Arcella megastoma Penard, 1902
Diagnosis: Shell very flattened with a wide aperture; ratio diameter aperture/shell usually 0.45-0.53; with 2-200 nuclei.
Dimensions: Shell diameter 140-402 µm; apertural diameter 40-216 µm. My measurements: Shell 197-370 µm, apertural diameter 87-190 µm.
Ecology: Freshwater, marshes and ponds, among algae. Common, but never abundant.
Remarks: I have measured the diameter of the aperture and the shell of a number of discoid Arcella specimens from several locations in the Netherlands. The results are plotted in the graph below and show the existence of at least two different groups, probably two species, A. discoides and A. megastoma.