494 Jownial of Comparative Neurology and Psychology. 



Cause of Backzvani Sivi))nning in Strong Currents. — The 

 observations described above on the direction of the effective 

 beat of the ciha as the current becomes stronger throw hght on 

 the disputed question as to the cause of the swimming back- 

 ward toward the anode in a strong current. Ludloff (1895) 

 explained this backward movement as due to the fact that in a 

 strong current the effectiveness of the reversed stroke of the 

 anterior (cathodic) cilia becomes nicreased, till it overcomes the 

 forward effect of the posterior cilia. According to Ludloff's 

 view, then, the animal swims actively backward in a strong 

 current, just as it swims actively forward in a weak current. On 

 the other hand Pearl (1900, p. 123) holds that in a strong cur- 

 rent the animals are borne passively backward to the anode by 

 the cataphoric effect of the current — the electrical convection — 

 while their active movements tend to carry them to the cathode. 

 In other words, he holds that in a strong current the electrical 

 convection becomes more effective than the stroke of the cilia, 

 thus carrying the animal backward. D.\lf (1901, p. 354) holds 

 the same view. Wallengren (1902) adopts this explanation, 

 for Opalina, without expressing an opinion in regard to Para- 

 mecium. Which of the two explanations is correct? 



As the account given on preceding pages (pp. 490 492) 

 shows, the observations on the direction of the effective beat 

 the cilia are throughout in accordance with the explanation 

 given by Ludloff, and no other factor is required to account 

 for the phenomena which actually occur. When the animal is 

 swimming backward to the anode the effective beat of a large 

 portion of the cilia is demonstrably forward, producing currents 

 equal or superior to those due to the backward stroke of the 

 other cilia. This forward stroke of the anterior cilia must in- 

 evitably tend strongly to drive the animal backward, so that at 

 the best only a very small part in the phenomena could 

 possibly be attributed to the electrical convection. The direct 

 impression from observations is that the result is fully accounted 

 for without bringing the electrical convection into the matter 

 at all. 



The further question arises as to whether the electrical 



