lo6 LIGHT AND THE BEHAVIOR OF ORGANISMS 



and with the dorsal surface exposed the same reaction is 

 induced by a change in the direction of the rays without a 

 decrease of intensity. These results lead to the conclusion 

 that a change of the organism from a position in which the 

 anterior end faces the so'urce of light to one in which the 

 dorsal surface faces it, results in a reduction of effective 

 Itght intensity of approximately 44 ca. m. in a total inten- 

 sity of 61 ca. m. Since the anterior end of Euglena is 

 nearly transparent, such a relatively large reduction seems 

 possible only if there is a highly sensitive bit of protoplasm 

 so situated that the eye-spot casts a shadow on it when the 

 light strikes the dorsal surface. 



k. Function of the eye-spot. — Wager (1900, PI. 32, 

 Fig. 2) observed an enlargement in the flagellum, situated 

 very near the concave surface of the eye-spot (see Fig. 11). 

 It may be that this is highly sensitive to changes in light 

 intensity and that the eye-spot functions as an opaque 

 screen casting a shadow upon the enlargement whenever 

 the dorsal surface is exposed. It may however also function 

 in absorbing the rays when the ventral surface or the 

 anterior end is exposed, much as the retinal pigment func- 

 tions in the eye of higher forms, or it may function some- 

 what like the pigment cups in planarians, amphioxus, etc. 



The only evidence we have with reference to the function 

 of the eye-spot aside from that presented above is given by 

 Engelmann (1882, p. 396). He says, in substance, refer- 

 ring to this structure in Euglena viridis, that if a sharp 

 shadow is gradually brought from the posterior end of a 

 swimming Euglena toward the anterior, there is no reac- 

 tion until the shadow reaches the colorless anterior portion 

 of the organism which contains the eye-spot. In the case 

 of large individuals moving into a shadow, the reaction 

 could be seen to be given before the eye-spot was in dark- 

 ness. The colorless anterior end is therefore the primary 

 light recipient region, but the eye-spot may still function 

 secondarily, as do the pigment cells in the retina of higher 

 animals. These observations of Engelmann have been 



