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Another hymenolepidid species, Diploposthe laevis (Bloch, 1782) Jacobi, 1897, developing in hosts in an aquatic environment was used to examine oncospheral envelope differentiation and uterus - egg interrelations at the ultrastructural level (SEM and TEM). Inside of the distal proglottids of two D. laevis specimens, the only gravid specimens from among 20 cestodes found, the uterus took the form of wide tubes whose folded walls created chambers in which oncospheres were present within their envelopes. The lumenal surface of the uterus was covered with numerous projections that linked to individual oncospheres forming the uterus-egg interface. Marked differences in the sizes of different oncospheres were visible. Finally, five layers around each Diploposthe laevis oncosphere were revealed: (1) the outer envelope, (2) the distal cytoplasmic portion of the inner envelope, (3) the non-cytoplasmic embryophore, (4) the proximal portion of the inner envelope and (5) the non-cytoplasmic oncospheral membrane. Striking features visible up to the end of the uterine period of formation of D. laevis infective egg were: (1) presence of well-developed outermost envelope, highly folded, rich in cytoplasmic structures, probably complex in origin, and (2) relatively weak development of the embryophore that was one-layered, moderate electron-dense, with the granular material never forming a very compact envelope.