Pelomyxa are giant amoebae, usually 500-800 μm but occasionally up to 5 mm in length. The most notable species is P. palustris; other described species may be synonyms, or have been moved to the unrelated genus Chaos. They are found in mud at the bottom of freshwater streams.
Pelomyxa have anywhere from two to several hundred nuclei. A moving cell is cylindrical in shape, with a single hemispherical pseudopod at the front and a semipermanent projection called a uroid at the back, which is covered in tiny non-motile flagella. They consume a wide variety of food, and have many vacuoles containing whatever particles that happen across, both food such as diatoms and debris such as sand.
This is one of several genera of amoebae that lack mitochondria. As such it has been considered as possibly one of the most primitive eukaryotes, and given its own phylum (Pelobionta or Caryoblastea). However the absence is now known to be secondary. It is grouped with the other amitochondriate genera as the Archamoebae, which are now placed among the Amoebozoa.