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 



28' 



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. 



