THE PILOBOLUS GUN AND ITS PROJECTILE 113 



zooids becat backwards obliquely, so that the colony progresses 

 forwards and, at the same time, rotates in a counter-clockwise 

 direction. When a colony which is swimming toward the light 

 with its anterior end facing the light is suddenly subjected to lateral 

 illumination only, in the zooids on the side of the colony facing the 

 light the hyaline portion of each eye is fully exposed to the light, 

 while in the zooids on the side of the colony facing away from the 

 light the hyaline portion of each eye is well-shaded by the pigment- 

 cup, with the result that the flagella of the zooids with shaded eyes 

 beat backwards more directly than before while the flagella of the 

 zooids with unshaded eyes beat backwards less directly than before 

 (Fig. 55). Owing to this change in the direction of the stroke of the 

 flagella on the two sides of the colony, the colony gradually turns 

 its axis until this again becomes parallel to the incident rays of 

 light and the anterior end of the colony faces the light directly 

 (Fig. 55). When the colony is in photic equilibrium, the incident 

 light falls on the eye-spots of any one lateral half of the colony in 

 the same way and with the same intensity as upon the eye-spots of 

 the corresponding other lateral half of the colony. 



My own investigations on Pilobolus and those of Mast on Volvox 

 thus seem to show in both these plants — the one a lowly and seden- 

 tary fungus and the other a highly organised motile alga — that 

 photic orientation is accomplished by means of optical sense organs, 

 and that re-orientation is due in Pilobolus to the asymmetrical 

 illumination of a single eye and in Volvox to the combined effect 

 of the asymmetrical illumination of many eyes.^ 



There is a red pigment in the eyes of both Pilobolus and Volvox ; 

 but, whereas that of Pilobolus is situated in the sensitive protoplasm 

 and is relatively diffuse, that of Volvox is situated below the sensitive 

 protoplasm and is aggregated into a dense and opaque cup-like 



1 Pilobolus and Volvox may be compared to those mythical monsters of antiquity 

 Cyclops and Argus : for Cyclops, like Pilobolus, looked out on the world with a single 

 median eye ; while Argus, like Volvox, had many eyes. It is said that Argus 

 had one hundred eyes some of which were always open and on the watch, and that 

 they were so splendid that, when he was killed, Juno transferred them to the Peacock's 

 tail ; but a Volvox globator, although not mentioned in the annals of Olympus, may 

 have more than twenty thousand eyes which never close and are ever ready for 

 service. 



VOL. VI. J 



