Life of the Waters 



371 



of closing nets have been designed, so constructed that they 

 can be lowered closed to the desired depth, and then opened, 

 either automatically by the pull on the net itself, or by a 

 messenger sent down a cable, and closed again by a mes- 

 senger, after which they are drawn to the surface without 

 danger of mixing the catch with organisms present at other 

 levels. 



For qualitative work the loss of a few organisms is a 

 matter of small consequence, and accordingly the net bucket 



TOW-NETS 



In use on the "Albatross" of the U. S. Bureau of Fisheries. 

 Courtesy of the Bureau. 



is a simple affair merely screwed into a ring at the lower end 

 of the net, or tied into the net itself. But for quantitative 

 work every organism possible must be saved, and the bucket 

 is accordingly more complicated, is readily detachable from 

 the net, and can then be opened and drained into the con- 

 tainer for receiving the catch. 



The problem of determining the number of organisms 

 present in a given volume of water, at any given time and 

 place, is one of the most important, and withal difficult .prob- 

 lems in oceanography. The quantitative study of the animals 

 and plants in the sea is essential to a knowledge of their rate 



