METHODS OF COLLECTING AND PHOTOGRAPHING 8i 



two liters capacity, and with a good-sized neck is provided with a 

 tight rubber stopper to which is attached the draw-cord by which 

 the bottle is to be lowered and the stopper drawn. Beneath the 

 bottle is attached a weight a little heavier than needed to sub- 

 merge the empty stoppered bottle. The bottle may be lowered 

 to a depth of a hundred feet or less and the stopper removed 

 by jerking on the draw-cord. 



E. Quantitative Study of the Net Plankton 



If the plankton net were a perfect instrument it should catch 

 all the organisms contained in the vertical column of water through 

 which it is drawn, that is, in a column of the diameter of the net 

 opening and equal in height to the distance through which the 

 net is drawn. But the net filters only a part of the column of 

 water through which it is drawn, a part which 'depends on the age 

 of the net, the rate at which it is drawn and upon 

 the rapidity with which it becomes clogged while 

 being drawn. If the net is of the form described 

 above, is cleansed by throwing a stream of water on 

 it after each haul and is drawn at about the rate 

 of one meter per second, it filters about 40 per cent 

 of the column of water which it traverses. Hence, 

 ^o know the total amount of plankton in the column 

 of water traversed by the net, we must multiply the 

 amount actually taken by two and one-half. This 

 number is called the coefficient of the net. The 

 coefficient depends on the construction of the net, 

 on the fineness of the gauze used, and on the rate 

 at which the net is drawn, and must therefore be 

 determined by calculation for each net for the 

 different rates. Not only does the net filter but a 

 part of the water and a different part at different 

 times, but it removes from the water filtered only b^otUe. Sfter 



. r .-, . . , . . -r-, Wiley and Jones.) 



a part 01 the organisms contamed m it. Even the 

 finest gauze permits a leakage through it of very many small 

 organisms. Owing to the sources of error indicated the net 

 method is useful chiefly with the larger organisms, such as crus- 



