502 TRANSACTIONS LTVEHrOOL BIOLOGICAL SOCIETY. 



quantity of water could be estimated These quantitative 

 nets are lowered perpendicularly in the water to a certain 

 depth and then raised to the surface, so that any towing 

 in a horizontal direction is avoided. Thus a vertical 

 column of water passes through the net, and the volume 

 of this has to be calculated before the net is used for 

 quantitative work, since it is obvious that not the same 

 quantity of water passes through the net as would pass 

 through the open mouth if no net was attached to it. In 

 short, the problem has been to devise an apparatus which 

 should be handy and workable from a small ship, which 

 would take up a definite quantity of water from the sea, 

 and abstract as thoroughly as possible the organisms 

 contained therein. 



Silk nets still form the apparatus most used for this 

 purpose, but as Lohmann (11, 13 and 18) has shown, the 

 results are only accurate to a certain point and must be 

 supplemented by other methods, according to the aim of 

 the research. The large llensen vertical net was described 

 in Dr. Jenkins' paper, and therefore need not be 

 mentioned further. Apstein has shown that a much 

 smaller net can be conveniently used with much saving 

 of time and labour, and under conditions where the larger 

 net is impossible, in the absence of a steamer. 



At the present time the net used in the German 

 investigations above all others, and thus the chief imple- 

 ment for plankton research, is the Middle Plankton Net 

 of Apstein (7). The large Tlensen net is used only for 

 special purposes, one of which is the quantitative estima- 

 tion of fish eggs, where a large catch of plankton is 

 desirable, or for other large organisms that do not occur 

 frequently enough for accurate measurements to be made 

 with the smaller nets. In form the Apstein net is almost 

 the same as the Hensen. It is shown in fig. 1 to consist 



