TROPISMS AND RELATED PHENOMENA 



143 



powerful stroke being directed backward, the animal is pushed 

 forward. Ludloff found that if a Paramacium is struck sidewise 

 by the current, the position of the cilia on the cathode side is re- 

 versed; namely, they are now turned forward (Fig. 31, B); while 

 on the anode side of the animal they remain practically unaltered. 

 Instead of striking symmetrically on both sides of the animal, the cilia 

 on the cathode side strike forward powerfully, those on 

 the anode side backward. The animal is thus under the 

 influence of a couple of forces which turn its oral pole 

 toward the cathode side. As soon as it is in this condition 

 the symmetrical ciha are struck equally by the current 

 curves, and they must assume a symmetrical position. 

 Such is, indeed, the case. They are now pointed forward 

 at the oral end, at the aboral end backward {B, Fig. 31). 

 As long as the current is not too strong, the oral region 

 where the ciha point forward is rather small, and therefore 

 the cilia which are pointed backward prevail, and the 

 organism moves forward toward J;he cathode. That the 

 motion of the organism to the cathode is exclusively due to 

 the position of the cilia, and not to a stimulating effect 



-B 



Fig. 31. — After Bancroft. 



A. Normal position of the cilia in a Paramacium. 



B. Forced position of the cilia when the Paramisciutit is in a trough through 

 which a constant galvanic current flows. The free ends of the cilia on the 

 cathode side of the organism point in this case toward its oral pole. 



of the current at the anode, as Verworn had assumed, follows from 

 observations made by Budgett and myself.* We found that in certain 

 solutions, e.g. 0.8 per cent NaCl solution, the Paramacia show a 

 tendency to swim backward. When exposed to a galvanic current 

 in such a solution, they show a tendency to go to the anode. The 

 explanation is that in such an organism the ciha are pointed forward 

 under the influence of the solution. Bancroft found that when the 

 current goes crosswise through such a Paramecium, the cilia on the 

 cathode side continue to point forward while those on the anode side 

 assume their natural position, pointing backward. The animal is 

 thus turned with its oral pole toward the cathode. As soon as this 



* Loeb and Budgett, Pfliiger's Archiv, Vol. 65, p. 518, 1897. 



