REACTIONS TO LIGHT IN C1LIATES AND FLAGELLATES. 



49 



THE FLAGELLATA. 



In the following pages we shall examine the method of reaction to 

 light in the flagellates Euglena viridis, Cryptomonas i 



ovata, and a species of Chlamydomonas. ' 



/ / 

 EUGLENA VIRIDIS. 



Euglena viridis swims in a spiral path, continu- 

 ally swerving toward that side which bears the larger 

 u lip " and the eye, the so-called dorsal side (Fig. 20). 

 Its motor reaction to most stimuli is by a sudden pro- 

 nounced turning toward the dorsal side ; that is, by 

 swerving still farther toward the same side toward 

 which it swerves in its normal swimming. Thus the 

 direction of its path is changed (Jennings, 1900). 



The general features of the reaction of Euglena to 

 light have been well worked out by Englemann 

 (1882, a) and Wager (1900). These authors show 

 that Euglena collects in lighted regions. The organ- 

 isms pass into a lighted area without reaction. But 

 on coming to the outer boundary of such an area, 

 where they would pass out into the dark, they react 

 by turning round and passing back into the light. 

 The collections of Euglenae in lighted areas are thus 

 brought about in much the same manner as the col- 

 lections of Paramecia in regions containing a weak 

 acid (Jennings, 1899). If diffuse light falls from one 

 side on water containing Euglenae, the organisms swim 

 toward the source of light. But if strong sunlight 

 falls upon them they swim away from the source of 

 light. 



Engelmann showed that the colorless anterior end 

 is the part that is chiefly sensitive to variations of light. 

 Often the organism in a lighted area, on reaching the 

 edge, reacts by turning when only the colorless tip 

 has passed into the darkness. 



The precise method of reaction to light, the direc- 

 tion of turning in becoming oriented or in passing 

 back into the lighted area, was not worked out by the authors named. 

 To this point we shall direct our attention. 



When a large number of Euglenas are swimming toward the source 

 of light, if the illumination is suddenly decreased in any way, they give 



* Fig. 20 shows the spiral path of Euglena in its ordinary swimming. 



FIG. 20.* 



