Everything about Ecdysozoa totally explained
The
Ecdysozoa are a grouping of
protostome animals, including the
Arthropoda (
insects,
chelicera,
crustaceans, and
myriapods),
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. A large study in 2008 by Dunn
et al. strongly supported the ecdysozoa as a natural grouping.
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, whilst others have contested the interpretation of the molecular data.
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 of molting is called
ecdysis and gives the group its name. The Ecdysozoans lack locomotory
cilia, produce mostly
amoeboid sperm, and their
embryos don't 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're 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.
One of the proposed solutions is to regard Ecdysozoa as a sister-group of
Annelida, however the controversy is still far from closure.
Further Information
Get more info on 'Ecdysozoa'.
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