of this interesting factf; and the 

 accuracy of his observations is readily demonstrable by a careful exa- 

 mination of the ganglionic chain of the Lobster and other large Crus- 

 tacean species. Each ganglionic enlargement is, upon close inspection, 

 clearly seen to consist of two portions : first, of a mass of cineritious 

 nervous substance forming the inferior aspect of the ganglion, and of a 

 cord of medullary or fibrous matter which passes over the dorsal or 



* Fide Swan, Illustrations of the Comparative Anatomy of the Nervous System. 

 London, 4to. f Phil. Trans. 1834. 



