Seashells are the external skeletons of a class of marine animals called mollusks. The external skeleton helps protect the creatures from predators, strong currents and storms, help camouflage the animal, and do many other things. Seashells are primarily made of calcium. When a mollusk dies, its shell is left behind and then sometimes the shell is taken and used as a home by other sea creatures, such as hermit crabs. After the animal that created the seashell dies, the shell often washes up onto the shore, or remains in the tide pool where the creature lived.  A great beach puzzle!