278 Birge—The Crustacea of the Plankton. 
and I have therefore put out the suggestions of the final sec¬ 
tions of each part of my paper, with the hope that they will 
stimulate others to similar attempts and thus lead to an en¬ 
largement of our knowledge and to the correction of whatever 
errors may be present in my conclusions. 
THE COEFFICIENT OF THE DREDGE. 
One of the most difficult and unsatisfactory portions of plank¬ 
ton investigation has been the determination of the coefficient 
of the dredge. It is well known that the net when raised 
through the water offers a certain resistance to the passage of 
the water, so that a part only is filtered by the net, while an¬ 
other fraction is displaced. The determination of the relative 
amounts of water filtered and displaced is the determination of 
the coefficient of the dredge. Many attempts have been made 
to determine this quantity. The most elaborate investigations 
have been made by Hensen (Hensen, ’87, p. 11, and Appendix; 
’95, pp. 67-86). Reighard (’94, p. 57) has also devised and car¬ 
ried out another method of determining the coefficient. Hensen 
has attempted to work out a formula by which the coefficient for 
a net of given cloth and given area could be determined, and 
has finally given the best and easiest method of determining the 
coefficient in lakes abounding in vegetable plankton (’95, p. 92). 
Reighard’s method depends upon mixing with the water a known 
number of particles and determining the relation between those 
caught by the net when drawn through the water and the num¬ 
ber known to be present. This method was entirely inappli¬ 
cable to a net constructed like mine, and it was impossible for 
me to enter upon any elaborate investigation of the coefficients 
of the cloth which I used. I confined myself, therefore, to a de¬ 
termination of the coefficient of my net under the conditions in 
which it was used. In the serial investigations which formed 
the greater and more essential part of my study, the dredge 
was raised through a distance of three meters. The speed was 
approximately one half meter per second, although ordinarily a 
little less, the total time occupied by raising the dredge through 
3 meters, being from 6.5 to 6.75 seconds. In order to ascertain 
the coefficient of the dredge I determined to ascertain the num- 
