METABOLISM IN THE SEA 183 



organisms present. This latter knowledge is what 

 Hensen tries to obtain by the use of a specially 

 constructed net, which is lowered down to the 

 bottom of the sea and is then slowly hauled up to 

 the surface. By a series of ingenious calculations, 

 in which the " filtration capacity " of the net, the 

 depth to which it is lowered, the speed with which 

 it is hauled up, etc., are factors, it is possible to 

 calculate what fraction of the water entering the 

 mouth of the net passes through its meshes. The 

 plankton present in the net is, then, that which 

 was present in a column of sea-water of a certain 

 cross area, and extending from the surface of the sea 

 down to the depth to which the net was lowered. 

 It is, in fact, a sample of the contents of the sea, 

 both in quality and quantity, in the place where 

 the experiment was made. A number of such 

 fishing experiments are made, and an average is 

 struck from their results and expressed in this way : 

 Every square yard (or square metre) of (say) the 

 North Sea or the Baltic contained so much plankton 

 at the time when the experiments were made. 



By a series of ingenious operations,-^ the plankton 

 " catch " is then removed from the net and is 



1 It is quite impossible to give even a brief account here of the 

 methods involved. The reader is referred to a paper by Jenkins in 

 Proceedings and Transactions of the Liverpool Biological Society, vol. 

 XV., Liverpool, 1901, or to an appreciative paper by Reighard in the 

 Bulletin of the U.S. Rsh Commission, vol. xvii., Washington, i8g8. 

 Hensen's original memoirs are published in the Wissenschaftliche 

 Meeresuntersuchungen of the Kiel Kommission. 



