30 



TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



Jt has been shown by Kofoid, by Lohmann, and by other? 

 that there are imperfections in the methods which were 

 not at first realised, and that under some circumstances 

 anything- from 50 to 98 per cent, of the more minute 

 organisms of the Plankton may escape capture by the 

 finest silk quantitative nets. The mesh of the silk is 



Pig. 7.— Closing Petersen-Hensen Net 

 going down open . 



Pn 



The same quantitative plankton 

 net coniing|up?closed. 



l-300th inch across, but many of the organisms are only 

 l-3000th inch in diameter, and so can readily escape. 



Other methods have been devised to supplement the 

 Eensen nets, such as the filtering of water pumped up 

 through hose-pipes let down to known depths, and also 

 the microscopic examination in the laboratory of the 

 centrifuged contents of comparatively small samples of 



