REACTIONS TO STIMULI IN CERTAIN ROTIFEKA. 85 



negative results were obtained with several of the Philodinadas (Rotifer, 

 Philodina), some species of Euchlanis and Salpina, Noteus quadri- 

 cornis, and a number of the Rattulidae. 



In Hydatina senta there is a reaction of a peculiar character which 

 perhaps furnishes a clue to the cause of the more pronounced reaction 

 to be described for Anura^a. With a current of moderate strength, 

 such as that to which Paramecia react most markedly, Hydatina shows 

 no reaction except when the head is directed toward the anode. But 

 in this position the animal at once retracts its cilia and sinks to the 

 bottom. Thus a Hydatina may swim freely about in water through 

 which the current is passing, provided it swims toward the cathode, or 

 transversely, or obliquely ; as soon, however, as it turns its head toward 

 the anode it stops swimming and sinks to the bottom. Thus if an 

 electric current is passed through a preparation containing a large 

 number of specimens of Hydatina, many will be seen swimming 

 toward the cathode and others at all sorts of angles with the current, but 

 none toward the anode. This is a phenomenon akin to what I have 

 elsewhere called the production of orientation by exclusion. If organ- 

 isms are prevented from swimming in any direction but one, after a 

 time, provided the course is frequently changed, all that are swim- 

 ming will be found moving in that one direction. This condition is 

 realized, as I have shown in the first of these contributions, in the 

 reactions of infusoria to heat and cold. But in the reactions of Hydatina 

 to the electric current the "exclusion" is less complete than in the 

 cases just mentioned ; the animal may swim in any direction except 

 one. 



The fact that the head is retracted when directed toward the anode 

 and not in other positions indicates that there is a greater stimulation 

 at the anode than elsewhere. This agrees with much that is seen in 

 the reactions of infusoria to the current. After Hydatina has sunk to 

 the bottom with anterior end to the anode, it repeatedly makes attempts 

 to unfold its cilia. But scarcely have they begun to operate when 

 they are withdrawn again. Each time that they are uncovered for an 

 instant, however, they turn the animal a little toward its dorsal side. 

 Thus, after a considerable number of attempts to unfold the cilia, the 

 head has become turned away from the anode ; then the cilia are spread 

 out and the animal goes on its way until it is so incautious as to turn 

 its head again toward the anode. 



Anurcea cocklearis shows marked electrotaxis similar to that found 

 in the infusoria. When the continuous current is passed through a 

 preparation containing large numbers of this species, all orient quickly 

 and swim toward the cathode. They thus agree, so far, in their 

 reaction to the electric current, with the ciliate infusoria. 



