504 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY 



column would be simply pushed aside without passing 

 through the net. Thus a reliable sample would not be 

 obtained. 



The Net, which is the actual filtering agency, must 

 be made of some material that will stand the work well- 

 filter the organisms as thoroughly as possible, and most 

 important of all, be so manufactured that the size and 

 shape of the meshes will not easily alter, so* that the 

 formula calculated for determining the filtration co- 

 efficient for the net may remain always applicable. The 

 best material has been proved to be " Miillergaze " or 

 " bolting silk," and the grade used for quantitative nets 

 is denoted by the number 20. For this particular tissue 

 the filtration capacity has been calculated. The net must 

 be carefully made, because in order to determine the 

 quantity of water from which the plankton has been 

 abstracted, an extremely complicated mathematical and 

 practical research is necessary, and this when once made 

 for a particular silk and shape of net should be applicable 

 to all others of the same size, shape and material. 

 Furthermore, in order to compare the results of workers 

 in different countries, it is of course advisable that a 

 standard net such as the Middle Plankton Net should be 

 used everywhere. 



The silk net is attached above to a strip of strong 

 linen cloth or canvas about an inch wide, which is 

 fastened to the lower brass ring of the conical mouth- 

 piece. It is conical in shape with a truncated end, which 

 forms the bottom of the net and is fastened to a metal 

 bucket by means of a clamp ring consisting of a 

 strip of thin brass, which is bent into a ring with the 

 two ends bearing projections perforated by holes so that 

 they can be screwed together. 



The brass bucket (fig. 1), being of considerable 



