20 Introduction 



(3). Collecting the commoner invertebrates, such as 

 water insects, crustaceans and snails involves ordinarily 

 the use of a hand net. A square rod of water is suffi- 

 cient area to ply it in; a satisfactory catch may be a 

 hundred specimens. 



(4). For collecting entomostracans and the larger 

 plancton organisms towing nets of fine silk bolting-cloth 

 are commonly employed. Possibly a cubic meter of 

 water is strained and a good catch of a thousand speci- 

 mens may result. 



(5). The microplancton organisms that slip through 

 the meshes of the finest nets are collected by means of 

 centrifuge and filter. A liter of water is often an ample 

 field for finding ten thousand specimens. 



(6). Last and least are the water bacteria, which are 

 gathered by means of cultures. A single drop of water 

 will often furnish a good seeding for a culture plate 

 yielding hundreds of thousands of specimens. 



Thus the field of operation varies from a wide sea to 

 a single drop of water and the weapons of chase from 

 a harpoon gun to a sterilized needle. Such divergencies 

 have from the beginning enforced specialization among 

 limnological workers, and different methods of studying 

 the problems of water life have grown up wide apart, 

 and, often, unfortunately, without mutual recognition. 

 The educational, the economic and the sanitary inter- 

 ests of the people in the water have been too often dealt 

 with as though they are wholly unrelated. 



The agencies that in America furnish aid and support 

 to investigations in fresh water biology are in the main : 



I. Universities which give courses of instruction 

 in limnology and other biological subjects, and some of 

 which maintain field stations or laboratories for investi- 

 gation of water problems. 2. National, state and 

 municipal boards and surveys, which more or less 

 constantly maintain researches that bear directly upon 



