Life of the Waters 375 



further this amount is a variable quantity depending on the 

 rate at which the net is hauled through the water, the extent 

 to which it has been used and various other factors, all of 

 which introduce an error which is inconstant and is not 

 exactly known. 



Many kinds of quantitative plankton nets, both open and 

 closing, have been designed since Hensen's original device 

 was conceived. They are all however subject to the same 

 errors as is Ilensen's net, and for exact work, especially on 

 the smallest forms or nanno-plankton (from the Greek word 

 meaning dwarf) are very unreliable. 



To obviate the uncertainty of the net method in quanti- 

 tative plankton work, the water bottle described above has 

 been used for small samples, while larger samples have been 

 pumped from shallow depths and the water strained through 

 a net to concentrate the catch. The former method is open 

 to the objection that a small sample is not necessarily repre- 

 sentative of a large area, as the organisms (especially the 

 larger ones) may vary in abundance from place to place, 

 while the pump method is only applicable to shallow depths, 

 because of the great amount of hose required at lower levels, 

 and is furthermore subject to one of the errors involved in 

 tlie net method, namely that the smallest organisms in large 

 measure press through the net, while a further error is 

 involved in the tendency of many actively swimming, though 

 minute animals, to swim away from the suction current 

 created by the pump, and thereby give the eager and hard- 

 working biologist the slip. For great depths and large catches 

 therefore the net method, with all its faults, is the only prac- 

 tical one as yet devised. 



But having made his collections how does the biologist 

 determine the number of organisms which they contain? To 

 do this the method in use among physicians for counting 

 the corpuscles in the blood has been adopted. Knowing the 

 amount of water filtered through the net, the catch is brought 

 to a known concentration, say 1/50 or 1/100 part of tlie 

 former, and a small quantity (usually one cubic centimeter) 

 of the latter is placed in a glass cell on the stage of the micro- 

 scope, and the total number of organisms (in the case of the 

 larger forms) is counted. In the case of the very small 

 organisms, such as the unicellular forms, the number present 

 in several small parts of the cell is counted and by averaging 

 these and multiplying by the ratio between the area of the 

 cell and the total area of the parts counted, the number con- 

 tained in the former can be estimated with a fair degree of 

 accuracy. The size of the parts is determined by means of 

 squares ruled on a small disk of glass in the eye-piece of the 



