The Structure of the Muscular and Nervous Systems of the Orthonectid Rhopalura Litoralis (orthonectida) or What Parasitism Can Do to an Annelid

Orthonectida is an enigmatic group of parasitic invertebrates with an unclear taxonomic position. Recent molecular studies demonstrated that Orthonectida belongs to Annelida; however, the lack of morphological data does not allow to follow the evolutionary pathway from free-living annelids to parasitic orthonectids. Here, we studied the nervous and the muscular systems in the male and female orthonectid Rhopalura litoralis using confocal laser scanning microscopy and immunohistochemistry. The muscular system is formed by four outer longitudinal muscular bundles and several inner transversal muscles. The nervous system of females is represented by a well-developed cerebral ganglion and a nerve plexus in the body. In males, the cerebral ganglion is significantly smaller, and the body plexus is absent. Instead, a pair of nerves with three pairs of serially organized nerve cells runs posteriorly from the ganglion along the lateral sides of the body. Analyses of the structure of all the orthonectids studied so far suggest that reduction and simplification of the free-living males and females are the dominant mode of evolution in orthonectids.