28 A W. GREELEY. 



first have to obtain a satisfactory explanation of the rhythmical 

 contraction of the ectosarc which controls the cilia. It is certain 

 that the surface tension relations of the muscular elements play 

 an important role in this process. In Amccba the problem is very 

 simple. The protoplasm contracts on the anodal side because 

 of the neutralization of the charge carried by the protoplasmic 

 particles and consequent increase in surface tension. Pseudopodia 

 are thrown out on the cathodal side, and movement occurs in this 

 direction because of the decrease in surface tension at this point. 

 But for paramoecia it suffices at present to show that the sense of 

 the response is not a fixed attribute of the organism which has 

 been acquired by natural selection, but that it is a purely physical 

 response to an external stimulus, and varies directly with the 

 conditions under which it occurs. The ultimate determining 

 factor of the response to the electrical current must be the elec- 

 trical conditions of the protoplasm itself. In paramcecia from a 

 normal alkaline culture the protoplasmic particles appear to be 

 negatively charged. These paramoecia collect about the cathode 

 where liquefaction of the protoplasm occurs. But with precisely 

 the same conditions, under which liquefaction is produced not by 

 anions but by cathions, i. c., an acid culture medium, we find that 

 the characteristic gathering about the cathode does not occur, 

 but the paramcecia tend to move toward the anode where the 

 protoplasm would now become liquefied. The only explanation 

 of this phenomenon that presents itself is the assumption, that in 

 an acid medium the protoplasmic particles become positively 

 charged in a portion of the organisms (for the reaction is never 

 completely reversed). The partial reversal of the reaction by 

 osmotic means is also due to an alteration in the electrical con- 

 ditions of the protoplasm, as has been shown. 



Hardy's acid and alkali-modified colloids reacted also in an 

 opposite manner to the electrical current because in the acid 

 solution the particles were positively, and in the alkaline solution 

 negatively charged ; and while the explanation is not so simple 

 in the case of Paramoeciuni, because movement is effected by a 

 complex motor apparatus, still the sense of the reaction must be 

 ultimately due to the same cause in both cases, /. t\, the charge 

 carried by the colloidal or protoplasmic particles. Moreover, 



