PROPHYLAXIS 187 



smaller proportion of the eggs survive than in a moister 

 country. 



(3) Superficial burial. If very superficial, i.e., merely 

 covered with a thin layer of earth, there is little advan- 

 tage over deposition on the surface as far as destruction 

 of eggs is concerned. If in a trench, and that used by 

 a considerable number of persons, during the decompo- 

 sition of the faeces oxygen will be absorbed and some of 

 the eggs and most of the larvae killed from want of 

 oxygen. In deep trenching probably few larvae survive. 



Deposition in Open Cesspits. These, for the use of small 

 gangs of men, fifty or so, are more convenient and more 

 suitable than the previous methods. In any accumulation 

 of excreta the deeper part is practically a septic tank all 

 the oxygen is absorbed. In the superficial layer muscid 

 larvae of the species that are capable of living in putrid 

 faeces accumulate and rapidly invade and destroy any 

 fresh deposits. Only the thick-shelled eggs survive their 

 passage through the intestines of these larvae, and these 

 gradually sink into the anaerobic deeper contents of the 

 pit and are slowly destroyed. If suitable muscid larvae 

 (Sarcophagidae) are present, such a cesspit has very little 

 smell, the faeces are rendered odourless and harmless very 

 soon after they are deposited. The risks are bacterial 

 contamination of water and the conveyance by flies of 

 portions of excreta to food material. 



Other Methods. A well-managed septic tank built and 

 managed on European methods is an effective way of 

 dealing with eggs and larvae of entozoa. Cement cess- 

 pits are also effective and can be rendered anaerobic by 

 covering the surface of the deposits with a layer of oil. 

 Ross's automatic oiler is suitable for this purpose (fig. 72). 



The use of human excrement for manure is free from 

 much risk if the dejecta have been kept sufficiently 

 long and there is no admixture of fresh faeces. The 

 Chinese store faeces and urine for some time before using 

 them in large earthenware pots, but often it is mixed with 

 fresh faeces and is then dangerous (fig. 73). 



