The mollusks that live inside the calcium carbonate shell are soft-bodied animals. The organism's exterior skeleton is its shell. It is typical for other marine creatures to move into the empty shell when the organism has died and is no longer inside to establish a home.
Seashells can have one, two or even many valves, and they may be made of shells (like those of snails) or plates that are hinged together. In some slugs, they also break down into tiny internal grains.
The spiraled shells of gastropods, bivalves, and shelled cephalopods are like equiangular (or similar) spirals. Shells use a variety of complex structures to protect themselves from predators, strengthen their skeletons, and help with movement.