Chap. 25 FLATWORMS VANGUARD OF THE HIGHER ANIMALS 509 



Fig. 25.10. A black and white flatworm of the Pacific coast (Pseiidoceros 

 montereyensis) called a polyclad because of its many-branched digestive tract. 

 Natural size. This and other polyclads swim and glide about through the water, 

 the fluted borders of their bodies undulating like living ruffles. They are among 

 the most beautiful of marine animals, comparable to the butterflies on land. 

 (Courtesy, MacGinitie and MacGinitie: Natural History of Marine Animals. 

 New York, McGraw-Hill Book Co., 1949.) 



Many other trematodes have an elaborate life history and develop in- 

 directly (Order Digenea). The fertilized eggs develop into young flukes that 

 look unlike their parents and go through several phases before they are adults. 

 During the life span they live in alternate hosts, the adults in a warm-blooded 

 vertebrate, the young ones in snails, crustaceans or other invertebrates. If there 

 are three hosts in one series, they are usually, first, a mammal occupied by the 

 adult; second, a snail; and third, a fish. 



Sheep Liver Fluke 



The liver fluke, Fasciola hepatica, is often chosen as a type for study because 

 of its large size, economic importance and its well-known life history (Fig. 

 25.11). The hosts of the adults are sheep, cattle and other herbivores, and 

 man. There are sheep flukes all over the world wherever sheep are raised, 

 especially in mild climates; in the United States, they are most common in the 

 states bordering the Gulf of Mexico. Where the cysts are thickly distributed 

 over pasture grass the infection of the sheep may be enormous, killing 50 to 60 

 per cent of a flock. 



The adult liver fluke looks like a small dead leaf. At the pointed tip of its 

 body is the muscular mouth with which it punctures the tissues of its host and 



