2 9 o A NATURALIST IN THE GREAT LAKES REGION 



of many animals like the protozoa, rotifers (Fig. 447) , and small 

 crustaceans. The waters of midstream are teeming with this 

 floating population, of which one is not aware until he strains 

 it out with a fine net. Kofoid found in the Illinois River at 

 Urbana almost five million organisms to the quart, a million of 

 which were animals. He estimates the total plancton carried 

 down by the river per year at more than two and one 



FIG. 448. Valley of lower Galien River, New Buffalo, Michigan. Note plant 

 zonation in foreground. 



quarter million cubic feet of solid organisms. Certain flagellates, 

 ciliated forms like Vorticella, and the shell-bearing rhizopods, 

 Difflugia and Arcella, are among the commoner protozoa. 

 Daphne, Bosmina, Acroperus, Leptobora, are common cladoceran 

 crustaceans, while Cyclops, Diapotamus, and Canthocampus are 

 the customary copepods. A very transparent midge larva, 

 Corethra, is sometimes very abundant. It is nearly an inch 

 long and so transparent that the internal organs are plainly 

 visible as if it were made of glass. The crustaceans of the 

 plancton are the staple article of diet for all of the small 



