16 MOLLUSCA. 
existence of a gullet is doubtful, as the food seems to enter 
the stomach immediately ; while, in others, there is a por- 
tion of the intestinal canal which has some claim to the de- 
nomination. 
The stomach, in many instances, is membranaceous, and 
can scarcely be distinguished from the remaining portion of 
the intestinal canal. In some cases, however, it is strong 
and muscular like the gizzard of a bird, and even fortified 
with corneous knobs for the reduction of hard substances. 
In some species, the stomach opens laterally into the py- 
lorus, and, in a few instances, possesses a spiral caecum 
attached to it. 
The liver is usually of very large dimensions, and seated 
close to the stomach, which it, in many cases envelopes. 
It is divided into many lobes, and receives numerous blood- 
vessels. There is, however, nothing analogous to the vena 
portarum of quadrupeds. The dz/e is poured, in some, into 
the stomach, and, in others, into the pyloric extremity of 
the intestine by different openings. There is no gall-blad- 
der. 
There is no division of the canal into small and large in- 
testines, as in the higher classes ; or rather, among the mol- 
lusca, the relative size of the different parts is reversed. 
Here the pyloric extremity is usually the largest, while the 
anal is more slender. The intestine, as in fishes, is short 
in proportion to the length of the body, and in its course, 
is subject to few turns. ‘The anus, is, in some, placed on 
one side of the body ; in others it is terminal, while ina few 
it opens on the back. 
The digestive system is thus more simple in its structure 
than in the higher classes. It possesses neither pancreas, 
spleen, nor mesentery. And, we may add, that the calls of 
