Fig. 8

Ultrastructure of the first septum and opisthosomal segments of the late metatrochophore of Siboglinum fiordicum, parasagittal sections. A The first septum formed by bottleneck-shaped myoepithelial cells. B Myoepithelial cell of the first septum bearing the myofilaments. B’—cells contacted with adherens junctions (aj) and septate junction (sj). C The sagittal section of the segmented opisthosoma, segments are divided by non-muscular septa, which are the second in order of formation (sII), marked by arrowheads. sII at this stage is just thin layers of the ECM (sometimes with blood), they have no muscle fibers, but they contacted to the myoepithelial cells of the body wall (at place of moc1-6). aj—adherens junctions, bv—blood vessel, co—opisthosomal chaetae, gr—gut rudiment, m—mesotroch, mcn—large nuclei of the longitudinal myoepithelial cells, mf—myofilaments, moc1-6—the first-sixth circular muscle bundles in the opisthosoma, py—pygidial area, sj—septate junction, sI—the septum that is formed the first in order of formation, dividing trunk and the first opisthosomal segments, sII—the septa that are formed second in order of formation, dividing the opisthosomal segments, simultaneously formed