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LECTUEES ON BIOLOGY 



preventing the flow of bile these parasites cause a disease, 

 called distomiasis, which gradually brings about the death of 

 the host. In order that the eggs of the liver fluke may develop 

 they must first of all reach the intestinal canal of their host, 

 whence they are liberated with the faeces. But only those 



I 



FIG. 45. LIPE-HISTOEY OF THE LIVER-FLUKE (Fasciold (Distomum) Jiepatica.) 



(1) Ovum ; (2) free- swimming ciliated larva ; (3) sporocyst with young redise 

 (second larval form) ; (4) redia with cercarise (third larval form) ; (5) adult free- 

 swimming cercaria ; (6) adult sexual fluke. 



which chance carries to a ditch or pool will reach maturity. 

 For having arrived in the water there emerges from the egg 

 a larva which by means of its cilia swims about until it happens 

 to meet a certain water-snail. Forthwith it bores its way into 

 the snail, sheds its cilia-dress, and changes in the interior of 



