“…Thus, the flows induced by a swimming creature's motions start and stop immediately when the creature's movements start and stop. This phenomenon is closely related to the "scallop theorem," of Purcell [2], which states that time reversible strokes in a Newtonian fluid, such as water, cannot produce net swimming [1], because there is an equal and opposite net translation for corresponding forward and backward stroke motions. As a result, swimming microorganisms use strokes with a clear time direction, such as the traveling wave patterns on the flexible flagella of mammalian sperm, or the rigid-body rotation of the helical flagellar filaments of Escherichia coli.…”