The morphological peculiarities of the typical spermatozoa of Theodoxus fluviatilis (L.) (Neritoidea) and their implications for motility

A detailed examination of the typical sperm of Neritoidea shows it to be characterized by a unique flagellar structure. During the spermiogenesis of Theodoxus fluviatilis(L.), the flagellum breaks into two parts, one folded back on the other. The first length of the flagellum is constituted of a normal axoneme with a protein fiber associated with doublet 1. It is encircled by a cytoplasmic sheath containing two mitochondrial cylinders and numerous glycogen granules. This first part of the flagellum terminated in correspondence with a cap-shaped electron-dense body which gives origin to the second part of the flagellum which contains only the axoneme. The second half of the flagellum therefore develops parallel to the first but in the opposite direction. This particular conformation could account for the abnormal movement of this sperm, in which the tail precedes the head. © 1982 Academic Press, Inc.