1911 Encyclopædia Britannica/Rhizopoda
RHIZOPODA, the name given by Dujardin (pro parte, 1838) to a group of Sarcodine Protozoa. They are distinguished by their pseudopods, simple or branched, passing by wide bases into the general surface, never fine radial nor fusing into complex networks; skeleton absent or a simple shell (“test,” “theca”), never (?) a calcareous shell, nor represented by a siliceous network, nor spicules. Reproduction by binary fission; by division or abstriction of buds after the body has become multi-nucleate; or by the resolution of the body into numerous uninucleate zoospores (amœbulæ or flagellulæ) which may conjugate as gametes; plasmodium formation unknown; encystment (in “resting cysts” or “hypnocysts”) common. Without a knowledge of the history it is impossible to distinguish a naked Lobose from the Amoebula (pseudopodiospore) of a Myxomycete or Proteomyxan. As to the name, Dujardin included the thecate Lobosa, the Filosa, and the Reticularia or Foraminifera (q.v.). The latter had already received the name Foraminifera (for their shells) from d'Orbigny; and as it is impossible to separate naked from thecate Lobosa we have merged his Amoebina (Amibiens) in the larger group. The Filosa were removed by Lang from the Reticularia; in habit and test they are inseparable from the Lobosa; and though their cytoplasm approximates to that of Reticularia, their ectosarc is much less granular, though not free from granules as stated by Lang.
The majority of Rhizopoda are fresh-water forms, some occurring in the film of water on mosses, among Sphagnum, or about the bases of grass-haulms; many, however, are exclusively marine. The aquatic forms generally may lurk among Confervae or higher weeds, or lie in the bottom of decomposing or excrementitious matter in still or slow-flowing waters. Of these some may become temporarily pelagic, floating up by the formation of gas vacuoles (containing probably CO2) in the cytoplasm. It is easy to verify this by placing Arcella (fig. 1, 7) in a drop of water on a glass cover and inverting this over a glass ring; the Arcella sink to the free convex surface of the drop and escape from this most unnatural position by secreting gas-vacuoles; when they float up to contact with the glass cover, so as to touch it by the convex back of the shell, they put forth long pseudopodia which attach themselves to the glass and by their contraction turn the animal over, so that it can crawl over (i.e. under) the glass. Amoeba (Entamoeba) histolytica, Schaudinn, is the cause of tropical dysentery and hepatic abscess in man. Pelomyxa (fig. 1, 5-6) is remarkable for containing symbiotic bacteria. Zooxanthellae (symbiotic green cells—Algae or Flagellates) occur in several species; and Paulinella contains two sausage-shaped blue-green bodies, “chromatophores,” which are probably symbiotic Cyanophyceae. The shell, even when not a simple membrane, has always a continuous inner membrane of a complex nitrogenous substance containing sulphur, allied to keratin and termed pseudochitin. The outer layer when present is composed of little hollow prisms (Arcella, fig. 1, 7), sand, or inorganic matter first swallowed by the animal (Difflugia, Pseudodifflugia), sometimes partially digested (Lecquereuxia), or else of plates secreted as “reserve plates” within the cytoplasm of the animal Cyphoderia (fig. 6, B), Quadrula, Nebelia, Euglypha (figs. 4, 6, A), &c. In Quadrula irregularis alone are the plates said to be calcareous; elsewhere they are always siliceous and simply refractive, so that the silica is probably hydrated (opal). The cement is possibly of silicified pseudochitin. This material is often permeated by a ferric oxide or hydrate, even when it is not coloured rusty brown. Shell formation of the membranous test is by simple surface-excretion; under budding we describe its accomplishment in the aggregated shells.
The “pylome,” or aperture for the protrusion of the protoplasm, is usually single. There are two pylomes at opposite poles in several Filosa (Ditrema), hence united by some authors into a distinct family (fig. 7, 1, 5, 11), and in the gelatinous theca of Trichosphaerium (fig. 5) are numerous permanent pylomic pores. The nucleus is variable in form and character. In Amoeba binucleata two nuclei are always present; and some genera are permanently plurinucleate (Pelomyxa, Arcella, fig. 1, 7). It often gives forth fragments into the cytoplasm, the “chromidia” of R. Hertwig, which, as in Foraminifera (q.v.), may play an important part in reproductive processes. The contractile vacuole (there are two in Arcella, fig. 1, 7) in actively progressing Rhizopods always discharges at the hinder end. Absent or sluggish in marine forms, it is of constant occurrence lin all fresh-water Rhizopods except Pelomyxa.
The pseudopods vary greatly in type. In Amoeba princeps (fig. 1, 4) they are mere promontory-like extensions of the body; in A. radiosa (fig. 1, 1-3) and Trichosphaerium (fig. 5) they are distinct slender processes, tapering, and either blunt or finely pointed at the apex; in Pelomyxa (fig. 1, 5, 6) as in A. (Lithamoeba) discus (fig. 2) they are “eruptive,” hemispherical, formed apparently by the rupture of the ectoplasm, and the outpouring of the endoplasm which at once differentiates a clear outer layer as a new ectoplasm; in Amoeba limax during progression the body is roughly oval with the apex truncated posteriorly and the wide anterior end forming a single anterior pseudopod. Progression chiefly takes place by a rolling over of the anterior end (fig. 3—see also Amoeba); but it may take place by the extension of a pseudopod, its attachment at the tip, followed by its contraction to pull up the rest of the animal; this is well shown in the thecate species. Another mode is that of A. radiosa (fig. 1, 1-3), which can roll over on the tips of its stiff pseudopods. The pseudopods of the Filosa (figs. 6, 7) are branched, but less rich in granules, and less viscid than those of Foraminifera; they rarely anastomose, and never coalesce to form perforated plates.
A process whose relations to reproduction are not fully made out is that of “plastogamy,” where two or more individuals unite completely by their cytoplasm, the nuclei remaining distinct; it may be temporary or permanent: in the latter case determining, of course, a much more rapid increase of size than that due to growth. Thanks to the labours of F. Schaudinn, we now know the full life cycles of at least half a dozen species; previously we only knew with certainty of two modes of fission—equal constriction (Amoeba—fig. 1, 1-3) and bud-fission (Diffugia). As in other Sarcodina, chromidia, or fragments of nuclear substance budded off from the nucleus into the endoplasm, play an important part in many reproductive processes. Equal binary fission is common. In the thecate forms, e.g. Diffugia, Euglypha (fig. 4), this is replaced by bud-fission; half the cytoplasm passes out through the pylome, and becomes invested with its covering there; the enclosed “reserve” skeletal elements pass to the surface in order, so that the pylome of the new shell faces that of the old; the original nucleus divides in situ and one daughter nucleus passes into what we may call the bud-cytoplasm; the two daughters of the original cell, which we may call the “bud-sister” and the “stock-sister” respectively, now separate. In the plurinucleate forms a true bud-formation takes place, nucleate masses of cytoplasm being constricted off at the surface. A simultaneous resolution into uninucleate cells may affect the multinucleate species (or the multinucleate state of habitually uninucleate species), this is termed schizogony.
In Trichosphaerium (fig. 5) it occurs at the close of two distinct periods in the life cycle which we may call A and B; the individuals of the A period being distinguished by the presence of radiating spicules of MgCO3 in the gelatinous theca; the resolution of period A is simple (fig. 5, 3) and the uninucleate brood-cells are amoebulae (pseudopodiospores) (fig. 5, 4) which grow into the multinucleate B type, with a nonspiculate theca (fig. 5, 5). The resolution of the B type is preceded by rapid multiplication of the nuclei by mitosis (fig. 5, 7), and the uninucleate cells are 2-flagellate zoospores (fig. 5, 9). These pair with zoospores of a different brood to their own (fig. 5, 10) (i.e. they are exogamous gametes); and the fusion cell (fig. 5, 11) so formed is the starting-point of the A type (fig. 5, 12). Brood formation by resolution of a multinucleate individual has been observed or conjectured in Amoeba, &c.
A formation of numerous pseudopodiospores within Pelomyxa has been repeatedly described, and these have been seen to conjugate equally, the zygote becoming multinuclear. But the possibility of the alleged reproductive cells being parasites has not yet been fully excluded.
Chlamydophrys stercorea is a small Filose, occurring in the faeces of several mammals, but only forming its characteristic shell outside the body; plastogamic monstrosities are frequent. The nucleus degenerates, and is expelled with some plasm. The chromidia remain inside the shell, and differentiate or aggregate into about eight nuclei; the cell is then resolved into as many 2-flagellate swarmers, which escape as isogamous exogametes. The zygote becomes surrounded by a brown cyst. When swallowed by a mammal it develops, and the ordinary form is found in the excreta.
Centropyxis aculeata is closely allied to Diffugia. It divides by fission and also at the end of a cycle by schizogony, the offspring being amoebulae. In some these acquire a shell directly; in others a second brood division into four takes place, and it is only then that shells are formed. The latter conjugate as males with the former as females; and the fusion cell encysts within the approximated shells; it emerges as a naked amoeba after a period of rest, forms a shell and assumes the type of the species. Other types of reproduction are known, Amoeba coli, an inhabitant of the gut of man, showing an endogamous pairing of closely related nuclei similar to that of Actinosphaerium (see Heliozoa).
Lobosa.—W. B. Carpenter. Cytoplasm with a clear ectosarc, not wetted by the medium; pseudo pods never finely branching, usually rounded at the apex; nucleus single or multiple; shell (“test,” “theca”) absent, gelatinous, membranous or of cemented granules of ingested sand, &c., or plates secreted in the endosarc.
Selected genera: § 1. Naked Amoeba (q.v.) (“Amibe,” Bory), with the subgenera Dactylosphaerium, Hertwig and Lesser (fig. 1, 1-3), with slender, pointed pseudopods; Lithamoeba, Lankester, always containing inorganic granules (fig. 2). Pelomyxa, Greeff (fig. 1, 5, 6), with blunt, eruptive pseudopods and numerous nuclei, 1 in. or more in diameter when contracted. Arcuothrix, Claparède and Lachmann, with one or more slender, very mobile, flagelliform pseudopods as well as the lobose ones.
§ 2. Test gelatinous, perforated by pseudopods: Amphizonella, Greeff; Trichosphaerium, Schneider (fig. 5).
§ 3. Test membranous: Cochliopodium, Hertwig and Lesser (fig. 1, 8).
§ 4. Test “chitinous,” shagreened: Arcella, Stein (fig. 1, 7).
§ 5. Test of ingested particles: Diffugia, Leclerc; Centropyxis, Stein; Lecqueureuxia, Schlumberger (shell material of diatomaceous tests fused into sausage-shaped masses).
§ 6. Test of secreted siliceous or chitinous plates: Quadrula, F. E. Schultze. (In Q. irregularis the plates are said to be calcareous.)
Filosa.—A. Lang. Cytoplasm without definite ectosarc; pseudopods branching, tapering to fine tips, somewhat granular; test present in all known species and varying as in the Lobosa.
Selected genera: § 1. Test membranous: Gromia, Dujardin (pro parte); Mikrogromia, Hertwig; Diplophrys, Barker (fig. 7, 1); Ditrema, Archer; Amphitrema, Archer (fig. 7, 11); the last three have a mouth-like aperture (pylome) at either end of the test.
§ 2. Test of ingested or incrusted particles: Pseudodiffugia, Schlumberger; Diaphorodon, Archer (fig. 7, 12).
§ 3. Test of secreted plates: Euglypha, Dujardin (figs. 4, 6, A); sphenoderia lenta (fig. 6, B); Paulinetta, Lauterborn.
Bibliography.—E. Penard, Faune rhizopodique du bassin du Léman (1902), and Les Rhizopodes des grands lacs (1905); James Cash, The British Freshwater Rhizopoda and Heliozoa (Ray Society), vol. 1. (1905)—these works contain full bibliographies of older literature. L. Rhumbler, “Beiträge zur Kenntniss der Rhizopoden” (Zeitsch. Wiss. Zoologie, lii. (1891), and numerous papers in Arch. Entwickelungsgeschichte and Arch. Protistenkunde; F. Schaudinn, “Untersuch. üb. die Fortpflanzung einiger Rhizopoden.” in Arb. Kaiserl. Gesundheitsamt, xix. (1903); S. Awerinzew, “Die chemische Zusammensetzung der Gehäuse der süsswasserrhizopoden,” Arch. Prot. viii. (1906); K. Boll, “Über die Fortpflanzung von Pelomyxa palustris,” Arch. Prot. viii. (1906). For bibliographies and a clear exposition we may also cite Y. Delage and, E. Hérouard, Traité de zoologie concrète, i. (La cellule et les proiozoaires) (1896); A. Lang, Lehrb. d. vergleich. Anatomie d. wirbellosen Thiere (ed. 2), i., “Protozoa” (1901); and Marcus Hartog, “Protozoa,” in Cambridge Natural History, i. (1906). Of the older literature we need only cite F. Dujardin, “Sur les organismes inférieurs,” Ann. Sc. Nat. Zool. iv. (1835), and “Zoophytes, infusoires” (1841). (M. Ha.)