Jennings, Behavior of Paraviccmin. 447 



the oral groove. It may be increased, under stimulation, by a 

 change in the beat of the body cilia of the anterior end at the 

 left side of the oral groove, by which they strike toward the 

 oral groove instead of away from it. On the number and 

 strength of action of the cilia showing the changed beat depends 

 the amount of swerving toward the aboral side. 



All the three factors in the spiral course may vary more 

 or less independently of each other, and on the amount of such 

 variations depends the width of the spiral, the number of turns 

 in a given distance, and the like. The effects of stimuli consist 

 largely, as we shall see, in changing the proportional parts 

 played by these various factors — decreasing or stopping one, 

 increasing another, etc. 



Fig. 2. A Paramecium swims toward an area containing India ink ; before 

 it reaches the boundary of the area a cone of the ink is drawn out by the action 

 of the oral cilia, reaching the anterior end and oral side. 



Owing to the stronger and more direct backward beat of 

 the oral cilia, in swimming forward a current of water is caused 

 to pass from in front in the form of a cone to the oral side and 

 mouth. This is rendered evident when a cloud of India ink is 

 added to the water containing many Paramecia. The cloud 

 has for a time a definite boundary surface. When the Para- 

 mecia swim toward this surface, the latter may be seen to ex- 

 tend out in the form of a cone, to meet the advancing animal 

 (Fig. 2). As soon as this black cone comes in contact with the 

 anterior end of the Paramecium, the latter stops and turns in 

 another direction — this occurring some distance from the gen- 



