34 STUDIES IN ANIMAL LIFE. 



reeling my attention to these entozoa, I became 

 aware of the fact that they were not true FHarice at 

 all, but belonged to a peculiar family of thread- 

 worms, embracing the genera of Gordius and Mer- 

 mis. 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 be- 

 cause 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 ox. The larvae of both these insects creep forth 

 in order to become chrysalises, and thence to pro- 

 ceed to their higher and perfect condition. I have 

 demonstrated that the perfect, full-grown, but sex- 

 less thread-worms of insects are in like manner 

 moved by their desire to wander out of their pre- 

 vious 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. Thread- 

 worms found in the damp earth, in digging up gar- 

 dens and cutting ditches, have often been brought 

 to me which presented no external distinctions from 

 the thread- worms of insects. This suggested to me 

 that the wandering thread- worms of insects might 



