408 Birge — The Crustacea of the Plankton. 



very greatly. On some occasions the catch of certain species 

 from the upper meter was larger than that obtained by a second 

 catch from the entire three meters. Such instances were due 

 to the presence of very large numbers of young in the upper 

 meter, with a somewhat irregular distribution, so that the 

 catches varied considerably. Upon the whole, however, the 

 average number derived from these twenty observations agreed 

 surprisingly in all the species. It was found that the upper 

 meter contained an average of 43 per cent, of the entire catch 

 of Diaptomus from the upper three meters; 47 per cent, of Cy- 

 clops; and 50 per cent, of Daphnia hyalina. These catches 

 were made during the day and may be taken as fairly indicating 

 the relative number of Crustacea in the upper meter during the 

 daylight hours. It will be seen that these observations fully 

 justify the statement made in my former paper (Birge, '95, 

 p. 479) that "a general movement of the Crustacea as much as one 

 meter would have been detected, " and indicates that at no time 

 is the population of the upper meter of the lake notably de- 

 ficient. The minimum percentages were very irregularly dis- 

 tributed and depended more upon the presence or absence of 

 young individuals than upon any influence of light, weather, or 

 wind. 



These observations also indicate the extent to which the lines* 

 of Figs. 29 and 30 should be altered in the upper three meters 

 in order to express the average distribution within that level. 



During 1897 observations were made with a view of deter- 

 mining the exact distribution of the Crustacea in the upper 

 meter. They were made by two methods : First, a net with an 

 opening ten centimeters in diameter was supported so that it. 

 could be drawn horizontally through the water for a known dis- 

 tance at an uniform rate of speed. The Crustacea so obtained 

 were counted and the number present at a given level was thus 

 determined. Second, a pump was taken out in the boat, by 

 whose aid the water of the lake was pumped through a hose and 

 strained by the plankton net, the mouth of the suction hose be- 

 ing placed at the successive levels. Water was taken from tho 

 surface at a depth varying from two or five centimeters in calm 

 weather, to ten when the lake was agitated by the wind ; at one- 



