Ultrastructure of the protonephridial system of the oncomiracidium of Zeuxapta seriolae [Meserve, 1938] [Monogenea, Polyopisthocotylea, Axinidae]

Flame bulbs are formed by interdigitating terminal and proximal canal cells. There are many internal but few (or no) external leptotriches. The proximal canal has a septate junction and a reticulate (vacuolated) wall. The internal surface area of the distal capillaries (and possibly of parts of the proximal capillaries) is increased by lamellae. The cilia of the flame bulb and the lateral flames are few, and there is no well defined excretory bladder. The terminal part of the distal capillary is in contact with the surrounding surface epidermis by a septate junction. Flame bulbs of the oncomiracidium of Zeuxapta differ from those of adult monogeneans mainly in their smaller size and a smaller number of cilia. Comparative analysis of literature data shows that there is an increase of the number of cilia in the flame bulbs of monogeneans with increasing body size to a size of about 2.5 mm² (length x maximum width).