SHALLOW WATER COMMUNITIES 77 
“When in its native element it is almost perfectly transparent and 
consequently invisible—a true microscopic ghost” (Forbes, 89). 
-The wheel animalcules are as a rule larger than the protozoa and are 
of a much higher structural organization, capable of making more 
complex movements. About thirteen species of these may be found in 
the waters of the lake in midsummer. WNotops pygmaeus Calm. (see 
Figs. 18-19) is a characteristic member of the group. 
In addition to these forms there are also worms, such as round worms, 
planarians, leeches, etc., found in the limnetic formation either inciden- 
tally or habitually. 
None of the adult fishes of the lake belong strictly to the limnetic 
formation. Fishes such as the whitefish, lake herring, and lake trout 
are sometimes found in the open water, and the young of some lake 
fishes may belong there strictly (go). 
b) Characters.—Specialists in the various groups of animals might be 
able to pick out some structural characters which would distinguish 
the forms:of such open-water situations from the forms living in among 
the vegetation or on the bottoms of this or smaller lakes. The only 
striking structural character is the transparent or translucent color of 
most of the forms. 
A large number, if not all, of the limnetic crustaceans are in deep 
water during the day and come to the surface at night. The behavior 
of the rotifers is somewhat different. Jennings (87) says: ‘During 
the day the limnetic rotifers are found in much greater numbers near 
the surface than near the bottom, reversing the condition commonly 
observed for the crustaceans. At night the distribution seems not to be 
materially changed: The immense numbers of crustaceans obscure the 
rotifers; but there was no greater number of rotifers near the bottom 
in the few towings made at night than in the day time.” 
The most striking characteristic of the limnetic formation is that it 1s 
independent of bottom and in its reactions 1s indifferent to the bottom. 
Jennings (44) states that pelagic forms have a more simple type of 
behavior than the attached and bottom forms. 
2. BOTTOM COMMUNITIES 
Forms inhabiting the bottom of lakes and also of the sea in a general 
way bear the same relation to the water that the terrestrial animals do 
to the surface of the land. Usually they do not leave it to rise to any 
considerable height above the bottom. The fishes of lakes correspond 
to the birds of the land. 
