CRUSTACEA. 7 



*^The nervous system of the Crustacea submitted to our 

 observation, say they, presents itself in two very different as- 

 pects, which constitute the two extremes of the modifications 

 visible in that class. Sometimes, as in the Talitrus, this ap- 

 paratus is constituted by numerous similar nervous inflations, 

 arranged in pairs, and united by cords of communication in 

 such a way as to form two ganglionic chains, separated from 

 each other and extending throughout the length of the animal. 

 At others, on the contrary, it is solely composed of two gan- 

 glions or knotty enlargements, dissimilar in form, volume, and 

 arrangement, but always simple and azygous, and situated, 

 one in the head and the other in the thorax. Such is the 

 case in the Maia. 



''^ These two modes of organization, at the first glance, cer- 

 tainly seem essentially diiferent, and if the study of the ner- 

 vous system of the Crustacea were limited to these two ani- 

 mals, it would be extremely difficult to recognize the analogy 

 between the central nervous mass in the thortix of the Maia, 

 and the two ganglionic chains which occupy the same region 

 of the body in the Talitrus. But if we remember the various 

 facts detailed in this memoir, we necessarily arrive at this re- 

 markable result.'' 



They were led to it by the exact and careful study of the 

 nervous system of various intermediate Crustacea, forming so 

 many links of the series, such as the CymothoiB(l ), the Phyllo- 

 somce(2), Astacus(3), Palcemon and Palinurus. They have also 

 supported their positions by the observations of Cuvier and 

 those of M. Treviranus. The consequence deduced by them 

 is, that notwithstanding this difference, the nervous system of 

 the Crustacea is formed of the same elements, which, insulated 

 in some and uniformly distributed throughout the length of 

 the body, present in others, various degrees of centralization, 

 at first from without inwardly, and then in a longitudinal di- 



ll) Isopoda. 



(2) Stomapoda. 



(3) For this subgenus and the two following subgenera see the Decapoda 



Macroura. 



