132 QUANTITATIVE PLANKTON INVESTIGATIONS [PART II 



to explore the seas in remote parts of the earth, its biological 

 interest is often so great as to exclude the possibility of a 

 chemical analysis. 



Of all the methods of investigation of the catches made, the 

 actual counting of the individual organisms is the most satis- 

 factory. In this way its biological composition is known, and if 

 necessary the chemical composition can also be determined, in an 

 approximate manner at least, by the application of constants 

 previously ascertained. Thus the chemical composition of the 

 principal planktonic organisms, such as diatoms, copepods, peri- 

 dinians and so on, can be determined by analyses of catches which 

 consist predominantly of these organisms, and if the average 

 number of each of these creatures, corresponding to a given mass, 

 be previously determined then the chemical composition can be as- 

 certained from the results of the enumeration. From most points 

 of view the direct counting of the catch is the most valuable method 

 of estimation. Now if, as often happens, a single catch may consist 

 of millions of individuals it becomes an utterly impossible pro* 

 ceeding to count these, and their number has therefore to be 

 estimated by the methods of dilution of the original catch which 

 are employed by the bacteriologists. The original catch is mixed 

 with a volume of water, so that it has a definite volume and then 

 a fractional part of this diluted catch is taken and the organisms in 

 it are identified and counted. The numbers so obtained are then 

 simply multiplied by the number of times the fraction is contained 

 in the whole diluted volume of the catch. 



Suppose that we have a catch which on a preliminary exami- 

 nation is seen to consist of comparatively few fish eggs, many 

 copepods, and a large number of the diatoms Coscwodiscus and 

 Biddidphia. Its rough volume is, say, 4 c.c. Now it is usually an 

 easy matter to pick out the fish eggs and count them directly. 

 The catch is emptied out into a flat dish and the eggs are picked 

 out with the aid of a hand lens, sorted into groups and identified 

 and counted. Then the remains of the catch are swept up into 

 a glass flask and diluted with water until the volume is 100 c.c. 

 One c.c. of this mixture is taken and counted. 



Now this is not so easy as it seems to be, for we are not dealing 

 with a bacterial emulsion, or with a simple fluid mixture in which 



