STIMULI AND THEIR ACTIONS 419 



be broken, the breaking frequently is no longer stimulating, and, 

 in order to obtain a result on remaking, considerably stronger 

 currents must be employed than before. The decrease of irritabil- 

 ity under long action of the current is also the' reason why in 

 Actinosphcerium, with the intensity of the current remaining equal, 

 the amalgamation-process constantly decreases in intensity. Living 

 substance loses in irritability under the long action of a stimulus. 

 The law of excitation of Pelomyxa runs, therefore, as follows : 

 Pelomyxa is stimulated to contraction at the anode upon making, and 

 at the kathode upon breaking. 



Another form of polar excitation, which is perhaps still more 

 interesting, is shown by Amoeba proteus. 1 If a constant current be 

 sent through the body of the amoeba when the latter is extending 

 its pseudopodia in various directions, it is seen that the body 

 assumes at once the typical form of Amceba Umax, i.e., the extended 

 form in which the protoplasm flows in a single direction, the body 

 in a certain sense representing a single, large, thickpseudopodium. 





FIG. 200. Pelomyxa palustris. I, Normal, spherically contracted. //, At the moment of making, 

 the protoplasm begins to disintegrate at the anode. 



It is thus shown that the extended body is stimulated to contraction 

 at the anode, for here the characteristic vacuoles of Btitschli develop 

 in the protoplasm, and the body retracts strongly upon this side. 

 At the kathode, on the contrary, there exists an excitation of 

 expansion, for here the protoplasm spreads out into a broad lobe. 

 The phenomena are seen best when the direction of the current is, 

 suddenly reversed, so that what was previously the anode now 

 becomes the kathode, and vice versa (Fig. 201). 



Wholly analogous relations have been demonstrated recently by 

 Ludloff ('95) in Paramoecium. By the making of the current 

 phenomena of contraction are shown in the external form of the 

 body at the anode ; by strong currents that end of the body is 

 compressed into a point, the liquid of the trichocysts is pressed out 

 and, becoming coagulated in the form of threads, surrounds the 

 end of the body with rays (Fig. 202, B). The polar excitation of 

 the ciliary motion is much more characteristic. The cilia at the two 



1 Of. Verworn ('96, 4). 



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