A North American lobster, Homarus americanus, off the coast of New Jersey, USA. Lobsters belong to the largely aquatic class Crustacea -- which includes some 26,000 species -- and the order Decapoda (Greek "deca" meaning ten) which means they have five pairs of legs. Decapoda contains shrimp, crayfish, crabs and lobsters. Lobsters and shrimp have similar body structures with large muscular tails they use to propel themselves backward through the water away from danger.