72 FRESH-WATER BIOLOGY 



formed is covered with wire cloth and the apex of the pyramid is 

 filled with lead to the depth of an inch and a half. An eye at each 

 angle serves to attach a cord. This dredge is very effective in 

 collecting bottom mollusca. 



C. Open Water Collecting Qualitative Methods 



i. The townet is the simplest device for collecting the plankton 

 organisms which abound in the open water. The following direc- 

 tions for making a townet are modified from 

 Kofoid (1898). The completed net (Fig. 15) 

 consists of a conical bag of India linen or better 

 of silk bolting cloth hung from a ring which is sup- 

 ported by three cords. The bolting cloth may 

 be number 12, 16 or 20 and is to be had from 

 dealers in mill supplies, but discarded cloth may 

 often be obtained from flour mills. Before cut- 

 ting the cloth should be shrunk by boiling in 

 soapsuds and then pressed. A pattern for cutting 

 two nets twelve inches in diameter from a yard 



FIG. 15. Simple townet , f , . , ., r ,, 1,1- /-&' \ 



without bucket. A, wire o f forty-inch wide bolting cloth is given (Fig. 10). 



rings for draw lines, dl, * *-f ^ *-* 



The c ^ ot ^ nas ^ een doubled length wise (with the 

 u Modiffrom warp) and is shown with the fold at the right and 



Kofoid.) the two f ree edges at the lef t with a ra( iius equal 



to the length of the cloth two arcs are struck from the points a 

 and b as centers. These arcs, which form the tops of the completed 

 nets, must be equal in length to one-half the circumference of the 

 net hoop and these lengths may be most readily determined by 

 rolling the net ring along the arcs. An additional width must be 

 allowed on the piece d, since this is in two parts and has two 

 seams. This is accomplished by cutting the two pieces apart 

 along the line ab a quarter of an inch to the right of the diagonal. 

 The pieces are then formed into cones and closed by a French 

 seam along the side and by the seam across the apex. The top 

 of the net is finished by sewing on a band made of a doubled 

 strip of butcher's linen, cut bias and provided with a heavy cord 

 sewed into its upper margin. The net is attached to the ring 

 by over-cast stitches of heavy thread. The ring r (Fig. 15) of 



