ectoplasm of the cell body interfere with strong mechanical stimuli in such a manner that a negative thigmotaxis develops; following weak mechanical stimuli a positive thigmotaxis results. Here is an instance of the relation between the intensity of the stimulus and the manner in which its effects interfere with an already existing excitation. [Illustration: _A_ _B_ _C_ Fig. 41. Thigmotaxis of Paramaecium aurelia.] However, the strength of the inhibitory effect of a weak contact stimulus upon another excitation is best appreciated when positive thigmotaxis is interfered with by the effect of a thermal or galvanic stimulus. _Jennings_[165] and especially _Pütter_[166] have, at my request, more thoroughly investigated my original observations and have given us a complete analysis of these interesting interference effects. If the freely swimming Paramecia are subjected 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 maximal. 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 milliampère is passed through, it is observed that the freely swimming individuals hasten towards the cathode. Those attached to objects, on the contrary, do not respond in this manner to the electrical current. (Figure 42.) The intensity of the current can be greatly increased 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 thigmotactically 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 produced sufficient to bring about a separation of the infusoria, whereupon they immediately swim toward the cathode. In this interference between the contact stimulus, on the one hand, and the thermal or galvanic on the other, the inhibitory effect of the former may overpower the strong excitation of the latter.