174 BULLETIN OF THE UNITED STATES FISH COMMISSION 



bottom to surface. It' it is possible to obtain by the pump such a column of water. 

 then the pump may very well replace the net so far as this part of the proce-- is 

 concerned. I do not say that this is not possible, but we should not assume that the 

 \\atcr drawn in by a pump through the submerged end of a hose, which is being 

 'slowly moved from top to bottom, or vice versa, is a vertical column of water. 

 Before the pump can replace the Hensen net there must be sufficient evidence that 

 this is so, and such evidence is not yet forthcoming. 



Having obtained the water by use of the pump, it is necessary to separate the 

 plankton from it. To accomplish this, the second process into which we have analyzed 

 the Hensen procedure, various means have been proposed. Frenzel, and at iirst 

 Kot'oid. 1 made use of the Hensen net to strain the water pumped. In order to 

 avoid the loss of plankton due to the permeability of the net to small organisms, 

 Kofoid later tried various other methods of separating the plankton from the water. 

 Thc-e were the sand filter, the filter paper, the centrifuge, and the Berk el eld 

 filter. By each of these methods a greater number of plankton organisms is retained 

 than by the Hensen net. (Nothing is said of volumes.) In some cases as much as J)S 

 .nt of the total number of organisms present is retained. By none of the>e 

 methods N it possible to obtain the plankton from a large volume of water in a short 

 time, and each has besides other disadvantages which are enumerated by Kofoid. In 

 the ease <>f the IJerkefcld filter, which was found to be the most efficient method, it 

 was necessary to remove the catch from the surface of the filter with a '-stiff brush." 

 The surface of the filter, which is composed of infusorial earth, was thereby disinte- 

 grated and the plankton contaminated by the fragments. It is to be hoped that the 

 disintegration i> confined to the filter. The large form of the Berkefeld filter (army 

 filter) filters about U liters of water per minute. This is a very slow rate of filtration 

 if one has to deal, as is sometimes desirable in plankton work, with a column of water 

 several hundred feet long and perhaps 10 inches in diameter. 



The methods which it has been proposed to substitute for the Hensen method are 

 thus seen to be deficient in two ways. For obtaining the water the pumping method 

 tar as yet shown) defective in that the source of the water pumped is uncertain. 

 It is not known that the pump can be, made to deliver with accuracy the contents of 

 a vertical column of water. For filtering the water the methods proposed, although 

 they remove the. plankton organisms more perfectly than the Hensen net, are yet 

 inferior to it in that they are incapable of handling large volumes of water. Is it 

 possible to >o modify the Hensen method or to so combine it with other methods as 

 to correct its errors and at the same time, retain its good points? Its errors are 1 he 

 variation in net coefficient, due. to clogging and shrinkage, and the permeability Of 

 the net for small plankton organisms. Its advantages are that it filters a representa- 

 tive vertical column of water, and that it filters rapidly very large volumes of water. 

 N.\\. if it is possible to measure the volume of water that passes through the net at 

 each haul the difficulties of clogging, shrinkage, and net coefficient at once, vanish. I 

 have not made any attempts in this direction, but I see no reason why a small current 

 meter can not be placed within the opening of the plankton net, so as to register the 

 rate of the current of water passing through the opening during each haul. If this 

 rate Were known the volume of water passing through the net could be calculated, 



'Itiillrtin Illinois State Laboratory of Natural History, vol. \, :irt !<! i. 



