§4] 



PHOTOTAXIS AND PHOTOPATHY 



193 



we find this capacity for rearrangement of pigment granules, 

 as ExNKR ('89 and '91, p. 104), Stepanowska ('90), SzczA- 

 ■wiNSKA ('91), Paekbb ('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 Pigmejit 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 rods and cones, whereas in the dark the 



