478 Birge — Vertical Distribution of Pelagic Crustacea. 



pected to confirm this result, at least partially, by finding a 

 migration within a limit of. several meters. At the same time 

 the senior author of this paper had so often found Crustacea at 

 the surface by day that he doubted whether the law 

 was as simple for lake Mendota as that found in lake Balaton. 

 Until our observations forced the conclusion upon us, we had no 

 idea that the Crustacea would remain at the same level by day 

 and by night. Our work was incited by this paper of France's, 

 and had at first two aims: (1) Assuming France's law to hold 

 for lake Mendota, we wished to determine how deep the Crus- 

 tacea go by day. While France found that the Crustacea go to 

 the bottom of lake Balaton, it was quite possible that in lake 

 Mendota, more than twice as deep, they would go only part 

 way. (2) We wished to determine the rate of ascent and de- 

 scent. These original purposes were, of course, interfered with 

 by the result of our observations, which were negative as re- 

 gards any diurnal movement of the Crustacea. 



It does not seem to us at all possible that any vertical move- 

 ment of numbers of the Crustacea can have taken place to an ex- 

 tent of 3 m. It is plain that there is no general movement of 

 any species to the surface at any time. There was never an 

 aggregation of all the Crustacea in the 0-3 m. level. There was 

 very rarely such a distribution as to make the number in the 

 3-6 m. level larger than that in the 0-3 m. level. In Diaptomus 

 this occurred six times, four by day and two by night. In the 

 greatest difference the 3-6 m. level contained 59 per cent, of the 

 catch, and the 0-3 m. level 34 per cent. In the same species 

 the average percentage of the sum of the 0-3 and 3-6 m. levels 

 was about 82 per cent. There were 17 cases where this sum 

 passed 90 per cent., of which fifteen were by day and two by 

 night. There were eleven cases where the sum fell below 70 

 per cent., of which eight were at night and three by day. The 

 3-6 m. level was by far the most constant in percentage. While 

 naturally varying a good deal, the averages were remarkably 

 constant, as the following table will show, which gives for 

 each of the four periods the average percentage which this level 

 contained, of the whole number of the species of Crustacea 

 named. 



