REGULATION OF REACTIONS 275 



longlcaudus. A large majority of all of these species were 

 strongly negative at 22°, In light of 250 ca. m., when first 

 mounted, but after they had been exposed from two to 

 three minutes they became strongly positive without any 

 change In temperature or light Intensity. The temperature 

 was now gradually lowered, and as this proceeded the organ- 

 isms became less and less active. At about 12° nearly all of 

 them came to rest and the Euglenae contracted and became 

 nearly spherical as If about to encyst. Thus the organisms 

 lay motionless as the temperature decreased to 8° and 

 finally to 5°. But after having been In this low temperature 

 for nearly five minutes, they gradually became active again 

 and swam about, first In an apparently aimless fashion, but 

 later as definitely and rapidly from the source of light as 

 they had been swimming toward It at 22°. They thus 

 became negative In the low temperature without any change 

 In the Intensity of the light. Is this reversal In the sense 

 of reaction due to the effect of changing the temperature, or 

 is it due to the absolute difference in temperature? The 

 following has reference only to Euglena viridls, although 

 the reaction of the other species mentioned above is similar 

 to that in this form. After the Euglenae used in the ob- 

 servations referred to above had been subjected to 5° for 

 some minutes, the temperature was gradually raised and it 

 was found that they were still negative at 8°, but positive 

 at 12°. After having been at 12° for six minutes, the tem- 

 perature was again decreased, and now It was found that the 

 organisms remained active and positive at a temperature 

 even below 5°. They did not come to rest at 8° as they 

 had when first exposed to decrease in temperature. The 

 temperature was now allowed to rise gradually to about 

 22° in 250 ca. m. About half of the Euglenae collected on 

 the side toward the light and the rest on the opposite side. 

 When the slide was turned end for end, the two groups 

 immediately began to swim in opposite directions in two 

 columns which met and passed near the middle of the field, 

 the positive column above, near the cover-slip, the negative 



