ROT IN SHEEP. 
351 
it becomes encased in a kind of shell again, and lies just 
underneath the skin of the snail, and from this state it goes 
on developing within the snail until it forms a distoma. 
So that there is no doubt, whatever may be the origin of 
these cercaria, so far as they are parasitic to snails, we have 
traced the whole history of them. By analogy, we imagine 
that, as we have germs of a similar nature which become 
parasitic from the ova of the true liver fluke, that these pass 
through a series of similar changes, and that in this imbedded 
condition they either remain in the water attached to plants, 
or to smaller creatures inhabiting water, especially slugs and 
snails; and that when these pass into the stomach of the 
sheep they find their proper habitat , and develop into flukes. 
It is important to bear in mind that the last transformation 
takes place, not in the liver, nor in the biliary ducts, but in 
the stomach or intestines of the sheep. You will see pre¬ 
sently what importance there is attaching to this fact, which 
has only very recently come to light. 
Now, if we accept this description as an approximation to 
the truth w 7 ith regard to the development of flukes, we shall 
find that it unravels the whole mystery with reference to rot. 
It shows at once why certain districts are dangerous. It 
explains the occurrence which has been recorded over and 
over again, that out of a hundred sheep, for example, ninety- 
nine having strayed over a common, and one having been 
accidentally prevented from doing so, that subsequently the 
ninety-nine have been attacked with rot, while the one that 
remained behind escaped. I have recently received a letter 
from a well-known agriculturist—Mr. Edward Umbers, 
of Wappenbury—in w 7 hich he has been kind enough to give 
me the particulars of tw 7 o cases of this kind. 
Many years ago some sheep belonging to his father w r ere 
found, subsequently to their purchase, to be affected w ith rot, 
and in tracing out the matter, it w 7 as found that the sheep had 
•been left in what was called a dangerous part of the country 
for a short space of time w r hile the man w 7 ent into a public- 
house close by to get some refreshment. If you look at 
Mr. Youatt’s w r ork vou w ill see numerous cases of this 
description. And what is the explanation of them ? I have 
know 7 n the time w 7 hen I have stood in the lecture-room of our 
own institution and combatted this view 7 , and never could 
become a convert to it till I became more enlightened upon 
the natural history of these entozoa. I now believe that it is 
perfectly possible for sheep to be free from the cause of rot at 
this minute, and to receive it at the next; that is, if they are 
placed under circumstances where they can obtain these 
