Arcella vulgaris Ehrenberg, 1830
Diagnosis: Shell hemispherical, helmet-shaped, with a distinct basal edge; height of the shell about one-half the diameter. Surface smooth; aperture invaginated, circular, with recurved collar; cytoplasm with epipodia attached to the shell; usually with two vesicular nuclei.
Dimensions: In literature the diameter is 100-152 µm; diameter of aperture 22-32 µm. My measurements: 80-150 µm, aperture 18-36 µm; ratio shell diameter/aperture = 0.2; ratio diameter/height = 1.5-1.8.
Ecology: Freshwater. 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 that Ehrenberg’s description is too broad. It says, translated from French:
Orbicular bell-shaped, often hemispherical or swollen on the back, with a smooth shell made up of very small grains stuck in regular series, yellow or reddish brown.