290 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



used, but au excess of osmic acid should be avoided. 

 Perhaps the best preservative is alcohol. This had better 

 be used at first in a weak solution. The strength of the 

 solution can be subsequently increased to the point desired. 



The Estimation of the Catch. 



The most difficult part of the whole undertaking is the 

 work on land, consisting, as it does in the estimation of 

 the catch in so exact a manner that the plankton in the 

 sea in different places or in the same place at different 

 times may be quantitatively compared. This estimation 

 may be conducted in several different ways, each of which 

 is summarised and discussed by Hensen. 



There are four chief methods of estimation, namely : — 

 1. Volume estimation. 2. Estimation by weight. 3. 

 By chemical analysis. 4. By enumeration of the indi- 

 vidual organisms. 



Taking first of all the method of estimation by volume, 

 we find that the simplest method is by allowing the 

 plankton to settle down in a glass vessel. The catch 

 is first of all thoroughly shaken up in alcohol in 

 a glass measuring cylinder, and allowed to stand for at 

 least twenty-four hours. This shaking up had better be 

 centrifugal when the plankton contains a large number of 

 Diatoms or Peridinese, as the subsequent deposition of 

 these organisms is thereby facilitated. At the end of 

 this time the plankton material has settled down at the 

 bottom of the vessel, and forms a more or less deep layer. 

 The depth of the layer can then be read off in centimetres, 

 which are marked on the outside of the glass cylinder. It 

 is of the utmost importance that the catch should remain 

 perfectly still, because the least disturbance causes the 

 layer of deposit to become thicker. By this means the 

 crude or rough volume (" Rohvolumen " of Schtitt) is 



