Arcella is a genus of testate amoebae in the order Arcellinida, usually found in freshwaters and mosses, and rarely in soils.

A key characteristic of Arcella is the circular test with a hole on its center from where finger-like pseudopods emerge.

An Arcella is typically enclosed in a chitinous, umbrella-shaped test (or shell) that has a single central aperture through which the pseudopods – which are used for locomotion – extend out.

The test is composed of organic material with a diameter of up to 300 μm and is transparent or light-yellow-colored in young Arcella, but browns while aging due to the progressive deposition of iron and manganese compounds.

Arcella inhabit freshwater pools, eutrophic waters, marshes, mosses, as well as wet foliage.