PARASITIC DISEASES OF THE LIVER. 



293 



cuticular membrane which is provided with a sting at its anterior 

 pole. This ciliated embryo penetrates into the respiratory cavities 

 of some mollusks, and mainly of the Limnœa peregra and L. 

 truncatula (Leuckart). According to Thomas^ it is especially the 

 latter which shelters the embryo. Of about half a centimetre in 

 length, very common, and very resistant, provided with a spiral- 

 shaped shell, living rather on the ground than in the water, it 

 would be more likely for the embryo to penetrate it than the 

 Limnœa peregra. This author believes that the ciliated larvse do 

 not penetrate directly into the respiratory cavities, but that they 

 reach the mollusks by perforating the wall of the body and the 

 intestine, or by ingestion. 



Within fourteen days in summer time ; from three to four weeks 

 in winter, the embryo is transformed into a sporocyst provided with 

 germinative cells which are developed in their length in order to 

 form the redies (cercaria sacs) ; these latter, in undergoing a new 

 metamorphosis, give birth — directly according to Leuckart, indi- 

 rectly according to Thomas, who describes female radies — to the 

 cercaria^ small microscopic beings provided with a tail, which live 

 in liberty in the water. A single distoma egg gives birth to nearly 

 1000 cercaria. 



Leuckart states that the sheep ingests the cercaria either by 

 swallowing the limnsea with the food, or by drinking water in 

 which they are suspended, or through the intermediary of a new 

 host (mollusk, worm, etc.), in whose body the tailed cercaria 

 are said to penetrate, and where they could be preserved for 

 more than two years. Quite recently Thomas has demonstrated 

 in a decisive way that the cercaria leave their host's body (redie 

 or mollusk) in order to swim in the water and to fix themselves 

 upon aquatic plants or damp meadow grass. They lose the tail, 

 become ball-shaped and surrounded with a clammy mass which 

 gives them a cyst of a diameter of two or three millimetres, and of 

 " snowy whiteness ; " they preserve their vitality for several weeks. 

 The sheep ingest them in browsing the grass to which they are 

 adhering. Spinola had already established by experiments that 

 healthy sheep contract distomiasis when they are made to ingest 

 limnsea containing cercaria, picked up in the infested pastures. 



Distribution of distomiasis. The disease has been known 

 from time immemorial. It appears in the ancient Germanic juris- 

 prudence as a vice setting aside a contract of sale." It is de- 



