20 THE BEHAVIOR OF LOWER ORGANISMS. 



away from the heat, much as is described by Mendelssohn for Para- 

 mccium bursaria. With a large number of individuals a general 

 orientation is evident, however, after the experiment has been some 

 time in progress. 



If ice water is used as the stimulating agent in place of heated water, 

 the phenomena to be observed are practically identical with those 

 above described. The organisms leave the colder region, giving the 

 same reaction as with heated water. As the cold has the additional 

 effect of decreasing the movements, many individuals are immobilized 

 by a very low temperature before they have succeeded in escaping 

 from it, so that they remain in the cold region. The reaction is thus 

 less clearly defined than that to heat. 



Oxytricha ceruginosa: This organism, though smaller, is in some 

 respects more favorable than O. fallax for observing the method of 

 reaction. This is because the individuals are more inclined to swim 

 freely through the water, so that their progress away from or toward 

 the heated region is more rapid than in O. fallax. O. czruginosa, 

 further, even when moving freely through the water, either does not 

 revolve on the long axis at all or revolves only very slowly. In con- 

 sequence of this it is easy to determine the relation of the direction of 

 turning to the differentiations of the body. 



The reaction to heat and cold is in essentials identical with that of 

 O. fallax, and this reaction is repeated till the animals are carried into 

 a region where the temperature is not such as to cause the reaction. 

 Those that are carried into such a region will of course be swimming 

 away from the stimulating region ; hence, in a large number of individ- 

 uals there is an evident orientation, with anterior end directed away 

 from the source of heat or cold. All the conditions and details as to 

 the production of this orientation are as set forth above for O. fallax. 



Stylonychia mytilus : This large Hypotrichan is still more favorable 

 for the study of the movements of individuals under the stimulus of 

 heat or cold coming from one side than are the two species of 

 Oxytricha. But I have found it less easy to obtain in large numbers, 

 and for this reason have not chosen it for the detailed description of 

 the reaction. Where comparatively few specimens are available, 

 the movements of individuals are easily studied, but there is little 

 impression of any real orientation, such as one gets clearly when large 

 numbers are used. 



The movements of the individuals are like those described for 

 Oxytricha fallax. The animal in reacting always turns to its right, 

 without regard to the relation of this to the direction from which the 

 heat or cold is coming. With an organism of the large size of 



