Printed in Great Britain © Company of Biologists Limited DIRECTIONAL CHANGE OF CILIARY BEAT EFFECTED WITH Mg2+ IN PARAMECIUM

Two types of Triton X-100 extracted model of Paramecium caudatum were prepared and effects of the ionic compositions of the reactivating medium on the swimming behaviour of the extracted models were examined. Type I, prepared by the method of Naitoh and Kaneko, did not change swimming direction depending on the concentrations of Mgt+ and K+ in the reactivating medium. Type II, prepared by a different method, however, reversed the swimming direction depending on the concentrations of Mgl+ and K+ in the reactivating medium, even when the concentration of Ca2+ was kept at an extremely low level (below io"8 M). Decreasing the concentration of Mg1+ induced the type II model to swim backwards. Intracellular injection of EDTA into a live paramecium cell induced reversal of the direction of ciliary beat. The concentration of Mg2+ at which the reversal of swimming direction occurred varied with the concentration of adenosine triphosphate (ATP) in a manner which suggested that the concentration of free ATP4 " might be the critical determinant of swimming direction