ИСТИНА

The axial complex is one of the most typical features of echinoderms. However, the descriptions of it in various publications are contradictory and the homologization of the whole complex and its organs is still a problem. The organization of the axial complex in two common species of sea stars – Asterias rubens and Asterias amurensis – was restudied with the help of histological, ultrastructural and modern 3D-reconsruction techniques. The axial complex is located in the wall of the interradius CD. It is elongated in the oral-aboral direction. The axial complex consists of the coelomic organs and the haemocoel (blood) structures that are morphologically and functionally integral. The axial organ consists of the blood vessel network between the basal laminae of the pericardial, axial, and perihemal coelothelia. The axial organ divides into the axial and pericardial parts; the latter includes vesicular heart. The extensive axial (oral) part of the axial organ opens into the oral blood ring. In its aboral region, the axial organ is connected with the genital haemal ring and one of the five gonad blood lacunae that is in the interradius CD along with the so called "gastric haemal tufts" (mesenterial swellings with haemal lacunae inside). The coelothelia of the axial part of the axial ogran contains podocytes and myoepithelial cells, whereas the coelothelia of the pericardial part of the axial ogran lacks podocytes. The podocytes can be also found in the coelothelia of the gastric haemal tufts. The possible functions of the axial complex are blood circulation and excretion. The blood circulates through the axial ogran mostly in oral direction. We suppose that the axial organ provides the ultrafiltration of the fluid from the blood lacunae through the basal lamina to the axial coelom, and from there the urine is removed through the madreporite.