THE MECHANISM OP LIFE 501 



flagellate swims straight away from the source of the stimulus. 

 In this way negative chemotaxis, etc., occur in the forms possessing 

 two flagella, by reason of unilateral excitation of contraction. 



The same thing that occurs in Polytoma and other forms as the 

 result of the activity of two flagella, occurs in ciliate Infusoria 

 by means of the beat of numerous cilia. The movements of Para- 

 mcccium are analogous to the movements of a long boat possessing 

 many oars. If all the oars upon the two sides move with exactly 

 equal force, the boat moves straight forward ; if the stroke of the 

 oars is stronger upon one side than upon the other, the boat turns 

 toward the opposite side. The same is true of the ciliary move- 

 ment in Faramcecium. If the cilia beat with equal strength upon 

 the two sides, the infusorian swims forward in a straight line ; if, 

 however, a contractile stimulus acts upon one side, so that the 

 cilia upon that side are made to beat more strongly than upon the 

 other (Fig. 2.50, a), the anterior end of the body must turn away 



Fig, 249. — Scheme of axial orientation of a bifiagellated infusorian cell, resulting from an excitation 

 of contraction upon the right side. The greater concavity of one flagellum indicates the 

 stronger excitation. The arrows indicate the direction of movement. 



from the source of the stimulus until its long axis is placed in the 

 direction of the latter. The cilia then become stimulated equally 

 upon corresponding points of the two sides of the body, and the 

 cell swims forward in a straight direction away from the source of 

 the stimulus. In this way negative chemotaxis, barotaxis, thermo- 

 taxis, and phototaxis occur in ciliate Infusoria from unilateral 

 excitation of contraction. 



The mechanism of axial orientation in the positively tactic 

 movements of ciliated cells is likewise simple. Orientation in 

 these cases can be called forth by a unilateral excitation of 

 expansion. If such a stimulus acts upon one side, the expansion 

 of the ciliary stroke, i.e., the return of the flagellum or the cilium 

 to the resting-position, will then take place more energetically 

 upon this side of the body than upon the opposite side. The 

 result will be the reverse of that when the contraction is more 

 energetic, i.e., the anterior end of the body will be turned 

 toward the side of the incident stimulus, until the long axis is 



