REACTIONS TO LIGHT IN CILIATES AND FLAGELLATES. 



half of the circular area, making a sharp line (x-y, Fig. 1 1) dividing the 

 light from the darkness. A mirror was placed above the microscope, 

 inclined in such a position as to project the image of the Stentors, very 

 much magnified, on the ordinary vertical screen used for receiving 

 lantern slide views. Thus the behavior of the Stentors could be studied 

 with great ease on the screen. 



The heat from the lantern was cut out, so far as possible, by placing 

 between it and the mirror of the microscope a glass cell three inches 

 thick, filled with cold water. In this manner the heat was excluded to 

 such an extent as to fall below the threshold for the stimulation of 

 Stentor by heat. This was demonstrated by comparing the reactions 

 of Stentor with those of Paramecium. Stentor is less sensitive to 

 changes in temperature than is Paramecium ; this was clear in my ex- 

 periments on the reaction to heat. Par- 

 amecium does not react at all on passing 

 into the area illuminated by the lantern, 

 but swims about indifferently in both the 

 dark and the light parts of the dish, show- 

 ing that the heat produced is below the 

 threshold for Paramecium ; it must then 

 be below the threshold for Stentor. 



The free Stentors in the unlighted part 

 of the vessel swim about at random. 

 Many individuals thus come by chance to 

 the line x-y, Fig. 1 1 , where they would 

 pass into the lighted area. These at 

 once back a little, then turn toward the 

 right aboral side, and swim forward 



again. The turning toward the right aboral side is usually through an 

 angle sufficient to direct the Stentor away from the lighted area (see 

 i, 2, 3, 4, Fig. n) ; if it is not, the Stentor repeats the reaction until, 

 after one or two trials, it swims into the unlighted region. 



Many of the individuals react as soon as the anterior end reaches the 

 lighted area, so that less than one-fourth of the body is in the light. 

 This shows that light falling upon the anterior end alone is sufficient 

 to cause the reaction. 



A few specimens swim completely into the lighted area, then react 



*FiG. ii. Method of studying the manner in which Stentor reacts to light. 

 The figure shows a circular area, illuminated from below, with the light cut oft 

 from the left side by a dark screen, the line x-y separating the light from the 

 dark area. The Stentors collect in the dark area. The reaction of a specimen 

 which comes to the line x-y is shown at i, 2, 3, 4. 



