Ultrastructure of the protonephridia of Monocelis (Proseriata, Monocelididae).

The ultrastructure of the protonephridia of Monocelis sp. (Platyhelminthes, Proseriata) is described. The terminal part of the protonephridia ('terminal complex') consists of a terminal cell, a proximal canal cell and at least one supporting cell. The nucleus of the terminal cell is close to the rootlets of the cilia which form the flame. The weir consists of longitudinal external (small) and internal (large) ribs containing cross-striated microfilaments and connected by a membrane. External ribs arise from cytoplasm of the terminal cell, internal ribs from the proximal canal cell, although continuity of an internal rib and a terminal cell was observed in one case. The terminal cell also gives rise to internal leptotriches. Two thick cytoplasmic cords connected by a desmosome extend along much of the flame. A maximum of two supporting cells with nuclei close to the weir were found along the weir, separated from it by cell membranes. Cells with nuclei some distance from the weir may be in contact with the weir as well. The internal surface area of protonephridial capillaries is enlarged by lamellae and reticula. The wall of some but not all capillaries possesses one or several desmosomes. There are many lateral flames. Differences between the protonephridia of Monocelis and other 'turbellarians' and the Neodermata are discussed.