(Fig. 141), is flat, but swollen in the middle, when it contains a large number of eggs, under which circumstances the edges only are flat. The last are of a dirty brown colour, while the middle of the body is greyish yellow. There is a triangular projection at the front end of the body with a sucker at the apex, and another at hinder end of the triangle. The outer surface of the body is covered with minute backwardly projecting spines, which present no hindrance to the forward creeping of the body in the bile-ducts, but make backward creeping or gliding impossible. [Illustration: FIG. 141.—The Liver Fluke (_Distoma hepaticum_), natural size. ] Eggs ¹⁄₁₈₀ of an inch long, with a red shell possessing a greenish sheen. They can only develop if they get into water, and in wet years the conditions are very favourable on damp meadows, for under such conditions the sheep-dung, with the contained fluke eggs, is liable to fall into pools of water. But many of the eggs can develop in years that are not so wet and on dryer fields, as they may be carried into a ditch by rain or on the feet of the sheep. The larva hatched from the egg of the liver-fluke is ¹⁄₁₂₅ of an inch long, elongated in shape, and swims about freely in the water by means of a covering of cilia (Fig. 142, _a_). It soon finds its way into the lung cavity of a small water snail (_Lymnæus truncatulus_, Fig. 142, _i_), loses its covering of cilia, and becomes broader, even almost spherical. In the free-swimming larva there are already to be found a collection of germinal cells in the hinder part of the body; later on, these cells repeatedly divide and form little heaps, which ultimately become fresh individuals. The larva has, therefore, passed into a second stage, and this is now known as a _sporocyst_ (Fig. 142, _b_, _c_). The stage developed asexually from the heap of germinal cells is distinguished from the sporocyst by a differently shaped body and by the possession of a gut (with mouth, pharynx, and intestine, but no anus), which is absent in the latter. This stage is termed a “_redia_” (Fig. 142, _d_). From five to eight of them are usually developed within one sporocyst. Germinal cells are also present in the redia, and they develop into little heaps of cells from which, while the weather is warm, new rediæ are developed, that live in the lung cavity of the snail, like their parents. When the weather becomes colder, “_cercariæ_” (Fig. 142, _f_) are developed from the germinal cells, some fifteen to twenty of them within a single redia (Fig. 142, _e_). The cercaria has a long tail, by means of which it can propel itself in the water, after leaving the mother redia and the snail. It possesses two suckers, and a forked intestine without anus. After the cercariæ have moved about in the water for some time they fix themselves, by means of their suckers, to various plants growing in the water. The tail being now superfluous is lost, the cercaria becomes