228 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 
THE FILTRATION COEFFICIENT OF 
PLANKTON NETS. 
By W. J. Dakin, MSc. 
1851 Hxhibitioner, Zoology, Liverpool University. 
Descriptions of the methods now used in the quanti- 
tative study of the plankton, and some of the results 
gained by the application of such methods both at Port 
Krin and abroad, have already appeared on_ several 
occasions in the Transactions of the Liverpool Biological 
Society and this series of Reports. So far, however, no 
account has been given of the methods adopted to deter- 
mine from what actual volume of water the catch (fished 
by a plankton net) has been filtered. 
This is naturally of fundamental importance, since 
with the various types and sizes of nets now employed the 
volume of water filtered varies considerably, even though 
they be pulled for the same length of time, or with the 
same speed. For this reason, it would be impossible to 
compare the abundance of any form or the amount of 
plankton present at two places if at one a catch was made 
with a Hensen net and at the other with a Nansen (taking 
these two examples), unless the volume or number of the 
specimens making up the catch was reduced in each case 
to a common measure, namely the volume or number in a 
unit quantity of water. In short, we must have for each 
net a coefficient by means of which the relation of the 
catch to the volume of water from which it was abstracted 
can be readily determined. 
The calculation of this coefficient 1s a somewhat 
dificult task, and at best we can only hope for approxi- 
mate results, but, on the other hand, we must have some 
means of comparing catches made at different places and 
with different size and shape of nets. As regards the 
