These species lack mitochondria, are variably aero-tolerant and inhabit environments with low oxygen where they feed on bacteria (Fig. 1.2). They are found in low-oxygen environments as commensals in the intestinal tract of animals or free living. Retortomonads consist mostly of small organisms 5-20 ^m long, with two pairs of basal bodies with usually four emerging cilia (e.g. Chilomastix). The basal body pairs are perpendicular to each other and connected by a striated fibre. The recurrent trailing cilium runs through the cytoplasm and emerges posteriorly through
a groove which functions as the cytostome. The recurrent cilium and its associated ribbons of cytoskeleton are more closely associated with the nucleus, and form an elaborate organelle that contributes to motility and cell rigidity. The cytostome is a long funnel into the cell and is used for phagocytosis of bacteria. The cell membrane and cytostome are lined by supporting microtubules. There is no Golgi, and the endomembrane network is poorly developed. The nucleus is held tightly by the cytoskele-ton close to the anterior of the cell. Mitosis in Retortomonadea is closed, with an intranuclear spindle. The resistant cyst stage is transferred between host individuals, and sexual stages are not known.
The Trepomonadea resemble the Retortomonadea on most points, with the following distinctions. There are additional supporting cytoskeletal fibres extending from the basal bodies to the nucleus (Enteromonas, Trimitus and Caviomonas). In the Trepomonadea, mitosis is semi-open, with a mostly intranuclear spindle. Some genera (Trepomonas, Hexamita, Spironucleus, Octomitus and Giardia) are double cells that are a mirror image of each other. Mitosis is a semi-open type with an intranuclear spindle. There are free-living aero-tolerant species (in Hexamita and Trepomonas), but most are intestinal parasites or commensals. The latter have lost a functioning cytostome and rely on osmotrophy or pinocytosis.
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