METHODS OF COLLECTING AND PHOTOGRAPHING 85 



weight is taken and the cone is weighed again after having been 

 ignited. The loss in weight represents the organic matter. 



Larger samples of material are needed for more accurate quan- 

 titative work, and especially for the study of the chemical com- 

 position of the nannoplankton. For the latter purpose at least 

 two or three grams of organic matter are required. In order to 

 secure this amount, even from a lake which is rich in plankton, it 

 is necessary to centrifuge one to two thousand liters of water. 

 This process requires an apparatus that will act continuously. 

 For this work the Wisconsin Geological and Natural History 

 Survey is using a De Laval clarifier and filter, belt style, A size, 

 in which the water is first centrifuged and then filtered. This 

 machine has a maximum speed of 6000 revolutions per minute and 

 will both centrifuge and filter from ten to twelve liters per minute. 

 In general about ninety per cent of the material is deposited in the 

 bowl of the centrifuge and ten per cent on the filter papers. This 

 method requires a special laboratory and equipment (cf. Juday, 

 1916). 



Very little is known of the bacterial portion of the nannoplank- 

 ton. The culture methods used for the other bacteria do not seem 

 to be well adapted to the strictly aquatic forms and only a small 

 part of them can be obtained with a centrifuge. Recently, how- 

 ever, it has been found that the direct count method of Brew can 

 be used for determining the number and distribution of aquatic 

 bacteria, but no results have thus far been published. 



G. Special Methods for Invertebrates 



Special methods for collecting and preserving various sorts of 

 fresh-water organisms are described in the chapters devoted to 

 invertebrate groups. To secure the best results it is necessary 

 to become familiar with the habits of the animals. The collection 

 of the larvae of aquatic insects is facilitated by the use of the 

 ingenious apparatus made by the Simplex Net Company. The 

 imagos of many such insects are readily collected at night by 

 some one of the forms of traps used by entomologists in which 

 a light serves as a lure. 



