294 Aquatic Societies 



For every species the possible range is determined 

 by climate; the possible habitat, by distribution of 

 water and land; the actual habitat, by the presence of 

 available food and shelter, and by competitors and 

 enemies. 



Our classification of aquatic societies finds its basis 

 in physiographic conditions. We recognize two princi- 

 pal ecological categories of aquatic organisms: 



I. Limnetic Societies, fitted for life in the open water, 

 and able to get along in comparative independence of 

 the shores. 



II. Littoral Societies, of shoreward and inland dis- 

 tribution. 



Fig. 178. Diagram illustrating the distribution of \^| 



aquatic societies, in a section extending from an 

 upland marsh to deep water. The littoral region 

 is shaded. 



The life of the open water of lakes includes very small 

 and very large organisms, with a noteworthy scarcity 

 of forms of intermediate size. It is rather sharply 

 differentiated into plancton and necton; into small and 

 large; into free-floating and free-swimming forms. 

 These have been mentioned in Chapter V, where their 

 main lines of adaptation were pointed out. It remains 

 to indicate something of the composition and relations 

 of these ecological groups. 



