SHELL-BEARING BRITISH MOLLUSCA.
A common garden Snail, observed crawling and feeding, is easily seen to possess a distinct head, upon which are four feelers or horns, with eyes at the end, and underneath which is a mouth; a broad disc, extending the whole length of the body, is the organ of locomotion; a spiral or coiled shell surmounts the whole.
If, on the other hand, an Oyster be opened, no distinct head, or place for eyes or mouth, can be seen, but only a seeming mass of pulp, with a round gristle in the centre (the muscle of attachment), the former enclosed in a soft skin with a double fringe round the edge (mantle), placed within a shell composed of two pieces.
All Mollusca are more or less completely represented by these two. The Snail and its class are named Gasteropoda (abdomen-creeping), and their shells Univalves; the Oyster and its class, Acephala (headless), and their shells Bivalves. We begin with the latter class at—
Pholadidæ, or "Borers," 1-13, have the power of piercing and inhabiting holes in submarine wood, rocks, and stones. The hinder part of the body is lengthened out in the form of a double tube, with openings for the ingress and egress of fluids. Teredo, 1-6, has short valves, two pallets fixed at the sides of the siphons, and lines its hole with a shelly tube. Pholas and Pholadidea, 8-13, have long valves and sometimes accessory plates. The shell of Xylophaga, 7, is like that of Teredo, but the animal has neither pallets nor tube.
Gastrochæna, 14, has a gaping shell, and encloses itself in a bottle-shaped covering of cemented stones and sand. Saxicava, 15, 16, Venerupis, 18, and Petricola, 17, burrow in sand, or live in holes of rocks.
Myadæ, or "Gapers," Mya, 19, 20, and Panopæa, 21, 21*, burrow downwards in sand and mud, their siphons protected by a coriaceous covering. Their shells are not closed all round, and those of Mya have a spoon-shaped process in the hinge of one valve.
Corbulidæ, 22-28, have short siphons with fringed edges, and the foot protrudes through a hole in the otherwise closed mantle.