2^2 Animal Instincts and Tropisms 



to the left, i. e., the anode, and only with difficulty to 

 the right, i. e., the cathode. This change in the position 

 of the legs occurs when the animal is not moving at 

 all, thus showing that the galvanotropic movements 

 take place not because the animal intends to go to 

 the anode, but that the animal goes to the anode be- 

 cause its legs are practically prevented by the galvanic 

 current from working in any other way. This is 

 exactly what happens in the heliotropic motions of 

 animals. ^ 



To understand what happens when the current goes 

 lengthwise through the body it should be stated that 

 Palcemonetes uses the third, fourth, and fifth pairs of 

 legs for its locomotion. The third pair pulls in the 

 forward movement, and the fifth pair pushes. The 

 fourth pair generally acts like the fifth, and requires 

 no further attention. If a current be sent through the 

 animal longitudinally, from tail to head, and the strength 

 be increased gradually, a change soon takes place in 

 the position of the legs (Fig. 46). In the third pair the 

 tension of the flexors predominates, in the fifth the 

 tension of the extensors. The animal can thus move 

 easily with the pulling of the third and the pushing 

 of the fifth pairs of legs, that is to say, the current 

 changes the tension of the muscles in such a way that 



* That the mechanisms by which heliotropic and galvanotropic 

 orientation is brought about are identical was shown by Bancroft 

 in Euglena (Bancroft, loc. cit.). 



