Ultrastructure of the flame bulbs and protonephridial capillaries of Rugogaster hydrolagi (Platyhelminthes, Trematoda, Aspidogastrea)

The ultrastructure of the terminal apparatus and protonephridial capillaries of R. hydrolagi is described. The weir of the flame bulb consists of alternating rows of ribs connected by a membrane of extracellular matrix. Internal ribs, internal leptotriches and cilia of the flame are continuous with the terminal cell, the perikaryon of which lies basal to and alongside the weir. External ribs, external leptotriches and a pair of cytoplasmic cords along the weir are continuous with the proximal canal cell. Distally, some ciliary membranes extend beyond the end of the microtubules but do not enter the cytoplasm. The distal half of the flame is surrounded by cytoplasm of the proximal canal cell with many electron dense inclusions. The lumen of proximal capillaries is relatively smooth, but in more distal capillaries the lumen surface is enlarged by increasing numbers of lamellae, and lateral flames are present. The phylogenetic position of Rugogastridae is discussed.