Coelomic System of the Sea Urchin Strongylocentrotus pallidus (Echinodermata, Echinoidea) and the Issue of Coelomic Metamery in Echinoderms.

The representatives of the main phylogenetic clades of Bilateria are characterized by metamery. In Deuterostomia, metamery is presented in hemichordates and chordates. Sea urchins have 7 metameric coelomic rings located along the oral-aboral axis of the body. A similar coelomic metamery is also a sign of representatives of other classes of echinoderms. We hypothesize that the metameric coelomic rings in echinoderms are inherent of the common ancestor of Deuterostomia. Asymmetry in the structure of echinoderm coeloms is the result of ancestral forms lying on the right side of the body, which led to a reduction of the coeloms on the right side. During the sedentary lifestyle, the left-side coeloms formed metameric coelomic rings.