Malacology is the field of science which studies the molluscs (phylum Mollusca) which includes familiar groups of invertebrates (animals without a back bone) including snails and slugs, clams and mussels, squids, octopuses, scallops, oysters, and chitons. The molluscs are one of the most diverse groups of animals on our planet with over 100,000 known species (up to 200,000 more likely). They occur within almost every type of habitat, from the deep seas to dry deserts, as important members of the ecosystem. Most mollusc species secrete hard shells which are collected by beach combers worldwide. Historically, humans have used molluscs as food, material for weapons, money, religion, and jewelry.