Subclass Spirotricha Bütschli, 1889

Syns.

Blepharismina, Bursari[i]na, Condylostomatina, Stentorina

With characteristics of class (above).

With characteristics of subclass s.s. (above). Body generally large to very large, sometimes highly contractile; somatic ciliation frequently holotrichous, with regular, even dense, rows of simple cilia (though not true of all suborders); wide distribution, both free-living and symbiotic forms.	Order 1. HETEROTRICHIDA Stein, 1859
Body small, laterally compressed, wedge- or helmet-shaped (nearly as wide as long), with armor-like cuirass and often short posterior spines or, occasionally, cirri; somatic ciliature reduced; buccal membranelles inconspicuous, < 10 in number; one to several rounded macronuclei; no cytoproct; fresh-water (rarely marine) polysaprobic forms.	Order 2. ODONTOSTOMATIDA Sawaya, 1940
Body form ovoid to elongate, sometimes tailed, with thickened pellicle; commonly reduced somatic ciliature; buccal paramembranelles extensive and conspicuous, often in two sections one inside buccal cavity proper, other ("somatic" portion) out onto body surface encircling anterior pole of organism; infraciliary base of paroral membrane composed of single, nonzigzag row of kinetosomes (monostichomonad); stomatogenesis apokinetal in a below-surface pouch; a perilemma present external to the cell (plasma) membrane in many species; possibly no cytoproct; macronucleus with reorganization bands; mainly (but not exclusively) marine, pelagic organisms, and free-swimming (even when loricate, as in suborder Tintinnina); all free-living except for two oligotrichine species associated with echinoids.	Order 3. OLIGOTRICHIDA Bütschli, 1887
Dorsoventrally flattened, oval or elongate, medium-sized forms, occasionally tailed; unique cursorial mode of locomotion; prominent zone of generally numerous adoral (para)membranelles on left-anterior portion of the ventral surface, bordering a broad peristomial field and sometimes continuing over apical end of body onto the dorsal surface; paroral membrane may be multiple (diplo- or polystichomonad condition); somatic ciliature commonly represented by rows or localized groups of cirri, conspicuous on the ventral surface, and rows of widely spaced pairs of short cilia ("sensory bristles") dorsally; a perilemma present in some species; stomatogenesis generally apokinetal, but apparently parakinetal in more primitive forms; macronuclear reorganization bands common; cytoproct, as well as a contractile vacuolar system, always present, and mucocysts plentiful; widely distributed free-living forms in many and diverse habitats, with a few ectocommensalistic on various invertebrates and one inquilinic in an echinoid.	Order 4. HYPOTRICHIDA Stein, 1859