FIGURE 1 in Higher classification of the Monostilifera (Nemertea: Hoplonemertea)

FIGURE 1. Three different interpretations on the character evolution of rhynchocoel-wall composition among Monostilifera, mapped on topologically the same cladogram. A, if the last common ancestor of monostiliferans had a two-layered rhynchocoel wall (t), the interwoven state (i) would evolve twice; B, C, if the ancestral state was the interwoven one, either the last common ancestor of Eumonostilifera (= Plectonemertidae + Oerstediidae + Amphiporina) would acquire the two-layered state (B), or Oerstediidae and Amphiporina independently evolved the two-layered rhynchocoel walls (C); in either case, the loss of a single vascular plug on the mid-dorsal vessel (l) must have happened at the last common ancestor of Oerstediina (= Plectonemertidae + Oerstediidae), in which cephalic vessels protrude into the rhynchocoel when passing through cerebral ring.