THE ECONOMIC IMPORTANCE OF ANIMALS 227 



of all mammals. Rats are believed to carry bubonic plague, the 

 " Black Death " of the Middle Ages, a disease estimated to have 

 killed 25,000,000 people during the fourteenth century. The rat, 

 like man, is susceptible to plague ; fleas bite the rat and then biting 

 man transmit the disease to him. A determined effort is now being 

 made to exterminate the rat because of its connection with 

 bubonic plague. 



Other Parasitic Animals cause Disease. - - Besides parasitic 

 protozoans other forms of animals have been found that cause 

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

 which have come to live as parasites on man and other animals. 

 A one-sided relationship has thus come into existence where the 

 worm receives its living from the host, as the animal is called on 

 which the parasite lives. Consequently the parasite frequently 

 becomes fastened to its host during adult life and often is reduced 

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

 absorbed. Sometimes a complicated life history has arisen from 

 their parasitic habits. Such is seen in the 

 life history of the liver fluke, a flatworm 

 which kills sheep, and in the tapeworm. 



Cestodes or Tapeworms. - These para- 

 sites 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 developing eggs are 

 passed off with wastes from the intestine 

 of man. The pig, an animal with dirty 

 habits, may take in the worm embryos 

 with its food. The worm develops within 

 the intestine of the pig, but soon makes its 

 way into the muscle or other tissues. It 

 is here known as a bladderworm. If man eats raw or undercooked 

 pork containing these worms, he may become a host for the tape- 

 worm. Thus during its complete life history it has two hosts. 

 Another common tapeworm parasitic on man lives part of its life as 

 an embryo within the muscles of cattle. The adult worm consists 



The life cycle of a tape- 

 worm. (1) The eggs are 

 taken in with filthy food 

 by the pig ; (2) man 

 eats undercooked pork 

 by means of which 

 the bladder worm (3) is 

 transferred to his own 

 intestine (4). 



