Fig. 1. Pharyngeal anatomy and pharyngeal pumping. (A) The pharynx is a tubular muscle divided into three regions: the corpus, connected to the mouth at the anterior end, the isthmus, and the terminal bulb, connected to the intestine at the posterior end. The corpus is further subdivided into a cylindrical or tapered anterior section, the procorpus (proCo), and a posterior bulb, the metacorpus (metaCo). (B) Simplified schematic cross-section through the pharynx. The lumen is shown almost closed, and is approximately to scale. The hatching in the muscle cells shows the orientation of their acto-myosin filaments. When they contract the muscle cells become thinner in the radial dimension and longer circumferentially, pulling the lumen open. Three intermediate filament-containing marginal cells anchor the apices of the lumen. (C) Pharyngeal pumping is a contraction-relaxation cycle involving the corpus, anterior half of the isthmus and terminal bulb. The cycle begins with the near-simultaneous contraction of these muscles. Contraction of the radially oriented muscles of the corpus and anterior isthmus opens the lumen. Because the posterior isthmus remains closed during this time, the lumen is filled by liquid sucked in through the mouth, carrying with it suspended food particles. Contraction of the terminal bulb rotates the plates of the grinder, breaking bacteria in the terminal bulb and passing the debris back to the intestine. After contraction there is a near-simultaneous relaxation, returning the grinder to its resting position and closing the lumen of the corpus and anterior isthmus. Liquid is expelled from the corpus and anterior isthmus, but food particles are trapped and transported posteriorly. A second motion of the pharyngeal muscles, posterior isthmus peristalsis (not shown here) carries food from the anterior isthmus back to the terminal bulb.