24b Differentiation of Schistosomes 
which these schistosomes may develop in other snails than those that serve 
as their common intermediary host. (See also Cort, 1918.) 
In a specimen of Physopsis africana which I sent him from Natal, Faust 
(ix. 1920) has reported the presence of the cercariae of S. haematobium and 
S. mansoni , as well as Cercaria octadena which he (ix. 1921) regards as a de¬ 
velopmental stage of S. bovis. Since S. bovis in Natal has only been recorded 
from experimental animals and possibly some Natal boys and, as S. mansoni 
has been seen in only one patient in Natal and in experimental animals, it is 
difficult to understand how one individual snail from the Durban suburbs 
can have been exposed to infestation by the miracidia of all three schistosomes. 
However, the observation shows that the development of miracidia in Physopsis 
africana is of no value as a means of differentiating these three schistosomes. 
The association of such closely allied species in the same individual host 
is of great interest and emphasises the need of identifying the various schisto¬ 
somes in their free-swimming stage (Faust vi. 1920). The methods of locomo¬ 
tion of the various cercariae and the relative length of their prongs are of more 
differential value than their various total lengths. A determination of the 
number of pairs of mucin glands is one of the most reliable means of deter¬ 
mining the species to which a cercaria belongs; but the chief means of differ¬ 
entiating cercariae is by a comparison of the adult forms and, where there is 
mixed infestation and the cercariae are not readily distinguishable, the task 
becomes more complex. Soparkar states that the ‘'structural correlation 
between a known cercaria and one under investigation may at best be sug¬ 
gestive of their possible identity, but the final proof must rest with the 
experimental rearing from them of identical adults.” 
I have treated guinea-pigs with cercariae from a snail which was 
heavily infested with eye-spotted cercariae resembling C. frondosa but, on 
post-mortem dissection, have found schistosomes which could not possibly 
have developed from eye-spotted forms. If the cercariae responsible for the 
infection were so few as to be overlooked amongst the commoner species of 
parasite, it will be readily understood how difficult it is to determine those 
cercariae which represent the larval stage of the rarer schistosomes. It is 
certainly surprising to obtain successful experimental infection by using 
schistosomes that are so few as to be overlooked, when other animals which 
are injected and fed on very numerous schistosomes on several occasions show 
no sign of infection at the end of four months. 
With a large number of snails bred in captivity, and exposed to infection 
with the various miracidia, it should be possible to correlate the various cer¬ 
cariae with the adult worms used in the experiments, but the rarer schisto¬ 
somes are hard to obtain and the miracidia of fasciolae take a long time to 
hatch out. However, from clean bred Physopsis africana to which numerous 
ova of S. haematobium (readily obtained in Natal) are added we can obtain 
typical cercariae of S. haematobium at will. We have yet to determine the life- 
history of many cercariae commonly encountered in fresh-water snails from 
