202 F. G. Can>sion 



more slender and round. I have not found any cercariae in thi^ 

 species of snail. The living animal is hard to remove from its 

 shell, and no cercariae have been observed in the water in which 

 numbers of the snail have been kept. Cerithidea decollata L. 

 is a brackish- water inhabitant. 



I have recently found eight specimens belonging to the genus 

 Sep tar ia in the Umbogintwini lagoon. Sepiaria have not pre- 

 viously been recorded in South Africa, though they are found 

 in Mauritius. Mr. H. C. Burnup, who kindly examined them 

 for me and has identified the other species, found the operculum 

 attached to the animal between the foot and the viscera. All 

 four snails were attached to floating sugar-cane, close to the mouth 

 of the river. 



I do not think that the presence of an operculum has been 

 sufficiently recognised as a means whereby the life of a snail in- 

 fested with cercariae can be materially prolonged. Though 

 several species possess much stouter opercula than those I have 

 described, it is interesting to note that, whilst the operculum of 

 Sepiaria is a relatively inadequate one, and that of Cerithidea so 

 slender as to be partly transparent. Tiara tuberculata is provided 

 with a stout well-fitting operculum. Furthermore, at least two 

 distinct cercariae are parasitic in Tiara tuberculata which, of the 

 three species of snail mentioned here, is the one which possesses 

 the stoutest operculum and is thus the best able to resist drought. 

 On the other hand, in breeding Limnaea natalensis^ a common 

 fresh-water snail which does not possess an operculum, I have 

 repeatedly observed specimens crawling up the sides of the tub 

 or glass jar which contains them, and often escaping on to the 

 table or floor. These specimens die if they are not put back into 

 the water before they have had time to become quite dry; for they 

 have no power of resisting dessication, so that the complete drain- 

 ing of a pool containing infested snails which do not possess 

 opercula is a reliable means of preventing infection by killing the 

 snails that harbour the cercariae. 



