In the classification of the mollusca, the characters furnished by the nervous system, from the difficulty of their detection and exhibition, have never come into use. But those furnished by the organs of perception are highly prized. Of these, the eye is the most obvious and constant. It varies in position in different species ; but among individuals of the same species its characters are constant.
5. Digestive System—In the cutaneous, muscular, and nervous systems, traces of a general plan may be observed, according to which they have been construct ed in the different tribes. In the organs which remain to be considered, there is less uniformity of structure, each family, almost, being constructed according to a model of its own.
The time when molluscous animals feed has not been carefully attended to. Those which live in the water are beyond the reach of accurate observation. Those that reside on land usually shun the light, and creep forth in the evenings to commit their depredations. During warm dry weather, they stir not from their holes.
The animals under consideration feed equally on the products of the vegetable and animal kingdom. Those which are p/rytivorous appear to prefer living vegeta bles, and refuse to cat those which are dried. We arc not aware that putrid vegetable matter is 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 determine whether they sojourn in an agiceable dwelling, or a well-stored larder. Those mollusca which are carnivorous, prey on minute animals in a living state, and many of them greedily attack pu trid matter.
The 'mans employed by molluscous animals to bring the food within the reach of the organs of deglutition, are exceedingly interesting, both on account of their va riety and success. Some are provided with tentacula for securing their prey, and conveying it to their mouth, as the cattle-fish ; others protrude a lengthened pro boscis, or an extended lip or tongue, and thus bring their food into the mouth. With many, however, which are fixed to the same spot during the continuance of ex istence, or only capable of very limited locomotive power, successful efforts are made by the animal to excite cur rents in the water, (for no permanently fixed animals reside on land,) whereby fresh portions of it are brought in contact with the mouth, and its animal or vegetable contents separated. This action is performed in some
cases, as among the bivalves, by the mechanical action of the valves of tile shell, aided, in many species, by the syphons, while, in others, it is excited by the motion of articulate feet, as in the bernacles. 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 corncous mandibles to perlOrm the office.
In the mouth there is scarcely any process performed analogous to that of mastication in the higher orders of animals. When the food is in the mouth, or entering into the gullet, it is mixed with saliva, as in the more pet feet animals. The salivary glands in which it Is se creted are, in general, of considerable size, divided into lobes, and, in some cases, separated into distinct m isses. 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 portion of the intestinal canal which has some claim to the denomination.
The stomach, in many cases, is membranaccous, and can scarcely he distinguished from the remaining por tion of the intestinal canal. In some cas s, 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 pylorus, and, in a few instances, possesses a spiral cecum attached to it.
The liver in the molluscs is usually of large dimen sions, and seated close to the stomach, which it in many cases envelopes. It is divided into numerous lobes, and receives numerous blood vessels. There is, however, nothing analogous to the vena portarum of quadrupeds. The bile is poured in some into the stomach, and in others into the pyloric extremity of the intestine by dif ferent openings. There is no gall-bladder.