On the organization and classification of the protozoon, Actinophrys sol Ehrenberg, 1830.

The organization of the heliozoon Actinophrys sol is described using light-microscopy, transmission and scanning electron-microscopy, freeze fracturing and X-ray microanalysis. The features of the trophic organism and its organelles are described in detail. The processes of encystment, autogamy and excystment are outlined. Encystment involves the production of siliceous scales which are destined to become one layer of a resting cyst with a multilayered cyst wall. Autogamy occurs before the resting cyst is formed. Cytoplasmic peculiarities of the trophic organism are almost entirely absent from the resting cyst but reappear upon excystment. The structural characteristics are used to assess the affinities of Actinophrys with other actinophryids, heliozoons, actinopods and sarcodina. It is concluded that the justification of the conventional classification of Heliozoa is questionable.