i6 



THE BEHAVIOR OF LOWER ORGANISMS. 



raised above the threshold value the animals begin to move about 

 rapidly. At first view this movement seems to be quite irregular, as 

 Mendelssohn describes it in Paramecium. But exact observation of 



FIG. 7.* 



the individuals taken separately shows that this movement is not so 

 entirely irregular as it at first appears. Most of the animals swim 



*Fio. 7. Method by which Oxytricha fallax reacts to heat or cold. The fig- 

 ure represents one end of a trough or slide, which is heated from the end #. An 

 Oxytricha in the position i is reached by the heat coming from the end x. The 



