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. Notops 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 w r ater, and the young of some lake 

 fishes may belong there strictly (90). 



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 is 

 independent of bottom and in its reactions is 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. 



