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A subphylum of Protozoa, including those forms that possess flagella or pseudopodia or both. Organisms have a single type of nucleus, except the developmental stages of some Foraminiferida. Sexuality, if present, is syngamy, the fusion of two gametes. Spores typically are not formed. Flagella may be permanent or transient or confined to a certain stage in the life history; this is true also of pseudopodia. Both flagella and pseudopodia may be present at the same time.
Three superclasses are included: (1) Mastigophora, commonly flagellates, contains 19 orders. (2) Opalinata includes 1 order; these organisms were once considered as ciliates, but further research has indicated flagellate kinships. (3) Sarcodina comprises organisms which normally possess pseudopodia and are flagellated only in the developmental stages; 13 orders possess irregularly distributed lobose or filose and branching pseudopodia, while 7 orders have radially distributed axopodia, often with axial filaments.
Most of the plant flagellates will live in either fresh water or in both fresh and salt water. The zooflagellates are small and are not sufficiently abundant to enter markedly into the food chain. But the parasites and symbionts are of considerable interest economically and theoretically, for example, the trypanosomes and the peculiar xylophagous (wood-eating) symbionts of termites. In the termites these parasites actually digest the wood eaten by the host. Conspicuous in the ecology of marine waters are the dinoflagellates, radiolarians, and acantharians, especially in tropical waters of otherwise low productivity. See Protozoa