Hypostomata

Subclass (3) Hypostomata Schewiakoff, 1896

Syns. Gymnostomatida-Cyrtophorina,
Gymnostomorida p.p.,
Homotricha p.p.,
Hypostomatida,
Hypostomina

Cytostome ventral (and fundamentally so: no migration ontogenetically from apical position); cytopharyngeal apparatus typically of the cyrtos type; oral area may be sunk into an atrium, with more or less organized atrial ciliature; body flattened dorsoventrally or cylindrical, often with reduced somatic ciliation; morphogenesis of fission may be complex, with stomatogenesis of an advanced telokinetal type or even parakinetal- or buccokinetal-like; fibrous trichocysts in one group; many free-living species, microphagous or algivorous, but ecto- and endocommensals (usually with invertebrate hosts) also common.

Hypostomial frange extensive, winding around anterior part of body; bipolar kineties; body cylindrical, with holotrichous ciliation; no atrium; stomatogenesis parakinetal-like; free-living, predominantly fresh-water forms, though some marine and a number interstitial. Order 1. SYNHYMENIIDA de Puytorac et al., 1974

Hypostomial frange often not extensive, limited to left side of ventral surface, and sometimes reduced to 3-4 "pseudomembranelles" ("pavés" of French workers) in an oral atrium; distinct preoral suture; stomatogenesis complex, parakinetal- or buccokinetal-like in many species; trichocysts especially characteristic of one suborder. Order 2. NASSULIDA Jankowski, 1967

Hypostomial frange not identifiable as such; oral area occupied by several short doublerows of kinetosomes (outer ciliferous) located anteriad to complex cyrtos (circlet of a few large nematodesmata enclosing numerous microtubular sheets radially oriented); preoral suture skewed far to left; body frequently dorsoventrally flattened, with somatic ciliature restricted to ventral surface and often an adhesive organelle at posterior end; macronucleus (except one suborder) heteromerous, stomatogenesis telokinetal (parakinetal, according to some workers), but involving extensive morphogenetic movements; a widespread group, mostly marine, with scores of species yet to be properly described; numerous free-living forms, but also many ectocommensals and a few species parasitic on fish gills. Order 3. CYRTOPHORIDA Fauré-Fremiet in Corliss, 1956

Ciliature in adult forms limited to atrial area (sometimes = entire ventral surface of body), which may be funnel-shaped with a simple cytopharyngeal apparatus (no surrounding circlet of nematodesmata) at the bottom; adhesive organelle produces stalk or pedicel, always noncontractile; macronucleus heteromerous; reproduction solely by budding; migratory tomites bear two fields of cilia on concave ventral surface and an adhesive gland posteriorly; conjugants, of unequal size invariably undergo total fusion; no cytoproct or contractile vacuoles; body often vase-shaped, with pellicle quite rigid and frequently adorned with collar, lobes, and/or spines; widely found as symphorionts on gills, mouthparts, and/or other appendages of marine, brackish, and freshwater crustaceans, principally amphipods, isopods, and copepods (but also decapods, nebaliids, and others); one species attaches to a marine alga. Order 4. CHONOTRICHIDA Wallengren, 1895

Single, knobbed, anteriorly located tentacle, equipped with toxicysts and serving as an ingestatory tube; adult forms small, either devoid of ciliature or with it mostly restricted to an anteroventral thigmotactic field; reproduction often by budding, with ciliated (in two fields) larval forms; symbiotic (parasitic) on gills or mouthparts of diverse invertebrates, fresh- and saltwater, but most often on gills of marine bivalve molluscs. Order 5. RHYNCHODIDA Chatton & Lwoff, 1939

Cytostome inconspicuous or absent in certain stages, typically with a unique rosette (secretory function? ) nearby in the oral area; stomatogenesis considered telokinetal, often with involvement of three or four specialized kinetofragments; mature forms show holotrichous somatic ciliation, generally arranged in fewer than 22 dextrally spiraled rows, with fields of strongly thigmotactic cilia; reproduction may involve palintomy; well developed true kinetodesmata present; typically a contractile vacuole plus pore, but no cytoproct; macronucleus homomerous in trophonts, heteromerous in tomites of many species; complex, polymorphic life cycles; symbiotic (parasitic) in or on predominantly marine hosts (rarely fresh-water, and only one possible terrestrial host - edaphic acari - reported), with a number of invertebrate groups involved; apostomes s.s. most commonly found in hermit crabs, shrimps, and copepods (with sea anemones as alternating host, for species with such an obligate cycle). Order 6. APOSTOMATIDA Chatton & Lwoff, 1928

Hypostomial frange extensive, winding around anterior part of body; bipolar kineties; body cylindrical, with holotrichous ciliation; no atrium; stomatogenesis parakinetal-like; free-living, predominantly fresh-water forms, though some marine and a number interstitial.	*Order 1. SYNHYMENIIDA* de Puytorac et al., 1974**
Hypostomial frange often not extensive, limited to left side of ventral surface, and sometimes reduced to 3-4 "pseudomembranelles" ("pavés" of French workers) in an oral atrium; distinct preoral suture; stomatogenesis complex, parakinetal- or buccokinetal-like in many species; trichocysts especially characteristic of one suborder.	*Order 2. NASSULIDA* Jankowski, 1967**
Hypostomial frange not identifiable as such; oral area occupied by several short doublerows of kinetosomes (outer ciliferous) located anteriad to complex cyrtos (circlet of a few large nematodesmata enclosing numerous microtubular sheets radially oriented); preoral suture skewed far to left; body frequently dorsoventrally flattened, with somatic ciliature restricted to ventral surface and often an adhesive organelle at posterior end; macronucleus (except one suborder) heteromerous, stomatogenesis telokinetal (parakinetal, according to some workers), but involving extensive morphogenetic movements; a widespread group, mostly marine, with scores of species yet to be properly described; numerous free-living forms, but also many ectocommensals and a few species parasitic on fish gills.	*Order 3. CYRTOPHORIDA* Fauré-Fremiet in Corliss, 1956**
Ciliature in adult forms limited to atrial area (sometimes = entire ventral surface of body), which may be funnel-shaped with a simple cytopharyngeal apparatus (no surrounding circlet of nematodesmata) at the bottom; adhesive organelle produces stalk or pedicel, always noncontractile; macronucleus heteromerous; reproduction solely by budding; migratory tomites bear two fields of cilia on concave ventral surface and an adhesive gland posteriorly; conjugants, of unequal size invariably undergo total fusion; no cytoproct or contractile vacuoles; body often vase-shaped, with pellicle quite rigid and frequently adorned with collar, lobes, and/or spines; widely found as symphorionts on gills, mouthparts, and/or other appendages of marine, brackish, and freshwater crustaceans, principally amphipods, isopods, and copepods (but also decapods, nebaliids, and others); one species attaches to a marine alga.	*Order 4. CHONOTRICHIDA* Wallengren, 1895**
Single, knobbed, anteriorly located tentacle, equipped with toxicysts and serving as an ingestatory tube; adult forms small, either devoid of ciliature or with it mostly restricted to an anteroventral thigmotactic field; reproduction often by budding, with ciliated (in two fields) larval forms; symbiotic (parasitic) on gills or mouthparts of diverse invertebrates, fresh- and saltwater, but most often on gills of marine bivalve molluscs.	*Order 5. RHYNCHODIDA* Chatton & Lwoff, 1939**
Cytostome inconspicuous or absent in certain stages, typically with a unique rosette (secretory function? ) nearby in the oral area; stomatogenesis considered telokinetal, often with involvement of three or four specialized kinetofragments; mature forms show holotrichous somatic ciliation, generally arranged in fewer than 22 dextrally spiraled rows, with fields of strongly thigmotactic cilia; reproduction may involve palintomy; well developed true kinetodesmata present; typically a contractile vacuole plus pore, but no cytoproct; macronucleus homomerous in trophonts, heteromerous in tomites of many species; complex, polymorphic life cycles; symbiotic (parasitic) in or on predominantly marine hosts (rarely fresh-water, and only one possible terrestrial host - edaphic acari - reported), with a number of invertebrate groups involved; apostomes s.s. most commonly found in hermit crabs, shrimps, and copepods (with sea anemones as alternating host, for species with such an obligate cycle).	*Order 6. APOSTOMATIDA* Chatton & Lwoff, 1928**