ИСТИНА

Phoronida is a small group of marine invertebrates, whose position among Bilateria was reappraised due to new data of molecular phylogeny. Although classical data of comparative anatomy and embryology evidence the position of phoronids among deuterostomes, new molecular analysis reveals their position among typical trochozoans. According to data of comparative anatomy and embryology, both phoronids and deuterostomes have three coelomic compartments: proto-, meso-, and metacoel. During phoronid embryogenesis, two sources of coelomic mesoderm arise: anterior and posterior. These sources can be found in all bilaterians and, apparently, were inherited from the last common ancestor and associated with two poles of the slit-like blastopore of diploblastic ancestor. In phoronids, larvae and adults have both bipartite and tripartite type of coelomic system organization. In adults, the metacoel contains one pair of lateral mesenteries, which can be regarded as dissepiments between two trunk segments. The closest relatives of phoronids, brachiopods, also exhibit two types of coelomic system organization: bipartite (in the most known brachiopods) and tripartite (in Lingula anatina). Brachiopods have two pairs of lateral mesenteries and their trunk consists of three segments. Among other lophotrochozoans bipartite coelom is known in annelids (Canalipalpata), which have postoral coelom in the palpi and metameric metacoel. In ecdysozoans, coelomic metamery is lost during embryogenesis, but they have deutocerebral segment, which bears long sensitive appendages. Most of deuterostomian have preoral, postoral, and metameric trunk coeloms. The presence of similar organization of the coelomic system in all bilaterians allows to suggest the presence of preoral, postoral, and metameric trunk coeloms in the last common bilaterian ancestor. This idea can be supported by data about expression of Hox genes in preoral, postroral, and trunk segments of annelids, arthropods, and deuterostomes. This study was supported by RFBR (project No. 14-04-00238).