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EVOLUTION OF ANIMALS 



Part V 



Host snail 



, natural 

 size 



snail on I 

 grass in ' 

 water 



A- free swimming 

 miracidium 



Fig. 25.11. Life history of the liver fluke of sheep, Fasciola hepatica. (After 

 Thomas. Courtesy, Storer: General Zoology, ed. 2. New York, McGraw-Hill 

 Book Co., 1951.) 



sucks up their fluids. With minor differences, the digestive, excretory, nervous, 

 and reproductive systems are similar to those of planarians. They are all 

 hermaphrodites. 



Life History. During its life cycle the liver fluke of sheep resides in two 

 hosts, the adults, usually in sheep, the larvae in fresh-water snails of the genus 

 Lymnaea. Without both of these hosts, the fluke cannot complete its life his- 

 tory. 



The adult flukes inhabit the ducts of the sheep's liver. The fertilized eggs are 

 carried down the bile duct, into the intestine, and from there are cast out of 

 the body. One sheep may support, on an average, 200 mature flukes. Although 

 each of these may produce its half million eggs, only those that happen to fall 

 into fresh water have any chance of survival. In the water, they hatch into 

 minute ciliated larvae (miracidia) that are active swimmers. In order to sur- 

 vive, the larvae in this particular stage bore their way into the body of the 

 common water snail Lymnaea (Fig. 25.11). In the liver of this snail, they 

 transform into stationary sporocysts within which the egglike cells develop into 

 very minute active larvae (rediae). These work their way about in the snail, 

 become stationary and then produce active larvae (more rediae). Several 

 generations of these active larvae may develop resulting in great increases of 

 numbers. Instead of changing into sporocysts, the later generations transform 

 into active tadpole-shaped larvae (cercariae), which are discharged into the 

 water by the snail. In order to survive, they must reach the grass and leaves 

 along the shore where they enclose themselves in resistant cysts and await 

 their fate of being eaten by a sheep or left to perish. Billions of them are lost. 



