PHYLUM PLATYHELMINTHES 169 



197. Occurrence and Economic Importance. — The phylum Platy- 

 hehninthes contains a large number of species, and a very large per- 

 centage of vertebrates are infected by the parasitic forms. From an 

 economic standpoint the free-living flatworms are of no importance, but 

 both trematodes and cestodes produce a great deal of injury to domes- 

 tic animals. Among trematodes several flukes are parasitic in man. 

 Human tapeworms often cause serious symptoms but are usually not 

 dangerous to life. Nevertheless the larvae of one tapeworm, Echinococcus 

 granulosus (Batsch), which lives as an adult in the dog, may occur in 

 man, where they form cysts known as hydatids and, if not removed by 

 operation, are often fatal. The injuries to domestic animals caused by 

 tapeworms, though not so serious as to cause the death of the animal, 

 are often sufficient to interfere somewhat with their usefulness. 



