SOME SOUTH AFRICAN SNAILS AND THE CERCAkLi: 

 WHICH Al TACK THEM. 



By Frederick Gordon Cawston, B.A., M.D., B.C., Al.k.C.S.. 



L.R.C.P. 



{Abstract.) 



During the last four years I have exposed to microscopic 

 examination 3,000 molluscs from various rivers and stagnant 

 pools in Natal and the Transvaal. 



Four different species harboured, trematode parasites in 

 tlieir cercarial stage : I procured the finest specimens of Linnuca 

 )iatalcitsis from Rustenburg. My observation oif Physopsis afri- 

 cana, containing cercarise at Klerksdorp established a new locality 

 for this mollusc. The genus Isidora is notoriously polymor- 

 phous, and the species very variable ; but I have obtained a large 

 number of a new variety from Potchefstroom and Klerksdorp 

 which agree very well with Jickeli's description and figure of 

 schakoi. Mr. H. C. Burnup has no hesitation in identifying 

 them as Isidora scliakoi Jickeli. Plauorbis pfciffcri is conmiiu 

 at Durban and Rustenburg. 



The other species showed no evidence of cercarial infection : 

 they included Lmvna^a tmncatiila, Isidora tropica^ Isidora annpta 

 M. & P. (from Pietemiaritzburg, a new locality), Isidora fors- 

 kali, Plauorbis leucochcilus, Planorbis gibbonsi Nelson (from 

 Potchefstroom, a new locality), Ancylus (from Pietemiaritz- 

 burg, Klerksdorp. Potchefstroom, Mulder's Drift, and Pretoria), 

 ferrissio (from Pretoria), Uiiio caffer Krauss and Corbiciila 

 radiata. 



Schistosomum hcctnatobium occurred in Physopsis africana 

 from Durban (Toll Gate), Pietermaritzburg (Umsindusi), Rus- 

 tenburg, Magaliesburg, and Mulder's Drift. No evidence of 

 redia formation could be seen in infested specimens. The head 

 of the cercaria varied in shape, but the average full length of the 

 cercaria was 0.6 mm. The prongs of the forked tail were a 

 quarter of the length of the tail. These flat prongs are said to 

 enable the cercaria to suspend itself head-downwards from the 

 surface of the water. 



Cercaria sccobii is a narrower cercaria, with the appearance 

 of an avian trematode, found only at Pietermaritzburg. 



Cercaria gladii occurred in Isidora scliakoi at I'otchef- 

 .stroom. It develops in sporocyst : the prongs are equal in 

 length to the rest of the tail, and are sword-Hke. There is no 

 pharynx. 



At Durban an eye-spotted cercaria, Cercaria oculata, can 

 only be differentiated from Schistosome because of its eye-si)ots. 



Various leptocercous cercarise occurred in Limncca imtal- 

 ensis, Physopsis africana, Plauorbis pfeifferi. and Isidora sclia- 



