ИСТИНА

The axial complex is one of the most typical features of echinoderms. However, there are many mismatching descriptions of its structure in different sources. The homology of the entire complex and its constituent parts within the Echinodermata or with other organs of Deuterostomia remains problematic. That is why the axial complex in the representatives of three echinoderm classes – the sea star Asterias rubens Linnaeus, 1758, the sea urchin Strongylocentrotus pallidus (Sars G.O., 1872) and the brittle star Ophiura robusta (Ayres, 1854) – was restudied with the help of histological and modern 3D-reconsruction techniques. The comparison of the axial complexes between echinoderm classes is important for understanding of the phylogenetic relations in Echinoidea. The axial complex of Asteroidea, Echinoidea, and Ophiuroidea consists of the coelomic organs and the haemocoel (blood) structures, which are morphologically and functionally integral. The structures of the axial complex are arranged in a similar way in all three classes. Even in brittle stars, in spite of the shift of the madreporite and its ampulla towards the oral side and the turning of the axial complex in oral direction, all its anatomical connections are retained. However, there are several minor differences in the axial complex of this three classes. First of all, starfish and brittle stars possess an axocoel perihaemal coelomic ring that is соnnects with the axial coelom, whereas sea urchins' axial coelom at its oral side ends blindly. The perioral coelom is represented in all the studied Echinoidea and Ophiuroidea, but it is not found in a number of Asteroidea species. There are also some differences in the organization of the haemal structures of the axial complex. The possible functions of the axial complex are the blood circulation and the excretion.