As a phylum they are morphologically diverse: gastropod species are most numerous, followed by bivalves, cephalopods, chitons, scaphopods, pteropods, aplacophorans. Although molluscs feature some unique anatomical features, no one feature is common to all Classes of mollusc. The major anatomical characteristics of molluscs are:
SHELL: a calcium carbonate exoskeleton which may contain entirely, or partially, the mollusc’s soft tissue. In some species the shell may be vestigial only, e.g. the land slug, Testacella.
BODY: the body consists of 3 regions; a head which can retract into a covered cavity, a foot with which the animal may creep, dig or swim, and a visceral hump which consists of the gut and reproductive organs.
MANTLE: this is also known as the pallium (Latin meaning’ cloak’) and is the skin of the dorsal body wall which covers the visceral hump. It is unique to molluscs and often secretes a calcareous shell with a matrix of protein (conchiolin).
MANTLE SKIRT: an extension of the mantle which forms a down-growth enclosing, beneath it, a pallial groove. This usually becomes a deep mantle cavity (known as the pallial cavity) which contains the anal and renal openings and paired gills (CTENIDIUM) which is a character unique to molluscs.
RADULA: a ribbon of minute teeth (denticles) used for feeding. The radula is unique to molluscs and is found in all Classes except bivalves. Radula morphologies are unique to species – they are a useful diagnostic feature for identification although sophisticated techniques are required to extract and examine the radula.
BUCCAL CAVITY: the ‘floor’ which contains an odontophore, consisting of cartilage and muscles which operate the radula.
OPERCULUM: a thin chitinous ‘disc’ whose shape may vary - it may be circular or tear-drop shaped, elongate with serrate edge – and which attaches to the sole of the gastropod foot. It forms a trap door to protect against desiccation or predation and is known by fishermen as a ‘shoe’. The opercula of trochaceans are calcareous.