Sensory systems of Lineus ruber (Nemertea, Pilidiophora)

The aim of this study was to examine the organization of sensory systems in the heteronemertean Lineus ruber (Muller, 1774). Sensory systems of the head region (cerebral and frontal organs), body wall, proboscis and digestive tract of L. ruber were studied using Golgi–Colonnier silver impregnation, glyoxylic acid-induced fluorescence method for catecholamines (CAs) and immunochemical staining for serotonin (5-HT) and FMRFamide-related peptides (FaRPs). The study revealed a large number of intra- and subepithelial sensory cells in all of these structures and body regions. Only some of the sensory cells identified by silver impregnation were positive for CAs, 5-HT or FaRPs; the remaining cells appear to have different neurotransmitter modality. CA-containing (CA-C) intra- and subepithelial sensory cells were the most common cell types and were present in the epidermis and all the organs studied. All CA-C cells had a stiff cilium, which showed glyoxylic acid-induced fluorescence in the blue–green range indicative of CAs. In addition to CA-C cells, the cerebral organs had FaRP-immunoreactive sensory cells and argyrophilic cells containing an unidentified neurotransmitter and the frontal organs contained the neurites of 5-HT-immunoreactive, most likely efferent neurons. The presence of presumably mechanoreceptive CA-C cells in the frontal and cerebral organs indicates that these organs may perform not only a chemosensory, but also mechanosensory function.