512 EVOLUTION OF ANIMALS Part V 



the stage is set for the human infection through the skin or by swallowing 

 infected water. Blood flukes with life histories similar to this are encountered 

 in the West Indies, the Philippine Islands, China and Japan. 



Lung Flukes. Known in Oriental countries, including the Philippine Islands, 

 and in Central America and Peru, lung flukes occasionally appear in the United 

 States in former residents of the Orient. The adults of one well-known species, 

 Paragonimus westermani, deposit their eggs in the cavities of the human lung, 

 and the fertilized eggs are set free in mucus coughed from the lungs. The 

 larvae first enter fresh-water snails, and next fresh-water crabs and crayfishes 

 in which they become inactive and encysted. They then have two chances to 

 live; meat from the crab must be eaten raw by human beings or water in 

 which larvae have been freed from dead crabs must be used for drinking. 



Intestinal Flukes. Probably the most destructive of these is the giant intesti- 

 nal fluke, Fasciolopsis biiski, common in man and pigs, particularly in Central 

 and South China, but also encountered in India, Siam, and Malaya (Fig. 

 25.12). The adult flukes, about two or three inches long, inhabit the small 

 intestine and produce the fertilized eggs. In order to live, these eggs must reach 

 quiet fresh water, the haunts of several species of snails which the larvae 

 (miracidia) may then enter. About 50 days later, the larval flukes leave the 

 snails and swim about freely as cercariae. They then encyst themselves on 



Fig. 25.12. Life history of the giant intestinal fluke, Fasciolopsis, abundant in 

 South China. In one stage the larvae are in cysts on water-chestnuts (water 

 caltrop) that are commonly eaten raw. (Courtesy, Mackie, Hunter and Worth: 

 Manual of Tropical Medicine. Philadelphia, W. B. Saunders Co., 1945.) 



