456 ESSENTIALS OF ZOOLOGY 



intestine with the feces. It continues to live and crawl slowly 

 through the feces like an independent animal for a few hours, then 

 usually dies, but the embryos within the proglottid remain alive 

 much longer. If gravid proglottids or separate eggs are eaten by 

 a hog, the six-hooked hexacanth larva hatches in the hog's intestine, 

 bores through the intestinal wall, and migrates to other parts of 

 the body where it changes into a bladder- worm or cysticercus, which 

 is a saclike larva with an inverted scolex. The cysticerci remain 

 in the flesh of the hog until the pork is eaten by man; under the 

 influence of human digestive juices the cysticerci become everted 

 so that the scolex is on the outside of the saclike part, then the 

 scolex attaches itself to the wall of the human intestine, proglottids 

 begin to bud off from the neck, and an adult tapeworm is formed 

 within a few weeks. 



Taenia saginata, the beef tapeworm, has a similar life history, but 

 uses cattle instead of hogs as intermediate hosts. Taenia serrata, 

 a common dog tapeworm often used as a laboratory specimen, is 

 very similar to the two human species in structure and life cycle. 



Other important cestodes are Hymenolepis nana of man and mice; 

 the broad fish tapeworm, Biphyllohothrium latum, which man gets 

 by eating raw or poorly cooked fish ; the peculiar Echinococcus granu- 

 losus, adult in dogs, whose cysticercus stages are dangerous parasites 

 of man (man being the intermediate host in this case) ; and many 

 tapeworms of domestic animals, such as Dipylidium caninum of dogs, 

 Thysanosoma and Moniezia of sheep, goats, and cattle, and many 

 others. 



References 



Chandler, A. C. : Introduction to Human Parasitology, New York, 1940, John 



Wiley & Sons, Inc. 

 Ewing, H. E. : Manual of External Parasites, Springfield, 111., 1929, C. C. Thomas. 

 Faust, E. C. : Human Helminthology, 1929, Lea & Febiger. 

 Riley, W. A., and Christensen, R. B. : Guide to the Study of Animal Parasites, 



New York, 1930, McGraw-Hill Book Company. 



