(a) ArSK/CCK1-immunoreactivity (ArSK/CCK1-ir) in a transverse section of the V-shaped radial nerve cord, with bilaterally symmetrical regional variation in the density of immunostaining in the ectoneural neuropile. (b) High magnification image of the boxed region in (a), showing stained cell bodies in the hyponeural (grey arrowhead) and ectoneural (black arrowhead) regions of the radial nerve cord. Regions of the ectoneural neuropile containing a higher (*) and lower (+) densities of immunostained fibres can be seen here. (c) ArSK/CCK1-ir in the circumoral nerve ring, with stained cells present in the hyponeural region (grey arrowheads) and in the ectoneural epithelium (black arrowheads); in the ectoneural neuropile there is regional variation in the density of immunostained fibres. Immunostaining can also be seen here in the sub-epithelial nerve plexus and basal nerve ring of an adjacent peri-oral tube foot. (d) ArSK/CCK1-ir in cells and fibres in a lateral branch of the radial nerve cord; the inset shows immunostained cells in the boxed region at higher magnification. (e) ArSK/CCK1-ir in the marginal nerve, the sub-epithelial nerve plexus of an adjacent tube foot (black arrowhead) and in branches of the lateral motor nerve (grey arrowheads). (f) ArSK/CCK1-ir in the sub-epithelial nerve plexus (white arrowheads) and basal nerve ring (black arrowheads) of a tube foot. (g) ArSK/CCK1-ir in the basi-epithelial nerve plexus of the peristomial membrane (black arrowhead) and the oesophagus (white arrowhead); immunostaining in the circumoral nerve can also be seen here. (h) ArSK/CCK1-ir in the lateral pouches of the cardiac stomach; note that the density of immunostained fibres is highest (black arrowheads) in regions of the mucosa adjacent to the intrinsic retractor strand; immunostaining in the intrinsic retractor strand, nodule and extrinsic retractor strand can also be seen here (white arrowheads). (i,j) High magnification images of cardiac stomach tissue showing ArSK/CCK1-ir in cell bodies (black arrowheads) and their processes in the basi-epithelial nerve plexus (white arrowheads); note that in (j) a process emanating from an immunostained cell body can be seen projecting into the plexus. (k) ArSK/CCK1-ir in the cardiac stomach and pyloric stomach; the boxed region is shown at higher magnification in the inset, showing immunostaining in cells (black arrowheads) and the basi-epithelial nerve plexus (white arrowhead). (l) ArSK/CCK1-ir in the pyloric duct and pyloric caeca; the boxed regions are shown at higher magnification in the insets, where immunostained cells (black arrowheads) and fibres (white arrowheads) can be seen. (m) ArSK/CCK1-ir in an oblique section of the intestine, with immunostained cells in the mucosa (black arrowheads) and intense immunostaining in the basi-epithelial nerve plexus (white arrowheads). (n) ArSK/CCK1-ir in the basi-epithelial nerve plexus of the body wall external epithelium (white arrowhead) and in the lining of a papula (black arrowhead). (o) ArSK/CCK1-ir in nerve fibres projecting around the base of a tube foot at its junction with the neck of its ampulla. (p) ArSK/CCK1-ir in nerve fibres located in the coelomic lining of the lateral region of the body wall. (q) ArSK/CCK1-ir in inter-ossicular tissue of the body wall. Abbreviations: am, apical muscle; Amp, ampulla; bnr, basal nerve ring; conr, circumoral nerve ring; cs, cardiac stomach; Ect, ectoneural; ers, extrinsic retractor strand; Hyp, hyponeural; int, intestine; irs, intrinsic retractor strand; ln, lateral nerve; mn, marginal nerve; nod, nodule; oes, oesophagus; os, ossicle; pm, peristomial membrane; pc, pyloric caecum; pd, pyloric duct; ps, pyloric stomach; rhs, radial hemal strand; rnc, radial nerve cord; snp, sub-epithelial nerve plexus; tf, tube foot. Scale bars: (g), (k), (l) = 120 μm; (a), (c), (f), (h), (o), (p) = 60 μm; (d), (e), (m), (q) = 32 μm; (d-inset), (i), (j), (k-inset), (l-insets), (n) = 16 μm; (b) = 6 μm. Graphs showing enzyme-linked immunosorbent assay (ELISA)-based characterisation of the antibodies to ArSK/CCK1 used here for immunohistochemistry are presented in Figure 5—figure supplement 1.