94 S. O. MAST. 



would hardly expect this in an organism so nearly radially sym- 

 metrical that it is impossible to see any fixed structural differences 

 between the various sides. 



That these animals do, however, always turn toward the same 

 side was ascertained in two ways : ( i ) By withdrawing the water 

 from under the cover-glass until they could no longer swim 

 freely; and (2) by adding Chinese ink to a thin solution of 

 quinced jelly and studying their reactions in this. Under the 

 cover-glass in only a very thin layer of water they could of course 

 turn only in two directions and thus it was very easy to see that 

 they always turned toward the same side under these conditions. 

 When free to swim in all directions, it was much more difficult 

 to follow their movements precisely. In the jelly solution, how- 

 ever, the avoiding reactions are so frequent and the rate of 

 motion is so much reduced that by selecting a specimen to the 

 surface of which particles of ink adhered in such a way as to 

 differentiate the sides, it could be definitely seen that they always 

 turn toward the same side. 



By direct observations on the movement of the cilia during the 

 process of turning, and by studying the currents produced, it 

 could be definitely seen that the cilia in the anterior band along 

 the edge toward which the animal turns, strike forward toward 

 the mouth, while the rest in this band and all those in the posterior 

 band strike backward. The fact that some of the cilia in the an- 

 terior band strike in one direction at the same time that others in 

 the same band strike in the opposite direction and that this occurs 

 only when the creature is turning, shows an interesting adaptive 

 differentiation in the function of cilia. 



In this organism there is no evidence of differential response 

 to localized stimulation. Didiniwn always turns toward the same 

 side, no matter which part of the anterior end comes in contact 

 with an object, the conical projection or various portions of the 

 edge. 



VARIATION IN FORM. 



Not infrequently one finds specimens of Didinium which appar- 

 ently consist of two or three individuals fused together at the 

 posterior ends. These specimens are about two or three times 

 normal size, depending upon the number fused, and all the 



