638 - Heredity and Evolution 



covered by a protective cuticle. At least one 

 sucker is present and often one sucker sur- 

 rounds the anteriorly placed mouth, which 

 leads into the enteron. The food generally 

 consists of the tissues of body fluids of the 

 host, sucked into the enteron by action of a 

 muscular pharynx (Fig. 32-13). A few are 

 external parasites of various fish, amphibians, 

 and reptiles and these tend to have a rela- 

 tively simple life cycle. Most, however, are 

 internal parasites. They may live in the in- 

 testine (intestinal flukes), bile ducts, or gall 

 bladder (liver flukes), or even in the blood 

 stream (blood flukes); and all have a complex 

 life cycle. At least one stage of the cycle is 

 spent in an invertebrate host, usually a snail 

 or other mollusk (Fig. 32-16). 



Some Trematode Diseases. A number of 

 Trematoda are human parasites of consider- 



able medical importance. More than half 

 the population in certain parts of China and 

 India carry chronic infections of the intes- 

 tinal fluke, Fasciolopsis buski. This parasite 

 infects man (and sometimes dogs and cats) 

 by way of an encysted larval stage found on 

 certain water plants that are eaten raw. Also 

 there is another larval stage found in the 

 tissues of a fresh-water snail. 



The commonest human liver fluke, Clo- 

 norchis sinensis, likewise has a wide distribu- 

 tion in the Orient. This fluke, as may be 

 seen in Figure 32-16, has a larval (redia) stage 

 in a snail; but the encysted stage (metacer- 

 caria) occurs in the skin and muscles of cer- 

 tain fish. Thus human infection occurs when 

 the raw fish is eaten by man. 



Various blood flukes, of the genus Schisto- 

 soma, are fairly frequent in certain coun- 



Embryonated egg 



Fig. 32-16. Life cycle of the "Chinese liver fluke" (C/onorch/s sinensis). Such a 

 cycle, in which the parasite must pass through a mollusk (the snail) before rein- 

 fecting a vertebrate, is typical of this group of trematodes. (From Genera/ Zool- 

 ogy, by Miller and Haub. Holt, Rinehart and Winston, Inc.) 



