THE SHEEP LIVER FLUKE 57 



habits the bile ducts of such herbivorous animals as sheep, 

 goats, deer and cattle. It is a cosmopolitan form, being found 

 in North and South America, Europe, Asia, Australia and Africa. 

 In many countries sheep raising is considerably hampered by 

 liver rot caused by the presence of these parasites. The species 

 undergoes a very characteristic complex metamorphosis, part of 

 which is passed in the body of a snail of the genus Limnaeus, 

 and part in the herbivorous host. 



Life history. The eggs are deposited by the hermaphroditic 

 adult liver fluke in the bile ducts of the host, thence are washed 

 out through these passages into the intestine and pass out of the 

 host with the faeces. If the ova chance to reach water, the 

 ciliated embryos, called miracidia, emerge. The miracidium 

 soon penetrates the intermediary host, usually a small snail 

 (Limnseus) entering through the pulmonary passage. Within 

 the snail the miracidium transforms into the sporocyst, in which 

 a number of bodies are formed, each of which develops into the 

 next stage, the redia. Thus one egg may give rise to a number 

 of individuals, through this sporulating process. The redia 

 next develop into the tailed cercaria, in which condition the 

 intermediary host is abandoned. The cercaria swim about in 

 the water, finally losing the tail, and becoming encysted on a 

 grass blade or other plant, which is eaten by the sheep or other 

 herbivorous animal, resulting in infestation. In some species 

 the cercaria bore into a second intermediary host, in which they 

 become encysted and are swallowed by the animal together with 

 this second intermediary host. Once within the alimentary 

 canal of the final host the young flukes find their way to the bile 

 ducts and reach sexual maturity. 



E. Structure. With several specimens of the adult liver fluke 

 before you, one of which should be stained and mounted in 

 balsam, note the following characteristics: 



a. Form, is leaf-like and flattened. 



b. Suckers, with a hand lens note a terminal oral sucker, and 

 a -ventral sucker, whence the name Distomum. 



