Most seashells belong to creatures called mollusks. Snails, oysters and clams are mollusks. Mollusks have soft bodies and no bones. They need shells for protection, says 3-2-1 Contact magazine.
A mollusk starts to grow a shell when it is very tiny. The shell starts out as bits of calcuim (four bones are made of the same stuff). The shell keeps growing along with the mollusk.
Snails, whelks and many mollusks grow shells in a spiral shape. Their body organs are arranged inside the shell.
Bivalves, such as clams and oysters, are mollusks with two shells that open and close like a book.
Seashells come in all shapes and sizes. The biggest is the giant clam: its 4-foot-long shell can weigh up to 500 pounds. Some people have used these shells as bathtubs!