Hymenostome

Taxobox | color = khaki
name = Hymenostomes
domain = Eukaryota
unranked_phylum = Alveolata
phylum = Ciliophora
classis = Oligohymenophorea
ordo = Hymenostomatida
ordo_authority = Delage & Hérouard 1896
subdivision_ranks = Typical families
subdivision = Suborder Tetrahymenina
Curimostomatidae
Tetrahymenidae
Turaniellidae
Glaucomidae
Suborder Ophryoglenina
Ichthyopthiriidae
OphryoglenidaeThe 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. Among these is the important species "Ichthyopthirius 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 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. 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.