194 LIGHT AND PROTOPLASM [Cn. VII 



pigment lies behind all these elements. Also, among the Crus- 

 tacea, the protoplasm of the outer pigment cells, surrounding 

 the cones of the compound eye, migrates centripetally in a strong 

 light, to return again to its peripheral position in darkness. 

 These movements may also be considered adaptive. They are, 

 in addition, movements which are discharged only by light. 



c. Metazoa. We may treat of the. control by light of the 

 movements of the higher animals somewhat more summarily 

 than we have the preceding classes. The facts will be arranged 

 by groups in systematic order. 



Among radial animals, Hydra has perhaps been for the 

 longest time an object for photopathic study. TREMBLE Y 

 (1744, p. 66) had noticed that Hydra viridis, and even muti- 

 lated pieces of it, came to the light side of the vessel. When 

 tbte light was admitted only through a chevron-shaped slit, the 

 Hydras were later found aggregated opposite the slit in the 

 form of a chevron. That it was not the warmth of the sunlight 

 that attracted was shown by turning the slit towards the cooler 

 air, whereupon the same response occurred. The observations 

 of TREMBLEY showed that the Hydras did not move in as straight 

 a line as possible towards the light (they must, of course, follow 

 a firm substratum), but gradually wandered towards it. The 

 response of Hydra must therefore be considered as photopathy. 

 More extensive studies were made upon Hydra by WILSON ('91), 

 who found that Hydra fusca is likewise responsive to light, and, 

 indeed, photophil with reference to diffuse daylight, and photo- 

 phob to direct sunlight. And it can be shown that it is an 

 advantage to Hydra to be photophil, since many of the Ento- 

 mostraca upon which it feeds are phototactic. 



Besides Hydra, I know of only one case of response by Cce- 

 lenterata to light. The larvae of the sponge Reniera are said 

 (MARSHALL, '82, p. 225) to flee from the light, probably 

 negative phototaxis. 



Among Echinodermata, Asteracanthion rubens (GRABER, '85, 

 p. 155) appears to be photophil, and Asterina gibbosa (DRIESCH, 

 '90, p. 155) to be photophob. 



Although, as we have seen, some radial animals may respond 

 to light, the phenomenon is more wide-spread in the bilateral 

 groups, flatworms, annelids, crustaceans, insects, molluscs, 



