Fig. 15

Syllis tyrrhena. Innervation of the stomatogastric nervous system. a: Schema of the stomatogastric nervous system. Five pairs of stomatogastric neurite bundles are present in the anterior part of the digestive tract. Stgn 1 and 2 originate from the brain, stgn 3 and 4 from the ventral root of the circumoesophageal connective and stgn 5 from a region just after drcc and vrcc have fused. They are interconnected by several links. The stomatogastric neurite bundles lead posteriorly until reaching a first ring neurite bundle (r1) in a region where the epithelium of the buccal cavity and the pharynx fuse. Here the neurite bundles turn (green arrow) towards the beginning of the pharynx. At the anterior end of the pharynx the stomatogastric neurite bundles again turn course posteriorly (yellow arrow), entering the pharyngeal epithelium underneath the muscular layer of the pharynx. The neurite bundles then meet a second pharyngeal ring neurite bundle. From there a pair of thick neurite bundles continues in direction of the proventricle, branching again into two bundles, which then enter proventricle and ventricle. Some species possess pharyngeal glands, which are sac like tubes sitting approximately at the region of the first ring neurite bundle. b: Semi-thin saggital section through the anterior digestive system of S. tyrrhena showing the different histological layers of the pharyngeal tube and the pharyngeal glands. C1-C3: Maximum intensity z-projections of the dissected pharynx, proventricle and ventricle of S. tyrrhena. C1: F-actin staining. C2: α-tubulin-lir (grey) and cell nuclei (magenta). C3: Serotonin-lir. The individual neurite bundles are ambiguous in serotonin-lir, the intensity of the signal differing between stainings. The serotonin-lir nervous plexus of the pharynx and both stomatogastric ring neurite bundles are clearly visible as is the innervation of the ventricle. Scale bars = 100 μm. Abbreviations: br – brain; bc – buccal cavity; bv – blood vessel; cae – caecum;cp – cuticularised layer of the pharynx; drcc – dorsal root of circumoesophageal connective; ep – epidermal layer of pharynx; glp – pharyngeal glands; mlp – muscular layer of pharynx; mpa – muscles of pharyngeal papillae; pxm – pharynx muscles; nl – nuchal lobe; np – nervous plexus; no – nuchal organ; ppa – pharyngeal papilla; psh – pharyngeal sheath; pv – proventricle; pxm – pharynx muscles; r1 – first stomatogastric ring neurite bundle; r2 – second stomatogastric ring neurite bundle; smp – striated muscles of proventricle; stgn1–5 – stomatogastric neurite bundles 1–5; ven – ventricle; vnc – ventral nerve cord; vrcc – ventral root of circumoesophageal connective