Parasitol Res (1992) 78:368-375 Parasitology Research 9Springer-Verlag1992 Ultrastructure of the developing protonephridial system of the cercaria of Philophthalmus sp. (Trematoda, Digenea)* K. Rohde and N.A. Watson Department of Zoology, University of New England, Armidale, NSW 2351, Australia Accepted February 12, 1992 Abstract. The fully developed flame bulb of Philopthal- mus exhibits the structure characteristic of Trematoda and Monogenea: external and internal ribs :forming a weir, external and internal leptotriches, and two longitu- dinal cytoplasmic cords connected by a septate junction. The proximal canal has a septate junction and surface lamellae. In developing cercariae, perikarya of terminal and proximal canal cells are close together, and sheet- like outgrowths of the terminal cell are externally sur- rounded by cytoplasm of the proximal canal cell con- taining a septate junction. Internal outgrowths and ex- ternal cytoplasm are connected by many "membranes", i.e. desmosome-like structures. Internal sheets break up into internal ribs, and the external cytoplasm breaks up into external ribs, external and internal ribs connected by the filtration '~ membrane". The developing distal ex- cretory duct possesses a septate junction and many branching and looping lamellae. A comparison of Phi- lopthaImus with the cestode Austramphilina elongata, the only other platyhelminth species in which the develop- ment of the protonephridia has been studied at the ultra- structural level, revealed that the two species differ in the presence and absence, respectively, of a septate junc- tion in the flame bulb at an early stage of development. The ultrastructure of the protonephridia of free-living and parasitic Platyhelminthes has received much atten- tion, mainly with the aim of contributing to a phyloge- netic system of the phylum (reviews by Rohde 1990, 1991 ; references in Rohde et al. 1992). Among the major groups of parasitic Platyhelminthes (Neodermata sensu Ehlers 1985), two types of protonephridia can be distin- guished, one characterized by cytoplasmic cords along the weir of the flame bulb connected by a septate junc- tion and by a septate junction in the proximal capillaries, * Financially supported by the Australian Research Council and the University of New England Offprint requests to: K. Rohde laries, and another without such cords or junction. The first type is found in the Trematoda Digenea, Trematoda Aspidogastrea and Monogenea and the second, in the cestodes, including the Amphilinidea and Gyrocotylidea, as well as in Udonella (Rohde et al. 1992). To date, the development of the protonephridia of only one species has been examined with the electron microscope: that of the amphilinid Austramphilina elongata (see Rohde and Watson 1988). In this paper, we examine the devel- oping protonephridia of the cercaria of Philophthalmus sp. Comparison of the trematode (weir with cytoplasmic cords) with the cestode (weir without cytoplasmic cords) will show whether there are fundamental differences in the development of the protonephridia in the two groups. Materials and methods Rediae containing developing and fully developed cercariae of Phi- lophthalmus sp. were dissected out of Pyrazus obeninus snails col- lected in Deception Bay near Brisbane in March 1987 and then fixed for 1-2 h in 3% glutaraldehyde in 0.1 M phosphate buffer (pH 7.2) at 4~ C. Specimens were washed in the same buffer at 4~ C, postfixed in 1% OsO~ in 0.1 M phosphate buffer (pH 7.2) at room temperature for 30 min, and dehydrated in an alcohol series. They were embedded in Spurr's resin and polymerized over- night at 60 ~ C. Sections measuring 60-70 nm in thickness were stained with alcoholic uranyl acetate and lead citrate and examined under a Jeol 1200 EX electron microscope operating at 60 kV. Results The fully (or almost fully) developed flame bulb and proximal capillary exhibit the structure characteristic of Trematoda and Monogenea (Figs. 1 5, 23): a terminal cell with a nucleus close to the weir and a proximal canal cell possessing internal and external ribs (rods), respectively, that interdigitate to form the filtration ap- paratus or weir. Ribs are connected by a filtration "membrane". External processes or leptotriches arise from the external ribs, and internal leptotriches arise