THE ROLE OF AN ORGANIC MATRIX DURING THE FORMATION OF SILICEOUS SCALES IN THE HELIOZOON ACTINOPHRYS-SOL (ACTINOPHRYIDA, PROTISTA)

Actinophrys sol is a freshwater heliozoon which has trophic and encyst ed body forms. During encystment, siliceous scales are laid down in si lica deposition vesicles. The scales form one layer of a multi-layered cyst wall. Scale production is described using light microscopy, tran smission electron microscopy, scanning electron microscopy, and X-ray microanalysis. Silica is laid down on an organic matrix which is visib le prior to silicification and after removal of silica with hydrofluor ic acid. Actinophrys sol can be cultured under silica impoverished con ditions, with the result that the siliceous plates are absent. The cys ts continue to form but are fragile. Silica is not a prerequisite for the processes of encystment and cyst formation.