2] EFFECT ON STRUCTURE AND FUNCTIONS 135 



making is distinctive, we may, call this the anode-excitation 

 tyr>e, or, briefer still, anex type. 



C Turning, now, to nerve and muscle tissue, we meet with a 

 type of response, on making and breaking the current, alto- 

 gether irreconcilable with this. As is well known, when a 

 constant current is made or broken, all the tissue lying between 

 the electrodes is not stimulated at one time, but the excitation 

 makes its appearance at the anode or kathode, and thence is 

 transmitted to the other pole. One can demonstrate this on 

 slow-moving (e.g. extremely tired or dying) muscle at the 

 extremities of which the electrodes are placed. The contrac- 

 tion begins at one electrode, and travels towards the other. 

 By using more refined methods, this relation, which holds for 

 nerves, striated and smooth muscle (cf . ENGELMANN, '70, p. 302) 

 has been formulated as follows : 



This is seen to be the very opposite of the response given by 

 Pelomyxa. It may be called the kathode-excitation type, or, 

 in brief, the katex type. 



Having now seen that two fundamentally different types exist 

 in the response of the two extreme groups of the animal king- 

 dom, the question arises, what is the distribution of these types 

 amongst the intermediate forms the Invertebrate Metazoa? j 

 Fortunately, through the investigations of NAGEL ('92 and '92 a ), 

 we have data upon this subject. In NAGEL'S experiments, the 

 whole animal was employed, the two electrodes were placed at 

 the opposite ends of its long axis, the metallic circuit was then 

 made or broken as required, and the pole (anode or kathode) 

 at which contraction first occurred was noted. Thus, NAGEL 

 found that when the current was made through the sea-hare, 

 Aplysia, there was strong excitation and momentary retraction 

 of the parts next to the ariode, while next to the kathode the 

 body showed a considerably weaker contraction. Upon break- 

 ing the current, there was some excitation of the parts of the 



