Cryptocotyle lingua (Creplin) (Digenea: Heterophyidae): observations on the morphology of the redia, with special reference to the birth papilla and release of cercariae

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Abstract
Summary: Rediae of Cryptocotyle lingua were studied using transmission and stereoscan electron microscopy. The mouth is surrounded by numerous ciliated sensory structures. Posteriorally, the body wall is concentrically folded and its surface area is greatly increased by the closely ridged configuration of the outer membrane of the tegument. Attached to the surface, especially at the anterior end, are numerous, small spherical vesicles. At the posterior end there is a retractable papilliform process. The appearance of the surface of the birth papilla is variable. The birth pore is lined by an epithelium which is specialized to withstand extreme distortion. A single cilium is frequently visible in the birth pore lumen. The release of each cercaria is preceded by the emission of a tiny drop of fluid from the birth pore. It is suggested that this fluid acts as a lubricant to the passage of the cercaria through the pore.