How Are Seashells Formed?
The protective shells of clams and other shell-forming mollusks are formed by a special part of the mollusk’s body called the mantle. This skin-like organ surrounds the mollusk’s soft body like an envelope. Glands inside the mantle produce the limy shell material in the form of a liquid.
As the mollusk grows, the mantle squeezes out the liquid, which quickly hardens and becomes part of the shell. The lines of growth are usually marked by the ridges on the outer surface of the shell. A mollusk gets its shell-building minerals from the food it eats.
Oysters, snails, and certain other kinds of mollusks make seashells. The shells help protect these animals’ soft bodies from dangers such as parasites, sharp objects, heavy-footed animals, and especially from animals that would try to eat them.
A shell grows as the surface of the mollusk’s body secretes a chemical that contains calcium, the same element that makes bones hard. The calcium helps form the hard shell, and when the mollusk dies, the shell is left behind.