306 



DISEASES OF THE LtVElt. 



and pass on to the pastures, where they continue then- Hfe cycle, thanks 

 to moisture and the presence of stagnant water. The embryos, having 

 escaped from the egg, enter the bodies of the snails found in or near 

 stagnant water {Limnaa truncatida), become converted into sporocysts, 

 and afterwards into rediaj and cercariae. The cercarise become encysted 

 on the lower surface of blades of grass in damp pastures, whence they 

 are transferred to the animals' stomachs along with the grass itself. 



As the LivDuea truncatida lives not only in marshy regions, but also 

 in all damp situations, the embryos of distomata are distributed over 

 enormous areas, and the disease itself is 

 equally widespread. The embryo, after in- 

 gestion, is set at libert}^, and passes from the 

 intestine into the innermost recesses of the 

 liver, being guided up the bile ducts by 

 the current of bile. At this point it attaches 

 itself to the wall of the bile duct, passes 

 through its various stages of evolution, and 

 attains the adult form. It then begins lay- 

 ing eggs, and thus starts a new evolutionary 

 cycle. 



The life cycle of Distoma lanceolatnm is 

 not yet known, and this variet}^ moreover, 

 is less widely distributed than the Distoma 

 liepaticum. 



The bile ducts are more easily pene- 

 trated by the distoma in young animals, a 

 fact which explains why calves and lambs 

 are particularly affected. Adults present a 

 less favourable nidus, a fact which renders 

 them less easily infected, but does not en- 

 tirely prevent the parasites from attacking 

 them. Old animals, although unable to resist entirely, seldom harbour 

 many of the parasites. 



Wet years appear to favour the extension and propagation of dis- 

 tomatosis in an extraordinary fashion, a fact which is easily understood, 

 if we regard the phases of evolution of the parasite. The autumn appears 

 particularly favourable to the infection of herds. This is explained by 

 the fact that, during the summer, the dryness of the fields entirely pre- 

 vents the development of such eggs as may be distributed over them ; 

 whilst wet periods during the autumn favour this development. 



On the other hand, the grass becomes eaten down in autunni, so that 

 the animals gather it almost level with the ground. As the cercariae 

 attach themselves to the lowest leaves they are then ingested in much 



Fig. 162. — Limncca trunca- 

 tula. Natural size and mag- 

 nified. (After Railliet.) 



