Ultrastructural studies on the oncospheral envelopes of hymenolepidid cestode, Wardium aequabile [Rudolphi, 1810] Spassky et Spasskaya, 1954, in the final stage on infective egg formation

The oncospheral envelope morphology, representing an useful criterion in the taxonomy of cestodes, were examined, at the ultrastructural level, in hymenolepidid Wardium aequabile, developing in hosts connected with an aquatic environment. Adult specimens of the cestode from swan, processed for ТЕМ, were analysed. Inside the uterus, around each oncosphere the following envelopes were observed: the outer and the inner envelopes, the embryophore and the oncospheral membrane. The lumenal side of the uterus forms compact layer, rich in cytoplasmic components and intruding between individual eggs. There were certain features characteristic for W. aequabile: (1) details of ultrastructure of the outer envelope and uterine wall; (2) the embryophore with thick electron-dense core layer; (3) the long hooked appendage at each pole of the egg, inside the inner envelope, with the embryophore penetrating into the appendages. Significance of ultrastructural features of W. aequabile in relation to life cycle of the cestode is discussed and, differences and similarity with other hymenolepidids developing in hosts in aquatic environment are presented.