82 FRESH-WATER BIOLOGY 



tacea. Smaller organisms escape in variable quantity and the 

 smallest are not caught at all. When the pump is used a known 

 volume of water is drawn from a known source and all of this is 

 filtered, so that the source of error arising from a varying and 

 uncertain net coefficient is eliminated. The leakage error remains 

 uncorrected so long as a net is used to separate the 

 plankton from the water. The plankton obtained 

 by nets whether directly or by aid of the pump 

 may be treated quantitatively by the following 

 methods : 



(a) The volume may be obtained by allowing the 

 alcoholic material to stand for 24 hours in gradu- 

 ated tubes (carbon tubes of the chemist) until it has 

 settled, when the volume may be read off. There 

 is thus obtained in cubic centimeters the volume of 

 one catch and from this may be calculated the vol- 

 ume per cubic meter or under one square meter of 



luted plankton. B, . . . , 



strong glass tube, in- the original water. 



side the tube is a pis- , . . , . t . 



ton made of alternate (b) The approximate weight may be obtained by 



layers of metal i and * 



b rk screws- d m| eth T drying the sample on filter paper and weighing it. 

 ; Tne net weight is obtained by deducting the weight 



of the filter paper, and from this the number of 

 spindfe a and the glass grams of plankton per cubic meter of water or under 



tube is of known vol- r r i i i ^ i 



ume; /, piston-rod one square meter of surface may be calculated. 



with handle; K, cover 11-1 



of vessel. (From AP (c) Chemical analyses may be made of the dried 



stem, after Hensen.) ' ' J 



material and from these the quantities of the 

 various constituents: ash, organic material, silica, etc., may be 

 calculated per cubic meter of water or per square meter of 

 surface. 



(d) The organisms may be counted in the Sedgwick-Rafter cell. 

 The ordinary plankton catch is so concentrated that it is impos- 

 sible to count the organisms in it until it has been diluted. A 

 measured quantity of water added to the plankton for this pur- 

 pose replaces the alcohol or fixing fluid. This water is then agitated 

 to distribute the organisms uniformly through it and a carefully 

 measured sample is taken from it with a specially constructed pipette 

 provided with a piston (Fig. 25). The organisms in the sample are 



