This is the second largest phylum of marine invertebrates. They range in size from snails less than 1mm long to the giant squid which is estimated to grow to more than 18m long. They include shelled snails, shell-less slugs, coral and wood boring wormlike bivalves, giant clams, blue-ringed octopus, and fragile sea-butterflies which swim and drift their whole lives in the plankton. Molluscs play an important role in the diverse web of reef life, each species having its own effect on the community it lives in. Most invertebrates and many algae have specific molluscs which feed upon them. The single spiral shell of a gastropod snail is the most easily recognised mollusc, but closely related to them are the limpets, with a cap-shaped shell and the nudibranch sea slugs which have lost their shell completely. The cockles, oysters and clams are bivalve molluscs, so named because they have two shells. The other major mollusc class are the cephalopods, which include the octopus and the squid. The cephalopods have the most highly developed nervous system and brain amongst the invertebrates.

• An unsegmented body with a protective shell of calcium carbonate (which may be reduced or absent).
• To have a
• A circulatory system with heart(s) and vessels but also with blood-filled body cavities.
• A nervous system with
• To have a unique veliger larvae.
• To have a unique 'tongue' or ribbon of teeth, called radula, for feeding (absent in bivalves).