New and rare species of microturbellarians of the genus Opisthocystis sekera, 1911 (Platyhelminthes, Kalyptorhynchia, Polycystididae) from Lake Baikal

Descriptions of four new species of the genus Opisthocystis Sekera, 1911 are given: Opisthocystis zaytsevae Krivorotkin et Timoshkin sp. n., Opisthocystis strictostylus Krivorotkin et Timoshkin sp. n., Opisthocystis sekeri Krivorotkin et Timoshkin sp. n. and Opisthocystis tenuistylus Krivorotkin et Timoshkin sp. n., all of them endemic to Lake Baikal. The former three species were found in the coastal zone of the lake: O. zaytsevae: in the shallow waters of Bolshoy Ushkany Island; O. strictostylus: in the shallow waters of Ayaya Bay (northern basin) and opposite the village of Khuzhir (Olkhon Island, Maloye More Strait); O. sekeri: in the shallow waters of Onokochanskaya Bay (northern basin); O. tenuistylus was found on the underwater Akademichesky Ridge at a depth of about 350 m. Illustrated descriptions, differential diagnoses, brief information on their distribution and ecology are provided for all new species. New extensive information on the morphology and zoogeography of three rare and poorly studied endemic Baikal species of the same genus is provided: Opisthocystis multifidus (Nasonov, 1935), Opisthocystis octopropagostylus Lukhnev et Timoshkin, 2014, and Opisthocystis cf. umylinae Timoshkin, 2010. Brief illustrated descriptions and generalized information on their distribution are provided for all species. Revision of the taxonomic status of five Baikal species is proposed: Opisthocystis bilobatus Nasonov, 1935, Opisthocystis bicurvatus Nasonov, 1935 Opisthocystis campanulatus Nasonov, 1935, Opisthocystis irregularis Nasonov, 1935 and Opisthocystis trifida Nasonov, 1935. These species were described very briefly, and the drawings were very schematic. Based on the poor descriptions and schematic drawings, their reliable identification is impossible. In this regard, we are compelled to assign the status of nomen nudum to the above species of N. V. Nasonov.