Daphnia hyalina. 339 



The table shows that the summer history of this spe 

 cies was very different in the three years of my study. In 

 July and August of 1894 the numbers were exceedingly smal 1 

 smaller than in any of the three winters during which I have 

 studied the species. In 1895 the numbers were large and re- 

 mained large throughout the summer, gradually declining in 

 September and October, and falling off rapidly in the latter 

 part of October to the winter minimum without showing any 

 marked reproductive period in late autumn. In 1894 and 1896 

 the numbers, which were small and nearly equal in the latter 

 part of August, rose steadily through September and October 

 to a maximum in the latter part of October, and then fell off 

 rapidly to reach the winter minimum in December or January. 

 In late October, 1896, there were present enormous broods of new 

 hatched Daphnias, which raised the number for that period beyond 

 the records of any other. In 1896 the spring maximum was 

 followed by a minimum about the middle of June, in which the 

 numbers were scarcely one-quarter of the maximum. From this 

 minimum there was a rapid recovery, which lasted for about a 

 month and was followed by another marked depression. In 1895 

 the spring maximum continued into June, and the early summer 

 minimum came about the first of July. Portions of this mini- 

 mum are included for the averages of the latter part of June and 

 the early part of July, so that the number at the minimum ap- 

 pears greater in the tables and diagram than it actually was. 

 As a matter of fact, there was very little difference in the num- 

 ber present in 1895 and 1896. In 1895 the recovery of the 

 species from the early minimum came on as in 1896, but there 

 was no reaction from the increase, and the number remained 

 substantially unchanged through the entire summer. 



No observations were made in the spring of 1894, but the proba- 

 ble history of the species was similar to that in the other years. 

 There was a spring maximum followed by a marked minimum 

 from which there was no reaction. This failure of the species 

 to develop a summer brood seems to have been due to the pres- 

 ence of Lyngbya in the upper strata of the water. 



The largest catches of this species were 331,000 per sq. m., 

 Oct. 17, 1894; 565,000 June 6, 1895, and 1,049,000 Oct. 26, 1896. 



