Factors Determining Vertical Distribution. 425 



case of the latter species a great majority of the old animals 

 are so affected by various diseases as to need no other explan- 

 ation of their death. 



Undoubtedly the condition of the water in summer causes the 

 rise of the survivors of the spring broods of D. pulicaria from 

 the bottom to the region of the thermocline. 



Light. 



In lake Mendota the direct effect of light is confined to the 

 upper meter or two, within which distance it has a powerful 

 influence in determining the position of the Crustacea. 



Laboratory study shows that the relation of the Crustacea to- 

 light differs in different species. Daphnia in all of the limnetic 

 species has a strongly negative movement. Diaptomus, Dia- 

 phanosoma, and Ghydorus are strongly positive while Cyclops 

 is, on the whole, positive, but is not very strongly affected 

 either way. Yet the vertical distribution of these species is 

 not very different when studied in the lake by three-meter inter- 

 vals. Compare Fig. 30, and the percentage tables on p. 393 

 Diaptomus and Daphnia show an especially close correspond, 

 ence in spite of their opposite relation to light. These species, 

 placed in a glass vessel near a window, will segregate, Diapto- 

 mus collecting near the surface and toward the light, while 

 Daphnia goes to the bottom and to the side furthest from the 

 light. This movement away from the light is not shared by every 

 Daphnia present; some may move toward the light, usually not 

 more than one per cent, of the adult or half-grown individuals. 

 Young Daphnias, especially the newly hatched, are attracted by 

 the light. The adult individuals of Diaptomus are found in a 

 higher level of the lake than those of Daphnia. 



The young Crustacea have a monopoly of the upper half-meter, 

 or thereabouts, during the day. It is easy to see the advant- 

 age of this arrangement to the species. In the upper meter, 

 plant-life is most abundant, and is represented chiefly by small 

 forms like Anabaena which are especially adapted as food to 

 the small Crustacea. On the other \haud, the adult Crustacea 

 find an abundance of food suited to their size and masticatory 



