Molluscs. Molluscs cover a huge array of sea creatures, some of which do not even bear shells. They consist of the gastropods, creatures such as the abalone, sea snail and limpet, which live in a single shell and are soft bodied, boneless; the bivalves which live in a double, hinged shell and include mussels, oysters, clams, cockles and scallops; and the cephalopods, including the octopus and squid. I have also heard bivalves described as the creatures which are exposed to the air in tidal regions, such as mussels and clams. This sounds much less likely. It was given as a reason why some creatures must be obtained and cooked in a live state, since, as they can live without being submerged in the water, it is important to keep them in this state. This, of course, also applies to lobsters and crabs which, unless fished out of the sea would generally prefer to be down in the depths. You don't meet many crabs or lobsters taking a breather on the beach in Cornwall, though some species do require to leave the sea to reproduce.