Fig. 7. Successive positions of a particle during repetitive pumping, showing its position at the beginning of each of 15 successive cycles. The particle was placed at the mouth (left) at the beginning of the first cycle. The 14th cycle carried it into the isthmus. In the anterior metacorpus some of the pictures of the particle are displaced vertically so that each successive position can be clearly seen, but this is not meant to imply an off-center position.