Fig. 2. Kinin-like immunolabeling (KLI) in the pericardial organ (PO) of Cancer borealis. (A) KLI in the ventral-most nerve trunk of the PO. This image, a brightest pixel projection of 11 optical sections taken at 1.95 µm intervals, shows that KLI axons, fine neurites and varicosities are present in the PO. Boxed region is shown at a larger scale in B. Scale bar, 100 µm. (B) A higher magnification image of the central core of the nerve trunk. Note that the large diameter immunopositive axons (arrowheads) are present in the center of the trunk, with labeled fine neurites radiating from them toward the perimeter of the nerve where they terminate in a dense network of immunopositive varicosities. Several of these labeled varicosities that appear to be in contact with the hemolymph space are indicated by arrows. This image is a brightest pixel projection of 12 optical sections taken at 0.2 µm intervals. Scale bar, 100 µm. Abbreviations: AB, anterior bar; PB, posterior bar. (C) Schematic representation of KLI in the PO. In brachyuran crabs, including C. borealis, each of the bilaterally symmetric POs consists of two or more longitudinal nerve trunks that are connected by vertical nerve bars. The trunks and bars that form each PO are elaborations of the segmental nerves (sn) that originate from the thoracic ganglion. In C. borealis the distribution of KLI neuropil in each PO, represented in this schematic by the local branching structures, was patchy, variable in location and often restricted to one or more small regions in the nerve trunks that form the PO.