25 



Order DIGENEA 



Exclusively endoparasites. Not more than two suckers, and 

 without hooks. Development with heterogony and with change of 

 host; the sexual generation in vertebrates, the asexual in inverte- 

 brates. Take as an illustration 



Fascolia hepatica — {Distomum hepaticum) — The liver-fluke of the 

 sheep and other ruminants. Adult in liver, causing "rot." Though 

 not common in the United States, it is one of the most dangerous 

 parasites of sheep, sometimes causing enormous loss in Europe. 

 Youatt estimated that in Europe alone more than 1,000,000 sheep 

 die annually from its attacks. 



Structure — Adult flat, unsegmented, oval, enlarged in front, 

 about 3.4 cm. (1.5 inch) in length. Two suckers near anterior end, 

 cuticle covered with minute backwardly directed spines. Digestive 

 system branched, ramifying all over the body. Excretory system 

 much branched, main duct opening to exterior caudal end. Re- 

 productive system hermaphroditic, with very prominent yolk glands. 



Life history — Adult in liver of a sheep; discharges of eggs via 

 the bile ducts of the sheep; development of eggs only in water 

 to form the ciliated embryo; entrance of embryo into the body of 

 a snail; change to a sporocyst; development of rediw in sporocyst; 

 second generation of redice; development of cercarice; passage of the 

 cercarise into the water; encysted cercarice; passage of young fluke 

 to the liver of a sheep; development into an adult fluke. 



Methods of control — Medical treatment of little avail and pre- 

 ventive measures very important. Drain pastures to destroy snails; 

 keep sheep away from damp pastures. Carp, frogs and toads may 

 play an important role by feeding on snails. Destroy bodies of 

 disease d animals and spread manure from such animals only on dry 

 ground. 



Related species — Stiles, '94, lists twelve species of liver-flukes 

 found in man and the higher animals. Of these the following are 

 especially deserving of mention: 



Fasciola magna — Widely distributed in the United States, espec- 

 ially Southern, being apparently much more common in sheep and 

 cattle in this country than T. hepatica. 



Opisthorchis pseudofelineus Ward 1901, found in gall ducts of the 

 domestic cat. Closely related species O.felineus in Europe occasional 

 parasite of man. 



