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Dinoflagellida(Dinophyceae, Pyrrophyta, dinoflagellates; subphylum Sarcomastigophora, superclass Mastigophora) An order of protozoons that are heterotrophs but closely allied to brown algae and diatoms (they are sometimes classified as algae). Many have brown or yellow chromoplasts containing xanthophyll and chlorophylls a and c; others are colourless. Typically, dinoflagellates have two flagella (see FLAGELLUM), one, propelling water to the rear and providing forward motion, attached just behind the centre of the body and directed posteriorly, the other, causing the body to rotate and move forwards, forming a transverse ring or spiral of several turns around the centre of the body. Some dinoflagellates are naked, others are covered with a membrane or plates of cellulose. Many species are capable of emitting light, and these are the main contributors to bioluminescence in the sea. Most are planktonic, some in freshwater but most in marine environments, and some live in symbiosis with animals (e.g. the flatworm Amphiscolops, sea anemones, and corals) with which they exchange nutrients. Some are colonial. There are many species.