388 



Birge — The Crustacea of the Plankton. 



tober, 1896, when great numbers of young appeared on several 

 occasioDS, and when the old animals were nearly all full grown, 

 so that there were very few half developed individuals. This is 

 given on p. 398. 



During November and December the population of the lake 

 falls oil" pretty uniformly in all levels, more rapidly in Novem- 

 ber than later, and at this time the distribution of the animals 

 may be more even than at any other period. If Daphnia puli- 

 caria is present it rises toward the surface in December and in- 

 creases the population of the upper strata. This occurred in 

 1895. In all years the distribution in November is more uni- 

 form than that of December, in which month the population of 

 the lower levels of the lake seem to decline more rapidly than 

 that of the upper stratum. 



Table XXXI. — Average percentile distribution Oct. 1 — Dec. 31. 





Average 

 No. 



Pee cent, in each 3 m. level. 





0-3 m. 



3-6. 



6-9. 



9-13. 



12-15. 



15-18. 



1894 



595,000 

 436,000 

 759,000 



25.8 

 29.7 

 25.9 



18.8 

 18.3 

 21.0 



16.0 

 14.3 

 15.3 



15.7 

 14.9 

 13.9 



14.0 

 12.2 

 12.4 



9.8 



1895 



10.6 



1896 



11.4 







Figures 22 and 23 represent the total population of each of 

 the 6 levels into which the lake was divided. The scale is 

 100,000 Crustacea to each vertical interval. If the scale be di- 

 vided by 3 the same diagrams will serve to show the population 

 of each level per cubic meter. The relations of the increase 

 and decrease of the population in the several levels are shown 

 very plainly from these diagrams. For instance in 1895 it will 

 be seen that while the two upper levels began to increase dur- 

 ing the latter part of April, the population of the lower levels 

 scarcely changed from the winter condition until about the first 

 of May. The population of the three upper levels reached its 

 maximum in the latter part of May, while in the lower part of 

 t he lake the population went on increasing, or at least remained 

 stationary, until near the middle of June. The 6-9 m. level 



