Abstract
1. In all species of Spirorbis examined, the first two or three abdominal segments of mature adults are female and the remaining abdominal segments are male. Both female and male gametes differentiate simultaneously in the same individual and arise from a discrete and persistent gonad composed of primordial germ cells arranged in two retroperitoneal rows, mesial to the ventral nerve cords, and running the length of the abdominal segments.2. Cytological events associated with the development of female and male gametes are described. Differentiation of oocytes and spermatocytes occurs freely within the coelomic cavity without the aid of attached nurse cells.3. Although coelomostome rudiments are present in both female and male segments, functional genital ducts do not develop. Spawning is assumed to take place by rupture of the body wall.
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