148 LIGHT AND THE BEHAVIOR OF ORGANISMS 



but not so in darkness. No evidence of this property was 

 seen in the eye-spot of Euglena. It therefore appears that 

 the eye-spots in these two forms differ in composition, and 

 it may be that they function differently. At any rate, 

 judging from their location I am unable to see how they 

 could function in Volvox, Pandorina or Eudorina by shading 

 structures in the interior, as they appear to in Euglena, 

 unless they function only when the colonies are negative. 

 The fact however that the eye-spots at the anterior are 

 much larger than those at the posterior end is strong evi- 

 dence in opposition to such a view. If these structures func- 

 tion in light reactions in these forms at all, they must 

 function either as an absorptive background somewhat like 

 the retinal pigment in the eye or as direct light recipient 

 organs as I suggested (1907, p. 112). 



In our work on Euglena it was pointed out that the 

 hyaline protoplasm at the anterior end condenses the light 

 and brings it to a focus in the neighborhood of the eye-spot 

 when the organism is oriented, possibly on the structure 

 most sensitive to light. In Pandorina and Eudorina each 

 zooid acts as a condensing lens. In direct sunlight a highly 

 illuminated spot can be clearly seen at the surface directed 

 away from the source of light in each zooid, even in those 

 well filled with chloroplasts. The focusing of the light is 

 much more definite in these forms than it is in Euglena. 

 It is evident that every lateral movement of the organism 

 causes a change in the location of the point on which the 

 light is focused in the zooids, and this of course produces 

 definite and marked changes in light intensity. It may be 

 that the eye-spots, located near the posterior end of the 

 zooids as they are, function in some way in connection with 

 such changes of the focal point. 



