OEIGIN OF INTESTINAL WORMS. ^ 9 



the'necessary moisture, they in a short time dry up. But their 

 fate is very different when the infested insects remain under 

 natural conditions ; the thread-worms, as they leave the bodies of 

 their hosts, then fall to the ground, and crawl away into the deeper 

 and moister parts of the soil. Thread-worms found in the damp 

 earth, in digging up garden-beds and cutting ditches in the fields, 

 have often been brought to me, which presented no external 

 distinctions from the thread-worms of insects externally. This 

 suggested to me that the wandering thread -worms of insects 

 might be instinctively necessitated to bury themselves in damp 

 ground, and I therefore instituted a series of experiments with 

 such entozoa (which I procured in numbers from the caterpillars 

 of a moth, Yponomeuta evonymella), by placing the newly emigrated 

 worms in flower-pots filled with damp earth. 1 To my delight, I 

 soon perceived that these worms 2 began to bore with their heads 

 into the earth, and by degrees drew themselves entirely in. For 

 many months (through the whole winter) I kept the earth in the 

 flower-pots moderately moist, and on examining the worms from 

 time to time I found, to my great astonishment, that the sexual 

 apparatus became gradually developed in them, and that, after a 

 time, eggs were formed and were eventually deposited by hundreds 

 in the earth. Towards the conclusion of winter I could succeed 

 in detecting the commencing development of the embryo in these 

 eggs. By the end of spring they were fully formed, and many of 

 them, having by this time left their shells, were to be seen creeping 

 about the earth in the flower-pots, which I still carefully kept 

 damp. 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 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 of Yponomeuta, of half a line in length, 

 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 thread- 

 worms had wintered, and of course satisfying myself that it con- 



1 These experiments and their results have been already published in the ' Entomo- 

 logische Zeitung,' 1848, p. 290. 



2 I have named this species of thread-worm Mennis albicans. 



