PARASITOLOGY. 163 



State. It loses its layer of ciliated epithelial cells and 

 the small mass of round cells forming the interior of 

 the body rapidly grow ; it soon develops into the Spo= 

 rocyst. From the cells in the sporocyst are formed 

 five to eight redia ; each redia in turn develops 

 from fifteen to twenty cercaria ; the cercaria is the 

 fully developed larval stage, oval in shape, provided 

 with an oral and ventral sucker, a rudimentary di- 

 gestive tract and a tail. It now leaves the snail and 

 is enabled to move about in the water by means of 

 its tail and crawls upon a blade of grass or an 

 aquatic plant ; it loses its tail at this period and be- 

 comes encysted with a gelatinous substance with 

 which it provides itself ; this is the fully developed 

 larval stage. The cysts are the most numerous on 

 the part of the grass close to the ground ; the larva 

 retains its vitality for a long time if it is in a moist 

 locality; these cysts appear as very small white 

 specks and are eaten by the herbivora grazing upon 

 these infested pastures. The cyst upon reaching the 

 stomach is digested and the larva, set free, soon tinds 

 its way to the bile duct, and thence to the liver. 



A7ihnals Infested, — The ox, sheep, goat, pig, horse, 

 and man. 



Part hiksted. — The liver. 



Conditions Produced. — The worms invade the bile 

 ducts and here develop into the mature worm ; here 

 they live for six months, then die. In autopsy work 

 one's attention is attracted by the enlargement of 

 the large bile duct on the posterior face of the 

 liver; a cut section of this part shows a greatly 

 thickened wall and many worms contained within 

 the duct ; the bile m.ay be somewhat thicker than 



