56 COLLEGE ZOOLOGY 



" The cilia in the oral groove beat more effectively than those 

 elsewhere. The result is to turn the anterior end continually 

 away from the oral side, just as happens in a boat that is rowed 

 on one side more strongly than on the other. As a result the 

 animal would swim in circles, turning continually toward the 

 aboral side, but for the fact that it rotates on its long axis. 

 Through the rotation the forward movement and the swerving 

 to one side are combined to produce a spiral course. The swerv- 

 ing when the oral side is to the left, is to the right; when the oral 

 side is above, the body swerves downward; when the oral side 

 is to the right, the body swerves to the left, etc. Hence the 

 swerving in any given direction is compensated by an equal 

 swerving in the opposite direction; the resultant is a spiral path 

 having a straight axis " (Fig. 36). 



Rotation is thus effective in enabling an unsymmetrical 

 animal to swim in a straight course through a medium which 

 allows deviations to right or left, and up or down. 



Reactions to Stimuli. — Paramecium responds to stimuli 

 either negatively or positively. The negative response is known 



Fig. 37. — Diagram of the avoiding reaction of Paramecium. A is a solid 

 object or other source of stimulation. 1-6, successive positions occupied 

 by the animal. (The rotation on the long axis is not shown.) (From Jennings.) 



as the " avoiding reaction " (Fig. 37) ; it takes place in the follow- 

 ing manner. When a Paramecium receives an injurious stimulus 



