274 Animal Instincts and Tropisms 



pair can push, and the fifth pair pull. The animal 

 will thus move backward easily and forward with 

 difficulty, and it is thus driven to the anode again. 



The explanation which Loeb and Maxwell proposed 

 for this influence of the current on the legs assumes 

 that there are three groups of ganglion cells in the 

 central nervous system of these animals which are 

 oriented according to the three main axes of the body ; 

 (i) right-left and left-right, (2) backward, and (3) for- 

 ward. It depends upon whether the ganglion cells 

 or the nerve elements are in anelectrotonus, which 

 muscles are bent and which relaxed. It would lead 

 us too far to recapitulate the theory in this place, and 

 the reader who is interested in it is referred to Loeb 

 and Maxwell's paper.' The importance of the ob- 

 servations lies in the fact that they show that any 

 element of will or choice on the part of the animal in 

 these motions is eliminated, that the animal moves 

 where its legs carry it, and not that the legs carry the 

 animal where the latter "wishes" to go. 



7. This may be the place to dispel an error which has 

 sometimes crept into the discussion of the tropistic 

 reactions of animals. It has been stated occasionally 

 that it is the energy gradient and not the automatic 

 orientation of the animal by the light which makes 

 the positively heliotropic animal move towards the 

 source of light and the negatively heliotropic away 



^ Loeb, J., and Maxwell, S. S., Arch.f. d. ges. Physiol., 1896, Ixiii., 121. 



