ИСТИНА

Axial complex of organs (ACO) of Crinoidea significantly differs from the ACO of other modern echinoderms. We have to decide, are these peculiarities plesiomorphic or derived? We studied the structure of ACO of Himerometra robustipinna (Carpenter, 1881) (Himerometridae, Comatulida). Axial organ is located within the cavity of axial coelom. Axial organ extends along the oral-aboral body axis, and only in the most oral part it deviates from the central axis to the interradius AB. In the axial organ, we identify the lacunar region and the tubular region. The tubular canals of the axial organ have the cuboidal epithelial lining, and end blindly both on the oral and aboral sides. Axial coelom, perihaemal coelom, and genital coelom are separated from the general perivisceral coelom, but they connect with it and with each other via numerous openings. The oral haemal ring lies within the cavity of the perihaemal coelom, and communicates with the axial organ. In addition, the axial organ communicates with the haemal vessels of the intestine and with the genital haemal lacuna. Numerous thin stone canals pierce the spongy tissue of the oral haemal ring and at the same time they serve as a support for it. On the oral side, each stone canal opens into the water ring. Stone canals do not communicate with the environment. Instead of them, the numerous slender tegmenal pores, which pierce the epidermis of the oral side of the calyx, open to the environment. Pore canaliculi of the tegmenal pores lead into the bubbles of the perivisceral coelom. For the crinoid's ACO, it is characteristic the polymerized condition of some structures, which are oligomerized in other echinoderms, for example, numerous tegmenal pores and numerous stone canals. Numerous tegmenal pores of Crinoidea are the mere openings, which are scattered over the oral surface of the calyx. They are not gathered into the specialized madreporic plate as in Echinoidea, Asteroidea, and Ophiuroidea. We can consider this feature as a plesiomorphic state. At the same time, the perihaemal coelom of Crinoidea, which surrounds the oral haemal ring, is, possible, the result of fusing left axocoel and both somatocoels, which remain separate in other modern echinoderms. Herewith, the histological structure of the axial organ of Crinoidea is more similar to the axial organ (glomerulus) of Hemichordata. There are numerous coelomic tubular canals, between which the haemocoelic lacunae lie, in the axial organ of Crinoidea and Hemichordata. Thus, ACO of Crinoidea has a plesiomorphic state and possesses a similarity to the ACO of hemichordates. It corresponds to the basal position of Crinoidea among the modern echinoderms.