, 104 PEACTICAL PARASITOLOGY 



glycerine treatment ; or, after clearing with creosote, they may be 

 mounted in Canada balsam. 



(b) PREPARATION OF SECTIONS. 



The best fixing mixture for flatworms is a cold saturated watery 

 solution of mercuric chloride, which as a general rule (and this 

 applies particularly to Trematodes and Cestodes fresh from the host), 

 should be used warm after previous boiling. Small parasites 

 (especially those from warm-blooded animals) which cease to move 

 at room temperature, should be put into flat watch-glasses with a 

 little normal saline and, as soon as they are straightened out, hot 

 solution of mercuric chloride should be poured over them. They 

 become fixed almost immediately, and in the course of a few minutes 

 the fluid may be withdrawn by means of a pipette. The specimens 

 should be washed out with a 0*6 per cent, normal saline, to which 

 it is as well to add a few drops of tincture of iodine. They are 

 next put through the alcohol stages, and this will take one to two 

 days. They are stored in 95 per cent, alcohol. 



Large specimens, or those which need straightening, should be 

 arranged with the finger or with a paint-brush before pouring the 

 hot solution of mercuric chloride over them. Large Cestodes, if 

 stretched out upon a plate of suitable length, may be readily killed 

 in this manner. They may also be arranged in zigzags on the glass 

 slide, and if the worm is straightened out with the fingers immediately 

 after immersion in the fixing mixture, the contractions will disappear. 

 This manipulation requires the greatest care and it can be performed 

 only while the tissues are still alive. In the case of thick muscular 

 worms, such as Tcenia crassicollis of the cat, several minutes are 

 required before death ensues. This is because the thin layer of 

 fixing fluid becomes chilled by contact with the cold glass slide, and 

 its action is, in consequence, retarded. 



There is yet another method of managing large worms. The 

 entire length of the worm ma} 7 " be lifted in the fingers by its hinder 

 end and then gradually lowered, head first, on to a warm glass plate 

 containing fixing fluid. The worm will at first contract, but, if the 

 temperature is not too high, it will generally straighten out of its 

 own accord as it dies. 



Cestodes which have attained a length of several yards should be 

 cut into lengths and each length fixed separately. 



In place of mercuric chloride, the following fixing reagents may 

 be employed : -J- to 1 per cent, chromic acid solution, or acetic acid 

 solution of chromium (chromic acid 2 to 2 parts, acetic acid 1 part, 

 water 3.000 parts) ; or, acetic acid solution of chromium and osmium 



