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Arcella vulgaris
Arcella vulgaris in lateral view

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; two vesicular nuclei.

Dimensions: In literature the diameter is 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 shell diameter : 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 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.

Arcella vulgaris
Arcella vulgaris, from Ehrenberg, 1838
Arcella vulgaris
Arcella vulgaris in dorsal view, with distinct basal border.
Arcella vulgaris
Arcella vulgaris
Shell diameter 122 µm, height 73 µm; D/H=1.7; Shell D 116 µm, H 76 µm D/H=1.5
Arcella vulgaris
Arcella vulgaris
Shell D 109 µm, H 72 µm D/H=1.5; Shell D 116 µm, H 74 µm D/H=1.6
Arcella vulgaris
These five shells were found in the same sample from the Naardermeer, the Netherlands. They differ slightly in height, diameter, shape and size of the basal rim and the invagination. The diameter-height ratio varies between 1.5 and 1.8. 1.6; Shell D 125 µm, H 69 µm D/H=1.8.
Arcella vulgaris
Arcella vulgaris, lateral view
Arcella vulgaris
Arcella vulgaris in lateral view
Arcella vulgaris
Arcella vulgaris in aboral view
Arcella vulgaris
Arcella vulgaris in lateral view
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