Suborder (2) Clevelandellina de Puytorac & Grain, 1976

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.

[Most important taxonomic step, recognition of this separate suborder!]

Body ovoid to slightly reniform, plump; buccal ciliature running from near-apical to subequatorial position, in a sigmoid-like curve as it enters conspicuous infundibulum of the buccal caviry sensu lato; single, large, compact macronucleus, in anterior half of body, supported by more or less well-developed karyophore; systèmes sécants vary significantly with included genera; endosymbionts in wide variery of hosts: from oligochaetes, insects (cockroach), and myriapods (centipede, millipede) to molluscs (shipworm), amphibians (frog, toad), and reptiles, but not fishes.	Family NYCTOTHERIDAE Amaro, 1972
Body plump-ovoid to ellipsoidal, occasionally tailed; sucker (often extensive) rypically present on concave surface of body, uniquely reinforced with polysaccharide skeletal elements (which are also elsewhere on body); buccal ciliature and macronucleus similar to those of preceding family; kinetal sutures vary with genus; endosymbionts in vertebrate hosts only: amphibians, reptiles, and (if Ichthyonyctus remains here) certain fresh-water fishes.	Family SICUOPHORIDAE Amaro, 1972
Body basically ovoid or elongate-ovoid, with posterior pole oddly shaped because of bearing entire buccal cavity and its (infundibular) opening; macronucleus and systémes sécants reminiscent of Nyctotheridae; endosymbionts in certain insects (termites, wood-feeding roaches) only.	Family CLEVELANDELLIDAE Kidder, 1938
Body roughly ovoid in shape, but distorted somewhat by huge sucker on right side at anterior end and by position of buccal caviry sensu stricto (infundibulum only) directly at the (truncate) posterior pole; other features generally similar to those of the Sicuophoridae, although a transverse suture is unique here (except for its presence in Ichthyonyctus); found only in intestine of certain fresh-water fishes.	Family INFEROSTOMATIDAE Ky, 1971
Body relatively large, uniformly ciliated; the adoral ciliature and "peristome" course from anterior to posterior end of body in a gentle sigmoidal curve; compact macronucleus; single contractile vacuole; organisms found in digestive tract of fresh-water siluroid fishes.	Family NATHELLIDAE Singh, 1953