Comparative morphology and evolution of the nephridia in Nemertea

Abstract

In various discussions on the phylogenetic position of the Nemertea, nephridial morphology seems to support current hypotheses for a close relationship to lophotrochozoan subtaxa. These arguments are based on isolated findings and suffer from the lack of a phylogeny-based inference. In order to fill in this gap, the structure of the nemertean nephridia is reviewed. Based on the structural data presently available for nemertean nephridia, 30 characters are described. Their ancestral states are inferred from cladograms derived from recent molecular analyses. According to these, paired protonephridia restricted to the foregut region and associated with the blood vascular system represent the primary state in nemerteans. The terminal cells are covered by the blood vessel endothelium and have no open connection to the lumen of the blood vascular system. Serially arranged protonephridia that could support the hypothesis of an annelid-nemertean-relationship are either apomorphic for Cephalothricidae, a nemertean subgroup, or a cephalothricid ingroup.

Disentangling ribbon worm relationships: multi‐locus analysis supports traditional classification of the phylum Nemertea

“…All analyses support the monophyly of Hoplonemertea [100% JF and BF; posterior probability (PP) = 1] and Heteronemertea (98% JF for direct optimization, 100% JF for TNT, PP = 1, and 100% BF), confirming previous results by Thollesson and Norenburg (2003). These results also agree with recent morphological approaches using sperm and nephridial structure ( Bartolomaeus and von Do¨hren, 2010;von Do¨hren et al, 2010). Based on these characters, both studies suggest that Heteronemertea and Hoplonemertea are monophyletic and palaeonemertean taxa retain the ancestral design in some of the structures.…”

“…This hypothesis also finds support in the study of Bürger (1895), where Hubrechtia desiderata is reported to have a protonephridial structure similar to that of heteronemerteans. However, this description is incomplete and requires verification (Bartolomaeus and von Döhren, 2010).…”

New insights into the phylogeny, systematics and DNA barcoding of Nemertea

“…The validity of the Pilidiophora clade is reinforced by several autapomorphic morphological and behavioural features, including the pilidium larva (Cantell 1969;Norenburg 1993;Maslakova 2010) and the sharing of similar protonephridial structures (Bürger 1895; but see also Bartolomaeus and von Döhren 2010). A recent study also suggests a relationship between Hubrechtella and Baseodiscus (Heteronemertea) based on their shared subendothelial diagonal muscles in the proboscis and the absence of outer diagonal musculature (Chernyshev et al 2013), but this would optimise as a plesiomorphy for Heteronemertea under the current scheme, where Valenciniidae constitutes the sister group of all the remaining heteronemerteans.…”

Nemertea

“…In most benthic polystiliferan species (Reptantia), a gradual fusion of both openings by sharing a common atrial chamber at the tip of the head is present. Recent molecular analyses and reassessment of morphological data, however, give a different picture putting the traditional classifi cation in jeopardy Bartolomaeus and von Döhren 2010 ;Andrade et al 2012Andrade et al , 2014Kvist et al 2014 ). Of the traditional higher-ranking taxa, only Hoplonemertea and Heteronemertea are recovered ( Fig.…”