Fig. 19

Syllinae. Segmental innervation. aPlakosyllis brevipes, bSyllis tyrrhena, cSyllis garciai. A1-A4, B1-B4, C1-C4: Maximum intensity z-projections. a1, b1, c1: α-tubulin-lir of dorsal planes. Diffuse nervous plexus of the epidermis. Four segmental ring neurite bundles form dorsal commissures. The dorsal longitudinal neurite bundle connects IIr to III, IV, I and IIr, proceeding from dorsolateral to mediolateral. a2, b2, c2: Ventral planes of α-tubulin-lir. P. brevipes: five connectives are present and three ventral nerves within the ganglion. Syllis species: only the two main nerves are visible. Yellow arrowhead: a fine neurite bundle connects to II and III. Adjoining segments are connected by a ventral and a lateral longitudinal neurite bundle. S. tyrrhena possesses one and S. garciai two additional fine segmental neurite bundles. The intersegmental neurite bundle IV is missing between some segments of P. brevipes (red arrowheads). a3, b3, c3: α-tubulin and cell nuclei (magenta). The ventral ganglia are situated slightly closer to the anterior edge of the segment. A cluster of somata accompanies segmental neurite bundle III in P. brevipes. a4, b4, c4: Serotonin-lir of ventral planes. White arrows: no Segmental neurite bundle IV. An elaborate serotonin-lir epidermal nervous plexus is visible.a5, b5, c5: Schematic drawing of the innervation of two segments. Green – neurite bundles, margenta – somata of the ventral nerve cord. a5: Plakosyllis brevipes. No additional segmental neurite bundles are present. The ventral longitudinal neurite bundle traverses over three segments. Segmental neurite bundle III sends fibres into a ventral cluster of somata. The parapodial innervation II consists of two neurite bundles. b5: Syllis tyrrhena. One additional segmental neurite bundle is present. The parapodial innervation II consists of four separate neurite bundles II.I.-II.IV. c5: Syllis garciai. Two additional segmental neurite bundles innervating the nervous plexus are present. Scale bars = 100 μm. Abbreviations: ap – anterior neurite bundle entering the parapodium; cr – ciliary receptor; dc – dorsal cirrus; dcn – neurite bundle innervating dorsal cirrus; dep – dorsal epidermal plexus; dln – dorsal longitudinal neurite bundle; int – intestine; lln – lateral longitudinal neurite bundle; pmn – paramedian ventral nerve; ppl – parapodial lobe; mn – median ventral nerve; mp – main parapodial neurite bundle entering parapodium; mvn – main ventral nerve; n - nephridium; scs – segmental cluster of somata on segmental neurite bundle III; snp – serotonin-lir epidermal nervous plexus; vc – ventral cirrus; vcn – neurite bundle innervating ventral cirrus; vgl – ganglion of ventral nerve cord; vln – ventral longitudinal neurite bundle. Segmental neurite bundles in yellow: 1 – first fine segmental neurite bundle; 2 – second fine segmental neurite bundle; I-IV – segmental neurite bundles forming ring commissures; II.I-II.V – neurite bundles comprising parapodial innervation; II.IIr – dorsal commissure of root 2 of II