REACTIONS TO HEAT AND COLD. 27 



only rendered inadmissible by the reaction method above described, 

 but it is rendered a priori improbable by certain other considerations. 

 First we have the fact that the anterior end is much more sensitive than 

 the posterior. Of course it is impossible to measure this difference in 

 sensitiveness, yet the experiments with mechanical and chemical stimuli 

 show that it is great. In many infusoria, while the slightest touch at 

 the anterior end causes a pronounced reaction, it requires a strong stroke 

 at the posterior end to produce even a slight reaction. (See Jennings, 

 1900, pp. 238, 243, 251.) Owing to the much greater sensitiveness of 

 the anterior end, it is probable that, with the posterior end but 0.01 

 warmer than the anterior, the reaction, if any, would be due to the tem- 

 perature of the anterior end. In other words, there is reason to suppose 

 that the threshold temperature for the anterior end would be considera- 

 bly lower than that for the posterior end. If this is true the usual tem- 

 perature reactions would be throughout due primarily to stimulation 

 at the anterior end ; and the reaction, as we have seen, is of just the 

 character which would be expected from this. The first stage in 

 the reaction is to swim backward, and this is true also when the animal 

 is dropped directly into water of uniformly high or low temperature, so 

 that the temperature of the anterior end is no greater than that of the 

 posterior end. There is no explanation for the swimming backward 

 under these circumstances on the theory that accounts for thermotaxis 

 by the different temperature of the two ends. 



A second factor which must be taken into consideration relates to the 

 currents produced by the cilia of the organism itself. As shown above 

 (p. 13) , the water of a higher temperature (supposing that we are deal- 

 ing with the reaction to heat), would as a rule first reach the anterior 

 end and pass at once down the oral groove, on the oral side (Fig. 6). 

 The natural result therefore would be a turning toward the opposite or 

 aboral side, and this is exactly what we find takes place. We should 

 therefore not expect the organism to turn directly away from that end 

 of the trough from which the heat comes, for the heated water may not 

 reach the Paramecium from that side at all. 



As will be seen, the facts adduced in the last paragraph are not incon- 

 sistent with the idea that the organism turns directly away from the 

 side stimulated. It is the oral side which is, as a rule, stimulated, and 

 the organism turns toward the aboral side. We seem thus to obtain 

 a most gratifying union of two apparently opposed views. But the 

 reactions to certain other stimuli do not admit of such a union. This 

 is notably true of the reactions to mechanical stimuli, as shown in a 

 previous paper (Jennings, 1900), and of the reactions to light, to be 

 described in the following paper. 



