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Encyclopedia Britannica - Main :: HEG-HIG |
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HELIOZOA , in zoology , a group of the Sarcodina (q.v.) so named by E. Haeckel, 1866. They are characterized by the radiate pseudopods, finely tapering at the apex, springing abruptly from the superficial protoplasm, containing a denser, rather permanent axial rod (figs. I (I), 2 (2); protoplasm without a clear ectoplasm or pellicle, often frothy with large vacuoles, like the alveoli of Radiolaria; nucleus I or numerous; skeleton absent, gelatinous or of separate siliceous fibres, plates or spicules, rarely complete and latticed; reproduction by simple fission or by brood-formation, often syngamous; form usually nearly spherical, rarely changing slowly. This group was formerly included with the Rhizopoda; but was separated from it by Haeckel on account of the character of its pseudopods, and its general adaptation to a semipelagic existence correlated with the frothy cytoplasm (fig. I (I)). Actinophrys sol and Actinosphaerium eichhornii (fig. 2), known as sun animalcules to the older microscopists, float freely in stagnant or slow-flowing waters, and Alyriophrys is able by an investment of long fiagelliform cilia to swim freely. The majority, however, lurk among confervae or the light debris of the bottom ooze ; and come under the head of " sapropelic " rather than pelagic organisms. The body
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The Heliozoa can move by rolling over on their extended pseudo-pods; Acanthocystis ludibunda traversing a path of as much as twenty times its diameter in a minute, according to Penard. several species (e.g. Raphidiophrys elegans) remain associated by the union of their pseudopods, whether into social aggregates (due to approximation) or " colonies " due to lack of separation1 ....a 8after fission, is not accurately known. The multinuclear species Actinosphaerium eichhornii (fig. 2), normally apocytial (i.e. the nuclei divide repeatedly without division of the cytoplasm), may increase in size by the fusion (" plastogamic ") of small individuals. If a large specimen be cut up or fragment itself under irritation, the small ones so produced soon approach one another and fuse completely. Reproduction.Binary fission has been repeatedly observed; in some cases one or both of the daughter cells may swim for a timeas a biflagellate zoospore (fig. i (6, 7)). The process may take place when the cell is naked or after preliminary encystment. Budding has been well studied in Acanthocystis; the cell nucleus divides repeatedly and most of the daughter nuclei pass to the periphery, aggregate part of the cytoplasm, and with it are constricted off as independent cells; one nucleus remains central and the process may be repeated. The detached bud may assume the typical character after a short amoeboid (lobose) stage, sometimes preceded by rest, or it may develop 2 flagella and swim off (fig. T (6)). Brood formation is only known here in relation to a syngamic process; this is a sharp
critical character.Equal conjugation was demonstrated fully by F. Schaudinn in Actinophrys; two individuals approach and enter into close contact, and are surrounded by a common cyst wall
Actinosphaerium (fig. 2) shows a still more remarkable process, fully studied by R. Hertwig. The large multinucleate animal withdraws its pseudopods, its vacuoles disappear, it encysts and its nuclei diminish in number to about th partly by fusion, 2 and 2, probably by digestion of the majority. Within the primary cyst the body
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Classification. Aphrothoraca. Body naked. Actinophrys Ehrb. (fig. 1 (I)) (nucleate), Actinosphaerium Stein plurinucleate (fig. 2 (I)), Camptonema (plurinucleate) Schaud., Dimorpha Gruber (some-times 2 flagellate). I. Chlamydophora. Investment gelatinous. Astrodiscus. II. Chalarothoraca. Body protected by an investment of spicules or fibre scattered or approximated, never fused into a continuous skeleton. 1. Spicules netted or free in the protoplasm. Heterophrys Arch. (fig. 1 (3)), Raphidiophrys Arch. (fig. 1 (4)), Pinacodocystis, Hertw. and Less. 2. Spicules approximated radially. Pinaciophora Greeff, Pompholyxophrys Arch., Lithocolla F. E. Schultze, Elaeorhanis Greeff (in the two foregoing genera the spicules represented by sand granules), Acanthocystis Carter (fig. i (5)), Pinacocystis (?) Hertw. and Less, Myriophrys Penard. (Astrodisculus). 2. Free Elaster, Grimin, Choanocystis. End of Article: HELIOZOA If you wish, you can link directly to this article.
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