c, > 

 Preface 



The plan and purpose of this little book in this its fourth re- 

 vision are the same as in the earlier ones: to facilitate the recog- 

 nition of freshwater organisms in the field and in the laboratory. 

 It offers for the purpose keys and tables and figures illustrating 

 genera. Only things commonly found in fresh waters are in- 

 cluded. Inhabitants of salt and alkaline waters and of caves are 

 omitted. Vertebrate animals and vascular plants are also 

 omitted because they are better known, and because aids to 

 their recognition are commonly available. The larger inverte- 

 brates are more fully treated than are the microscopic ones. 

 Genera and not species are illustrated; and generic names are 

 applied in an inclusive sense to groups of species that a beginner 

 may be able to recognize by external differences. To provide 

 him with a tool that he can use has been our aim, and we trust 

 that that will explain our disregard of some of the much sub- 

 divided genera of systematic specialists. The keys have been 

 expanded to include genera whose immature aquatic stages were 

 formerly unknown. 



The program of 25 practical exercises for use by classes stands 

 as before, these having yielded satisfactory results in practical 

 acquaintance with waterTife. A good background of field ex- 

 perience is needed for proper appreciation and enjoyment of the 

 world we live in. Knowledge of the place and role that these 

 organisms fill in that world is the best basis for further and more 

 technical work with them. 



James G. Needham 

 Paul R. Needham 

 Ithaca, New York 

 March 1, 1938 



