ON MOLLUSCA. 935 
of the organs to which they appertain, and that consequently 
it is much greater in the brachiocephala, which stand at the 
head of the class, than in the patella, &c., which close it. 
This observation is equally applicable to the nervous system 
of the acephalous malacozoaria; in fact, it is so very little 
developed in them that its very existence was for a long time 
unperceived. Their brain is no more than a double ganglion, 
or rather a sort of flatted cord, situated always above the 
cesophagus. It appears that there are no threads which 
should form round the latter a true ring, as in the cephalous 
mollusca. From this sort of brain there proceed two long 
cords, but they go much farther behind, and proceed to estab- 
lish the communication between this organ and the ganglion 
of locomotion which is found underneath the adductor and 
posterior muscle, and which, in fact, receives threads from 
them, as do the mantle and the tubes, when there are any. 
In the common mussel (Mytilus edulis), the nervous system 
seems to be more evident than in any other species of acepha- 
lous mollusca. It is composed of three pairs of ganglia; the 
first, the most anterior, is certainly placed under the cesopha- 
gus, or rather under the anterior retractor muscle of the foot, 
in part covered by the posterior edge of the union of the 
second pair of labial tentacula. The lesser ganglia of which 
it 1s composed are of triangular form, and of a white opake 
colour: they furnish, first, a very fine transverse net, which 
serves as a commissure between them; second, a thicker 
branch, which is distributed to the anterior adductor muscle, 
and to the labial appendages ; and third, from their hinder 
termination, a very thick thread, which proceeds externally, is 
applied on the membrane of the liver, traverses obliquely 
the anterior retractor muscle of the foot, follows the sides of 
the abdomen above the termination of the ovary, and pro- 
ceeds to join the posterior ganglion. 
The second pair of ganglia, which alone can be regarded as 
