502 GENERAL PHYSIOLOGY 



placed in the direction of the stimulus. It depends then upon 

 the relative extent of the phases of contraction and expansion of 

 the cilia, whether the motor effect will be so directed that the cell 

 in taking the axial position will move toward the source of the 

 stimulus or away from it. The axial position must always be 

 taken, however, whether the cell moves by means of one cilium, 

 by two, or by many (Fig. 251). 



If, finally, an excitation of contraction occurs upon one side of 

 the infusorian cell and an excitation of expansion upon the other 



cL 



Fig. 250. — Scheme of axial orientation of a ciliate infusorian, resulting from an excitation of con- 

 traction upon the right side. The greater concavity of the cilia backwards indicates the stronger 

 excitation. The arrows indicate the direction of the movement and are placed at the anterior 

 pole of the body 



side, it is evident that as regards the axial position of the body, 

 the two must act in the same sense, i.e., so that the anterior end 

 is directed toward the side of the expansory stimulus. Whether 

 in this position a movement in one or the other direction or a 

 standstill takes place, depends wholly upon the extent and 



Fig. 251. — Scheme of axial orientation of a ciliate infusorian, resulting from an excitation of ex- 

 pansion upon the right side. The arrows are at the anterior pole of the body and indicate the 

 direction of movement. The greater concavity of the cilia forward indicates the excitation. 



direction of the motor effect exerted by the ciliary stroke at the 

 two ends of the body. That all these three possibilities are 

 realised, is shown most beautifully by galvanotaxis, in which, 

 according to the intensity of the galvanic current, swimming 

 forward or backward, or standstill, can be obtained. 



From the foregoing considerations the mechanism of axial 

 orientation resulting from a depression of contraction or expansion 

 upon one side of the body may be at once understood. In such 

 a caso thoi'f exists a difference in the activity of the organoids of 



