Ichthyophthirius multifiliis from freshwater fish.  This is the ectoparasite that troubles tropical fish enthusiasts.  This is a photo of the trophozoite sectioned through the cytostome, revealing the forming pinocytotic vesicles (unlabeled) to the immediate left of the cytostome (CYT).  Secretory vesicles (SV) can be seen being discharged.  It is thought these secretory vesicles contain digestive enzymes which lyse the skin of the fish, providing nutrition for this ciliate.25,000x.