any member of the protozoan class Heliozoea (superclass Actinopoda). Heliozoans are spherical and predominantly freshwater and are found either floating or stalked. They are frequently enveloped by a shell (or test) composed of silica or organic material secreted by the organism in the form of scales or pieces in a gelatinous covering. The secretions exhibit a wide variety of shapes, which may help in species identification. The numerous radiating cytoplasmic masses, called pseudopodia (pseudopodium) (axopodia), are used more for capturing food than for locomotion. Heliozoans ingest protozoans, algae, and other small organisms and reproduce asexually by binary fission or by budding. Flagellated forms, which may be gametes, have been described in several genera.

Actinophrys sol is a common species often referred to as the sun animalcule. Acanthocystis turfacea is a similar species commonly called the green sun animalcule because its body is coloured by harmless symbiotic green algae (zoochlorellae). Actinosphaerium species are multinucleate, often reaching a diameter of 1 mm (0.04 inch).