METHODS OF PLANKTON RESEARCH. 525 



The filter is carefully washed down towards the point 

 of the cone when all the water has been filtered, and this 

 is perforated by a pointed glass rod, whilst held over a 

 bottle for the reception of the catch. The filtrate is then 

 carefully washed through the perforation by means of a 

 wash bottle. provided with a strong indiarubber ball, in 

 order to get a powerful current. The original volume of 

 water collected being known, the catch as now obtained 

 can be diluted and portions extracted for counting as 

 explained above. 



By fishing with the net and also with the pump and 

 tube simultaneously, or directly after one another, and 

 comparing the results, it is possible to determine the loss 

 by the net due to its inability to retain the smaller forms. 



If, however, exactly like methods are employed at 



the same time and place, and close to one another, the 



catch is different. This divergence, due to an irregularity 



in the distribution of the organisms in the sea, has been 



emphasised by Herdman and will be mentioned later, it 



must be borne in mind, therefore, when comparing the 



unlike methods, that a certain irregularity in distribution 



already exists. After reckoning the volume of water 



filtered by the net and reducing the number of organisms 



found in the whole catch to the number present in a 



volume of water equal to that collected by the pump, it 



was found by Lohmann that in the case of Copepod 



nauplii the net lost 74'5 per cent. This is a very 



important constituent of the plankton fauna, Other 



organisms were present in 1,000 litres of water in the 



following numbers : — 



Silk net Pump, tube 



No. 20 and filter. 



Globigerina 250 2,125 



Eadiolarians 2,350 3,860 



Cystoflagellates 20 20 



Tintiimidae 475 19,900 



Naked ciliates some 35,300 



