Chydorus sphaericus. 349 



The above table shows that the number of this species is sub- 

 ject to very great variation; yet there is a certain degree of reg- 

 ularity in its appearance. The years 1894 and 1896 resembled 

 each other in having a maximum in autumn, which was wholly 

 absent in 1895. A large number was also found in July, 1895 

 and 1896, while practically none were present in 1894. In the 

 winter of 1895-6, Chydorus was regularly present; while in that 

 of 1894-5 there were found only isolated individuals from time 

 to time. I believe that these periods of abundance are correlated 

 with the abundance of Anabaena and allied algae in the water. 

 The autumn of 1894, and the whole season of 1896 were charac- 

 terized by a great abundance of these plants; while they were 

 exceedingly rare in 1895 after the spring and early summer. 

 The summer of 1894 was marked by an enormous development of 

 Lyngbya, an alga quite too large to serve as food for Chydorus, 

 and at the same time occupying the upper stratum of the water 

 to the exclusioQ of the smaller algae. 



The development of Chydorus is therefore dependent on the 

 kind of food to a degree unusual among the limnetic Crustacea. 

 It is also dependent on temperature. In both 1895 and 1896 it 

 was the last of the perennial Crustacea in its development, no 

 marked rise occurring before the last of June or the first of July. 

 This is the more noteworthy, since eggs may be found in the brood 

 sac at any time during the winter. 



In 1894 and 1896 the maximum came about the middle of 

 September, while in 1895 only one small maximum was present, 

 and that was in July. In 1896 there was no decline of the 

 species in August, but rather an increase, and in this season 

 Anabaena and allied forms were abundant throughout the 

 ; summer. 



In 1894 the number increased very greatly between the 6th 

 and 10th of June, as is shown by the following record of the 

 .number of individuals caught. 



June 3 90 



June 5 •....., 450 



June6 , • 120 



June 10 4,200 



June 13 4,430 



June 17 1,740 



June 19 4,100 



