Online Encyclopedia

SARCODINA

Online Encyclopedia
Originally appearing in Volume V24, Page 209 of the 1911 Encyclopedia Britannica.
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SARCODINA, a principal group or phylum of Protista, defined by O. Butschli as those which during their active and motile existence discharge the functions of motion and nutrition by simple flowing movements of their protoplasm or by the extension of simple pseudopods, which merge without trace into the protoplasmic body (Bronn's Tierreich, vol. i. pt. i., 1882). Thus defined, it is co-extensive with the older group Rhizopoda (Dujardin), and comprises five classes: Proteomyxa (Lankester), Rhizopoda (Dujardin), Foraminifera (d'Orbigny), Heliozoa (Haeckel) and Radiolaria (Haeckel). The delimitation of Sarcodina is not unattended with difficulties. A very few of those we include possess in addition to the pseudo-pods one or more flagella, such as Dimorpha and Myriophrys (Heliozoa), Arcuothrix (Rhizopoda), and might equally be referred to the Flagellata (q.v.). The Sporozoa differ in that their active state is usually (not always, e.g. Haemosporidia, &c.) a wriggling, sickle-shaped cell, that growth takes place in the whole surface of the body, and not by ingestion of food and consequently without the active deformations that characterize Sarcodina, and that the life-cycle embraces at least two alternating modes of brood formation. The subdivision of the phylum is no less difficult. The character of the pseudopods (see AMOEBA) is the most obvious one to select, as it appears to be fairly constant. The surface may be a " precipitation-pellicle," not wetted by water, and the cytoplasm immediately within (" ectosarc ") free from granules, so that no streaming movement is visible at the surface of the pseudopods, which are blunt or taper sharply to a point (Rhizopoda Lobosa); or the cytoplasm has no such protective outer layer, and the granules extend to the surface where they show a constant streaming, and the pseudopods are fine-pointed, and taper very slowly to the tip, as in all the other groups. For convenience, however, from general similarity of habit, habitat and general structure, we have been obliged to give a minor importance to this character within Rhizopoda. The divisions then stand thus: I. PROTEOMYXA.—Pseudopods fine granular, not branching freely; fission usually multiple, in a cyst; no conjugation process known. 2. RHIZOPODA.—Simple forms, sometimes with a simple shell, chitinous, siliceous or of cemented particles, never calcareous; pseudopods lobose, in the tapering and branching never either stiff or reticulate. 3. HELIOzoA.—Pseudopods granular, finely radiate, and gradually tapering, stiff ; skeleton variable, never calcareous nor of cemented particles. 4. FoRAMINIFERA.—Pseudopods branching freely and anastomosing, flexible except in a few pelagic forms where they are more radiate; shell variable, mostly of cemented sand-grains, calcareous, very rarely siliceous in a few deep-sea forms, not generically separable from 5. RADIOLARIA.—Cytoplasm divided into a central and a peripheral region by a perforated membranous central capsule; pseudo-pods radiate flexible branching or not; skeleton either of a proteid (?) substance (" a canthin ") or siliceous, of spicules or forming an elegant lattice, more rarely continuous. 6. LABYRINTHULIDEA.--Body a reticulate plasmodium, formed by cells more or less coalescent, and connected by a network of anastomosing threadlike pseudopods. Cells aggregated into loose networks without distinct boundaries, the minor aggregates connected by fine threadlike pseudopodia. 7. MYxOMYCETES.—Cells at first free, finally aggregated to form a coalescent fructification, usually preceded by a continuous or fenestrated plasmodium stage in which all cytoplasmic boundaries may be lost. The reproduction processes of the Sarcodina are (I) Binary fission, equal or nearly so. (2) Multiple fission or " sporulation " (also termed " brood formation "). Conjugation (equal or unequal) usually occurs between cells produced by the latter mode (microgametes) ; or if not, there are antecedent processes suggesting that brood formation has been lost. Conjugation is entirely unknown in Proteomyxa, Labyrinthulidea and Myxomycetes, even at stages where it occurs in other groups, and it has only been definitely made out in a very limited number of genera in the remaining groups. The zygote or product of cell fusion is usually here, as in the majority of types of conjugation, a resting cell. (See the separate articles on the classes.) The young of the Sarcodina, formed from' the outcome of multiple fission, or single resting cells (spores), may be provided with pseudopodia from the first (myxopods or amoebulae), or come into active life for a short time with flagella (mastigopods or flagellulae). (M. HA.)
End of Article: SARCODINA
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