ИСТИНА

Development of muscular system in Mooreonuphis stigmatis was studied by phalloidin staining and confocal laser scanning microscopy. M. stigmatis has lecithotrophic larvae developing inside the parental tubes. In each hatch, embryos develop synchronously and are released at a stage of 18–20 chaetigers. The first muscles appear in the morphologically undifferentiated embryo. Seven pairs of segmental transversal elements, five parapodial bundles and two longitudinal muscles develop on the ventral side, the irregular thin longitudinal fibers develop on the dorsal side. Two-chaetiger stage bears complex pharyngeal muscles; five-chaetiger stage forms seven pairs of segmental muscles and parapodial complex consisting of two bundles of fibers. Complete set of muscular elements with muscles of head tentacles develops in 12-chaetiger larvae. M. stigmatis develops body muscular before the musculature of the digestive system. Similar pattern of myogenesis was reported in non-feeding larvae of Capitella while planctotrophic polychaete larvae (Pomatoceros lamarckii and Sabellaria alveolata) form the digestive system before the body wall musculature. Therefore heterochrony in myogenesis of the digestive system and the body wall in polychaetes appears to be related to feeding modes in larval stages.