The structured bodies of the syncytial cytoplasm were randomly oriented except near the apical plasma membrane where they were perpendicular to the surface. They were described from three profiles: in narrow L.S. (N-profile), broad L.S. (B-profile), and in transverse (T-profile), and their shape was generally that of a flattened, rectangular structure but with curved edges. Although of differing sizes, the approximate maximum dimensions were: 311 nm long, 166 nm broad, and 69 nm thick. They had a striated appearance in L.S. profiles, with a maximum of eight electron-dense striae in the N-profile and 20 in the B-profile. The striae range from 2.5 to 2.8 nm wide with an interstrial distance of 4.8 to 5.5 nm. The T-profile showed a substructure consisting of hexagonal subunits with a wall thickness of 2.5 nm and an inter-wall distance of 5.5 nm. In the center of each hexagon was a 'rod' 2 nm wide. Reconstruction diagrams show the relationship of the T-profile hexagonal subunits to the striated substructure of the N- and B-profiles. The structured bodies appear to pass into the syncytium via the internuncial connections of the tegumentary cells. The bodies often were seen in close association with the plasma membrane and also with the 'rootlets' of microtriches which had not fully emerged. They were not considered to be involved in microthrix-spine formation, but their complex substructure does not necessarily preclude them from being precursors of apical plasma membrane. They may also have a cytoskeletal role. The presence of electron dense bodies in the outer, syncytial cytoplasm of parasitic platyhelminthes is well documented (see Lumsden, 1975; Lyons, 1977) and in all but one instance (Featherston, 1972) the bodies appeared membrane-bound. Although of ran- dom orientation within the proximal regions of the syncytium, they have been shown, in many species of monogeneans, trematodes and cestodes, to be aligned with their long axis perpendicular to the apical plasma mem- brane in the distal syncytial cytoplasm.