14 Illinois State Laboratory of Natural History. 



was found that when the water was delivered from a 1-inch 

 hose into the plankton net and the filtrate refiltered in a second 

 net of the same silk (No. 20), a considerable quantity of the 

 more minute forms, Rot if era and Protozoa, were forced through 

 the meshes of the first net. This led to the adoption of the 

 l^-inch discharge hose, and of a net devised hy the writer 

 to reduce the force of the discharge, to protect the silk from 

 direct contact with the current, and to equalize the pressure 

 upon the filtering surface. 



2. The Net (Plate IV., Fig. 8-10).— The net consists of 

 the cover with its accessories and the net proper, the two 

 being so constructed as to he readily separable. From the 

 under side of the conical copper top to which the hose 

 is attached hangs the silk net. When in use the net 

 is supported in a wooden frame, which also serves as a 

 float. The rim fits into the circular central opening and rests 

 upon a projecting ledge of the frame (Fig. 8, fl.) in such a 

 manner that the silk does not come in contact with the wood. 

 The frame is so proportioned that the net projects about 8 cm. 

 above the level of the water. Experience has demonstrated 

 that even when the water is full of silt or the plankton is very 

 abundant this elevation is sufficient to provide for filtration 

 without forcing the water into the net by the pump. In ordi- 

 nary circumstances the water does not rise more than 2-4 

 cm. above the level of the water in which the net is sub- 

 merged. Thus practically the whole straining surface is under 

 water. Two turn buttons (t. b.) hold the net firmly in place 

 so that it cannot be dislodged by the action of the waves when 

 the water is rough. 



The cover (Plate IV., Fig. 8, cov.) is an obtuse 

 cone of sheet copper, 33 cm. in diameter and 20 cm. on the 

 side. The apex bears a curved connector {con.) upon which 

 the H-inch hose can be slipped. The cover is beaded for 

 rigidity, and carries two handles (//.) for lifting the net from 

 the frame. • After the water enters the net two devices are 

 employed to check the force of the discharge. The first is an 

 inner copper cone (i. c), with diameter of 13 cm. and side of 10, 

 placed in the axis of the net immediately below the orifice of 



