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 and come back to it.
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.
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. So a shell may be spotted, all one color, or marked with lines.