REACTIONS OF INFUSORIA TO ELECTRIC CURRENT 161 



as the current is weak, only the cilia on the convexity of the right side 

 strike powerfully with their reversed stroke, — these being the cilia that 

 are reversed in the usual avoiding reaction. The other reversed cilia 

 strike only weakly. In consequence the animal must turn toward the 

 right side, reaching the position shown in Fig. 107, 4. Here most of 

 the strong cilia x of the convexity are still striking forward, hence the ani- 

 mal still turns toward the right. A little beyond 4, — between this and 

 5, — the animal reaches a position where the tendencies to turn in oppo- 

 site directions are equal. 1 But the turning which has been initiated in 

 positions 1-4, as a rule has given the animal sufficient momentum to 

 carry it past this dead point, so that it reaches the anode pointing posi- 

 tion (Fig. 107, 7). Here the cilia of both sides of the anterior end are 

 directed backward. When striking backward the cilia x of the convexity 

 are no more powerful than those of the opposite side. Hence there is 

 now no tendency to turn farther, and the anode-pointing position is 

 retained. Since the backward stroke of the anterior cilia is more power- 

 ful than the forward stroke of the reversed posterior cilia, the animal is 

 carried forward to the anode. Thus in a weak current the position with 

 anterior end directed to the anode is the stable one, so that in the course 

 of time, after some oscillation, the animals reach this position and swim 

 toward the anode. 



Now if the current is considerably increased in strength, the cathodic 

 cilia are caused to strike more strongly forward than before. Their 

 motor effect therefore nearly equals that of the anodic cilia, so that the 

 forward movement toward the anode is made much slower. If at the 

 time the current is made the Opalina is in an oblique position, as will 

 usually be the case, or if as a reaction to other stimuli during the passage 

 of the current it passes out of the position with anterior end to the anode, 

 then another effect is produced. Suppose it comes thus into the position 

 shown in Fig. 107, 8. Then the larger number of cilia tend to turn it to 

 the right, as is shown by the arrows at 8. It thus comes into position 1, 

 where all the cilia assist in turning it to the right ; it continues in the 

 same way through position 2 to position 3, with anterior end pointing 

 to the cathode. With a weak current, as we have seen, this position 

 is not a stable one; the stronger forward beating of the cilia on the 

 convexity of the right side cause the animal to continue to turn to the 

 right. But with a stronger current this becomes changed. Since even 

 in a weak current the cilia of this convexity strike as strongly forward 

 as they can, their forward stroke is not increased when the current is 



1 If the animal at this point or earlier turns on its long axis, as it frequently does in its 

 usual locomotion, it must now swing back through the cathode-pointing position, till it 

 again reaches a position corresponding to 4 or 5. 

 M 



