Some shells are so small they can hardly be seen without a microscope; the largest shell—that of the giant clam—may be four feet (120 cm) wide and weigh 500 pounds (230 kg). Most shells are either bivalves (made up of two parts hinged together, as in clams) or univalves (one part, as in snails). One group of mollusks, the chitons, have shells made up of eight plates.Wentletraps are marine gastropods with long, white, spiral shells.
Although not all mollusks have shells, there are thousands of species that do. The purpose of the shell is to provide protection. No two species have the same kind of shell, there being great variation in both shape and color. Often there are differences within the same species. Some shells have a tough glassy, porcelain-like surface; others have numerous spiny projections; still others have rough, granular, or ridged surfaces. Some species, such as the nautilus, have shells made up of many chambers.
Shell-bearing mollusks are found in sea-water and freshwater, on land, and on plants. Univalves can be found in almost any type of habitat. Bivalves are mostly marine, although some species such as certain clams and mussels are found only in fresh water. There are no land- or plant-dwelling bivalves. The warmer sea waters contain the largest number of species of shelled mollusks and the most colorful shells.
A shell is typically made up of three distinct layers. The outer layer, called periostracum, is made up of a protein-like material called conchiolin. The two layers under the periostracum are composed of a framework of conchiolin in which is deposited calcium carbonate (the chief component of limestone) with traces of calcium phosphate and magnesium carbonate. In some mollusks, the molecules of the innermost layer are arranged in such a way that it is hard and iridescent; such a layer is commonly called mother-of-pearl.
Mollusks begin secreting their shells very early in life—while still in the larval forms. The shell is formed by various specialized cells in the mantle—a thin layer of tissue that covers all or part of the soft portion of the mollusk's body. The materials secreted by the mantle quickly solidify into the appropriate layer of shell. The shell increases in size when new material is added to the edge of the existing shell. In general, the older the animal, the larger the shell.
Shells are widely collected, traded, or bought for their beauty and rarity. Shells are often collected on beaches where they are cast ashore by waves. They are also collected by diving or dredging, digging at low tide, or, in the case of land mollusks, by searching for them in their natural habitat. Shells obtained with the animal still inside should be cleaned. In most instances, the animal is removed after the shell is boiled in water for a few minutes. The shell's exterior can be cleaned with a stiff brush.
Shells have many uses in the arts. Their designs appear in many paintings and other works of art. Jewelry and decorative inlays on musical instruments and other objects are made from mother-of-pearl. Shells can be used as ornaments in themselves or to decorate various objects.
Shells are used to make roadbeds in some parts of the world. They are burned and slaked to make lime for fertilizer, and they are crushed for use in animal feed. Shells can be useful in archeological studies. For example, shells known to be from a certain area but found in distant burial sites or ruins can help determine such things as trade routes. The fossil remains of shells play an important role in geology and paleontology.