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The World of Protozoa, Rotifera, Nematoda and Oligochaeta

Ref ID : 2042

Harald Netzel; [Test Morphologie and Ultrastructure of Centropyxis discoides (Rhizopoda, Testacea)]. Arch.Protistenk.Bd. 117:369-392, 1975

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The test of Centropyxis discoides is dorsoventrally flattened and bilaterally symmetrical. Measuring on the average 215 µm in length and 245 µm in width, it is among the largest thecamoeban shells (volume about 1,700,000 µm3; surface area about 100,000 µm2). The opening of the shell is excentrically located on the ventral side at the bottom of a funnel-shaped invagination. The margin of the aperture inverts to form (usually four) pillars, which are continuous with the roof of the shell. The outer face of the roof, the spines and a sickle-shaped zone on the floor at the posterior edge has a rough appearance and is polygonally reticulate, as is the inner face, which appears smooth and is provided with irregularly-spaced grooves. The depth of the grooves measures 1-1.5 µm, and is almost equal to the thickness of the wall. The inner face of the floor, its outer face in a region concentric to the aperture, and a narrow band parallel and dorsal to the anterior edge of the shell are similar in sculpture to the inner face of the roof, except for the grooves, which are shallower and less numerous. The wall is composed out of (up to four) layers of hollow polyhedral chamberlets, giving to it the reticulate appearance. There may be between 140,000 and 560,000 chamberlets per shell. Some evidence indicates that the main portion of the test substance is a keratinoid scleroprotein. The cytoplasm is organized into a inner plasm, containing food vacuoles, and an electron dense outer plasm, free of gastrioles. Both parts have a tendency to lacunize and to disintegrate into interconnecting cytoplasmic strands. The cell contains a well-developed ribosome-bearing endoplasmic reticulum, indicating the synthesis in quantities of extrusible proteins. The most conspicuous cellular products are ovoid electrodense granules with an excentric lumen; these seem to originate in vesicles at the dictyosomes. Since the granules are the immediate precursors of the polyhedral chamberlets, they might be termed "thecagenous granules". The overall volume of the granules is about 2-7% of the maximum volume of the cell (= capacity of the shell). A description of morphogenesis will be given in the subsequent paper.