Axial complex of Crinoidea: Comparison with other Ambulacraria.

The anatomy of Crinoidea differs from that of the other modern echinoderms. In order to see, whether such differences extend to the axial complex as well, we studied the axial complex of Himerometra robustipinna (Himerometridae, Comatulida) and compared it with modern Eleutherozoa. The axial coelom is represented by narrow spaces lined with squamous coelothelium, and surrounds the extracellular haemocoelic lacunae of the axial organ. The latter is located, for the most part, along the central oral-aboral axis of the body. The axial organ can be divided into the lacunar and tubular region. The tubular coelomic canals penetrating the thickness of the axial organ have cuboidal epithelial lining, and end blindly both on the oral and aboral sides. The axial coelom, perihaemal coelom, and genital coelom are clearly visible, but they connect with the general perivisceral coelom and with each other via numerous openings. The haemocoelic spaces of the oral haemal ring pass between the clefts of the perihaemal coelom, and connect with the axial organ. In addition, the axial organ connects with intestinal haemal vessels and with the genital haemal lacuna. Numerous thin stone canaliculi pierce the spongy tissue of the oral haemal ring. They do not connect with the environment. On the oral side, each stone canaliculus opens into the water ring. The numerous slender tegmenal pores penetrate the oral epidermis of the calyx and open to the environment. Tegmenal canaliculi lead into bubbles of the perivisceral coelom. Some structures of the crinoid axial complex (stone canaliculi, communication between different coeloms) are numerous whereas in other echinoderms these structures are fewer or only one. The arrangement of the circumoral complex of Crinoidea is most similar to Holothuroidea. The anatomical structure and histology of the axial complex of Crinoidea resembles the "heart-kidney" of Hemichordata in some aspects.

Axial complex and associated structures of the sea urchin Strongylocentrotus pallidus (Sars, G.O. 1871) (Echinodermata: Echinoidea).

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.