4] 



PHOTOTAXTS AND PHOTOPATHY 



193 



we find this capacity for rearrangement of pigment granules, 

 as EXNER ('89 and '91, p. 104), STEFANOWSKA ('90), SZCZA- 

 WINSKA ('91), PARKER ('95), and others have shown. In the 

 higher Crustacea, for example, the pigment granules of the 

 pigment cells surrounding the rhabdome (or " spindle ") are, in 

 the dark, below the level of the spindle. Upon illumination, 

 however-, these granules migrate (or are carried) upwards, and 

 partly envelop the rhabdomes. I believe it has not been deter- 

 mined what rays are involved in producing this result. This 

 response to light is considered to be an advantageous one, since 



cu. ep. 



FIG. 58. Vertical section of a whitish-yellow dermal papilla ; lettering as in Fig. 57. 

 p', processes of black pigment cells containing no pigment. (After KELLER, '95.) 



the pigment thus cuts off side rays from the perceptive organ 

 -the rhabdome. 



It is interesting that we should find cells containing two so 

 diverse kinds of pigment as chlorophyll and the retinal pig- 

 ment responding to light in so similar a fashion. In most of 

 the cases, if not all, this response is an adaptive one. 



7. The Migration of Pigment Cells in the Metazoan Body. - 

 It has been shown, apparently first by ENGELMANN ('85), that 

 the pigment cells of the retina vary their movements with the 

 light. Thus, when a strong light is thrown upon the retina 

 of the frog, the pigment cells send out pseudopodium-like 

 processes between the roo"s and cones, whereas in the dark the 



