GERMAN PLANKTON INVESTIGATIONS. 291 



obtained. The results obtained by this method can be 

 readily compared with one another, but do not by any 

 means give the trne volume, because a certain amount of 

 fluid occupies the spaces between the individual 

 organisms. 



It is possible that the plankton on several different days 

 might yield like results according to this method, and 

 yet be essentially quite different. For instance, the 

 plankton might be one ccm. per day for several successive 

 days. Then according to the volume estimation method 

 one would be constrained to say that the plankton had 

 remained the same. The enumeration of the individual 

 constituents might however show that while the total 

 volume had remained the same, the individual species had 

 undergone a remarkable variation. Hence the importance 

 of the last of the four methods, the estimation by counting. 

 The presence or absence of certain forms makes a great 

 difference in the volume of the catch, which may be out 

 of all proportion to their importance for the particular 

 object of the experiments. Again, certain forms take a 

 much longer time to settle down than others. Catches 

 in which Dinoflagellata preponderate settle down very 

 rapidly, and only require a few hours. Copepoda also 

 settle down quickly, and after twenty-four hours a correct 

 result can be obtained. The Diatomacese are very trouble- 

 some, and when they are present in large quantities they 

 tend to nullify the results, and a true estimation of the 

 volume can only be obtained after waiting for a very long 

 time. When Salpa? and similar animals are present, the 

 estimation becomes so imperfect that the large individuals 

 require to be separately estimated. 



Another objection to the estimation of volume by this 

 method is that a varying amount of fluid fills up the spaces 

 between the organisms. An attempt has been made to 



