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Hymenostome
Tetrahymena thermophila
Scientific classification
Domain:	Eukaryota
Clade:	Diaphoretickes
Clade:	SAR
Clade:	Alveolata
Phylum:	Ciliophora
Class:	Oligohymenophorea
Subclass:	Hymenostomatia Delage & Hérouard, 1896
Order:	Hymenostomatida Delage & Hérouard 1896
Typical families
Suborder Tetrahymenina     Curimostomatidae     Tetrahymenidae     Turaniellidae     Glaucomidae Suborder Ophryoglenina     Ichthyopthiriidae     Ophryoglenidae Suborder Peniculina

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.^[1] 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.^[2]

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