GASTEROPODA OCEAN LIFE "Molluscous animals furnished with a distinct head, and a fleshy crawling foot ; either naked or protected by a shell, for the most part formed of a single piece, and generally spirally turned; mouth provided with lips, and a ribbon-like tongue, armed with rows of teeth ; eyes two ; tentacles two or four.— Young subject to metamorphosis.
" Extensively distributed over the surface of the land, or in habiting the waters either fresh or salt, there exists a very nu merous body of Mollusca, differing widely among themselves in construction and habits, but distinguished by a peculiar locomo tive apparatus common to the entire class, by means of which they are able to fix themselves to plane surfaces, and to move from place to place by a slow or gliding motion. The Slug, the Snail, the Limpet, and the Whelk, afford familiar examples of their general form and appearance.
" The upper parts in the majority of species, are covered with a fleshy cloak or mantle, the edges of which are free, in a greater or less degree, and in some cases are expanded into great lateral wings. In the Sea-hare (Aplysia), these expansions serve, by their waving motion, as swimming fins. The proper function'of the mantle-edge is, however, the formation and increase of the shell ; and hence it is always found, in shell-covered species, on that part of the body which is near the orifice. In a few species, the shell is included within the substance of the mantle. A dis tinct head is always present, more or less conspicuous according to the degree in which it. projects from beneath the front of the mantle. It is furnished with various organs of sense. The organs of touch are one or two pairs of contractile tentacles, placed com monly on the back of the head. In some cases these can be inverted and everted ; but more generally they are solid. The hinder pair, where there are two, are often more complex in structure than the others, and are, perhaps, the seat of the sense of smell. Well-developed eyes are almost invariably present in this class. They never exceed a single pair, and are generally placed on some part of the tentacles,—the hinder ones, when there are two pairs. The most common position for these organs is at the extremity of a short fleshy column, springing from the base of the tentacle. In many species the eyes present an elaborate structure ; the great Strombidce of the tropical seas, for example, have eyes with a distinct pupil and a double iris, equalling, in beauty and correctness of outline, those of birds and reptiles ;' and many of our native genera, as Bucsinum and Murex, are scarcely inferior, in this respect, to the Strombidce.
" There is always in these animals a distinct mouth, placed at the front of the head, furnished with swollen, contractile lips, often prolonged into a cylindrical proboscis. In many species, as in Rolls, Tritonia, &c., there are two horny, sharp-edged plates, which act as jaws, playing over each other, like the blades of shears. Sometimes there is only a single jaw-plate inserted in the palate. There is, beside the jaws, a fleshy band, longer or shorter according to the genus, which performs the function of a tongue. It is sometimes grooved along its surface, and is always armed with horny teeth or spines, which are ar ranged in regular rows, both longitudinally and transversely. The form, number, and pattern of these teeth vary greatly, though always constant in the same species. They ' are amber colored, glassy, and translucent; and being siliceous (they are in soluble in acid), they can be used like a file for the abrasion of very hard substances. With them the Limpet rasps the stony mullipore, the Whelk bores holes in other shells, and the Cuttle fish doubtless uses its tongue in the same manner as the cat.' "In each transverse row, there is generally a variously-toothed plate, pointing backwards, and overlapping the base of its pre decessor ; while on each side of this there are several lateral teeth in the form of curved spines which arch inwards. The tongue of the Limpet is longer, when extended, than the whole animal ; that of the Whelk has a hundred rows of teeth, while the great Slug has one hundred and sixty rows, with one hun dred and eighty teeth in each row. The stomach is sometimes armed with horny plates and teeth, as in the Sea-hare (Aplysia), which feeds on leathery Fuci.
" The shell is formed in the same manner as in the CoNem FERA, by the folding back of the edge of the mantle. Very many species carry on the hinder part of the body a horny or shelly plate (Operculum), which accurately closes the aperture of the shell, when the animal has withdrawn into its recesses. The form of this appendage is ordinarily that of a very flat cone, made by successive layers, each a little larger than its predeces sor, or a flattened spire. The common Top-shells (Trochus,) afford good example of the spiral operculum."