1919] Esterly: Reactions of Various Plankton Animals 39 



above the vertical movements of the animals are practically all down- 

 ward except in water of low temperatures. There is then a com- 

 paratively marked tendency to ascend. There is no marked change in 

 behavior with change in salinity. (4) "When the animals are lighted 

 from below 7 there is marked movement upward at ordinary tempera- 

 tures, and this increases in water of high salinity. The amount of 

 movement away from the bottom decreases at low temperatures, espe- 

 cially as regards the distance covered. The animals remain at the 

 bottom more persistently when the water in the lower part of the 

 column is chilled than when it is at room temperature. (5) Animals 

 in weak, diffuse daylight do not show any tendency to ascend. But 

 after nightfall a noticeable upward migration begins to appear in a 

 column of water whose lower portion has been cooled to 8° C or 9° C, 

 although no change that might affect the direction of movement has 

 taken place in the surroundings. Some of the animals swim into water 

 eight or nine degrees warmer than that in which they started. This 

 behavior is not shown in a cylinder where the temperature throughout 

 is 16°-18° C. 



POSSIBLE BEARING OF THE EXPERIMENTS ON DIURNAL MIGRATION 



The general facts of the distribution of Calanus in the ocean are 

 well established by our field work. The animals are less abundant at 

 the surface by day and more abundant in deeper water ; at night they 

 are less abundant in deeper water and more abundant at the surface. 

 It is assumed that upward migration leads to the increase in numbers 

 at the surface, and that downward migration leads to the greater 

 numbers in deeper water. How can the results of experiments be 

 applied to explain this general condition? Since the copepods are 

 strongly negative to light at temperatures of 14°-20° C, that fact 

 would explain their absence from the surface of the sea during the 

 day, assuming that the reactions to vertical and horizontal light is 

 the same. Furthermore, since the phototropism becomes positive at 

 lower temperatures we might expect that the animals would begin to 

 move toward the surface of the sea if, in the course of their descent, 

 they reach water of a sufficiently low temperature while the surface 

 is still lighted. There is experimental evidence showing that there is 

 movement toward a light at the top of a column of water when its 

 lower strata are cold enough. But the laboratory results indicate that 

 there is practically no upward movement in darkness or in diffuse light 

 if the observations are made during the hours of daylight. In view 



