MOLLUSKS, ARTHROPODS AND VERTEBRATES 223 



forms which orient after one eye is destroyed difference of 

 effective intensity on opposite sides does not regulate ori- 

 entation. " The head is . . . [not necessarily] bent until 

 its symmetrical photosensitive points are . . . struck at 

 the same angle by the rays of light," nor is it necessary 

 that both eyes be influenced alike, as Loeb's theory de- 

 mands. It appears that these organisms, as Holmes says, 

 referring to the fiddler crab, follow light " much as an 

 animal pursues any other object of interest, such as prey, 

 or its mate." 



Graber (1884, p. 226) found that Rana esculenta tends to 

 collect in light of relatively low intensity and we have seen 

 that Miss Torelle found that the frogs she studied tended 

 to collect in shaded places, but Parker (1903) and Cole 

 (1907) found that when frogs are exposed to light in a dark 

 room they are positive, apparently regardless of the inten- 

 sity of the light. I found the same true with reference 

 to Bufo. How can these contradictory phenomena be 

 accounted for? In a dark room containing but a single 

 compact source of light such as was used by Parker, Cole 

 and myself in these experiments, it is not likely that an 

 animal can see anything but the source of light. If then 

 the frogs and*toads are guided by sight in their movements, 

 as their reactions indicate, it is evident that they must go 

 toward the source of light if they go at all. In these reac- 

 tions light no doubt acts continuously as an orienting stimu- 

 lus, and the direction of the rays must necessarily, if they 

 go toward an object because they see it, guide them on 

 their way. This of course does not imply that the reac- 

 tions are controlled by psychic phenomena. The process 

 of orientation is however, without doubt, far more compli- 

 cated than the theory of Loeb demands. 



4. Caprella 



What can be said with regard to orientation in the 

 higher forms with eyes incapable of forming images ? Such 

 animals are of course not able to see. They cannot follow 



