• 



50 NATHAN FASTEN 



descend into deep water with the rising of the sun, because 

 they are negatively phototropic to strong light (their negative 

 geotropism being overcome by their negative phototropism) ; 

 the males follow the females in migration, because they are 

 probably positively chemotropic toward the females." 



Juday (1904), has studied the diurnal migrations of fresh 

 water Plankton Crustacea, and found that most of these organ- 

 isms come to the surface of the water at night, especially during 

 the early part of the evening. Through most of the day, how- 

 ever, the animals occupied deeper water. He calls attention 

 to the fact that diurnal movements are rather complex phenom- 

 ena, and that they cannot be attributed to a single factor alone. 

 Each lake presents different conditions, and these modify the 

 behavior accordingly. The downward movement of the Plank- 

 ton Crustacea he attributes to light, but he maintains that other 

 factors, such as temperature, food, and the substances dissolved 

 in the water may control the upward migrations of the animals. 



Esterly (1907), working with Cyclops also emphasizes light 

 as the directive agent in the migrations of this form. 



It is thus seen that amongst marine as well as fresh water 

 organisms we find a periodic migration. Light seems to play 

 the most important factor in these movements. We must not, 

 however, leave out of consideration such important factors as 

 food, temperature and gravity, for these may also influence 

 migrations, as was shown by Loeb (1893), Parker (1901) and 

 Juday (1904). 



In the case of Lernaeopoda we meet with an organism that 

 offers an exception to the general rule of vertical migrations. 

 Here we find the larva attracted by strong light, and it swims 

 about close to the surface of the water during the day time, 

 even overcoming its natural tendency to sink downward. At 

 night, on the other hand, the animal does not react to weak 

 light, and responds to the pull of gravity, thereby sinking to 

 deeper regions. Clearly this behavior is of great value to the 

 stability of the species. The migrations of the copepod are 

 identical with the movements of its host. During the day 

 trout generally feed near the upper surface of the water, whereas, 

 at night they frequent lower regions. 



