CREATURE FEATURE - MOLLUSCS
The soft-bodied molluscs are a very successful group with around 110,000 species, including fresh water and terrestrial forms. They make up the largest group of marine animals and include snails, mussels, limpets, sea slugs, octopuses and the like.
Molluscs are characterised by a soft body protected by a calcium carbonate (chalky) shell. The shell is produced by the mantle, a thin layer of tissue covering the body. The body has a muscular foot, usually used for locomotion, and most species have a radula - a tongue with small teeth used for rasping food from surfaces. They possess eyes, which are particularly well developed in the octopus. Molluscs gain oxygen from the water through gills.
There are three main kinds of molluscs.
Gastropods (literally meaning 'stomach-footed') have a creeping foot and a single, often coiled, shell. Snails, limpets, whelks and cone shells are all in this group. Snails and limpets are vegetarians, using their radula to graze algae from rocks, while cone shells and whelks are carnivores. Whelks, common on the sea shore, have modified their radula for use as a drill with which to penetrate the hard shells of prey such as barnacles and mussels.
Bivalves (mussels, clams, scallops and razor shells) don't have a radula but they are recognised by having two parts to the shell, known as valves. They tend to be filter feeders and either burrow, attach to rocks or live freely on the sea bed. For example, clams burrow using their foot, sessile mussels produce strong byssus threads that attach them firmly to rocks, while scallops clap the two valves of their shell together to propel themselves away from danger.
Cephalopods are the third group of molluscs, it includes octopus, squid and cuttlefish and they have become specialised for active hunting. The typical molluscan shell is either internal, in squid and cuttlefish, or is lost altogether as in the octopus.