Hypothetical model of extension of the excretory canal lumen. Diagram of distal end of a canal showing the canal lumen and terminal web within the cytoplasm. Most proteins are shown in a narrow plane of focus, but ACT-5 actin and SMA-1 spectrin are shown covering the luminal membrane on all sides; these are presumed to be added as the canal grows during development (right-hand side of the diagram). EXC-1/IRG is linked to the luminal membrane (via presumed myristoylation site), while most EXC-9 is retained by the C-terminus of the EXC-2 component of the terminal web. In regions of osmotic stress (“Gap”) or rapid growth (right-hand end), the terminal web is thinned or missing, and EXC-9 can make contact with and activate EXC-1. EXC-1 then directly or indirectly (thick dashed arrows) stimulates RAB-8 to deliver vesicles (thin dashed arrows), containing cargo of unknown identity, that help rebuild or strengthen the cytoskeleton at that location. Overexpressed EXC-2 CTD (short red lines on right) blocks EXC-1 activity.