REACTIONS OF INFUSORIA TO ELECTRIC CURRENT 163 



differences in the strength of beat of the cilia of different regions of the 

 body under currents of different strength. The different effects pro- 

 duced may be classified, as to their causes, in the following way: 



1. The orientation with anterior end to the cathode is due to the 

 fact that the cilia of the cathodic side strike forward ; of the anodic side 

 backward. This may be assisted or hindered by the usual tendency of 

 the organisms to turn when stimulated toward a certain structurally 

 defined side. 



2. The movement toward the cathode in weak or moderate currents 

 is due to the fact that under these conditions the backward stroke 

 of the anodic cilia is more powerful than the forward stroke of the 

 cathodic cilia. 



3. The cessation of progression in a stronger current, with reten- 

 tion of the cathode-pointing orientation, is due to the fact that as the 

 current is increased the forward stroke of the cathodic cilia becomes 

 more powerful, till it equals the backward stroke of the anodic cilia. 



4. The swimming backward toward the anode in a still stronger cur- 

 rent is due to a continued increase in the power of the forward stroke of 

 the cathodic cilia, so that they overcome the tendency of the anodic cilia 

 to drive the animal forward. (In Opalina, Wallengren believes that this 

 backward movement is due, at least partly, to the cataphoric effect of 

 the current.) 



5. The unstable transverse position seen in some cases (Spiro- 

 stomum) is due primarily to the fact that the cilia of one side ojf the 

 elongated body are more powerful, when striking either backward or 

 forward, than are the corresponding cilia of the opposite side. As a 

 result, neither the position with anterior end to the cathode nor that 

 with anterior end to the anode is a stable one, and the animal is com- 

 pelled to oscillate about a transverse position. This result is accen- 

 tuated by the slenderness and suppleness of the body in these species. 



6. The orientation with anterior end to the anode seen in certain 

 cases (Opalina in a weak current) is due to the fact that the cilia of one 

 side of the anterior half of the body are more readily reversed than the 

 opposing cilia, and their reversed stroke is more powerful, though their 

 usual backward stroke is not. The result is that the position with an- 

 terior end to the cathode becomes unstable, while the position with 

 anterior end to the anode is stable so long as accidental causes do not 

 produce slight deviations from it. 



7. The transverse or oblique position, at rest or with movement 

 Athwart the current, is due to interference between the contact reaction 

 and the effect of the current. This position is maintained only when 

 the more powerful cilia of the peristome are striking forward ; that is, 



