Modern taxonomy does not study criteria such as "higher" and "lower" states of development or distinctions between "main" and "side" branches—only family relationships indicated by cladistic methods are considered relevant. From this point of view, several common characteristics still support the Articulata hypothesis — segmented body; paired appendages on each segment; pairwise arrangement of waste-elimination organs in each segment; and above all, a rope-ladder-like nervous system based on a double nerve strand lying along the belly. An alternative concept, most widely accepted today, is the so-called Ecdysozoa hypothesis. This places the annelids and Panarthropoda in two very different groups: the former in the Lophotrochozoa and the latter in the Ecdysozoa. Mitochondrial gene sequences also provide support for this hypothesis. Proponents of this hypothesis assume that the aforementioned similarities between annelids and velvet worms either developed convergently or were primitive characteristics passed unchanged from a common ancestor to both the Lophotrochozoa and Ecdysozoa. For example, in the first case, the rope-ladder nervous system would have developed in the two groups independently, while in the second case, it is a very old characteristic, which does not imply a particularly close relationship between the annelids and Panarthropoda. The Ecdysozoa concept divides the taxon into two, the Panarthropoda into which the velvet worms are placed, and the sister group Cycloneuralia, containing the threadworms (Nematoda), horsehair worms (Nematomorpha) and three rather obscure groups: the mud dragons (Kinorhyncha); penis worms (Priapulida); and brush-heads (Loricifera).
modern taxonomy does not study criteria such as "higher" and "lower" states of development or distinctions between "main" and "side" branches—only family relationships indicated by cladistic methods are considered relevant from this point of view several common characteristics still support the articulata hypothesis — segmented body; paired appendages on each segment; pairwise arrangement of waste-elimination organs in each segment; and above all a rope-ladder-like nervous system based on a double nerve strand lying along the belly an alternative concept most widely accepted today is the so-called ecdysozoa hypothesis this places the annelids and panarthropoda in two very different groups the former in the lophotrochozoa and the latter in the ecdysozoa mitochondrial gene sequences also provide support for this hypothesis proponents of this hypothesis assume that the aforementioned similarities between annelids and velvet worms either developed convergently or were primitive characteristics passed unchanged from a common ancestor to both the lophotrochozoa and ecdysozoa for example in the first case the rope-ladder nervous system would have developed in the two groups independently while in the second case it is a very old characteristic which does not imply a particularly close relationship between the annelids and panarthropoda the ecdysozoa concept divides the taxon into two the panarthropoda into which the velvet worms are placed and the sister group cycloneuralia containing the threadworms (nematoda) horsehair worms (nematomorpha) and three rather obscure groups the mud dragons (kinorhyncha); penis worms (priapulida); and brush-heads (loricifera)