The fine structure of the mature spermatozoon of the pseudophyllidean tapeworm Eubothrium crassum, a parasite of salmonid fishes, has been studied by transmission electron microscopy for the first time. The mature spermatozoon of E. crassum is filiform and tapered at both extremities. It contains two axonemes of unequal length showing the 9 + "1" pattern of Trepaxonemata. The anterior extremity exhibits a crested body 50-100 nm thick. It spirals around the outside of the anterior region of the spermatozoon. The nucleus is electron-dense, exhibiting a fibrous appearance in its middle (the broadest) region. The cortical microtubules are of two types and are situated parallel to the spermatozoon axis. The cytoplasm is slightly electron-dense and contains numerous electron-dense granules in region II of the spermatozoon. A ring of electron-dense, centred microtubules surrounds the axoneme, together with the underlying ring of moderately electron-dense, subjacent submicrotubular material in region V. The anterior and posterior extremities of the spermatozoon lack cortical microtubules and contain a single axoneme. Our results reveal several peculiarities, in which the spermatozoon of E. crassum differs from those of other pseudophyllidean cestodes.

Spermiogenesis and ultrastructure of the spermatozoon of the pseudophyllidean cestode Triaenophorus nodulosus (Pallas, 1781), a parasite of pike Esox lucius, has been studied by transmission electron microscopy. Spermiogenesis involves firstly the formation of a zone of differentiation with two centrioles associated with striated roots, and an intercentriolar body between them, subsequent growth of the two flagella of unequal length, and a formation of a median cytoplasmic process exhibiting patches of dense material. The nucleus penetrates into spermatid body after flagellar rotation and proximo-distal fusion has started. The mature spermatozoon of T. nodulosus is filiform and contains two axonemes of 9+"1" pattern of the Trepaxonemata, nucleus, cortical microtubules parallel to the spermatozoon axis, and electron-dense granules. The anterior extremity of the gamete contains a single centriole surrounded by numerous electron-dense tubular structures exhibiting spiral arrangement and giving rise to lateral projections, which correspond to the crested body. When the crested body disappears, the spiral pattern of electron-dense tubular structures is changed into a ring, persisting until the centriole of the second axonemes appears. This structure of the crested body of T. nodulosus is unique among the Eucestoda.

Spermiogenesis and the ultrastructural features of the spermatozoon of Diphyllobothrium latum (Cestoda, Pseudophyllidea) are described using transmission electron microscopy. Spermiogenesis is characterized by the development of two flagella of unequal length that grow asynchronously. When the first growing flagellum starts to rotate, the second one develops. Flagellar rotation is thus asymmetric and asynchronic. It is followed by proximodistal fusion with the median cytoplasmic process. Electron-dense material is present in the apical region of the zone of differentiation in the early stages of spermiogenesis. The intercentriolar body consists of seven plates: three are electron-dense. Four attachment zones occur in the median cytoplasmic process. An atypical arrangement of striated roots was occasionally observed. The mature spermatozoon possesses two axonemes of 9 + "1" trepaxonematan pattern, nucleus, cortical microtubules, electron-dense granules, and lacks mitochondria. The ultrastructure of the anterior extremity of the spermatozoon in D. latum clearly differs from that in the bothriocephalid pseudophyllideans, mainly in the absence of a crested body and a ring of electron-dense tubular structures. The spermatological data support the assumption that the order Pseudophyllidea is formed by two unrelated clades, "Bothriocephalidea" and "Diphyllobothriidea."