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 net-work, 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 (amcx bulx or flagelluhe) 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 excre, mentitious 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 Amelia (fig . 1, 7) in S. drop of water on a glass cover and inverting this over a glass ring; the Arcellasink 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 pseudo-podia which attach themselves to the glass and, by their con-traction 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 (Amelia, fig . 1, 7), sand, or inorganic matter first swallowed by the animal (Difflugia, Pseudodijiugia), 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 Amelia, fig . 1, 7) in actively progressing Rhizopods always discharges at the hinder end .

Absent or sluggish in marine forms, it is of

constant occurrence in 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 M . Schultze, in three stages of equal binary fission during fifteen minutes; a, nucleus; b, contractile vacuole (after M . Schultze) . 4, Amoeba princeps, Ehr.; a, nucleus; b, c, vacuoles; food vacuoles shaded (after Auerbach) .

5, 6, Pelomyxa palustris: 5, a small example in. in

diameter, moderately extended; 6, a portion more highly magnified; a, ectosarc; b, vacuoles; c, d, pseudopods formed by eruption and containing endosarc; e, vesicles containing a solution of glycogen; f, nuclei; the numerous little pods are symbiotic bacteria . 7, Arcella vulgaris: a, shell; b, cytoplasm; c, lobose pseudopods; d, d, d, 3 nuclei; e, one of the contractile vacuoles; the dark shaded circles represent bubbles or gas vacuoles . 8, Cochliopodium pellucidum: a, " vesicular " nucleus, with dense central mass or " karyosome " (a frequent type of Protistic nucleus) . (From Lankester.) 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 ani=mal; this is well shown in the thecate species . Another mode is that of A. radiosa (fig. i, I-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 cone cone f, food particles; conc., concretions, insoluble in dilute HCI and KHO, soluble in strong HCl; n, nucleus . 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 . I, 1—3) and bud-fission (Dijjlugia) . 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 . Difflugia, Euglypha (fig . 4), this is replaced by bud-fission; half the cytoplasm passes out through the pylome, and becomes A x x' a b c From Jenning's Contributions to the Study of the Behavior of Lower Organisms, by permission of the Carnegie Institution of Washington, D.C . 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 dcd' 2 b P From The Cambridge Natural History, after Schaudinn, vol. i., Protozoa, by permission of Macmillan & Co .

Ltd . 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, to) (i.e. they are exogamous gametes); and the

fusion cell (fig . 5, II) so formed is the starting-point of the A type (fig . 5, 12) . Brood formations 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 out- D

side 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 which escape as isogamous exogametes . The becomes surrounded by a brown cyst . When From Eugene Penard, Faune rhizopodique du Bassin du L€man . B, Sphenoderia lenla . I, Animal, lateral view; 2, same from above; 3, shell, lateral view; 4, shell, oral view of the pylome; 5, optical section through empty shell and pylome; 6, nucleus; 7, surface view of pylome (dotted lines represent its opposite.side as seen at a lower focus) . distinct periods in the life cycle which we may call A and B; the individuals of the A period being distinguished by the C A B From Calkin's Protozoa, by permission of the Macmillan Co., New York . Fm . 4.—Bud-fission of Euglypha alveolata .

A, passing out of secreted plates to surface of bud . B, bud completely invested ; nucleus preparing to

divide by mitosis . C, D, later stages . presence of radiating spicules of MgCO3 in the gelatinous theca; the resolution of period A is simple (fig . 5, 3) and the uninucleate swarmers, zygote swallowed by a mammal it develops, and the ordinary form is found in the excreta . Centropyxis aculeata is closely allied to Difugia: It divides by fission and also at the end of a cycle by schizogony, the j 5 6 7 S 9 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 HELIoz0A) . CLASSIFICATION Lobosa.—W . B . Carpenter .

Cytoplasm with a clear ectosarc, not wetted by the

medium; pseudopods 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: § I . 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, T in. or more in diameter when contracted . Arcuothrix, Claparede 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 . I, 7) . § 5 . Test of ingested particles: Diflugia, 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, I); 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: Pseudodifflugia, Schlumberger; Diaphorodon, Archer (fig . 7, 12) . § 3 . Test of secreted plates: Euglypha, Dujardin (figs . 4, 6, A); sphenoderia lenta (fig .

6, B) ; Paulinetllaa, Lauterborn . bellosen 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 inferieurs," Ann . Sc . Nat . Zool. iv . (1835), and " Zoophytes, infusoires " (1841) . (M .