taxonID	type	description	language	source
03A387C8FFDBFFECFF209A47ABF6C108.taxon	description	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 2005 b); the brackishwater Sacconemertella lutulenta Iwata, 1970; the ascidian-symbiotic Gononemertes parasita Bergendal, 1900; and the enigmatic Verrillianemertes schultzei Senz, 2001.	en	Kajihara, Hiroshi (2021): Higher classification of the Monostilifera (Nemertea: Hoplonemertea). Zootaxa 4920 (2): 151-199, DOI: 10.11646/zootaxa.4920.2.1
03A387C8FFDBFFE9FF209F9BA937C56F.taxon	description	As given by Chernyshev (2003 a, 2011 a), members are characterised by having a wickerwork rhynchocoel wall and a single vascular plug; the cerebral organs are located in the brain region, with the possible exception of Cratenemertes amboinensis (Bürger, 1890). Clade definition Monostiliferans that are more closely related to Cratenemertes amboinensis (Bürger, 1890) than to Amphiporus lactifloreus (Johnston, 1828). For practical purposes, however, Cratenemertes amboinensis will have to be proxied by other cratenemertids for which sequences are available, such as Nipponnemertes pulchra (Johnston, 1837) (cf. Andrade et al. 2012), and until molecular data become available for the former. Confusion occurs, therefore, if Cratenemertes amboinensis does not nest within the clade represented by such proxies, once the former is placed in a molecular phylogenetic context. Constituent subtaxa Currently, 28 species in seven genera are recognized in the Cratenemertea (Table 1). * Collarenemertes Chernyshev, 1993 shares the type species Amphiporus bimaculatus Coe, 1901. Remarks Thollesson & Norenburg (2003) performed an extensive molecular phylogenetic analysis covering all the major nemertean subtaxa — Palaeonemertea, Heteronemertea, Monostilifera, Polystilifera (Pelagica and Reptantia), and Bdellonemertea — based on 28 S rRNA, histone H 3, 16 S rRNA, and COI sequences. Within Monostilifera, Thollesson & Norenburg (2003) introduced two clade names, Cratenemertea and Distromatonemertea, stating that “ We believe that effective communication is served by providing a name to the sister clade of cratenemertids, which we propose renaming as Cratenemertea ... ” (op. cit., p. 414). The problem is that the taxon concept of Cratenemerteacomprising “ cratenemertids ” — is ambiguous. Cratenemertea can be either the Cratenemertidae per se, or something like ‘ hoplonemerteans with the rhynchocoel wall composed of a wickerwork of interwoven longitudinal and circular muscle fibres, ’ as the name indicates (the Latin crâtis, meaning ‘ wickerwork’). One of the most remarkable recent findings in hoplonemertean systematics is that Uniporus Brinkmann, 1914 – 1915 has proven to belong actually in Monostilifera (Chernyshev & Polyakova 2018 b), rather than in Reptantia (Polystilifera), as has long been thought (Brinkmann 1914 – 1915, 1917; Stiasny-Wijnhoff 1934; Härlin & Sundberg 1995). The structure of the stylet apparatus is not known for any of four congeners [U. acutocaudatus Brinkmann, 1914 – 1915; U. alisae Chernyshev & Polyakova, 2018 b; U. borealis (Punnett, 1901 a); U. hyalinus Brinkmann, 1914 – 1915], as the proboscis tends to be lost while dredging. Uniporus was placed in Reptantia due to anatomical similarities to other reptantians, such as the interwoven rhynchocoel wall and the rhynchocoelic lateral diverticula; the latter is branched in Uniporus and unique among Reptantia. Chernyshev & Polyakova (2018 b) carefully avoided the possibility of DNA contamination for their phylogenetic analysis, in which U. alisae was nested among Cratenemertea. Another recent finding that might be of significance to cratenemertean systematics is that two of the planktonic forms, Achoronemertes scoresbyi (Wheeler, 1934) and Korotkevitschia pelagica (Korotkevitsch, 1961), may be juveniles of benthic adults (Chernyshev & Polyakova 2019). Among the yields of a vertical tow net from a depth of 250 m to the surface in the Northwest Pacific off Simusir Island in the Kuril Islands, Chernyshev & Polyakova (2019) found a planktonic nemertean. It was 6 – 7 mm in length, having transverse lateral cephalic furrows without secondary grooves, an orange-pink gut, no eyes, small cerebral organs with bifurcated canal, an interwoven rhynchocoel wall, and remarkably, neither the stylet apparatus nor bulb musculature (ibid, fig. 6 A, F, G). In many respects, this planktonic specimen ‘ Cratenemertea 25 DS’ resembles Achoronemertes scoresbyi and Korotkevitschia pelagica. Chernyshev & Polyakova (2019) suggested that Cratenemertea 25 DS represented a swimming juvenile stage of a benthic species with a 0.4 % difference in COI barcode sequence between a benthic adult specimen, ‘ Cratenemertidae sp. IZ 45644 ’ (Kvist et al. 2015). The latter was collected in the Sea of Okhotsk off Iturup Island in the Kuril Islands from depths of 183 – 213 m; it was 7.5 cm in body length, having well-developed lateral cephalic furrows with numerous secondary grooves (Chernyshev & Polyakova 2019, fig. 6 B), no eyes (op. cit., fig. 6 C), an interwoven rhynchocoel wall (op. cit., fig. 6 K), and a stylet apparatus with doubled central stylets on the basis (op. cit., fig. 6 D). The implications are that i) Korotkevitschiidae and Cratenemertidae may be junior synonyms of Uniporidae; and ii) Achoronemertes and Korotkevitschia may be junior synonyms of Nipponnemertes. Proper application of family names requires examining the placement of type species of each family in a molecular phylogeny, but no sequence data are yet available for Cratenemertes amboinensis (Bürger, 1890), Korotkevitschia pelagica (Korotkevitsch, 1961), or Uniporus hyalinus Brinkmann, 1914 – 1915. Nomenclatural notes Chernyshev (2003 a) argued that the Cratenemertea of Thollesson & Norenburg (2003) was nomenclaturally unavailable because it was “ accompanied with neither a diagnosis, nor even a single synapomorphy; therefore, it should be referred to as nom. nudum ” (op. cit., p. S 63). In fact, however, Article 13 of the Code (ICZN 1999) is not relevant to names at the ranks of phylum, class, order, etc., as Article 1.2.2 provides that “ Articles 1 – 4, 7 – 10, 11.1 – 11.3, 14, 27, 28 and 32.5.2.5 also regulate names of taxa at ranks above the family group ”. That is, higher taxon names above the family group are supposed to be available even if they were established without being “ accompanied by a description or definition that states in words characters that are purported to differentiate the taxon ” as stipulated in Article 13.1.1 of the Code (ICZN 1999), as long as these names satisfy other articles given in Article 1.2.2., i. e., Articles 21 – 22 (Date of Publication) and 50 – 51 (Authorship) of the Code (ICZN 1999). There is thus no rule to be followed to determine the authorship of the name Cratenemertea under the current Code (ICZN 1999). For this reason, in this paper, author names and date of publication have intentionally been omitted for names at ranks above the family group. The name Nipponnemertes was originally proposed by Friedrich (1968: 34) for the following seven nominal species without type designation: Amphiporus bergendali Gering, 1912; Amphiporus drepanophoroides Griffin, 1898; Amphiporus pacificus Coe, 1905; Amphiporus occidentalis Coe, 1905; Amphiporus punctatulus Coe, 1905; Cratenemertes danae Friedrich, 1957; and Cratenemertes madagascarensis Kirsteuer, 1965. In accordance with Article 13.3 of the Code (ICZN 1999), Nipponnemertes had been nomenclaturally unavailable before Gibson & Crandall (1989: 463) made it available by giving a diagnosis and designating Amphiporus drepanophoroides Griffin, 1898 as the type species. Crandall (2001: 106) invoked Articles 69.1 and 69.1.1 of the Code (ICZN 1999), argu-ing Amphiporus punctatulus Coe, 1905 be the type species for Nipponnemertes. Crandall’s (2001: 106) “ subsequent designation ” is invalid as a nomenclatural act, because the type fixation had been already done in Gibson & Crandall (1989). The authorship of Nipponnemertes is thus ascribed to Gibson & Crandall (1989).	en	Kajihara, Hiroshi (2021): Higher classification of the Monostilifera (Nemertea: Hoplonemertea). Zootaxa 4920 (2): 151-199, DOI: 10.11646/zootaxa.4920.2.1
03A387C8FFDEFFE9FF209BB9AAB2C15B.taxon	description	Given by Chernyshev (2011 a: 191). Monostiliferans having cerebral organs not extending behind the brain (exceptions include Proamphiporus Chernyshev & Polyakova, 2019), with simple canal, but rarely with forked canal bearing a sac-shaped cavity; mouth and rhynchodaeum merge into rhynchostomodeum, or open into a common atrium; rhynchocoel wall of either two-layered (inner longitudinal and outer circular) or interwoven musculature; myofibrils located in fibrous core of lateral nerve cord. Clade definition Monostiliferans that are more closely related to Amphiporus lactifloreus (Johnston, 1828) than to Cratenemertes amboinensis (Bürger, 1890). Remarks Thollesson & Norenburg (2003: 414) defined Distromatonemertea as “ the most inclusive clade comprising all monostiliferous nemertea except Cratenemertea ”. Ambiguity remains in this definition, because Cratenemertea was not clearly defined (see above). In descriptive terms, Distromatonemertea sounds as though it consists only of members with a two-layered rhynchocoel wall, which excludes Plectonemertidae. The name Distromatonemertea in the sense of Thollesson & Norenburg (2003) is thus problematic. Chernyshev (2003 a) placed Enopla (= Hoplonemertea) at the rank of subclass and included in it five orders — Pelagica, Reptantia, Cratenemertea, Eumonostilifera, and Bdellomorpha, abandoning Brinkmann’s (1917) Polystilifera and Monostilifera. Chernyshev (2003 a, 2005 a) originally regarded Eumonostilifera as not encompassing Malacobdella. Later, Chernyshev (2011 a: 191) amended the diagnosis of Eumonostilifera to accommodate Malacobdella. Although Distromatonemertea has nomenclatural precedence over Eumonostilifera, I here adopt the latter due to the taxonomic uncertainty in the former.	en	Kajihara, Hiroshi (2021): Higher classification of the Monostilifera (Nemertea: Hoplonemertea). Zootaxa 4920 (2): 151-199, DOI: 10.11646/zootaxa.4920.2.1
03A387C8FFDEFFE8FF209F6DAB90C509.taxon	description	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	en	Kajihara, Hiroshi (2021): Higher classification of the Monostilifera (Nemertea: Hoplonemertea). Zootaxa 4920 (2): 151-199, DOI: 10.11646/zootaxa.4920.2.1
03A387C8FFDEFFE8FF209F6DAB90C509.taxon	discussion	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).	en	Kajihara, Hiroshi (2021): Higher classification of the Monostilifera (Nemertea: Hoplonemertea). Zootaxa 4920 (2): 151-199, DOI: 10.11646/zootaxa.4920.2.1
03A387C8FFDFFFE7FF209B9AAB49C49B.taxon	description	Oerstediina having a wickerwork rhynchocoel wall; the cephalic vessels protrude into the rhynchocoel as they pass through the cerebral ring to form a pair of vascular plugs; the mid-dorsal vessel does not penetrate the rhynchocoel to form a vascular plug. Clade definition Eumonostiliferans that are more closely related to Plectonemertes sinensis Gibson, 1990 a than to Oerstedia dorsalis (Abildgaard, 1806). Plectonemertes sinensis has hitherto been known only from the original morphological description (Gibson 1990 a). For practical purposes, therefore, it has to be proxied by other plectonemertids for which sequence data are available, such as Argonemertes australiensis (Dendy, 1892) (cf. Kvist et al. 2014), until Plectonemertes sinensis is eventually sequenced. If Plectonemertes sinensis appears in a different clade, e. g., Cratenemertea, then this taxon will be referred to as either Acteonemertidae Chernyshev, 2005 a or Potamonemertidae Chernyshev, 2005 a. Constituent subtaxa Currently, 14 species in eight genera are recognized in Plectonemertidae (Table 2). Remarks Crandall’s (2001) morphology-based cladistic analyses of monostiliferous hoplonemerteans with interwoven rhynchocoel musculature suggested that i) the marine genus Plectonemertes Gibson, 1990 a, ii) the freshwater genera Campbellonemertes Moore & Gibson, 1972 and Potamonemertes Moore & Gibson, 1973, and iii) the terrestrial genera Acteonemertes Pantin, 1961, Antiponemertes Moore & Gibson, 1981, Argonemertes Moore & Gibson, 1981, Katechonemertes Moore & Gibson, 1981, and Leptonemertes Girard, 1893 could each warrant a separate family. Chernyshev (2005 a) implemented Crandall’s (2001) results and established Acteonemertidae (for Acteonemertes, Antiponemertes, Argonemertes, Katechonemertes, and Leptonemertes) and Potamonemertidae (for Campbellonemertes and Potamonemertes). However, recent molecular phylogenetic results are not necessarily concordant with Crandall’s (2001) morphology-based tree topology. For instance, in Kvist et al. (2014), Argonemertes australiensis (Dendy, 1892) is more closely related to Potamonemertes percivali Moore & Gibson, 1973 than to Leptonemertes chalicophora (Graff, 1879), making Acteonemertidae (or, Acteonemertinae, if placed at the subfamily rank) nonmonophyletic. Implementation of a subfamily classification for Plectonemertidae (in the sense of this paper) thus seems premature. The type species for the Plectonemertidae, Plectonemertes sinensis Gibson, 1990 a (type locality: Hong Kong, subtidal, 21 m depth), has been described morphologically from only a single specimen. Sequence data for Plectonemertes sinensis are a prerequisite for plectonemertid systematics, but attempts to collect more individuals of this species have so far been unsuccessful (Chernyshev 2011 b, 2016). Nomenclatural note As discussed by Crandall (2001: 105), the authorship of the generic names Argonemertes and Antiponemertes is ascribed to Moore & Gibson (1985), who made the names nomenclaturally available, although these were first proposed without type designation by Moore & Gibson (1981).	en	Kajihara, Hiroshi (2021): Higher classification of the Monostilifera (Nemertea: Hoplonemertea). Zootaxa 4920 (2): 151-199, DOI: 10.11646/zootaxa.4920.2.1
03A387C8FFD0FFE4FF209D2DAD9BC0BD.taxon	description	Oerstediina having a two-layered rhynchocoel wall; the cephalic vessels protrude into the rhynchocoel as they pass through cerebral ring; the mid-dorsal vessel does not penetrate the rhynchocoel to form a vascular plug. Clade definition Eumonostiliferans that are more closely related to Oerstedia dorsalis (Abildgaard, 1806) than to Plectonemertes sinensis Gibson, 1990 a. See the clade definition for Plectonemertidae for the potential problem stemming from the absence of molecular sequence data for Plectonemertes sinensis. Remarks Thollesson & Norenburg (2003) were the first to recognize the clade Oerstediidae, as treated herein, which in their molecular phylogeny included Antarctonemertes varvarae Chernyshev, 1999; Nemertellina yamaokai Kajihara et al., 2000; Oerstedia polyorbis Iwata, 1954 [two specimens, erroneously identified as Oerstedia venusta Iwata, 1954 and Oerstedia zebra (Chernyshev, 1993), respectively; see Akhmatova et al. (2012) for the corrected identifications]; ‘ Tetrastemma ’ elegans (Girard, 1852); and ‘ Tetrastemma ’ wilsoni Coe, 1943. Thollesson & Norenburg (2003) referred to this clade as Tetrastemmatidae, probably because it included two species of ‘ Tetrastemma ’; furthermore, Gibson (1982 a) had placed Nemertellina Friedrich, 1935 b in Tetrastemmatidae, and Bürger (1895) had once placed Oerstedia Quatrefages, 1846 in Tetrastemmatidae. In a subsequent study by Strand & Sundberg (2005 a), however, Tetrastemma flavidum Ehrenberg, 1828 (type species of Tetrastemmatidae) and Oerstedia dorsalis (Abildgaard, 1806) (type species of Oerstediidae) appeared in different clades, representing the Amphiporina and Oerstediina, respectively, as treated herein. The name Tetrastemmatidae should thus be used to denote a subclade in the Amphiporina, given the species identification of T. flavidum (see Nomenclatural notes below). Listed in Table 3 are species that have been confirmed to constitute Oerstediidae by means of molecular phylogenetics. Among the 24 species listed, none is known to possess a mid-dorsal vessel that penetrates to form a single vascular plug, and 18 are known to lack a single vascular plug on the mid-dorsal vessel (see also Table 4; Fig. 2 A – I); the vascular-system anatomy is unknown for the remaining six species. Instead of a vascular plug from the mid-dorsal vessel, these 18 species tend to have cephalic vessels protruding into the rhynchocoel lumen as they pass through the cerebral ring (Fig. 2 F – G; Kajihara et al. 2000: fig, 19; Kajihara et al. 2011: fig. 24), although the protrusions can be subtle in smaller specimens (Fig. 2 H, I). In larger species and specimens, the vascular epithelium in these protruding portions may be thickened (Fig. 2 A) and specialized (Fig. 2 D), so that these could be referred to as ‘ vascular plugs’ (perhaps a different terminology for the ‘ plugs’ in Oerstediina may prevent confusion: Gibson [pers comm.] suggests ‘ monovascular plug’ and ‘ bivascular plugs’ respectively, which I now propose). The ‘ vascular plug’ (or, vascular protrusion into the rhynchocoel) in hoplonemerteans presumably functions to facilitate material transfer between the blood vascular system and the rhynchocoel (Crandall 1993 a). Therefore, small-bodied species may actually lack a vascular plug because diffusion could compensate for its function. * Lacks a single vascular plug from the mid-dorsal vessel (see Table 4; Fig. 2). † Judging from the shape of the head, Antarctonemertes unilineata (Joubin, 1910) of Taboada et al. (2018) is different from Tetrastemma unilineatum Joubin, 1910 of Gibson & Tait (1984); furthermore, the latter has been reported i) to lack an accessory lateral nerve and ii) to possess a single vascular plug from the mid-dorsal vessel (see Table 6), each of which suggests a non- Antarctonemertes identity. ‡ “ Prosorhochmus nelsoni ” of Andrade et al. (2012) represents a different species, most likely as the result of mislabelling or a sample mix-up. Among specimens collected in Coquimbo (Chile) on 2 February 2009 by Per Sundberg were NemPhyl 27 (DNA 105586 at Museum of Comparative Zoology, Harvard; COI accession number HQ 848606) and the three specimens NemBar 0842 – 0844 (COI accession numbers KU 840164 – KU 840166; identical sequences); NemPhyl 27 was used in Andrade et al. (2012). BLAST searches with KU 840164 – KU 840166 (NemBar 0842 – 0844) indicate a 100 % match with EF 157586 [P. nelsoni identified by Maslakova & Norenburg (2008 a)] and close similarity to other congeners [P. belizeanus (EF 157591), P. claparedii (MH 106532), and P. americanus (HQ 848595)]. However, HQ 848606 (NemPhyl 27) differs from KU 840164 – KU 840166 and EF 157586 by 15.8 % in uncorrected p - distance. Prosorhochmus nelsoni of Maslakova et al. (2005) (Table 6) has been reported to possess a vascular plug.	en	Kajihara, Hiroshi (2021): Higher classification of the Monostilifera (Nemertea: Hoplonemertea). Zootaxa 4920 (2): 151-199, DOI: 10.11646/zootaxa.4920.2.1
03A387C8FFD0FFE4FF209D2DAD9BC0BD.taxon	description	...... continued on the next page	en	Kajihara, Hiroshi (2021): Higher classification of the Monostilifera (Nemertea: Hoplonemertea). Zootaxa 4920 (2): 151-199, DOI: 10.11646/zootaxa.4920.2.1
03A387C8FFD5FFFFFF2098A5AD99C089.taxon	description	Monostiliferous hoplonemerteans that are more closely related to Amphiporus lactifloreus (Johnston, 1828) than to Oerstedia dorsalis (Abildgaard, 1806). Remarks A clade of eumonostiliferan species more closely related to Amphiporus Ehrenberg, 1831 than to Oerstedia Quatrefages, 1846 was consistently recovered in previous molecular studies (Thollesson & Norenburg 2003; Strand & Sundberg 2005 a; Sundberg & Strand 2007; Sundberg et al. 2007, 2009; Kajihara et al. 2011; Andrade et al. 2012; Kajihara & Kuris 2013; Taboada et al. 2013, 2018; Kvist et al. 2014, 2015; Chernyshev & Polyakova 2018 a, b), but had lacked a formal scientific name before Chernyshev & Polyakova (2019) referred to it as Amphiporina, a rankfree clade name. The taxon is here assigned infraorder rank. As with the case in Oerstediina, I leave the ending as it was proposed by Chernyshev & Polyakova (2019), although the suffix - ina should be used for the name of a subtribe in accordance with Article 29.2 of the Code (ICZN 1999) when the taxon is placed in any of the family-group ranks. The name Amphiporina was first proposed by Ehrenberg (1828 – 1831: 60) (see Nomenclatural notes below). Table 5 lists species that have been molecularly ascertained to be affiliated with Amphiporina. Of the 54 species listed, 29 are known to have a single vascular plug (Table 6; Fig. 3 A – E). Among the remaining 25, the six species, Nemertovema hadalis Chernyshev & Polyakova, 2018 a, Nemertovema norenburgi Chernyshev & Polyakova, 2019, Ototyphlonemertes correae Envall, 1996, Ovicides paralithodis Kajihara & Kuris, 2013, Proamphiporus crandalli Chernyshev & Polyakova, 2019, and Proamphiporus rectangulus (Strand et al., 2014) have been reported to lack a vascular plug; for the rest of 19 species, the anatomy of the blood vascular system is unknown. Within Eumonostilifera, species with a single vascular plug have so far been found only in Amphiporina; no species in Oerstediina have been ascertained to possess a single vascular plug (Tables 2, 3, 5). Therefore, it appears certain that the 147 species listed in Table 6 — species reported to possess a single vascular plug — likely belong in Amphiporina. As noted above, however, the absence of a single vascular plug does not necessarily denote an affiliation to Oerstediidae. Interestingly, Amphiporina harbours more species than Cratenemertea + Oerstediina combined. One of the significant findings of Thollesson & Norenburg (2003) was that Malacobdella Blainville, 1827 nest-ed among monostiliferans. Species in Malacobdella live in the bivalve mantle cavity, attached to their host by their posterior ventral sucker (Vázquez et al. 2009). Unlike other nemerteans, they are suspension feeders (Gibson & Jennings 1969), and their proboscis completely lacks a stylet apparatus (e. g., Riepen 1933). Having a highly specialized morphology, Malacobdella had been placed outside Monostilifera in a different higher taxon variably named Bdellomorphae (Blanchard 1847), Bdellonemertini (Wijnhoff 1913), Bdellonemertea (Coe 1943; Iwata 1960), or Bdellomorpha (Chernyshev 2003 a); Thollesson & Norenburg (2003) brought an end to these higher taxa. No mention has been made in previous literature to any contact between the rhynchocoel and the blood vascular system in Malacobdella (e. g., Kennel 1877 – 1878; Oudemans 1885; Maclaren 1901; Riepen 1933; Gibson & Jennings 1967). My examination of Malacobdella japonica Takakura, 1897 revealed that the mid-dorsal blood vessel — while it does lack a single vascular plug — sends off at least three branches of vessels anterodorsally near its anterior end, each penetrating into the ventral portion of the rhynchocoel muscular wall without protruding into the rhynchocoel lumen, and terminating in a blind end that contains basophilic substances (Fig. 4 A – D). While these branches may compensate for the function of the vascular plug in terms of material transfer between the rhynchocoel and blood vessel, their fine structure and taxonomic distribution should be further studied to understand their physiological function and evolutionary significance. In the tree in Thollesson & Norenburg (2003), Carcinonemertes cf. carcinophila imminuta (K ̂ lliker, 1845) was the sister group to all other Distromatonemertea. This topology was probably due to long-branch attraction (Felsenstein 1978; Philippe et al. 2005) caused by an increased substitution rate in these decapod-egg parasites. Subsequent molecular studies showed Carcinonemertes as either the sister group to all other Amphiporina (Andrade et al. 2012; Kajihara & Kuris 2013) or nested among the latter (Kvist et al. 2014). Species in Carcinonemertidae lack a middorsal vessel (Humes 1942) and thus do not have a vascular plug. In Ovicides paralithodis Kajihara & Kuris, 2013, the cephalic vessels do not protrude into the rhynchocoel as they pass through the cerebral ring, although they are tightly attached to the rhynchocoel wall (Kajihara & Kuris 2013: fig. 3). ...... continued on the next page * Possesses a single vascular plug on the mid-dorsal vessel (see Table 6). † Reported to lack a vascular plug (see Table 4). ...... continued on the next page	en	Kajihara, Hiroshi (2021): Higher classification of the Monostilifera (Nemertea: Hoplonemertea). Zootaxa 4920 (2): 151-199, DOI: 10.11646/zootaxa.4920.2.1
03A387C8FFC1FFF6FF209CD9AA94C0EB.taxon	discussion	Remarks Established based on four specimens, up to 3.7 mm in length, collected from 19 – 25 m depth near Wauwermans Islands, Antarctica. With four eyes. The rhynchocoel is “ composed of single fibres spaced apart from one another in a partially woven lattice ” (Crandall 2010: 2412), a character state that cannot readily be interpreted as decisively homologous with that in Cratenemertea and / or Plectonemertidae. The cerebral organs are large, located behind the pre-cerebral septum, lying beside and beneath the brain, and thus conforming to those in Cratenemertea, but also in Proamphiporus Chernyshev & Polyakova, 2019. The vascular plug is “ broad and flat, lying tightly against ventral wall of rhynchocoel ” (Crandall 2010: 2421), but one of the original figures (Crandall 2010, fig. 12) depicts it as if there are two vascular plugs, reminiscent of the character state in Oerstediina. The taxonomic placement of the family Cinclidonemertidae Crandall, 2010, the genus Cinclidonemertes Crandall, 2010, and C. mooreae among Monostilifera thus requires further scrutiny.	en	Kajihara, Hiroshi (2021): Higher classification of the Monostilifera (Nemertea: Hoplonemertea). Zootaxa 4920 (2): 151-199, DOI: 10.11646/zootaxa.4920.2.1
03A387C8FFC2FFF5FF2099A0AB2FC548.taxon	discussion	Remarks Described based on four specimens from Umm al-Dalkh oil field in the Persian Gulf, off the United Arab Emirates, up to 3 mm in body length. With two to six eyes. The rhynchocoel is half the length of the body; its wall is mainly composed of circular muscle fibres, but longitudinal muscle fibres are also interwoven to varying degrees, a condition otherwise not hitherto reported in Monostilifera. The cerebral organs are simple, reaching back to 66 % of the pre-septal region; the position of the organs thus differs from that in Cratenemertea. A stylet apparatus is missing from V. schultzei. Whether or not the mid-dorsal vessel forms a vascular plug is unknown. While the type species of the genus Verrillianemertes rathkei Senz, 2001 likely belongs in Amphiporina, V. schultzei cannot be placed with certainty in any of the higher taxon recognized in this paper.	en	Kajihara, Hiroshi (2021): Higher classification of the Monostilifera (Nemertea: Hoplonemertea). Zootaxa 4920 (2): 151-199, DOI: 10.11646/zootaxa.4920.2.1
