TraiifiDiission 41 



cercaria, which on morphological grounds was only provisionally 

 distinguished from that occurring in Planorhis boissyl, was also found 

 at Marg in a certain number of specimens of Bulliniis (fig. 15, a). 



At Marg the Planorhis boissyl (fig. 15, b) were so commonly 

 infected with non-eyed Bilharzia cercaria that half-an-houi's 

 collecting sufficed to ensure a large supply of active larva\ An ex- 

 tended series of experiments was instituted to determine the specific 

 character of these forms. 



Three species of Bilharzia worms are supposed to occur in 

 Egypt : In man the Schistosoma hamatohium (both varieties), in 

 cattle the 8. bovis, and in ducks the Bilharziella polonica. As 

 bifid-tailed cercariae with eye spots have been found, though not 

 identified, in snails both in Central Europe and in North America, 

 and as this cercaria departed somewhat from the two other forms, 

 it seemed a reasonable conjecture that this cercaria was the larval 



A B 



Fig. 15. — a, Biilliinis cnrtortus ; b, Plaiwrbis boissi/i. 



stage of an avian Bilharzia, and that the two remaining and similar 

 cercarioe probably attained their maturity in the two known mam- 

 malian hosts. Attempts were made to verify this conjecture 

 experimentally. In order to exclude possible fallacies it was 

 essential to infect ducklings immediately after hatching and before 

 they had come in contact with other than filtered water. These 

 were not obtainable in Cairo. 



One was struck by the apparent entire absence of rats in 

 the banks of the canals and in the fields. The authorities 

 at the Giza Zoological Gardens stated that the common water- 

 vole does not occur in Egypt. A professional rat-catcher was 

 commissioned to obtain rats and field-mice from around Marg. 

 His search proved quite fruitless. It was possible that this 

 extraordinary absence of rodents might be due to a susceptibility 

 to Bilharzia disease. 



The animals frequenting the canal at Marg were man, cattle, 



