REACTIONS OF INFUSORIA TO ELECTRIC CURRENT 



153 



orientation takes place varies in different species. The direct effect 

 of the current is, as in Paramecium, to cause the cilia on the cathode 

 side to strike forward, while those on the anode side strike backward. 

 This would result, taken by -g-g o 

 itself, in turning the animals 

 directly, by the shortest path, 

 toward the cathode. But in 

 many species, as our study 

 of the reactions to other 

 stimuli has shown us, there 

 is a strong tendency to turn 

 toward one side rather than 

 the other, usually toward 

 the aboral side, — that oppo- 

 site the peristome. The ciUa 

 of the peristome are usually 

 more powerful than those of 

 the remainder of the body, 

 so that the direction in which 

 the animal turns depends 

 largely upon the way these 

 ciha strike. When the peris- 

 tomal cilia strike strongly 

 backward, the organism 

 turns toward the opposite or 

 aboral side, with little regard 

 to the beat of the remainder 

 of the ciha. These peristo- 

 mal cilia are as a rule lim- 

 ited to one of the four 

 quarters into which the sur- 

 face of the body can be di- 

 vided, as illustrated in Fig. 

 100. They, of course, beat 

 backward when either the 

 end bearing them, or the side 

 bearing them, is directed 

 toward the anode (1-3, Fig. 

 100), so that in these positions the animal turns toward the aboral side 

 in order to reach the position of orientation, just as it does in response 

 to other stimuh. It is only when the side bearing the peristome is di- 

 rected toward the cathode that these cilia beat forward, and hence tend 



