ИСТИНА

Although molecular data have confirmed the position of phoronids among typical Trochozoa, supporting data from comparative embryology and morphology are lacking. [Ph. harmeri] produces spermatophores, which have a sail and can float to female. The development is holopelagic. Egg cleavage can be regarded as transitory between a typical radial and typical spiral pattern. The presence of oblique furrows in early phoronid development should not be interpreted to mean that phoronid cleavage is typically radial. At the same time, phoronids lack a specific mosaic of blastomeres, whose presence is characteristic of true spiral development. Phoronids also lack trochoblasts, rosettes, and cross cells. Nondeterministic cleavage with an oblique position of furrows allows the blastula to form at the 16-cell stage, which is important for species with holopelagic development. 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 (esophagus and proctodaeum). The entoblast gives rise other parts of the digestive (cardiac sphincter, stomach, and midgut). Mesoblast is the source of the coelomic lining of the preoral coelom, muscles, and blood corpuscles. Phoronids combine features of Protostomia and Deuteroatomia. Apparently, these features (nondeterministic cleavage of the egg; formation of blastule early in development; two sources of mesoderm; etc.) 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.