Since fish are commonly eaten raw in the countries 

 where Opistliorchis occurs, the young flukes emerge 

 unharmed into the human intestine, make their way 

 up the bile duct to the smaller bile passages, there at- 

 tach by their suckers, and feed on blood. 



Another well-known liver fluke is Fasciola hepat- 

 ica, whose cercarias leave the snail host and encyst 

 on grasses and other vegetation in nearly all parts of 

 the world. It thrives best in the dense concentra- 

 tions of hosts provided by man's herding of cattle, 

 sheep, and goats, and it may also be found in pigs, 

 horses, and many wild animals. In some countries it 

 finds its way into man by means of cercarias that 

 cling to wild watercress. 



Far more serious as a human problem is the blood 

 fluke, Schisiosoiua, referred to in the introductory 

 part of this chapter. A member of one of the several 

 families of elongated flukes that live in the blood ves- 

 sels of fishes, turtles, birds, and mammals. Schisto- 

 soma differs from the liver flukes not only in shape 

 but in some other ways. For one thing, this fluke oc- 

 curs as separate males and females, and the sides of 

 the male fold over to form a groove in which the even 

 longer and more slender female is held. Three wide- 

 spread species debilitate an estimated 114,000,000 

 people. Schistosoma haematobinm infects primarily 

 the small veins of the urinary system and is found in 

 much of Africa, the Middle East, and part of Portugal. 

 Schistosoma mausoui, which occupies small intestinal 

 veins, spreads misery in most of Africa, in South 

 America from Brazil to Venezuela, and in some of 

 the West Indies. Schistosoma japonicitm, also a para- 

 site of intestinal veins, accounts for an estimated 

 46,000,000 cases in Japan, China, the Celebes, and 

 some of the Philippine islands. 



For each of the three species of flukes there are 

 particular species of fresh-water snails that serve as 

 hosts to the larval stages. The fork-tailed cercarias 

 that emerge from the snail burrow through human 

 skin or are taken in with drinking water. Wherever 

 schistosomes that infect man are prevalent it is haz- 

 ardous to drink untreated water, or to bathe, wade 

 in, or dip the arms in fresh waters. Millions of Chi- 

 nese and Japanese become infected during the plant- 

 ing of rice as they stand bare-legged in flooded rice 

 fields. In recent years the extension of irrigation sys- 

 tems in Africa and in the Near East has steadily mul- 

 tiplied the habitats for fresh-water snails, speeding 

 the increase of this serious disease despite many con- 

 trol measures. 



The temperate and more sanitary parts of the 

 world are not free of blood flukes, for wherever suit- 

 able snail hosts occur, there may be swimming cer- 

 carias of some kind of schistosome. The adults often 

 live in wild birds, especially ducks. Though the cer- 

 carias of bird schistosomes do not reach the human 

 liver, their penetration of the skin causes a skin irri- 



tation known as "swimmer's itch." Repeated expo- 

 sures may so sensitize an individual that he becomes 

 prostrate and develops a severe rash. Swimmer's itch 

 is especially serious in certain lakes in the north- 

 central United States, but many other fresh-water 

 and marine shores are affected. Chandler's Intro- 

 duction to Parasitology lists as victims of swimmer's 

 itch: vacationers in Quebec and New England west 

 to Manitoba and Oregon, carp-breeders in Ger- 

 many, rice-growers in Japan and Malaya, lake bath- 

 ers in Australia and New Zealand — also sea bathers 

 and clam-diggers on the American North Atlantic 

 and Florida coasts, fishermen in San Salvador, and 

 naturalists on the rocky shores of southern California 

 and Mexico. Wherever bathers are aware of this an- 

 noyance they should wipe the skin dry immediately 

 after leaving the water, and should avoid getting al- 

 ternately wet and dry by playing in shallow water. 



(Class Cestoda) 



The Tapeworms 



The cestodes, named from the Greek word for 

 "girdle" or "ribbon," are mostly long, flattened, 

 opaque white or yellowish ribbon-like parasites. The 

 adults live inside vertebrates, almost always in the 

 intestine, but the larval stages develop in either ver- 

 tebrate or invertebrate hosts. The life cycle is com- 

 plex, involving one or two intermediate hosts in ad- 

 dition to the vertebrate "final host" that nurtures the 

 adult. 



Aside from the enormous length, 50 feet or more, 

 attained by some tapeworms, their most notable fea- 

 ture is a complete lack of a mouth or any digestive 

 apparatus. The body is covered with a protective 

 cuticle, as in flukes, and the worms absorb much of 

 their nutrition directly through the body wall from 

 the intestinal contents of the host. 



The scolex is a very small knob at the narrow or 

 front end of the long body, and it bears the only or- 

 gans of attachment. These may be suckers, hooks, or 

 sometimes glandular adhesive areas. Behind the sco- 

 lex there is usually a short, narrow, undivided neck 

 region or growing region, and from this there is a 

 constant budding off of body segments. Those closest 

 to the neck are smallest and youngest, those farthest 

 away the largest and most mature. Thus the chain of 

 segments represents every stage of development, and 

 widens gradually along the body's length. 



Tapeworms have no specialized sense organs, not 

 even the poor ones seen in turbellarians and in some 

 flukes, though the body wall, especially that of the 

 scolex, is well supplied with sensory cells. These 

 worms are all business, and their energies are chan- 

 neled into a prodigious reproductive effort which in- 

 sures that a sufficient number of the young will find 

 new hosts and keep the species going. 



126] 



