Stein, 1859
Dorsoventrally flattened, oval or elongate, medium-sized forms, occasionally tailed. Prominent zone of adoral membranelles on left-anterior portion of ventral surface. Somatic ciliature represented by rows or groups of cirri on ventral surface and rows of basal body pairs on dorsal side; perilemma present in some species. Stomatogenesis generally apokinetal, but apparently parakinetal in more primitive species. Macronuclear reorganization bands common. Mucocysts plentiful. Widely distributed free-living forms in many and diverse habitats, with few ectocommensal on various invertebrates and rarely inquilinic in echinoids. Data on morphogenesis often needed to arrange families and genera into proper taxonomic groupings. Thus, classification is in many instances provisional.
Suborder Stichotrichina
Family Urostylidae
Suborder Sporadotrichina
Family Oxytrichidae
Family Aspidiscidae
Family Euplotidae