ON MOLLUSCA. 294 
derable, and which more or less resemble clusters of grapes, 
especially when the eggs are of a black colour, as those of 
sepia. Often they are united by a gelatinous substance, in 
which they are plunged, like those of the limnzz, planorbes, 
and aplysiz. At other times many of these eges are enclosed 
in sorts of corneous envelopes, piled like pods one after the 
other, an arrangement which is observable in some species of 
fusus. 
We next proceed to the nervous system of the mollusca, or 
that portion of the organization on which the irritability of 
the animal and its susceptibility to external impressions, en- 
tirely depends. 
This apparatus of the nerves, as may be seen in the general 
characters of the type, presents a disposition somewhat sin- 
gular. It is, however, always composed of a central portion, 
or brain, situated above the intestinal canal; of ganglions, for 
the special organs of the senses, where these exist, as well as 
for the apparatus of locomotion; of some visceral ganglia ; 
and finally, of conducting threads or nerves, the structure of 
which is sometimes singular, in that they have a fibrous en- 
velope larger than the nervous cord, so as to permit, as some 
say, their injection, which appears very doubtful. 
The general disposition, and especially the proportion of 
the parts of the nervous system, are very different in the two 
classes of the malacozoaria, and especially in the sub-type of 
the malentozoaria. 
In the cephalous mollusca the brain, composed of two 
similar parts, more or less bulky, more or less united by a 
sort of commissure, is sometimes contained in a sort of 
cranium or cartilaginous box, which serves as a support to the 
contractile fibre. But in a great number of cases it is scarcely 
covered with cellular tissue, and it is placed at the origin of 
the cesophagus, behind the buccal mass, so that it follows 
their movements. 
