13 



Unionidae, whose limnetic sojourn is at the best but brief, and the 

 larvas of certain dipterous insects, such as Chironomus and Corethra. 

 The limnetic habit of these larvas is hardly established as yet. The 

 small size of fresh-water planktonts as contrasted with those of the 

 sea is very striking. Representatives of the same group for exam- 

 ple, the Dinoflagellata and the Entomostraca in the two habitats 

 exhibit this contrast. The largest entomostracan of fresh water is 

 less than a centimeter in length, and there is nothing to compare 

 with the pelagic ccelenterates, Mollusca, or such tunicates as Salpa 

 and Pyrosoma. The smaller size of fresh- water planktonts may 

 be due to the lower specific gravity of the environing medium, and 

 perhaps also to the effect of smaller quantities of dissolved salts 

 upon the metabolic processes of limnetic animals. 



Notwithstanding this absence of large individuals in the plank- 

 ton of fresh water, the total quantitative production of plankton 

 per cubic meter is greater here than in the sea. For example, the 

 average production in the Illinois River is 2.71 cm. 3 , and the average 

 amount in adjacent backwaters rises as high as 22.55 cm. 3 (in Phelps 

 Lake) . These measurements were made by the centrifuge, and the 

 results of the "Plankton Expedition" of Hensen reduced to this 

 basis of measurement by Kramer ('97) show that the Atlantic 

 Ocean at the time of this expedition had in the upper strata exam- 

 ined but 0.12 to 0.48 cm. 3 of plankton per cubic meter 



ORGANISMS OF THE PLANKTON. 



The groups of plants represented in the plankton of the Illinois 

 River are principally algas, of which the Bacteriacea are but partially 

 retained in the collections and are usually omitted in plankton 

 investigations. The Sckizophyce&, or blue-green algae, furnish a 

 few important representatives and a number of adventitious .species. 

 The ChlorophycecB, or green algas, on the other hand, abound both 

 in species and individuals, and afford an element of great impor- 

 tance in the primal food supply. The Bacillariacece are exceedingly 

 abundant, and are represented by a number of eulimnetic, as 

 well as many adventitious, species. They also constitute one of 

 the primal sources of food for the zooplankton. The Conjugate 

 furnish but few species and individuals -principally desmids to 

 the phytoplankton. The phanerogams afford a few species which 



