ALBERT

Notes: Summary The mesogastropod Pyrazus ebeninus, produces true spermatozoa (here termed euspermatozoa) and multi-flagellate, mobile cells (here termed paraspermatozoa). The mature paraspermatozoon consists of an elongateconical ‘head’ (6.5–8.5 μm in length), constructed of an electron-dense mosaic sheath surrounding a similarly dense, rod-shaped nuclear core (which runs almost the full length of the head). An acrosome-like structure forms the apex of the head. Five to eight axonemes are fixed to the posterior extremity of the nuclear core, each by means of an attachment complex (dense attachment rod, centriolar cap and centriole). A short (3–4 μm) ‘midpiece’ zone follows the head and consists of the multiple axonemes interspersed with very elongate mitochondria. A tuft of short (20 μm) tails (termed minor tails) emerges from the midpiece in addition to one very long tail (termed the major tail) ensheathed in dense granules which resemble glycogen granules. A single membrane surrounds head, midpiece and tails whilst the nuclear core retains the original double nuclear membrane. Developmentally, the multiple axonemes arise from one of a pair of wheel-shaped arrangements of centrioles and attach to posterior indentations in the nucleus prior to its transformation into the nuclear core. Dense vesicles, derived apparently from the endoplasmic reticulum, accumulate along and around the developing nuclear core and (in the presence of microtubules) condense into the mosaic head sheath. Cytoplasmic mitochondria elongate and collect at the posterior axis of the cell, where, together with the axonemes, they form the midpiece. Features not previously reported in other ultrastructural studies of paraspermatozoa include the acrosome-like structure of the head, the structure of the midpiece zone, the glycogen sheath of the major tail, the dense annular structure at the junction of the midpiece and major tail and the presence of microtubules in the final phase of head and midpiece maturation. Some features of the euspermatozoon are also described and the comparative ultrastructure of mature and developing paraspermatozoa and their possible functions in the Gastropoda, are reviewed.