Von Siebold’s description of another form of emigration he has observed in parasites will be read with interest. “For a long time,” he says, “the origin of the threadworm, known as _Filaria insectorum_, that lives in the cavity of the bodies of adult and larval insects, could not be accounted for. Shut up within the abdominal cavity of caterpillars, grasshoppers, beetles, and other insects, these parasites were supposed to originate by spontaneous generation, under the influence of wet weather or from decayed food. Helminthologists (students of parasitic worms) were obliged to content themselves with this explanation, since they were unable to find 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.