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.