68 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



the surface in the same periodicity as they did in their former home. In this case I 

 believe the periodical depth migration is due to periodical changes in the amount of 

 water contained in the animal. These changes are due to metabolic processes which, 

 however, are influenced in their periods by the change of day and night. Thus these 

 metabolic changes adapt themselves in their periodicity to the change of day and night 

 and this periodicity remains the same even if the animal later on is carried to the region 

 of the midnight sun. 



