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). Test 100-150 μm in diameter, 50-80 high (Illustrated Guide, 1985).

31 μm 63 μm 94 μm; x 640

Arcella vulgaris, test 47 μm wide, μm heigh, x 640, Senjo-ga-hara (marsh), Nikko city, Tochigi Pref., Japan, September 2003 by Y. Tsukii