Fig. 12

Schematic representation of sense organs, protocerebral centers, and different neuron types in Pycnogonida. The structures depicted are primarily based on results obtained in E. spinosa (Endeidae), which in the case of some neuron types have been complemented by findings repeatedly observed in representatives of other families (e.g., the orcokinin-ir sensory cells of the lateral sense organ) and therefore represent a more general feature in Pycnogonida. Question marks and stippled neurite branches mark regions in which projections could not be traced with certainty or were not followed any further posteriorly into the central brain neuropil. Abbreviations: 5HT – serotonin; AB – arcuate body; ABL – lower arcuate body layer; ABU – upper arcuate body layer; AE – anterior eye; AMN – antero-median neuropil; AMT – antero-median tract; BRN – brain neuropil; CENV – (dorsal) cephalon nerve; DPL – dorso-posterior lobe; HIS – histamine; LO – lateral sense organ; LT – lateral thickening; ORCO – orcokinin; ONV – optic nerve, PE – posterior eye; PNV – proboscis nerve; SC – soma cortex; SVD – sub-visual domain, TH – tyrosine hydroxylase; VN – visual neuropil