ИСТИНА

The Phoronida is a group of marine animals whose position in the Bilateria has radically changed as a result of recent molecular phylogenetic analysis. Because embryological data have significant value for rate of phylogenetic position, the study of Phoronid embryogenesis is relevant. Phoronid egg cleavage can be regarded as transitory between a typical radial and typical spiral pattern because of presence of the oblique furrows of the fourth cleavage in some phoronid species. The coelomic mesoderm arises by anterior and posterior sources, which are multicultural. Organogenesis starts in the later gastrula. The ectoblast gives rise to the epidermis, ciliated bands, nervous system, excretory system, and some parts of the digestive tract (the esophagus and the lower portion of the proctodaeum). The excretory system forms as an unpaired ectodermal protrusion that develops two branches (left and right), which then separate and form two protonephridia. The stomach, the midgut, and the upper portion of the proctodaeum arise from the entoblast. The coelomic lining is endoblast derivate, because metacoel arises as dorsal outgrowth of the proctodaeum upper portion. The mesoblast is the source of the coelomic lining of the protocoel and mesocoel, muscles, and blood corpuscles. In summary, phoronids combine trochozoan- and deuterostome-like features (nondeterministic cleavage of the egg; formation of blastule early in development; two sources of mesoderm; simultaneous occurrence of the mouth, nervous system, and muscles; etc.), which are plesiomorphic and inherited from the common Bilateria ancestor. On the phylogenetic tree, phoronids should therefore be regarded as one of the basal groups of the Lophortochozoa. This work was supported by RFBR grants 11-04-00690, 12-04-33045.