THE FLATWORMS— PLATYHELMINTHES 151_ 



rare in the United States, but in some sections of the world, such as Tibet, 

 nearly all the people have them due to their habit of broiling large 

 chunks of beef over open fires, which thoroughly sears the outside, but 

 does not penetrate to the center. 



The longest tapeworm of man is the fish tapeworm, Dibothriocephalas 

 latus, that has been measured to a length of 60 feet, with a width of about 

 an inch. There may be as many as 4,000 proglottids in such a worm. 

 The cycle is similar to the ones just studied, with, the fish substituting for 

 the hog or cow ; however, in this case, another animal is also involved in 

 the life cycle. This is a small fresh-water crustacean about a sixteenth 

 of an inch long that eats the tapeworm embryos and then, in turn, is eaten 



Photo by Winchester 



Fig. 10.11. Dwarf tapeworms Echinococcus granulosus, of the dog. These tapeworms 



are only about a quarter of an inch long and do little harm to the dog, but if the eggs 



are eaten by man they cause a huge cyst to develop in the muscles. 



by a fish. Thus, the embryos can find their way into the fish's muscle, 

 later to be devoured by man in the raw or poorly cooked condition. This 

 parasite is found rather abundantly in regions of the Great Lakes which 

 often serve as a means of sewage disposal and also as a source of fresh 

 fish. Many of the people who live near the lakes, especially those of 

 Scandinavian ancestry, like to eat smoked fish, and smoking apparently does 

 not always kill the larvae in the fish. Because there are so many popular 

 vacation spots in these regions, vacationists may become infected, taking 

 the tapeworms home with them, and thus the infection seems to be spread- 

 ing in the United States. 



Man, himself, may sometimes harbor the bladderworm through the in- 

 gestion of the eggs of tapeworms that live in other animals. One of these 



