Two phases, cyst and motile, of three species of Halosphaera (Prasinophyceae), H. viridis Schrn., H. minor Ostenf. and H. russellii Parke, sp.nov., are described and the distinguishing characters tabulated. The stages in development of the two phases are detailed and a possible periodicity in their production is suggested. Only the motile phase can persist as an independent form reproducing asexually by fission. Sexuality is not recorded for the genus nor can the reported production of aplanospores be substantiated. The supposed ‘heterokont motile phase of Halosphaera’ of Pascher, who placed the genus in the Xanthophyceae, is believed to be a colourless phagotrophic flagellate parasitizing the cyst phase. All cyst phases, with plain or punctate walls, resembling those of Halosphaera in structure, do not necessarily belong to this genus. The study of living material is therefore essential.
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