13 
Unionide, whose limnetic sojourn is at the best but brief, and the 
larvee of certain dipterous insects, such as Chironomus and Corethra. 
The limnetic habit of these larve 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 1s nothing to compare 
with the pelagic coelenterates, 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.*, and the average 
amount in adjacent backwaters rises as high as 22.55 cm.* (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.’ of plankton per cubic meter. 
ORGANISMS OF THE PLANKTON. 
The groups of plants represented in the plankton of the Illinois 
River are principally algee, of which the Bacteriacee are but partially 
retained in the collections and are usually omitted in plankton 
investigations. The Schizophycee, or blue-green alge, furnish a 
few important representatives and a number of adventitious species. 
The Chlorophycee, or green alge, on the other hand, abound both 
in species and individuals, and afford an element of great impor- 
tance in the primal food supply. The Bacillariacee 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 zodplankton. The Conjugate 
furnish but few species and individuals—principally desmids—to 
the phytoplankton. The phanerogams afford a few species which 
