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Pelomyxa are giant amoebae, usually 500-800 μm, but occasionally passing 3 mm in length. The most notable species is P. palustris; other described species may be synonyms. They are found in mud at the bottom of freshwater streams. A moving cell is cylindrical in shape, with a single hemispherical pseudopod at the front and a semipermanent bulb called the uroid at the back, which is usually covered in thin non-motile extensions. The cytoplasm streams forward through the center of the organism and back along the outside, allowing the creature to slide along the substratum. They are not picky eaters, and are full of vacuoles containing whatever food they happened across, along with sand and other debris.
Pelomyxa have anywhere from two to several hundered nuclei, which undergo mitosis independent of cell division. They are one of several genera of amoebae that lack mitochondria, as well as dictyosomes. As such, they have been considered as a possibility for one of the earliest eukaryotes to diverge, and given their own phylum (Pelobionta or Caryoblastea). However, the absence is now known to be secondary, as with the parasitic entamoebae and flagellated mastigamoebae. Genetic studies show these groups to be close relatives, especially Pelomyxa and the mastigamoebae, which are grouped together as the pelobionts; all of them are now placed among the Amoebozoa.
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