226 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES 



liigh. A great number of other animals have been experimentally 

 infected. 



Nearly all of the taenioid tapeworms have a rather simple life 

 cycle. In the case of the beef and pork tapeworms, for example, the 

 infective stage occurs in the flesh of the host in a milky white cyst. 

 When this larval tapeworm, or cysticerus, consisting of an inverted 

 head or scolex and its outer cyst wall, is ingested by man, the head 



if imppcfparly 

 cooked. , pork with 

 "^ cysLs In musda. 

 f ibens may develop 

 vhan eaten 'by 



encysts in pork 

 if hog' is "host- 



vulva 



oviduct 



tacome adults 



in smoll intestine, 



vithin afev dajs- 



■females burro*^ into 



U5C mucosa , depos-it 



over 10,000 larvae^ 



intestine 

 of hog- 



encyst in human 

 Yntcscl© if man 

 is host 



larvae enter blooeC 

 stream , are carried 

 to vol untoj'y muscles 



•muscles ^ , . , , 



of xnan which larvae penetrate and then 



The life cycle of Trichinella. 



becomes everted, and then attached to the intestinal wall, where the 

 worm starts budding segments or proglottids and soon reaches sexual 

 maturity. Proglottids of Taenia saginata, or proglottids together 

 with free eggs in the case of T. solium, are passed with the feces and, 

 when eaten by the proper intermediate host, develop into cysticerci. 

 Cattle, buffalo, giraffes, and llamas may harbor the larval form of the 

 beef tapeworm, while the hog, camel, monkey, dog, and man are the 

 only known hosts for the pork tapeworm. The chief difference 

 between the cycle of these two parasites centers around the possibility 

 of auto-infection in the case of the latter. This occurs by ingesting 

 the eggs destined for the outside, which hatch in the intestines, 



