80 'Be])o?i of the Billiarzia Mission in Egypt, 1915 



^ The cercariae can survive on a damp surface 



Drying from which the visible water has disappeared. 



They are immediately killed if the drying process 



is allowed to proceed to the extent of abstracting fluid from their 



bodies. They cannot withstand the slightest desiccation. 



Although the bilharzia cercariae, with one excep- 

 OxYGEN ^^°°' ^^^ -^^^ provided with eye-spots, the bulk are 

 found near the surface of the water. They accumu- 

 late there irrespective of exposure to light. If a thin layer of oil 

 or refined paraffin is poured on to the surface of the w^ater the 

 length of life of the cercarige is reduced to a few hours. This 

 may also be observed when a drop of water containing cercarise is 

 periodically examined under a sealed cover-glass. If a bubble of 

 air has been left in the preparation it will be noticed that the 

 cercarige course round and round its circumference like moths 

 around a flame. 



The obvious purpose is to obtain oxygen from those portions 

 of the water nearest the air. 



The free-swimming bilharzia cercarige can sur- 

 Tempek\ture ^^^^ ^ temperature of 45° C. They are killed, 

 however, when the temperature is momentarily 

 raised to 50° C. This corresponds very closely to the clinical finds 

 of Conor [116] in Tunis. He noted that bilharziosis is acquired from 

 the waters of the thermal springs at Gafsa, Tozeur and Gabes, 

 which have a temperature of from 28" C. to 45° C, while the disease 

 was quite absent in the neighbourhood of other springs in Tunis 

 where the temperature ranges from 50° C. to 10° C. 



Effect of Very weak alkalies were found to have a 



CHEMicAii stimulant action and weak acids an inhibitory 

 Eeagents. effect on the movements of the bilharzia cercariae. 

 One in five hundred hydrochloric acid kills immediately. The 

 following acids, acid salts, essential oils and antiseptics were found 

 in dilute solutions of varying strengths to have a lethal effect on 

 the cercariae : — 



