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THE CERCARIAE OF THE TRANSVAAL. 
By F. G. CAWSTON, M.D. (Cantab.). 
During the months of September, October, November and December 1917 
I collected 1250 different specimens of fresh-water snails from various parts 
of the Transvaal, more especially from the sources of the AAal and Crocodile 
rivers. All the larger specimens I submitted to microscopic examination for 
possible infection with cercariae. From the mud at the bottom of the Mooi 
river at Potchefstroom, which is a tributary of the Vaal, I came across some 
very finely preserved specimens of Unio caffer Krauss, a large number of 
which contained small pearls attached to the shell. The amount of lime in 
the bed of this river probably accounted for the perfect state of the umbones 
of these Uniones. Amongst the small stones at the bottom of the river I 
collected some remarkably large specimens of Corbicula radiata Parreyss. The 
river also contained some specimens of Pisidium, a much smaller variety, not 
yet identified. On the decomposing rushes at the edge of this river there 
were numerous specimens of Ancylus , for which Mr Henry C. Burnup has 
suggested the name Ancylus cawstoni ; I have seen similar specimens at 
Klerksdorp and in the Hex .river at Rustenburg, a branch of the Little 
Crocodile. All these specimens were free from cercarial infection. 
In the tributary of the Little Crocodile at Magaliesburg, I collected several 
specimens of Limnaea natalensis. One of these was infested with motile 
rediae, provided with oral sucker and a pair of lateral locomotor appendages. 
These rediae contained fully developed leptocercous cercariae with a chain 
of about twenty minute “cysts” on either side of their divided gut. The 
life-history of this trematode worm is at present unknown, but the cercaria 
and redia had the appearance of those I had isolated from Limnaea natalensis , 
Planorbis pfeifferi and Physopsis africana at Durban and described under the 
heading Cercaria catenata in the (American) Journal of Parasitology , March 
1917. A specimen of Physopsis africana from the same pool at Magaliesburg 
contained cercariae typical of the Schistosome group developing in a branching 
sporocyst. The cercariae were not fully developed. The full-sized Bilharzia 
cercaria which I found in specimens of Physopsis at Rustenburg was 0-6 mm. 
in length. At this point on the Hex river, a tributary of the Vaal, 70 % of 
the boys and 10 % of the girls harbour the Bilharzia parasite. The flat, 
fin-like prongs of the tail of the Schistosome cercaria are about one-quarter 
the length of the tail itself. I have found Physopsis along the course of the 
Little Crocodile river at Mulder’s drift also infected with this cercaria. At 
