The mollusks (Mollusca) comprise over 120,000 recent species. This very diverse group of animals includes bivalves (Bivalvia), gastropods (Gastropoda), and cephalopods (Cephalopoda), but also largely unknown representatives such as tusk shells (Scaphopoda), chitons (Polyplacophora), and aplacophorans (Aplacophora, worm-like mollusks). They are all characterized by a soft body. Most mollusks produce a shell of calcium carbonate to protect their soft bodies, which, in some taxa, has been reduced or internalized as a supporting element. In some cultures, the shells of mollusks served as raw materials for manufacturing tools and were considered a substitute for money or jewelry. Mollusks are also an essential part of the food in many cultures.