INVERTEBRATE PHOTORECEPTORS 



627 



fitting the concavity of the pigment and concentrating Hght. Presuma- 

 bly the pigmented structure contains photosensitive materials, since the 

 organism responds to light — orienting fairly precisely in directed beams — 

 with a maximum sensitivity in the green near 500 m^. 



The stigmata of Volvox, Gonium, and other chlorophyll-bearing colonial 

 protozoans consist of a cup-shaped pigment mass, with a lens at the 

 mouth of the cup and the light-sensitive material located between the 

 lens and the bottom of the cup. Again the photoreceptive substance 

 appears connected with the flagella by means of a differentiated neuro- 

 motor or conducting system. The lens is inferred from the fact that 



EYESPOT 



FLAGELLUM 



Fig. 14-3. A pigment mass within an organism may cast a shadow. This seems to be 

 the basis of directional movements in some unicellular forms. If fiagellum activity 

 is inhibited by light on. the bases and full action follows shading there, then average 

 (random) movements become directional toward a source because of the pigment spot. 



(After Wager, 1900.) 



incident directed light is concentrated into two foci — a yellowish one in 

 the wall of the pigment cup near its outer surface, and a bluish-green one 

 in the cup near the inner surface. The walls of the cup transmit longer 

 light waves (red to yellow) but reflect the shorter waves to the second 

 focal point. Mast believed the concentration of longer waves to be inci- 

 dental. But since each zooid in the colony has one such stigma, the 

 orientation of the pigment cup is definitely related to the structure of 

 the colony in such a way that lateral illumination (in terms of the axis 

 of symmetry) produces alternate shading and exposure of the lens mecha- 

 nism in each stigma as the colony rotates on its longitudinal axis. Illumi- 

 nation from directly in front or behind furnishes no concentration of light 

 behind the lenses. In general, zooids tow^ard the anterior pole of the 

 colony in Volvox and Gonium possess larger stigmata — as much as 3 m in 

 diameter — but zooids at the opposite pole contain similar structures many 

 times smaller. Mast concluded that the stigmata served as direction eyes 



