The sea urchin histamine 1 receptor (suH1R) is localized in the plasma membrane of ectodermal cells in early pluteus stages. Whole mounted embryos were labeled with anti-suH1R antibodies (in red) and counterstained with Hoechst (in blue) to show nuclei. A and B show confocal images of the surface of the embryo; A’ and B’ show images taken at the equatorial plane; A” and B” show DIC images. St: Stomach; POA: Post-oral arm. Scale bars: 20 μm.