142 



PEACTICAL PAEASITOLOGY 



hours, as small whitish bodies in the water. Their movements are 

 somewhat rapid and they remain at first in the neighbourhood of 

 their host. They should be lifted out of the water with a pipette 

 and put into a watch-glass or hollow glass slide with a little water. 

 If they are now examined with a low-power lens, the peculiar nature 

 of their movements will seen. The anterior portion of the body is 

 drawn up into a ball (fig. 64), while the tail moves so rapidly as to 

 be scarcely discernible. After a varying period of time, this movement 

 slackens and the animalcule rests, or, with the aid of the suckers at 

 the anterior .part of the body, crawls about for a time at the bottom 



of the vessel before recom- 

 mencing its swimming 

 movements. The struc- 

 ture of the cercariae should 

 be studied in living speci- 

 mens under the micro- 

 scope. This is done by 

 putting them in a little 

 water on to a glass slide, 

 and using supports to pre- 

 vent crushing by the cover- 

 glass. The supports (strips 

 of paper) should be of 

 sufficient thickness to pin 

 the parasite between the 

 cover-glass and slide and 



thus prevent swimming, but not crawling, movements. The cercariae 

 are sufficiently transparent to allow of the organs being clearly seen, 

 and especially is this the case just before they die. Many varieties 

 have a boring organ within the oral sucker, and occasionally, though 

 more rarely, eye-spots are seen. 



Cercarise are easily fixed and coloured ; they should be mounted in 

 balsam. Large numbers should be treated in watch-glasses, single 

 specimens on glass slides, the method being the same in both cases. ' 



The sporocysts and the rediae from which the cercarise are evolved, 

 are obtained by breaking a snail, which is known to be infected, 

 out of the shell, when the sporocysts will be seen as long yellowish 

 bodies underneath the mantle. Sometimes they occur, in the liver or 

 in the sexual glands, and here also their yellow colour renders them 

 easily recognizable. The material should be diluted with snail's blood 

 or with normal saline, which should be spread out upon a glass slide 

 and examined with a low-power lens. There is very little to observe 

 in the sporocysts themselves ; they are quite simple structures, 

 entirely filled with cercariae in various developmental stages. The 



FIG. 64. Cercarise of Echinostomum sp. (from 

 LimncBUs stagnalis). 25 : 1. 



