SOME creatures, such as mammals, have a skeleton of bones inside their body. But other creatures have skeletons outside their body, for example, mollusks like clams, mussels and oysters. The shell of a mollusk is part of the creature, just as our bones are part of us.
Mollusks have glands that take particles of limestone and other substances from the water and deposit them around the soft parts of their bodies to form shells. - bigsiteofamazingfacts.com; chacha.com; sciencedaily.com