245 
SOME NOTES ON THE DIFFERENTIATION 
OF CLOSELY-ALLIED SCHISTOSOMES. 
By F. G. CAWSTON, M.D., 
First Streatfeild Research Scholar, Durban, Natal. 
When fresh-water snails are examined microscopically, specimens are oc¬ 
casionally encountered that are infested with the cercariae of more than one 
species. Sometimes these cercariae are easily differentiated. A snail may 
harbour numerous eye-spotted amphistomes and a few distomes without eye- 
spots, or furcocercous forms may be associated with cercariae possessing un¬ 
divided tails. In some collections of semi-stagnant water, however, the same 
individual snail may be found infested with at least two distinct schistosomes. 
In 1916, I found Physopsis africana (in an overflow pool along the course of 
the Umsindusi river at Pietermaritzburg) heavily infested with two distinct 
schistosomes, and it was not uncommon, as Dr E. E. Warren also observed, 
for the two forms to develop in the same host. One of these cercariae I named 
Cercaria secobii, the other was probably the cercaria of Schistosoma haemato¬ 
bium. Soparkar (1921 a and b) has moreover noted a double infection of 
Planorbis exustus near Bombay. 
The two trematode species which cause Bilharziasis in Africa were long 
consideied to be identical, but careful work, conducted during recent years, 
has demonstrated essential differences in the cercarial stages of the worms. 
At one time it was assumed that the worms could be determined in accordance 
vith the species of snail in which the cercariae were found. Thus Leiper (1919) 
stated that cercariae ‘ developing in the Bullinus molluscs always produce 
bilharzia worms which give rise solely to terminal-spined eggs, while those 
which have developed in Planorbis boissyi always become worms which pro¬ 
duce solely lateral-spined eggs.” 
But Physopsis africana has been shown to be the common intermediary 
host for both S. haematobium and S. bovis and less commonly for S. mansoni. 
the cercariae of these species resembling each other even more closely than do 
those of S. haematobium and C. secobii, the latter being a longer and narrower 
form with particularly long prongs to its divided tail (Cawston, 1917). 
Though I have never succeeded in following the development of schisto¬ 
somes in any other species of fresh-water snail than Physopsis africana, the 
presence of schistosomes in various other species which I have found infested 
m Natal streams and the development of S. mansoni and S. haematobium in 
other forms recorded by Porter (1920) shows that there are conditions under 
