28 THE BEHAVIOR OF LOWER ORGANISMS. 



SUMMARY. 



The ciliate infusoria react in the same manner to heat and cold as 

 to most other classes of stimuli ; the response on coming into a region 

 where the temperature is above or below the optimum is by backing 

 and turning toward a structurally defined side, followed by a movement 

 forward. This reaction is repeated as long as an effective supraoptimal 

 or suboptimal temperature continues. The result is to prevent the 

 organisms from entering regions of marked supraoptimal or suboptimal 

 temperature, and to cause them to form collections in regions of opti- 

 mal temperature. The common orientation of a large number of 

 individuals sometimes produced in this way is an indirect result of the 

 method of reaction. Since movement in any other direction than a 

 certain one is stopped, the organisms after many trials come into this 

 direction. Orientation is therefore by " exclusion/' or by the method 

 of trial and error. In many of the organisms orientation is not a 

 noticeable feature of the reaction. 



