When I place a drop of Paramecium culture on a slide having on two sides parallel pieces of baked clay which serve as electrodes and allow a constant current of about .2 milliampère to flow through, it will be seen that the infusoria at room temperature 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 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,[164] 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 powerfully backward than forward. If now the Paramecia, while moving forward, meet with a resisting body, they withdraw sideways 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 of the whole body suddenly cease their movement. (Figure [41], A.) At other times, this cessation is limited to the cilia in the anterior portion of the body (Figure [41], B), while the movements of those on the posterior portion of the body are of less amplitude or are irregular and weak. In all cases the infusorium remains quiescent in the water in contact with the resistance, and it is not uncommon to find numerous individuals in apposition with particles of ground, slimy detritus, plant fibers and so forth. (Figure [41], C.) In short, the rhythmic activity of the cilia of the Paramecia receiving their normal impulses of excitation from the 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.
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.
