Molluscs
This diverse group includes chitons; clams, mussels, scallops, and other bivalves; sea snails and limpets; octopuses and squid; and nudibranchs. The basic mollusc body form has a shell, though in some molluscs the shell is reduced or internalized, or has been lost altogether. Other general traits common to molluscs include gills, a muscular foot which is often used to attach to substrates, and a rough tongue-like structure called a radula. Despite these commonalities, molluscs vary widely in body form and size. Click here for more information on molluscs.
Chitons
Chitons belong to Class Polyplacophora (poly = many, placo = tablet, phoros = bearing); they have eight overlapping calcareous shell plates that are partially or completely embedded in tough girdle tissue. These animals are very slow-moving.
Bivalves
Bivalves have two hinged shell valves. Many bivalve species are harvested for food.
Snails
Grouped here are the sea snails and other similar univalves (single-shelled gastropods) that have coiled or curved shells.
Limpets
Grouped here are the true limpets, keyhole limpets, and similar cone-shaped univalves (single-shelled gastropods).
Nudibranchs
Also known as sea slugs, these gastropods have lost the typical mollusc shell. There are various nudibranch body forms: some are relatively smooth, flat, and oval-shaped, while others are more elongated and have elaborately-branched or frilled structures projecting off their backs.
Other Opisthobranchs
Nudibranchs belong to a subclass of gastropods known as Opisthobranchia. This group includes bubble shells, pteropods, sea hares, and more. The majority of opisthobranchs have a reduced shell, internalized shell, or no shell at all. Listed here are the other opisthobranchs, which tend to be recognized less than nudibranchs.
Octopuses and Squid
Squid and octopuses, as well as cuttlefish and nautlilus, are cephalopods (cephalo = relating to the head or skull, pod = foot). These animals have diverse body forms. Squid have an internalized and reduced shell in the form of a chitinous pen, while octopuses have lost the typical molluscan shell altogether.
