Serotonergic head neurons in free-living Rhabditid nematodes. All panels show serotonin immunoreactivity in whole-mount heads of adult nematodes. Anterior is to the left, ventral view. Calibration bars in each panel are 20 μm. In B-H, arrows indicate NSMs, open arrowheads indicate other serotonin immunoreactive somata, and closed arrowheads indicate serotonin immunoreactive processes. (A) C. elegans (N2) with labeled neurons. (B) C. briggsae (AF16). (C) Caenorhabditis sp. 3 (PS1010). The two large bright spots in the posterior bulb of the pharynx are large NSM-associated varicosities, and not somas. [Double-labeling with DAPI staining shows no nuclei are associated with these blobs.] The filled arrowhead indicates a neurite extending to the amphid associated with a putative ADF homolog. (D) Oscheius myriophila (DF5020). (E) Rhabditella axei (DF 5006). Inset: two serotonin-immunoreactive neurons posterior to the NSMs in a different R. axei animal. (F) Pristionchus pacificus (PS312). The asterisk represents the position of serotonin immunoreactive processes occasionally visible extending from a pair of neurons immediately anterior to the NSMs. Inset: a serotonin immunoreactive process extending from a paired neuron located posterior and lateral to the NSM in a different Pristionchus pacificus animal. (G) Pellioditis typica (DF5025). (H) Panagrellus redivivus (PS1163). Inset: a serotonin immunoreactive process extending from a paired neuron located posterior and lateral to the NSM in a different Panagrellus redivivus worm.