Vertical Distribution of Individual Species. 391 



In October the distribution of all of the species of Crustacea is 

 approximately equal. In the winter the equality of distribution 

 is brought about by the excess of Dap/mia and Diaptomus in 

 the upper strata, nearly balancing the excess of Cyclops near 

 the bottom. (See Fig. 30.) The curve for August shows a 

 very large percentage in the upper 3 meters and a very small 

 number in the lower water. It is a characteristic distribution 

 for middle summer. 



THE VERTICAL DISTRIBUTION OF THE INDIVIDUAL SPECIES. 



After this full discussion of the vertical distribution of the 

 total crustacean population I do not intend to describe that of 

 the individual species in similar detail, but I shall confine my- 

 self to pointing out the individual peculiarities of each species, 

 devoting more space to those which depart in a marked way 

 from the average vertical distribution. One general law holds 

 for nearly all the species, as already stated: the broods of young 

 appear first in the upper water of the lake and the increase of 

 population extends downward, becoming approximately uniform 

 at all depths as the species reaches its maximum, and later in 

 its life becoming more numerous in the deeper water of the 

 lake. To the first part of this rule the only exception is Daph- 

 nia pulicaria during summer. There are, however, several fac- 

 tors which prevent the full carrying out of the latter part of the 

 rule. The most important of these is the formation of the ther- 

 mocline, by which all of the crustacean life is confined to the 

 upper waters of the lake during that period when the develop- 

 ment of several species is going on actively. In the late au- 

 tumn also the numbers of the Crustacea decline so rapidly after 

 the fall broods appear that it is not easy to find any accumula- 

 tion at any low level of the lake. The downward movement of 

 the older forms is shown most clearly by Cyclops and Daphnia 

 hyalina during the spring, and by the accumulation of Cyclops 

 in the deeper water of the lake during the winter, by the dis- 

 appearance of D. hyalina and D. retrocurva in autumn. Sim- 

 ilar, though less striking, illustrations can be found in all of the 

 species of limnetic Crustacea. 



