Subclass (1) Hymenostomata Delage & Hérouard, 1896

Syns.

Homoiotricha p.p., Tetrahymenophora s.s.

Buccal structures, when not absent altogether, usually inconspicuous; typically uniform, often heavy, somatic ciliation; body generally of medium size; found in diverse habitats, with nonsymbiotic forms predominantly from fresh-water biotopes.

[(Grain et al. (1976) have very recently erected a new order, the Parahymenostomatida, which they assign to this subclass. The (few) organisms involved have been allocated in this book to the order Nassulida (class Kinetofragminophora); but the matter certainly deserves further serious consideration.]

Well-defined buccal cavity (with rare exception), typically with paroral membrane plus tripartite AZM (or peniculi ± quadrulus); fusiform, explosive trichocysts in one suborder; mostly free-living, fresh-water forms.	Order 1. HYMENOSTOMATIDA Delage & Hérouard, 1896
Body typically small to medium in size; ciliation uniform, though sometimes sparse, often including specific thigmotactic area(s) and one or more caudal cilia; a director-median common to many species; paroral membrane sometimes the dominating feature of the oral ciliature, which may lie in an extended but shallow buccal caviry; stomatogenesis buccokinetal with complex morphogenetic movements and unique involvement of a scutica (which may persist in the trophont as a scutico-vestige); mucocysts prominent; large, elbngate, cortically located, often-fused mitochondria conspicuously present; cysts widespread; particularly abundant in marine habitats, free-living or in symbiotic association with primarily invertebrates (especially molluscs, echinoids, and annelids), but also some edaphic and fresh-water forms.	Order 2. SCUTICOCILIATIDA Small, 1967
Body relatively large, especially often lengthy, cylindrical or flattened-ovoid, uniformly ciliated, mouthless; an infraciliary endoskeleton of considerable complexiry may be present, frequently with elaborate holdfast organelle at anterior end of body; typically anisotomic fission, sometimes involving budding with chain-formation; contractile vacuoles present, in one or two rows, but cytoproct absent; universally endosymbiotic (possibly one exception: see first family, below), with majority found in digestive tract of oligochaete annelids from soil, fresh-water, brackish, or marine habitats; but some species in polychaetes, leeches, turbellarians, or molluscs, and one major group exclusively in tailed amphibians.	Order 3. ASTOMATIDA Schewiakoff, 1896