INTERFERENCE OF EXCITATIONS 193 



positive thigmotaxis is interfered with by the effect of a thermal 

 or galvanic stimulus. Jennings^ 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 43.) 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 Putter: "Studien iiber Thigmotaxis bei Protisten.'' Arch. f. Anat. und Physiol- 

 ogic, physiol. Abt. Suppl. 1900. 



