DEVELOPMENT OF HAIR-WORMS. 173 



When in this stage it incessantly protrudes and retracts 

 its armed head, the spines being directed backward when the 

 head is out. 



In the first period of larval life the worm lives encysted 

 in the bodies of aquatic fly larvae. The vessel in which 

 M. Yillot put his Gordius eggs also contained the larvae of 

 Tanapus, Corethra, and Chironomus, small gnat-like flies. 

 He found that each of these larvae contained numerous cysts 

 with larvae of Gordius. He then removed the larvae 

 from the cysts, placed them on the gnat-larva, and saw the 

 larval hair-worm work its way into the head of the gnat- 

 larva through the softer part of the integument ; during the 

 process the spines on the head, reversing their usual position, 

 enabled the worm to retain its position and penetrate farther 

 in. Then, finding a suitable place, it came to rest, and re- 

 mained immovable. Then the fluids bathing the parts co- 

 agulated and formed a hard, granulated sac. Tin's sac at 

 first closely envelopes the body, then it becomes looser and 

 longer, the worm living in the anterior part, the front end 

 of the sac being probably never closed. In this first larval 

 state the worm is. active. 



In the second larval period the young hair-worm lives mo- 

 tionless and encysted in the mucous layer of the intestines 

 of such small fish as prey on the gnat-larvae. A minnow, for 

 example, swallowing one of the aquatic gnat-larvae, the en- 

 cysted larva becomes set free by the process of digestion in the 

 stomach of the fish ; the cyst dissolving, the young hair- 

 worm itself becomes free in the intestine of its new host. 

 Immediately it begins to bore, aided by the spines around 

 the head, into the mucous membrane lining the inner wall 

 of the intestine of the fish, and there becomes encysted, the 

 worm itself lying motionless in its new home, with its head 

 retracted and the tail rolled in a spiral. The cyst is either 

 spherical or oval. (Fig. 119, G.) 



The return to a free state and an aquatic life occurs in the 

 spring, five or six months after the second encystment. It 

 then bores through its cyst, and passes into the intestinal 

 cavity of the fish, and from thence is carried out with the 

 f^ces into the water. On contact with the water great 



