SPERMATOZOON STRUCTURE AND SPERMIOGENESIS IN NASSARIUS-KRAUSSIANUS (GASTROPODA, PROSOBRANCHIA, NASSARIINAE)

The testis of Nassarius kraussianus (Nassariinae) produces two types o f spermatozoa, a motile euspermatozoon and a non-motile paraspermatozo on. The euspermatozoon is filiform and about 95 mum long. The elongate d head (40 mum long) is comprised of a slender nucleus (about 0.5 mum diameter) which is penetrated throughout by an intranuclear canal hous ing the anterior portion of the axoneme. A short (about 2 mum long) co nical acrosome surmounts the nucleus anteriorly. The mid-piece (23 mum in length) consists of six to seven modified mitochondria which are h elically arranged around the axoneme. Posterior to the mid-piece the t ail is composed of a short glycogen piece and an end piece. The parasp ermatozoon is spindle-shaped (about 50 mum long) and contains multiple (16-20) axonemes the basal bodies of which fuse anteriorly. Posterior ly, numerous small mitochondria and electron-dense bodies lie between the axonemes. Structural changes during eu- and paraspermiogenesis mir ror those described for other species of gastropod mollusc with dimorp hic spermatozoa. However unlike, other molluscs, the cytoplasmic bridg es which connect developing spermatids contain well developed stacks o f endoplasmic reticulum which form a continuum with that in the cytopl asm of the spermatids. These structures may in some way facilitate the synchronous development of the spermatozoa.