CLASS CRUSTACEA. 135 



observation, presents itself, as they inform us, under two 

 very different aspects, which constitute the two extremes of 

 the modifications which it exhibits in the Crustacea. Some- 

 times, as takes place in the talitrus, this apparatus is formed 

 by a great number of nervous enlargements, similar among 

 themselves, disposed in pairs, and united by cords of com- 

 munication, so as to form two ganglionic chains, distant one 

 from the other, and occupying the whole length of the ani- 

 mal. Sometimes, on the contrary, it is merely composed of 

 two ganglia, or knotty swellings, dissimilar in form, volume, 

 and disposition, but always simple and uneven, and situated 

 oi^e at the head, and the other at the thorax. This is what 

 we meet with in the maja. 



" Certainly, at the first glance, these two modes of organi- 

 zation appear to be essentially different, and were we to limit 

 the study of the nervous system of the Crustacea to those two 

 animals, it would be very difficult to recognize in the central 

 nervous mass of the thorax of the maja the analogue of the 

 two ganglionic chains, which occupy the same part of the 

 body in the talitrus. But if we recall to mind the divers 

 facts which we have mentioned in this memoir, we must 

 necessarily arrive at this remarkable result." 



They were conducted to this by the exact study of the 

 nervous system of various intermediate Crustacea, forming so 

 many links of this series, such as cymothoe, phyllosoma, pale- 

 mon, and palinurus ; they also rest on the observations of 

 M. le Baron Cuvier and M. Treviranus : they deduced from 

 them this consequence, that in spite of these differences of 

 disposition, the nervous system of the Crustacea is neverthe- 

 less formed of the same elements, which, isolated in some, and 

 uniformly distributed through the whole length of the body 

 in others, present various degrees of centralization, at first from 

 without inwards, afterwards in a longitudinal direction ; and 

 that, finally, this approximation is carried to an extreme, 



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