130 QUANTITATIVE PLANKTON INVESTIGATIONS [PART II 



Useful results are generally afforded by the mere estimation of 

 the volume of the catches made : thus it is easy to compare, in a 

 rough sort of way of course, the relative abundance of plankton in 

 the areas investigated, merely by a comparison of the bulks of 

 organisms contained in similar volumes of sea water. The catch 

 is taken out of the bottle in which it is contained and is put into 

 a measuring glass and shaken up with the preservative, preferably 

 alcohol, since the organisms settle down more quickly in this fluid. 

 Then after the whole has been allowed to stand for some time, at 

 least a day or two, the level to which the catch rises in the 

 measuring glass is read off on the graduations of the latter. 

 This procedure requires some considerable time if a larger number 

 of catches have to be examined, and when this is the case the 

 separate bottles are allowed to stand for the necessary time and 

 then the level to which the plankton rises in each is marked on 

 the outside by means of a pencil which will write on glass. 

 Each catch then is emptied out into another vessel and the 

 original bottle is filled up with water to the level of the mark from 

 a burette and the quantity of liquid discharged from the latter 

 gives the volume of the catch. But obviously this volume is not 

 the true volume of the organisms but is that plus the volume of 

 the liquid which was present in the interstices between the sepa- 

 rate organisms. Now this latter value will vary with the closeness 

 with which the separate organisms lie together and obviously this 

 depends on the nature of the catch. Thus copepoda lie together 

 closely, but diatoms furnished with long spines will leave consider- 

 able interspaces between them. 



If the catch is filtered through a very fine silk cloth it is 

 obtained in a wet condition, and only a very small quantity of 

 liquid is contained in it. Then a measuring cjdinder is filled with 

 water, or some other liquid, up to a certain mark and the wet 

 catch is added to this liquid. The level of the latter will rise and 

 the difference of level is the volume of the catch plus the liquid 

 remaining in its interstices. But if minimal catches have to be 

 measured this difference is very small, and this is the defect of the 

 method. Again the volume of the catch in its preservative liquid 

 may be exactly measured and then the whole is filtered. The 

 volume of the filtrate is again measured and the difference is the 



