CLASS CRUSTACEA. 
135 
observation, presents itself, as they inform us, under two 
very different aspects, which constitute the two extremes ot 
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 
one 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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