REACTIONS TO STIMULI IN CERTAIN ROTIFERA. 83 



just how the reaction took place. Of course, only sharply defined posi- 

 tive observations are of value in deciding between two opposing possi- 

 bilities ; hence, although I have studied a number of other rotifers in 

 this connection, I give the results only where absolutely sure of them. 

 But in the two or three other rotifers I have examined in this connection 

 the reaction is apparently the same as that in Anurcea cochlearis, to 

 be described at once. 



The specimens of Anurcea cochlearis studied had been in a small 

 aquarium in the laboratory some months, and were distinctly negative 

 to light, gathering at the side of vessel farthest from the window. The 

 freshly collected animals are, I believe, usually positive to light. 



These negative individuals were placed in a small flat-bottomed 

 rectangular glass vessel, on a dark background, in a dark room. At 

 opposite sides of the vessel and somewhat above were clamped two 

 incandescent electric lights, A and B, at a distance of about 10 inches* 

 in the manner described for Stentor (p. 41 and Fig. 15). One of 

 these lights could be extinguished while the other was simultaneously 

 turned on. In this way the direction of the light falling on the rotifers 

 could be reversed. 



When only one of the lights, as A, was turned on, the AnuraBas all 

 collected at the opposite side of the vessel, next to B. When A was 

 extinguished and B turned on, they turned and swam in the opposite 

 direction, toward A. By reversing the direction of the light while the 

 animals were crossing the vessel their course could be reversed while 

 in full career. 



Focusing the Braus-Driiner on the vessel, and reversing the lights 

 when the animals were well in the field of observation, the following 

 could be observed : Some turned at once, with some sharpness, 

 toward the dorsal side> the turning continuing until the direction of 

 swimming was reversed and the animals were again swimming away 

 from the light (Fig. 29) . In these cases the direction of turning was 

 clear and could be observed without great difficulty. 



Other individuals continued for a short time to swim in the same 

 direction as before, then turned, either sharply, as just described, or 

 more slowly, in the manner to be described. 



Where the turning was sharp, as described above, there was no 

 great difficulty in determining with certainty the nature of the reaction. 

 But in many cases the turning took place more slowly, in the following 

 manner : Either as soon as the light was reversed, or very soon after, 

 the width of the spiral in which the animal was swimming became 

 much greater. In other words, the animal swerved more toward the 

 dorsal side and progressed less rapidly than usual. Thus it described 

 rather wide circles, and the swerving toward the dorsal side increased, 



