The actinophryids are an order of heliozoa, a polyphyletic array of stramenopiles, having a close relationship with pedinellids and Ciliophrys.

Axopodia are a type of pseudopodia that are supported by hundreds of microtubules arranged in interlocking spirals and forming a needle-like internal structure or axoneme.

[4] The organisms can be either mononucleate, with a single, well defined nucleus in the center of the cell body, or multinucleate, with 10 or more nuclei located under the outer vacuolated layer of cytoplasm.

[7] Contractile vacuoles are common in these organisms, which are presumed to use them to maintain body volume by expelling fluids to compensate for the entry of water by osmosis.

Contractile vacuoles are visible as clear bulges from the surface of the cell body that slowly fill then rapidly deflate, expelling their contents into the environment.

[10] This behavior has been documented in many species, including Actinosphaerium nucleofilum, Actinophrys sol, and Raphidiophrys contractilis.

[13] The axopodial contractions have been shown to be highly sensitive to environmental factors such as temperature and pressure[9][14] as well as chemical signals like Ca2+ and colchicine.

[17] Cells undergoing this process withdraw their axopodia, adhere to the substrate, and take on an opaque and grayish appearance.

Actinosphaerium are several times larger, from 200 to 1000 μm in diameter, with many nuclei[11] and are found exclusively in fresh water.

It has been observed once and was treated as a junior subjective synonym of Actinosphaerium by Mikrjukov & Patterson in 2001,[20] but as a valid genus by Cavalier-Smith & Scoble (2013).

[1] Heliorapha is a further debated taxon, it being a new generic vehicle for the species azurina that was initially assigned to the genus Ciliophrys.