The locomotive powers of the mollusca are confined to creeping and swimming. The former action is per fot med by alternate contraction and relaxation of the foot, or muscular expansion, which serves as a sucker, and is analogous to the motion of serpents. The motion of swimming is executed either by the serpentine undu lations of the foot and the body, or by the action of ten tacula, or expanded portions of the integuments. Many species are aided in swimming by being able to vary the specific gravity of their body at pleasure, and either rise or sink in the water, as circumstances may require. In some, as the Janthina, them is a cellular organ pecu liat ly destined for this purpose, which may be regarded as in some measure analogous to the air-bladder of fishes. In all these exertions, their is bially slow. Sonic bivalve shells have the power of leaping, or shifting their position by a sudden jerk, pro duced by shutting the valves rapidly. This is strikingly displayed in the common Scallop, and is less perfectly exhibited in the river mussels. In a few instances, especially among the slugs, a thread is formed of the viscous secretion of the skin, by which the animal is enabled to suspend itself in the air from the branches of trees.
Although the progressive motions of molluscous ani mals are comparatively slow, the other muscular ac tions are executed with ordinary rapidity. The irri tabilility of some parts, as the tentacula and branchix, is so great, that the protecting movements are executed almost instantaneously, and the organs are contracted or withdrawn into the body. But these rapid exertions are only called forth in the moments of danger ; the or dinary movements are all executed with characteristic slowness.
The characters furnished by the muscular system are of great value in the discrimination of species, and in the construction of genera and higher divisions. They are intimately connected with the habits of the animal, and merit the attentive examination of the philosophical naturalist.
4. Xervous System.—In the molluscous animals, the nervous system is less complicated in its structure than in the higher classes, and the brain is not restricted in its position to the head. The whole nervous system appears in the form of ganglia and filaments. The prin cipal ganglion, or the one to which the term brain is usually applied, is seated above the gullet or entrance to the stomach. It sends out nerves to the parts about the mouth, to the tentacula, and to the eyes. It may be considered as analogous to the cerebrum of the ver tebral animals. From this ganglion proceed two fila ments, one on each side, which, in their descent, inclose the gullet, and unite underneath to form a second gang lion. From this last, which has been compared to the cerebellum, numerous filaments arc likewise distributed to the parts around the mouth, and to the other regions of the body. These filaments in some eases again unite, and form subordinate ganglia. In many cases
the brain and ganglia are of a white colour, and granu lated structure ; while the nerves which issue from them are white and unifortn. The covering of the first gang lion, which is analogous to the dura mater, does not ad here to it closely, but leaves a space filled with loose cel lular matter. The tunics of the nerves are equally de tached ; and as they can he inflated or injected readily, they have led some to suppose that the nerves were hol low, and others, that the tunics were the vessels of the lymphatic system.
The organs of perception, common to the higher classes of animals, do not all exist in an obvious manner among the 'nausea. The touch, that universal sense, is here displayed in many cases with great delicacy ; and the tentacula, and other cuticular elongations which we have already referred to, contribute to augment its resources. The sense of sight is by no means univer sally enjoyed by the inhabitants of this class. In a few species the eye is constructed on the plan of the same organ in the vertebral animals. In general, however, it appears only as a black point, whose peculiar functions can only be inferred from analogy. In many animals there is no trace of an eye, consequently they cannot possess that varied information which the higher animals obtain from that organ. Where eyes exist in this class, they are uniformly two in number. In one tribe only, namely, the cuttic-fish, have the rudiments of the organs of hearing been detected. The organs adapted to smell ing cannot be exhibited, but the existence of the sense is demonstrated by the facility with which they discover suitable food, when placed within their reach. The sense of taste exists, but it is difficult to point out the particular parts of the mouth fitted for its residence. But as they select particular articles of food in preference to others, we reasonably conclude that taste regulates the choice.
It is difficult to form an accurate estimate of the know ledge of external objects which molluscous animals can obtain by means of their organs of perception, joined to their powers of locomotion. The kind of life which they enjoy is so widely different from our own ; and attempts to tame them, or vary their habits by education, are limited by so many circumstances, that we must ever re main in ignorance of their mental powers or capabilities. M. Lamarck ranges them among his ilnimeaux Sensibles, and considers that, by means of their sensations, they acquire only perceptions of objects, or simple ideas, which they arc incapable of combining by any mental process. glad this opinion rested on observations, or even on arguments, it might have been worth while to controvert it, but, as it forms a part of the metaphysical speculations of that zoologist, built upon gratuitous as sumptions, it requires no farther notice.