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.