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. 1 (1), 2 (2)); protoplasm without a clear ectoplasm or pellicle, often frothy with large vacuoles, like the alveoli of Radiolaria; nucleus 1 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. 1 (1)). Actinophrys sol and Actinosphaerium eichhornii (fig. 2), known as sun animalcules to the older microscopists, float freely in stagnant or slow-flowing waters, and Myriophrys is able by an investment of long flagelliform cilia to swim freely. The majority, however, lurk among confervae or the light débris of the bottom ooze; and come under the head of “sapropelic” rather than pelagic organisms. The body is usually of constant spherical form in relation to the floating habit. Nuclearia, however, shows amoeboid changes of general outline. The pseudopods are retractile, the axial filament being absorbed as the filament grows shorter and thicker and disappearing when the pseudopod merges into the ectoplasm, to be reformed at the same time with the pseudopod. There is often a distinction, clear, but never sharp, between the richly vacuolate, almost frothy ectoplasm and the denser endoplasm. One or more contractile vacuoles may protrude from the ectoplasm. The endoplasm contains the nucleus or nuclei. The nucleus when single may be central or excentric: in the latter case, the endoplasm contains a clear central sphere (“centrosome”) on which abut the axial filaments of the pseudopods. The ectoplasm contains, in some species, constantly (Raphidiophrys viridis) or occasionally (Actinosphaerium), green cells belonging to the genera Zoochlorella and Sphaerocystis, both probably—the latter certainly—vegetative stages of a Chlamydomonad (Flagellata, q.v.) and of symbiotic significance.

Fig. 1.—Heliozoa. 1. Actinophrys sol, Ehrb. a, food-particle lying in a large food-vacuole; b, deep-lying finely granular protoplasm; c, axial filament of a pseudopodium extended inwards to the nucleus; d, the central nucleus; e, contractile vacuole; f, superficial much vacuolated protoplasm. 2. Clathrulina elegans, Cienk. 3. Heterophrys marina, H. and L. a, nucleus; b, clearer protoplasm surrounding the nucleus; c, the peculiar felted envelope. 4. Raphidiophrys pallida, F. E. Schultze. a, food-particle; b, contractile vacuole; c, the nucleus; d, central granule in which all the axis-filaments of the pseudopodia meet. The tangentially disposed spicules are seen arranged in masses on the surface. 5. Acanthocystis turfacea, Carter. a, probably the central nucleus; b, clear protoplasm around the nucleus; c, more superficial protoplasm with vacuoles and chlorophyll corpuscles; d, coarser siliceous spicules; e, finer forked siliceous spicules; f, finely granular layer of protoplasm. The long pseudopodia reaching beyond the spicules are not lettered. 6. Bi-flagellate “flagellula” of Acanthocystis aculeata. a, nucleus. 7. Id. of Clathrulina elegans. a, nucleus; b, granules. 8. Astrodisculus ruber, Greeff. a, red-coloured central sphere (? nucleus); b, peripheral homogeneous envelope.

The Heliozoa can move by rolling over on their extended pseudopods; 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 separation after 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.

Fig. 2.—Heliozoa. 1. Actinosphaerium eichhornii, Ehr.; a, nuclei; b, deeper protoplasm with smaller vacuoles and numerous nuclei; c, contractile vacuoles; d, peripheral protoplasm with larger vacuoles. 2. A portion of the same specimen more highly magnified and seen in optical section. a, Nuclei; b, deeper protoplasm (so-called endosarc); d, peripheral protoplasm (so-called ectosarc); e, pseudopodia showing the granular protoplasm streaming over the stiff axial filament: f, food-particle in a good-vacuole. 3, 4. Nuclei of Actinosphaerium in the resting condition. 5-13. Successive stages in the division of a nucleus of Actinosphaerium, showing fibrillation, and in 7 and 8 formation of an equatorial plate of chromatin substance (after Hertwig). 14. Cyst-phase of Actinosphaerium eichhornii, showing the protoplasm divided into twelve chlamydospores, each of which has a siliceous coat; a, nucleus of the spore; g, gelatinous wall of the cyst; h, siliceous coat of the spore.

Reproduction.—Binary fission has been repeatedly observed; in some cases one or both of the daughter cells may swim for a time as a biflagellate zoospore (fig. 1 (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. 1 (6)).

Brood formation is only known here in relation to a syngamic process; this is a sharp contrast to Proteomyxa (q.v.) where brood formation is the commonest mode of reproduction, and plasmodium-formation, rare indeed, is the nearest approach to syngamy observed. Indeed, if we knew the life-history of all the species this difference in the life cycle would be a convenient 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. The nucleus of either male divides; and one nucleus passes to the surface at either side, and is budded off with a small portion of the cytoplasm as an abortive cell; the two remaining nuclei which are “first cousins” in cellular relationship now fuse, as is the case with the cytoplasts. The resulting coupled cell or zygote divides into two, which again encyst.

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 1/20th partly by fusion, 2 and 2, probably by digestion of the majority. Within the primary cyst the body is now resolved into nuclear cells, which again surround themselves with secondary cysts. The cell in each secondary cyst divides (by karyokinesis), and these sister cells, or rather their offspring, pair in much the same way as the individual cells of Actinophrys—the chief difference is that after the first division and budding off of a rudimentary cell, a second division of the same character takes place, with the formation of a second rudimentary cell, which is the niece of the first, absolutely in the same way as the 1st and 2nd polar bodies are formed in the maturation of the ovum in Metazoa. The actual pairing cells are thus second cousins, great-granddaughters of the original cell of the secondary cysts. Complete fusion now takes place to form the coupled cell, which is now contracted and forms a gelatinous wall within the siliceous secondary cyst wall (fig. 2 (14)), During a resting stage nuclear divisions occur and finally a brood of young 1-nuclear Actinosphaerium leave the cyst.


Aphrothoraca. Body naked. Actinophrys Ehrb. (fig. 1 (1)) (nucleate), Actinosphaerium Stein plurinucleate (fig. 2 (1)), Camptonema (plurinucleate) Schaud., Dimorpha Gruber (sometimes 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. 1 (5)), Pinacocystis (?) Hertw. and Less, Myriophrys Penard. (Astrodisculus).

III. Desmothoraca. § 1 attached by a stalk. Clathrulina Cienk. (fig. 1 (2, 7)), Hedriocystis, Hertw. and Less.

§ 2. Free Elaster, Grimin, Choanocystis.

Literature.—The most important English original papers on this group are those by W. Archer, “On some Freshwater Rhizopoda, new, or little known,” Quarterly Journal of Microscopic Science, N.S. ix.-xi. (1869–1871), and “Résumé of Recent Contributions to the Knowledge of Freshwater Rhizopods,” ibid. xvi., xvii. (1876–1877). See also R. Hertwig and Lesser, “Über Rhizopoda und denselben nahestehenden Organismen,” in Archiv für mikroscopische Anatomie, x. (1874), p. 35; R. Schaudinn, “Heliozoa” in Tierreich (1896); E. Penard, Les Héliozoaires d’eau douce (1904); the two last named contain full bibliographies.  (M. Ha.)