Arcella megastoma Penard, 1902
Diagnosis: shell very flattened with a wide aperture; ratio diameter aperture/shell usually 0.45-0.53; with 2 to 200 nuclei.
Dimensions: diameters of the tests 140-402 µm; diameter of aperture 40-216 µm. My measurements 197-370 µm, diameter of aperture 87-190 µm.
Ecology: 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 shows the existence of at least two different groups, probably two species, Arcella discoides and Arcella megastoma.