In most of the PectinibranchiaM, the brain consists of two ganglia united by a transverse cord ; from these two centres arise the principal nerves, two of which unite to form a small ganglion beneath the esophagus, from which that tube derives its peculiar supply.
It is in the Nudibranchiate division, how ever, that the nervous centres exist in their most concentrated form, and in these it is doubtful whether there are any ganglia, except the large supra-oesophageal brain. Wo take Tritonia as an example of this form of the nervous system. In this beautiful Gasteropod the brain consists of four tubercles placed across the commencement of the resopliaglis, the nervous collar being completed by a simple cord ; all the nerves which supply the skin, the muscular integument, the tentacles, the eye, and the muscles of the mouth arise from the brain, and anatomists have not hitherto detected any other source of nervous supply, although Cuvier suspected two minute bodies, which he found beneath the oesophagus appa rently connected with the brain, to be of a ganglionic nature.
The slow-moving and repent tribes of which we are now speaking have their powers of sense almost entirely limited to the perception of objects in actual contact with their bodies, and instruments adapted to touch and vision are the only organs of sense which the anato mist has been able to distinguish. The utter want of an internal skeleton or of an external ar ticulated crust forbids us to expect that any of them are provided with an apparatus specially calculated to appreciate sonorous undulations. Their tongue, coated as it is with horny plates, studded with spines, or absolutely corneous in texture, is obviously rather an instrument of deglutition than an organ of taste. No re searches have hitherto detected any part of the body which could be looked upon as devoted to smell ; the eye is generally a mere point, rather inferred to be such by analogy than clearly adapted to vision ; and the sense of touch in fact is the only one which anatomical evidence would intimate to be perfectly deve loped. Yet in spite of these apparent defi ciencies, observation teaches us that many genera are not utterly deprived of the power of appre ciating intimations from without connected with the perception of odours ; it has been found by direct experiment that some of them are pe culiarly sensible of the approach of scented bodies; thus the snail, although at rest within the Shelly covering which forms its habitation, will with great quickness perceive the proximity of scented plants which are agreeable articles of food, and promptly issue from its concealment to devour them. Some anatomists have sup
posed that it is at the entrance of the respiratory cavity that we are to look for the special seat of smell, where, as the air alternately enters and is expelled by the movements of respiration, the odorous particles with which it may be impreg nated are rendered sensible. Others with scarcely less probability conceive that the whole surface of the body which is exposed to the at mosphere may be endowed with a power of smelling, the quantity of nerves which are dis tributed to the integument, and the moisture with which it is constantly lubricated, seeming to adapt it perfectly to the performance of this function, giving it all the characters of a Schneiderian membrane. It is not impossible that sounds may be perceived in a somewhat analogous manner, although no proof has yet been adduced that any of the Gasteropoda are sensible to impressions of this nature. The sense of touch is exquisitely delicate over the whole surface of the animal, but more especially so in the foot, which is extremely vascular and abundantly supplied with nerves ; yet in spite of this delicacy in the organisation of the skin which makes it so sensible of contact, it appears to have been beneficently ordered that animals so helpless and exposed to injury from every quarter, are but little sensible to pain, and that such is the case, Al. Ferussac, a diligent ob server of their economy, bears ample testimony. " 1 have seen," says he, " the terrestrial gaste ropods allow their skin to be eaten by others, and in spite of large wounds thus produced, skew no sign of pain." But besides the sen sation generally distributed over the skin, we may observe in most instances organs of variable form which seem peculiarly appropriated to touch. These are the tentacles, or horns as they are usually termed, which occupy a va riable position upon the anterior part of the animal.