GERMAN PLANKTON INVESTIGATIONS. 299 



arranged so that not more than one thousand and not less 

 than one hundred of the organisms are found at one time 

 on the glass plate. When the number of a single species 

 only is enumerated at one time the counting is a simple 

 matter, and is easily carried out. The plate is carefully 

 looked over and every individual as it comes into the 

 field of view is counted. In this way the total number of 

 the individuals on the glass plate is arrived at, and the 

 amount of the dilution being known the total number of 

 individuals of that species in the catch can be calculated. 

 Suppose, for example, the dilution to be ten times the 

 original volume and that one ccm. of the diluted portion 

 is used for the purpose of enumeration, and 55 Clathro- 

 cystis were found, then the total number in the catch of 

 50 ccm. would be 55 x 10 x 50, that is, 27,500 individuals. 



When it becomes necessary to count the different 

 species of a genus, or a number of different species of 

 plants or animals from the same plate, it is found expe- 

 dient to adopt a labour-saving device suggested byHensen. 

 A box, divided into as many different compartments as 

 there are species to be counted, and having each compart- 

 ment labelled with the names of the species, is used for 

 this purpose. The plate containing the various organisms 

 is now brought under observation, and as each individual 

 of a given species comes into view, the observer drops a 

 token of some kind (button or coin) into the compartment 

 labelled with the name of the species. In this way a 

 plate with fifty different species can be easily investigated, 

 The method is, of course, more practicable for the rarer 

 individuals. The first organisms to be enumerated are 

 usually the Diatomacese, as they are at certain seasons the 

 most numerous constituents of the plankton. Firstly a 

 strongly diluted portion of the catch is taken, and about 

 one-tenth of a ccm. of the fluid is placed on the glass plate. 



