REACTIONS TO STIMULI IN CERTAIN ROTIKERA. 



S7 



turning is continued still farther, so that the head of the animal de- 

 scribes a complete circle ; indeed, this may continue so that the animal 

 whirls around several times, always towards the dorsal side. The 

 reaction thus far is the same as that produced by heat (Fig. 28). In 

 reacting to the electric current the whirling finally ceases with ante- 



FiG. 29.* 



rior end directed toward the new cathode. The animal then swims 

 forward in the direction so indicated. These turnings, even when 

 several times repeated, require but a moment, so that very soon prac- 

 tically all the specimens are swimming toward the new cathode. The 



*FiG. 29. — Diagram of method by which Anuraea becomes oriented to rays of 

 light, or to the electric current. Taking the latter for example, the animal is at 

 first swimming toward the cathode, in direction indicated by arrow a;; it thus 

 follows a spiral path from a to i>. At b the electric current is reversed. The 

 animal thereupon swerves strongly toward its dorsal side, describing a semicircle, 

 b, c, <i, until its anterior end is directed toward the new cathode, in the opposite 

 direction from before. It now follows the spiral path d to <>, in the general direc- 

 tion indicated by the arrow j*. The facts are similar for the reversal of light, or 

 for the reaction when the current or the light is first set in operation. 



