Ultrastructure of the protonephridium in Prostoma graecense (Bohmig) (Rhynchocoela, Enopla, Hoplonemertini)

Fine structure studies show that (1) the terminal cell is an elongated thin-walled and fenestrated basket with a multiciliary flame. Many short curved microvilli are confined to the cell lumen, while longer straight microvilli project from the cell's apical end into the proximal part of the protonephri-dial capillary, forming a sheath around the flame. The filtration area consists of slits between narrow cytoplasmic bars and is entirely confined to the terminal cell, which consequently is defined as a flame bulb, not closely similar to those of other phyla. (2) The protonephridial capillary is short and narrow, with few scattered cilia and luminal microvilli. (3) The coiled tubule is thick-walled, with several ciliated cells very rich in glycogen. The luminal border shows specializations probably concerned with modifying the ultrafiltrate.