PARAMECIUM 



1 



usually inclined backward, and their stroke then drives the animal 

 forward. They may at times be directed forward; their stroke 

 then drives the animal backward. 

 The direction of their effective 

 stroke may indeed be varied in 

 many ways, as we shall see later. 

 The stroke of the cilia is always 

 somewhat oblique, so that in addi- 

 tion to its forward or backward 

 movement Paramecium rotates on 

 its long axis. This rotation is over 

 to the left, both when the animal 

 is swimming forward and when it 

 is swimming backward. The revo- 

 lution on the long axis is not due 

 to the oblique position of the oral 

 groove, as might be supposed, for if 

 the animal is cut in two, the pos- 

 terior half, which has no oral groove, 

 continues to revolve. 



"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 to- 

 ward 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 spi- 

 ral course. The swerving when the 



