MOTOR RESPONSES 297 



rotation no longer produces changes of intensity on the opposite sur- 

 faces (Fig. 107). Photic orientation in Stentor is therefore the result 

 of a series of shock reactions, as is the case in Euglena. There is no evi- 

 dence in support of the view that it is the result of a continuous quanti- 

 tative difference in the activity of the cilia on opposite sides, in pro- 

 portion to the difference in the illumination of these sides. The process 

 of orientation in Stentor is therefore not in accord with the Ray-Verworn 

 theory. 



If stentors are exposed in a field of diffuse, non-directive light which 

 contains a dark spot, they aggregate in this spot. The process of ag- 

 gregation is, in principle, precisely the same as the process of aggrega- 

 tion of photonegative euglenae in a dark spot. They reach the dark spot 

 by random movements. No reaction occurs when the organisms enter the 

 unilluminated area. However, at the periphery on the way out, as the 

 light intensity rapidly increases, they stop suddenly, turn sharply to- 

 ward the aboral surface, and then proceed in a different direction. The 

 dark spot therefore acts like a trap (Jennings, 1904; Mast, 1906, 1911) • 



The relative stimulating efficiency of different regions in the spectrum 

 has not been investigated; no observations have been made on the 

 quantitative relation between the different phases of the shock reaction, 

 the state of adaptation, and the extent of change in luminous intensity. 

 Indeed, very little is known about the body processes involved in stimula- 

 tion and response. 



D. COLONIAL ORGANISMS 



Response to light is essentially the same in all of the colonial forms 

 in which it has been studied, but it has been more intensively investigated 

 in Volvox globator than in any of the other species. 



Volvox is a slightly elongated, globular colonial organism somewhat 

 less than one mm. in diameter. It consists of numerous cells (zooids), 

 each of which contains two flagella and an eyespot. The zooids are ar- 

 ranged in a single layer at the surface of the colonies. The eyespot in each 

 zooid is directed toward the posterior end of the colony, but those at the 

 anterior end are much larger than the rest (Fig. 108). 



Mast (1927a) presented evidence which demonstrates that the eye- 

 spots consist of a cup-shaped pigmented portion, a lens-like structure 

 near the opening of the cup, and photosensitive substance between this 



