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; macronu- cleus, two to many globular to ellipsoid nodules; micronucleus, present; contractile vacuole, present; cytoproct (?); feeding on bacteria, microalgae, and smaller protists; in marine and freshwater habi- tats, including sand, and one species well-known ectocommensal on Hydra ; four genera. \u2013 Kerona Müller, 1786 \u2013 Keronopsis Penard, 1922 \u2013 Paraholosticha Wenzel, 1953 \u2013 Parakeronopsis Shi, Song, & Shi, 1999 [nomen nudum] Family PLAGIOTOMIDAE Bütschli, 1887 Size, medium; shape, laterally flattened, elongate- ovoid, with right side slightly concave; free- swimming; somatic ciliation, holotrichous, dense, of small polykinetids or cirri with no differentia- tion into cirral groups on either body surface ; oral ciliature of extensive adoral zone on its left side, coursing from apical end to subequatorial posi- tion and then entering a deep oral cavity and with paroral and endoral along right wall of oral cavity; stomatogenesis, parakinetal; macronucleus, as an irregular bunch of nodules; micronuclei, several, relatively large; contractile vacuole, present; cyto- proct (?); feeding on bacteria and organic matter in host\u2019s digestive system (?); in terrestrial habitats as an endocommensals solely in certain species of lumbricid oligochaete annelids ; one genus. \u2013 Plagiotoma Dujardin, 1841 Family PSILOTRICHIDAE Bütschli, 1889 (syn. Psilotrichinae ) Size, small to medium; shape, oval to elliptical in outline, with posterior spiny extensions in some species, and sometimes with zoochlorellae in cyto- plasm; free-swimming; somatic ventral ciliature as long and sparse cirri in seven slightly curved cirral files with the postoral oblique cirral file developing from the anlage file IV ; frontal cirri; marginal cirri, strongly reduced; transverse cirri, present; caudal cirri, present; dorsal somatic cili- ature as several files of monokinetids (?); peristo- mial areal limited to anterior third of organism with oral ciliature typical of order, including paroral and endoral; macronucleus, two globular to ellipsoid nodules; micronucleus, present; contractile vacu- ole, present; cytoproct (?); feeding (?); in freshwa- ter and terrestrial habitats; one genus. \u2013 Psilotricha Stein, 1859 \u2013 Hemiholosticha von Gelei, 1954 Family SPIROFILIDAE von Gelei, 1929 (syns. Atractidae , Chaetospiridae , Chaetospirina , Chaetospirinae , Chaetospiroidea , Hypotrichidiidae , Microspirettidae , Spiretellidae , Spirofilopsidae , Stichotrichinae , Strongylidae , Strongylidiidae , Strongylidiinae , Strongylidioidea ) Size, small to medium; shape, varied, with some forms tailed and others more elongate at the ante- rior end; free-swimming, but lorica produced by some species; somatic ventral ciliature as incon- spicuous ventral cirri in files curved or spiralling obliquely around body, some ending on dorsal surface ; transverse cirri, present or absent; caudal cirri, present or absent; somatic dorsal ciliature as several dorsal files of bristle dikinetids in \u201cdor- sal strip\u201d helically winding around the body ; adoral zone not highly prominent with oral cili- ature typical of order and with paroral and endoral; macronucleus, globular to ellipsoid, single to sev- eral nodules; micronucleus, present; contractile vacuole, present; cytoproct, likely present; feeding on bacteria, algae, and smaller protists; in marine and freshwater habitats, both benthic and plank- tonic; 12 genera and one genus incertae sedis . \u2013 Atractos Vörösváry, 1950 \u2013 Chaetospira Lachmann, 1856 \u2013 Hypotrichidium Ilowaisky, 1921 17.3 The Ciliate Taxa to Genus 359 \u2013 Microspiretta Jankowski, 1975 \u2013 Mucotrichidium Foissner, Oleksiv, & Müller, 1990 \u2013 Parastrongylidium Fleury & Fryd-Versavel, 1985 \u2013 Pelagotrichidium Jankowski, 1978 \u2013 Planitrichidium Jankowski, 1979 [nomen nudum] \u2013 Spirofilopsis Corliss, 1960 \u2013 Stichotricha Perty, 1849 \u2013 Strongylidium Sterki, 1878 \u2013 Urostrongylum Kahl, 1932 Incertae sedis in Family Spirofilidae \u2013 Kahliela Tucolesco, 1962 Incertae sedis in Order Stichotrichida \u2013 Balladinopsis Ghosh, 1921 [nomen dubium] \u2013 Klonostricha Jankowski, 1979 \u2013 Psilotrix Gourret & Roeser, 1888 \u2013 Stylonethes Sterki, 1878 [nomen dubium] Order Sporadotrichida Fauré-Fremiet, 1961 (syn. Halteriina p.p ., Oxytrichina p.p ., Pleurotrichina p.p ., Sporadotrichorina ) Size, small to large; shape, sometimes elongate, even tailed, but often oval to elliptical in outline; free-swimming; somatic ventral ciliature as fron- toventral cirri, typically heavy and conspicuous, arranged in specific, localized frontal and ven- tral groups, except in a few taxa (e.g., Family Halteriidae, Laurentiella , Onychodromus , and Styxophrya ); marginal cirri, typically present; transverse cirri, may or may not be present; caudal cirri, may or may not be present; dorsal somatic ciliature, typically as files of dikinetids with a single bristle cilium; oral ciliature as for subclass; stomatogenesis, apokinetal, usually with five or six anlagen streaks in two groups for differentia- tion of ventral somatic ciliature ; in marine, fresh- water, and terrestrial habitats, widely distributed, primarily benthic with some forms planktonic, others interstitial, and a few species symbiotic, either as ectocommensals on the integument or in the branchial cavity of several invertebrates or as intestinal inquilines of echinoids; three families. NOTE : Gene sequences of both actins and small subunit rRNA unambiguously place several genera of the Family Halteriidae (e.g., Halteria , Meseres ) close to the oxytrichid clade (Croft et al., 2003; Foissner et al., 2004; Hewitt et al., 2003). We have therefore transferred this family to this order from the Subclass Oligotrichia . We consider halteriids as planktonic descendants of this primarily benthic lineage. Family HALTERIIDAE Claparède & Lachmann, 1858 (syn. Meseridae [for Lieberkuehnidae ]) Size, small; shape, spheroid to subovoid and conical; free-swimming, often darting through the water; somatic ciliature as somatic kinetids with long cilia (i.e., >10 µm long), often as cirrus-like \u201cbristles\u201d or stiff cilia ; oral cili- ature, with the \u201ccollar\u201d as an \u201copen\u201d circle of apical oral polykinetids, the \u201clapel\u201d on the left side of the oral cavity, and only with the paroral (or endoral?) on the right side of the oral cavity; macronucleus, single, ellipsoid ; micronucleus, present; contractile vacuole, present; cytoproct (?); feeding on bacteria and microalgae; predominantly in freshwater habitats, typically planktonic but some in terrestrial habi- tats; six genera and two incertae sedis . \u2013 Cephalotrichium Meunier, 1910 (subj. syn. Strobilidium ) \u2013 Halteria Dujardin, 1841 \u2013 Halterioforma Horváth, 1956 [ nomen oblitum ] \u2013 Meseres Schewiakoff, 1893 \u2013 Pelagohalteria Foissner, Skogstad, & Pratt, 1988 Incertae sedis in Family Halteriidae \u2013 Jeannellia Tucolesco, 1962 (subj. syn. Halter- ioforma ) \u2013 Octocirrus Madhava Rao, 1929 [ nomen dubium ] Family OXYTRICHIDAE Ehrenberg, 1830 (syns. Ancystropodiinae , Gastrostylidae p.p ., Gastrostylina p.p ., Onychodromusidae , Oxytri- chinae , Oxytrichoidea , Pattersoniellidae , Pleuro- trichidae , Rigidotrichidae (?), Stylonychinae ) Size, small to large; shape, relatively elongate; free-swimming; somatic ventral ciliature as fron- toventral and transverse cirri, typically 18 in number, large and distinctive, scattered over mid-area of ventral surface, between right and left marginal cirral files, usually with three fron- toventral cirri posterior to posterior vertex of the oral region, but several genera with conspicuous files of cirri (e.g., Laurentiella , Onychodromus ) ; 360 17. The Ciliate Taxa Including Families and Genera right and left marginal files of cirri, obvious;