Some stylet cercariae and a microphallid type in British freshwater molluscs

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Abstract
Cercaria helvetica XXXIII Dubois and the cercaria of Sphaerostoma bramae (Müll.) are redescribed and their metacercariae, which occur in various invertebrates, are described. Additional observations are made on C. parvus Khan and C. tarda Khan and their metacercariae, occurring mainly in arthropods, are described. C. wentloogensis sp.nov. has four pairs of penetration glands, a slender, thickened stylet 0·03 mm long and the flame-cell formula2[(2 + 2 + 2) + (2 + 2 + 2)].C. rumniensis sp.nov. has four pairs of penetration glands lying dorsally to the acetabulum, a slender thickened stylet 0·03 mm long and at least 20 flame cells arranged in pairs. C. octoglandulata sp.nov. is a virgulate species with four pairs of penetration glands, a partially thickened stylet 0·029–0·032 mm long and the flame-cell formula 2[(3 + 3 + 3) + (3 + 3 + 3)]. Each of these new species develops in sporocysts in the digestive gland of the prosobranch mollusc Bithynia tentaculata. A cercaria occurring in the digestive gland of B. tentaculata and assigned to the Microphallidae on the structure of its metacercaria is considered to encyst in the digestive gland without emerging. It bears no stylet or penetration glands and the cysts occur in the snail in exceedingly large numbers.All of these cercariae were found in freshwater molluscs collected at the Wentloog level near Cardiff, South Wales.