(act-in-o-sphere-ee-um) (=Echinosphaerium), one of the largest freshwater heliozoa, may measure up to 500 microns, often with outer layer being very vacuolated. Arms taper from base to tip. Multiple nuclei and nucleating sites for the arms line at the interface between the inner and outer regions of cytoplasm. Brown inclusions are food vacuoles. Phase contrast illumination.