232 THE CONTROL OF LIFE 



inside the spot- winged gnat or mosquito {Anopheles). 

 He goes on to show that if we can avoid being bitten by 

 a mosquito we may live without malaria in the midst 

 of a malaria-rife swamp ; that if we can keep the 

 mosquito from sucking the blood of malarial patients 

 we can greatly lessen the disease ; and that a film of 

 parafiin on the pools in which the larval mosquitos 

 develop will abolish the disease from the area. 



xA.nother very instructive case concerns the disease 

 known as bilharziasis. It must be explained, first of 

 all, that a common disease in sheep, called liver-rot, 

 is due to a fluke-worm (Distomum hepaticum) which 

 spends the early part of its life in a tiny fresh-water 

 snail (LymncBus truncatulus). The more of these snails 

 that wagtails and lapwings eat, the less liver-rot in 

 sheep. Now% in warm countries, such as Egypt, there 

 is a distantly related worm, called Bilharzia, which 

 occurs in man. It has the peculiarity, shared by the 

 formidable hookworm, of being able to pass as a micro- 

 scopic larva by boring right through the human skin. 

 It causes very serious and painful disease and occurs 

 in not less than a half of the total population of Lower 

 Egypt, and in every third child born in Cairo. Its 

 life-history, which involves several kinds of fresh-water 

 snail, e.g. Melania and Bulinus, has been revealed by 

 Dr. Leiper's careful tracking, so thoroughly revealed 

 that he has been able to protect all who are willing to 

 take a few simple precautions. Thus, the free-swim- 

 ming larva cannot live for more than thirty-six hours in 

 drawn water. Eventually, by getting rid of the snails 



