688 FRESH-WATER BIOLOGY 



to settle before pouring off the water and the Crustacea through the 

 funnel. As little as possible of the weed and debris should be in- 

 cluded in the catch, since it is a wearisome task to search for the 

 various species in a catch which contains great masses of weed with 

 but few Crustacea. It is well to retain a part of the weed in a 

 separate bottle, so that species which go to the bottom may not 

 be overlooked. It is also well to cover the cup with one's hat, or 

 otherwise, while the debris is settling so that species which fly from 

 the light may not go into the weed at the bottom. Numerous hauls 

 of the net should be made and concentrated so as to give abundant 

 material. Considerable experience with collections sent me by 

 students of Cladocera has shown me that the chief faults of the 

 collector are including too much debris in the[^catch and taking 

 too few hauls of the dredge. 



The best preservative I have found to be strong, 95 per cent, 

 alcohol. This keeps the shape of the species as well as or better 

 than anything else. Certain soft-bodied forms with strong muscles 

 may be distorted by any fluid which kills and hardens quickly. 

 Such are Pseudosida, Latona, and Latonopsis, and, to a less degree, 

 Moina and Diaphanosoma. For these also we have nothing better 

 for field use than alcohol. If their forms are to be well preserved 

 they should be killed individually by some poison like osmic acid, 

 or chloral hydrate, and then hardened gradually, according to regu- 

 lar microscopic methods. Formalin distorts many species. 



I have found no better mounting fluid than pure glycerine. I 

 place the animal with a small drop of glycerine in the center of the 

 slide and support the cover glass by three bits of paper thick enough 

 to permit the cover to press slightly on the specimen. The cover 

 glass is put on carefully so that the glycerine occupies its center 

 only. A bit of soft paraffin (melting point about 50° C.) is placed at 

 the edge of the cover glass, and, on warming the sHde, the paraf- 

 fin melts and runs in, sealing the mount. The cover may after- 

 wards be cemented down by any microscopic cement, and this 

 should be done if the preparation is to be kept; but for purposes 

 of study it is well not to do so, since an advantage of the method is 

 the ease with which the specimen can be unmounted for study or 

 dissection. 



