Phoronida

Abstract

AbstractPhoronids are marine invertebrates with biphasic life cycles. Development of the nervous system starts at the stage of late gastrula on the animal pole of the embryo, which acquires several 5HT-like immunoreactive (-lir) cells in the epidermis of the apical plate. With time, the apical organ—a larval brain—is formed here. It consists of numerous 5HT-lir and FMRF-lir perikarya, which then give rise to the postoral neurite bundle. The development and organization of the nervous system in phoronid larvae has more in common with the deuterostomian than with the protostomian nervous system. During metamorphosis the apical organ is consumed by the juvenile, whereas the postoral neurite bundle retains and gives rise to the definitive tentacular nerve ring. The juvenile inherits from the larva the complex structure of the lophophore nervous system: the regular alternation of the intertentacular and abfrontal neurite bundles. In adult phoronids, the nervous system is represented by the intraepithelial nerve plexus, which is thickened in several areas and forms here the dorsal ganglion and tentacular nerve. All phoronids have two (right and left) or one (only left) giant nerve fibres, which start from the dorsal ganglion and pass along the left and right sides of the trunk. Each tentacle is innervated by six longitudinal neurite bundles connected to the tentacular nerve ring. All nerve elements have a similar cytological organization and stratified structure that is related to the cellular layers. Simplification of the definitive nervous system in comparison with complex larval nervous system may correlate with a sessile life style of adult phoronids.