The ultrastructure of the contractile vacuole of Ochromonas and Poterioochromonas species is described. It appears to be a permanent structure in the anterior end of the phytoflagellate, consisting of a central cavity with a unit-membrane lining from which flattened saccules protrude in all directions.

The contracted saccules are about 40 nm in diameter and are made up of a single trilammellar unit membrane about 9 nm thick and continuous with the central cavity membrane. In their contracted state the saccules contain transverse fibrils which may be involved in the contraction of the saccules; these fibrils are no longer seen when the central cavity and the saccules swell. The saccules seem to merge and become continuous with the central cavity during the swelling of the contractile vacuole.

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