ULTRASTRUCTURE OF THE MATURE SPERM AND SPERMIOGENESIS IN CALLISTA-CHIONE (MOLLUSCA, BIVALVIA)

The ultrastructure of the mature spermatozoon and spermiogenesis of th e bivalve Callista chione (Heterodonta, Veneridae) are described. As o ur TEM and SEM observations show the spermatozoon of C. chione may be considered of primitive type. The head contains a slightly curved nucl eus bearing at the apex a short cone-shaped acrosome. The structure of the acrosome is typical of heterodont bivalves and, as in other vener id bivalves, it comprises a distinct axial rod. The mid-piece is an an nular band of five mitochondria which surround the proximal and the di stal centrioles. Occasionally spermatozoa with one or more misplaced f lagella may be observed. The structural changes that occur during sper miogenesis are similar to those described for other bivalves.