ABSTRACT
The axial organ of Asterias rubens is a mesenteric organ made of a lacunar -or haemal-connective tissue surrounded by a coelomic epithelium (mesothelium). The axial organ is enclosed in a narrow coelomic cavity (the axial sinus) toped by a minute closed cavity (the dorsal sac) in which the uppermost part of the axial organ (the head process) occurs. The place where the head process arises from the axial organ is a major haemal junction: it is also the place where the haemal circuits of both the gonads and the digestive system connect the axial organ. These haemal structures are in perfect haemal continuity. However, each of them is enclosed in a distinct non-communicating coelomic compartment. The mesothelium that lines the dorsal sac lumen also occurs around the head process. It consists of a contractile pseudostratified epithelium made of two cell layers: an upper layer of adluminal cells and a lower layer of myoepithelial cells. The inner part of the head process consists in a loose connective tissue that includes wide haemal lacunae. The possible involvement of the dorsal sac/head process complex in the haemal fluid translocation from the digestive system to the gonads is discussed in the light of its topographical and morphological characteristics.
