EN
The aim of the present study was to describe ind compare the ultrastructure of oncospheral envelopes in closely related species within the same genus, Diorchis. The envelopes of the following species were examined: Diorchis brevis, D. ovofurcata, D. inflata and D. elisae (filiform, infective eggs), and D. parvogenitalis and D. stefanski (oval shaped eggs). Two categories of oncospheral envelopes are distinguished: the primary cytoplasmic and secondary non-cytoplasmic envelopes. All diorchids examined, in distinction to other hymenolepidids, show at the end of the oncospheral differentiation: (1) a much thicker, clearly striated oncospheral membrane; (2) a three-layered embryophore; (3) the presence of an additional zone of electron-dense aggregates resembling a honeycomb; and (4) an asynchronous differentiation of the derivatives of the inner envelopes. Some ultrastructural differences in oncospheral envelopes of diorchids with oval shaped and filiform oncospheres are described and their significance is discussed.