Next Question 5. About half of all mollusk species have probably been identified. Next Question 4. Crustaceans are not a class of mollusk. Lobsters and crayfish are two types of crustaceans.
Gastropods are a class of mollusk that includes snails, conchs and whelks.
They are also called univalves. Bivalves are a class of mollusk that includes clams, oysters and scallops. Cephalopods are a class of mollusk that includes squid, nautilus and octopus. Next Question 3. An operculum is a snail s trapdoor .