METHODS OF PLANKTON RESEARCH. 513 



is applied as follows: — The filtering bucket which has 

 been unscrewed from the net is inclined so that what little 

 water remains in it lies over the silk. By carefully 

 tapping or rubbing the latter, this water can be got rid 

 of; but in doing this great care must be taken that the 

 water, and consequently part of the catch, does not run 

 over the edge of the bucket. 



The filtering bucket is now held over a glass tube or 

 bottle, the tap opened, and the whole catch washed out 

 by a strong stream of alcohol directed from a wash bottle 

 directly on to the organisms on the inside of the bucket. 

 By this means the catch is easily removed by the fixing 

 fluid itself, the sea water is reduced to a minimum, and 

 the catch is fixed and put in its preserving fluid as soon 

 as possible after leaving the water, by means of one 

 operation. The bottles can be stored away and taken to 

 land for further investigation. To run the contents of 

 the filtering bucket into salt water and carry to land is, 

 even when only an hour intervenes, not at all advisable 

 and, of course, impossible on a long cruise. 



The Estimation of the Catch. 



There are two methods at present in use by which 

 the plankton tables are constructed. One is a simple 

 method of estimation by examining the catch under the 

 microscope, noting down the forms that occur and 

 denoting their frequency by letters such as c. c. (very 

 common), c. (common), + (neither common nor rare), 

 r. (rare), r. r. (very rare). This method is still the most 

 general one in use. The other method is that carried out 

 by the Hensen School, and forms as essential a part of 

 the quantitative work as the nets themselves. By this 

 latter, the actual organisms present in a known fraction 

 of the catch are counted. Since the first method still is 



