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The subtleties of parasitology are often almost incredible. There is a remarkable fluke called Leu- cochloridium that lives in the intestine of singing-birds in European countries. The miscroscopic eggs are voided on the meadow and may be eaten by a small snail (Succinea), in whose stomach they hatch. Each larva gives rise to strange branched forms, with ramifications which extend up to the snail's horns, where they pulsate with great rapidity, sometimes twice a second. The swollen and agitated horns are all the more conspicuous because they have become banded with red and green pigment. They have been, so to speak, painted by the parasite. If a Blackcap or some similar perching songster is attracted by the snail on a leaf and pecks off the pulsating horn, the branch of the parasite, which is now full of larvae of another generation, closes up automatically at its base, so that there is no loss of the multitudinous microscopic progeny. Now the investigators tell us that if the Blackcap swallows the horn then and there, nothing happens. The fluke-parasites are digested. But if the bird gives the tidbit to its nestling, which has a weaker digestion, then infection occurs and the extraordinary life-cycle begins again.