The desert snailsSphincterochila zonataandS. prophetarumare hermaphrodites which have a distinct spermatogenic cycle. Snails aestivate (240–280 days/year) throughout the dry season. In mid-summer (May) the gonad is small and completely inactive. In August and September whilst the snails are still dormant, the ovotestis increases in size and spermatogenesis commences, continuing until winter. Oogenesis occurs in winter (December/January) just prior to egg laying. Spermatozoa produced in August and September are stored in the seminal vesicle region of the hermaphrodite duct, thus enabling the snails to copulate and exchange spermatozoa immediately after the first winter rains. During aestivation the seminal vesicle region of the hermaphrodite duct and its constituent cells decrease in size and the duct becomes a region of sperm degradation. Electron microscopy has shown that the wall of the seminal vesicle is composed of two types of cell which undergo considerable morphological change during the different phases of the reproductive cycle.