200 



NERVOUS SYSTEM. 



luscous classes. The motor and sensitive columns are seen 

 on a larger scale in the Crustacea, and they occupy the same 

 relative position as those long known in the other entomoid 

 classes. The supra-cesophageal ganglia are generally larger 

 than those of arachnida, and smaller than those of insects ; they 

 are, for the most part, united into a single cerebral ganglion de- 

 voted chiefly to the large organs of the senses, and their nerves 

 unite with the sympathetic, as in insects and mollusca. The ner- 

 vous ring of the oesophagus is here very wide, but the columns 

 are small which form it, and they give offlarge branches to the 

 stomach and the sympathetic while they pass along the sides 

 of the oesophagus. The ganglia of the cephalo-thorax vary 

 much in their number, their magnitude, and their degree of 

 approximation according to the form of that part of the trunk, 

 and the size of the several pairs of legs. The motor columns 

 are seen in the large macrourous decapods, as in the post 

 abdomen of the lobster, (Fig. 85. a,) passing over the upper sur- 

 face of the sensitive columns (#,) and their ganglia (c,) as a broad, 

 thin, white, fibrous layer, and giving off lateral branches 

 chiefly behind each pair of ganglia. The largest trunks and 

 mixed or moto-sensitive nerves of the 

 columns come off at their ganglionic 

 spaces (c, d.} These inter-ganglionic 

 motor nerves (e, e,) in the posterior 

 portion of the trunk come off at a great- 

 er distance behind the ganglia, as they 

 do in insects ; but as we advance to the 

 fore part of the body, their origins be- 

 come approximated to the ganglia. The 

 posterior terminal pair of ganglia have 

 generally a high position, and are of great size where the 

 caudal appendices of the trunk are much developed, as in the 

 long-tailed decapods. The same transverse and longitudinal 

 approximation of the nervous columns and their ganglia,seen in 

 the inferior articulata, is perceived in the development of the 

 Crustacea ; and the most concentrated form of the nervous 

 system met with in the highest brachyourous decapods, gra- 

 dually acquires this concentration of all its ganglia in two 

 points of the body, above and below the oesophagus (Fig. 86. 

 D,) by passing through all the inferior conditions which pre- 

 sent themselves as permanent or adult forms in this class. 



