INTERFERENCE OF EXCITATIONS 193 



positive thigmotaxis is interfered with by the effect of a thermal 

 or galvanic stimulus. Jennings 1 and especially Putter- have, at 

 my request, more thoroughly investigated my original observa- 

 tions and have given us a complete analysis of these interesting 

 interference effects. If the freely swimming Paramecia are sub- 

 jected to a constantly increasing temperature, the movements of 

 these infusoria become more and more active. At 30 C, the 

 rapidity is very violent and at about 37 C. they reach their maxi- 

 mal. If now the same experiment is repeated with Paramecia 

 which have in consequence of thigmotaxis fixed themselves to 

 particles of slime, the temperature may be increased to 30 C. 

 without an observable effect. The infusoria remain throughout 

 in contact with the resistance. Only when the temperature is 

 37 C. do they release their contact and move violently through 

 the water. If a drop containing Paramecia is placed on a slide, 

 between parallel pieces of fired clay which serve as electrodes, 

 it will be seen that some freely swim about, whereas others 

 remain thigmotactically in contact with particles of slime. When 

 a constant current of about .2 of a milliampere is passed through, 

 it is observed that the freely swimming individuals hasten 

 towards the cathode. Those attached to objects, on the con- 

 trary, do not respond in this manner to the electrical current. 

 (Figure 42.) The intensity of the current can be greatly in- 

 creased without bringing about detachment of the individuals 

 from their position of fixation. The typical influence of the 

 strong current upon the movement of the cilia of the thigmotacti- 

 cally fixed individuals can be clearly seen. Nevertheless, the 

 inhibition, brought about by the contact stimulus, predominates 

 over that of the excitating effect of the current, so that a freeing 

 of the organisms from their position does not occur. Not until 

 the current becomes very strong is the excitation thereby pro- 

 duced sufficient to bring about a separation of the infusoria, 

 whereupon they immediately swim toward the cathode. In this 



1 Herbert S. Jennings: "Studies on reactions to stimuli in unicellular organisms. 

 I. Reactions to chemical, osmotic and mechanical stimuli in the ciliate infusoria." 

 Journal of Physiology, Vol. XXI, 189 F. 



2 Piitter: "Studien iiber Thigmotaxis bei Protisten." Arch. f. Anat. und Physiol- 

 ogic, physiol. Abt. Suppl. 1900. 



