The Molluscs are a large phylum of about 100,000 species. They are characterised by the shell and the mantle that secretes it, the radula which is a peculiar file-like feeding structure, and the moluscan gills. The phylum includes, as well as certain primitive groups, the Bivalves which include the cockle and mussel, the Cephalopods which include the cuttlefish, squid and octopus, and the Gastropods which include the limpet, whelk, snails and slugs.