Pelobionts

The Pelobionts, or Caryoblastea, are a small group of amoeboid protists lacking both mitochondria and dictyosomes. These conditions are probably primitive. The most notable member is Pelomyxa, called the giant amoeba because of its size: usually 500-800 μm, but occasionally passing 3 mm in length.

Pelomyxa are found in the 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. There are anywhere from two to several hundred nuclei, which undergo mitosis independently of cell division. Pelomyxa are not picky eaters, and are full of vacuoles containing whatever food they happened across along with sand and other debris. Symbiotic bacteria take the place of mitochondria. The most notable species is P. pelustris; several other species have been described, but they may be synonyms.

The monoflagellate amoebae Mastigamoeba and Mastigella show various ultrastructural similarities to Pelomyxa, and so are probably related. Other than these it does not appear to have any especially close relatives, and probably diverged from the rest of the eukaryotes early on.