ELECTRIC CURRENTS 93 



normal beat is augmented, but is temporarily reversed when the 

 current is switched off. Rather different results are obtained with 

 higher current intensities, as can be seen from the observations 

 quoted in Table 7. Naitoh found that it was possible to restore 

 the ciliary beat of an animal that was dying, and had undetectable 

 membrane potential, by the application of a suitable inward 

 current. The restored beat was co-ordinated metachronally, and 

 the animal survived for several minutes, provided that the 

 application of inward current was continued. 



It has long been known that potassium chloride would cause 

 the reversal of Opalina cilia, and Naitoh found that the membrane 

 potential of this animal is linearly related to the log potassium 

 concentration at higher concentrations, although the slope of the 

 curve decreases at concentrations below about 10 m mol. The 

 resting potential of squid giant axons is related to potassium 

 concentration in much the same way (Curtis and Cole, 1942). 

 This suggests that a diffusion potential of potassium ions is the 

 basis of the membrane potential in Opalina. Perhaps the change 

 in potassium concentration changes the membrane conductivity, 

 for the excitability (increase in current required to produce a 

 given change in beat direction) is virtually constant over a wide 

 range of potassium concentrations, and therefore over a wide 

 range of membrane potentials. This finding led Naitoh to suggest 

 that the induction of reversed beating of cilia is not directly related 

 to the membrane potential. There can, however, be little doubt 

 about the close connexion between ciliary reversal and the 

 movements and concentrations of ions. 



The movement of flagellates may also be affected by electric 

 currents, and Mast (1927) found that the colonial flagellate 

 Volvox swims towards the cathode when the organism is photo- 

 positive, while a photonegative organism swims towards the anode. 

 These changes are due to the stoppage of flagellar movements on 

 one side of the colony, which Ma^t explained on the basis of 

 ionic movements. The difference in response between photo- 

 negative and photopositive Volvox seems to result from a difference 

 in charge on the colony, photonegative colonies being positively 

 charged and photopositive colonies negatively charged. The 

 activity of flagella on the anodal side of positively charged colonies 

 seems to be inhibited, so that the colony turns towards the anode 



