The protecting shells of clams and other shell forming mollusks are built by a special part of the mollusk’s body called the mantle. This skin-like organ surrounds the animal’s soft body like an envelope.
Glands inside the mantle produce the substances that make the shell, in the form of a liquid. The mantle squeezes out the liquid shell materials and adds them to the shell as the mollusks grows.
The lines of growth are usually clearly marked by ridges on the outer surface of the shell. The food eaten by a mollusk provides the minerals that form the shell and give it color.–Dick Rogers