166 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



appendages, and the tail also, are stretched backward 

 or dorsad to their fullest extent. This proves that the 

 tension of the muscles that move those organs back- 

 wards is greater than that of their antagonists. The 

 shrimp can thus swim forwards toward the anode 

 easily under the influence of such a current, but back- 

 wards only with difficulty. If the current passes 

 through in the opposite direction (from tail to head), 

 however, the tail and the ventral appendages are 

 turned forward. The tension and the development 

 of energy now predominate in those muscles which 

 move the swimming appendages backwards. In this 

 way the animal can swim backwards easily, while it is 

 difficult or impossible for it to swim forwards. Hence 

 the nervous elements, which determine the forward- 

 swimming, must also have the same orientation in re- 

 gard to the longitudinal axis of the animal as those 

 elements which determine the walking forwards, while 

 the nervous elements for swimming backwards have 

 the opposite orientation. 



The fact yet remains to be considered that Palse- 

 monetes, like many other Crustacea, can also move 

 sideways. 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 the extensors). 

 If a current be sent transversely, say from right to 

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

 assume the flexor-position, those of the left side the 

 extensor-position. In the legs of the right side the 



