ИСТИНА

Phoronids are marine invertebrates with biphasic life cycle. Planktotrophic larvae of phoronids have complex nervous system, which is different in holopelagic and brooding phoronid species. We have discovered new type of larval nervous system in a new phoronid species that has unique for phoronids type of development – viviparity, in which mothers release fully developed early larvae into the environment. Early larva has apical organ and two main nerves. These nerves form two circles around the remnant of the blastopore. First of these nerves underlies the preoral and postoral ciliated bands. Such a unity of the preoral and postoral ciliated band is known in deuterostomian larvae. Second nerve extends along the preoral ciliated band and gives rise to the neurites of the oral field. The main nerve ring, which is the first neurite bundle for appear in other phoronid larvae, is absent in early larvae of viviparous phoronids. Although the main nerve ring is presence in competent larvae of viviparous phoronids, it does not give rise to the T-like intertentacular neurite bundles, which are characteristic of larvae of other phoronids. The minor nerve ring, which is prominent in all planktotrophic phoronid larvae, is not developed in larvae of viviparous phoronids. In competent larvae of viviparous species, the apical organ does not contain basal neurons as it does in larvae of other phoronids. The development and simple organization of larval nervous system in viviparous phoronids may correlate with specific type of development as well as it may reflect the ancestral state of larval nervous system in phoronids.