An electron-microscope study of Gonyaulax polyedra, a holophytic marine dinoflagellate which is bioluminescent, has revealed 2 structures not previously described in free-living dino-flagellates: a polyvesicular body occuring at intervals along the cell's periphery, and a large spherical body which may function in intracelluar digestion. Coated vesicles were found in the nuclear-Golgi region. Chloroplast lamellae were widely and regularly spaced in portions of the chloroplasts at the interior of cells harvested during the light period of the culture cycle. The lamellae were closer together and not so regularly arrayed in peripherally located chloroplast branches, or in chloroplasts harvested during the dark period. Other aspects of the ultrastructure of G. polyedra are described and discussed.

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