A mollusc is an animal with a soft body. Many — snails, clams, oysters — have a hard shell covering that body, and that is the picture most school books give you. Though actually, the group is wider than “shell on the outside.” An octopus and a squid are molluscs too, and a slug is a mollusc with no shell you can pick up. Some have only a small shell hidden inside. It is a biology word, not a dinner-table word, though people eat many molluscs. The spelling mollusc is the usual British one; American English almost always writes mollusk. In Taiwan school science you will see 軟體動物 either way. Do not stretch it to mean any sea creature: a fish is not a mollusc, and a crab is not one either.