[Illustration: Compound stomach of Sea-Hare.]
Though not so gifted as the cephalopods, many of the gasteropods possess all the organs of sense. Like them, they have an apparatus specially calculated to appreciate sonorous undulations, and consisting of a membranous vesicle attached to an auditive nerve, and containing either a single spherical otolithe or a larger number of similar smaller calcareous bodies, which by their vibrations communicate the impression of sound to the nerve. Their minute eyes are short-sighted, it is true, and frequently either entirely wanting or, as in the Nudibranchiates, scarcely able to distinguish light from darkness; but their inactive habits require no wide field of vision, and thus they see as much of the external world as is necessary for their humble sphere of existence. The organs of sight are generally situated either on a prominence at the base of the superior pair of tentacles or, as, for instance, in the Murex, at the extremity of these organs (_a_, _b_), a position which enables the animal to direct them readily to different objects.
[Illustration: Tentacles and eye of Murex.
Many of the Gasteropods are evidently capable of perceiving odours; thus, animal substances let down in a net to the bottom will attract thousands of Nassæ in one night. We also may infer that they are not deficient in taste from the presence of papillæ at the bottom of their mouth, analogous to those found on the tongue of other animals; but, of all their senses, that of touch is undoubtedly the most perfect. The whole soft surface of the body is indeed of exquisite sensibility, but more especially the vascular foot, and the tentacles, or horns, which vary both in number and in shape in different genera. 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. Although they are deprived of all higher instincts, we find among the Gasteropods a few examples of concealment under extraneous objects, which remind us of the masks and artifices frequently employed by the insects and crustaceans.