212 
A SURVEY OF CAWSTON’S SPECIES OF SOUTH 
AFRICAN CERCARIAE. 
By ERNEST CARROLL FAUST, Ph.D., 
Parasitologist, Union Medical College, Peking , China. 
(From the Department of Pathology, Union Medical College, Peking.) 
(With 4 Text-figures.) 
For several years Dr F. G. Cawston of Durban, Natal, has been making 
examinations of various mollusks taken from the rivers and pools of the Trans¬ 
vaal and Natal in order to discover the fluke infection of the region. These 
examinations have been extensive as regards the number of individuals of 
each species examined, ranging up into the thousands for the more common 
gasteropods. The investigations have been carried on through the various 
seasons of the year so that the data are representative. I have been enabled 
to re-examine much of this material through the courtesy of Dr Cawston, who 
has also placed at my disposal many of the biological data concerned with 
these researches. 
It is common knowledge that the mollusk is the obligatory host of the 
larval fluke; that in regions where the geological or biological conditions are 
unfavourable for molluscan life trematodes do not occur; and that one usually 
expects to find fluke infection where molluscan life is abundant. But while 
the presence of the mollusk is an essential factor in the distribution of fluke 
diseases, it does not always follow that an abundance of trematodes follows 
an abundance of mollusks. Thus in certain parts of Japan and in the Yangtse 
Valley, China, fluke infection of man and other animals is common. Yet in 
North China, where the sanitary conditions are equally bad, trematodes are 
relatively rare. 
Taken as a whole, one is impressed with the large number of individuals 
of such species as Lymnaea natalensis and Physopsis africana which Dr 
Cawston has examined. Infection is heavy in April and decreases toward 
July as the cold season comes on. Infection of L. natalensis is relatively light 
in the Transvaal and heavy at Durban. The number of infected individuals 
of P. africana is moderately few in both Natal and across the Yaal. Linked 
up with this fact is the paradoxical one that P. africana is infected with the 
highest number of species of cercariae (nine) and is the only host known for 
six of these species. L. natalensis has the next highest number of species (six) 
