Family: Test rigid, chitinoid, often areolar, smooth; pseudostome ventral, round (Illustrated Guide, 1985).
Genus: Test variable, chitinoid; binucleate (Illustrated Guide, 1985).
Group [Vulgaroides]: Test hemispherical; height 1/2 ~ 4/5 diam.; lacks external shelf or rim.
Species: Test 30-100 μm in diameter; height of test about 1/2 the diameter; dome of hemispherical test evenly convex; pseudostome cicular, central; colorless, yellow, or brown; two vesicular nuclei; several contractile vacuoles (Kudo, 1966).

Arcella vulgaris, x 100, x 200, x 400, Japan, 1998 by Y. Tsukii