Centropyxis aculeata (Ehrenberg, 1838)
Diagnosis: Shell variable in contour and size, cap-shapes, yellow or brown, ovoid or circular and usually with about four or more lateral spines; in dorsal view spherical and tapering towards the aperture; shell surface rough, except for a smooth region around the aperture, often covered with quartz grains and/or diatom frustules; aperture invaginated, oval and sub-terminal.
Dimensions: Literature: shell 92-178 µm in diameter.
Ecology: In sphagnum, other mosses, in the ooze of ditches and lakes.
Remarks: Specimens cultured in the absence of extraneous particles produce a shell that is proteinaceous and similar in structure to species of Arcella.
When encysted, the body assumes a spherical form in wider part of test; granulated, colorless or with green globules.
Cash (1905) observed that specimens appear to differ widely from the type description and also from each other in size and structure. Deflandre (1929) in his review of the genus described three varieties of C. aculeata which differ in size, shape and the number of spines. In clonal culture both large and many spined specimens are produced, but these forms represent only a small percentage of the population.