A new variant of the neodermatan-type spermiogenesis in a parasitic 'turbellarian', Notentera ivanovi (Platyhelminthes) and the origin of the Neodermata

Spermiogenesis is characterized by fully incorporated (in the testes of mat ure worms) or partially free (submature worms) axonemes in spermatids. Form ation of free flagella correlates with tight arrangement of cells in the te stes and small size of the zone of differentiation and vice versa. In both cases the axonemes elongate within the growing shaft, so that the organizat ion of the resulting spermatids is different only with regard to the distal end. In late spermatids, the nucleus occupies the proximal half, the two m itochondria and the axonemes directed distally lie in the distal half. Afte r detachment of the spermatid, a migration of the nucleus takes place. In t he resulting mature sperm, the proximal (anterior) half is occupied by the mitochondria and axonemes the basal bodies of which lie at the anterior end of the spermatozoon; the nucleus occupies the distal (posterior) half. Bec ause of the distal orientation of the axonemes and a peculiar mode of the m igration of the nucleus, the spermiogenesis of Notentera should be classifi ed as a new variant of the type characteristic of the Neodermata (parasitic Platyhelminthes). Based on the analysis of the available morphological and other relevant data it is argued (i) that a high-ranked taxon, the Fecampi ida, should be established within the Neoophora to include Notentera and th e closely related Fecampiidae and (ii) that all the Platyhelminthes with ne odermatan-type spermiogenesis form a monophyletic taxon, the Revertospermat a, which includes the sister groups Fecampiida and Mediofusata.