ELECTROTAXIS 59 



ments, trusting to luck to get it into a more favorable situa- 

 tion. A comparatively large element of random movement 

 may be combined with a tendency to turn directly away 

 from the stimulus, as is apparently the case in planarians. 

 The subject of orientation to heat rays has been little 

 studied. With aquatic organisms it would be difficult 

 to separate the reaction to heat rays and reaction to warmer 

 or colder regions of the water. Of the effect of radiant 

 heat alone on the movements of terrestrial organisms we 

 have as yet very inadequate knowledge. 



ELECTROTAXIS 



Reactions to the electric current form no part of the be- 

 havior of animals under natural conditions. Nevertheless, 

 many forms respond to the electric current in a very precise 

 manner. An attempt to review the rather extensive 

 literature on this subject would lead to a considerable 

 amount of technical discussion, and an adequate treatment 

 would require much more space than can be devoted 

 to it here. The subject is one, however, which has an 

 important bearing on the general theory of tropisms, and 

 we shall therefore describe briefly some typical cases of 

 electrotactic response. 



An Amceba proteus placed in the pathway of a weak electric 

 current assumes an elongated form and creeps toward the 

 cathode. The protoplasm toward the end nearest the anode 

 apparently contracts and with a stronger current may 

 undergo a granular disintegration. Pseudopods are put out 

 on the side toward the cathode and cause the flow of the 

 protoplasm hi that direction. 



When Paramcecium is subjected to the action of the cur- 

 rent it swims with a fairly accurate orientation of the body 

 toward the cathode. If two poles of a galvanic battery are 



