Morphological circumscription
Eumonostiliferans with the cephalic blood vessels protruding into the rhynchocoel as they pass through the cerebral ring, often forming a pair of vascular plugs; the mid-dorsal vessel not penetrating into the rhynchocoel to form a single vascular plug.
Clade definition
Eumonostiliferans that are more closely related to Oerstedia dorsalis (Abildgaard, 1806) than to Amphiporus lactifloreus (Johnston, 1828).
Constituent families
Oerstediidae Chernyshev, 1993 and Plectonemertidae Gibson, 1990a.
Remarks
As far as I am aware, Moore & Gibson (1988) were the first to envisage Oerstedia Quatrefages, 1846 as closely related to Plectonemertidae by having two vascular plugs. This conjecture was later confirmed with molecular phylogenetic analyses, first by Mateos & Giribet (2008) and then more convincingly by Kajihara & Kuris (2013), Kvist et al. (2014, 2015), and Chernyshev & Polyakova (2018 a, b, 2019) [Oerstedia was not included in the extensive analysis by Andrade et al. (2012)]. The clade comprising Acteonemertidae + Oerstedia and their kin was named Oerstediina (Chernyshev & Polyakova 2019), with neither Linnaean rank nor morphological circumscription. Here I place it at the infraorder rank. The suffix -ina must be used for the name of a subtribe when the taxon is placed on any of family group ranks in accordance with Article 29.2 of the Code (ICZN 1999); however, the Code (ICZN 1999) does not forbid the usage of -ina for ranks above the family group. While this may lead to confusion, I leave the ending as it was proposed by Chernyshev & Polyakova (2019).