578 Wisconsin Academy of Sciences, Arts, and Letters. 
bag about 4 cm. wide and 10 cm. long. The open end of 
the bag is securely tied with a string and the bag is then 
placed in the preserving liquid. A number placed on the 
bag with water proof ink enables one to keep a record of 
the catch. The net can be used as a tow net or can be 
thrown either from the shore or from a boat. In making 
catches among weeds, it is better to haul the net with a jerk¬ 
ing motion rather than a steady pull, giving a brief time 
between jerks for the net to settle among the weeds. 
THE PLANKTON TRAP. 
A general idea of the plankton trap and the method of 
operating it may be obtained from Plate XXXV, figs. 1 and 2. 
The principle of it is similar to that of the tube trap used by 
Birge 1 for obtaining the coefficient of his plankton nets. 
It consists essentially of a box with upper and lower frame¬ 
works which carry sliding doors that close the ends of the 
box. The two parts of the framework are 33.7 cm. wide 
and 66 cm. long and they are made of brass bars, two side 
and three cross pieces, 6 mm. thick and 19 mm. wide. These 
frames are held together by pieces of brass tubing at the 
four corners. The box part is made of thin sheet brass and 
it is 30 cm. square by 50 cm. long so that it holds 45 liters of 
water. The top and bottom are closed by sliding doors 
which operate in grooves in the side pieces of the framework. 
The lower door has an opening 20 cm. in diameter from which 
a band of brass extends down 3 cm. and to this the net is at¬ 
tached by a clamp which is operated with a screw. Three 
ears on this clamp serve for the attachment of the lines 
which support the bucket of the net. Pieces of brass fastened 
to the back of the sliding doors project outward beyond the 
framework and serve for the attachment of the long coil- 
springs which operate the doors. In front these springs are 
attached to pieces of brass which project from the frame¬ 
work. 
The doors fit tight enough to prevent contamination of 
the catch. A cover of sheet brass prevents organisms from 
entering the net when the bottom door is open. A hole in this 
cover which is protected by a small cloth bag, permits the 
air to escape from the net when the trap is lowered into the 
1 Trans. Wis. Acad. Sci., Arts, and Let., vol. XI, 1898, p. 278. 
