Abstract
This cercaria was found to be very common in specimens of Littorina rudis (Maton) and L. obtusata (Linné) collected at Aberystwyth during February and March, 1935, occurring in twenty out of fifty specimens of L. rudis and in four out of forty specimens of L. obtusata. It was, on the other hand, very rare in L. littorea, only two infested individuals being found out of a total of two thousand molluscs examined. In all three species of mollusc the parts of the digestive gland most heavily infested appeared white or creamy-white in colour.The form described here appears to be almost identical with Cercaria ubiquitoides recorded by Stunkard (1932) from L. littorea and L. rudis occurring on the coast of Brittany. Certain differences are found between my specimens and those of Stunkard. It is difficult to determine whether these differences are individual or specific because there are so few larval characters in cercariæ of the Ubiquita group which can be used for separating species. The problem is rendered more difficult by the close resemblance of the adult trematodes (species of the genus Spelotrema).
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