Every year, thousands of shells wash up on beaches around the world. Have you ever found a shell and wondered where it came from or what its function once was?

These shells come from a large phylum of animals known as mollusks which includes members such as snails, clams, oysters, scallops, squid, and octopuses. One of the prominent characteristics of most mollusks is a hard exterior shell, although some mollusks, including the squid and the sea hare, produce internal shells.

The bodies of mollusks are very soft. Their shells protect them from predators. The body of the mollusk is formed of a combined structure called a "head-foot" that is used for locomotion. If threatened, shelled mollusks can retreat the "head-foot" quickly inside their shells.

A thin, fleshy fold of tissue called the mantle covers the internal organs of most mollusks. The mantle, which is made up of specialized...