158 BEHAVIOR OF THE LOWER ORGANISMS 



half of the body strike more powerfully forward, and at a certain strength 

 their effect, tending to drive the animal backward, becomes about equal 

 to that of the anodic cilia, tending to drive it forward. The result is 

 that the animals move neither forward nor backward, or only very 

 slowly in one direction or the other. They thus sink to the bottom 

 before much progress has been made. Now, if in this position the an- 

 terior end is directed toward the cathode (Fig. 106, A], of course the 

 cilia of the anterior (cathodic) half of the body tend to push the animal 

 backward, while those of the opposite half tend to push it forward. This 

 push in opposite directions bends the supple body near its middle. More- 

 over, in the cathodic half the peristomal cilia have a more powerful for- 

 ward stroke than do the ordinary cilia on the opposite side, hence the 

 anterior half of the body tends to bend toward the peristomal or oral 

 side. The general result is that the animal is bent into the position 

 shown in Fig. 106, B. The bending of the anterior part of the body 

 toward the oral side continues, until this part of the body becomes trans- 

 verse to the current (Fig. 106, C). The body may now become com- 

 pletely straightened (Fig. 106, Z>), or it may not. But in either case 

 the peristome is now turned toward the anode. The powerful peris- 

 tomal cilia therefore strike backward, causing the anterior end to swing 

 toward the aboral side, directing it again toward the cathode, as indi- 

 cated by the arrow in D. On becoming directed toward the cathode, 

 the original condition (Fig. 106, A] is restored. The animal therefore 

 again takes the positions B, C, and D. It thus continues to squirm 

 from side to side. But during its movements Spirostomum, like Para- 

 mecium, frequently revolves on its long axis. This often happens when 

 in the position shown in Fig. 106, C, so that the animal becomes placed 

 transversely to the current, with peristome to the cathode (Fig. 106, E). 

 In this position the peristomal cilia are directed forward and have there- 

 fore comparatively little motor effect. If at the same time the animal 

 comes in contact with the bottom, the contact reaction may overcome 

 for a time this slight motor effect, so that the animal lies nearly quiet, 

 in the transverse position. If now the current is reversed, so that the 

 peristome is at the anode (Fig. 106, Z>), the animal at once swings again 

 toward the aboral side. Even if the current is not reversed, the animal 

 usually does not remain long in the position shown at E. The peri- 

 stomal cilia being more effective than the opposing ones, gradually swing 

 the anterior half toward the oral side. Soon a bending takes place 

 again, as in J5, and the organism is forced to squirm about from side 

 to side, as before. 



Thus Spirostomum finds in a strong current no position of equilib- 

 rium, because the peristomal cilia have always a more powerful effect 



