Transmission electron micrograph studies of developing oncospheral envelopes of Fimbriaria fasciolaris (Hymenolepididae).

Studies of the ultrastructure of oncospheral envelopes of Fimbriaria fasciolaris in utero using transmission electron microscopy revealed three primary layers: uterine, outer and inner envelopes. A capsule was not found. The uterine envelope surrounds up to 12 oncospheres enclosed by the outer and inner oncospheral envelopes. The secondary envelopes arise during the embryos' development and, in part, are formed in a way different from that described for other Hymenolepididae. A thin, folded shell is a derivative of the embryo and the mother organism. The embryophore and oncospheral "membrane" originate intracellularly from the inner oncospheral envelope. The mode of formation and final shape of the definitive oncospheral envelopes in F. fasciolaris and other Hymenolepididae species as well as their terminology are discussed.