SPERM ULTRASTRUCTURE AND SPERMATOGENESIS IN THE INTERSTITIAL POLYCHAETE SPHAEROSYLLIS-HERMAPHRODITA (SYLLIDAE, EXOGONINAE)

The mature spermatozoa of Sphaerosyllis hermaphrodita (Polychaeta: Syl lidae) were reconstructed from electron micrographs. Their filiform sh ape (length 60 mu m), the elongated, helically coiled acrosome and the similarly shaped nucleus identify them as belonging to the modified s perm type often found in interstitial polychaetes. No discrete middle piece is present; the single mitochondrion, a long oval structure, is situated within the proximal urns of the nucleus. The axoneme of the l ong tail section contains microtubules in the conventional 9x2+2 arran gement, except that in the distal part the central microtubules are ab sent. A notable feature of the tail section is a cortex comprising a f lat cuff, which coils around the flagellum in closely packed helicoid turns. The mature sperm are grouped in very orderly bundles of 20 or m ore, presumably held together by a matrix that surrounds the tail sect ions and is characterized by tubular structures oriented parallel to t he long axis of the sperm cells. The primary spermatocytes are joined to one another by cytoplasmic bridges; a striking characteristic of th ese stages is the presence of two pairs of centrioles. The functional significance of this sperm morphology and their;grouping in bundles re garding the reproductive biology of this species are discussed, as is the recent debate about the direction in which sperm structure has evo lved within the Bilateria.