68 



FRESH-WATER BIOLOGY 



aquatic vegetation may be obtained and searched for smaller 



organisms. Many forms that are detected with difficulty in the 

 field appear in abundance in the water of small 

 dishes containing aquatic plants, when allowed to 

 stand undisturbed for some days (annelids, flat 

 worms, rotifers, hydras, protozoa, etc.). Sub- 

 merged vegetation which grows in deeper water 

 and cannot be reached by other means may be 

 obtained by dragging behind a boat the grapple 

 (Fig. n) described as follows by Pieters (1901): 

 "This is made by passing four or five bent steel 

 wires through a piece of i^-inch pipe and bending 

 back the free ends to make hooks. The pipe was 

 filled with lead to make it heavier and a rope 



fastened through the loops of the wires." 



3. The cone dredge. Many organisms are too small to be readily 



collected with dip nets and many escape when aquatic vegetation 



is gathered. These may be readily obtained 



by this ingenious device of Professor E. A. 



Birge, which may be run among aquatic plants 



where the townet cannot be used. 



The cone dredge (Fig. 12) now used by 



Professor Birge consists of four parts. 



A. The body is a cylinder of sheet copper 

 three inches in diameter and one inch deep, 

 wired at its lower edge to form a lip on the 

 outside. A brass wire bent into a V with an 

 eye at its apex is soldered by its free ends 

 inside the body while its apex extends upward 

 like the bail of a pail. 



B. A cone of brass wire netting of about 

 twenty meshes to the inch fits over the bail. 

 Its base is soldered to the body and its apex 

 to the eye of the bail which projects through 

 it. Two flat loops of wire soldered to the 



outside of the body serve for the attachment of cords. 



C. The net is a conical bag of cheesecloth eighteen to twenty- 



FIG. 12. Cone dredge. At 

 bottom fxmnel-filter for use 

 with the dredge. (Original 

 photograph from appara- 

 tus loaned by Professor 

 Birge.) 



