  The eukaryotic flagellum beats with apparently unfailing periodicity, yet
responds rapidly to stimuli. Like the human heartbeat, flagellar oscillations
are now known to be noisy. Using the alga \textit{C. reinhardtii}, we explore
three aspects of nonuniform flagellar beating. We report the existence of
rhythmicity, waveform noise peaking at transitions between power and recovery
strokes, and fluctuations of interbeat intervals that are correlated and even
recurrent, with memory extending to hundreds of beats. These features are
altered qualitatively by physiological perturbations. Further, we quantify the
recovery of periodic breastroke beating from transient hydrodynamic forcing.
These results will help constrain microscopic theories on the origins and
regulation of flagellar beating.
