16 



relations existing between the different groups. The plants are 

 more abundant (and generally smaller) than the animals, outnum- 

 bering them nearly 5 to 1 . Computation shows that for each one of 

 the Cladocera there are 7 Copepoda, the predominance of the latter 



CONSTITUENT GROUPS OF THE ANNUAL PLANKTON OF THE ILLINOIS RIVER. 

 AVERAGE OF 52 WEEKLY COLLECTIONS IN 1898 NUMBER PER M 3 . 



Number of 



forms 

 recorded. 



Number of 

 individuals. 



being accounted for in part by the fact that their larval stages are 

 free-s Dimming and appear in the enumerations, while the young of 

 the Clidocera are not set free until nearer maturity. About 10 to 

 20 per cent, of the Copepoda are adults. The relative numbers of 



* Represents fragments of filaments, and is not included in totals. 



