The metamonads are microscopic eukaryotic organisms, a large group of flagellate amitochondriate Loukozoa. Their composition is not entirely settled, but they include the retortamonads, diplomonads, and possibly the parabasalids and oxymonads as well. These four groups are all anaerobic (many being aerotolerant anaerobes), occurring mostly as symbiotes or parasites of animals, as is the case with Giardia lamblia which causes diarrhea in mammals. Characteristics A number of parabasalids and oxymonads are found in termite guts, and play an important role in breaking down the cellulose found in wood. Some other metamonads are parasites. These flagellates are unusual in lacking mitochondria. Originally they were considered among the most primitive eukaryotes, diverging from the others before mitochondria appeared. However, they are now known to have lost mitochondria secondarily, and retain both organelles and nuclear genes derived from them. Mitochondrial relics include hydrogenosomes, which produce hydrogen, and small structures called mitosomes. It now appears the Metamonada are, together with Malawimonas, sister clades of the Podiata. All of these groups are united by having flagella or basal bodies in characteristic groups of four, which are often associated with the nucleus, forming a structure called a karyomastigont. In addition, the genera Carpediemonas and Trimastix are now known to be close relatives of the retortamonad-diplomonad line and the oxymonads, respectively. Both are free-living and amitochondriate. Classification The metamonads make up part of the Excavata, a eukaryotic supergroup including flagellates with feeding grooves and their close relatives. Their relationships are uncertain, and they do not always appear together on molecular trees. It is possible that the metamonads as defined here do not form a monophyletic subgroup. The following higher level treatment is based on works of Cavalier-Smith with amendments within Fornicata from Yubukia, Simpson & Leander Metamonada were once again proposed to being basal eukaryotes in 2018. Phylum Metamonada (Grassé 1952) Cavalier-Smith 1987 emend. Cavalier-Smith 2003 Subphylum Anaeromonada Cavalier-Smith 1997 emend. 2003 Class Anaeromonadea Cavalier-Smith 1997 emend. 1999 Family Paratrimastigidae Zhang et al. 2015 Order Trimastigida Cavalier-Smith 2003 Family Trimastigidae Saville Kent 1880 Order Oxymonadida Grassé 1952 emend. Cavalier-Smith 2003 Family Polymastigidae Bütschli 1884 Family Saccinobaculidae Brugerolle & Lee 2002 ex Cavalier-Smith 2013 Family Pyrsonymphidae Grassé 1892 Family Streblomastigidae Kofoid & Swezy 1919 Family Oxymonadidae Kirby 1928 Subphylum Trichozoa Cavalier-Smith 1996 emend. Cavalier-Smith 2003 stat. n. 2013 Superclass Fornicata Simpson 2003 stat. n. Cavalier-Smith 2013 Genus Ergobibamus Genus Hicanonectes Family Kipferliidae Cavalier-Smith 2013 Order Chilomastigida Cavalier-Smith 2013