INTRODUCTION 9 



following chapter. An adequate consideration of methods and 

 apparatus demands more space than is available here and for 

 further information the student is referred to manuals dealing with 

 that phase of aquatic investigation. General methods of collect- 

 ing and photographing aquatic organisms form the subject of a 

 separate chapter while such methods as are applicable to the study 

 of each special group are discussed in the chapter on that group. 



The environment of water organisms as of all others is a com- 

 plex of many elements. The physical factors are determined by 

 the materials held in suspension or in solution in the water, by its 

 temperature, depth, movement, illumination, shore and bottom. 

 Chemical factors are found in the acidity or alkalinity of the water 

 and in the gases, salts, and other materials in it. The organisms 

 themselves make the biological environment. Living or dead, as 

 food or feeder, parasite or host, friend, enemy, or neutral, each 

 living thing contributes to the sum total of the biological complex 

 by which each living unit is surrounded. It is the problem of 

 science to unravel this tangle and to determine the relation of each 

 constituent, living or non-living, to the others. The conditions of 

 existence to which organisms are subject in different aquatic en- 

 vironments and the influence which these environments exert on 

 organisms in general are discussed in the following chapter. In 

 subsequent chapters an attempt has been made to present these 

 relations as illustrated by each group of organisms. To become 

 thoroughly acquainted with a single group involves a knowledge 

 of the relations its members bear to every other organism in the 

 community. 



No climate is too rigorous for fresh-water life. It exists in 

 fresh-water lakes at 77 N. L., hardly if ever free from ice, often 

 only slightly melted and with a maximum temperature of less than 

 2 C. at the bottom. The Shackleton expedition described an 

 extensive rnicrofauna at 77 30' S. L. from Antarctic lakes that 

 are frozen solid for many months, often for several years. At the 

 other extreme of temperature evidence is less complete but Cypris 

 is recorded from hot springs at 50 C., cilia tes and rotifers from 

 waters at 65 C., Oscillaria and nostocs from places that are 

 recorded at 70 to 93 C. 



