come adapted to living in the shelter of these cavities 

 in fishes, amphibians, and aquatic reptiles. Polystoina 

 hangs on with a rear disk equipped with six large 

 suckers. The different species live, as larvae, on the 

 gills of frog tadpoles; the adults of Poly stoma are 

 found in the urinary bladder of amphibians and also 

 in the bladder, nose, and mouth of turtles. 



THE DIGENETIC FLUKES 



The digenetic flukes, as their name implies, have 

 complex life histories involving two or more hosts. 

 Typically they have four larval stages and live suc- 

 cessively in three or even four hosts. As adults they 

 are internal parasites of practically every kind of 

 vertebrate — fresh-water, marine, and terrestrial. The 

 larval worms live in snails, fishes, and other small 

 animals. So large is this group of parasites, and so 

 varied in structure and habit, that it can barely be 

 touched on here. The five thousand or so known spe- 

 cies are added to constantly. Details must be sought 

 in zoological textbooks or in specialized books on 

 parasitology suggested in the bibliography. 



Adults of the digenetic flukes live mostly in the 

 vertebrate intestine or in organs that connect with it, 

 such as the liver. The Chinese liver fluke, Opisihor- 

 chis (Clonorchis) sinensis, occupies the bile passages 

 of the human liver, causing serious anemia and liver 

 disease from Japan and Korea through China to In- 

 dochina and India. The adult fluke is about % of an 

 inch long, and has two suckers, one at the front end 

 surrounding the mouth, one a short distance back 

 from the first. As in many flukes, the digestive tube 

 rather promptly forks into two long digestive sacs 

 with blind ends. The flattened baglike body is stuffed 

 mostly with reproductive organs, and covered on the 

 outside with a cuticle that resists digestion by the 

 host. Hundreds or even thousands may obstruct the 

 ducts of one host, shedding fertilized eggs that pass 

 from the liver into the intestine and out with the 

 feces. If it reaches fresh water, as is commonly the 

 case, the egg may be eaten by an aquatic snail of a 

 species favorable to the growth of the parasite. 

 Within such a snail the egg hatches and the parasite 

 develops through three larval stages, two of which 

 multiply asexually. Finally a fourth stage, the cer- 

 caria, which has the two suckers and the forked in- 

 testine of the adult but has also a long tail, escapes 

 from the snail in large numbers and swims about ac- 

 tively. On encountering fishes of certain species, the 

 cercarias pjenetrate into the flesh and encyst there. 



The Chinese human liver fluke, Opisthorchis (Clo- 

 norchis) sinensis, is about Vi inch long. The front 

 sucker, the muscular sucking pharynx, the two- 

 branched digestive tract, and both male and female 

 sex organs show in this stained preparation. (General 

 Biological Supply House) 



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