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 a better. Those who dissected these threadworms and submitted them to a careful inspection, could not deny the probability, since it was clear that they contained no trace of sexual organs. But on directing my attention to these entozoa, I became aware of the fact that they were not true Filariæ at all, but belonged to a peculiar family of threadworms, embracing the genera of Gordius and Mermis. Furthermore, I convinced myself that these parasites wander away when full-grown, boring their way from within through any soft place in the body of their host, and creeping out through the opening. These parasites do not emigrate because they are uneasy, or because the caterpillar is sickly; but from that same internal necessity which constrains the horsefly to leave the stomach of the horse where he has been reared, or which moves the gadfly to work its way out through the skin of the oxen. The larvæ of both these insects creep forth in order to become chrysalises, and thence to proceed to their higher and perfect condition. I have demonstrated that the perfect, full-grown, but sexless threadworms of insects are in like manner moved by their desire to wander out of their previous homes, in order to enter upon a new period of their lives, which ends in the development of their sex. As they leave the bodies of their hosts they fall to the ground, and crawl away into the deeper and moister parts of the soil. Threadworms found in the damp earth, in digging up gardens and cutting ditches, have often been brought to me, which presented no external distinctions from the threadworms of insects. This suggested to me that the wandering threadworms of insects might instinctively bury themselves in damp ground, and I therefore instituted a series of experiments by placing the newly-emigrated worms in flower-pots filled with damp earth. To my delight I soon perceived that they began to bore with their heads into the earth, and by degrees drew themselves entirely in. For many months I kept the earth in the flower-pots moderately moist, and on examining the worms from time to time I found they had gradually attained their sex-development, and eggs were deposited in hundreds. Towards the conclusion of winter I could succeed in detecting the commencing development of the embryos in these eggs. By the end of spring they were fully formed, and many of them having left their shells were to be seen creeping about the earth. I now conjectured that these young worms would be impelled by their instincts to pursue a parasitic existence, and to seek out an animal to inhabit and to grow to maturity in; and it seemed not improbable that the brood I had reared would, like their parents, thrive best in the caterpillar. In order, therefore, to induce my young brood to immigrate, I procured a number of very small caterpillars which the first spring sunshine had just called into life. For the purpose of my experiment I filled a watch-glass with damp earth, taking it from amongst the flower-pots where the threadworms had wintered. Upon this I placed several of the young caterpillars. The result was as he expected; the caterpillars were soon bored into by the worms, and served them at once as food and home.