Snails.—A common name used for any mollusk with a coiled shell. In the narrower meaning it includes only those forms which occur on land. These land-dwelling forms have a slight shell, into which the whole body can be retracted. They feed exclusively on vegetation, which they rasp by means of a long ribbon, just inside the mouth, the surface of which is covered with thousands of minute teeth, so that the whole is a flexible file. The animal creeps about on a broad sole, and has four tentacles on the head, one pair of them bearing the simple eyes at the tip. Snails do considerable damage where they are numerous. One species is eaten by many in Europe, especially in France and Italy. Over ten thousand species are known.
The shells of sea snails are often of great beauty, and large sums have been given by [230] collectors for specimens of unusual elegance or rarity, fifty pounds having been paid for a single example of a species of cone shell (Conus). The helmet shells (Cassis) are made up of differently colored layers, and on account of their beauty have been largely employed for the carving of cameos.
Squid.—A mollusk nearly related to the cuttlefish. It has a barrel-shaped body, with a head in front bearing ten pairs of tapering tentacles, each with numerous suckers. On the side of the head is a well-developed eye. Squid live largely on small fishes which they catch with the tentacles, biting them with a pair of parrot-like jaws. They are largely used as bait in fishing, and to a limited extent as food. The average length is a foot or two, but in the seas around Newfoundland and Japan giants are occasionally found with bodies a dozen feet in length and tentacles adding thirty feet to this.
Tusk Shells are a small group of burrowing marine forms, in which the body is covered by a long, curved shell resembling a tusk in shape. There is a small hole at its tip, through which the water which has been used in breathing makes its exit. An imperfectly developed head and a rasping organ are present, and burrowing is effected by a long foot with a three-lobed end. The food consists of small organisms, which are apparently secured by the agency of a bunch of filaments with thickened sticky tips that can be protruded from the mouth of the shell. In some respects these animals are intermediate in structure between typical sea snails and bivalve molluscs.
JOINTED-LIMBED ANIMALS (Arthropoda)
CRABS AND LOBSTERS, SCORPIONS AND SPIDERS, INSECTS AND GRASSHOPPERS
This great division of the animal kingdom includes far more numerous species than any other, and is abundantly represented in both salt and fresh water, on the land and in the air. It consequently includes both air-breathers and gill-breathers: the former, typical land insects; and the latter, chiefly crustaceous.
CRABS AND THEIR ALLIES (Crustacea)
The Crustaceans breathe by means of gills, and their bodies consist of rings. They have two pairs of feelers and two pairs of jaws, to which are mostly joined one or more pairs of jaw feet. All the remaining rings of the body may have a pair of limbs each. The head bears two pairs of feelers. Crustaceans commonly hatch out as free-swimming larvæ, like the adult in form.
This large class of jointed-limbed animals includes lobsters, prawns, shrimps, crabs, and other familiar forms, the great bulk of which are aquatic, though the wood-lice have become adapted to a life on land.
