Characteristics of ventral nerve cords and distribution of serotonin-like immunoreactive neurons in Metaperipatus blainvillei (Onychophora, Peripatopsidae). (A) Histological cross-section of a mid-body segment (Azan staining, adult specimen) to show the ventrolateral position of the nerve cords (vn) widely separated from each other. Arrows point to a median commissure connecting the nerve cords. Scale bar: 500 μm. (B) Histological cross-section of a nerve cord (Azan staining, adult specimen). Dorsal is up, median is left. Arrowheads indicate giant fibers within the neuropil (np). Scale bar: 100 μm. (C) Serotonin-like immunoreactivity within the nerve cord of an embryo almost ready for birth (vibratome section). Dorsal is up, median is left. Neuropil contains a high number of 5-HT-lir fibers. Most somata of 5-HT-lir neurons (arrowheads) are situated in the periphery of the perikaryal layer (pe). Depth-coded image of stacks of confocal micrographs (75 × 0.75 μm). The colors range from warm (red/yellow) indicating anterior optical sections to cooler (blue/purple) indicating posterior optical sections. Scale bar: 50 μm. (D) Details of 5-HT-lir neurons within a nerve cord of the same specimen (as in C). Each neuron gives rise to a single neurite that targets the neuropil without fasciculating with other fibers (arrows). Scale bar: 20 μm. Abbreviations: co, commissure; gl, gut lumen; lm, longitudinal musculature; ne, nephridial end bladder; np, neuropil; pe, perikaryal region; pn, peripheral nerves; so, somata of 5-HT-lir neurons; vn, ventral nerve cords.