70 



FRESH-WATER BIOLOGY 



poured through an ordinary tin funnel into the bags, which are 

 then tied and placed in the preservative. 



An "improved" form of cone dredge has been described by Wol- 

 cott (1901), who has worked out a standard type of holder for cone 

 dredge, dip net, sieve, and scoop. A folding-cone dredge is sold 

 under the name simplex plankton net. Its cone is made of cloth. 



The plankton pump may also be used for collecting free swim- 

 ming forms among aquatic vegetation. 



In making collections along the margin of a pond or stream, or 

 in the puddles of a bog or half-dried ditch, it is advantageous to 

 use a dipper with a cane or short bamboo handle. One may 

 fasten to such a handle a wide-mouth bottle, a dipper with fine 

 metal gauze bottom, a pruning hook or other apparatus for 

 securing samples of the plant or animal Hfe in such places as are 

 somewhat inaccessible. A shallow glass dish or white soup plate 

 is very useful in examining immediately refuse obtained from the 

 margin or bottom of such pools. By some such means the heavier 

 particles of sand and silt may be separated from the collection 

 before it is preserved. 



B. Bottofn Collecting 



The dredge that is commonly used in deep-sea work is of little 

 value in fresh water owing to the relative barrenness of lake bottoms. 

 The larger bottom vegetation may be obtained at any depth by the 

 use of Pieters' grapple already described. For the smaller organ- 

 isms that Hve in the superficial ooze of the bottom, the cone dredge 

 or the townet may be used. A weight heavy enough to bring the 

 line to the bottom is attached to the towline two or three feet in 

 front of the net. The cone dredge when attached to a weighted 

 line may be made to run along the bottom by weighting the screw 

 tip, but in that case it is well to fasten a band of cloth about the 

 base of the wire cone so as to leave only the upper part free. 

 The net, while admitting water through the tip of the wire cone, 

 then gHdes over the bottom without scraping up mud. A townet 

 mounted on runners, as shown here (Fig. 13), has been found 

 very useful by the writer for taking organisms just above soft 

 bottom. From the iron ring which supports the mouth of the 



