LIMNETIC COMMUNITY 75 
city water in a day in August, all together they would be consuming 
about ro quarts of solid plant and animal substance—enough to make 
a meal for about forty people. 
One does not think of the lake as an area of luxuriant vegetation, 
teeming with animal life, but rather as a barren waste of water. How- 
ever, if one’s vision for small objects were only better, he would see as 
he passes over the water in a boat, thousands of small animals and plants 
such as are shown in Figs. 12-18 together with about fifty other forms of 
protozoa, wheel animal- 
cules, crustaceans, insects, 
and small fish. Most of 
these spend their entire 
existence freely floating or 
freely swimming. With 
the exception of the fish 
and insects they consti- 
tute the plankton which is 
the basis of the food of the 
millions of pounds of fish 
taken from Lake Michigan 
every year. - 
From the standpoint 
of our economic interests, 
the limnetic formation 
is of great importance. 
It deserves comment also Fic. 12.—A sun animalcule (Actinophrys sol 
Ehrbg.); 330 times natural size (after Leidy). 
because of its scientific 
interest, and the aes- 
thetic value of the vari- 
ous forms of which it is 
composed. 
a) Its composition (85, 86, 87, 88, 89).—The minutest animals of 
this formation are the protozoa. About thirteen species have been found 
to inhabit the open waters of the lake. Of these the sun animalcule 
(Actinophrys sol) (Fig. 12) and the shelled protozoan (Difflugia globu- 
losa) (Fig. 14) are easiest to recognize. Nine of the thirteen common 
species are mixotrophic in their nutrition (i.e., contain chlorophyll and 
manufacture their own food) (Fig. 13) and share with the algae and 
diatoms the important function of furnishing food for the rotifers (wheel 
animalcules) and the crustaceans. 
Fic. 13.—Protozoan (Peridinium tabulatum 
Ehrbg.); 400 times natural size (after Kent). 
Fic. 14.—A shelled protozoan (Difflugia globu- 
losa Duj.); 130 times natural size (after Leidy). 
