Sometimes placed in the Cryptophyta. Planktonic organisms, usually unicellular, found mainly in the sea but also in freshwater. Have no mitochondria but instead have symbiotic bacteria that seem to perform a similar function thus illustrating how mitochondria might have started out prior to their total integration into the cellular machinery. Known only from a few species in the genus Pelomyxa that occur in the Northern Hemisphere in low oxygen freshwater muds. Single-celled flagellates, once placed in the Zoomastigina.
Most species are commensal in animal intestines but some are free-living in stagnant fresh water.
Giardia is a diplomonad that gets into fresh water streams and causes hiker's diarrhoea. Genera include Giardia, Hexamita, Spironucleus, Trepomonas. Entamoeba histolytica is a pathogen of humans found in water and which infects the large intestine, causing diarrhoea and death of cells it comes into contact with. Other species sometimes found in people's intestines include Entamoeba coli, Dientamoeba fragilis, Iodamoeba butschlii and Endolimax nana . Usually unicellular but some are colonial.