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Opalina ranarum, one of the larger opalines - and found usually in the posterior section of the intestine of the frog Rana. This differential interference contrast image is in focus on the surface of the cell, and shows the crests of the waves of flagellar movement. To achieve this, each of the flagella - of which there are thousands in rows over the body surface - must beat slightly out of phase with their neighbours - a kind of protistan Mexican wave. Animations by Rosemary Arbur of flagellar beat patterns are available here. The front is to the left.