Most of the previous studies on the functional ultrastructure of oncospheral envelopes in cestodes are restricted to hymenolepidids with terrestrial life cycles, mainly parasites of mammals. The purpose of the present study is to describe and compare origin, differentiation and functional ultrastructure of oncospheral envelopes of 12 cestode species with aquatic life cycles examined by means of transmission (TEM) and scanning electron microscopy (SEM) techniques. Results of our comparative electron microscopical studies revealed that despite the general similarities in the ultrastructure of the primary envelopes surrounding developing embryos, there exist important differences both in the type of morphogenesis and in the final form and arrangement of the secondary envelopes between the 12 examined hymenolepidids. In all examined species, the embryophore develops within the syncytial layer of the primary inner envelope and is transformed into a heterogeneous structure in the final phase of infective egg formation. Some ultrastructural features of oncospheral envelopes are very characteristic for the cestode species examined, and may indeed represent a new useful criterion for differential diagnosis. Our data on ultramorphology of the envelopes and their connections with the uterine wall may also be useful for better understanding of the developmental physiology and biology of the oncospheral stage of hymenolepidids with aquatic life cycles. The comparative analysis allows determination of some ultrastructural features that adapt oncospheres to the behavior and habitat of their intermediate hosts. Interrelations among the ultrastructure of the oncospheral envelopes, habitat of crustacean intermediate hosts, and cestode life cycles are drawn and discussed.