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

Abstract

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.

Axial complex of Crinoidea: Comparison with other Ambulacraria

“…Organization of the madreporic plate, madreporic ampulla (left axocoel), stone canal, water ring, and left somatocoel in different classes of echinoderms (Hayashi, 1935; Cuénot, 1948; Hyman, 1955; Ivanova‐Kazas, 1978; Erber, 1983a, 1983b; Ivanov et al, 1985; Balser & Ruppert,1993; Ziegler et al, 2009; Ezhova et al, 2013, 2014, 2015, 2017, 2018; Ezhova, Malakhov, & Martynov, 2016). amp, madreporic ampulla; apc, axocoelomic perihaemal ring; axc, axial coelom; hgc, hypogastric coelom (left somatocoel); paxc, perihaemal axocoelomic ring; pvc, perivisceral coelom; sc, stone canals; spc, somatocoelomic perihaemal ring; wr, water ring [Color figure can be viewed at wileyonlinelibrary.com]…”

“…Speaking about the arrangement of the circumoral complex, it is important to note first of all that its arrangement in Asterozoa differs from Echinozoa (Ezhova et al, 2018, p. 805, Figure 9): the oral haemal ring of Echinozoa lies between the water ring (left hydrocoel) and the perioral coelomic ring (one of derivatives of the left somatocoel), and in Asterozoa, the oral haemal ring lies between the special perihaemal rings, axocoelomic and somatocoelomic (i.e., the derivatives of the left axocoel and the left somatocoel). The water ring and the perioral ring in Asterozoa do not closely adjoin to each other like in Echinozoa, but lie at a distance from each other.…”

“…Sometimes, the number of the stone canals may secondarily increase during the ontogenesis of holothuroids (Ivanova-Kazas, 1978). According to some paleontological data (Haugh, 1973;Haugh & Bell, 1980), in fossil camerate crinoids, "function and morphology of the water vascular F I G U R E 1 1 Organization of the madreporic plate, madreporic ampulla (left axocoel), stone canal, water ring, and left somatocoel in different classes of echinoderms (Hayashi, 1935;Cuénot, 1948;Hyman, 1955;Ivanova-Kazas, 1978;Erber, 1983aErber, , 1983bIvanov et al, 1985;Balser & Ruppert,1993;Ziegler et al, 2009;Ezhova et al, 2013Ezhova et al, , 2014Ezhova et al, , 2015Ezhova et al, , 2017Ezhova et al, , 2018Ezhova, Malakhov, & Martynov, 2016). amp, madreporic ampulla; apc, axocoelomic perihaemal ring; axc, axial coelom; hgc, hypogastric coelom (left somatocoel); paxc, perihaemal axocoelomic ring; pvc, perivisceral coelom; sc, stone canals; spc, somatocoelomic perihaemal ring; wr, water ring F I G U R E 1 2 Circumoral complex in different classes of modern echinoderms.…”

“…The axial complex of Eleutherozoa (except Holothuroidea) consists of the following main organs: axial coelom + madreporic ampulla (derivatives of the left axocoel), pericardial coelom (derivative of the right axocoel), axial organ, which consists of an axial part (a mesh of the haemocoelic spaces between the folds of the coelothelium of the axial coelom) and a pericardial part including a heart (a haemocoelic vesicle and capillaries surrounded by the coelothelium of the pericardial coelom), and calcareous stone canal, which supports the soft organs of the axial complex and communicates with the water ring at the oral end and with the madreporic ampulla and the environment at the aboral end. The axial complex is morphologically and functionally associated with the gonads and genital haemal ring, with the perihaemal coeloms and the oral haemal ring, with the intestinal vessels, with the perioral coelom, and with the nerve structures associated with the perihaemal coeloms and epineural canals (Cuénot, 1948; Erber, 1983a, 1983b; Ezhova, Ershova, & Malakhov, 2017; Ezhova, Lavrova, Ershova, & Malakhov, 2015; Ezhova, Lavrova, & Malakhov, 2013, 2014; Ezhova, Malakhov, & Egorova, 2018; Hyman, 1955; Ivanov et al, 1985; Ziegler, Faber, & Bartolomaeus, 2009). In Holothuroidea, the left and right axocoels are reduced, so the axial complex retains only the madreporic ampulla and the stone canal surrounded by the haemal lacuna, which connects with the genital haemal lacuna, with the oral haemal ring, and with the intestinal vessels (Balser, Ruppert, & Jaeckle, 1993; Erber, 1983a, 1983b; Ezhova et al, 2017).…”

A new ophiocistioid with soft-tissue preservation from the Silurian Herefordshire Lagerstätte, and the evolution of the holothurian body plan

“…The tube feet may have been connected to the radial canals through additional canals or branches and large ambulacral cavities, as in modern elasipodid holothurians with greatly enlarged tube feet [34,35]. A stone canal presumably linked the ring canal to the madreporite, as in living echinoids and holothurians [31][32][33].…”