Excavate flagellates, small (less than 30µm), free-living or parasitic in animals; anaerobic or microaerophilic; no mitochondria; although there may be Golgi function there is no Golgi apparatus (dictyosome); with one or two karyomastigonts formed of 4 basal bodies and flagella closely associated with a nucleus; in each karyomastigont one flagellum is always directed backward and is generally associated with a ventral cytostome; mitosis is semi-open, the nuclear envelope persists and the two hemi-spindles are extranuclear with kinetochore microtubules penetrating in the nucleus by pores; form resistant cysts which ensure the transmission in parasitic species; free-living species found in water sites rich in organic matter and deficient in oxygen such as sediments, stagnant reservoirs, marshes, water treatment plants and also in brackish or salt water; feed on bacteria and also on dead cells of other protists, plants and animals which are engulfed by the cytostomal apparatus.