F. G. Cawston 
07 
CONCLUSION. 
It would seem that the specimens of snails found in the flowing rivers are 
considerably less infected with trematode parasites than those occurring.in 
stagnant pools. The specimens obtained from the river-bed of the Vaal and 
its tributaries are remarkably well preserved, possibly from the amount of 
lime in the river-bed. The inference is allowable that the absence of cercarial 
infection in river snails is also due to the lime; this will also account for a 
relative infrequency of Bilharziasis along the course of the Vaal river, as 
compared with those portions of the Low Veldt through which the Crocodile 
river and its tributaries flow. 
Parasitology xi 
7 
