The nervous system of the actinotroch larva of Phoronis muelleri (Phoronida)

Summary The nervous system of the actinotroch larva of Phoronis muelleri has been investigated with the transmission electron microscope (TEM). Attempts have been made to localize all of the major nerves and to reveal the cytoarchitecture of the apical ganglion. The nervous system is intraepithelial in position and consists of an apical ganglion, located on the epistome, with at least four different cell types, including monopolar sensory cells and mono- or multipolar neuron-like cells. From the anterior part of the apical ganglion three median nerves extend to the edge of the epistome; two of these nerves connect to nerves which follow the edge of the epistome all the way to the junction of the epistome and the mesosome. From the posterior part of the ganglion extend two lateral nerves which continue along the tentacular ring. Each tentacle has three nerves located on the frontal side which connect to the nerve ring along the tentacles. Along the posterior ciliary band is a minor nerve ring. In addition, a nerve net is found on the epistome, mesosome, and metasome, but no longitudinal nerves were observed between the posterior ciliary band and the apical ganglion. All nerve cells were found in the apical ganglion and none was observed along the nerves. Sensory cells (probably mechanoreceptors) are located in two rows on each tentacle; sensory organs such as eyes and statocysts were not observed.Abbreviations ac accessory centricle - aen anterior epistome edge nerve - af abfrontal cells - bl basal lamina - bl.c blastocoel coelomocyte - ci cilium - co collar - cp cell process - cr ciliary root - ec₁ epistome edge cell type 1 - mne mouth nerve ring - mo mouth - mp metasomal pouch - ms mesosome - mt metasome - mu muscle - n nerve process - ne nerve - np neuropil - nu nucleus - pc₁ posterior ciliary band cell type 1 - ec₂ epistome edge cell type 2 - ec₃ epistome edge cell type 3 - epi epidermis - es epistome - ese epistome edge - fc frontal cell - gc₁ type 1 ganglion cells - gc₂ type 2 ganglion cells - gc₃ type 3 ganglion cells - ge gut epithelium - ij intermediate junction - laf lateroabfrontal cell - lc lateral cell - lfc laterofrontal cell - lgc lateral ganglion cell - me metacoel epithelium - lne longitudinal median epistome nerves - pc₂ posterior ciliary band cell type 2 - pc procoel - pe procoel epithelium - pen posterior epistome edge nerve - pr posterior ciliary band - p.rec proximal recess of procoel epithelium - prne nerve ring along posterior ciliary band - sj septate junction - sne secondary nerve along the tentacular ring - t tentacle - tr tentacular ring - trne horseshoe-shaped nerve along the tentacular ring