Distinctive euclitellate features of the spermatozoon of Eudrilus eugeniaeare the acrosome tube followed by an elongate nucleus; a cylindrical midpiece interpolated between the basal body and the nucleus; and location of the sole, distal, centriole (basal body) behind the midpiece. Features specific to oligochaete sperm are the absence of a large dense fibre (marginal fibre) peripheral to the axoneme in the Branchiobdellidan sperm ground-plan, and absence of an extension of the acrosome tube, anteriorly to the acrosome vesicle, co-occurring with a single midpiece mitochondrion, which is typical of the Hirudinea. Arrangement of the mitochondria radially around a central axis or rachis is shared with branchiobdellidans. An axial rod (perforatorium) is shared with hirudineans and with other oligochaetes. Unique features of the Eudrilus sperm relative to other oligochaetes include the spiral ridge, and great elongation, of the acrosome tube; the presence of an endonuclear canal (containing the axial rod) unknown in other euclitellates; presence of a dense sheath beneath the plasma membrane of the anterior region of the axoneme (known elsewhere in euclitellates only in the leech Acanthobdella); and replacement of the two glycogen granules usually associated with each axonemal doublet in euclitellates with radial rows of glycogen granules which occupy a wide band of cytoplasm peripheral to the axonemal doublets. These apomorphies, not seen in other oligochaete euspermatozoa, are deduced to be adaptations for aspects of internal fertilization.