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Description of ArcellaTestate amoebae Species with a more or less circular shell, aperture central, invaginated, in many species surrounded by a tubus or a circle of pores. Test completely organic, composed of box-like building units arranged in a single layer and cemented together resulting in an areolar surface. Size of the building units varies between species. Young shells are colourless, older cells become brown due to iron and manganese storage into the building units. Most species are binucleate, but several species have more; A. megastoma may have up to 200. These nuclei are always vesicular. Several contractile vacuoles. Cell does not fill the test, fixed with small pseudopodia at the shell wall. Lobose pseudopodia emerge from the aperture. Cyst: round, within test. About 50 species and many more varieties and forms. Type species: A. vulgaris Ehrenberg, 1832. Deflandre (1928) in his monograph has subdivided this genus in four artificial sections and four derived groups on the basis of the height/diameter ratio, the presence of angular facets or a flat circular rim.