Subphylum 1. POSTCILIODESMATOPHORA: Class 2. HETEROTRICHEA – Once Close to the Top
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Ciliates in this class were thought to represent the pinnacle of ciliate evolution, along with the spirotrichs. However, small subunit rRNA gene sequences and the presence of postciliodes-mata in the somatic cortex strongly relate members of this class to the Class KARYORELICTEA. The heterotrichs are typically majestic ciliates of large cell size and with a conspicuous adoral zone of polykinetids or membranelles (AZM) that extend out over the peristomial surface. The ciliates in this class are not subdivided, and so there is one order —Order Heterotrichida. Heterotrichs are found in a diversity of habitats, from the marine benthos and hydrothermal vents to the plankton of high altitude oligotrophic lakes. They feed on a diversity of prey, ranging from bacteria up to small metazoa, like rotifers, and sometimes are conspicuous by carrying symbiotic zoochlorellae. Their body is highly contractile, elongated by postciliodesmal micro-tubules and shortened by contractile myonemes. The oral structures have a paroral and multiple paramembranelles. Stomatogenesis is parakinetal. Macronuclei can be nodular, and are divided by extramacronuclear microtubules. Conjugation has not been studied in any breadth in the class with the gamone-receptor system of Blepharisma being the only model. The heterotrich Spirostomum has been developed as a bioassay model for heavy metals.
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