Ultrastructural observations on the reserve bladder system of Cyathocotyle bushiensis Khan, 1962 (Trematoda: Strigeoidea) with special reference to lipid excretion.

The reserve bladder system of Cyathocotyle bushiensis (Trematoda: Strigeoidea) consists of a series of lacunae extending over the dorsal and lateral regions of the body. The lacunar wall is composed of a layer of cytoplasm containing nuclei, mitochondria, and ribosomes. The wall possesses an outer plasma membrane elevated to form stacks of lamellae and a basal plasma membrane demarcating the layer from other structures. In the adult (5-day) parasite the cytoplasm contains lipid droplets only, which become associated with the lamellae and are then released into the lumen of the lacuna. In the 18-hr parasite the cytoplasm and lamellae are associated with excretory corpuscles consisting of layers arranged concentrically around a central core. At 3 days, the excretory cytoplasm possesses both fat and excretory corpuscles. Histochemical tests suggest that the corpuscles contain calcium salts and the lipid droplets neutral fat. The lamellae exhibit acid and alkaline phosphatase activity. The excretory system of the strigeoid trematodes is distinctive in that the excretory bladder becomes enlarged to form an extensive system described as the reserve bladder (Hughes, 1927). These trematodes also possess the basic protonephridial system and this is in continuity with the reserve bladder at specific points. In some genera (e.g., Diplostomum) the reserve bladder comprises narrow tubules terminating in vesicles containing excretory corpuscles. In other genera (e.g., Cyathocotyle, Holostephanus, and Apatemon) the reserve bladder consists of a series of large, fluid-filled lacunae. It has been suggested that the reserve bladder lacuna may function as a hydrostatic skeleton, as a vehicle for the circulation of dissolved nutrients through the body, as well as being involved in excretion (Van Haitsma, 1931; La Rue, 1932; Erasmus and Ohman, 1963; Ohman, 1966a, b). The lappets, adhesive organ, and reserve bladder characteristic of the adult strigeoid make their appearance during the development of the metacercaria in the second intermediate host. The development of the reserve bladder at this period has been followed by Komiya (1938), Erasmus (1958), and Pearson (1961) and these authors have demonstrated that the system is fully formed and functional in the completely developed metacercaria. In this way the system exhibits morphological and physiological continuity between metacercaria and adult. Received for publication 12 August 1966. The appearance of lipids in the excretory system of adult Digenea has been conclusively demonstrated in only a few genera: Fasciola hepatica (by von Brand and Weinland, 1924; Vogel and von Brand, 1933; Stephenson, 1947); Diplostomum sp. , Apatemon gracilis minor, and Holostephanus luiihei (by Ohman, 1965, 1966a, b). The present study was undertaken to demonstrate the presence of lipid in the reserve bladder system of C. bushiensis and to determine as far as possible the place of origin of the free droplets and the nature of the wall enclosing the excretory lacuna. In addition, age variation in relation to the structure of the reserve bladder lining and the associated excretory products was