Ultrastructure of the Flame Bulbs and Protonephridial Capillaries of Artioposthia sp. (Platyhelminthes,Tricladida, Geoplanidae)

The protonephridial terminal complex of Artioposthia is formed by one or two terminal cells, each with a nucleus located in the lateral wall of the flame bulb, and probably two proximal canal cells forming the wall of the protonephridial capillary. The weir is restricted to the proximal parts of the flame bulbs and consists of convoluted slits separated by thick cytoplasmic columns. Cross-striated ciliary rootlets running parallel with and obliquely or transversely to the longitudinal axis of the flame bulbs strengthen the walls of the flame bulbs and, to a lesser degree, that of the capillary. Numerous cristate mitochondria are present in the terminal and proximal canal cells. Cytoplasmic processes extend from the terminal cells into the adjacent tissue, and narrow internal leptotriches extend from the cytoplasm of the terminal cells into the lumen of the flame bulbs. The wall of the capillary contains many interconnected, liquid filled spaces that communicate with the lumen of the capillary, and two septate junctions. Phylogenetic implications of the findings are discussed.