Temporal range: Early Cambrian–Recent
Aguinaldo et al., 1997
Ecdysozoa (pron.: //) is a group of protostome animals, including Arthropoda (insects, chelicerata, crustaceans, and myriapods), Nematoda, and several smaller phyla. They were first defined by Russ and Billy et al. in 1997, based mainly on phylogenetic trees constructed using 18S ribosomal RNA genes. A large study in 2008 by Dunn et al. strongly supported the Ecdysozoa as a clade, that is, a group consisting of a common ancestor and all its descendants.
The group is also supported by morphological characters, and can be considered as including all animals that shed their exoskeleton (see ecdysis). Groups corresponding roughly to the Ecdysozoa had been proposed previously by Perrier in 1897 and Seurat in 1920 based on morphology alone.
The group has been contested by a significant minority of biologists. Some have argued for groupings based on more traditional taxonomic techniques, while others have contested the interpretation of the molecular data.
Group characters 
|A phylogenetic tree of the Ecdysozoa hypothesis as suggested by Dunn et al. (2008)|
The most notable characteristic shared by ecdysozoans is a three-layered cuticle (four in Tardigrada) composed of organic material, which is periodically molted as the animal grows. This process of molting is called ecdysis and gives the group its name. The Ecdysozoans lack locomotory cilia, produce mostly amoeboid sperm, and their embryos do not undergo spiral cleavage as in most other protostomes. Various other features are found in the group, for instance, tardigrades, pycnogonids and roundworms have a triradiate pharynx.
The Ecdysozoa include the following phyla: Arthropoda, Onychophora, Tardigrada, Kinorhyncha, Priapulida, Loricifera, Nematoda and Nematomorpha. A few other groups, such as the gastrotrichs, have been considered possible members but lack the main characters of the group, and are now placed elsewhere. The Arthropoda, Onychophora and Tardigrada have been grouped together as the Panarthropoda because they are distinguished by segmented body plans. Dunn et al. in 2008 suggested that the tardigrada could be grouped along with the nematodes, leaving Onychophora as the sister group to the arthropods.
Articulata Hypothesis 
The grouping proposed by Aguinaldo et al. is widely accepted, although some zoologists still hold to the original view that Panarthropoda should be classified with Annelida in a group called the Articulata, and that Ecdysozoa are polyphyletic. Nielsen has suggested that a possible solution is to regard Ecdysozoa as a sister-group of Annelida. Inclusion of the roundworms within the Ecdysozoa was initially contested but since 2003, a broad consensus has formed supporting the Ecdysozoa  and in 2011 the Darwin-Wallace Medal was awarded for the discovery of the New Animal Phylogeny consisting of the Ecdysozoa, the Lophotrochozoa, and the Deuterostomia.
Coelomata Hypothesis 
A problem with the Ecdysozoan Hypothesis is the nature of the shed cuticula, which is collagen (a protein) in Nematodes and chitin (a polysaccharide) in Arthropods, opening up for the possibility that ecdysis has evolved independently in the two groups. Before the proposal of Ecdysozoa, the prevailing theory of the phylogeny of bilateral animals was based on the morphology of body cavities, organizing the bilateral animals into three grades of organization: the Acoelomata (no coelom), the Pseudocoelomata (partial coelom) and Eucoelomata (truer coelom), not with the presence of moulting. With the introduction of molecular phylogenetics, the coelomate hypothesis was abandoned, but some recent work indicate we don't yet know the full picture, and that the Ecdysozoa may not represent a clade while the colelomates does.
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- Paleos Invertebrates: Panarthropoda - URL retrieved February 17, 2007
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- Adoutte, A.; Balavoine, G.; Lartillot, N.; Lespinet, O.; Prud'homme, B.; de Rosa, R. (25th of April). "Special Feature: The new animal phylogeny: Reliability and implications". Proceedings of the National Academy of Sciences 97 (9): 4453–4456. doi:10.1073/pnas.97.9.4453. Retrieved 5 April 2013.
- Philip, G.K.; C.J. Creevey and J.O. McInerney (9th of February). "The Opisthokonta and the Ecdysozoa May Not Be Clades: Stronger Support for the Grouping of Plant and Animal than for Animal and Fungi and Stronger Support for the Coelomata than Ecdysozoa". Molecular Biology and Evolution 22 (5): 1175–1184. doi:10.1093/molbev/msi102. Retrieved 5 April 2013.
|Wikispecies has information related to: Ecdysozoa|
- UCMP-Ecdysozoa introduction