To find out, I wrote a program simulating the motions of bacteria within the pharyngeal lumen. Setting up the simulation The pharyngeal lumen is triradiate. When closed, it forms a Y-shape. There s no easy way to see it open, but it probably pulls out to an approximately triangular shape. In these pictures the size of the lumen relative to the pharynx is exaggerated.
Fluid can move around it in the partly closed shaped.
The picture to the left shows how I formalize the geometry and motions. The pharyngeal lumen has a maximum radius r max to which it can open, which varies along its length. It s biggest in the metacorpus and smallest in the isthmus. At any given time it s open to some radius r , which varies between 0 and r max with time. I define the opening, o , as the ratio of r to r max .