298 MOTOR RESPONSES 



and the inner surface of the cup. The evidence also indicates that the 

 lens-hke structure brings the longer incident waves of light to focus in the 

 wall of the cup; and that the shorter wave lengths, after being reflected 

 from the inner surface of the cup, are focused in the photosensitive sub- 

 stance (Fig. 109). 



Figure 108. Camera drawing showing the zooids in about one half of an optical sec- 

 tion through the longitudinal axis of a colony of Volvox. l-a. longitudinal axis of 

 colony; a, anterior end; z, zooids, /, flagella; e, eyes. Note that the eyes are located at 

 the outer posterior border of the zooids and that they become larger as the anterior 

 end of the colony is approached. (After Mast, 1927.) 



Movement, response, and orientation in V . globator have been thor- 

 oughly studied by Mast (1907, 1926b, 1927b, 1932b). The more im- 

 portant of the results obtained in this study lead to the following con- 

 clusions. 



Shock reaction. — Volvox colonies rotate on the longitudinal axis as 

 they swim. This is due to the diagonal stroke of the flagella. In a beam 

 of light they usually orient and go almost directly either toward or away 

 from the light, i.e., they may be photopositive photonegative, or neutral. 



If, while the colonies are swimming toward the light, the intensity is 

 rapidly decreased without any change in the direction of the rays, rota- 

 tion on the longitudinal axis stops and forward movement increases 



