MODIFIED SPERM ULTRASTRUCTURE AND SOME DATA ON SPERMIOGENESIS IN PROVORTEX TUBIFERUS (PLATHELMINTHES, RHABDOCOELA) - PHYLOGENETIC IMPLICATIONS FOR THE DALYELLIOIDA

Spermatozoa of Provortex tubiferus are thread-shaped, totally enclosed by peripheral microtubules and devoid of cilia or flagella. A periphe ral layer of dense bodies underlies the cortical microtubules. Roundis h mitochondria are arranged in a single row in the middle region of th e sperm body. A special feature is documented in the appearance of a p erinuclear condensation consisting of leaflet-like compartments. Such a perinuclear condensation is not known for any other species of the P lathelminthes. This characteristic may have functional roles in the lo comotion and stabilization of the filiform aciliary spermatozoon. Duri ng spermiogenesis, two centrioles in a diplosome configuration differe ntiate near each nucleus of an early spermatid. With proceeding sperm maturation, a structure resembling an intercentriolar body is formed a s typical for the taxon Trepaxonemata. However, axonemes are not diffe rentiated and the intercentriolar body becomes disorganized. It is hyp othesized that the Provorticinae (with Provortex) and the Graffillinae (with Paravortex) constitute a new monophylum with the autapomorphy ' 'special type of aciliary spermatozoon''. The Provorticidae and the Gr affillidae, respectively, are non-monophyletic groupings; the Kirgisel linae, Pseudograffillinae and Bresslauillinae do not belong to the mon ophylum Provorticinae + Graffillinae.