The ultrastructure of infective eggs of the hymenolepidid cestode, Ditestolepis tripartita, a parasite of shrews, was examined with emphasis on cellular organisation of mature oncospheres. Each hexacanth larva is surrounded by three main egg envelopes: thin layer of amorphous outer envelope, relatively thick layer of inner envelope with an embryophore and a delicate oncospheral membrane. The outer and inner envelopes of infective eggs of D. tripartita are usually amorphous; the embryophore is relatively thin and moderately electron-dense. Five major types of oncospheral cells have been distinguished. These consist of: (1) about ten germinative cells; (2) about 30 somatic cells (= myocytons of somatic and hook muscles); (3) a bi-nucleate medullary centre (= tegumental perikaryon); (4) a bi-nucleate, U-shaped penetration gland and (5) two cells of neurosecretory type with characteristic dense-cored vesicles. The hook-muscle system with complex interconnections between different muscle fibers provides a structural basis for coordinated hook action.
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