190 



IRRITABILITY 



indeed, quite common that two stimuli interfere with each other 

 and manifold effects follow, depending upon the specific reaction 

 of the cell and the quality, intensity and duration of the inter- 

 fering stimuli. Sometimes the interference effect is readily 

 understandable from a knowledge of the specific effect of the 

 individual stimuli concerned. At other times, however, the 

 specific reaction seems entirely different in nature than would 

 be expected from a study of the effects of the individual stimuli. 



Fig. 39. 

 Galvanotaxis of Paramaecium aurelia. 



When I place a drop of Paramecium culture on a slide having 

 on two sides parallel pieces of baked clay which serve as elec- 

 trodes and allow a constant current of about .2 milliampere to 

 flow through, it will be seen that the infusoria at room tempera- 

 ture move toward the negative pole at a rate averaging 1-1.4 mm. 

 per second. (Figure 39.) If I increase the temperature, the rate 

 of movement is increased. Here the galvanic and the thermal 

 stimuli influence each other in such a manner that the reaction 



