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Arcella peruviana
A. pyramidalis – after Van Oye, 1926

Arcella pyramidalis   Van Oye, 1926

Diagnosis: Test forms an octahedral figure, dome surmounted by a four-faced pyramid. In profile is a dome more or less hemispherical with four ridges, surmounting a prism most often octahedral.

Dimensions: Height 80-85 μm, diameter 100 μm.

Ecology: Freshwater; River Ruki near Eala, Congo (1923)

Remarks: This species is probably synonymous with Arcella conica.

Van Oye (1926): “I have met this species several times. I had first called it A. octaedra because of its eight sides. Later, I encountered forms with six lateral faces. The dome is always formed by four facets. As the presentation of six or eight facets perpendicular to the buccal disk does not allow the distinction of two different species, nor even of two varieties, I have changed the name of A. octaedra into A. pyramidalis. It is, indeed, an absolutely typical character to have the shell surmounted by a dome in the form of a pyramid with four faces. I also met individuals with a rounded base surmounted by a pyramid with four faces, without intermediate facets between the buccal disk and the pyramid. I believe that all these forms must be considered as belonging to the same species. There is a point in the Arcella system that deserves our attention. We usually encounter the typical A. vulgaris, I have met this species more than twenty-five times in about five months. In addition to the typical, smooth shape, Penard already speaks of a rough form. For my part, I have found, as I say later, a form of A. pyramidalis composed of a buccal disk surmounted by a pyramide on four sides. I once found a copy that I took for an ill-formed A. costata, which presented two straight sides like A. costata flanked on two oblique sides like the pyramidalis. At that time, I did not attach much importance to this form, but now it seems to me it deserves mention. We can then mention the A. costata Ehrenberg to which follows the A. pyramidalis with six sides and with dome in the form of a pyramid with four sides; Then A. pyramidalis with eight sides and four-sided dome. Next are: the A. pyramidalis with two sets of superimposed facets, each series consisting of eight sides each, the whole surmounted by a pyramid of four sides, and finally, the A. rukiensis which can be considered as a distortion of the previous one. My observations are not yet sufficiently numerous to permit a conclusion. However, they seem to me quite important To draw the attention of naturalists to the evolution of the Arcella. Other observations must confirm or deny mine, before I can reach a final conclusion. The fact remains that one cannot deny a certain sequence in the transition from one form to the other and that the authors have already had to recognize that the A. vulgaris, which is the most common species, Must be considered as a species which presents very varied forms.”

Literature:
Van Oye, P., 1926. Six rhizopodes nouveaux du Congo Belge. Arch. Zool. Exp. et Gen. 65:3,64-74.

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