V SCHISTOSOMA 169 



and burrow through the skin into a lymphatic or blood-vessel. They are 

 carried round in the blood-stream and normally settle down for a time 

 in the liver, whence as sexual maturity approaches they migrate to their 

 definitive home, e.g. the veins of the mesentery. 



5. haematobium while best known in connexion with Egypt — ^where 

 in places as many as 90 per cent of the population are infected — occurs 

 also all over the continent of Africa where conditions are favourable 

 and extends into Asia at least as far as Persia. It has apparently been 

 introduced into Australia, and as a result of the Great War there will 

 doubtless take place a wide extension of its distribution in parts of the 

 world where suitable climatic conditions are combined with the presence 

 of water-snails capable of acting as hosts to the sporocyst stage. When 

 once established in a particular district it will be difiScult to get rid of 

 owing to small mammals such as rats and mice being able to play 

 the part of warm-blooded host. Freedom from infection in such a 

 district can only be assured by taking precautions against infected water 

 coming in contact with the skin. Even getting the feet wet by walking 

 through swampy ground may communicate infection as the cercariae 

 are able to penetrate wet stockings. If water containing cercariae is 

 taken into the mouth they are liable to penetrate the lining of the 

 mouth and so reach the blood : those actually swallowed are in all 

 probability killed by the digestive juices of the aUmentary canal. Water 

 used for drinking or washing should be freed from cercariae by the use 

 of an efficient filter, by boiling, by the use of a chemical disinfectant 

 such as sodium bisulphate (-i %), or by keeping the water stored for 

 48 hours before use so as to allow any cercariae to die off. In the 

 case of stationary camps and settlements the last -mentioned method 

 of sterilizing the water so far as Schistosoma is concerned is obviously 

 the simplest and most practical. 



5. mansoni occurs also in Egypt and other parts of Africa, as well 

 as in Tropical America and the West Indies. While agreeing in its general 

 features with S. haematobium it differs in certain details. The sharp 

 spine is situated not at the end of the egg-shell but at one side (Fig. 77, 

 B, 2), and the eggs are usually set free not in the bladder but in the 

 intestine, causing dysentery-like symptoms. 



The sporocyst stage is found not in Isidora but in the flattened disc- 

 shaped water-snails of the genus Planorbis. 



S. japonicum occurs in China, Japan and the Philippines. The eggs 

 are — as is the case also with the adult — rather smaller than those of the 



