Mollusc shell

The mollusc (or mollusk) shell is typically a calcareous exoskeleton which encloses, supports and protects the soft parts of an animal in the phylum Mollusca, which includes snails, clams, tusk shells, and several other classes. Not all shelled molluscs live in the sea, many live on the land and in freshwater.

The ancestral mollusc is thought to have had a shell, but this has subsequently been lost or reduced on some families, such as the squid, octopus, and some smaller groups such as the caudofoveata and solenogastres, and the highly derived Xenoturbella. Today, over 100,000 living species bear a shell; there is some dispute as to whether these shell-bearing molluscs form a monophyletic group (conchifera) or whether shell-less molluscs are interleaved into their family tree.

Malacology, the scientific study of molluscs as living organisms, has a branch devoted to the study of shells, and this is called conchology - although these terms used to be, and to a minor extent still are, used interchangeably, even by scientists (this is more common in Europe).