| 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. |
