Arcella vulgaris Ehrenberg, 1830
Diagnosis: shell hemispherical, a distinct dome, evenly convex, with a distinct basal collar or rim; surface of dome may be undulated; height of test about one-half the diameter; surface smooth or with regular undulations; aperture invaginated, circular, and bordered by small lip; two vesicular nuclei.
Dimensions: in literature diameter 30-152 µm; diameter of aperture 22-32 µm. However, these measurements may cover a large range of specimens which were considered to be A. vulgaris and are therefore not reliable. My measurements: 80-150 µm, aperture 18-36 µm; ratio diameter of shell - aperture 0.2; ratio diameter - height 1.5-1.8.
Ecology: in the ooze and vegetation in stagnant water and also in soil, among algae and other plants.
Remarks: Probably to many shells have been labeled with the name "vulgaris", in the past but also still today. The problem is we don't know what species Ehrenberg really has seen, because his original description is to vague. A redescription will be needed.