PARASITISM AND DEGENERATION 185 
of the intestine. They migrate to the voluntary muscles 
of the hosts, especially those of the limbs and back, and 
here each worm coils itself up in a muscle fiber and be- 
comes inclosed in a spindle-shaped cyst or cell (Fig. 109, 8). 
A single muscle may be infested by hundreds of thousands of 
these minute worms. It has been estimated that fully one 
hundred million encysted worms have existed in thé mus- 
cles of a “trichinized” human body. The muscles undergo 
more or less degeneration, and the death of the host may 
occur. It is necessary, for the further development of the 
worms, that the flesh of the host be eaten by another mam- 
mal, as the flesh of the pig by man, or the flesh of man by 
a pig or rat. The Trichine in the alimentary canal of 
the new host develop into active adult worms and produce 
new young. 
In the Yellowstone Lake the trout are infested by the 
larve or young of a round-worm (Bothriocephalus cordiceps) 
which reaches a length of twenty inches, and which is 
often found stitched, as it were, through the viscera and 
the muscles of the fish. The infested trout ‘become feeble 
and die, or are eaten by the pelicans which fish in this 
lake. In the alimentary canal of the pelican the worms 
become adult, and parts of the worms containing eggs 
escape from the alimentary canal with the excreta. These 
portions of worms are eaten by the trout, and the eggs give 
birth to new worms which develop in the bodies of the 
fish with disastrous effects. It is estimated that for each 
pelican in Yellowstone Lake over five million eggs of the 
parasitic worms are discharged into the lake. 
The young of various carnivorous animals are often 
infested by one of the species of round-worms called “ pup- 
worms” (Uncinaria). Recent investigations show that 
thousands of the young or pup fur-seals are destroyed each 
year by these parasites. The eggs of the worm lie through 
the winter in the sands of the breeding grounds of the fur- 
seal. The young receive them from the fur of the mother 
