36 TROPISMS 



and the fifth pair pushes. The fourth pair acts like the 

 fifth and requires no special discussion. If a current be 

 sent through the animal longitudinally from head to tail 

 and the intensity be increased gradually, a change soon 

 takes place in the position of the legs. In the third pair 

 the tension of the flexors predominates (Fig. 2), in the 

 fifth the tension of the extensors. The animal can thus 

 move easily by pulling of the third and by pushing of the 

 fifth pair of legs, that is to say, the current changes the 

 tension of the muscles in such a way that the forward 

 motion is facilitated, while the backward motion is ren- 

 dered difficult. Hence it can easily go toward the anode 

 but only with difficulty toward the cathode. If a current 

 be sent through the animal in the opposite direction, 

 namely from tail to head, the third pair of legs is extended, 

 the fifth pair bent (Fig. 3) ; i.e., the third pair, can push, 

 the fifth pair can pull backward. The animal can thus 

 go backward with ease but forward only with difficulty. 

 This again will lead, to a gathering of such animals at 

 the anode, this time, however, by walking backward. 



The phenomena thus far described recall the forced 

 movements mentioned in the third chapter, where certain 

 injuries of the brain accelerate forward motion while 

 other lesions in the opposite parts of the brain make 

 forward motion difficult if not impossible. 



Palcemonetes can also walk sidewise. This movement 

 is produced by the pulling of the legs on the side toward 

 which the animal is moving (contraction of the flexors), 

 while the legs of the other side push (contraction of ex- 

 tensors). If a current be sent transversely, say from left 

 to right, through the animal, the legs of the left side 

 assume the flexor position, those of the right side the 



