Healy,
J.M. and Jamieson, B.G.M. 1991d. Ultrastructure of spermiogenesis in
the
gastropod Heliacus variegatus (Architectonicidae), with
description of a banded periaxonemal helix. Marine Biology 109,
67-77.
Spermiogenesis
and
spermatozoa of the marine gastropod Heliacus variegatus
Gmelin were
examined using transmission electron microscopy (TEM). Mature
spermatozoa are
composed of an acrosomal vesicle, helical nucleus, elongate midpiece,
annulus, glycogen piece and short end piece. The midpiece consists
of a 9+2 axoneme, nine coarse fibres, and a banded helix, all
enclosed by a continuous mitochondrial sheath (with multiple,
helically coiled
grooves). Anterior extensions of the mitochondrial sheath and banded helix
form a double sheath around the basal half of the nucleus - an
arrangement possibly unique in the Mollusca. During spermiogenesis,
dense plaques delineating the anterior and posterior poles of the spermatid
nucleus become attachment sites for the acrosomal vesicle and the axial
complex (respectively). As the nucleus condenses and elongates,
midpiece formation involves fusion of numerous, oblong mitochondria along
the length of the axoneme. The coarse fibres and banded helix of the
midpiece probably are derived through centriolar activity. Results of the
study support inclusion of the Architectonicidae within the Heterobranchia,
but in view of midpiece specializations, do not clarify the precise
relationship of the family within this subclass.