400 



Birge — The Crustacea of the Plankton. 



the largest number being found in the bottom stratum. A 

 second reproductive period came on in the latter part of May, 

 in which the upper water was again crowded, although the 

 numbers increased so rapidly that the population of all the 

 upper levels of the lake was greatly increased. During the 

 •early part of June the distribution became once more equal, 

 with the largest number again in the bottom level, and during 

 the latter part of the month the population rapidly declined, 

 falling off most in the upper levels. At this time more than 60 

 per cent, of the species was found below the 12 m. level and 

 less than 2 per cent, in the upper level. 



Late in June the species began to move away from the bot- 

 tom water, or perhaps it would be more correct to say that 

 the individuals at the bottom of the lake died off more rapidly 

 than those in the levels immediately above, so that in the early 

 part of July nearly 60 per cent, of the species was between 12 

 and 15 meters and only 6.5 between 15 and 18 meters. As the 

 species declined in numbers the decline took place chiefly in the 

 lower levels of the lake, so that in July and August the few 

 representatives of the species that were left were concentrated 

 in the region of the thermocline, thus occupying the same posi- 

 tion that they had held in the corresponding months of the pre- 

 ceding year. The following table shows the numerical relations. 



Table XXXV. — D. pulicaria, 1896. Population per cu. m. of each 

 level stated in thousands. 



.Depth, meter. 



April 1-15. 

 April 16-30 

 May 1-15.. 

 May 16-31.. 

 -June 1-15.. 

 June 16-30. 

 July 1-15 .. 

 July 16-31 . 



0-3 



1.0 

 41.6 

 10.4 

 55.4 

 10.3 



0.4 



3-6 



1.5 

 5.2 

 12.8 

 33.7 

 5.9 

 1.5 

 0.1 



6-9 



3.2 



0.4 

 15.5 

 37.4 

 8.8 

 2.8 

 1.1 

 3.2 



9-13 



2.5 



0.7 

 9.2 

 28.8 

 12.5 

 3.7 

 3.5 

 1.7 



13-15 



1.3 



0.9 

 13 3 

 19.8 



5.9 

 10.9 



7.3 



0.1 



15-18 



0.6 



0.2 



17.8 



23.4 



10.9 



4.4 



0.8 



0.1 



Fig. 31 shows the movement of D. pulicaria during the late 

 summer and autumn of 1895. Points were established indicat- 



