38 TROPISMS 



It is not difficult to suggest by aid of a diagram the 

 arrangement of the elements in the central nervous system 

 required to bring about the phenomena of galvanotropism 

 just described for Palcemonetes. We take it for granted 

 that the regular phenomena of anelectrotonus and cate- 

 lectrotonus of motor nerve elements suffice for the ex- 

 planation of these phenomena. We assume that if the 

 cell body of a neuron is in the state of catelectrotonus 



Fio. 5. Fig. G. 



Figs. 5 and 6. — Show the same effects of current on the common crayfish as those on 



Palcemonetes in Figs. 2 and 3. 



its activity is increased, when it is in anelectrotonic con- 

 dition activity is diminished. Neurons having the same 

 orientation will always be affected in the same sense by 

 the current. 



Fig. 7 is a diagram of the arrangement of neurons 

 giving rise to the bending of the legs on the side of the 

 anode and to the extension of the legs on the side of the 

 cathode when the current goes sidewise through the ani- 

 mal. This diagram assumes that the nerves innervating 

 the extensors come from the opposite side of the central 



