SOME INHABITANTS OP MAN 145 



remained for many years a matter of pmrely zoological interest. 

 Steenstrup had published his researches in 1842. At that time 

 the fluke parasites of man had not been discovered. In 1853 

 Bilharz found adult flukes (now known as Bilharzia worms) in 

 the portal veins of a patient who had died of Egyptian haema- 

 turia. The Asiatic liver-fluke, Clonorchis sinensis, was first 

 seen by McConnell in the bile-ducts of a Chinaman who died in 

 Calcutta in 1874, and in 1880 Sir Patrick Manson forwarded 

 to Cobbold the first specimens of the lung-fluke, Paragonimus 

 ringeri, found in a native of Formosa. 



During the succeeding years information gradually accumu- 

 lated which showed that each of these "flukes frequently gave 

 rise to widespread and incurable disease in the regions in which 

 it was endemic. The need for preventive measures became 

 more and more appreciated and the physician once more 

 turned to the natiuraUst for guidance. 



The questions calling for solution were not those which 

 concerned the mode of development of the flukes outside the 

 body, for the work of Steenstrup and others had definitely 

 shown that the flukes during their free stage parasitised and 

 multiplied within some species of mollusc. The special 

 information required was (a) what was the exact means used 

 by the " infective stage," i.e. the cercaria, of each species to 

 enter the hixman body ? and (6) was there any phase in the 

 development of the parasite outside the body in which it 

 was specially susceptible and could be easily and cheaply 

 destroyed ? 



For the three chief types of flukes which invade man satis- 

 factory answers to these questions have been forthcoming. 

 It remains only for the sanitarian to put into effective use the 

 scientific data available. 



The lung-fluke and liver-fluke of man are confined almost 

 entirely to the Far East. It has been proved that these and 

 a third species, which, though a common intestinal parasite, 

 causes no symptoms in man, require for their larval develop- 

 ment a special fresh-water snail of the genus Melania, viz. 

 Melania libertina. After metamorphosis the " cercaria " in 

 each case becomes free-swimming and then attacks a second 



