Pleuronematina

Suborder (2) Pleuronematina Fauré-Fremiet in Corliss, 1956

Syns. Deuterostomatina p.p.,
Pleuronematorina

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.

Dominating feature is the paroral membrane, sometimes present as a stiff velum and distinctly curling around the subequatorial cystome; multifragmented paroral (segment "b") in one species; long, stiff caudal cilia; some species possibly have trichocysts?; in fresh-water and (especially) marine habitats, occasionally associated with some invertebrate as an ectocommensal (e.g., Pleurocoptes on hydractinian coelenterates). Family PLEURONEMATIDAE Kent, 1881

Body ovoid to elongate-ovoid, generally small to very small, with sparse ciliation and distinctive caudal cilium; cytostome variable in position and buccal ciliature less prominent than in the Pleuronematidae; macronucleus and large micronucleus often located in anterior third of body; widely distributed forms, from fresh-water, brackish, marine, interstitial, edaphic, and even coprozoic habitats. Family CYCLIDIIDAE Ehrenberg, 1838

Body elliptical in outline, with right side slightly concave and anterior end a little narrower than posterior; generally of large size; dense somatic ciliation, interspersed with longer bristles; prominent buccal caviry, medially located on ventral surface, containing distinctive tetrahymenal organization of buccal ciliary organelles plus scutico-vestige, though paroral membrane not prominent; two sizable macronuclei, with several micronuclei; commonly facultatively psammophilic forms, both fresh-water and marine habitats. Family HISTIOBALANTIIDAE de Puytorac & Corliss, n. fam.

Body kidney-shaped, large, strongly flattened laterally, densely ciliated; oral area subequatorial on ventral margin, with buccal ciliature clearly detectable; huge macronucleus; commensal in mantle caviry of marine bivalve mollusc (Mytilus edulis) but also in fresh-water clams and certain sea urchins. Family PENICULISTOMATIDAE Fenchel, 1965

Body tlattened laterally, bluntly tapered posteriorly, with clearly delineated area of thigmotactic ciliature on concave surface; posterior half of body sparsely ciliated; cytostome at an equatorial level, with buccal ciliature not highly developed; endoparasitic in renal organ of terrestrial snails. Family THIGMOCOMIDAE Kazubski, 1958

Body laterally compressed, generally ellipsoidal to broadly reniform in shape, with dense and uniform ciliation; buccal caviry relatively small, located nearly equatorially on ventral surface; commensals in mantle caviry (on gills) of fresh-water bivalve molluscs. Family CONCHOPHTHIRIDAE Kahl in Doflein & Reichenow, 1929

Incertae sedis in suborder Pleuronematina:

Larvulina Penard, 1922.
Single bonafide species.

Dominating feature is the paroral membrane, sometimes present as a stiff velum and distinctly curling around the subequatorial cystome; multifragmented paroral (segment "b") in one species; long, stiff caudal cilia; some species possibly have trichocysts?; in fresh-water and (especially) marine habitats, occasionally associated with some invertebrate as an ectocommensal (e.g., Pleurocoptes on hydractinian coelenterates).	*Family PLEURONEMATIDAE* Kent, 1881**
Body ovoid to elongate-ovoid, generally small to very small, with sparse ciliation and distinctive caudal cilium; cytostome variable in position and buccal ciliature less prominent than in the Pleuronematidae; macronucleus and large micronucleus often located in anterior third of body; widely distributed forms, from fresh-water, brackish, marine, interstitial, edaphic, and even coprozoic habitats.	*Family CYCLIDIIDAE* Ehrenberg, 1838**
Body elliptical in outline, with right side slightly concave and anterior end a little narrower than posterior; generally of large size; dense somatic ciliation, interspersed with longer bristles; prominent buccal caviry, medially located on ventral surface, containing distinctive tetrahymenal organization of buccal ciliary organelles plus scutico-vestige, though paroral membrane not prominent; two sizable macronuclei, with several micronuclei; commonly facultatively psammophilic forms, both fresh-water and marine habitats.	*Family HISTIOBALANTIIDAE* de Puytorac & Corliss, n. fam.**
Body kidney-shaped, large, strongly flattened laterally, densely ciliated; oral area subequatorial on ventral margin, with buccal ciliature clearly detectable; huge macronucleus; commensal in mantle caviry of marine bivalve mollusc (Mytilus edulis) but also in fresh-water clams and certain sea urchins.	*Family PENICULISTOMATIDAE* Fenchel, 1965**
Body tlattened laterally, bluntly tapered posteriorly, with clearly delineated area of thigmotactic ciliature on concave surface; posterior half of body sparsely ciliated; cytostome at an equatorial level, with buccal ciliature not highly developed; endoparasitic in renal organ of terrestrial snails.	*Family THIGMOCOMIDAE* Kazubski, 1958**
Body laterally compressed, generally ellipsoidal to broadly reniform in shape, with dense and uniform ciliation; buccal caviry relatively small, located nearly equatorially on ventral surface; commensals in mantle caviry (on gills) of fresh-water bivalve molluscs.	*Family CONCHOPHTHIRIDAE* Kahl in Doflein & Reichenow, 1929**

Incertae sedis in suborder Pleuronematina:
Larvulina Penard, 1922. Single bonafide species.