Suborder (3) Thigmotrichina Chatton & Lwoff, 1922

Syns.

Arhynchodina, Diplohymenina, Hysterocinetina, Parastomatina, Stomatina, Stomodea, Thigmotricha, Thigmotrichida

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.

Body of small to medium size, occasionally elongate, with anterior thigmotactic ciliature not set apart from other somatic kineties; ventral oral area courses nearly length of body, with cytostome moving progressively posterior-poleward; buccal ciliature conspicuous, winding in arc of >360° around antapical pole in some species; widely found in mantle cavity and, less often intestine of marine and fresh-water molluscs (prosobranch limpets, pulmonates, lamellibranchs) and respiratory organ of holothurian echinoderms	Family ANCISTRIDAE Issel, 1903
Body often small, with somatic ciliature in reduced number of spiraled rows in many species (becoming oblique and even almost horizontal in some); distinct thigmotactic area of reduced dorsal kineties enclosed in a système sécant, very pronounced in certain genera; arrangements of buccal ciliature parallel those seen in the Ancistridae, but the ciliature is often reduced when at the posterior pole and forms an arc of < 180°; in mantle cavity of marine and fresh-water molluscs or on integument of certain echinoderms.	Family HEMISPEIRIDAE König, 1894
Body of medium to large size, somewhat flattened laterally, densely ciliated~ prominent thigmotactic sucker, essentially at apical end of body, comprised of segments of left-anterior kineties surrounded by nonciliated strip or field (often giving it a horseshoe shape) and generally strengthened by fibers or other skeletal structures; buccal apparatus, reduced or even rudimentary, at antapical pole; reproductive methods include posterior budding or catenulation in some species; widespread as commensals in intestine of oligochaete annelids, aquatic or terrestrial, with a few species of two genera (Hysterocineta, Ptychostomum) in gut of certain fresh-water snails.	Family HYSTEROCINETIDAE Diesing, 1866
Body laterally compressed and highly Astomatida-like in appearance: often elongate (up to 1,500 µm), with lengthy macronucleus, thickened pellicle, two rows of numerous contractile vacuoles, dense and uniform ciliation, and budding possible; buccal apparatus, reduced and inconspicuous, located short distance from apical pole; commensal in intestine of prosobranch snails.	Family PROTANOPLOPHRYIDAE Miyashita, 1929
Large, cylindrical, with spiraled rows of somatic ciliature running nearly at right angles to long axis of body; prominent aboral sucker or adhesive disc; buccal ciliature appears to be at posterior pole; huge, branching macronucleus; in mantle caviry of shipworm (eulamellibranch mollusc).	Family NUCLEOCORBULIDAE Santhakumari & Nair, 1970

Incertae sedis in suborder Thigmotrichina:
Syndaetor Berger [unpublished] . Single (or more? ) species, in echinoids [deserves family of its own?] .
Incertae sedis in order Scuticociliatida:
Agigea Lepsi, 1965. Single species [small organism, figured (Lepsi, 1965) but not described verbally] .