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A group of heliozoan protists, found world-wide in fresh water, salt water, and wet soil. Actinophryids are single cells, spherical in shape and without any shell or test, covered in radial pseudopods (projections) that allow the creature to capture food and roll about. These contain grains and are supported by microtubules in a unique double coil pattern that characteristically arise on the surface of a nucleus. The mitochondria have tubular cristae, and fresh water forms have several contractile vacuoles to maintain the osmotic balance.

There are two main genera of Actinophryids. Actinophrys has a single central nucleus, with a body typically around 50 μm in diameter and pseudopods around 100 μm long. Actinosphaerium is several times larger and multinucleate, and is found exclusively in fresh water. Two more genera, Echinosphaerium and Camptonema, have been described but may be equivalent to Actinosphaerium.

Under unfavorable conditions, the cell will form a cyst, which is multi-walled and covered in spikes. Sexual reproduction occurs through a process of autogamy (self-fertilization) inside of the cyst: the cell undergoes meiosis and divides in two, then fuses back together. Otherwise reproduction is usually by binary fission.