Ecdysozoa

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Ecdysozoa
Centipede
Scientific classification
Kingdom: Animalia Subkingdom: Eumetazoa (unranked) Bilateria Superphylum: Ecdysozoa Aguinaldo et al., 1997
Phyla
Panarthropoda   Arthropoda   Onychophora   Tardigrada Scalidophora   Kinorhyncha   Priapulida   Loricifera Nematoda Nematomorpha

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 be 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 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 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 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 Panarthropoda are distinguished by a segmented body plan, and as such were traditionally believed to have evolved from the Annelida (segmented worms), together comprising the Articulata. However, they do not have many other characteristics in common, so it now appears that they developed segmentation separately.

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. A minority of zoologists still hold to the view that Panarthropoda should be classified with Annelida and that Ecdysozoa are polyphyletic.^{[citation needed]}

[edit] References

• 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.
• Camatini, M., E. Franchi, A. Saita, 2005. Ultrastructural investigation of spermiogenesis in Peripatopsis capensis (Onychophora) Journal of Morphology. Volume 159, Issue 1 , Pages 29-47.
• Jamieson, B. G. M. 1987. The Ultrastructure and Phylogeny of Insect Spermatozoa. Cambridge University Press.
• Pilato, G., M. G. Binda, O. Biondi, V. D. Urso, O. Lisi, A. Marletta, S. Maugeri, V. Nobile, G. Rappazzo, G. Sabella, F. Sammartano, G. Turrisi, F. Viglianisi, 2005. The clade Ecdysozoa, perplexities and questions. Zoologischer Anzeiger. 244. 43-50
• Wagele, J. W., T. Erikson, P. Lockhart, & B. Misof, 1999. The Ecdysozoa: Artifact or monophylum? J. Zoo. Syst. Evol. Research 37: 211-223.
• Tree of Life web project - Bilateria - accessed January 8, 2006

[edit] 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