Chap. 25 FLATWORMS VANGUARD OF THE HIGHER ANIMALS 517 



vessels and are distributed through the body. Parasites in general not only 

 have their own hosts but their particular niches in the host to which they are 

 chemically and physically adjusted. So it is with young tapeworms. Their par- 

 ticular niche is the subcutitneous tissue and muscle, usually voluntary muscle 

 such as that in the shoulders and back — ham and spare rib. In these tissues, 

 they become encysted and begin their waiting period. 



Within 60 to 70 days the encysted embryos have metamorphosed into 

 bladder worms (about 5 mm. long and 8 mm. broad), the cysticercus stage, 

 often confusingly called Cysticercus cellulosae as if they were a separate species 

 as they were first thought to be. Bladder worms are capable of growth into 

 adult worms if they are freed from their enclosure in the muscle and reach 

 the human intestine (Fig. 25.13). This is the point at which eating infected and 

 inadequately cooked pork is a favor to tapeworms. In the intestine, the worm 

 becomes mature in five to ten weeks but it may live there for several years 

 continuing to produce and cast off proglottids as well as millions of fertilized 

 eggs free in the intestinal contents. 



Larval tapeworms may make their way out of the human intestine and be- 

 come encysted in the muscle of the same person. They remain there a long 

 time and are ultimately absorbed. Cannibalism would be their only gate to 

 freedom. 



Fish Tapeworm 



The broad or fish tapeworm, Diphyllobothrium latum, is common in per- 

 sons living in the Baltic countries, northern Wisconsin, Minnesota, Michigan, 

 and regions of Canada bordering these states (Fig. 25.15). The adults live 

 in the human intestine. In order to progress further, the developing eggs must 

 reach fresh water, where the larvae, then free-swimmers, are eaten by various 



IN MAN 



larvae in 

 raw fish eaten 

 by man 



adult worm develops 

 in human intesfine 



copepod 

 eaten 

 by fish 



egg 



swimming embryo 



IN MUSCLES"^OF FISH 



IN COPEPOD 



Fig. 25.15. Life cycle of the fish tapeworm. Diphyllobothrium latum. Adult 

 fish much reduced; larval stages variously enlarged. (Courtesy, Storer: General 

 Zoology, ed. 2. New York, McGraw-Hill Book Co., 1951.) 



