Order 2. SCUTICOCILIATIDA Small, 1967

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.

Infraciliature of paroral membrane may show reduced "a" and no "c" segment, while "b" is always clearly present; scutico-vestige separate and posterior to paroral, often in anterior part of distinct director-meridian; mucocysts and mitochondria very prominent; body size generally small; most commonly in brackish or marine habitats, including sand; numerous species freeliving, but number of others occur as inquilines in sea urchins or as commensals in molluscs, coelenterates, annelids, sipunculids, and even the sea horse.	Suborder (1) Philasterina Small, 1967
Paroral membrane often prominent, its infraciliary base tripartite, with a short "a" and an elongate "b" segment and with "c" as a permanent scutico-vestige; cytostome equatorial or subequatorial in location; caudal cilia conspicuous in many species; rarely (room for) a directormeridian; two types of mucocysts sensu lato; body size rypically small to very small (with occasional striking exception); widely distributed as free-living (including psammophilic) marine species, but some commensalistic in molluscs and others in fresh-water and edaphic habitats, with a few coprozoic forms.	Suborder (2) Pleuronematina Fauré-Fremiet in Corliss, 1956
Buccal ciliature mostly subequatorial in location, often spiraled around posterior pole of the body or there in reduced form; segment "c" of paroral membrane (the scutico-vestige) usually indistinct; strongly developed thigmotactic ciliature and/or pronounced sucker or adhesive disc characteristically present at apical pole; director-meridian and cytoproct apparently often absent; somatic ciliation uniform, frequently heavy, and body laterally compressed in many species; all symbionts: one major group widely occurring in lamellibranch molluscs fresh-water or marine, and another mainly in oligochaete annelids, although other hosts occasionally involved.	Suborder (3) Thigmotrichina Chatton & Lwoff, 1922