REACTIONS TO HEAT AND COLD. 21 



Stylonychia this is very evident. This turning to the right under the 

 stimulus of heat and cold in Stylonychia has already been described 

 by Putter (1900), incidentally to his study of the effect of contact 

 stimuli in this organism. 



Stentor cceruleus : Mendelssohn includes in his paper a note stat- 

 ing that positive and negative thermotaxis occur in some species of 

 Stentor, and giving the optimum ; but he made no study of the mech- 

 anism of the reactions in this animal. Had he done so, it seems to me 

 that he could not have maintained his theory of the way in which 

 the reaction takes place. 



When one end of the trough is warmed the Stentors near that end 

 begin after a few seconds to move about more rapidly. In most cases 

 the movement is as follows : The animals swim backward some 

 distance, then turn toward the right aboral side and swim forward 

 (the typical motor reaction). Thus the general effect is as of an 

 irregular movement in all directions. Those individuals which swim 

 forward toward the other end of the slide pass out of the heated region ; 

 hence the motor reaction no longer takes place, and the animals con- 

 tinue to swim forward. Those which start in any other direction do 

 not escape from the heated region, and therefore soon give again the 

 motor reaction, backing and turning again to the right. Thus only 

 those that swim away from the heated region continue their course ; 

 the others are stopped and turned until finally they too get started in 

 the same direction. Therefore, after a period of apparently disordered 

 swimming, there is an evident orientation of many individuals, with 

 anterior ends away from the heated region. This orientation is caused 

 as it were by exclusion ; in animals swimming in any direction but one 

 the motor reaction is produced, so that only this direction can be main- 

 tained. After a time, therefore, a large proportion of the individuals 

 are swimming in this direction, with a common orientation. 



Thus the direction in which the animals turn is determined, as in 

 the Hypotricha, by the structure of the body, and not by the direction 

 from which the heat comes. 



Those outside the region where the heat has reached the threshold 

 temperature often swim for some distance toward the heated region ; 

 then arriving at a point where the heat is effective, they give the motor 

 reaction, backing and turning to the right. They are thus prevented 

 from entering the heated region. 



If the temperature is rapidly raised, the animals may not succeed in 

 escaping from the heated region until they are injured. In this case 

 the specimen contracts strongly and swims backward a long time. It 

 becomes distorted, places the disk against the bottom or other surface, 

 becomes motionless, and finally dies. 



