REACTIONS TO HEAT AND COLD. 



13 



ceived to be greatest to the right of the figure and to fall off regu- 

 larly toward the left, the lines indicating regions of equal temperature. 

 The last line to the left is marked 28°, this being about the threshold 

 temperature for the negative reaction of Paramecium, according to 

 Mendelssohn. The space about the Paramecium (without lines) is at 

 a temperature below 28° — say at the room temperature — so that it 

 does not act as a stimulus to cause movement. Now, as the diagram 



Fig. 6.* 



shows, a cone of water is drawn toward the anterior end of Para- 

 mecium, from a considerable distance away, necessarily therefore 

 including water above the threshold temperature of 28°. This cone or 



•Fig. 6. — Diagram of currents produced by the cilia in Paramecium when 

 the animal is nearly or quite at rest. At right of the line marked 28^ the tem- 

 perature is above the optimum (above 28^) while at the left of this line it is at 

 the normal or optimum temperature. The heated water first reaches the Para- 

 mecium at the anterior end on the oral side, passing down the oral groove to 

 the mouth. 



