[Comparative ultrastructural study of spermiogenesis in Monogenea. 1. Megalocotyle (Monopisthocotylea Capsalidae)]

In Megalocotyle the zones of differentiation (ZD) are short and incomplete; there is no median cytoplasmic process or free flagella. Some peripheral microtubules are present in early ZD, but they disappear. Each nucleus of the common cytoplasmic mass inserts one extremity into a ZD. The mitochondria fuse around this extremity and form a beadlike perforated sphere. Later this sphere slides along the nucleus away from the plasmic membrane like a bead on a string. Each mitochondrial bead (MB), as it moves, trails part of its bulk in the form of a solid streamer; two parallel centrioles follow the MB, each generating a trailing 9 + "1" flagellum. The result is a four-part ensemble extending from each MB to each ZD: the two axonemes, the streamer, and the nucleus, with no membrane separating it from the surrounding cytoplasm. Subsequently a cytoplasmic canal (CC) is formed. The arching membranes, originally located at the base of the ZD, migrate along the four-part ensembles as far as the MB, leaving behind two membranes which define the CC. This formation of the CC is remarkable in that it occurs after migration of the centrioles. At the end of spermiogenesis, the cytoplasmic mass reveals 64 parallel CC each containing an immature spermatozoid whose free rear extremity lengthens out. Each spermatozoid is then detached at its anterior extremity set in the cytoplasmic mass. As in most monopisthocotylean monogeneans, the mature spermatozoid of Megalocotyle lacks peripheral microtubules, although its early ZD has microtubules which subsequently disappear. This feature distinguishes it from trematodes and polyopisthocotylean monogeneans, whose mature spermatozoid keep their ZD microtubules.