Parasites of the Liver. 349 



the fluke is present in these three countries. Neumann suggests 

 that the I/imnoea humilis may be the host in North America and 

 lyimncea visitor in Buenos Ayres, but this has not been demon- 

 strated. It is a remarkable coincidence that the distoma hepatica 

 and Ivimnoea truncatula are both practically unknown in most parts 

 of North America. I^euckart obtained encystment of the embryo 

 in the Ivimnoea peregra but it failed to mature into cercaria. Ex- 

 periments with a number of other snails have also failed. 



On becoming encysted the embryo, sheds its layer of ciliated 

 cells, contracts into an ovoid or rounded mass, and develops 

 within it round clusters of cells. It is now a sporocyst. The cell 

 clusters, at first round, become ovoid, then oblong, acquire a deli- 

 cate cuticle, and at one end an opening which leads into a simple 

 blind digestive sac. Behind the pharynx there is a circular ring, 

 and near the posterior extremity two short obtuse processes. This 

 new organism is called a redia (sometimes a nurse from its devel- 

 oping cercaria within it). The sporocyst reaches maturity, and a 

 size of 0.5 mm. to 0.7 mm. in 15 days in summer or 30 days in 

 autumn. It may contain but one redia but usually it encloses 

 two large ones and four to six small ones. 



A redia having attained the requisite development bursts 

 through the walls of the cyst, which at once closes up and goes 

 on hatching the younger ones. The released redia is very active in 

 boring through the tissues of the snail and mostly seeks a home 

 in the liver on which it feeds and tends to destroy the snail in 

 three weeks (Thomas J. The redia grows rapidly to 1.5 mm., 

 becomes elongated, cylindroid, acquires a mouth and digestive 

 canal, and like the sporocyst develops within it clusters of 

 germinative cells which develop into daughter redice in summer 

 or cercaria in winter. 



The daughter redia has a similar organisation to its parent, and 

 like it may develop within it a second crop of daughter redics or, 

 in other cases, cercaria. As there may be in a vigorous snail as 

 many as three or four successive generations of daughter redia, 

 and as each redia develops as many as 15 or 20 in a generation it 

 follows that 1000 cercaria (Thomas) or even 100 times as many 

 might possibly come from a single embryo fluke or sporocyst. 



The cercaria developed in the redia or daughter redia to the 

 number of 15 or 20 are ovoid, show an oral and a ventral sucker. 



