Page:Popular Science Monthly Volume 23.djvu/765

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from the time the encysted cercaria is swallowed by the sheep, before the fluke is fully grown and begins to lay eggs. The flukes do not always pass from the sheep in summer-time, as was once thought by leading veterinarians. There is no time of year when sheep-livers

PSM V23 D765 Free cercaria in water and cysts attached to the grass.jpg
Fig. 10. Fig. 11.

containing flukes can not be obtained. Cases are known where the flukes have been known to live for more than a year.

The summary of the life-history of the liver-fluke is given as follows: "The adult fluke in the liver of the sheep produces enormous numbers of eggs, which are distributed with the droppings of the sheep. If these eggs have moisture and a suitable degree of warmth, they continue to live, and in each is formed an embryo. The embryo leaves the egg and swims in search of the particular snail (Limnæus truncatulus) within which its future life and growth take place. The embryo bores into the snail, and then grows into the form which is called a sporocyst. The sporocyst gives rise to the second generation. This generation is known as the redia. The rediæ in turn produce the third generation, which has the form of a tadpole, and is called cercaria. The cercaria? quit the snail and inclose themselves in envelopes or cysts, which are attached to the grass. When the grass to which the cysts adhere is eaten by sheep or other suitable hosts, the young liver-fluke comes out of the cyst and takes up its abode in the liver of its host, and the fatal circle is thus completed."

The fluke-disease alternates between a kind of snail and the sheep. One sheep can not contract the "rot" directly from another member of the flock, and one snail can not take it from its neighbor. The sheep drop the eggs, and hatching, the embryos, find the snails, and