Arcella discoides Ehrenberg, 1843
Diagnosis: test generally circular in apical view, plano-convex in lateral view, sometimes arched; diameter about three to four times the height; without a distinct rim or border. Two nuclei or more. Aperture circular, invaginated, bordered by a shallow lip, usually surrounded by a ring of numerous small pores. Ratio diameter aperture/shell usually 0.30-0.45. Height of the shell about 1/3 that of the diameter. Shell yellow or brown.
Dimensions: literature: shell diameter 70-157 µm; aperture diameter 21-52 µm. My measurements: shell diameter
Ecology: Common in pond water
Remarks: this species forms together with A. megastoma and A. polypora a species complex. They are hard to separate from each other. I think A. polypora isn't a true species. I have based my division between A. discoides and A. megastoma on morphometric data. A.megastoma has a larger apertural diameter, though there is some overlap with A. discoides. I sampled my data from numerous individuals on several locations in the Netherlands. The number of nuclei could be a taxonomic character, but as most observed shells are empty, it is hard to get enough data. It is probably a variable character because the number of nuclei apparently increases with the size of the test.