Actinosphaerium

The actinophryids are a small, familiar group of heliozoan protists. They are the most common heliozoa in fresh water, and are especially frequent in lakes and rivers, but a few are found in marine and soil habitats as well. Each is unicellular and roughly spherical in shape, without any shell or test, and with many pseudopodia supported by axopods radiating outward from the cell body, which adhere to passing prey and allows it to roll or float about. The outer portion of the cell, or ectoplasm, is distinct and is filled with many tiny vacuoles, which assist in flotation. A few contractile vacuoles around the periphery of the cell expel excess water, and are visible as clear bulges when full.
There are two genera included here. Actinophrys have a single, central nucleus. Most are around 40-50 μm in diameter, with axopods up to 100 μm in length, though this varies. Actinosphaerium are several times larger, from 200-1000 μm in diameter, with many nuclei, and are found exclusively in fresh water. Two other genera, Echinosphaerium and Camptonema, have been described but appear to be synonyms.