When searching for turbellarian roots of parasitic flatworms - Neodermata - Fecampiid turbellarians always attract attention. Most Fecampiids have exactly the same mode of spermiogenesis (with proximo-distal fusion of axonemes with the spermatid shaft) as Neodermata, and thus they were believed to be sister-groups (Revertospermata). The nervous system structure of two fecampiids Notentera ivanovi and Urastoma cyprinae was studied by different combinations of histochemical and immunohistochemical methods.
N. ivanovi, parasite of a polychaete gut, has a roundish flattened body, entirely lacks digestive system and has a simple reproductive system with a posterior opening. The brain is shaped like a 6-rayed star with three pairs of brain roots radiating to the longitudinal nerve cords. The latter are interconnected by 6 ring commissures and plexuses. Notentra's NS pattern belongs to the radial orthogon type.
U. cyprinae, parasite of mussel gills, has a cylindrical body and a normal gut with a posterior pharynx. The brain is trapeze-shaped, the brain roots start from its posterolateral angles and reach the longitudinal cords. The only transverse commissure lies near the pharynx. This orthogon belongs to the concentrated type, the most derived type among the flatworms.
Therefore, two representatives of Fecampiida were shown to have more derived types of nervous system (radial and concentrated orthogons) than those in basal Neodermata (primitive regular orthogons). So the supposed sister-group relationship of Fecampiida and Neodermata (Revertospermata) is refuted based on the nervous system structure.
The study was performed using equipment of the “Taxon” Research Resource Center (www.ckp-rf.ru/ckp/3038/?sphrase_id=8879024) of the Zoological Institute RAS and Research Resource Center "Molecular and Cellular Technologies” of St.-Petersburg State University (Russia). Supported by ZIN RAS (NIOKTR: АААА-А17-117030110029-3) and RFBR 16-04-00593 А.