An order of ciliates (subclass Peritrichia, phylum Ciliophora) characterized by a cylindric shape with the cilia usually limited to the zone surrounding the mouth opening; includes the suborder Mobilina, the members of which are all ectoparasites or endoparasites of aquatic invertebrates and vertebrates, of which the genus Trichodina includes economically significant gill parasites of fish. [peri- + G. thrix, hair]