Higher classification of the Monostilifera (Nemertea: Hoplonemertea) Author Kajihara, Hiroshi text Zootaxa 2021 2021-01-28 4920 2 151 199 journal article 8401 10.11646/zootaxa.4920.2.1 ec1a3d33-651e-499c-b775-b42a131ec75a 1175-5326 4475126 C369AA2D-29D5-4DDF-BFE2-EA4FC4AD2703 Order MONOSTILIFERA Morphological circumscription Hoplonemerteans with stylet apparatus containing a basis with single central stylet; exceptions include some forms that are mostly symbiotic and pelagic (see below). Clade definition Hoplonemerteans that are more closely related to Amphiporus lactifloreus ( Johnston, 1828 ) than to Drepanophorus rubrostriatus Hubrecht, 1874 . Practically, until molecular data become available for D. rubrostriatus , the latter will have to be proxied by other polystiliferans, such as Drepanophorus spectabilis ( Quatrefages, 1846 ) , for which sequences are currently available (cf. Andrade et al . 2012 ). Remarks Brinkmann (1917) established Monostilifera originally at the suborder rank, diagnosing the taxon as “Hoplonemerteans with one stylet on the stylet basis” ( op. cit. , p. 4). Strictly, this diagnosis fails to accommodate bivalve-symbiotic species in the genus Malacobdella Blainville, 1827 , which supposedly secondarily lost the stylet apparatus (diaphragm, stylet basis, central stylet, accessory-stylet pouches, and accessory stylets). Other monostiliferans that have been reported to lack the central stylet and basis include: the pelagic forms Achoronemertes scoresbyi ( Wheeler, 1934 ) and Korotkevitschia pelagica ( Korotkevitsch, 1961 ) ( Crandall &Gibson 1998 ; Chernyshev 2005b ); the brackishwater Sacconemertella lutulenta Iwata, 1970 ; the ascidian-symbiotic Gononemertes parasita Bergendal, 1900 ; and the enigmatic Verrillianemertes schultzei Senz, 2001 .