Jennings, Behavior of Paranuxiuiii. 487 



be. Indeed, in any position from a through b and c to d. Fig. 

 14, the animal attains orientation by turning toward the aboral 

 side, as in reactions toother stimuli. These results follow even 

 when the movements of the cilia are precisely those described 

 as typical by Ludlokf, the greater effectivess of the oral cilia 

 determining the direction of turning. 



On the other hand, if the animal is transverse to the cur- 

 rent with the oral side toward the cathode (Fig. 14, e\ it turns 

 directly toward the oral side, until the position of orientation is 

 reached. In this turning toward the oral side the electrotactic 

 reaction differs from the motor reactions to other stimuli, the 

 factor peculiar to the action of the electric current playing here 

 the essential part. In the typical case where the cilia act as 

 described by Ludloff, all the cilia tend to produce the turning 

 toward the oral side, as Fig. 14, r, shows. 



Between the position shown in Fig. 14, r, in which the 

 animal turns toward the oral side, and that in Fig. 14, a, in 

 which it turns toward the aboral side, there is of course an in- 

 termediate position in which the tendencies to turn in the oppo- 

 site directions are in equilibrium. In such cases the animal retains 

 its position until the normal revolution on the long axis has 

 occurred, bringing the body into the position shown in Fig. 14,/", 

 with aboral side to the cathode. The animal then of course 

 turns at once toward the aboral side, into the position of orien- 

 tation. A similar method of reaction in certain positions has 

 been described by Pearl (1900, p. 10 1, "type III") for Col- 

 pidium, and by Wallengren (1902, p. 365) for Opalina. The 

 tendency to turn in two opposite directions at once, as it were, 

 so that the animal no longer reacts in a co-ordinated way, is 

 very characteristic for the reaction to the electric current, dis- 

 tinguishing this reaction from all others. 



Altogether, in nearly three-fourths of all possible positions 

 the animal attains orientation by turning toward the aboral 

 side; that is, the "action system" of Paramecium — Pearl's 

 "reflex factor" — determines to this extent the reactions to elec- 

 tricity, as it does still more completely the reactions to other 

 stimuli. In practical experimentation with free swimming Para- 



