Dimorphic, and sometimes polymorphic, spermatozoa are a feature of many caenogastropod and some archaeogastropod (s.l.) taxa. The two types of sperm, named euspermatozoa (fertilizing) and paraspermatozoa (non-fertilizing) by Healy and Jamieson (1981), are produced simultaneously in the same testicular acini. Paraspermatozoa develop from spermatogonia which are similar in structure to those producing euspermatozoa. Formation of parasperm from paraspermatocytes proceeds by atypical meiotic divisions. Despite the variability in the structure of parasperm of prosobranchs, the morphological changes which occur during paraspermatogenesis are remarkably similar between taxa. Multiple flagella develop from numerous basal bodies which in turn originate from two parent and their satellite centrioles (procentrioles). In parasperm which loose all the chromatin (apyrene sperm), the nucleus initially fragments into a number of vesicles. The chromatin in the vesicles gradually degenerates, the remaining material being discharged from the cell by exocytosis. In those parasperm in which a proportion of the chromatin is retained, the nucleus gradually decreases in size as the chromatin condenses. Electron-dense bodies (vesicles) which are produced either by granular endoplasmic reticulum or Golgi bodies begin to form in paraspermatocytes. These vesicles gradually increase in size and coalesce to form larger dense glycoprotein blocks which form the bulk of the head region of the parasperm. It is suggested that the material which forms these electron-dense blocks is derived from nuclear degeneration.