INTRODUCTION 13 



formation adequate to outline a picture of the life in fresh water. 

 A comparison of the records shows conspicuously the uniformity 

 of fresh-water life on the surface of the globe, especially among 

 plankton organisms. Many of the forms discussed on later pages 

 are identical with those that occur in Europe and many more are 

 closely related species. The uniformity noted is not confined to 

 Europe and North America, but extends, within the limits of records 

 already made, to other continents also and even to the islands of 

 the sea. It is most striking perhaps among the lowest groups 

 as was emphasized by Schewiakoff for Protozoa. 



This uniformity is due in part at least to the ease of dispersal 

 that the lower forms in the fresh- water fauna and flora enjoy. 

 They uniformly have hard-shelled resting spores, gemmules, or 

 eggs which are very resistant to adverse conditions, and are pro- 

 duced in enormous numbers. These structures are carried from 

 point to point on the feet of birds and other migrating animals 

 and are blown about in the dust until suitable conditions, e.g., 

 temperature and moisture, incite development and the beginning of 

 a new life cycle. 



Fresh-water life includes both plant and animal organisms of 



various types. The number of groups represented among the plants 



is not so great as the animals furnish. For details on individual 



groups the student is referred to the appropriate chapter. The 



following plant groups are found in fresh water: 



Schizomycetes Lowest type of plant life in the water; either 



or Bacteria saprophytic or parasitic in habit; found in great 



variety in different sorts .of aquatic environment. 



For a general discussion of their relations to 



fresh water consult Chapter IV, page 90. 



Algae Characteristic and abundant aquatic plants, 



nearly all free-living, found in all kinds of water 

 bodies; represented by a great variety of genera 

 and species. 



For Cyanophyceae or Blue-Green Algae, see 

 Chapter V, page 100. 

 For other classes of Algae see Chapter VI, page 



