The origin of parasitic flatworms, Neodermata, is one of the key questions of zoology. Fecampiida is a small group of rare commensal or parasitic turbellarians displaying the same mode of spermiogenesis (Revertospermata) as Neodermata and therefore proposed as their sister-group. Here is the first description of the nervous and muscular systems of a fecampiid, Notentera ivanovi Joffe, Selivanova, Kornakova, 1997, parasitic in a polychaete gut, studied by means of immunohistochemistry, histochemistry and confocal microscopy.
N. ivanovi entirely lacks digestive system and has a simple reproductive system with posterior opening. Its body wall has three muscle layers: the outer circular one, the previously overlooked middle diagonal one, and the inner longitudinal one with a wide marginal muscle band likely functioning as a ventral sucker.
The brain looks like a 6-rayed star with three pairs of brain roots radiating to the longitudinal nerve cords. The latter are interconnected by 7 ring commissures and plexuses. 5HT- and FMRFamide immunoreactive elements mostly follow the cholinergic ones with some exceptions: only cholinergic and FMRFamide immunoreactive fibers are present in the brain neuropile and in the frontal nerve plexus. In contrast, the fine subepidermal plexus shows only 5HT immunoreactivity.
Notentra's NS pattern belongs to the radial orthogon type, which seems to be more derived than the regular orthogon patterns found in basal Neodermata and their larval stages. Therefore the analysis of NS patterns points against the monophyly of Revertospermata.
The study was supported by ZIN RAS (NIOKTR: АААА-А17-117030110029-3), RFBR 16-04-00593 А and 15–29–02650