CH. Vl] QUANTITATIVE PLANKTON INVESTIGATIONS 131 



volume of the catch, again plus the interstitial liquid. When very 

 small catches have to be measured it is better to regard these as 

 minimal ones, in much the same way as we regard the results of 

 chemical analyses which are just on the limits of quantitative 

 expression. All methods of plankton volume measurement are 

 more or less unsatisfactory, and a very serious objection is the 

 frequent presence of the larger organisms in the catch. Thus if a 

 catch of 5 c.c. volume contains two Pleurohrachiae w^hile a similar 

 catch contains none of these organisms, it would obviously be a 

 misleading result if the volume of the first catch included the 

 ctenophores. The very large planktonic animals must therefore 

 first be picked out from the catch before the volume estimation is 

 made. Since the nets of finer Mlillergaze are not adapted for 

 catching these larger things they can be omitted without error from 

 the catches made, or at any rate they can be estimated separately. 



The mass of a plankton catch is very easily estimated. The 

 collection is washed with fresh w^ater until the salt has been re- 

 moved, and then it is put into a weighed glass capsule and heated 

 in a hot water oven at a temperature of 100° C. Then it is put 

 into a desiccator and when the weight becomes constant it is 

 recorded. 



It is not possible here to consider the methods of chemical 

 analysis. It wall be sufficient to state that the estimation of 

 carbon, hydrogen and nitrogen by the ordinary methods of organic 

 combustion are the fundamental results. Other constituents of 

 the catch are often estimated. Thus fat, which is abundant in 

 copepods and other Crustacea, is determined by making ether 

 extracts. Carbohydrates, chitin and cellulose may also be de- 

 termined. The inorganic constituents usually estimated are 

 chlorine, phosphoric acid and calcium (both the latter of importance 

 as forming the materials of the skeletons of many organisms), and 

 silica (which is an important constituent of the skeletons of 

 diatoms and radiolaria). The total ash which remains after 

 ignition is also often estimated. A knowledge of the chemical 

 composition of the plankton is, of course, of value in the discussion 

 of many general problems, but unfortunately it cannot be obtained 

 without destroying the catch, and when the latter has been obtained 

 at some considerable cost, as in the case of expeditions fitted out 



9—2 



