480 FRESH-WATER BIOLOGY 



be allowed to rest for from two to four minutes in a vessel two to 

 three inches deep the nematodes will have largely settled to the 

 bottom and the supernatant muddy water may be carefully de- 

 canted away. The residue will contain an abundance of nema- 

 todes that may be captured as described above. 



Fresh-water nematodes are so active that it is practically impos- 

 sible to examine them without first anesthetizing or killing them. 

 They may be rendered unconscious by the use of a small amount 

 of chloroform dissolved in water. Ether, chloral hydrate, tobacco 

 smoke and other anesthetics and narcotics are also used in this 

 way. Specimens treated thus are wonderfully transparent, and 

 display to a maximum advantage certain features of the anatomy. 



Permanent preparations may be made by killing and fixing with 

 Flemming's solution or Bouin's solution, washing, and then chang- 

 ing to water containing 5 per cent glycerine arid very slowly evap- 

 orating in a closed, preferably warm, space such that the solution 

 becomes fully concentrated in the course of a few days. The 

 cuticula of some nematodes is so thin and flexible, and at the 

 same time so impervious, that this evaporation process sometimes 

 has to be prolonged to several weeks to prevent crumpling, but 

 many kinds can be successfully treated in two to three days. If 

 the specimens have been blackened by the Flemming's solution, 

 they may be satisfactorily bleached in a few hours or days by 

 adding a few drops of dioxide of hydrogen solution to the glycerine 

 in which they lie after evaporation. They are removed to pure 

 glycerine one by one as they become bleached, and then are 

 mounted in glycerine jelly. Specimens treated in this way make 

 excellent material for examination, but may deteriorate in the 

 course of years. Again, the specimens may be killed by suddenly 

 heating in water on a glass slide until they become motionless, 

 and can then be examined at once, or evaporated as above de- 

 scribed in 5 per cent glycerine. 



The residue from the subsidence and sifting methods, already 

 described, may be added suddenly to an equal volume of boiling- 

 hot concentrated solution of corrosive sublimate and allowed to cool. 

 When the specimens have remained in this solution for twenty- 

 four hours or more they may be picked out one by one on the point 



