448 Birge — Vertical Distribution of Pelagic Crustacea. 

 The greatest variations in successive hauls were : 



July 27. 



July 19. 



July 27. 



6 P. M . . 





....3759 



12 Midnight 



.. .3898 



3A.M . 





....4021 



9 P. M . . 





7134 



3A.M 



6 A. M 



4564 

 , ..5478 



6 A.M.. 





8703 



The last is the only case where one catch is double the 

 preceding one. 



Averaging the total catch of the periods, we find : 



Period 1 3808 



Period II 4253 



Period III 5144 



Period IV 3428 



In spite of this absolute increase in numbers the percentage of 

 Diaptomus to the whole number of Crustacea declined, chiefly on 

 account of the more rapid increase of Cyclops. In Period I 

 Diaptomus constituted 79 per cent, of the Crustacea; in Period 

 II 66.3 per cent; in Period III 57.1 percent. ; in Per .od IV 53. 3 

 per cent. 



These numbers would indicate that the period of maximum 

 number of Diaptomus falls in July. This conclusion is confirmed 

 by subsequent observations. Catches made on August 24 and 

 25, 3 in number, gave from 1000 to 1500 Diaptomi. After Sep- 

 tember 18, when regular observations were resumed, very few 

 catches passed 1000, and in October the number declined still 

 more. 



A curious fact, for which no full explanation is possible, is 

 that the catches made by night average smaller than those by 

 day. No cause can be assigned for this at present beyond 

 chance. All but one of the exceptionally large catches occurred 

 during the day, and although the smallest catch was made at 6 

 a. m., most of the exceptionally small ones were made at 

 night. The night catches of Daphnia hyalina average some- 

 what larger than those of the day. 



Distribution in depth. — As has already been stated, the water 

 of the lake at the point of observation was slightly over 18 m. 

 in depth, so that it was possible to obtain material from six 

 levels of 3 m. each. In the lowest level, that from 15 m. to 



