Methods in Plankton Investigations. 21 



The enumerative or counting method involves a recogni- 

 tion of all of the different organisms composing the plank- 

 ton and the enumeration of the individuals of each spe- 

 cies present in a part or the whole of the catch. The number 

 present under a square meter of surface or in a cubic meter 

 of water can then be computed. This work is the basis of the 

 discussion of the seasonal range, local distribution, and 

 interrelations of the components of the plankton. The 

 method of counting at present employed by us is that described 

 by Rafter ('92) as a part of the Sedgwick-Eafter method of 

 microscopical examination of potable waters. This method 

 was employed by Professor J. I. Peck ('96), and I am indebted 

 to him for many kind suggestions on its use. The apparatus 

 consists of a brass cell, 20x50 mm. and 1 mm. in depth, 

 cemented upon a glass slide, a 1-cubic-cm. pipette, a mechan- 

 ical stage, and an area-stop for the eyepiece. After the 

 plankton to be examined is diluted to the desired degree and 

 thoroughly stirred, one cubic centimeter is transferred with the 

 pipette to the cell, in which one cubic millimeter underlies each 

 square millimeter of the cover-glass. By means of the 

 mechanical stage any desired cubic millimeter of the cell can 

 l)e placed in the center of the field. The area-stop is a circle 

 of black paper to be placed in the eyepiece, which cuts off 

 all the field except that visible through a square opening at its 

 center. This opening should be of such a size that with the 

 objective employed for the counting work exactly one square 

 millimeter of the cell is subject to inspection. 



Ordinarily the counting of from ten to twenty squares 

 suffices for a fair test of the occurrence of organisms in plank- 

 ton ; but in the work upon the richly diversified plankton at 

 Havana we have found it necessary to increase the number to 

 fifty or even one hundred for the commoner and smaller species, 

 while for the larger and the rarer forms a great part or even the 

 whole of the catch must be examined. 



Ukbana, Illinois, November 23, 1896. 



