hymenostome 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. among these is the important species ichthyophthirius multifiliis, a common cause of death in aquaria and fish farms. hymenostomes tetrahymena thermophila scientific classification domain: eukaryota clade: diaphoretickes clade: sar clade: alveolata phylum: ciliophora class: oligohymenophorea subclass: hymenostomatia order: hymenostomatidadelage & 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.