Order 1. HETEROTRICHIDA Stein, 1859

Syns.

Heterotricha, Heterotrichorida, Heterotrichorina

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.

With characteristics of order s.s. (above). Somatic ciliature well developed, but buccal organelles (paramembranelles) also numerous and prominent; of large body size, often contractile; single conspicuous contractile vacuole at posterior end of body; mucocysts in abundance, and pigment vesicles common in many species; no loricae, but cysts common; essentially all freeliving forms [endosymbiotic families formerly included here now comprise separate suborder: see below] , widely distributed in great variety of habitats.	Suborder (1) Heterotrichina Stein, 1859
As in preceding suborder, somatic ciliature well developed, buccal organelles numerous and prominent (though here unique as heteromembranelles), and body size often above average (i.e., > 150 µm). But additionally, here, are kinetodesmal (as well as cathetodesmal and retrodesmal) fibers in some species; complex "systèmes sécants," characteristic of specific kinetal patterns; macronuclear karyophore in many species; sometimes conspicuous dorsoanterior sucker; cilia-lined cytoproct in several forms; and all species endosymbionts in digestive tract of either insects (or certain related arthropods) and lower vertebrates or, less often, in oligochaetes and molluscs.	Suborder (2) Clevelandellina de Puytorac & Grain, 1976
Body generally small, top-shaped (often with very long caudal spine), with somatic ciliature absent except for caudal tuft and several anteriorly located cirri; buccal ciliature (accompanied by a "ciliary stripe") extensive, encircling body and spiraling posteriad, with cytostome thus near antapical pole; pellicle rigid; large ovoid macronucleus, multiple in several species; free living polysaprobic forms, typically in fresh-water habitats.	Suborder (3) Armophorina Jankowski, 1964
Universally loricate, sessile forms, with vermiform migratory larval stage; body large and elongate, especially in neck region in some species, with conspicuous pair of "peristomial wings" extending from the lorica (a structure generally cylindrical, with a definite bend in some species) and bearing the prominent buccal ciliature; somatic ciliation uniform and holotrichous; body highly contractile and often pigmented; stomatogenesis parakinetal; widely distributed in marine habitats (few species, fresh-water), with loricae attached to algae, higher aquatic plants, or integument or shells of invertebrates (molluscs, various crustaceans, bryozoa, coelenterates, etc.).	Suborder (4) Coliphorina Jankowski, 1967
Body laterally flattened, elongate-ovoid, with right side slightly concave; somatic ciliation uniform, but cilia in small, cirrus-like groups; no adhesive sucker; buccal ciliature extensive, coursing from apical end to subequatorial position; stomatogenesis parakinetal; macronucleus dendritic, with no karyophore; no mucocysts; endocommensals solely in certain species of lumbricid oligochaete annelids.	Suborder (5) Plagiotomina Albaret, 1974
Body in form of hourglass, with prominent oral disc apically and conspicuous basal disc at posterior pole; essentially without cilia except for buccal organelles and posterior ciliary rings; stomatogenesis apokinetal; basal disc, fiber-rich and quite complex, serves as organelle of attachment; macronucleus moniliform or fragmented; ectocommensals on a variety of marine organisms ranging from an alga (substratum for one unique species) to tunicates, with coelenterates, annelids, molluscs, and echinoderms (sea cucumber) also involved.	Suborder (6) Licnophorina Corliss, 1957