Morphology and fine structure of oncospheral envelopes in gravid proglottids of Diploposthe laevis (Bloch, 1782) Jacobi, 1897 (Cestoda, Hymenolepididae)

Another hymenolepidid species, Diploposthe laevis (Bloch, 1782) Jacobi, 189 7, developing in hosts in an aquatic environment was used to examine oncosp heral envelope differentiation and uterus-egg interrelations at the ultrast ructural level (SEM and TEM). Inside of the distal proglottids of two D. la evis specimens, the only gravid specimens from among 20 cestodes found, the uterus cook 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 individu al oncospheres forming the uterus-egg interface. Marked differences in the sizes of different oncospheres were visible. Finally, five layers around ea ch Diploposthe laevis oncosphere were revealed: (1) the outer envelope, (2) the distal cytoplasmic portion of the inner envelope, (3) the noncytoplasm ic embryophore, (4) the proximal portion of the inner envelope and (5) the non-cytoplasmic oncospheral membrane. Striking features visible up to the e nd of the uterine period of formation of D. laevis infective egg were: (1) presence of well-developed outermost envelope, highly folded, rich in cytop lasmic structures, probably complex in origin, and (2) relatively weak deve lopment of the embryophore that was one-layered, moderate electron-dense, w ith the granular material never forming a very compact envelope.