Mollusca. evenings to commit their depredations. 
=~ weather, they stir not from their holes. 
MOLLUSCA. 
During warm, dry 
The animals under consideration, feed equally on the 
products of the vegetable and animal kingdom. Those 
whichare phytivorous appear to prefer living vegetables, and 
refuse to eat those which are dried. Weare not aware that 
putrid vegetable matteris consumed by them, although 
many of the snails and slugs are found under putrid leaves 
and decayed wood. In these places there is shelter from 
the sun, together with dampness, so that it is difficult to de- 
termine, whether they sojourn in an agreeable dwelling, or 
a well-stored larder. Those mollusca which are carnivo- 
rous, prey on minute animals in a living state, and many of 
them greedily attack putrid matter. 
The means employed to bring the food within the reach 
of the organs of deglutition, are exceedingly interesting, 
both on account of their variety and success. Some are 
provided with ¢entacwla for securing their prey, and con- 
yeying it to their mouth, as the cuttle-fish ; others protrude 
a lengthened probosis, or an extended lip or tongue, as the 
limpet, and thus bring their food into the mouth. By many, 
however, whichare fixed to the same spot during the continu- 
ance of existence, or only capable of very limited locomotive 
power, successful efforts are made to excite currents in the wa- 
ter, whereby fresh portions of it are brought in contact with the 
mouth, and its animal or vegetable contents separated. 
Where part only of any kind of food is taken into the mouth - 
at once, the lips are possessed of sufficient firmness to cut 
off the requisite portions, or there are corneous mandibles 
to perform the office. 
In the mouth, there is scarcely any process performed 
analogous to that of mastication, in the higher orders of ani- 
mals. When the food is in the mouth, or entering into the 
gullet, it is mixed with saliva, as in the more perfect ani- 
mals. The salivary glands in which it is secreted, are in 
general of considerable size, divided into lobes, and, in some 
cases, separated into distinct masses. In many species the 
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 seme species, the stomach opens laterally into the py- 
lorous, and, in a few instances, possesses a spiral coecum at- 
tached 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 dile is poured, in some, into 
the stomach, and, in others, into the pyloric extremity of 
ans intestine by different openings. There is no gall-blad- 
er. 
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 in a 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 
hunger are often at distant intervals, and the power of ab- 
stinence very great. 
OAT 
The characters furnished by the digestive system are ex- Mollusca. 
tensively used in the inferior divisions of molluscous ani- == 
mals. The form of the lips, the position of the mouth and 
anus, and the structure of the stomach, deserve to be atten- 
tively considered, as indicating the habits of the species. 
CreuLatine System.—tThe process by which the food Circulating 
is converted into chyme, has not been satisfactorily traced, system. 
nor has the existence of the lacteals for the absorption of 
the chyme been demonstrated. In this class of animals the 
veins seem to perform the offices both of lacteals and lym- 
phatics. The blood is white, or rather of a bluish colour 
Its mechanical and chemical constitution yet remains to be 
investigated. 
The circulating system of molluscous animals, exhibits 
very remarkable differences in the different classes. In all of 
them, however, there is a systemic ventricle ; but the other 
parts of the heart are not of constant occurrence. 
The circulating system furnishes few characters which 
can be employed in systematical arrangements. The struc- 
ture of the systemic and pulmonary vessels does not ap- 
pear to be co-ordinate with any particular plan of external 
configuration and manner, as we see in the case of the 
pteropoda and gasteropoda. In these, the organs of circu- 
lation are very much alike, while the external forms exhibit 
very obvious differences. 
The molluscous animals which respire by means of lungs 
are few in number, and form a very natural tribe, which 
Cuvier has termed gasterpodes pulmones. In them the res- 
piratory organ is simple, consisting of a single cavity, on the 
walls of which the extremities of the pulmonary artery are 
spread. This cavity communicates externally by an aper- 
ture which the animal can open or shut at pleasure. 
The mollusca which breathe by means of gills, exhibit 
very remarkable differences, in their number, structure, and 
position. In some cases, there is a single cavity communi- 
cating by an aperture, through which, the water enters. 
The walls of this cavity exhibit an uneven surface, dispos- 
ed in ridges, which are the gills, and on which the pulmo- 
nic artery is expanded. This structure exhibits itself in 
the Gasteropoda pectini-branchia. In many cases the gills, 
though seated in a cavity, like the former, and equally ex- 
posed to the contact of the surrounding element, are two in 
number, one on each side, as in the Cephalopoda. In the 
Bivalvia, they are four in number, two on each side like 
leaves, and extend the whole length of the body. In these, 
the water is admitted at the pleasure of the animal. 
The gills of other mollusca are seated externally, and 
consist either of aborescent productions, or simple cuticular 
elongations, within which the pulmonary artery terminates. 
In some of these, as the Péeropoda, the branchial surface is 
constantly exposed to the action of the surrounding water 5 
whilst in others, the cuticular expansions, which are analo- 
gous to gills, are retractile at the will of the animal. In 
several examples, these cuticular elongations, which execute 
the functions of respiration, are covered with moveable cilia, 
well calculated to excite currents in the water, thus renew- 
ing the portions applied to the surface. 
By means of the characters furnished by the circulating 
and respiratory systems, the molluscous animals may be 
divided into several distinct classes. But as we shall em- 
ploy these characters in the construction of the different 
divisions to be employed, it is unnecessary, in this place, to 
enter into their details. 
PECULIAR SECRETIONS.— The molluscous animals are con- Secretions. 
sidered as destitute of organs for the production of urine, 
but they possess various organs for the secretion of peculiar 
fluids or solids, some of which are useful in the arts. 
The coloured fluid, which is secreted by the Cephalopo- 
da and some of the aquatic gasteropoda, appears to consist 
chiefly of a peculiar mucus, united to a peculiar pigment. 
The animals which furnish this secretion, eject it when in 
