in ELECTRICAL EXCITATION OF MUSCLE 307 



stimuli, e.g. light, warmth, chemical reagents, are known to 

 exercise a directive influence, if they act locally, or with differ- 

 ent intensity at different parts of the excitable substance. 

 Certain light rays (those with short vibrations in particular) 

 cause many of the Flagellata, as well as the spores of many 

 Algae, to move in the direction of the rays that strike them, either_ 

 from or towards the light (positive and negative heliotropism). 

 Pfeffer has demonstrated an analogous effect of chemical reagents 

 in solution upon Bacteria and Flagellata, i.e. attraction or repul- 

 sion of the organisms, a cheinotropic action positive or negative. 

 In all these cases, as Yerworn correctly remarks, we have to do 

 with some polar excitation of the protozoa, that gives rise to an 

 axial orientation of the organism in accordance with the direction 

 of excitation, or, more correctly speaking, of the differences in 

 intensity of such excitation. On the assumption that when 

 current passes through an amoeba, the make excitation, as in 

 other rhizopods, is primarily anodic, it is evident that a formation 

 of pseudopodia due to a forward current of the protoplasm can 

 take place only in the direction of the kathode; the forward 

 movement in the direction of the current is thus accounted for. 



This directive action of the current (" Galvanotropism ") is 

 still plainer in the case of many rapidly-moving ciliated Infusoria, 

 e.g. Flagellata. If a few drops of hay infusion swarming with 

 paramsecium are placed between clay electrodes (as described 

 above) upon an object-glass, and traversed by a sufficiently strong 

 current, the following effects (as pointed out by Verworn) are 

 observed, either with the naked eye or with a magnifying lens. 

 " At closure the Paramsecia turn altogether as if at word of com- 

 mand, with the anterior pole of the body towards the negative 

 electrode, and swim in this direction with uniform speed " 

 (Fig. 99). 



In a short time the anodic side of -the drop is completely 

 cleared of Paramsecia, not one being left behind ; the whole mass 

 has crowded to the kathode. As long as the circuit is closed the 

 protozoa remain thus ; if the current is broken the paraimecia 

 turn instantly with their anterior ends towards the anode, and 

 swim away in this direction. The kathode is quickly deserted, 

 and the majority of the organisms are now collected round the 

 anode. The crowding is, however, by no means so complete as 

 that produced at the kathode by make of the current, and the 



