Ultrastructure of the protonephridial system of larval Austramphilina elongata (Platyhelminthes, Amphilinidea).

The ultrastructure of the flame cell, protonephridial capillaries and excretory pore of larval and embryonic Austramphilina elongata is described. The weir apparatus of the flame cell consists of internal and external longitudinal rods (ribs) connected by a membrane and with many internal and some external leptotriches. Cilia are densely packed and have cross-striated and branching vertical rootlets. Axonemal doublets lose one tubule and decrease in number towards the tip of the cilia. The cell body proximal to the weir apparatus contains a nucleus, numerous mitochondria, Golgi complexes, and many ribosomes. Serial cross-sections of flame cells in embryos revealed that there are no desmosome-connected cytoplasmic cords anywhere along the flame cells; external ribs are in contact with the proximal cytoplasm by means of desmosomes and fuse to form the distal cytoplasmic tube; internal ribs continue as free processes between the flame and the distal cytoplasmic tube. Numerous 'filaments' (microtubules?) extend between the ciliary rootlets and into the cytoplasm of embryonic flame cells. Some free axonemes were seen in the cell body of embryonic flame cells.