Fig. 1. Schematic Paramecium cell. The plasma membrane (pm) is covered with cilia (ci). Along the oral groove, they beat rhythmically to wash food towards the oral cavity (oc). The phagosomal apparatus consists of several elements: as, acidosomes mediating acidification of a food vacuole (fv) after pinching off; cytopharyngal fibers (cf), post oral fibers (pof), cytoproct (cp, site of exocytotic release of spent phagosomes). Dense-core vesicles (trichocysts, tr), are attached to the plasma membrane. Osmoregulation is executed by two contractile vacuole complexes (a, ampula; cv, contractile vacuole; ds, decorated spongiome; ss, smooth spongiome). The red arrows indicate transport routes during phagosomal processing. Discoidal vesicles (dv) and other recycling vesicles (rv) are responsible for return transport of membranes. The endosomal system, composed of parasomal sacs (ps) and early endosomes (ee), is arrayed in a regular fashion. er, endoplasmatic reticulum; ga, golgi apparatus.