Mast, Light Reactions in Lower Organisms. 155 



Unterschied in der Orientierung, sondern nur in der Lokomotion 

 ist; dass das Tier in beiden Fallen gegen die Lichtquelle orientiert 

 ist, jedoch nicht gleiche Muskeln spannt." 



This explanation will not hold for Volvox or Euglena, for both 

 of them turn the anterior end from the source of light when they 

 are negative. 



When Volvox colonies are negative they orient in all essentials 

 as they do when positive, except that they direct the anterior end 

 from the source of light. In swimming horizontally from a source 

 of light they seldom move parallel with the light rays. If the 

 position of the light is changed after they have oriented, they 

 change the direction of motion until the course again bears the 

 same relation to the ray-direction it did before. If exposed to 

 • light from two*sources, so arranged that the rays make a definite 

 angle with each other, they move from a point between the two. 

 If one source is more intense than the other, the point from which 

 they move is nearer that source. 



These facts and others are established by the following experi- 

 mental results, which are presented in graphic form (Fig. 14). 



By referring to path A it will be seen that the colony introduced 

 at n was positive to light from the three glowers as well as to that 

 from the arc, but that it became negative after swimming toward 

 the arc for a short distance from c, turned about and moved across 

 the aquarium to c'. That is, at the end of the experiment the 

 colony was negative to a much lower light intensity than at the 

 beginning. The arc was approximately 250 candle power. It 

 was 15 cm. from the point where the organism became negative. 

 The light intensity at this point was therefore 1 1,1 11 ± candle 

 meters. But the colony was still negative after having crossed 

 the aquarium, a distance of nearly 8 cm., or nearly 23 cm. from 

 the arc, 1. e., in an intensity of 4726 ± candle meters, which is 

 6385 ± candle meters less than the intensity in which it first be- 

 came negative. Similar results are represented in path B and 

 the paradoxical nature of the results is even more striking than 

 in the case of path A. Unfortunately, the distances between the 

 sources of light and the aquarium, in this ^exposure, were not 

 recorded. 



The colony which produced path B was positive to the light 

 from the arc when first put into the aquarium at c, but after mov- 

 ing toward the source of light a few centimeters, it became negative. 



