Hymenostome
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The hymenostomes are an order of ciliate protozoa. Most are free-living in freshwater, such as the commonly studied genus Tetrahymena, but some are parasitic on fish or aquatic invertebrates.<ref>Template:Cite journal</ref> Among these is the important species Ichthyophthirius multifiliis, a common cause of death in aquaria and fish farms.
The hymenostomes are fairly typical members of the Oligohymenophorea. Their body cilia are mostly uniform, sometimes with a few long caudal cilia, and arise from monokinetids or from dikinetids at the anterior. The oral cilia are in general distinctly tetrahymenal, with three membranelles and a paroral membrane, which corresponds only to the middle segment of the tripartite membranes found in certain scuticociliates. Mouth formation during cell division usually begins next to a postoral kinety.
The hymenostomes were first defined by Delage & Hérouard in 1896. Initially the scuticociliates and peniculids were included, then later treated as separate orders of a subclass Hymenostomatia, to which the astomes are sometimes added. More recently each of these groups tends to be treated as a separate subclass.<ref>Template:Cite book</ref>