1 82 BRITISH FISHERIES 



Victor Hensen, who was the first to make a 

 systematic study of the plankton of the sea from 

 this latter point of view, attempted to answer two 

 main questions : (i) What does the sea (or any 

 given small volume of it) contain at a given time 

 in the way of organisms ; and (2) how does this 

 material vary from season to season and in different 

 places ? Formerly the quality only of the plankton 

 had been taken into account ; Hensen endeavoured 

 also to determine the quantities of the different 

 organisms in it. To attain these results it was 

 necessary to devise an entirely new scientific 

 method, and by the exercise of great patience it 

 became possible at length to devise quantitative 

 fishing apparatus. The ordinary plankton net is a 

 conical bag of silk cloth, the meshes of which are 

 so fine as to strain out practically all the smaller 

 microscopic animals and plants found in the sea. 

 This net is usually one to two feet in diameter at 

 the mouth, which is kept open by an iron ring, 

 and it may be a yard or more long. It is simply 

 dragged through the water from a vessel travelling 

 slowly, and a portion of the water entering its 

 mouth is strained through the fabric of the net, 

 which retains the organisms present. Now, 

 though it may be possible, by the use of such a 

 simple contrivance, to obtain relative ideas of the 

 quantities of plankton in the sea at different times 

 and places, it is practically impossible to obtain 

 any correct estimate of the absolute number of 



