514 



EVOLUTION OF ANIMALS 



Part V 



Most tapeworms are hermaphrodites. Each proglottid contains at least 

 one set of reproductive organs of each sex, and in some species two sets 

 (Fig. 25.13). The eggs may be fertilized by sperm cells from the same 

 proglottid. However, mating proglottids have been observed in tapeworms 

 immediately after being taken from the intestine. The physiology of tape- 

 worms is difficult to investigate since they live only a short time outside the 

 intestine, even in normal salt solutions. The youngest proglottids, behind the 

 neck, constitute a zone of growth. Those farther back have definite organs; in 

 the middle parts of the worm they contain mature reproductive organs; toward 

 the posterior end these organs lose their form and the proglottids become sacs 

 filled with hordes of eggs and embryos. Although lilies and tapeworms are far 

 kin, the stages of development in the chain of proglottids are comparable to a 



^XENIA SOLlUAf 



EGGS 

 passed in 

 human feces 



/ 



swallowed by 

 PIG or MAN 



cystlcercus cellulosae 



may lodge in brain, 



eye, muscle etc. 



produces 

 Cytlicercus cellulosae 

 in men 



CYSTICERGUS CELLULOSAE 



(larva) develops in pig Cysticercus 



cellulosae 



INFECTED MEAT 

 produces 

 ADULT TAENIA SOLIUM 

 in man 



Phyllis Smith, 1944. 



Fig. 25.13. Life cycle of pork tapeworm, Taenia solium. (Courtesy, Hunter 

 and Hunter: College Zoology. Philadelphia, W. B. Saunders Co., 1949.) 



