PREFACE 



For the ordinary student and teacher on this continent fresh- 

 water life has a significance heretofore greatly underestimated. 

 In most parts of the country it lies at one's very door, readily ac- 

 cessible, and is indeed the only type of aquatic existence which can 

 be studied Uving and at work. This fact gives to fresh-water life, 

 once the student has been introduced into its domain, an appeal- 

 ing interest that fetters his attention and stimulates his desire 

 to know something more of it. Among the most remarkable of 

 early works that followed hard upon the first use of the micro- 

 scope are some great classics which represent work in this very 

 field. 



Various European countries possess elaborate monographs on 

 fresh-water organisms as a whole and on single groups, but no 

 attempt has been made heretofore to deal with North American 

 fresh-water hfe in its entirety, and few treatises have essayed to 

 cover completely any group of fresh-water organisms. American 

 workers in general have accordingly avoided this field and the few 

 who have attempted to engage in its study have found their prob- 

 lems very difficult to solve. 



The preparation of the present work was undertaken many years 

 ago with the purpose of stimulating the study of the material so 

 easily obtainable and of aiding workers of all grades to acquire 

 some definite and precise knowledge of the organisms met in such 

 study. Each chapter has been handled by a specialist on the group 

 and the results achieved by this method have a significance that 

 could not have been attained in any other way. Conditions en- 

 tirely unavoidable led to the completion of the different parts of 

 the work at somewhat different dates. It is believed that this 

 will not, in fact, impair the value of the work as a whole and 

 will find an excuse in the magnitude of the task. Individual 

 chapters represent a survey of the group treated that is complete 



