4/2 Journal of Comparative Neurology and Psychology. 



acter, such as may be called plus and minus, it reacts to one of 

 these changes, but not to the opposite one (above, p. 466). 

 In its spiral course the unoriented organism is subjected, under 

 the action of a water current, to plus and minus changes in re- 

 sistance. As a rule it is the minus change that induces the re- 

 action, while the plus does not. This is perhaps intelligible, 

 from the fact that Paranicciuvi normally receives some resist- 

 ance in its swinging toward the aboral side, so that when the 

 pressure of the current comes from the oral side, driving the 

 animal toward the aboral side, the change from the usual con- 

 dition is a very marked one. Therefore, whenever the Para- 

 mecium swings from a to b. Fig. 11, a reaction is induced, caus- 

 ing strong swerving toward the aboral side. This is effective 

 in the next phase of the spiral, causing the animal to swing far 

 in the direction b — a (since the aboral side is now toward a); thus 

 the animal becomes more nearly oriented. Since this movement 

 from ^ to ^ involves only a plus change, it causes no reaction ; 

 the ordinary spiral swimming is resumed, so that in the next 

 phase the animal swerves only a short distance toward b. But 

 this involves the minus change, inducing reaction again ; so in 

 the next phase of the spiral the animal swings still farther in 

 the direction b — a, and is now nearly oriented. This process 

 continues, the animal swinging far in the direction b — a and 

 only slightly in the direction a — b, until the axis of its path co- 

 incides with the direction of the current ; then the plus and 

 minus changes cease, and there is no cause for further reaction. 

 The general principle on which the orientation depends is this : 

 whenever moving in a certain direction causes increased swerv- 

 ing, this increased swerving must show itself chiefly in the suc- 

 ceeding phase of the spiral, thus causing the animal to swerve 

 farther than usual in the opposite direction. 



In cases where it is the plus change which induces the re- 

 action, the organism must, in the way just described, finally 

 come into orientation with anterior end directed down stream. 

 If both plus and minus changes induce reaction, then the ani- 

 mals become oriented in either direction, the essential point be- 

 ing only that the axis of the spiral coincides with the current 



