RESUMEN
The spermiogenesis of Tetrabothrius erostris is characterized by the following events: formation of a differentiation zone containing 2 basal bodies and a pair of rootlets; one of the basal bodies gives rise to a free flagellum, the other induces formation of a flagellar bud; rotation at 90 degrees of the flagellum prior to its fusion with the middle cytoplasmic process of the differentiation zone and partial rotation of the flagellar bud; penetration of the nucleus between the rootlets and appearance of a spur-like protrusion in the differentiation zone; elongation and twisting of the differentiation zone, resulting in twisting of the peripheral microtubules and migration of the nucleus; formation of a crested body; proximal densification of the spermatozoon prior to its detachment from the spermatid rosette. The mature spermatozoon has a single axoneme of 9+"1" type and twisted peripheral microtubules. It consists of 3 portions: a proximal part with a crested body, a middle region rich in beta-glycogen, and a distal part containing the nucleus. The pattern of spermiogenesis resembles most closely that in phyllobothriid tetraphyllideans, and probably reflects a relationship of the family Tetrabothriidae with this group.