Fig. 11

Protocerebral sense organs and their connection to the brain in Pycnogonum litorale (Pycnogonidae). A 3D-reconstructed volume rendering of protocerebral sense organs and brain, μCT scan, lateral view. A neurite bundle (arrow) extends between the eyes, but the lateral sense organ is lacking. The stippled bracket and black line indicate the positions of (extended) optical sections shown in C and E–I. B–I Immunolabeled samples shown in blend mode (B) or MIP (C–I). B Synapsin (SYN), whole-mount brain in dorsal view. Note indistinct demarcation of the small antero-median neuropil. C Tubulin (TUB, magenta) and serotonin (5HT, green). A neurite bundle (yellow arrowheads) connects the optic nerve and antero-median neuropil. A tract (black arrowhead) connects the visual neuropil and arcuate body. D Tubulin (magenta) and histamine (HIS, green) with nuclear counterstain and autofluorescence (AF + NUC, gray), para-sagittal section showing histamine signal in the eyes’ retinae. E Histamine (green) with nuclear counterstain. Histamine-ir axon terminals of the R-cells target the visual neuropil. Note somata of the lateral thickening (arrow). The arrowhead marks a neuron projecting posteriorly (not shown). F,G Tyrosine hydroxylase (TH, magenta) and histamine (green, only in G). TH-ir interneurons (arrows) target the visual neuropil and sub-visual domain and project into the arcuate body. Black arrowheads indicate somata in the lateral thickening. White arrowheads highlight somata of posteriorly projecting dorsal neurons (not shown). H Serotonin. The antero-median neuropil lies on top of the serotonin-ir antero-median tract. I Orcokinin (ORCO). Arrows mark neurons projecting into the sub-visual domain. Note virtual absence of signal in the area housing the antero-median neuropil. Abbreviations: AB – arcuate body; AE – anterior eye; AMN – antero-median neuropil; AMT – antero-median tract; BR – brain; DPL – dorso-posterior lobe; ONV – optic nerve, PE – posterior eye; PNV – proboscis nerve; RTA – R-cell axons; SVD – sub-visual domain, VN – visual neuropil