The fine structure of the embryonic envelopes before and after hatching in bothriocephalids: physiological and ecological significance.

The fine structure and differentiation of the embryonic envelopes before and after hatching in two bothriocephalid species of teleostean fish are described. Scanning electron microscopic observation of the egg surface confirmed the specific ornamentation of the capsule. Its thickness and topographic morphology reflect the physiocochemical characteristics of the environment. Beneath the eggshell, two syncytial layers were observed around the oncosphere. The outer envelope, visible during the preoncospheral phase, degenerated before the liberation of the coracidium. Glycoconjugate labelling confirmed the nutritive character of this layer. After hatching, the inner envelope was the only layer covering the oncosphere. The apical plasma membrane of the coracidial sheath bore cilia and numerous clublike microvilli. Beneath the cortical zone, the syncytial cytoplasm was vacuolated, resembling a honeycomb; the electron-dense zone, described from other pseudophyllideans, was not present. The basal membrane of the inner envelope, uniformly electron-dense, was assimilated by the oncospheral membrane. Comparison of the composition of the embryonic envelopes of pseudophyllideans with those of other tapeworm orders and trematodes revealed ontogenetic, structural and functional similarities between the embryotrophic layers. The physiological and ecological significance of the structure of the free-swimming coracidium is discussed. In this respect, the inner envelope around the oncosphere appears to be responsible for the inability of the larva to detect and invade the intermediate host.