THE BEHAVIOR OF INFUSORIA; PARAMECIUM 



71 



Mendelssohn found that the optimum temperature for Paramecium 

 lies, under ordinary conditions, between 24 and 28 degrees C, 

 and that when there is a difference of but 3 degrees C. between the 

 two ends of a trough 10 centimeters in length, the Paramecia gather 

 at the end of the trough 

 nearest the optimum 

 (see Fig. 54). If the 

 end a has a tempera- 

 ture of 26 degrees, the 

 end b 38 degrees, the 

 Paramecia gather at 

 the end a; if now the 

 temperature of the two 

 ends is interchanged, 

 the Paramecia travel 

 from a toward b, and 

 collect there. The 

 same results are pro- 

 duced if one end has 

 a temperature of 10 



degrees, the Other of Fig. 54. — Reactions of Paramecia to heat and cold, after 



26 de°TeeS save that Mendelssohn (1902). At a the infusoria are placed in a trough, 



both ends of which have a temperature of 19 degrees. They are 

 equally scattered. At b the temperature of one end is raised to 

 38 degrees while the other is only 26 degrees. The infusoria col- 

 lect at the end having the lower temperature. At c one end has 

 a temperature of 2 5 degrees, while the other is lowered to 1 o degrees. 

 If Para- '^^ le aruma l s now collect at the end having the higher tempera- 

 ture. 



10- 



25- 



in this case the Para- 

 mecia gather at the 

 end having the higher 

 temperature, 

 mecia are kept for 

 some hours at a temperature of 36 to 38 degrees, the optimum be- 

 comes higher, — about 30 to 32 degrees; otherwise the phenomena 

 remain the same. 



Observation of the movement of the individuals shows that the re- 

 actions in these experiments take place in the following manner. As one 

 end of the trough is heated above the optimum, the Paramecia in that 

 region are seen to become more active, darting about rapidly in all direc- 

 tions. Those that come against the sides or end of the vessel respond by 

 the avoiding reaction ; they are thus directed elsewhere. Individuals 

 that are swimming toward the hotter region likewise give the avoiding 

 reaction, — at first in but a slightly marked form, stopping, swinging 

 the anterior end about in a circle, as illustrated in Figs. 37-39, and "try- 

 ing" forward movement in a number of different directions. This con- 

 tinues as long as they are moving toward the warmer region; but as 

 soon as their direction of movement leads them toward the cooler region, 



