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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 consider Ecdysozoa are:

• Panarthropoda
  • Arthropods
  • Tardigrades
  • Onychophora
• Nematoda
• Nematomorpha
• Cephalorhyncha[?]
  • Kinorhyncha
  • Priapulida
  • Loricifera

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 ciliae
• 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 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.

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