The Ecdysozoa are a major group of protostome animals, including the arthropods (insects, arachnids, crustaceans and relatives), roundworms, and several smaller phyla. They were first defined by Aguinaldo et al. in 1997, based mainly on trees constructed using 18s rRNA genes. However, the group is also strongly 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 Ecdysozoa include the following phyla:
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 most notable characteristic shared by ecdysozoans is a trilayered cuticle, composed of organic material, which is periodically molted as the animal grows. This process is called ecdysis, and gives the group its name. Ecdysozoans lack locomotory cilia, producing 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.
- 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.
- Wagele, J. W., T. Erikson, P. Lockhart, & B. Misof, 1999. The Ecdysozoa: Artifact or monophylum? J. Zoo. Syst. Evol. Research 37: 211-223.