Arcella hemisphaerica
Arcella hemisphaerica, shell 60 µm in diameter

Arcella hemisphaerica Perty, 1852

Diagnosis: Shell colorless, yellow or brown, circular in apertural view, and hemispherical or somewhat domed in lateral view; aboral hemisphere turns into the ventral surface at an acute or somewhat rounded corner, with or without a basal border; shell surface is either smooth or with depressions giving the test an undulate appearance; aperture invaginated, circular, and bordered by a small curled lip. Two vesicular nuclei.

Dimensions: Literature: diameter of shell 38-80 µm; height 23-45 µm; diameter of aperture 10-20 µm. My measurements: shell diameter 36-56 µm.

Remarks: In a recent study Lahr and Lopes (2009) show that Arcella hemisphaerica and A. rotundata are indistinguishable. They consider both species as a single entity with 9 non-distinguishable nominal taxa within.These are:
Arcella hemisphaerica Perty, 1852
Arcella hemisphaerica Perty, 1852
Arcella hemisphaerica fma undulata Deflandre, 1928
Arcella hemisphaerica var. depressa Playfair, 1918
Arcella hemisphaerica var. tuberculata Stepanek, 1963
Arcella rotundata Playfair, 1918
Arcella rotundata var. stenostoma Deflandre, 1928
Arcella rotundata var. stenostoma fma undulata Deflandre,1928
Arcella rotundata var. aplanata Deflandre, 1928
Arcella rotundata var. alta Playfair, 1918
Arcella rotundata var. alta fma undulata Stepanek,1963

The presence or absence of depressions is a natural variation of tests, as I could observe in several natural populations. Therefore it is not a qualitative taxonomic character.
The shells below were all photographed in one drop of a sample. Their size is about 49 µm in diameter. There is no doubt that all these shells are related. It shows the variation in shape within a natural population. Three shells, to the right, show a distinct basal rim. One shell, the second to the left, has depressions.

Arcella hemisphaerica
All above shells about 49 µm in diameter, all from the same sample.
Arcella hemisphaerica
Arcella hemispaerica
Arcella hemisphaerica, both shells 58 µm
Arcella hemisphaerica
Arcella hemisphaerica
Arcella hemisphaerica, 51 µm and 53 µm
Arcella hemisphaerica undulata
Arcella hemisphaerica undulata, 53 µm
Arcella hemisphaerica undulata
Arcella hemisphaerica undulata 60 µm
Arcella hemisphaerica
Arcella hemisphaerica undulata 60 µm
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