Calcium ion regulation of chirality of beating flagellum of reactivated sea urchin spermatozoa. Near an interface, sea urchin spermatozoa swim almost in circles. The direction is usually clockwise at the lower surface of a coverslip and counterclockwise at the upper surface of a glass slide, when viewed from above. Examination of demembranated spermatozoa has shown that Ca2+ regulates the direction of the circular motion of spermatozoa reactivated with adenosine triphosphate (ATP). This finding suggests that Ca2+ changes the chirality of the three-dimensional bending waves of sperm flagella.