Chap. 25 FLATWORMS VANGUARD OF THE HIGHER ANIMALS 511 



However, in the infected grass it now takes only the right nibble from one 

 sheep to insure a fluke population. In the sheep's stomach, the digestive juices 

 free the larvae (cercariae) which then migrate to the liver, chemically and 

 physically their home niche. They attach themselves by means of the ventral 

 suckers and in three to six weeks develop into adult flukes, the parents of an- 

 other generation. 



The two greatest gambles in the fluke's life history are on its chances of 

 entering its hosts, the pond snail and sheep. Both ends are achieved by the pro- 

 duction of vast numbers of young, the chief tool of a parasite's existence. Prob- 

 ably one fluke among untold numbers secures the necessary lodging in both 

 hosts. Yet, the great reproductive capacity of that one hermaphroditic fluke — 

 half a milUon eggs from a single adult, 300 larvae from a single egg — main- 

 tains the exuberant success of the species. 



Salmon-poisoning Fluke 



The salmon-poisoning fluke, Troglotrema salmincola, is prevalent in the 

 extreme northwestern United States. The adult flukes live in the intestines of 

 dogs, foxes, bears, bobcats, and other mammals. In dogs, the parasites cause 

 salmon-poisoning — violent illness often resulting in death. 



In order to live, the fertilized eggs must reach the water and enter their inter- 

 mediate host, a snail called a periwinkle {Goniobasis pUcijera). Larvae similar 

 to those of the sheep liver fluke develop and finally the active ones (cercariae) 

 make their way into the water. When these come in contact with trout or 

 salmon they bore into the muscles and become encysted. If a dog or other pos- 

 sible host eats salmon raw or semi-cooked, the young flukes are freed from 

 their cysts and take up their ultimate residence in the intestine and their busi- 

 ness of creating the next generation. 



Important Human Parasites 



Human flukes are frequent in tropical and Oriental countries; none is native 

 to North America. However, infections are occasionally discovered in per- 

 sons who have been residents of countries where they abound and these may 

 be a source of further infection. There are four main types of human parasites 

 in this group. 



Blood Flukes. The adults live in the blood vessels of man and several do- 

 mestic animals. Like those of other flukes the larvae inhabit water snails. One 

 species. Schistosoma haematobium, is distributed in parts of southern Europe, 

 Asia, and Australia. It causes the disease called bilharzia in about .fifty per 

 cent of the population of Egypt. The fertilized eggs are expelled from the 

 human body in the urine. The embryos hatch in fresh water and ultimately 

 enter mainly one kind of snail (Bulimus) and undergo part of their develop- 

 ment within it. Then, the active young cercariae swim out into the water and 



