its surface, and the flow of water through it, though continuous, is
slow. The tributary streamlet meanders through a succession of pasture
meadows, in one of which the beds themselves are situated, and here
throughout most of the year the fleecy victims of the human carnivore
carry on the industry of converting grass into mutton. Now it happened
some years ago that the sheep frequenting these pastures became affected
with the disease known as 'liver-rot'; and here we must make a short
digression into the domain of pathology.

"'Liver-rot' is a disease of quite romantic antecedents. Its cause is a
small, flat worm--the liver-fluke--which infests the liver and
bile-ducts of the affected sheep.

"Now how does the worm get into the sheep's liver? That is where the
romance comes in. Let us see.

"The cycle of transformations begins with the deposit of the eggs of the
fluke in some shallow stream or ditch running through pasture lands. Now
each egg has a sort of lid, which presently opens and lets out a
minute, hairy creature who swims away in search of a particular kind of
water-snail--the kind called by naturalists _Limnaea truncatula_. If he
finds a snail, he bores his way into its flesh and soon begins to grow
and wax fat. Then he brings forth a family--of tiny worms quite unlike
himself, little creatures called _rediae_, which soon give birth to
families of young _rediae_. So they may go on for several generations,
but at last there comes a generation of _rediae_ which, instead of
giving birth to fresh _rediae_, produce families of totally different
offspring; big-headed, long-tailed creatures like miniature tadpoles,
called by the learned _cercariae_. The _cercariae_ soon wriggle their
way out of the body of the snail, and then complications arise: for it