Description
Heliozoa with axonemes arising from centroplast comprised of a tripartite disc sand- wiched between two hemispheres of dense material . The nucleus is located eccentrically. The pattern of microtubules in the axoneme is one of hexagons and triangles (Bardele, 1 977) . Mitochondrial cristae are lamellate ; the kinetocysts are structurally complex ball-and- cone-shaped structures which can kill motile prey (Bardele , 1976) . Members of two genera have a mucous stalk . Division in the order usually by binary fission, but there are records of budding and one account of the production of biflagellated forms . Many species form cysts . This order contains the most (85) species of heliozoa (more then 70 %). The species level taxonomy has been recently reviewed (Mikrjukov , 1996a, 1996b ; 1997) . Most are described from freshwater habitats, but they also occur in marine and terrestrial sites. The ultrastructure has been well studied (e .g. Bardele , 1975 ; Siemensma , 1981) and , as the group is defined here , is structurally very conservative . One species has a naked cell surface ; members of one genus have a mucous cell envelope ; one genus has tangential organic spicules ; and one genus has radial organic spicules . The remainder have a periplast of tangential and/or radial siliceous rods , plate- scales , or tube-like , trumpet-like , or spine-like radial spicules . There are 14 genera in three families . 1. Without siliceous elements in periplasts ....... 2 1'. Periplasts with siliceous elements .... ..... .... ... 5 2. Cell surface is naked ................. ... ... Oxnerella 2'. Cell surface is covered by a mucous coat and may bear tangential or rad ial organic spicules ......... .. ... .......... ...... ...... ... ... .. .... .. ....... ... 3 3. Organic spicules are absent .... Chlamydaster 3'. Organ ic spicules are present. .. .. .................... . 4 4. Spicules are tangential. .... ....... Phaerastrum 4'. Spicules are radial. ... ... ....... .... .. Heterophrys 5. Periplast comprised of tangential elements only .... .......... .................... .. ..... ... ............ ...... .... 6 5'. Periplast with tangential plate scales and radial elements ................ ............ ..... ..... ...... .. .. 9 6. Perip last comprised of tangential rod-like spicul es ........ .............. ... Parasphaerastrum 6'. Periplast comprised of plate-scales which may be elliptical or spindle-shaped ... ..... ....... 7 7. All scales with a broad marginal rim ...... .. ..... 8 7' . Two types of plate scales, both lacking broad marginal rim .. ........... Pseudoraphidiophrys 8. Scales with smooth or reticulate texture . . . . .. . .. . . . . .. . . . . .. . . .. . . . .. . . .. . . . . . .. . . . . Polyplacocy 8'. Plate scales with radial slits ............... ........... . .. ... .... .............. ........... ....... .. .... . Raphidiophrys 9. Rad ial elements are tube -shaped , trumpet- shaped or funnel -shaped ............................... 10 9'. Radial elements are spine-like (spicules) ; plate scales do not have a broad marginal rim ....... ... ........ ........ .... ...... ................. .. .... .. .. . 11 10. With one type of funnel-shaped radial e I e- ments; plate scales without broad marginal rim ................ ... .... ... ... . Pseudoraphidocystis 10'. Two types of radial elements: funnel-shaped and trumpet-shaped or tube-like ; plate scales with broad marginal rim ....... . Raphidocystis 11. Spicules are bilaterally symmetrical and consist of a shaft and a base formed as a continuation of a membranous flange to the shaft .... ........... ... .. .... .. ...... .......... ........ .. .... ... .. .. 12 11 '. Spicules consist of a shaft and a discrete base plate ... ........ .. ... ...... ....... ... ..... ....... ................... 13 12. The membranous base continues as two lateral wings extending along the shaft , and a further basal wing forms the base of the spicule extending perpendicularly to the shaft and lateral wings .......... ..... .. ... .... ... ... Echinocystis 12'. Without well-developed basal wing ; spicules often with stalk .. .. ...... .... ... .... .... ... Pterocystis 13. Spicule base-plates are heart-shaped ; the shafts attach centrically .... ...... Choanocystis 13'. Base plates of spicule are circular ; shafts attach centrally .... ... .... .. ... ... .. . Acanthocystis
Type species
Figures
No linked figures.
Raw text
Heliozoa with axonemes arising from centroplast comprised of a tripartite disc sand- wiched between two hemispheres of dense material . The nucleus is located eccentrically. The pattern of microtubules in the axoneme is one of hexagons and triangles (Bardele, 1 977) . Mitochondrial cristae are lamellate ; the kinetocysts are structurally complex ball-and- cone-shaped structures which can kill motile prey (Bardele , 1976) . Members of two genera have a mucous stalk . Division in the order usually by binary fission, but there are records of budding and one account of the production of biflagellated forms . Many species form cysts . This order contains the most (85) species of heliozoa (more then 70 %). The species level taxonomy has been recently reviewed (Mikrjukov , 1996a, 1996b ; 1997) . Most are described from freshwater habitats, but they also occur in marine and terrestrial sites. The ultrastructure has been well studied (e .g. Bardele , 1975 ; Siemensma , 1981) and , as the group is defined here , is structurally very conservative . One species has a naked cell surface ; members of one genus have a mucous cell envelope ; one genus has tangential organic spicules ; and one genus has radial organic spicules . The remainder have a periplast of tangential and/or radial siliceous rods , plate- scales , or tube-like , trumpet-like , or spine-like radial spicules . There are 14 genera in three families . 1. Without siliceous elements in periplasts ....... 2 1'. Periplasts with siliceous elements .... ..... .... ... 5 2. Cell surface is naked ................. ... ... Oxnerella 2'. Cell surface is covered by a mucous coat and may bear tangential or rad ial organic spicules ......... .. ... .......... ...... ...... ... ... .. .... .. ....... ... 3 3. Organic spicules are absent .... Chlamydaster 3'. Organ ic spicules are present. .. .. .................... . 4 4. Spicules are tangential. .... ....... Phaerastrum 4'. Spicules are radial. ... ... ....... .... .. Heterophrys 5. Periplast comprised of tangential elements only .... .......... .................... .. ..... ... ............ ...... .... 6 5'. Periplast with tangential plate scales and radial elements ................ ............ ..... ..... ...... .. .. 9 6. Perip last comprised of tangential rod-like spicul es ........ .............. ... Parasphaerastrum 6'. Periplast comprised of plate-scales which may be elliptical or spindle-shaped ... ..... ....... 7 7. All scales with a broad marginal rim ...... .. ..... 8 7' . Two types of plate scales, both lacking broad marginal rim .. ........... Pseudoraphidiophrys 8. Scales with smooth or reticulate texture . . . . .. . .. . . . . .. . . . . .. . . .. . . . .. . . .. . . . . . .. . . . . Polyplacocy 8'. Plate scales with radial slits ............... ........... . .. ... .... .............. ........... ....... .. .... . Raphidiophrys 9. Rad ial elements are tube -shaped , trumpet- shaped or funnel -shaped ............................... 10 9'. Radial elements are spine-like (spicules) ; plate scales do not have a broad marginal rim ....... ... ........ ........ .... ...... ................. .. .... .. .. . 11 10. With one type of funnel-shaped radial e I e- ments; plate scales without broad marginal rim ................ ... .... ... ... . Pseudoraphidocystis 10'. Two types of radial elements: funnel-shaped and trumpet-shaped or tube-like ; plate scales with broad marginal rim ....... . Raphidocystis 11. Spicules are bilaterally symmetrical and consist of a shaft and a base formed as a continuation of a membranous flange to the shaft .... ........... ... .. .... .. ...... .......... ........ .. .... ... .. .. 12 11 '. Spicules consist of a shaft and a discrete base plate ... ........ .. ... ...... ....... ... ..... ....... ................... 13 12. The membranous base continues as two lateral wings extending along the shaft , and a further basal wing forms the base of the spicule extending perpendicularly to the shaft and lateral wings .......... ..... .. ... .... ... ... Echinocystis 12'. Without well-developed basal wing ; spicules often with stalk .. .. ...... .... ... .... .... ... Pterocystis 13. Spicule base-plates are heart-shaped ; the shafts attach centrically .... ...... Choanocystis 13'. Base plates of spicule are circular ; shafts attach centrally .... ... .... .. ... ... .. . Acanthocystis