414 



Birge — The Crustacea of the Plankton. 



Table XXXVIII— Continued. 







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Sept. 13. 9 p. m. -{ 



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67.0 







These observations (and I could adduce many more) show that 

 there is a clearly marked diurnal movement of the Crustacea 

 in lake Mendota but that it is confined within the narrow limits 

 of the upper meter, or meter and a half. The day population 

 of the upper centimeters, especially in bright, calm weather, is 

 very small, but the number at one-half meter, even under such 

 conditions, is nearly or quite as large as that at any greater 

 depth, and may be the maximum number. The day population 

 of the upper meter consists chiefly of young and immature Crus- 

 tacea; most of the older individuals of all species being found 

 at greater depths. This relation of age to distribution is most 

 marked in the Daphnias and Diaptomus and least marked in Cy- 

 clops. At night the population of the upper meter agrees in 

 general character with that of the water below, the older indi- 

 viduals ascending, and the younger descending. I have found 

 no evidence of an aggregation of adult Crustacea close to the 

 surface at night, but my observations have been confined to the 

 hours before midnight. 



In general, these conclusions regarding the diurnal movement 

 of the Crustacea agree with those of France, ('94, p. 35), with 

 the important difference that while the movements described by 

 him are measured by meters, those which I have observed take 

 place within the narrow limits of the upper meter, or even 

 within a smaller distance. There are, however, some note- 

 worthy exceptions to the agreement. I do not find that the 



