150 ELECTRICITY AND PROTOPLASM [Ch. VI 



cephalad, so that they come to lie with head towards anode ; 

 the weakest current is that by which (following Ewald) 

 the organisms are irritated as it flows caudad, so that they 

 come to lie facing the kathode; finally, the strongest cur- 

 rent is that at which a violent stimulation leading to paraly- 

 sis is produced by the cephalad-flowing current (as well as 

 the caudad ?). 



In seeking for an explanation of electrotaxis in Metazoa, it is 

 necessary, first of all, to notice that there is a close relation 

 between response to the make-shock (as described on pp. 136, 

 137) and the direction of orientation of the body in electro- 

 taxis. Thus all gastropods studied are, upon making, excited 

 chiefly at the anode, and, correspondingly, all gastropods hitherto 

 studied when subjected to the current face the kathode ; so on 

 the other hand, such Crustacea as have been studied are stimu- 

 lated at the kathode, and they accordingly come to face the 

 anode. In regard to Vertebrates, we have apparently a double 

 electrotactic orientation varying with the current, and corre- 

 spondingly we have, as Nagel has shown (p. 137), a double 

 irritability depending on the current. A medium current pro- 

 duces a kathode excitation and an anode orientation; while 

 the weakest current produces an anode excitation and a 

 kathode orientation. So we may lay it down as a general 

 law : Positively electrotactic organisms exhibit the katex type of 

 irritaiility ; and negatively electrotactic organisms exhibit the 

 anex type or, in general, the electi'otactic organism turns tail to 

 the exciting pole. 



Ewald ('94, pp. 611-615) accounts for this difference of 

 response of Vertebrates to weak and strong currents, by the 

 aid of certain observations that he made upon the excitation 

 of the nerve cord. We have already seen that the making 

 of the medium constant current stimulates at the kathode, so 

 that an animal turns tail to the kathode. Ewald found that 

 the two parts of the dorsal nerve were differently stimulated 

 by the current ; the brain was stimulated chiefly by a caudad- 

 passing current ; the spinal cord chiefly by a cephalad-passing 

 current. This conclusion was established by two experiments. 

 First, the two electrodes, placed a few millimeters apart, are 

 brought into contact with different parts of the body of a fish. 



