Phylogenetic relationships of the nematomorpha - a discussion of current hypotheses

Although the Nematomorpha are mostly considered to be closest relatives to the Nematoda, there is a number of similarities with the Mermithida and the Scalidophora (Priapulida, Kinorhyncha and Loricifera). A sister group relationship between Nematoda and Nematomorpha is the most parsimonious and most convincing hypothesis. Autapomorphic characters of the taxon Nematoida (Nematomorpha + Nematoda) are: Ventral and dorsal epidermal cords with mainly unpaired ventral and dorsal nerve cords, cloaca in both sexes and aflagellate spermatozoa; furthermore reduction of the circular body musculature and the protonephridia. The two other hypotheses are based on superficial similarities, on plesiomorphies or on convergently evolved characters. The existence of introverts in different taxa within the Nemathelminthes is critically revised and it is concluded that introverts among the taxa are probably not homologous.