192 COMPARATIVE PHYSIOLOGY OF THE BRAIN 



of Balanus perforates which were known to make 

 periodic depth-migrations (2). As one of our results, 

 we found that these animals are sometimes negatively, 

 sometimes positively heliotropic, and that we were 

 able to make them positively or negatively heliotropic 

 at desire. By weak light, especially gaslight, which 

 contains comparatively few of the heliotropically effect- 

 ive blue rays, they became and remained positively 

 heliotropic, while in strong light they soon became 

 negatively heliotropic. This circumstance determines 

 the periodic depth-migration of these animals. When 

 they are near the surface of the ocean in the morning, 

 the strong light makes them negatively heliotropic and 

 forces them to go downwards vertically, because in 

 the open sea only the vertical components of the re- 

 flected skylight have any effect. As soon as they 

 approach a depth where the light is sufficiently weak, 

 they become positively heliotropic. They must then 

 begin to migrate upward again, but cannot reach the 

 surface, because they soon come to a region where the 

 light is so strong that they again become negatively 

 heliotropic. Hence during the day they are held at a 

 certain depth, which is, however, less than four hun- 

 dred metres. But as soon as it becomes dark and the 

 intensity of the light decreases more and more, they 

 are forced to rise to higher regions on account of their 

 positive heliotropism, until during the night, while the 

 intensity of the light is weak, they are held at the sur- 

 face of the water. Toward morning, when it begins 

 to dawn, they again become negatively heliotropic and 



