356 



EVOLUTION AND ANIMAL LIFE 



and here each worm coils itself up in a muscle fiber and becomes 

 inclosed in a spindle-shaped cyst or cell (Fig. 213)/ A single 

 muscle may be infested by hundreds of thousands of these 

 minute worms. It has been estimated that fully one hundred 

 miUion encysted worms may exist in the muscles of a '^ trichin- 

 ized'^ human body. The muscles undergo more or less de- 

 generation, 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 mammal, as the 

 flesh of the pig by man, or the flesh 

 of man by a pig or rat. The Trichince 

 in the alimentary canal of the new 

 host develop into active adult worms 

 and produce new young. 



In the Yellow^stone Lake the trout 

 are infested by the larvse or young of 

 a roundworm {Bothriocephalus cordi- 

 ceps) which reach a length of twenty 

 inches, and which are often found 

 stitched, as it were, through the vis- 

 cera 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 con- 

 taining eggs escape from the ali- 

 mentary canal with the excreta. These 

 portions of worms are eaten by tJie 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 roundworms 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 and the worm de- 



FiG. 213. — Trichina spiralis, 

 the terrible parasite of pork: 

 a, Male; 6, cyst; c, female. 



