ORIENTATION AND FUNCTION 165 



position of the legs. In the third pair the tension 

 of the flexors predominates, in the fifth the tension of 

 the extensors. The animal can thus move off 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 the 

 forward movement is rendered easy, the backward 

 difficult. Hence it can easily go toward the anode, 

 but only with difficulty toward the kathode. If a 

 current be sent through the animal in the opposite di- 

 rection, namely, from tail to head, the third pair of legs 

 is extended, the fifth pair bent ; that is, the third pair 

 can push and the fifth pair pull. The animal will 

 thus go backward easily and forward with difficulty. 

 We see here again that the nervous elements of the 

 central nervous system which bring about the for- 

 ward movements have the opposite orientation as re- 

 gards the longitudinal axis of the animal from the 

 nervous elements which bring about the backward 

 movement. But we can go still further in the devel- 

 opment of this law. Palsemonetes can not only walk, 

 but is also a good swimmer, and it can swim back- 

 wards as well as forwards. In swimming forwards 



^ 



the swimming appendages, among which the tail fin 

 must be counted, push backwards forcibly and for- 

 wards gently ; in swimming backwards the opposite 

 occurs. If the current be sent through Palaemonetes 

 in the direction from head to tail J the swimming 



1 The tail fin behaves toward the current like the abdominal swimming ap- 

 pendages and not like the body. This must be taken into consideration in 

 galvanotropic experiments. 



