ULTRASTRUCTURE OF ONCOSPHERAL ENVELOPES IN HYMENOLEPIDIDS (CESTODA) WITH AQUATIC LIFE-CYCLES

The ultrastructure of the oncospheral envelopes of five species of hym enolepidid cestodes, namely, Fimbriaria fasciolaris, Dicranotaenia cor onula, Sobolevicanthus gracilis, Diorchis elisae and Diorchis ovofurca ta are described. These cestodes are parasites of aquatic hosts and ut ilise aquatic invertebrates as intermediate hosts. The ultrastructure of the oncospheral envelopes was essentially similar among these speci es. However, the structure of the components of the inner oncospheral envelopes displayed significant structural variation among the five sp ecies examined, The small eggs of D. coronula and S. gracilis have a t hin embryophore and oncospheral membrane. The eggs of these species se ttle at the bottom of the water body and are eaten by benthic crustace ans. Eggs of F: fasciolaris, D. elisae and D. ovofurcata have thick mu ltilayered embryophores with specific ornamentation. Moreover, in Dior chis species the eggs are elongate, and the oncospheral membrane is th ick and striated. Zones of electron-dense aggregates penetrate the pol ar filaments in these species. Eggs of F: fasciolaris float passively in water whereas those of Diorchis species are attached by long filame nts to aquatic plants and are eaten by crustaceans in the littoral zon e. The interrelationships among the ultrastructure of the oncospheral envelopes, cestode life cycles and habitat of crustacean intermediate hosts are drawn and discussed.