Page:Popular Science Monthly Volume 23.djvu/763

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while it remains within the infested sheep. The eggs are, however, naturally washed away by the bile into the intestines, and finally pass from the sheep and are distributed with the droppings. If the eggs fall upon wet land, further changes take place during warm weather, and an embryo is formed. Fig. 3 shows a fluke-egg with the embryo fully formed within the shell. The body of the embryo is covered with cilia, by the motion of which the young trematode is propelled through the water. Both of the engravings (Figs. 3 and 4) are highly magnified. In swimming, the broader end is directed forward, and in its center is a projection, used as a boring-tool. The embryo has very simple eye-spots, which render it sensitive to light, and aid it in finding its future home. When the swimming embryo comes in contact with any object, it feels about, and, if not suited to its wants, starts off again. If the object met with is the snail (Limnæus truncatulus), shown in Fig. 5, it at once bores into it. In boring through the shell of the snail, the peg-like projection is extended, and the embryo spins around rapidly by means of the cilia. The natural place for the further growth of the embryo is in or near the lung of the snail, and when once lodged there its eye-spots and cilia disappear, and the body becomes oval in shape. Figure 6 shows the embryo while the changes are taking place. When the changes are completed the animal is called a sporocyst, meaning a sac of germs. The sporocysts live at the expense of the snail, and will, in July weather, reach their full growth, 140 of an inch, in a fortnight. Fig. 7 shows a full-grown

PSM V23 D763 The trematode becomes a sporocyst in a snail.jpg
Fig. 5. Fig. 6. Fig. 7.

sporocyst, or first generation of the liver-fluke. It contains a number of germs, the lower one of which is ready to hatch out. This is the second generation, and is named redia, after Redi, the celebrated anatomist. The young redia, when ready, breaks through the wall of the parent, and the wound thus formed closes up, and the re-