Microscopic anatomy of the axial complex in the starfish Asterias rubens (Echinodermata, Asteroidea)

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 perihemal 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.