SOME INHABITANTS OF MAN 149 



to do something to assist in the eradication of this disease, 

 especially as there are good grounds for believing that its 

 incidence has become much more intense since the introduction 

 of perennial irrigation. 



In other lands, where the snails live in natural water- 

 courses and the climatic conditions are less constant, the 

 problem of Bilharzia-prevention becomes more difficult and 

 necessarily more dependent on personal prophylaxis. 



We pass now to the roundworms or Nematoda, which form 

 by far the greater proportion of the Entozoa of man. The 

 various species present very different modes of migration 

 but space does not permit of details. In some cases, as in the 

 common roundworms met with in this country, development 

 takes place within the egg-shell, which, after a necessary 

 period of delay, again enters the body as an accidental con- 

 tamination of food. 



The Trichina worm develops in the pig. The infective stage 

 is encysted in the muscles, but is usually destroyed by our 

 British custom of cooking pork until it is white. In Germany, 

 where pig-flesh is used as raw food, trichinosis is common. 



In the case of the Filaria worms, which cause diseases of 

 the lymphatics and are responsible for the onset of elephanti- 

 asis, the young worms, circulating in the blood, are sucked 

 up by a mosquito and undergo development in the tissues of 

 this insect, returning again from the mosquito to another 

 human host. 



Many of the roundworms, after hatching from the egg, 

 spend their entire " period of delay " as free-living forms, 

 finding sufficient nutriment in the organic matter in the soil, 

 where they undergo their metamorphosis and remain quiescent 

 until taken again into the body. 



The manner in which the migrations of the Guinea-worm 

 and the Hookworm were elucidated call for special notice. 



The adult Guinea -worm (Dracunculus medinensis) is a 

 parasite which lives wholly in the connective tissues under the 



