Hymenostome

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.

Hymenostome
Tetrahymena thermophila
Scientific classification
Domain:	Eukaryota
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 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]