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.