Ultrastructure of the protonephridial system of the oncomiracidium of Encotyllabe chironemi (Platyhelminthes, Monopisthocotylea).

In the first electron-microscopic study of the protonephridium of a monopisthocotylean oncomiracidium, flame bulbs, capillaries, and excretory bladders of larval Encotyllabe chironemi were examined. Flame bulbs are formed by a terminal and a proximal canal cell, whose cytoplasmic processes interdigitate to form a typical weir. There are many internal leptotriches (cytoplasmic outgrowths into the lumen of the flame bulb), but no (or very few) external leptotriches. A septate junction, some surface lamellae, as well as numerous vacuoles, some of which open into the canal lumen, are found in the wall of the proximal canal. Lateral flames are present in the larger capillaries. Two excretory bladders, each with a nucleus and containing many excretory concrements, are located dorso-laterally. They open dorso-laterally through narrow ducts lined by a thick wall containing electron-dense and lucent vacuoles. Phylogenetic implications of the findings are discussed.