spherical, and contracts somewhat within an investment which it secretes (Fig. 142, _g_). When, later on, the pools in which the cercariæ live dry up, the plants which were before submerged are now on dry land, and may be devoured by sheep, together with the encysted cercariæ found upon them. The wall of the cyst is dissolved in the intestine of the sheep, and the contained cercaria gradually becomes a young liver-fluke, which at first is oval and very small (Fig. 142, _h_), while, later on, its anterior third becomes the triangular projection at the front end of the adult fluke, and its posterior two-thirds grow very considerably. Meanwhile the young liver-fluke has travelled from the intestine of the sheep into its bile-ducts. It appears that the fluke is completely grown within a few weeks after its introduction into the body of the sheep. Infection almost always takes place in summer or autumn. The life history of the liver-fluke exemplifies change of host and metagenesis (p. 16). From the egg of a sexual worm a free-swimming larva is developed, which becomes an asexual _sporocyst_ within the snail. This sporocyst produces asexual _rediæ_, and from these, also asexually, are developed _cercariæ_, which leave the snail, pass into water, attach themselves to plants, and reach the intestine of the sheep later on. The cercariæ become flukes by a process of metamorphosis, and are to be regarded as fluke larvæ. [Illustration: FIG. 142.—Liver Fluke: _a_, free-swimming ciliated larva; _b_, young _Sporocyst_, a few days after migration into the snail; _c_, _sporocyst_, containing rediæ in various stages of development; _d_, _redia_, containing balls of germinal cells; _e_, mature _redia_, containing its brood of cercariæ; _f_, _mature cercaria_; _g_, just-encysted _cercaria_; _h_, young fluke, soon after entering sheep; _i_, the water snail, _Lymnæus truncatulus_. _a_, _b_, _c_, _d_ × 200; _e_ × 150; _f_ × 300; _g_ × 150; _h_ × 25; _i_, natural size and × 3½. ] It is easily explained why the “fluke disease,” or “liver rot,” caused by the fluke should appear much oftener in some regions than others. Low-lying pastures which are now and then flooded are best suited for the development of the disease, since the fluke eggs have a better chance of developing upon them. This is because at certain times of the year there are in such spots numerous submerged plants on which _cercariæ_ encyst, and which, later on, when the pools dry up, are nibbled by sheep. It seems that the _cercariæ_ mostly attach themselves to certain kinds of plants, hence the local nature of the disease, which of course also depends on the presence of _Lymnæus truncatulus_ (Fig. 142, _i_).