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The ultrastructure of the vegetative cell and zoospore of Characiochloris acuminata Lee et Bold (Chlorangiellaceae, Tetrasporales, Chlorophyceae) is described. The vegetative cell is distinctive in having numerous contractile vacuoles which are randomly distributed in the cytoplasm and visible through the fissures of the parietal chloroplast. A single pyrenoid, embedded in the chloroplast, is penetrated by cytoplasmic canals which are lined by the chloroplast envelope. The vegetative cell is attached to the substrate or host by two flagellar remnants (retained from the zoospore stage), each of which is ensheathed in a gelatinous tube through the cell wall at the cell base. The basal bodies are apparently abscissed from the flagellar shaft by a unit membrane which becomes continuous with the plasma membrane. The zoospore is biflagellate, with the flagella equal in length, smooth and longer than the cell body. The flagellar sheath is characteristically undulate and the two flagellar bases are connected by a...