DEVELOPMENT OF HAIR-WORMS. 131 
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 larve. The vessel in which 
M. Villot put his Gordius eggs also contained the larve of 
Tanapus, Corethra, and Chironomus, small gnat-like flies. 
He found that each of these larvee contained numerous cysts 
with larve of Gordius. He then removed the larve 
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. This 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-larve. A minnow, for 
example, swallowing one of the aquatic gnat-larve, 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. 89, @). 
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 
feces into the water. On contact with the water great 
