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
        o Arthropods
        o Tardigrades
        o Onychophora
   * Nematoda
   * Nematomorpha
   * Cephalorhyncha
        o Kinorhyncha
        o Priapulida
        o 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 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.

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