ON CRUSTACEA. 
245 
any, are not visible. In the paguri, the nervous cord is longi- 
tudinal, as in the astaci, but the ganglia of the part corres- 
ponding to the tail, are less numerous. In the squill®, there 
are ten ganglia, without reckoning the brain. That which is 
at the union of the two cords which have formed the collar, 
gives out nerves to the two large claws, and to the three pairs 
of feet which immediately follow them, and which in these 
animals are almost ranged on a transverse line ; accordingly 
this ganglion is the largest of all. Each of the three follow- 
ing pairs has its particular ganglion. Then follow six in 
the length of the tail, which distribute their threads to the 
thick muscles of this part. The brain gives out immediately 
four trunks on each side, namely, the optic, those of the an- 
tennas, and the cord which forms the collar, and as the an- 
tennas are placed more behind than the brain, their nerves 
are directed backwards to repair to them.” 
“ In oniscus, the two cords which compose the middle part 
of the nervous system, are not altogether inter-approximate. 
They can be perfectly distinguished through their whole ex- 
tent. There are nine ganglia, without reckoning the brain, 
but the first two, and the last two, are so much approximated, 
that they may be reduced to seven.” 
In the entomostraca, the brain is often the only part which 
is visible. That of apus is a small transparent globule, situ- 
ated under the interval of the eyes. The medullary cord is 
double, and has an enlargement at each of the numerous arti- 
culations of the body ; but the whole is so slender, and so 
transparent, that it is difficult to ascertain the true nature of 
this organ. In daphnia, and branchipus, the brain is ap- 
parent, as well as the optic nerves, of which even the divisions 
can be observed. 
Among the Crustacea, without doubt, many degrees may be 
distinguished relative to the perfection of vision. Certain of 
them, as the crabs, and especially the land-crabs, appear to 
