In (a), the three tube forming cells of the excretory system of the nematode are drawn (cell outlines, red; junctions, green; apical membrane, gold); seamless tubes form in the large H-shaped canal cell. In (b) a cross sectional view of one arm of a canal cell tube is illustrated, with the lumen and canalicular channels and vesicles indicated. In (c), the effect of osmotic challenge on the canalicular network and lumen size is schematized.