INTERFERENCE OF EXCITATIONS 191 



to the galvanic is increased by the thermal stimulation. This 

 summation of excitation is readily understood on the basis of the 

 laws concerning the effect of temperature upon the velocity of 

 chemical change established by van't Hoff. If, however, the 

 Paramecia are in a 1 per cent, alcoholic solution, then, as was 

 shown by Nagai,^ the rapidity of movement following galvanic 

 stimulation is decidedly reduced. The interference effect between 

 the galvanic and chemical stimulation is, because of the depressing 

 effect of the latter, likewise readily understood. 



Fig. 40. 

 Thigmotaxis of Paramaecium aurelia. (After Jennings.) 



Greater difficulty meets us, however, in the following instance. 

 The forward movements of the Paramecia follow in consequence 

 of the fact that the individual cilia of the body lash more power- 

 fully backward than forward. If now the Paramecia, while 

 moving forward, meet with a resisting body, they withdraw side- 

 ways while executing a sudden strong forward ciliary stroke. The 

 strong mechanical stimulation brings about retraction of the 

 organism. Entirely different are the results when the impact is 

 weak. If Paramecia while slowly swimming touch a resisting 

 object with the anterior portion of the body, withdrawal does not 

 occur. The infusoria remain under proper conditions in contact 

 with the resistance, and the rhythmic activity of the cilia directly 

 against resistance, as well as those on the other side toward the 

 posterior portion of the body, are more or less inhibited. (Figure 

 40.) The degree of inhibition brought about by this weak 

 mechanical stimulation may vary considerably. At times the cilia 



1 Nagai: "Der Einfluss verschiedener Narcotica, Gase und Salze auf die Schwimm- 

 geschwindigkeit von Paramsecium." Zeitschr. f. allgem. Physiologic Bd. VI, 1907. 



