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 mature worms) or partially free (submature worms) axonemes in spermatids. Formation of free flagella correlates with tight arrangement of cells in the testes and small size of the zone of differentiation and vice versa. In both cases the axonemes elongate within the growing shaft, so that the organization of the resulting spermatids is different only with regard to the distal end. In late spermatids, the nucleus occupies the proximal half, the two mitochondria and the axonemes directed distally lie in the distal half. After detachment of the spermatid, a migration of the nucleus takes place. In the 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. Because of the distal orientation of the axonemes and a peculiar mode of the migration of the nucleus, the spermiogenesis of Notentera should be classified 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 Fecampiida, should be established within the Neoophora to include Notentera and the closely related Fecampiidae and (ii) that all the Platyhelminthes with neodermatan‐type spermiogenesis form a monophyletic taxon, the Revertospermata, which includes the sister groups Fecampiida and Mediofusata.