70 BEHAVIOR OF THE LOWER ORGANISMS 



case removed from the water. The experimental results described 

 cannot be verified unless this is done. 



In general it cannot be too much emphasized that in all experimental 

 studies on the behavior of Paramecium close attention to their reac- 

 tions with reference to carbon dioxide is necessary. When inconstant 

 results are obtained, or results seeming to contradict those noted by 

 other observers, it will often be found that inattention to the carbon 

 dioxide produced by the animals is at the bottom of the difficulty. 



3. REACTIONS TO HEAT AND COLD 



As we saw in the preceding chapter, a change to a temperature de- 

 cidedly above or decidedly below the optimum causes Paramecia to 

 give the avoiding reaction, while a change leading toward the optimum 

 does not. As a result the animals collect in temperatures as near the 

 optimum as possible. 



The effects of heat and cold differ slightly, since heat increases the 

 rapidity of movement, while cold reduces activity. Both produce the 

 avoiding reaction in the same way, but in heated water the reaction is 

 continued violently till the animals escape or are killed, while in ice 

 water the animals after a time become benumbed and sink to the bottom. 



The reactions to heat and cold are seen in a striking way when the 

 Paramecia are placed in a long tube or trough, one end of which is heated 

 while the other is maintained at the normal temperature, or is cooled. 

 The Paramecia then pass to the region that is nearest the optimum, 

 forming here a collection. By changing the temperature of the ends 

 or of the middle, the Paramecia may be driven from one end to the other 

 or caused to gather in any part of the trough. Such experiments were 

 devised by Mendelssohn (1895, 1902, 1902 a, b). He passed tubes 

 beneath the middle and ends of the trough or slide bearing the animals, 

 and through these tubes he conducted water of different temperatures. 

 By changing the connections of the tubes, that end of the trough which 

 is at first heated may later be cooled, etc., without disturbing the ani- 

 mals in any other way. 1 



If in this way we heat the water at one end of the trough to 38 degrees 

 while we cool the opposite end to 10 degrees, the Paramecia collect in 

 an intermediate region. By varying the temperatures at the two ends, 

 the infusoria may be driven back and forth, as represented in Fig. 54, 

 taken from Mendelssohn. By grading the temperatures properly, 

 the sensitiveness of Paramecium to changes in temperature may be 

 measured, and the optimum temperature determined very accurately. 



1 A simple apparatus of this sort is described and figured in Jennings, 1904. 



