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, V). 

 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 the 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 Trichinm 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 

 larvae 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 



