88 THE BEHAVIOR OF LOWER ORGANISMS. 



reaction in Anursea is in a general view as striking and clear-cut as 

 that of Paramecium. 



Thus in the rotifer Anuraea the orientation to the continuous elec- 

 tric current is produced through a motor reaction, the essential features 

 of which are determined by the structure of the organism. The organ- 

 ism turns always toward the dorsal side, continuing or repeating the 

 turning until the anterior end is directed toward the cathode. In these 

 respects it agrees with hypotrichous Ciliata, where the direction of 

 turning is determined by the action of the adoral cilia. The method 

 of reaction is quite incompatible with the tropism schema. 



SUMMARY. 



The reactions of those Rotifera of which an account is given in this 

 paper take place in a manner essentially similar to the reactions of the 

 ciliate infusoria. 



In the reactions to mechanical stimuli, to chemicals, and to heat, 

 orientation is not a striking feature. The organism turns when stimu- 

 lated toward a structurally defined side as a rule toward the dorsal 

 side ; in this way it avoids the source of stimulus. 



In the negative reaction to light the organism becomes oriented with 

 anterior end directed away from the source of strongest light, but this 

 orientation is brought about in the same manner as in Stentor ; the 

 animal turns toward the dorsal side without relation to the side on 

 which the light strikes it, and continues the turning or repeats it until 

 the anterior end is directed away from the source of light. 



To the continuous electric current the rotifer Anuraea orients and 

 swims directly toward the cathode. The reaction is brought about in 

 the same manner as the orientation to light. When the current is 

 made or reversed the animal turns toward the dorsal side and continues 

 the turning until the anterior end is directed toward the cathode. 



Thus the direction of turning is throughout dependent on an internal 

 factor, not primarily on the way in which the stimulus impinges on 

 the organism. These reactions of the Rotifera are thus inconsistent 

 with a theory of tropisms which regards orientation as a primary 

 feature of the reactions, and which holds that the action of the stimu- 

 lating agent is a direct one on the motor organs of that part of the 

 body on which it impinges. The reactions of the Rotifera, so far as 

 described in the preceding pages, are brought about, like those of the 

 infusoria, by what may be called the method of " trial and error." 

 The reaction to any stimulus is of such a nature as to head the organism 

 successively in many different directions. That direction is followed in 

 which there is no stimulus to induce further turning. 



