MOTOR RESPONSES 325 



function differently, owing to different internal factors. But neither Jen- 

 nings nor Koehler offers any explanation of how the responses are reg- 

 ulated. 



Among the most important of the known facts concerning the re- 

 sponses in Protozoa to an electric current are those discovered by Lud- 

 loff in observations on ParamecJutn. Ludloff (1895) found, as has been 

 abundantly confirmed, that when the circuit is closed, the direction of 

 the stroke of the cilia on the surface of the paramecia directed toward 

 the cathode reverses; but that if the longitudinal axis of the organisms 

 is directed obliquely to the direction of the current, reversal occurs on 



Figure 116. Paramecium showing reversal in the direction of the stroke of the cilia 

 in a galvanic current. A, weak current; B, strong current; +, anode, — , cathode. (After 

 Ludloff, 1895.) 



all sides of the end of the body nearest the cathode, extending to a line 

 around the body produced by passing a plane through it at right angles 

 to the direction of the current. The extent of the portion of the body 

 on which such reversal occurs depends upon the strength of the current: 

 the stronger the current, the larger the portion affected (Fig. 116). 



The fact that the cilia in different regions on the same side of the 

 paramecia are not always affected equally by the current seems to show 

 that the responses observed cannot, as Koehler points out, be due to 

 direct action on the cilia or to surface phenomena alone; for if they were, 

 all the cilia on either side should act alike, with the possible exception 



