130 LIGHT AND THE BEHAVIOR OF ORGANISMS 



for over a month and Increased in numbers. When they 

 are about to divide they crawl out through the opening in 

 the test at the base of the flagellum and then they may 

 swim about in the naked elongated state (Fig. 16) for some 

 time before they divide and form new tests. When the 

 test is new it is nearly transparent. It was in specimens in 

 this condition and in those in the naked state that the reac- 

 tions were observed. 



The orienting reactions were studied just as they were 

 in the forms already described. Trachelomonas was found 

 to orient just like Euglena in the free-swimming state. If 

 the light intensity is moderately decreased without changing 

 the direction of the rays they swim in a wider spiral; if 

 much decreased, they turn sharply in all directions. They 

 always turn toward the convex surface of the eye-spot. If 

 the ray direction is changed without a change of inten- 

 sity only those with the convex surface of the eye-spot 

 directed toward the light react immediately, the rest not 

 until this surface becomes exposed in the process of rotation. 

 The reaction consists in simply swerving farther toward 

 the light each time that the eye-spot faces the source of 

 light; thus they soon become oriented. If the ray direction 

 is changed with a simultaneous decrease in the light inten- 

 sity, all react at once. Under such conditions they turn 

 from the light as well as toward it. 



It is clearly evident that turning the convex surface of 

 the eye-spot tow^ard the source of light produces the same 

 effect as a decrease in the light intensity of the field. There 

 is practically the same amount of protoplasm on all sides 

 around this structure, and as far as can be seen under the 

 best oil immersion lens this protoplasm is the same on all 

 sides. If this is true the shadow of the eye-spot should 

 have the same effect whether illuminated from the concave 

 or the convex surface. The fact that it does not indicates 

 that there is a highly sensitive bit of protoplasm close to 

 the eye-spot on the concave surface. 



