264 GENERAL ZOOLOGY 



set free from its cyst in the digestive tract and is given 

 the chance to grow to maturity. All the higher trematodes 

 pass through a life history similar to this. In each species 

 a vertebrate serves as host to the adult worm, and the 

 young worm cannot enter another vertebrate directly but 

 must first undergo a series of stages at least one of which is 

 passed in the body of a snail. Draining of swamps, fencing, 

 and care in the selection of grazing range have done much 

 to reduce the ravages of this parasite. 



Human Flukes. In tropical countries, and especially in 

 the Orient, trematodes become serious enemies of man. 

 Blood flukes (Schistoso'ma) are especially common in Asia 

 and in Africa. They are small worms, less than one-half 

 inch long, living in the blood vessels of man. They cause 

 disease by blocking the blood vessels. The eggs, which are 

 provided with sharp spines, bore their way through the 

 tissues. There are other flukes of grave importance in the 

 Orient. A lung fluke (Paragon' imus) causes a disease very 

 similar to tuberculosis. Liver flukes and various intestinal 

 flukes also produce diseases in man. 



The Simplest Flukes. The simplest of all the trematodes 

 live on the body surface and gills of the lower vertebrates, 

 especially of fish. Hooks and spines on the posterior end 

 catch into the skin of the host and prevent the worm's 

 being dislodged easily. Frequently muscular organs known 

 as suckers also aid in this attachment. These external 

 parasites, or ectoparasites as they are called, undergo di- 

 rect sexual development. Nothing but accidental dislodg- 

 ment would prevent the young from immediately becoming 

 a parasite of the same individual host which sheltered its 

 parent. There are no free larval stages and no complicated 

 life history in these ectoparasitic trematodes. 



Tapeworms. Tapeworms are likewise exclusively para- 

 sitic. There are but few species of animals from fish on up 



