OBSERVATIONS ON UNICELLULAR FORMS 103 



every time this surface is turned toward the light, each 

 reaction resulting in directing the anterior end more nearly 

 toward the source of light, until both surfaces are so nearly 

 equally illuminated throughout the entire rotation that 

 the change of intensity is no longer sufficient to cause a 

 reaction. 



If the light intensity is decreased at the same time that 

 the ray direction is changed, the reaction described above 

 always occurs immediately after the change is made, regard- 

 less of the surface illuminated. All however turn toward 

 the dorsal surface. This results in movement in all direc- 

 tions and apparent confusion. When Euglenae are taken 

 from the culture jars and first exposed in the aquarium they 

 are more sensitive and respond to slighter changes than 

 they do after they have been exposed for some little time. 

 Among these one finds many specimens which always re- 

 spond immediately after the ray direction is changed, even 

 if the intensity remains the same, no matter which surface 

 faces the light after the change is made. This indicates 

 that a change in the direction of the rays from that in 

 which the anterior end is illuminated to one in which the 

 ventral surface is illuminated causes the same response as 

 a decrease of intensity. It may therefore be concluded 

 from this and preceding observations that the organism is 

 most stable^ when the anterior end faces the source of light, 

 less stable when the ventral surface faces it, still less stable 

 when the dorsal surface is exposed, and least stable when 

 the posterior end is directed toward the source of light. I 

 was able to ascertain roughly the amount of reduction in 

 light intensity required to induce the avoiding reaction 

 with each of these different surfaces illuminated excepting 

 that on the posterior end. The results of this work to- 

 gether with a description of methods will be found below. 



The orienting reaction of free-swimming specimens is 

 in all essentials like that of the crawling specimens. It 



* It is meant by this that it requires a greater change of light intensity 

 in the field to induce a reaction. 



