The word "mollusk" comes from the Latin molluscus, meaning "soft." The name is apt enough, for all mollusks have soft bodies. In most cases the body is protected by a shell, made up largely of calcium carbonate. This shell is secreted by the body covering known as the mantle. Most mollusks also have an unusual structure called the foot, which takes quite different forms in various species. In clams, for example, the foot is a muscular extension of the body and is used in plowing through mud and sand; in snails, it is flat and is used for creeping. In squids and octopuses is divided into arms. which serve to seize the animals' prey. Certain oysters have no foot.
Among the best-known of the mollusks are the striking creatures known as squids and octopuses. They belong to the class of the Cephalopoda, or cephalopods ("head-feet," in Greek; so called because the foot, which is separated into a number of "arms," encircles the head). The cephalopods differ from most other mollusks in one important respect: they generally do not develop shells, the mantle forming the outer part of the naked body. In some species, however, there is an inner skeleton, lying under the surface of the body.
The cephalopods all dwell in the sea. They are provided with arms (also called tentacles), which have suckers or hooks, or both. Almost all cephalopods secrete an inklike fluid, which is stored in a special sac. When they wish to escape a pursuer, the animals squirt the ink into the water, making it turbid and thus confusing the foe. Most cephalopods are capable of chameleonlike color changes. The skin contains cells, called chromatophores ("color-bearers"), which contain different pigments. When these cells become larger or smaller, the color of the skin changes rapidly. Because of such color changes, the animals generally blend effectively with the background.
The expert cephalopod swimmer called the squid is a streamlined, spindle-shaped creature; it is sometimes called the sea arrow because of the way in which it darts through the water. The foot is divided into ten arms, of which two are longer than the rest; these arms, which bear suckers, are used to seize and hold prey. The eyes have no lids; otherwise they look startlingly like human eyes.
The squid draws water through a central cavity of the body — the mantle cavity — and forces it out through a flexible tube, the siphon, when the mantle is contracted. The siphon is located just back of the arms; the jet of water that spurts through it serves to propel the animal swiftly backward. (Ink is also discharged through this siphon.) The fins, which are two flaplike extensions of the mantle, are used chiefly for steering; they also serve to propel the squid slowly forward or backward.
One of the most familiar species is the common squid, or Loligo pealei, which is found along the eastern coast of North America and in Mediterranean and Oriental waters. It is used by some fishermen as bait; it also serves as human food, particularly in the Mediterranean area and the Orient. The squid known as the flying squid (Ommastrephes bartrami) has been compared to the flying fish; it often shoots out of the water, particularly when the weather is rough, and sometimes lands on the decks of ships.