The Gyrocotylidea, a small and enigmatic group of intestinal parasites of chimaeras, has been considered to be related either to the Monogenea, or, more frequently, to the most primitive monozoic tapeworms (Cestoda), i.e., the Amphilinidea and Caryophyllidea. The present study, based on transmission electron microscopical observations of a species of Gyrocotyle from the rabbit fish, Chimaera monstrosa, in the North Atlantic, demonstrates for the first time the presence of microtriches as surface structures of gyrocotylideans. Because microtriches are considered to be an autapomorphy of tapeworms (Cestoda), in which they differ from other Neodermata (Monogenea and Trematoda), the present data represent another source of evidence in support of a close relationship between the gyrocotylideans and the tapeworms sensu stricto (Eucestoda). Simple morphology, small size, and shape uniformity of the microtriches of Gyrocotyle sp. may indicate they represent an original (plesiomorphic) form that then evolved in more derived cestode groups into a variety of types present mainly on the scolex. The microtriches of Gyrocotyle sp. resemble those found in caryophyllidean, spathebothriidean, pseudophyllidean, and trypanorhynch cestodes, which are considered to represent the most basal groups of the Eucestoda.

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