500 Journal of Comparative Neurology and Psychology. 



may alternate frequently in the direction of swimming, accord- 

 ing as it comes in contact with the surface, or becomes free 

 from it. To obtain these results in a sharply defined way, it is 

 necessary to vary the strength of current until exactly the 

 proper intensity is found. 



The cause of this peculiar effect of contact seems to be as 

 follows: Putter (1900) has shown that one effect of the con- 

 tact reaction is to cause the cilia of the region posterior to the 

 place of contact to cease effective action. In the strong cur- 

 rent the cilia of the anterior half of the body tend to drive the 

 animal backward, while the posterior cilia force it forward; the 

 latter are a little the more effective, so that the animal on the 

 whole moves forward. In the spiral course the body, swerv- 

 ing toward the aboral side, comes in contact with the surface 

 at about its middle. Thereupon, in accordance with the ob- 

 servation of PiJTTEK, above mentioned, the cilia behind this 

 spot, driving the animal forward, cease to beat, while the cilia 

 in front of this spot, driving it backward, continue their action. 

 Hence the anterior cilia now gain the upper hand, forcing the 

 animal backward. 



In his recent valuable papers (1903, pp. 46-47; 1903 a, 

 pp. 46-56), Statkewitsch maintains that there is no real in- 

 terference between the contact reaction and the reaction to the 

 electric current, but that the animal in contact with a solid is 

 reached only b}' a weaker current than the free swimming indi- 

 viduals, hence it reacts less markedly. Animals showing the con- 

 tact reaction are usually in contact with a heap of detritus ; 

 Statkewitsch holds that the electric current divides, a portion of 

 greater intensity passing through the water, a weaker portion 

 through the heap of detritus and the Paramecium. 



This simple physical theory would of course be very satis- 

 factory if it explained the observed facts, but this it does not 

 do. It is based on the assumptions (1) that the so-called inter- 

 ference is shown only when the animal is in contact with a heap 

 of detritus; (2) that the interference appears only as a 

 weakening of the reaction, not as a change in its character. 

 Both of these assumptions, as I have shown above, are 



