Transmission electron micrographs of cross section of canals in wild-type and homozygous mutant animals. (a–c) N2 wild type (a); exc-2 (rh90) mutant (b); and exc-4(rh133) mutant (c); electron-dense intermediate filament–rich terminal web (arrows) is closely apposed to the apical membrane, and canalicular vesicles (asterisks) are connected to the lumen and to each other (n = 3 animals, multiple sections each). In large cyst of exc-2(rh90) mutant (b), the terminal web is missing, as is the bulk of canalicular vesicles. (d)sma-1(e30) mutant shows frequent detachment of the terminal web (arrows) from the apical membrane, with visible gaps (arrowheads) between sections of the terminal web (n = 6 animals, multiple sections). (d’) Enlargement of boxed section of d; in spite of terminal web disruption, abundant canalicular membrane is visible, though the vesicles appear disrupted in areas where the terminal web is separated from the membrane.