PARASITIC DISEASES OF ANIMALS. 
739 
Flukes .—Liver rot among sheep is a well-known affection, 
which is always more or less prevalent in wet lands, bogs, or 
marshes; so clearly is this fact recognised, that certain 
plants, which only grow in those situations, have been looked 
upon as the cause of the disease. Liver rot also occurs in 
wet seasons on pastures which are not known as “ rotting 
lands,” but, on the contrary, are perfectly sound. It is 
clearly established that liver rot is due to the presence, in 
considerable numbers, of the liver fluke (Distoma hepati- 
cum ) ; and, reasoning from what has been observed as to the 
development of other varieties of the fluke, the life-history 
of the liver fluke of the sheep is briefly as follows : 
Mature parasites (flukes) are commonly present in the 
liver ducts of sheep, even when no symptoms of rot are 
apparent; indeed, healthy, well-conditioned animals are 
found to harbour them. The flukes, like parasites in general, 
are amazingly prolific, and during their residence in the gall 
ducts deposit thousands, perhaps millions, of eggs, which 
are carried along with the bile into the intestinal canal, and 
thence are expelled and fall on the land along with the 
manure. 
It will be observed that the eggs of the fluke are not 
hatched in the organisms of the infested sheep. If they 
w 7 ere, the animals must in a short time be overrun with 
them; but before the eggs advance a step in development 
they must be placed under totally new conditions. 
From investigations which were made by Steenstrup, 
Kuchenmeister, Siebold, and others, it appears that the 
eggs of the fluke, if they fall on a moist surface or into 
stagnant pools, give exit to ciliated embryos, which move 
about freely in the water, and after a time attach themselves 
to the bodies or shells of freshwater molluscs. In this situa¬ 
tion the parasite becomes encysted, and advances a stage in 
structure. 
The essential condition for the perpetuation of the fluke race 
is moisture. Falling on a dry soil, or being exposed to the sun 
of a hot dry summer, the eggs shrivel up and die; but 
finding a favorable situation, they quickly become fruitful, and 
furnish the larval forms of new generations of the parasite, 
ready to occupy the organs of warm-blooded animals which 
may feed on the pastures where they abound. 
Rotting lands, it is easy to understand, keep up the 
number of flukes in the form or stage of development which 
fits them for a new 7 residence, because sheep feeding on those 
lands, if not already infested, soon become so, and, by ex¬ 
pelling mature eggs from their intestines into the pools and 
