ExcFIG 5: Development of the excretory cell and the canals.

A. The excretory cell is born at the end of gastrulation, ~270 minutes after first cell division at 22°C. At this stage, it has only a basal (outside) surface.
B. At ~300-330 minutes, one or two vacuoles with apical surfaces (light gray) appear within the cell. At the same time, the cell starts to grow bilateral arms dorsolaterally.
C. At 400-430 minutes of development, the apical surface expands and the tubular arms extend from the vacuoles into the canals. The bilateral canals have reached the lateral hypodermal ridges by now. The basal surface and the apical surface join at the junction of the duct cell. A thick electron-dense material (black line) accumulates in the lumen.
D. Past the two-fold stage. On reaching the lateral hypodermis, the canals bifurcate extend anteriorly and posteriorly. Lumenal material disappears and is replaced by the terminal web within the cytoplasm surrounding the canal lumen (light gray lining).
E. Post-hatching and adult stages. The excretory sinus and the canals are fully formed with numerous canaliculi originating from the central canal. The lumens of the canals assume a flattened shape. The terminal web persists around the apical surface.

Click on picture for full resolution image.