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Testacea and Shells

The Testacea, or Shell-fish, have 4500 species, and those without shells 600; of radiata, as star-fish and medusa, there are 1000 species, and of polypus and corals 1500. Mollusca have no skeleton, and the muscles are attached to the skin, which encloses the viscera and nerves. In many, a shell is added to the skin, and, in all, there are circulating, respiring, and digesting systems, with senses of taste and sight. Aristotle divided Shells into three orders: univalves, of one piece, and bivalves, and turbinated; and the same arrangement is still preserved, except that the order is changed by Linnæus, and the turbinated are called multivalves; and he divides them into three genera of multivalves, fourteen of bivalves, and nineteen of univalves. In univalves, in some species, the cavity is divided into chambers, with a pipe of communication; the base is the mouth, and opposite is the apex. The convolutions are called whorls. The hinge in bivalves is sometimes furnished with teeth. It is usually joined by a strong ligament. They adhere to rocks by a thread-like substance called the beard.

Crustacea cast their shells once a year, and after remaining some time in a tender soft state, secretions of carbonate and phosphate of lime renew them. They also renew lost limbs, which after two or three moultings resemble the originals. Their remains, often very perfect, different from modern species, are found in cretaceous rocks, as oolite, lias, chalk, with crabs and lobsters often above the chalk, and in London clay.

Shell-fish form a vacuum, so that they adhere to rocks by a force equal to 15 lbs. to the square inch of their contact. Lobsters and crabs change their shells annually. For three or four days they are naked, defenceless, and preyed upon. Their limbs shrink before drawn out, and they grow only in the soft state. They lose their limbs in fighting.