Spermiogenesis and ultrastructure of the spermatozoon of Triaenophorus nodulosus was 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. The mature spermatozoon is filiform and contains two axonemes of 9 + “1” pattern, 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 that correspond to the crested body. When the crested body disappears, the spiral pattern of electron-dense tubular structures is changed into a ring. This structure of the crested body is unique among the Eucestoda.
