MOTOR RESPONSES 321 



tions, whereas Bancroft thinks that it is identical with the process of 

 orientation in hght. Further details concerning the processes involved 

 are much desired. Moreover, it is noteworthy that some of the flagel- 

 lates have proved to be excellent material for quantitative study of the 

 relation between stimulus and response. 



C. CILIATES 



The responses to electricity have been more intensively and extensively 

 studied in the ciliates than in any of the other groups of Protozoa. 



Jennings (1906) presents an excellent review of all the earlier in- 

 vestigations concerning these responses. He maintains that the results 

 obtained show the following: 



The principal feature in the response of all of the different species 

 studied consists of reversal in the direction of the effective stroke of the 

 cilia on the cathodal surface. In those species in which other ciliary 

 actions are only slightly or not at all involved (e.g., Paramecium) , this 

 results in direct turning until the anterior end is directed toward the 

 cathode. The organism then moves toward this pole. The extent of the 

 cathodal surface affected varies directly with the strength of the current. 

 If the current is strong enough to produce reversal over more than half 

 of the surface of the Favamecium, it swims backward toward the anode. 

 A still stronger current causes marked swelling of the anterior end and 

 contraction of the posterior end, changes which are followed by disin- 

 tegration beginning posteriorly (Fig. 115). The effect of the reversal 

 on the cathodal surface is variously modified by the normal action of 

 the cilia in other regions, in such a way that "with different strengths of 

 current, and with infusoria of different action systems, this results some- 

 times in movement forward to the cathode; sometimes in movement 

 forward to the anode; sometimes in cessation of movement, the anterior 

 end continuing to point to the cathode; sometimes in a backward move- 

 ment to the anode; sometimes in a position transverse to the current, the 

 animal either remaining at rest or moving across the current." 



Jennings holds that all the responses of the ciliates to electricity are 

 due to stimulation at the cathodal surface, resulting in local reversal in 

 the direction of the effective stroke of the cilia on this surface. 



Bancroft (iyo6), in his experiments with Paramecium, observed that 

 if certain salts (especially potassium, sodium, or barium salts) are added 



