Family: Test rigid, chitinoid, often areolar, smooth; pseudostome ventral, round (Illustrated Guide, 1985).
Genus: Test variable, chitinoid; binucleate (Illustrated Guide, 1985).

50 μm 100 μm 150 μm; x 400 : 31 μm 63 μm 94 μm; x 640

Arcella, x 400, x 640, Uruguay Nov. 1999, photo by Yuuji Tsukii

Arcella discoides Ehrenberg, 1871: Diam. 90-150 μm, height 25-30 μm; pseudostome, round, 30-40 μm; with or without ring of pores (Illustrated Guide, 1985).