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.
Tentacles and eye of Murex.
c. Eye highly magnified.
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.
The Agglutinating Top (Trochus agglutinans) covers itself with small stones and fragments of shells, and thus shielded from the view escapes the voracity of many an enemy but little suspecting the savoury morsel hidden under the mound of rubbish which he disdainfully passes by.
In animals which are only provided with passive means of defence, we may naturally expect a considerable degree of caution, and in this respect the gasteropods might give many useful lessons to man. How carefully they protrude their tentacles as far as possible to sound every obstacle in their way, before they creep onwards, and how rapidly they withdraw into their shell at the least symptom of danger! What an example to so many of us that leap before they look, and frequently break their necks in the fall!
Yet, in spite of all their prudence and of the protection of their stony dwellings, they serve as food to a host of powerful enemies. The sea-stars, their most dangerous foes, not only swallow the young fry but also seize with their long rays the full-grown gasteropods, and clasp them in a murderous embrace.
They are preyed upon by fishes, crustaceans, and sea-birds, who pick them up along the shores; but it will sometimes happen that a crow, while endeavouring to detach a limpet for its food, is caught by the tip of its bill, and held there until drowned by the advancing tide.
Man also consumes a vast number of sea-snails, for on every coast there are some edible species; and it may be said that, with the exception of very few that have a disagreeable taste, they are all of them used as food by the savage. The miserable inhabitants of Tierra del Fuego chiefly live upon a large limpet that abounds on the rocky shores of their inhospitable land, and but for this resource would most likely long since have been extirpated by hunger.
Many of the univalve shells are, moreover, highly prized as objects of ornament or use both by savage and civilised nations. The South Sea Islander makes use of a Triton as a war conch; the Patagonian drinks out of the Magellanic volute, the Arab of the Red Sea employs a large Buccinum as a water-jug, and the Cypræa moneta is well known in commerce as the current coin of the natives of many parts of Africa. In Europe the iridescent Haliotis is frequently used for the inlaying of tables or boxes, and various species of Helmet-shells and Strombi (Cassis rufa madagascariensis, Strombus gigas), peculiar as being formed of several differently coloured layers, placed side by side, are in great request for the cutting of cameos, as they are soft enough to be worked with ease, and hard enough to resist wear. More than two hundred thousand of these shells are annually imported into France, and the value of cameos produced in Paris alone amounts to more than a hundred thousand pounds. A large number are also cut in the small town of Oberstein on the Nahe (a river flowing into the Rhine at Bingen), which has long been famous for the manufactory of agate ornaments and trinkets, and has now added this new branch of industry to the more ancient sources of its prosperity.