Merriam-Webster Collegiate Dictionary - mollusk

 or mollusc  noun  Etymology: French mollusque, from New Latin Mollusca, from Latin, neuter plural of molluscus thin-shelled (of a nut), from mollis  Date: 1783 any of a large phylum (Mollusca) of invertebrate animals (as snails, clams, or squids) with a soft unsegmented body usually enclosed in a calcareous shell; broadly shellfish  • molluscan also ~an adjective