Parasites and Predators 



example of the large group of parasites which require 

 two hosts to complete their life cycle. 



The adult fluke, as it infests the liver, has a flattened 

 leaf-like body and measures nearly an inch in length: 

 There are no males and females ; each individual is 

 capable of producing about five hundred thousand very 

 minute eggs. These eggs pass from the body of the host 

 and then their troubles begin. Should they fall on dry 

 ground, their careers are at an end, but, Fortune favouring 

 them, they may fall into a pond at which the sheep is 

 drinking. Should this happen, small tailed larvae will 

 hatch therefrom, and they swim about in the water for 

 a day. 



Again, at this period they are in danger of extinction. 

 No wonder the liver-fluke lays hundreds of thousands of 

 eggs, for many dangers await the youngsters. Should the 

 swimming larvae come in contact with a certain water- 

 snail, all will be well. They bore into the soft tissues of 

 the snail and undergo certain changes within its body. 

 Eventually they leave the snail and become encysted that 

 is to say, covered with a resistant shell. Within this shell 

 further changes take place till the little creatures assume 

 the form of minute liver-flukes. Sheep in drinking 

 swallow these little shells with their contained flukes and 

 by the action of the digestive juices the flukes are soon 

 liberated from their imprisonment ; they rapidly work 

 their way to the liver and develop into adult flukes. 



The mention of the snail which plays so important 

 a part in the development of the liver-fluke reminds us 

 that some of the shell-fish are parasitic during a portion of 

 their lives. In our chapter on fishes we mentioned the 

 case of the bitterling and the pond-mussel. 



We need not trace the development of the young 

 mussel from its egg to its parasitic stage. Quite early in 

 its life, however, soon after it has attained the dignity of 

 a pair of valves, it possesses a long sticky thread which 

 floats, from between its valves, in the water. Should any 



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