Fig. 4. Particle trapping by the walls of the pharyngeal lumen. The pharyngeal lumen has a triradiate shape, with apices anchored by intermediate-filament-containing marginal cells and muscle cells on each of the three sides. Contraction of the muscles pulls the lumen open. We do not know the shape of the open lumen, but have assumed an equilateral triangle for simplicity (Fully open). (Note that the size of the lumen is exaggerated for clarity.) In this configuration particles are free to move with the fluid. As the muscles relax the lumen closes. At the point where the diameter of the lumen (defined as the diameter of an inscribed circle) equals the diameter of a particle contained within it, we assume that the particle is held by the walls and no longer moves (Partly open). Liquid can still flow through the three radii. As relaxation continues the particle remains immobile and the walls deform around it (Closed).