ИСТИНА

The existence of a preoral coelom in phoronids is still debated. We studied the development and organization of the preoral coelom in [Phoronopsis harmeri] using TEM, histology, and light microscopy. The first cavity of the preoral coelom appears at the early gastrula stage (40 h after spawning) and originates from the mesodermal cells that migrated from the upper wall of the archenteron. The small, closed cavity is situated above the mouth on the ventral side. As the form of the embryo changes with development, this cavity migrates under the apical plate. In young larvae, the preoral coelom is closed, cylinder-shaped, and situated under the apical plate; it is easily observed if the larvae are living. The cells of the coelomic cylinder vary in structure. In general, the walls of the preoral coelom consist of monociliate epithelial (or myoepithelial) cells, which are connected via desmosomes and are underlaid by a thin ECM. Thus, the lining of the protocoel should be considered a true epithelium. The preoral coelom is retained throughout all stages of metamorphosis. In adults, the preoral coelom has a complex shape with a central part and two branches. It is a closed cavity that is situated above and contacting the lophophoral coelom, forming two complete dissepiments on the lateral sides and a partition with many holes in the center. The cavity is completely lined with an epithelium consisting of myoepithelial cells that are interconnected via median adherens junctions. The cells rest on the ECM. Some coelothelial cells form the radial musculature of the epistome and cross the coelomic cavity from the frontal to abfrontal side. We infer that some phoronids (e.g., [Phoronopsis harmeri]) have a true closed preoral coelom through all stages of the life cycle.