260 



HEAT AND PROTOPLASM 



[CH. VIII 



desired. Starting now with the trough filled with infusion 

 water, the Paramecia are seen to be uniformly distributed 

 (Fig. 71, a). Hot water is run through tubes under the right 

 end of the trough. After 10 minutes the thermometers show 

 the temperature of the water at the right end to be 38, at the 

 left end 26. At this moment, all Paramecia are in the left 

 third of the trough (Fig. 72, ). If now the hot water be 

 passed through the left tube only, the temperature rises at 

 that end to 36 or 38, falling to 27 or 28 at the other, and 



19- 





26 s 



38* 





10 g > 25 



FIG. 71. Distribution of Paramecium in a trough of water with variable temperature 

 at the ends. (From MENDELSSOHN, '95.) 



the Paramecia swim to the right end. Thus, with reference to 

 a temperature of 38, Paramecia are negatively thermotactic. 



If, now, cold water be passed through the left tube so that 

 the temperature of the left end of the trough falls to 10, 

 while the right end is at 25, the Paramecia migrate to the 

 right end. Towards a temperature of 25 Paramecium is thus 

 positively thermotactic (Fig. 72, <?). 



Finally, if cold water be passed through the tube at one end 

 of the trough and hot water at the other, the organisms will be 

 found accumulated in the middle of the trough where the tern- 



