Fine structure of the infective eggs of the dilepidid cestode Hepatocestushepaticus (Baer, 1932), a parasite of shrews

Ultrastructural characteristics of the eggs of the dilepidid cestode. Hepat ocestus hepaticus, is described. The mature oncosphere is surrounded by thr ee envelopes: (1) an outer envelope; (2) an inner envelope consisting of th ree layers - an extraembryophoral cytoplasmic layer, a thick, fibrillar, lo w electron-dense embryophore, and intraembryophoral cytoplasmic layer, (3) a thin oncospheral membrane, surrounding the oncosphere. The whole surface of the oncosphere is covered by the cytoplasmic oncospheral tegument with n umerous short microvilli and a layer of subtegumental somatic muscles. The following cell types were distinguished in the mature oncospheres: the U-sh aped penetration gland a containing a large amount of tightly packed secret ory granules; bi-nucleate perikaryon of the oncospheral tegument; somatic c ells (= myocytons of somatic and hook muscles); and the germinative cells. The hook-muscle system consists of three pairs of embryonic hooks and a com plex system of specialised muscle fibers responsible for the coordinated ho ok movements. The ultrastructural characteristics of H. hepaticus oncospher e are discussed in comparison with those of previously studied dilepidids a nd other cyclophyllideans.