WORMS 



213 



to the shore of the pond, and encyst themselves in the grass near 



its border. If this grass is eaten by sheep, the encysted larvje 



(called cercarice) are taken 



into the digestive tract and 



then develop into adult 



hukes. 



Cestodes or Tapeworms. — 

 These parasites infest man 

 and many other vertebrate 

 animals. The tapeworm 

 (Tcenia solium) passes 

 through two stages in its 

 life history, the first within 

 a pig, the second within the 

 intestine of man. The eggs 

 of the worm are taken in 

 with the pig's food. The 

 worm develops within the 

 intestine of the pig, but 

 soon makes its way into the 

 muscles. If man eats pork 

 containing these worms, he 



may become a host for the tapeworm. The animal, which at this 

 stage consists of a round headlike part provided with hooks, fastens 

 itself to the wall of the intestine. This head now buds off a series 

 of segmentlike structures, which are practically bags full of eggs. 

 These structures, called proglottids, break off from time to time, 

 thus allowing the eggs to escape. The proglottids have no separate 

 digestive systems, but the whole body surface, bathed in digested 

 food, absorbs it and is thus enabled to grow rapidly. 



Roundworms. — Still other wormlike creatures called round- 

 worms are of importance to man. Some, as the vinegar eel found 

 in vinegar, or the pinworms parasitic in the lower intestine of 

 man, do little or no harm. The pork worm or Trichina, however, 

 is a parasite which may cause serious injury. It passes through 

 the first part of its existence as a parasite in a pig or other verte- 

 brate (dog, cat, ox, or horse), where it encysts itself in the muscles 



Development of the liver fluke; A, ciliated larva; 

 B, sporocyst, containing new sporocyst (r), and 

 redia(m.); C, redia, containing daughter rediaand 

 tadpolelike cercaria; D, fully developed cercaria. 



