General Conclusions. 481 



trustworthy method could hardly have led to such different re- 

 sults, and to results so constantly different. 



When in August Diaphanosoma began to appear in numbers 

 sufficient to count, its distribution agreed with that of Diap- 

 tomus. This agreement persisted through September, when the 

 number of Diaphanosoma caught in each series increased to 

 nearly 1,000, to disappear early in October. 



While, therefore, we are not disposed to urge that the re- 

 sults reached by us are to be considered as mathematically ac- 

 curate, we believe that they are essentially correct. Even taking 

 into account all the various sources of possible error, the gen- 

 eral results are so strongly marked that they can hardly be af- 

 fected by such errors. When the number in the upper level, 

 0-3 m. , is ordinarily 50 per cent, to 100 per cent, more than 

 that from any other level, with an average number of over 3000 

 Crustacea in the upper level, it is impossible to be mistaken in 

 the general fact. When this large excess of Crustacea in the 

 upper level is found at all times of day and night, it is impos- 

 sible to believe in a descent of the Crustacea by day, and a re- 

 ascent at night. 



Zacharias, in the Third Report of the Biological station at 

 Plon (p. 126), announces that the plankton of that lake shows 

 no diurnal movement. His observations were made on Septem- 

 ber 16, 17, and the report came to hand just as this paper was 

 going to press. 



GENERAL CONCLUSIONS. 



1. The plankton Crustacea of lake Mendota during July con- 

 sist chiefly of Diaptomus oregonensis (64 per cent.), three species 

 of Cyclops (30.5 per cent.), Daphnia hyalina (3.3 per cent.), 

 and D. pidicaria (1.5 per cent.). 



2. All of the Crustacea Dut the last named agree in vertical 

 distribution, having 50 per cent, or more in the upper 3-4 m. 

 of the lake. D. pidicaria is found chiefly between 6 m. and 12 m. 

 and almost wholly between 6 m. and 15 m. is distribu- 

 tion is maintained throughout the month. 



3. During July, only the upper 12 m. of lake Mendota are 

 tenanted by Crustacea, and over 90 per cent, are in the upper 9 



