Molluscs, also known as shellfish, are the collective name for animals in the phylum Mollusca. They are the largest group except arthropods, with about 100,000 species. The structures vary greatly, but they have common characteristics: the body is soft and unsegmented, and is generally divided into two parts: head-foot and viscera-mantle. The group of molluscs includes squid, octopus, nautilus and the extinct ammonites and belemnites.