Jamieson, B.G.M., Afzelius, B.A. and Franzén, A. 1985b. Ultrastructure of the acentriolar, aflagellate spermatozoa and the eggs of Histriobdella homari and Stratiodrilus novaehollandiae (Histriobdellidae, Polychaeta). Journal of Submicroscopic Cytology 17, 363-380.

The chief features of the filiform histriobdellid spermatozoon and late spermatid are a very long, complex acrosome, with preformed perforatorium; an elongate nucleus, a long anterior portion of which forms a narrow shaft surrounded by regularly cristate mitochondria (four in Stratiodrilus, eight or nine in Histriobdella); anterior invagination of the tip of the nucleus to form a beaker-shaped pedestal, fenestrated in Histriobdella, and subdivided into four prongs in Stratiodrilus or eight in Histriobdella, supporting the base of the acrosome; and posterior narrowing of the wide elongate hind portion of the nucleus to form the posterior end of the spermatozoon. The anterior region of the acrosome is composed of spiral laminae (4 in Stratiodrilus, 6 in Histriobdella). A flagellum is totally absent at all stages of development and centrioles are absent from the mature spermatozoon of either species though  transiently present in the spermatid (Stratiodrilus). The egg envelope consists of the oolemma overlain by a thick vitelline envelope which is cuticle-like but lacks true epivitelline projections. Outside this in Stratiodrilus there is a thick reticular layer. Euspermatozoa have three  functions: putative fertilization, and, on degeneration, contribution to the substance of the egg and formation of the reticular layer, the latter from the mitochondrial cristae.