Spermatogenesis of Haplopharynx rostratus (Platyhelminthes, Haplopharyngida)

During spermatogenesis, nuclei of spermatids are at first large with an irregular outline, containing chromatin granules; then condense to become round containing dense chromatin; then elongate containing loose fibrillar chromatin. Golgi bodies produce dense granules. Immature sperm in the testis are amoeboid, aflagellate, with numerous peripheral microtubules. Mature sperm in the sperm duct have a nucleated part with a narrow layer of cytoplasm drawn out into two flanges, and a thicker non-nucleated part. Peripheral microtubules and dense thickenings of the plasma membrane are present in both parts. Two symmetrically arranged blunt bristle-like structures, apparently modified large granules that do not reach the surface, are located in the nucleated part. Sperm structure supports the view that Haplopharyngida and Macrostomida are closely related.