CONTKOL OF ANIMAL PARASITES 263 



fed the ripe proglottides from man to calves, and was thus able to dis- 

 cover how man acquires this tapeworm from eating measly beef. The 

 tiny egg hatches in the stomach of the cow, burrows through the wall 

 of the intestine, and in from three to six months has grown to a 

 bladder, or cyst (the cysticercus), the size of a small bean, and is then 

 found in the muscles. After the cysticercus passes through the human 

 stomach, the head everts (pops out like turning a glove-finger), bringing 

 the hooks and suckers to the 

 outside ; these anchor in the Man 



intestine and begin a new life 

 cycle. While the beef tape- 

 worm (Tcenia sayinata) may 

 cause some irritation, and un- 

 doubtedly steals some digested 

 food, it seldom does serious 

 injury. This is due to the 

 simple fact that its eggs can- 

 not hatch and pass into cysti- 

 cerci in the muscles or other 



organs of man. The cysticer- '^^^ ^^^ Man 



cus stage is confined closely to ^^- ~"^li y - 1 cell\ 



cattle, and the adult stage as jy _ ^ * / 



closely to man. The eggs of sev- (cyst cell} 



eral of the other species do, how- V / 



ever, develop cysticerci in man, FlG< 115. L jf e cyc i e of pig tapeworm ; 

 which renders them much more infection from uncooked pork 



dangerous and sometimes fatal. 



The pig tapeworm T&nia solium. This parasite is distributed the 

 world over, wherever the pig is raised and eaten raw or rare. It is found 

 also in the wild boar, sheep, deer, dog, cat, bear, and monkey. The eggs 

 and newly hatched embryos (oncospheres) are microscopic, the latter 

 only 0.02 millimeter in diameter so small that they are easily carried 

 to foods on dirty hands, eaten with polluted vegetables, or even swal- 

 lowed by flies and carried to foods anywhere. These eggs, if swallowed, 

 may find their way to any part of the body muscles, eyes, brain, and 

 even heart, and there become cysticerci. These, too, are large (6-20 milli- 

 meters long by 5~10 millimeters thick), so that even one may prove fatal. 

 In expelling this tapeworm great care must be used to avoid causing nau- 

 sea, for a single ripe proglottis, forced back into the stomach and releas- 

 ing its myriad embryos, would leave little chance for a patient's recovery. 



