Arcella excavata  Cunningham, 1919

Diagnosis: Shell colorless or yellowish in young specimens and brown in older cells, oval or circular in apertural view and croissant-like in lateral view; large axis of the shell (length) usually in perpendicular direction to the shell’s protuberances, when the shape of the shell in apertural view is not circular; sometimes the large axis of the shell is parallel to the shell’s protuberances; apertural surface deeply invaginated forming two pronounced protuberances with circular or elliptical aperture at the bottom of invagination; aperture bordered by a small lip; aboral hemispherical region smooth or with regular depressions; shell wall composed of numerous alveoli, made of a proteinaceous material, and arranged in one layer; shell surface smooth or irregular with numerous small pores; usually 3-4 nuclei, sometimes 5-6, with central nucleolus.
According to Cunningham (1919) the shape of the shell is “somewhat like a quarter-section of cantaloupe,
the mouth being situated in the cup. The color is brown to almost black. The sizes of the shell are: length 55 μm, width 50 μm, total depth 45 μm, depth of depression 25 μm, mouth 15 x 20 μm”.
Dimensions: Cunningham 55 µm, Stepanek 63-67 µm, Todorov and Golemansky 60-70 µm; my measurements 44 µm (average).

Ecology: Freshwater, on aquatic plants; also reported from waste water (Todorov and Golemansky, 2003). Rare. I found this species once in considerable numbers in a sample from Crailoo, The Netherlands.

Literature: Todorov, M. and Golemansky, V., 2003. Morphology, Biometry and Ecology of Arcella excavata Cunningham, 1919 (Rhizopoda: Arcellinida). Acta Protozool. 42:105-111.