290 STUDIES IN GENERAL PHYSIOLOGY 



at the surface down to a depth of perhaps 400 m. and at the 

 bottom of the sea. Some of the surface, or pelagic, animals 

 show a periodical depth-migration. They come to the sur- 

 face at night and move down during the daytime. In the 

 Mediterranean, Chun found another migration of a greater 

 period. Animals which always come to the surface in win- 

 ter, or at least during certain hours of the day, live at greater 

 depths in summer. 



The first experimental investigations on the cause of 

 these depth- migrations were made by Groom and myself, 

 and led to the conclusion that in the Nauplii of Balanus per- 

 foratus heliotropism alone suffices to account for the fact 

 that they rise to the surface at night and move down during 

 the day. 1 These animals are positively heliotropic to weak 

 light, but strong light soon makes them negatively helio- 

 tropic. They are, in consequence, driven into the depths 

 during the day from the surface of the water. They can, 

 however, not go very deep, as the intensity of the light 

 decreases with an increase in the depth of the illuminated 

 layer of water, and becomes so weak at a certain depth that 

 the Nauplii again become positively heliotropic. They must, 

 in consequence, again move toward the surface. As soon, 

 however, as they again come into more intense light, they 

 become negative again. It can therefore be easily seen why 

 these animals do not go to the bottom of the ocean during 

 the day, but are forced to remain in a layer of water which 

 is not too far below the surface. When the light becomes 

 weaker toward evening and in the night, the Nauplii are 

 again forced to move to the surface of the water in conse- 

 quence of their positive heliotropism. 



2. The question may now be asked whether all animals 

 which are found at the surface of the sea are constantly, or 

 at least under certain conditions, positively heliotropic. I 



1GKOOM UND LOEB, Biologisches Centralblatt, Vol. X (1890) 



