PARASITES 



287 



man. In 1900 the plague gained entrance on our western coast. 

 It killed more than 100 persons during the next four years, and has 

 broken out occasionally since. The ground squirrels of California 

 have become infected with the plague, doubtless from the rats 

 which live in their burrows, so now the danger of other outbreaks 

 of the plague will be present until the ground squirrels are exter- 

 minated. Over a million rats were killed in fighting the last plague 

 outbreak in California and efforts are being made in all large cities 

 to eradicate this pest. 



Other Parasitic Animals cause Disease. — Besides parasitic 

 protozoa other forms of animals have been found that cause 

 disease. Chief among these are certain round and flat worms, 

 which live as parasites not only on man but on many animals and 

 plants. A one-sided relationship has thus come into existence 

 where the worm receives its living from the host, as the animal 

 which maintains the parasite is called. Consequently the parasite 

 frequently becomes fastened to its host during adult life and is 

 reduced to a mere bag through which the fluid food prepared by its 

 host is absorbed. Sometimes a complicated life history results 

 from parasitic habits. Such is the life history of the liver fluke, 

 a flat worm which kills sheep (page 228), and of the tapeworm. 



tian eats 

 underdone pork 



f^/ 



Cyst in muscle 

 of pig 



The tapeworm may spend part of its life cycle in a mammal other than man, or in 

 a fish, and be taken with improperly cooked food into the human body, where it may 

 become a parasite. 



Ces'todes or Tapeworms. — ^ These parasites infest man and many 

 other vertebrate animals. One tapeworm {Tcenia solium) passes 

 through two stages in its life history, the first within a pig, the sec- 

 ond within the intestine of man. The developing eggs are passed 

 off with wastes from the intestine of man. The pig, an animal with 

 dirty habits, may take in the tapeworm embryos with its food. 



