CH. Vl] QUANTITATIVE PLANKTON INVESTIGATIONS 129 



ship, the angle of the tow-rope, and other factors. Probably 

 the volume of the filtered water is estimated very imperfectly, 

 and neither of the two nets just described can be said to be very 

 successful. Further they can be worked only in a horizontal 

 direction. 



Estimation of the catch. Now we may return to our 

 original vertical net. We have obtained a catch which consists of 

 all the larger organisms which w^ere present in a column of sea 

 water of depth equal to the depth of the sea in situ, and of 

 a known cross section. The nature and amount of this catch 

 remain to be determined. In ordinary qualitative plankton work 

 the usual procedure is to measure the volume of the catch, then 

 pick out the larger organisms with the aid of a hand lens, and 

 then examine a small portion of it in detail under the micro- 

 scope. The organisms present are identified and their relative 

 abundance is estimated. It may be said, for instance, that the 

 constituents are " abundant," " frequent," " common," " rare," or 

 " very rare," or any other scale of relative abundance may be 

 employed. But these terms only express the relative frequency 

 of occurrence of the organisms identified ; they are purely 

 subjective, and may differ with the personal equation of the 

 observer ; and no precise significance can be attached to them. 

 Obviously the smaller organisms must be less correctly estimated 

 than the larger ones^. They have therefore little value, and 

 perhaps all that ought to be stated with regard to a plankton 

 catch examined in this manner is that one or more organisms are 

 abundant, and that others are rare. The Kiel planktologists 

 employ much more exact methods, and a plankton catch 

 may be examined quantitatively by one or more of four methods : 



(1) by the estimation of the volume of the catch; 



(2) by chemical analysis ; 



(3) by weighing ; and 



(4) by the enumeration of the individual organisms contained 

 in it. 



According to the end in view one or other of these methods is 

 employed. 



^ Apstein, "Die Schatzungsmethode in Planktonforschung," Wiss. Meeresunt. 

 Kiel Komm. Bd. viii. Abth. Kiel, 1905. 



J. F. 9 



