290 S. O. MAST AND K. S. LASHLEY 



ration were turning through only a small angle such as that 

 shown in figure 3. In the reaction represented in figure 4, the 

 aboral side of the anterior end of the organism first came in con- 

 tact with the acidified ink, then the animal began to turn toward 

 the aboral side and swim backward. Soon after this the ink 

 was drawn out in the form of a cone which increased in size and 

 followed the animal as it backed away, but only for a short time. 

 As soon as the ink no longer came in contact with the Para- 

 mecium it stopped backing and turning, the cilia reversed and it 

 swam away. In the reaction represented in figure 5, the or- 

 ganism entered a depression in the edge of the cloud of ink and 

 was stimulated while at right angles to this edge. The aboral 

 swerving continued only until the Paramecium was carried out 

 of the depression and until the cone no longer reached the body. 

 A large number of cases of this sort were observed but no con- 

 clusive evidence that the degree of turning resulted from the 

 duration of stimulation was obtained. In the same preparation 

 different paramecia give quite different reactions to the same 

 cloud of acidified ink. Some swim far into the cloud before 

 giving any reaction, some give a weak avoiding reaction at the 

 edge of the cloud, and some give a violent avoiding reaction at 

 the first contact with the ink. Jennings has shown that the 

 second phase of the strong avoiding reaction, the aboral rotation, 

 may be continued for a long time after violent stimulation and 

 it is possible that the weak avoding reactions in which the 

 degree of turning seemed to be controlled by the position of the 

 cone were in reality due merely to the chance intensity of stimu- 

 lation. The proportion of cases in which forward swimming 

 seemed to follow the removal of the stimulating agent from the 

 oral groove is, however, large enough to make an explanation on 

 the basis of chance rather doubtful. 



OBSERVATIONS ON STENTOR, SPIROSTOMUM AND ROTIFERS 



Free-swimming stentors do not ordinarily produce a feeding 

 cone unless they are mechanically retarded, as often occurs owing 

 to the collection of debris on the mucus so copiously secreted by 

 these organisms, especially at the posterior end. In approaching 



