Ghydorus 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 exclusion 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 
June 6. 120 
June 10. 4,200 
June 13.4,430 
June 17. 1,740 
June 19. 4,100 
