Cyclic membrane flow in the ingestive-digestive system of peritrich protozoans. I. Vesicular fusion at the cytopharynx.

Food vacuoles in peritrichs form by pinching off the distal half of the cytopharynx; and thus the pharyngeal membrane must be renewed during feeding. Correlation of lightand electronmicroscopic observations indicates that the membranes of the ingestive—digestive system recycle. As the young food vacuoles enter the first stage of digestion (condensation of the vacuole), they pinch off cup-shaped coated vesicles which, in association with the microtubular post-oral fibres, return to the pharynx. In the peripharyngeal region, the o-2$-fim diameter cups flatten into o-^2-/im diameter disks, which then fuse with the pharyngeal membrane, thereby providing membrane for future food vacuoles. Ultrastructural evidence for similar patterns of cyclic membrane flow and the functional implications of the association of the pharyngeal membrane with the microtubules of the pharyngeal ribs are considered for other protozoa, including suctorians.