[Phylum Orthonectida: morphology, biology, and relationships to other multicellular animals]

New data on morphology of the ciliary cells, sex pore, cuticle formation, sperm cell, and plasmodium in the Orthonectida were obtained. Interdigitating ciliary cells constitute a single-row epithelial layer underlied by the intercellular matrix but the basal plate formed by the matrix is not solid. Orthonectids are shown to possess the genuine muscle system consisting of the outer circular and inner longitudinal muscles. Metameric arrangement of the circular muscles in Intoshia linei, Rhopalura litoralis, and Rhopalura philinae corresponds to the integument netamery, the circular muscles underlying the circles of non-ciliary cells. In different species the longitudinal muscles form 4 to 6 bilateral cords. The longitudinal muscles provide body bending during pivots of the ciliary locomotion; they are also involved in the copulation as well as in the extrusion of genital pore cells, sperm cells, and larvae. Formation of the muscle cords in the embryogenesis is preceded by the stage of a solid layer of presumptive myoblasts at the boundary between outer and inner cell layers. Multicellular receptor in adult orthonectids is described. Finding both the muscle system and the multicellular receptor evidences for the existence of the nervous system in the orthonectids. A group of cells forming a putative ganglion is revealed in the anterior part of the ciliary individuals. Orthonectid cuticle is of microvillar type resembling that of Annelida. Comparison of reproductive individuals in Orthonectida and Dicyemida indicates that these taxa are distinct phyla belonging to quite separate branches of the Metazoan phylogenetic tree. New data on morphology and biology of the orthonectid plasmodium are obtained. The mechanism of the orthonectid leaving their hosts is described: the reproductive individuals move within the plasmodium processes that grow through the host tissues and contact with the environment. The plasmodium is shown to be covered with two membranes and is proved to contain multiple nuclei. The data obtained evidence for the parasitic nature of the orthonectid plasmodium. The orthonectid life cycle includes alternation of the sexual, asexual, and parthenogenetic reproduction phases and can be characterized as a combination of the metagenesis and heterogony. The term "metaheterogony" is suggested to denotate the orthonectid life cycle. By the entire set of the morphological characters, the orthonectids are to be allocated to the Spiralia.