Fig. 10. The four-stage pharynx: a simple model, which captures the essence of our proposed mechanism for particle transport in the corpus. It is so named because the motions occur in four non-overlapping stages: corpus contraction, anterior isthmus contraction, corpus relaxation and anterior isthmus relaxation. (The contractions of the isthmus and corpus overlap, in the sense that both are often contracted at the same time, but the motions do not overlap: when one is moving, the other holds its position.) Additionally, the corpus and isthmus are each uniform in diameter and motion along their entire lengths. In this figure the movement of a particle placed at the mouth at the beginning of a cycle is followed, and the results are shown for the particle moving at mean fluid velocity (particle 1) or at twice mean fluid velocity (particle 2). The particle has a diameter 1/3 that of the maximum corpus diameter, and the volume of the anterior isthmus is 5% that of the corpus. (B,E) Snapshots taken when the corpus is 1/3 open; it is at these points that the particles become free to move (B) or stop moving (E), because they are held by the walls of the corpus lumen.