GERMAN PLANKTON INVESTIGATIONS. 281 



work on the quantitative estimation of tlie smaller 

 plankton organisms (11). The other figures are repro- 

 ductions of photographs that I have taken of the apparatus 

 as at present in use. 



The Nets and Methods. 



The nets devised by Hensen for the quantitative deter- 

 mination of the plankton are of two kinds, vertical and 

 horizontal, according to the way they are used. Of these 

 the vertical alone are used, the difficulties in the case of 

 the horizontal nets being at present insuperable. The 

 principle of the use of the vertical net consists in that it 

 is, in the form of an inverted truncated cone, lowered 

 perpendicularly in the water to a required depth, and then 

 raised to the surface also perpendicularly. By this 

 method a cylindrical column of water filters through the 

 net, and its planktonic contents are captured. Now the 

 volume of this cylindrical column of water can be calcu- 

 lated since the depth to which the net is sunk is known, 

 as is also the area of the net opening. The first and most 

 important requisite of the Hensen net is that it should 

 capture the whole of the plankton in an exactly known 

 volume of water, so that on every occasion not only must 

 the whole of the plankton remain in the net, but the exact 

 volume of water which has filtered through must be 

 calculated. 



It is perfectly obvious that not so much water 

 passes through the net as would pass through a ring of 

 equal diameter to that of the mouth of the net, which had 

 nothing attached to it. It is also clear that a square 

 centimetre of the net would let through more water if it 

 were composed of a single mesh than if, as is really the 

 case, it is composed of a large number of minute meshes, 

 each bounded by a square of silk' fibre. Therefore it is 



