Molluscs are invertebrate animals that do not possess a backbone. They have unsegmented soft bodies with a muscular foot and tentacles. Molluscs form one of the largest groups in the animal kingdom with over 80,000 known species. Most molluscs have an external shell covering. Molluscs that are covered by one shell are called snails and molluscs that live inside two hinged shells are called bivalve molluscs.
Snails are found on land, in freshwater environments and in the sea. Sea snails are commonly known by other names such as the limpit, the whelk and the periwinkle. Similarly bivalve molluscs are more known by their common names such as cockles, mussels, razorshells, oysters and scallops. Some bivalve species of mussels and cockles also live in freshwater.
The sea contains a huge variety of other molluscs including the armour-plated chitons and molluscs without an external shell such as the octopus, squid, cuttlefish and slugs. There are an enormous variety of slugs in the sea. Slugs are also commonly found on land, but there is only one species of freshwater slug known to exist in the world and that is in Australia.