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The axial complex is one of the most characteristic features of echinoderms. Descriptions of it in classic papers are contradictory. The present paper provides data on the microscopic anatomy of the axial complex in the starfish Asterias rubens restudied using histological techniques. The axial complex is located in the wall of the interradius CD; it is elongated in the oral-aboral direction. The stone canal in its aboral end communicates with the axial coelom; in its oral end, it communicates with the water vascular ring canal. The axial coelom opens to the madreporic ampulla of the stone canal on its aboral end and to the axocoel perihe mal coelom on the oral end. The axial organ is formed by the blood vessel network between the basal laminae of the pericardial, axial, and perihemal coelothelia. In Asterias rubens, the heart is a part of the axial organ, which divides the latter into the true axial and pericardial parts. The extensive axial (oral) part of the axial organ opens to the oral blood ring, which is situated in the mesentery between the perihemal coeloms. The smaller, pericardial (aboral), part of the axial organ opens to the hemocoel of the body wall. The genital coelom represents an integral part of the axial complex since one of the five gonad blood lacunae located in the interradius CD communicates with the heart and axial organ vessels. The mistakes and inaccuracies extant in the scientific literature about the anatomy of the asteroid axial complex are discussed. The blood circulation into the axial organ occurs predominantly from the aboral side to the oral side of starfish.

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Studies of echinoid microscopic anatomy over the last two centuries have created a number of inaccuracies and mistakes that have accumulated in the descriptions of the intricate organization of the coelomic system of Echinoidea. To clarify the situation, we reconstructed the axial complex and radial complex of the echinoid Strongylocentrotus pallidus. The water ring is located between the perivisceral coelom and the perioral coelom. The oral haemal ring lies between the coelothelia of the water-vascular, perivisceral, and perioral rings. The axial part of the axial organ communicates with the oral haemal ring in interradius CD, but the axial coelom does not form the axocoelomic perihaemal ring. The ventral intestinal haemal vessel originates from the oral haemal ring in radius A, and then branches into a network of capillaries, from which the dorsal intestinal vessel starts. The pericardial coelom envelopes the pericardial part of the axial organ, the lacunae of which communicate with the haemocoel of the body wall and with the axial part of the axial organ. The genital haemal ring and the dorsal intestinal vessel communicate with the axial organ. The genital coelom passes in the CD interradius on the side opposite to the hindgut. There is a somatocoelomic perihaemal ring, which sends a pair of coelomic outgrowths into each radius, accompanied by a radial haemal vessel in the oral part. The mistakes and inaccuracies of early descriptions of the echinoid axial complex are listed. The axial complex and associated structures of sea urchins are compared with other eleutherozoans.

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