Formation of the Paruterine Capsules and Embryonic Envelopes in Cylindrotaenia-Hickmani (Jones, 1985) (Cestoda, Nematotaeniidae)

Abstract

The development of embryonic envelopes and paruterine capsules in Cylindrotaenia hickmani (Jones 1985) Jones, 1987 were studied using transmission electron microscopy. The embryonic envelopes of C. hickmani form in a similar way to those of other cyclophyllideans. The extant embryonic envelopes in fully developed eggs are: the thin, featureless outer envelope; the inner envelope, which forms an oncospheral membrane; embryophore; and an electron-dense peripheral cytoplasmic layer that lies immediately internal to the external plasma membrane of the inner envelope. Processes of uterine epithelial cells envelop the outer capsule of early embryos. Both the uterine envelope and outer capsule are lost as eggs develop. Paruterine organs are complex, paired structures that form lipid and flattened cellular processes for inclusion within paruterine capsules. Paruterine capsules are complex parenchymal structures containing eggs and the products of the paruterine organs. The possession of such complex paruterine envelopes suggests that the life cycles of nematotaeniids are terrestrial. The sequence of events in paruterine capsule formation in nematotaeniids is unlike that seen in other cyclophyllidean groups with paruterine capsules and it is unlikely that nematotaeniids are closely related to such cestodes.