Mast, Light Reactions in Loiver Organisms. Ill 



the enlargement in the flagellum, while the other side is exposed, 

 and thus produce a difference in light intensity on opposite sides 

 of the enlargement; or (3) it may cut off the light from one side of 

 the sensitive portion of the anterior end of the organism when it is 

 not oriented, and thus produce unequal illumination on opposite 

 sides of this end. 



It seems impossible to test the suggestions of Wager experi- 

 mentally, but it may be possible to arrive at a tentative conclusion 

 concerning the matter, from what we know about the structures 

 and reactions of these organisms. Jennings ('04, p. 54) shows 

 that Euglena swims in a spiral course with the larger lip constantly 

 farthest from the center of the spiral. In thus swimming the 

 longitudinal axis never points toward the source of greatest illumi- 

 nation, so that when the organism is oriented the side of the ante- 

 rior end containing the larger lip is always more shaded by the 

 eye-spot than that containing the smaller lip (see Fig. 4). From 

 this it seems evident that the eye-spot in Euglena does not func- 

 tion in accordance with Wager's third suggestion. That it does 

 not function in accordance with this suggestion in Volvox is still 

 more clearly evident, for here the eye-spots are located near the 

 posterior surface in the projection of the zooid, so that if this pro- 

 jection is sensitive to light there certainly is no possibility of oppo- 

 site sides being equally stimulated when the organism is oriented, 

 for under such conditions the shadow of the pigment granule falls 

 on the posterior surfaces while the anterior surface is fully exposed 

 to the light. With reference to the second suggestion, it is prob- 

 ably true that the eye-spot does prevent the light from reaching 

 one side of the enlargement in the flagellum in Euglena, but by 

 referring to Fig. 4 it will be seen that the difference in light inten- 

 sity thus produced on opposite sides of the enlargement must be 

 practically the same when the light strikes the organism nearly 

 parallel with the longitudinal axis, as it is when it strikes it an at 

 angle from the side containing the smaller lip. If this be true, 

 there is no change in stimulation when the organism is slightly 

 thrown out of orientation. It therefore does not seem probable 

 that the eye-spot in Euglena functions in accordance with the 

 second suggestion of Wager. In Volvox we know of no enlarge- 

 ment in the flagellum such as that found in Euglena, and if there 

 were one, or some other similar structure, the criticism offered 

 above with reference to Euglena would hold here also. 



