72 



worms," described by him, and which occur in great numbers in 

 the interior of snails, especially Limnseus stagnalis. It is, con- 

 sequently, in these yellow worms, which are about two lines 

 long, that the cercarise (which are the larvae of the actual fluke) 

 are developed (4 a, plate 11.) Cercaria armata appears the 

 nearest species I have found. There are, however, some points 

 about these free swimming larvae and Cercaria which appear to 

 ms still involved in obscurity. 



In Dr. Cobbold's instructive work on the Entozoa, he thus 

 summarises the known changes and reproduction of the Fasciola 

 hepatica, or liver fluke : — 



1. The Fasciola hepatica, or sexually mature liver fluke, is 

 especially prevalent in sheep during the spring of the year, at 

 which time it constantly escapes from the alimentary canal of 

 the host, and is thus transferred to open pasture grounds. 



2. It has been shown by dissection that the liver of a single 

 sheep may, at any time, harbour several hundred specimens of 

 the fluke, and it is certain that every mature entozoon will con- 

 tain many thousand of minute eggs. 



3. The escaped flukes do not exhibit powers of locomotion 

 sufficient to prove them capable of undertaking an extended 

 migration, but their movements may subserve the purpose of 

 concealing them within the grass.* 



4. The ova developed from the fluke eventually find their way 

 into ponds and ditches. The free eggs at the time of maturity 

 contained ciliated embryos capable of active progression. 



5. The ciliated embryo, after a larger or shorter period of 

 activity, loses its ciliated covering and becomes comparatively 

 inert. It alters its form, and probably soon after gains access to 

 the body of a fresh-water mollusc. 



6. Once within the viscera of this intermediate host, the 

 non-ciliated larva are called nurses, or spowcysts. 



7. The nurse progeny, when fully developed, constitute the 

 well-known cercariae. 



It is not certain whether the cercarios are taken into the 

 bodies of quadrupeds when the latter are drinking water or 

 eating solid food, but it is probable that they are positively 

 transferred in either way. 



From the digestive organs of sheep or cattle, the cercariae 

 make their way to the liver and become encysted. The pupa 

 thus encysted for many weeks, or even months, attains a higher 

 organization, at last becoming converted into the sexually 

 mature fasciola hepatica. Remarkable periodic outbreaks of 

 liver rot are recorded by Simonds as occurring in England in 



* All the living specimens of the sheep fluke died immediately after 

 removal from the living animal, and I could keep none of them alive in water, 

 they seem to require the warmth of the living animal. —G.D. 



