78 SUMMARY OF CUKHENT RKSEAliCHES RELATING TO 



the whole body gyrates and the cercaria progresses with the pronged tail 

 foremost. Swimming is not continuous. Brief periods of activity are 

 regularly alternated with periods of rest. During the latter the cercaria 

 very slowly sinks. When seen with a hand-lens their attitudes recall 

 those of minute mosquito larvfe. As a rule they frequent the surface, 

 but when a small mammal like a mouse is placed in the water they at 

 once attack the skin. As successful infection resulted in a young mouse 

 after only ten minutes' immersion on a single occasion, they appear to be 

 able to pierce the skin very rapidly. An infected mollusc, such as 

 Flanorhis boissyi, may coui,inue to discharge cercarijfi for three weeks, 

 but the cercariiK do not live in the water for longer than about thirty- 

 six hours. In ordinary tap-water they usually live about twenty-four 

 hours. They cannot withstand the slightest desiccation ; they are very 

 sensitive to scarcity of oxygen ; they are killed if the temperature of the 

 water be momentarily raised to 50° C. ; they are inhibited by weak acids 

 and stimulated by very weak alkalis. Free-swimming cercarise pass 

 through the finest silk mesh, through stocking material, and, given time, 

 through several inches of sand if there is a continuous flow of water. 

 Unlike the Ankylostome larvae, they are unable to traverse ordinary 

 filter-paper. 



For chemical sterilization of water, tabloids of sodium bisulphite are 

 effective. A dilution of 1 in 1000 is rapidly lethal to the Bilharzia 

 cercariffi. Personal contact of any kind with unfiltered water is risky. 

 When water is stored, in order to destroy the cercaria) care must bo 

 taken to screen the intake pipe with gauze to keep out infective 

 molluscs. The water of wells and " sakias " is uuich safer than that 

 from other sources. Hitherto molluscs have not been found in these 

 wells. Shallow barrel sand-filters are open to suspicion, for the cercaria- 

 can get through the sand. The reproductive activity of Bilharzia in the 

 molluscs is probably most intense during the summer months, but the 

 occurrence of mature cercaria^ in infected molluscs in February shows 

 that there is a certain liabilitv to infection throuschout the vear. 



In a third paper * the author continues his account of Bilharzi;i. 

 The life-history is that of a typical digenetic Trematode. The mira- 

 cidium gives rise to a sporocyst, which in turn produces daughter- 

 sporocysts. After leaving the mother-cyst the daughter-sporocysts 

 migrate into the tissue of the digestive gland of the water-snail and 

 grow rapidly. They become greatly elongated and eventually ramify 

 throughout the organ, so increasing its bulk that an infected Planorbis 

 can be detected at a glance. The colom' of the organ is also changed, 

 that of P. hoissyi turning from dark brown or black to ochre. 



The ends of the daughter-sporocysts are solid, but the walls of the 

 tubular bodies are very delicate and transparent. As the cercarise 

 develop within them, the sporocysts may be constricted by the host 

 tissue and division may result. As there is no oral sucker or gut, the 

 nutriment must be absorbed. The sporocysts are capable of travelling 

 by wrigghng movements. The cercariae leave the sporocyst by rupture 

 of the wall, and they are discharged from the snail in " puffs," a number 



* Journ. R..\.M.C., xxvi. (1915) pp. 253-67 (figs. 41-55). 



