14 FRESH-WATER BIOLOGY 



Higher Plants Among these plants which are more typically 



land organisms, a few species of various sorts have 

 become a part of the fresh-water flora. In this 

 change they have undergone important modifica- 

 tions adapting them to an aquatic existence. No 

 synoptic treatment of these forms has been at- 

 tempted. 

 For general biological relations involved see 

 Chapter VII, page 178. 

 Animals are represented in fresh water by many more types and 

 varieties than are plants. A brief outhne of the various animal 

 groups indicates in general the part played by each in aquatic life 

 and will serve to correlate the various chapters dealing with in- 

 dividual groups. Zoologists are not agreed as to the number and 

 rank of the subdivisions of the animal kingdom which should be 

 recognized; and other texts will show some variations from the 

 system used here. The student should bear in mind that the order 

 in the printed text does not express the relationship between higher 

 and lower groups and no arrangement in a linear series can show 

 that relationship. The phyla are indicated by full-faced type. 

 Protozoa Characteristic water-Kving forms with niunerous 



or Single- parasitic types; represented in fresh water by 



celled Animals .V. \, j- , . , , , 



many species frequently found m great abundance ; 



in all regions and in all types of water bodies. The 



following four sub-phyla are usually recognized. 

 Sarcodina The amoeboid Protozoa furnish both free-living 



and parasitic species. 



For the former see Chapter VIII, page 210. 

 Mastigophora Flagellate Protozoa include both free-living and 



parasitic species; forms of the first type are 



treated in Chapter IX, page 238. 

 Infusoria CiUate Protozoa include both free and parasitic 



species. 



For the former see Chapter IX, page 271. 

 Sporozoa Exclusively parasitic forms; certain types are 



abundant in fresh-water animals everywhere. 



North American forms almost unknown. Group 



not treated in this book. 



