ИСТИНА

Structure and function of the coelom in brachiopods are still poorly studied. The coelomic system of the articulate brachiopod Hemithyris psittacea is composed of the perivisceral cavity, the canal system of the lophophore, and the periesophageal coelom. The perivisceral cavity is the largest part of the coelomic system, it connects to the exterior through nephridia. There is no doubt that the nephridia play an important role in excretion. However, down to here, it was unknown, where does process of ultrafiltration take place? Recently, podocyte-like cells, and therefore supposed site of the ultrafiltration, were described in the periesophageal coelom in articulate brachiopods Hemithyris psittacea, but this coelom is not connected with the perivisceral cavity drained through nephridia (Kuzmina, Malakhov, 2011). We studied the microscopical anatomy of the perivisceral coelom of Hemithyris psittacea by light and transmission electron microscopy. The perivisceral cavity is subdivided by two types of mesenteries into interconnected compartments. First type is a dorsoventral mesentery. It is located in the sagittal plane and attaches the intestine to the body wall. Second type is represented by lateral mesenteries: gastroparietal and ileoparietal. The gastroparietal mesentery runs along the dorsal side of the stomach and continues along both sides of the digestive tract, attaching it to the lateral body walls. The ileoparietal mesentery runs along the dorsal side of the intestine (in the region where the stomach transits into the midintestine) and continues on both sides of the digestive tract, attaching to the lateral walls of the body. The gastroparietal and ileoparietal mesenteries are linked with each other by lateral bands running along the stomach symmetrically on the right and on the left. The species studied has two pairs of nephridia, the dorsal and ventral ones. The funnels of the dorsal pair are located on the gastroparietal mesentery where it touches the lateral body walls. The funnels of the ventral pair are located on the ileoparietal mesentery in the regions where it is in contact with the lateral body walls too. The mesenteries consist of two coelomic epitheliums and extracellular matrix (ECM) composed of two laminae densa and electron transparent lamina fibroreticulosa between them. The striated fibers of collagen are loosely dispersed in the electron-light lamina fibroreticulosa. Amoebocytes with numerous processes occur among these fibers. The coelomic epithelium of mesenteries are formed by epithelial monocilliary cells, which are joined by zonulae adhaerens and attach to the basal lamina with sparse hemidesmosomes. Sections of the mesenteries located close to intestine are partly formed by podocyte-like cells with long processes that extend along the basal lamina. The diameter of the processes ranges between 0.1 and 2 µm. There are spacious gaps between podocyte-like cell processes where the basal lamina contact directly with the perivisceral coelom. Perhaps, these areas are the sites of ultrafiltration of fluid from ECM through thin basal lamina into the perivisceral coelom. We suggest that lamina fibroreticulosa of the ECM in mesenteries is fluid and functions as blood sinus. This assumption is supported by presence of amoebocytes in the ECM of mesenteries as well as in ECM of intestine, mantle, and in the tentacle vessels. ECM surrounding the intestine of brachiopods corresponds to the circumintestinal blood sinus, which connects with heart cavity and with dorsal blood vessel. The sinuses of the mesenteries connect with the intestine blood sinus. Besides, the sinus of dorsoventral mesentery connects with the dorsal vessel while the sinuses of lateral mesenteries connect with the nephridium’s vessels.