Hymenostomatida

The Hymenostomatida are an order of ciliate protozoa, included in the class Oligohymenophorea. Most are free-living in freshwater, such as the commonly studied genus Tetrahymena, but some are parasitic on fish or aquatic invertebrates, generally with a complicated life-cycle. Among these is the important species Ichthyopthirius multifiis[?], a common cause of death in aquaria and fish farms.

As is typical for the class, the 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 distincly 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. Intially 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.