140 



the change in illumination produced is great. The variations in 

 illumination due to the spiral course are doubtless much accentuated 

 by the fact that one side of the anterior end bears a pigment spot, which 

 in certain positions of the unoriented Euglena cuts off the light. A 

 decrease of illumination causes, as we know, the avoiding reaction; 

 the anterior end swings in a wider circle (Fig. 91). This still further 

 increases the variations in the illumination of the anterior end. Every 

 time the illumination is decreased, this causes the animal to swerve still 

 more ; so that its anterior end becomes pointed in many different direc- 

 tions, till it comes into one where such changes in illumination no longer 

 occur. Such a position is found when the animal is swimming toward 

 the source of light. Now the axis of the body retains always the same 

 relation to the direction to the rays of light, so that the anterior end is 

 not subjected to variations in intensity of illumination. There is then 

 no further cause for reaction. Orientation is thus reached by trying 

 various directions. This will be best understood by an examination 

 of Fig. 95, together with its explanation. 



Euglena responds most readily to light of a blue color (Engelmann, 

 1882). Passage from blue light to light of other colors has essentially 

 the same effect as passage from stronger to weaker light. If the differ- 

 ence between the two is sufficiently decided, Euglena responds by the 

 avoiding reaction in passing to the other color; it therefore remains in 

 the blue. If a small spectrum is thrown on a slide containing many 

 Euglenae, they gather in larger numbers in the blue, especially in 

 the near vicinity of the Frauenhofer's line F. 



Very strong light, such as direct sunlight, has an effect on Euglena 

 precisely the opposite of that produced by weaker light. If the organ- 

 isms are subjected suddenly to sunlight, they give the avoiding reac- 

 tion. They tend therefore to gather in less lighted regions. If the 

 sunlight falls upon them from one side, they become oriented with an- 

 terior ends away from the source of light, and swim in that direction. 

 The orientation takes place in exactly the way described above, save 

 that now it is the increase of light at the anterior end that causes the 

 avoiding reaction. If a vessel is placed in such a position that the sun 

 shines on it from one side, while the half of the vessel away from the sun 

 is shaded with a board, the following result is produced : The Euglense 

 gather in a band at the edge of the shadow (Fig. 96). They do not 

 pass into the dark area beneath the shadow, nor do they remain in 

 the region affected by direct sunlight, but in an area of intermediate 

 illumination. 1 



'This experiment is due to Famintzin (1867, p. 21). 



