Seashells, often found washed up along the shores of oceans, are the skeletons of marine animals known as mollusks. Mollusks have exterior skeletons, whereas many mammals, including humans, have interior skeletons. The skeleton of a mollusk helps to protect it against the elements and camouflages it from potential predators.
Like human bones, seashells are primarily made up of a hard mineral known as calcium. Seashells are the remains of mollusks that have died and left their skeletons behind, just as mammals with interior skeletons eventually do after the process of decomposition.
There are two primary types of seashells. Bivalves are two shell halves that make up a whole. Examples of mollusks that leave behind bivalve shells include clams and oysters. Univalves, on the other hand, are spiral-shaped, one-piece shells. They are usually very elaborate and resemble the shell of a snail. Some common types of spiral seashells include conchs, whelks and nautilus.
Seashells are used by other marine creatures after the mollusk has died and abandoned its skeleton. The skeleton generally remains in the tide pool where the creature lived, or it washes up to the shore. Hermit crabs are known to take up residence within certain types of seashells, usually univalves.