Ecdysozoa

The Ecdysozoa are a large group of protostomian animals, erected by Aguinaldo et al. in 1997 primarily based on 18s rRNA data. They are sometimes ranked as a superphylum. The name comes from the Greek words ecdysis (molting) and zoon (animal) after a character common to the group.

Phyla considered Ecdysozoa are:

Panarthropoda
- Arthropods
- Tardigrades
- Onychophora
Nematoda
Nematomorpha
Cephalorhyncha

The non-pararthropoda have sometimes been considered a monophyletic group, occasionally called the Cycloneuralia, but they are more likely paraphyletic. The rotifers and Chaetognatha have also been considered possible members. Gastrotrichs are seen as a close sister taxon to the Ecdysozoa.

Besides the rRNA (and other nucleinic acid) sequencing data, some morphologic characters are common to the Ecdysozoa:

cuticle built of organic material
molting (ecdysis), initiated by hormones
trilayered cuticle

Further characters are

inner insemination in many species
lack of locomotory cilia
females and males present (though some groups are parthenogenetic and may have lost males during evolution)

The Ecdysozoa concept resolves some morphologic problems like the occurrence of a triradiate muscular sucking pharynx in tardigrades (whose bauplan reminds one strongly of arthropods) and roundworms.

The Ecdysozoa concept is contradictory to the more traditional Articulata concept, where the Panarthropoda are combined to one taxon with the annelids. The annelids do not belong to Ecdysozoa but instead to the Trochozoa.

Links:

Literature

Aguinaldo, A. M. A., J. M. Turbeville, L. S. Linford, M. C. Rivera, J. R. Garey, R. A. Raff, & J. A. Lake, 1997. Evidence for a clade of nematodes, arthropods and other moulting animals. Nature 387: 489-493.