Chap. 25 FLATWORMS VANGUARD OF THE HIGHER ANIMALS 515 



bud, a perfect flower, and finally a seed pod. Fertilized eggs and early embryos 

 are shed freely into the intestine (Figs. 25.13, 25.14). A ripe proglottid at the 

 end of the body occasionally separates off, carries the pregnant uterus with it, 

 and sets free the eggs wherever it may fall with the waste from the intestine. 

 Proglottids may be eaten by animals of many kinds. They will survive only 

 if they are swallowed by their secondary hosts. In them, they hatch out in the 

 intestines and bore their way into voluntary muscle where they become 

 encysted. Within the cysts they develop into minute bladder-shaped worms, the 

 cysticercus stage. Their lives now depend on having their final host feed upon 

 the secondary one, such as a cat or man eating raw fish or pork. The encysted 

 worm is then freed in the intestine and begins its growth as an adult. 



Physiology and Ecology of Adult Tapeworms. Tapeworms live in the dark, 

 in very special chemical surroundings; shifting hosts is a gamble for life; they 

 endure a long waiting period (cysticercus); and they perish by thousands. This 

 is the price of parasitism which tapeworms pay and yet survive. 



In making its home in the intestines of vertebrates, the adult tapeworm 

 adjusts itself within an elaborate canal that is functioning for another animal. 

 Such canals are in no way modified for the tapeworm. The worm must main- 

 tain its location against the constant shifting of the walls and the pressure 

 of moving food. Yet its only anchor is its minute head (scolex) hanging 

 attached by hooks and suction to the intestinal wall. 



Tapeworms live regardless of the presence or absence of oxygen in their 

 environment. There is very little of it in the intestines. 



The content of the host's intestine, the tapeworm's only source of food, is 

 absorbed through its body wall, but little is known of the process. Glycogen 

 constitutes about 60 per cent of the dry weight of tapeworms, however, and 

 is essentially similar to that stored as a reserve food in the bodies of the 

 majority of animals. 



Pork Tapeworm 



The two common tapeworms of man are the pork tapeworm and beef tape- 

 worm. Taenia solium and T. saginata. The latter is distributed throughout most 

 of the world, especially in parts of Africa and eastern Europe. The rate of in- 

 fection is high among the Mohammedans who merely sear the outside of large 

 chunks of beef. In the United States, less than one per cent of inspected beef 

 has been found infected. The pork tapeworm is also distributed throughout 

 the world, wherever raw or inadequately cooked pork is eaten. Adult pork 

 tapeworms rarely occur among Jews and Mohammedans since they seldom eat 

 pork. 



The beef and pork tapeworms are similar in structure and plan of life his- 

 tory. Man is the only final host of the pork tapeworm and the hog the usual 

 intermediate host. The adult pork tapeworm lives in the human intestine with 



