SNAILS AS TYPES OF GASTROPODS
The gastropods (Gastropoda), including the snails and slugs, limpets, whelks, periwinkles, sea hares and the like, are Mollusca having the mantle completely enveloping the body, and the shell, when present, in a single piece, and usually in spiral form. There is a well-developed ventral foot, on which the animal creeps, and in front of it a distinct head bearing eyes and tentacles. These organs retain their normal bilaterality, but the body is, as a rule, inequilateral. The cause of this is the fact that on the animal's back is developed from the first a shell, which, with its contents, amounts to a relatively large weight, and it naturally falls over to one side. The mouth is armed with a flat, distensible, ribbon-like organ, studded with rows of chitinous teeth, that serves as a rasp and a boring instrument, and which is called an odontophore, or, in snails, a radula. Most gastropods are carnivorous.
The lowest in rank are the shell-less, or "naked" gastropods known as "sea slugs," "sea hares," and so forth. One Mediterranean species of Aplysia secretes a purple liquid utilized by the ancients as a dye, and this is still sought for in Portugal, where storms sometimes cast vast quantities of the mollusk on the beaches.
We come now to the great group of mollusks inhabiting fresh waters and dry land—the snails, whose group name is "pulmonates," that is, possessors of lungs, and breathing air. On the generally accepted theory that all these are descended from marine ancestors, and have gradually acquired the faculty of living on land, it would be natural to look for a series of mollusks that were amphibious, and, as it were, half-way fitted for a terrestrial existence, and such intermediates exist in all parts of the world. The little black Melampus, which covers the mud of tide flats on both the Atlantic and Pacific coasts in tens of thousands, and seems just as happy when the tide is out as when it is in, or when it is simply refreshed by the spray, is a good example. A near relative, Carychium, is still more emancipated from the sea.
First among these pulmonates are those common in ponds and still streams the world over, of the family Limneidæ, called limneids or pond snails. They are in various forms. Some are limpet-shaped (Ancylus), some are flatly coiled (Planorbis), but most of them have shells drawn out into a graceful spiral; in all cases the shell is not composed of lime, but of the thin, fragile, horny substance "chitin." The best known one is Limnea stagnalis, which sometimes reaches a length of two inches, and inhabits almost every quiet piece of water in North America, and in Europe and all Asia except India and China.
These water snails of our ponds and ditches are exclusively vegetable feeders, and must come to the surface at frequent intervals to breathe, letting out a bubble of vitiated air, and taking in a fresh supply. Should the pond dry up in summer the limneids burrow down into the mud, and remain in that heat trance called æstivation until the autumnal rains refill the basin and let them come forth. The small kinds called "physas," exceedingly common everywhere in this country and Europe, differ from Limnea in having the shell partly enveloped in the turned-up fringed edges of the mantle, and by being coiled from right to left instead of clockwise. This reversal occasionally occurs in individuals of all gastropods, which are then said to be "sinistral," as opposed to the normal "dextral" coiling; but in the physas it is the rule.
Next come the wholly terrestrial pulmonates—snails and slugs, distinguished from the pond snails, which have only one pair of tentacles at the bases of which the eyes are embedded in the skin, by having two pairs of "horns," one of which carries the eyes on their tips—good eyes, which may be quickly withdrawn out of harm's way by inversion of the tubular stalks. The thick, extensible foot is surmounted by a body coiled within the shell; and this foot secretes a viscid fluid that lubricates the creature's path, and often leaves a silvery trail.
Snails are mainly vegetarians. The mouth lies just under the front tentacles, and its upper lip is armed with a horny, crescentic "jaw." Within the mouth is the lingual ribbon, which may be brought up against the cutting edge of the jaw. This tongue is studded with rows of infinitesimal, flinty teeth, the radula of our big white-lipped snail, a quarter of an inch long, furnishing room for 11,000 of these denticles; and as all of them point backward the tongue easily seizes and draws into the mouth whatever the jaw nips off. Substantially the same sort of "tongue" is possessed by all the gastropods, but the arrangement and shape of the microscopic denticles is different in every species, and this is one of the "characters" used in classification. With it the carnivorous rasp away their food; and by bending it double and using it as a gimlet bandits like Nassa, the oyster pest, drill through other shells and devour the occupant. You may pick up on any seabeach scores of examples of the work of these borers. In Europe some kinds of slugs and snails do great damage in gardens, but we have little to complain of in this respect.
Largely dependent on moisture, the young snails that are hatched in midsummer at once seek retreats, and may be looked for under leaves, logs, and loose stones in the woods and pastures. Most American snails are solitary, and will be found lurking in the moss beside mountain brooklets—a favorite spot for the glassy vitrinas—hiding in the crevices of rocky banks and old walls, crawling at the edge of swampy pools, creeping in and out of the crannies of bark on aged trees, or clinging to the underside of succulent leaves. Some forms, very beautiful in their ornamentation when magnified, are so minute that they might be encircled by the letter o in this type, yet you will soon come to perceive them amid the grains of mud adhering to the undersurface of a soaked chip or rotten log.
For fresh-water species various resorts are to be searched. Go to the torrents with rocky bottoms for the paludinas and periwinkles (Melania); to quiet brooks for physas and coil shells (Planorbis); for limneas to the reeking swamps and weedy ponds. By pulling up the weeds gently, you may get small species that otherwise easily escape your dipper or net. In the Southern States and in the tropics certain forms are to be picked off bushes and mangrove trees like fruit, especially the round "apple snails" (Ampullaria) as big as your fist.