Ecdysozoa

The Ecdysozoa are a superphylum 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.

Related Topics:
Superphylum - Protostome - Arthropod - Roundworms - 1997 - RRNA - Morphological - Exoskeleton - Ecdysis - 1897 - 1920

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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.

Related Topics:
Arthropoda - Onychophora - Tardigrada - Kinorhyncha - Priapulida - Loricifera - Nematoda - Nematomorpha - Gastrotrich - Panarthropoda - Annelida - Articulata

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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. 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.

Related Topics:
Cuticle - Ecdysis - Cilia - Amoeboid - Embryo - Spiral cleavage - Pharynx

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