168 



THE RISE OF ANIMAL LIFE 



Fig. 9-9. A stained adult human liver fluke (Opisfhorch/s 

 sinensis). Compare this photograph with the drawing 

 in Fig. 9-8 in order to make out the parts. 



hence the suckers are small, but in order 

 to complete its life cycle an enormously 

 prolific reproductive system has been de- 

 veloped. Not only is the production of large 

 numbers of potential offspring necessary, 

 but also various kinds of larval stages that 

 are able to pass through several hosts, all 

 of which are instrumental in spreading the 

 parasite far and wide. Let us consider two 

 examples of flukes that infect man, one 

 that lives in the liver {Opisthorchis sinen- 

 sis) and another that lives in the blood 

 ( Schistosonm haematobium ) . 



The life cycle of the human liver fluke 

 can serve as a typical example of most 

 related flukes that are so prevalent in wild 

 and domestic animals ( Fig. 9-8 ) . It involves 

 two intermediate hosts which harbor the 

 larval stages of the parasite, and, of course, 

 one final host in which the adult lives. The 

 human liver fluke infects 75-100 per cent 

 of the people in certain parts of China, 

 Japan, and Korea, constituting a real health 

 problem in these regions. This situation 

 should be alleviated with the advent of 

 improved sanitation and a better educa- 

 tional program. 



The adult fluke (Fig. 9-9) lives in the 

 small bile ducts of the liver, where toxic 

 products excreted by the flukes and the 

 subsequent mechanical occlusions of the 

 ducts may cause serious damage. For a 

 heavily infested individual, this may even- 

 tually develop into cirrhosis, together with 

 complicating infectious disease which usu- 

 ally terminates his life. The adult fluke is 

 about three-fourths of an inch in length 

 and has two suckers, one at the anterior 

 end, another about one-third of the way 

 from the posterior end. It feeds on blood 

 which is drawn in through the anterior 

 mouth. 



Eggs laid by the adult pass through the 

 bile duct into the gut and eventually pass 

 out of the body in the feces. Because of the 

 oriental habit of using human excrement 

 (night soil) as fertilizer in the rice paddies, 

 the eggs usually get into water. Unlike most 



