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com- pressed, smooth in outline except for two promi- nent anterior spines and one posterior spine ; free-swimming; somatic ciliature, sparse and frag- mented into several cirrus-like clusters posteriorly; a preoral band of cilia on a conspicuous trans- verse ridge anterior to the oral cavity ; oral cavity in posterior half with several oral polykinetids and a paroral; macronucleus, globular; micronucleus (?); contractile vacuole, present; cytoproct (?); feeding (?); in freshwater anaerobic habitats; one genus. \u2013 Discomorphella Corliss, 1960 Family EPALXELLIDAE Corliss, 1960 (for Epalcidae and Epalxidae ) Size, small; shape, box-like, generally with short posterior spines, and some species with well-devel- oped spine overhanging hidden oral cavity; free- swimming; somatic ciliature, relatively dense in its short anterior and posterior linear kinetof- ragments, with those at the anterior end parallel to and anterior of the oral opening on both left and right sides of the body ; oral cavity, roughly equatorial, with several oral polykinetids; paroral, may be absent; macronucleus, globular to ellipsoid; micronucleus, present; contractile vacuole, present; cytoproct (?); feeding (?); in marine and freshwa- ter anaerobic habitats, principally freshwater, but a few Epalxella species also in marine biotopes; three genera. \u2013 Epalxella Corliss, 1960 \u2013 Pelodinium Lauterborn, 1908 \u2013 Saprodinium Lauterborn, 1908 Family MYLESTOMATIDAE Kahl in Doflein & Reichenow, 1929 (syns. Atopodiniidae , Mylestomidae ) Size, small; shape, discoid; free-swimming; somatic ciliation very sparse, with anterior kinetofragments restricted to the ventral (= oral) surface, extending only slightly onto the left and right surface, and with longer cilia at posterior, 17.3 The Ciliate Taxa to Genus 411 cirrus-like in Myelostoma ; oral cavity, in posterior half, with several oral polykinetids; paroral, may be absent; macronucleus, globular to ellipsoid; micronucleus, present; contractile vacuole, present; cytoproct (?); feeding (?); in marine and freshwater anaerobic habitats, predominantly in freshwater but a few Mylestoma species are marine; two genera. \u2013 Atopodinium Kahl, 1932 \u2013 Mylestoma Kahl, 1928 Class OLIGOHYMENOPHOREA de Puytorac et al., 1974 (syns. Aspirigera p.p ., Aspirotricha p.p ., Cyrtostomata p.p ., Holotricha [ Holotrichasina , Holotrichia ] p.p ., Homoiotricha p.p ., Kinetodesmatophora p.p ., Membranellophora [ Membranellata ] p.p ., Stomatea p.p ., Axotrichidea + Peritrichidea ; Hymenotricha (sensu Raabe) + Peritricha; Tetrahymenophora + Cyclohymenophora ) Size, small to medium, rarely large; shape, typi- cally ovoid to elongate ovoid; free-swimming, but sessile and sedentary in Subclass Peritrichia ; alve- oli, well-developed; somatic ciliation, holotrichous; somatic monokinetids with anteriorly directed, dis- tinct, overlapping kinetodesmal fibrils, divergent postciliary ribbons, and radial transverse ribbons (except in Subclass Peniculia , which has tangential transverse ribbons); posterior kinetosome of somatic dikinetids similar to that of a monokinetid unit while anterior kinetosome bears only a tangential transverse ribbon; parasomal sacs, typically to left or anterior of kinetosomes; somatic extrusomes as mucocysts, but trichocysts common in Subclass Peniculia in which mucocysts are rare; oral appara- tus with a distinct right paroral of dikinetids (i.e. stichodyad) and typically three left oral polyki- netids, but oral apparatus absent in Subclass Astomatia, and highly modified in the Subclass Apostomatia ; stomatogenesis varies with subclass, of buccokinetal or parakinetal types; division while free-swimming, but typically in cyst in parasitic forms; macronucleus, homomerous, typically sin- gle; micronucleus, one to many; conjugation, usu- ally temporary, but total in Subclass Peritrichia ; cytoproct, typically located in the director meridian; feeding on bacteria and microalgae, but occasionally carnivorous in larger forms with the endocommensal astomes entirely osmotrophic; in marine, freshwa- ter, and terrestrial habitats, distributed widely as free-living forms with many species of the Subclass Peritrichia as symphorionts and the entire Subclass Astomatia endocommensalistic; six subclasses. Subclass Peniculia Fauré-Fremiet in Corliss, 1956 (syns. Trichohymenostomata and Vestibulata sensu von Gelei) Size, medium; shape, ovoid; free-swimming; alveoli, well-developed; somatic ciliation, holotri- chous, typically dense, with distinct pre- and post- oral sutures, but Order Urocentrida with girdle-like ciliature; caudal cilia, often conspicuous; somatic kinetids with tangential, not radial, transverse ribbons, but other fibrillar associates as for class ; somatic extrusomes as trichocysts, but mucocysts in Order Urocentrida ; oral structures, typically three left oral polykinetids with the long axis of the polykinetid (peniculus) parallel to the long axis of the oral cavity, and with alveoli between kinetosomal rows of oral polykinetids ; paroral, reduced, but present throughout interphase; no cyrtos, but nematodesmata may be associated with oral and perioral kinetosomes, sometimes loosely basket-like; stomatogenesis, ophryobuccoki- netal ; many species with endosymbiotic algae or bacteria; microphagous bacterivores, algivores, and some species carnivorous (e.g. Neobursaridium ); cysts, rare; distributed widely, predominantly in freshwater habitats, but some marine species; two orders. NOTE : Lynn and Small (2002) presented a single Order Peniculida divided into the Suborders Frontoniina and Parameciina . However, small sub- unit rRNA gene sequences do not strongly support this subdivision (Fokin, Andreoli, Verni, & Petroni, 2006; Strüder-Kypke, Wright, Fokin, & Lynn, 2000b), but do provide support, corroborated by morphology, for separation of the urocentrids at a higher level (Didier & de Puytorac, 1994). Order Peniculida Fauré-Fremiet in Corliss, 1956 (syns. Frontoniina ; Frontoniina + Quadrulina p.p .; Lembadionina , Parameciina , Peniculina ) Size, small to large; somatic ciliation, holotri- chous; somatic kinetids, predominantly dikinetids; somatic extrusomes as trichocysts ; seven families. 412 17. The Ciliate Taxa Including Families and Genera Family CLATHROSTOMATIDAE Kahl, 1926 (syn. Clathrostomidae ) Size, medium; shape, ovoid; free-swimming; somatic ciliation, holotrichous, with preoral and postoral sutures; ophryokineties, absent; oral region in anterior 1/4 of body; oral apparatus with six oral polykinetids, each one a file of dikinetids; oral nematodesmata, forming a ring around cytopharynx ; macronucleus, elongated ellipsoid; micronucleus, present; contractile vacuole, present; cytoproct, present; feeding on bacteria and micro- algae; in freshwater habitats; one genus. \u2013 Clathrostoma Penard, 1922 Family FRONTONIIDAE Kahl, 1926 (for Chiliferidae ) Size, medium to large; shape, ovoid to bluntly ovoid; free-swimming; somatic ciliation, holotri- chous, with postoral suture only; ophryokineties, often many, to right of oral region ; oral region, subapical, elongate in anterior 1/2\u20131/3 of body; prebuccal area, shallow or absent ; oral appa- ratus, typically three long, large polykinetids in oral cavity with the paroral along its right border; cytostome expansible along postoral suture ; oral nematodesmata not forming a ring around the cytopharynx, but more prominent to its side and posterior; macronucleus, ellipsoid to elongate ellipsoid, sometimes band-form; micronucleus, present; contractile vacuole, single; cytoproct, present; often containing zoochlorellae; feeding on bacteria and microalgae; in marine, freshwater, and terrestrial habitats, typically free-living but one species commensal (?) on gills