REACTIONS TO HEAT AND COLD. 19 



from the direction indicated by the straight arrows. The heat is supra- 

 optimal, so that the organism moves away from it. In what direction 

 will the organism turn in order to reach the position of orientation 

 b-bl According to the theory of Mendelssohn, that the orientation 

 is due to an increase of the effective beat of the cilia on the side from 

 which the heat comes, the animal must turn in the direction indicated 

 by the arrow #, and this is of course what one would naturally 

 expect, since this is the most direct method of becoming oriented. 

 But as a matter of fact the organism turns in the opposite direction, as 

 indicated by the arrow jy, thus demonstrating the incorrectness of the 

 theory that orientation is due to increase of the effective beat of the 

 cilia on the side from which the heat comes. I have made this obser- 

 vation hundreds of times, not only upon Oxytricha, but on other 

 Hypotricha and on infu- 

 soria belonging to other 

 groups (see below). The 

 direction of turning is de- 

 termined, under the heat 

 stimulus, by the differen- 

 tiation of the animal's 

 body. Oxytricha turns to 

 the right, without regard 

 to the direction from which 

 the heat comes. This is 

 very striking when the 

 trough is covered and part 

 of the animals are creep- FlG - 8- Method of orientation in Oxytricha. 

 ing on the cover-glass with For details ' see text " 



ventral side up, while the remainder are creeping on the bottom of the 

 trough with ventral side down. When stimulated by heat approaching 

 from one side, all the members of the first group will be 9bserved to 

 turn counter clock-wise, while those of the second group turn in the 

 same direction as the clock hands ; that is, each specimen turns toward 

 its right side. 



For becoming completely oriented an animal in the position a-a in 

 Fig. 8 usually requires a number of reactions, as indicated in Fig. 7, 

 but the turning in every case is as indicated by the arrow y (Fig. 8). 



After it has become oriented with the anterior end away from the 

 source of heat, Oxytricha by no means maintains this position with 

 rigidity ; on the contrary the individuals shoot back and forth, in a way 

 that might be anticipated from the method in which the reaction 

 occurs. They thus form groups here and there, which gradually move 



