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exact knowledge of the numbers of these minute crustacea 
in a given volume of water, over the whole area of the 
great lakes, and at each season of the year; and not until 
such determinations have been made will the fishculturist 
know accurately where his fry may be most advantageously 
planted. 
In April of this year, the Michigan Fish Commission re- 
quested me to make an examination of the waters in which 
the white-fish fry were at that time being planted. As 
the planting operations had already begun, only a short 
time remained in which to make preparation for the work 
contemplated, so that it was not possible to have apparatus 
of precision made for the purpose. 
The only entirely accurate method of determining the 
volume of living forms (7. e., the plankton) contained in a 
given volume of water is by the use of the vertical plank- 
ton-net. This isa conical net of fine silk bolting-cloth 
attached to a metal ring. It is lowered to the bottom and 
then drawn slowly upward to the surface. The net thus 
strains the water in a vertical column, the base of which 
is equal to the size of the opening of the net, while its 
height is equal to the distance through which the net is 
drawn in raising it. Not quite the whole of this column 
of water is thus strained through the net, since the resist- 
ance offered by the meshes of the cloth causes about one- 
tenth of it to pass over the edge of the net. Whatever 
was contained in the column of water strained is found at 
the bottom of the net, and may be preserved in alcohol for 
future examination. After the material has been brought 
to the laboratory, the volume taken at each haul is meas- 
ured by means of graduated glass tubes reading to tenths 
of a centimetre. This volume is that of all the minute 
animals and plants living under an area of lake surface 
equal to that of the opening of the net. From this it is 
possible to compute the volume of minute living forms to 
be found over any given area of water or in the entire 
