. The crustacea of the plankton, July, 1894-Dec., 1896 . es as present from June to November in Green lake. Allfind it a little earlier in the spring than I have done. Chydorus sphaericus O. F. M. Figure 20. —Table J, Appendix. Table XXII.—Chydorus sphaericus. Average number per square meter expressed in thousands. January 1-15 —January 16-31 ..February 1-14 ..February March 1-15 March 16-31 April 1-15 April 16-30 May 1-15 May 16-31 June 1-15 June 16-30 July 1-15 July 16-31 August 1-15 August 16-30October 1-15 ....October 16-31 ...November
. The crustacea of the plankton, July, 1894-Dec., 1896 . es as present from June to November in Green lake. Allfind it a little earlier in the spring than I have done. Chydorus sphaericus O. F. M. Figure 20. —Table J, Appendix. Table XXII.—Chydorus sphaericus. Average number per square meter expressed in thousands. January 1-15 —January 16-31 ..February 1-14 ..February March 1-15 March 16-31 April 1-15 April 16-30 May 1-15 May 16-31 June 1-15 June 16-30 July 1-15 July 16-31 August 1-15 August 16-30October 1-15 ....October 16-31 ...November 16-30December 1-15 ..December 16-31. 1894. 1895. a a a a a a CO a > u0w ?8 O$5 s s 163,4 s No obs. s No obs. 1 .6 1896. No obs. 02 3 8 o Q a o c CD 313 d c 5 a CO CD P 00CO Gi GO ~ CD MCD OC 00 p l-i i—?O EL X Q o o. 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 resembledeach other in having a maximum in autumn, which was whollyabsent in 1895. A large number was also found in July, 1895and 1896, while practically none were present in 1894. In thewinter of 1895-6, Chydorus was regularly present; while in thatof 1894-5 there were found only isolated individuals from timeto time. I believe that these periods of abundance are correlatedwith the abundance of Anabaena and allied algae in the autumn of 1894, and the whole season of 1896 were charac-terized by a great abundance of these plants; while they wereexceedingly rare in 1895 after the spring and early summer of 1894 was marked by an enormous development ofLyngbya, an alga quite too large to serve as food for Chydorus,a
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