Hymenostome
（重定向自Hymenostomatida）
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 Ichthyopthirius multifiliis, a common cause of death in aquaria and fish farms.