THE THREE-LAYERED ANIMALS 



eqq 



167 



sperm duct excretory pore 

 jtestis I 



ora\ sucker 



pharynx 



intestine 



metacercana 



cercaha redia Sporocyst 



Fig. 9-8. Life cycle of Opisttiorchis sinensis, the human liver fluke. 



have lost their ciha and have developed 

 hold-fast organs called suckers. They have 

 no apparent sense organs, but the nervous 

 and excretory systems are well developed. 

 In general, the entire anatomy is adapted 

 to a parasitic mode of life. 



All grades of parasitism are found in this 

 group, from those that live on the outside of 



their hosts to those that inhabit the internal 

 cavities. The life history of parasites that 

 cling to the outside of animals such as fast- 

 moving fish is relatively simple, its main 

 characteristic being the development of 

 hold-fast organs or suckers. The case of the 

 internal parasite is an entirely different one. 

 There is no great problem of staying put. 



