Ecdysozoa

Ecdysozoa Temporal range: Early Cambrian - Recent
Centipede
Scientific classification
Kingdom:	Animalia
Subkingdom:	Eumetazoa
Superphylum:	Ecdysozoa Aguinaldo et al., 1997
(unranked):	Bilateria
Phyla
Kinorhyncha Loricifera Priapulida Nematoda Nematomorpha Lobopodia † Onychophora Tardigrada Arthropoda

The Ecdysozoa are a group of protostome animals, including the Arthropoda (insects, arachnids, crustaceans, et cetera), Nematoda, and several smaller phyla. They were first defined by Aguinaldo et al. in 1997, based mainly on trees constructed using 18S ribosomal RNA genes. 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.

Group characters

The most notable characteristic shared by ecdysozoans is a three-layered cuticle composed of organic material, which is periodically molted as the animal grows. This process 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, both 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.

The non-panarthropod members of Ecdysozoa have been grouped as Cycloneuralia but they are more usually considered paraphyletic.

The grouping proposed by Aguinaldo et al. is not universally accepted. Some zoologists still hold to the view that Panarthropoda should be classified with Annelida and that Ecdysozoa are polyphyletic. Especially the highly derived roundworms, with their many parasitic taxa and a considerable number of autapomorphies continue to pose problems.

1. Cuticular epithelia are widely spread over diverse phyla of invertebrates (including some groups outside Ecdysozoa) and show a considerable degree of variation. They are believed to have evolved independently, at least in some groups. In Nematoda and Panarthropoda, the cuticle is different in both chemical composition and ultrastructure, which means that it may have evolved independently.
2. Molecular evidence for the monophyly of Ecdysozoa is also ambiguous.

One of the proposed solutions is to regard Ecdysozoa as a sister-group of Annelida, however the controversy is still far from closure.

A partial phylogenetic tree of the Ecdysozoa hypothesis is as follows.

	Deuterostomia Ecdysozoa Arthropoda Onychophora Tardigrada Nematoda Nematomorpha Kinorhyncha Loricifera Priapulida Annelida Bryozoa Sipuncula Mollusca Nemertea Entoprocta Phoronida Brachiopoda ? Platyhelminthes ? Chaetognatha ? Gastrotricha ? Rotifera ? Gnathostomulida ? Micrognathozoa ? Cycliophora ? Mesozoa

External links

• UCMP-Ecdysozoa introduction
• http://www.palaeos.com/Kingdoms/Animalia/Ecdysozoa.html
• http://www.nematodes.org/tardigrades/Tardigrades_and_Ecdysozoa.html
• http://chuma.cas.usf.edu/~garey/articulata.html
• http://chuma.cas.usf.edu/~garey/essential.html