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.
Molluscs form the largest marine phylum with an estimated 85,000 living species. They are invertebrates and have colonised a wide range of terrestrial and aquatic habitats. In addition to occupying equable habitats in temperate zones and the tropics, there are mollusc species living in hostile and extreme environments: in deserts, at the Poles and in the hot waters at hydrothermal vents on mid-ocean ridges.
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 following list of categories provides further information on UK molluscs:
- Encyclopedia - The Encyclopedia is an emerging on-line reference source for the species of mollusc found within Britain and Ireland which includes illustrations, descriptions, and distribution maps. It is under development but already contains over 250 accounts and 1500 images of species.
- Identification guides - The Society is actively developing a range of identification guides catering for a range of skill levels.
- Glossary - There are a variety of terms describing aspects of molluscs used to describe the appearance and parts of molluscs. A reasonably comprehensive list is included in the glossary.
- Conservation - Molluscan conservation is an important part of the Society's activities, and the Society works closely with a wide range of other governmental and non-governmental organisations in order to include molluscs in programmes maintaining Britain's species and habitat diversity.