Shells are found in woodlands, rivers and ponds as well as the sea. When people speak of shells, they usually mean those of soft-bodied animals known as mollusks.
Most mollusks have shells outside their soft bodies. The shell is a mollusk's skeleton. It is part of the animal, and the mollusk is attached to it by muscles. The soft animal inside can never leave its shell.
The shell is made of a form of limestone and is built by the mollusk itself. Certain glands in the mollusk are able to take limestone from the water and deposit it in tiny particles at the edge of and along the inside of the shell.
As a mollusk grows in size, its shell increases in thickness and size. You can see the lines of growth that are marked by ridges that run parallel to the outer edge. You've probably noticed these growth lines in the shells of oysters. The other ridges are caused by ridges in the "mantle" of the mollusk, or by muscles in its body.
The shell of a mollusk consists of three layers. The outside is covered with a thin layer of hornlike material that contains no lime. Under this is a layer of carbonate of lime. The inside layer is the "mother-of-pearl," or nacre. It is made up of very thin alternate layers of carbonate of lime and a horny substance.
The coloring of the shell comes from some glands of the mollusk that contain coloring matter.