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Description of PelomyxaFree-living polymorphic species from anaerobic or micro-aerophilic habitats, with large multinucleate amoeboid feeding form, up to 5000 microns long, with small non-motile flagella, endosymbiotic bacteria, refringent cytoplasmic granules (sand); movement is directed by fountain-flow cytoplasm and anterior monopodium. Posterior uroid active in food uptake; cysts about 100 microns in diameter also present. Found in freshwater and soils. The cells have many cones of microtubules each arising from the base and sides of a single basal body; sometimes enclosing a nucleus. One to many nuclei, may be surrounded by endosymbiotic bacteria. Cytoplasm contains 2 to several glycogen bodies as large as the nuclei (10 µm). With or without 9+2 arrangement of microtubules in flagellar axoneme. There is no clear synapomorphy, but this genus is distinguished because the most form most usually encountered is a large multinucleate amoeboid stage with many inactive flagella. This currently monotypic genus has been the generic vehicle for 25 nominal species, of which, 18 have been synonymised with Pelomyxa palustris by Whatley and Chapman-Andresen, 5 have been transferred to other genera and 2 are nomina nuda. This species has a widespread distribution. Many of the nominal species assigned to Pelomyxa are regarded different forms (life stages) of one species. A complex life cycle has been described. In spring, cysts release small binucleate amoebae, which grow and become multinucleate and acquire endosymbiotic bacteria. They become elongate with a posterior uroid, and flagella are evident at this stage. The cells later become spherical, and endosymbiotic bacteria congregate around the nuclei. These cells fragment into rosettes giving rise either to cysts in winter or to small amoebae which may undergo another cycle of development. There are endosymbionts that include a methanogenic bacterium, a long bacilliform bacterium resembling Methanobacterium formicium, and a large cylindrical bacterium with a characteristic internal cleft. Type species: Pelomyxa palustris.