44 



BEHAVIOR OF THE LOWER ORGANISMS 



2. MOVEMENTS 



Paramecium swims by the beating of its cilia. These are usually 

 inclined backward, and their stroke then drives 

 the animal forward. They may at times be di- 

 rected forward ; their stroke then drives the ani- 

 mal 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 addition to 

 its forward or backward movement Paramecium 

 rotates on its long axis. This rotation is over to 

 the left (Fig. 33), both when the animal is swim- 

 ming forward, and when it is swimming back- 

 ward. The revolution 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 posterior half, which has no oral groove, 

 continues to revolve. 



The cilia in the oral groove beat more effec- 

 tively 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 pro- 

 duce a spiral course (Fig. 33). The swerving 

 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 swerv- 

 ing in any given direction is compensated by an 

 equal swerving in the opposite direction ; the re- 

 of Pa 1 ram 3 eciu^ Dpl The P fig n - sultant is a spiral path having a straight axis. 



ures i, 2, 3, 4, etc., show the 



positions occu- The spiral course plays so important a part in the be- 



FIG. 33. Spiral path 



successive 

 pied. The 



dotted 



, ' havior of Paramecium that we must analyze it farther, 



with small arrows show the .... , , , 



currents of water drawn The spiral swimming is evidently the resultant of three 



from in front. factors, the forward movement, the rotation on the long 



