316 THE CRUSTACEA. § 270. 



CHAPTER III. 



NERVOUS SYSTEM. 



§ 270. 



The !^ervous system of the Crustacea, is developed in dlfiFerent de- 

 grees according to the various orders."^ 



Its central mass consists of an abdominal cord, connecting, usually, with 

 the cerebral ganglia by an oesophageal ring. With the long-bodied species, 

 this abdominal cord is composed of numerous ganglia, arranged in succes- 

 sive pairs from before backwards, and connected together by longitudinal 

 commissures. But when the cutaneous skeleton is shortened by a dimi- 

 nution or a fusion of the segments, the ganglionic chain is lessened in a like 

 manner by a coalescence or a disappearance of several of its ganglia. 



With the Macrura, the Stomapoda, the Amphipoda, and Isopoda, the ab- 

 dominal cord consists of ten to thirteen pairs of unequal ganglia, situated, 

 usually, on the median line, and shielded by septa given off from the inter- 

 nal surface of the thoracic and abdominal segments of the skeleton. 



The size of these ganglia is in direct ratio with the development of the 

 segments and their appendages, to which they belong. Those of the 

 thorax, — the anterior abdominal ones, as well as the last caudal one, are 

 consequently very large, for they send filaments to the various chelate, 

 prehensile, ambulatory, and natatory appendages, and to the caudal lam- 

 ellae, which are usually highly developed. With the Myriapoda, the ab- 

 dominal cord is remarkable for the great number of its ganglia, which are of 

 equal size. Quite often, the ganglia of the same pair are fused into a 

 single mass ; in which case, the two interganglionic commissures are more or 

 less approximated or even blended together. With some species, a portion 

 of the abdominal ganglia are so closely approximated, successively, that the 

 interganglionic commissures are wholly wanting. AVith the Brachyura, the 

 whole abdominal cord is concentrated into a large central mass. 



The peripheric nerves arise from the ganglia, rarely from the intergan- 

 glionic commissures. The cerebral mass, which is situated above or in 

 front of the oesophagus, is composed of a pair of considerable ganglia, 

 more or less fused together. The nerves sent off from these, go princi- 

 pally to the organs of sense ; and in the inferior Crustacea, where these last 

 are wanting, the cerebral mass is absent also. In such case, there are 

 usually wanting likewise the two cerebral commissures, which are given 

 off from the anterior thoracic ganglion, and surround the oesophagus.^ 



I Audouin and Milne Edwards (Ann. d. Sc. Milne. Edwards, Hist. Nat. d. Crustac. I. p. 126, 

 N.it. XIV. 1828, p. 77, PI. II.-VI.) have given a Tl. XI., and his article Crustacea in the Cyclop- 

 general review of tlie disposition of the nervous sys- of Anat. loo. cit. p. 762. 

 tern in the different orders of Crustacea ; see also 



* [ End of § 270.] It is regretted that no example, inferior Crustacea, such as Califrus, a.ntl some of 



illustrative of this last statement, is given, for not all of the Cyclops tribe, the coi)halic, thoracic, 



certainly none ia now recollected where the grand and abdominal ganglia, are fused into a single 



typical structure is not present, — in other words, mass through the anterior jjart of which the oesopha- 



wliure the oesophagus does not pierce the cerebral gus passes ; see Dana, loc. cit. Caligus. Amer. Jour, 



nervoua system at some point. In many of tbe Sc. XXXIV. p. 250. — Ed. 



