76 



PLATYHELMINTHES 



as a sporocysty develops germ cells which produce embryos. These 

 embryos grow into rediae which have germ cells and a primitive 

 digestive tube including a mouth, a pharynx and an intestine but no 

 anus. If the weather is warm, the rediae continue to multiply in 

 the lungs of the snail. If it is cold they multiply for a short time 

 and then pass into the liver of the snail where they give rise to tailed 

 forms called cercariae which have an oral and a ventral sucker, and 

 a forked intestine with no anus. The cercariae escape from the 



P'e-nis ~--.^ 



Ovary 



Shell (j/and 



Vas c/eferens 



''rot sucker 



Uterus 



Vas deferens 



- Yolk-duct 



Testis 



Yolk-qlonc/s- 



Fig. 22- -A" adult liver fluke. (After Kerr. Courtesy of Macmillan and Co., Ltd.) 



body of the snail and, after swimming about, settle down on grass, 

 later becoming encysted. When their plant substrate is eaten the 

 cercariae develop in about six weeks into adult liver flukes and travel 

 from the intestine of the sheep to its bile ducts. Thus from one 

 single egg come the larval stages, miracidia, sporocysts, rediae^ 

 cercariae and finally many adult flukes. 



The adult liver fluke is a tailless cere aria with well-developed 



