Ultrastructure of the neodermal sclerites of Gyrocotyle urna Grube and Wagener, 1852 (Gyrocotylidea, Cestoda).

Body sclerites of Gyrocotyle urna Grube and Wagener, 1852, parasites of Chimaera monstrosa L., were studied by transmission electron microscopy. Each sclerite consists of ten to 15 concentric layers varying in electron density and thickness. The sclerites insert in pockets whose epithelia are continuous with the body neodermis but are 14 times thinner. The pocket neodermis bears surface projections with an inner electron-dense cylinder and a small electron-dense cap at their tip. It is surrounded by a well-elaborated basement lamina and bundles of musculature; the perikarya of this part of the neodermis lie underneath the basement membrane. The luminal plasma membrane of the pockets is covered by a coat originating from vesicles and dense bodies most probably discharged from the pocket neodermis, supporting the assumption that the deposition of this material builds up the sclerites. Therefore, sclerites are derivatives of the neodermis. With regard to their ultrastructure, sclerites resemble the calcareous corpuscles of other cestode taxa. They represent a useful phylogenetic character and are an autapomorphy of the Gyrocotylidea.