ИСТИНА

Metamerism is present in all the main bilaterian groups viz. Lophotrochozoa, Ecdysozoa, and Deuterostomia. Among the latter, metamerism is apparent in chordates and hemichordates. In echinoderms metamerism can be seen in the arrangement of the left somatocoel, which gives rise to several rings along the oral-aboral axis: in echinoids – 5, in holothuroids and ophiuroids – 4, in asteroids – 4 or 3, in crinoids – 2. This allows to hypothesize that the common ambulacralian ancestor had paired preoral coeloms (homologues of the proboscis coeloms), paired circumoral coeloms that gave rise to tentacles (homologues of the collar coeloms) and few metameric trunk coeloms. Echinoderm ancestors lay on the right side of the body and that led to the reduction of tentacles on the right side and the right hydrocoel and to the prevailing development of left trunk coeloms. Carpozoans are considered to be ancient echinoderms that lived lying on their right side, most likely on soft soils. The incomplete radial symmetry developed secondarily as a result of the attachment of pelmatozoans to a substratum and retained after the detaching of eleutherozoan ancestors. Within that hypothesis, echinoderms retain 3-5 coelomic segments in addition to the axocoel and hydrocoel, which means that echinoderms can be considered metameric animals.