Ecdysozoa (/ˌɛkdɪsoʊˈzoʊə/) is a group of protostome animals,^[4] including Arthropoda (insects, chelicerata (including arachnids), crustaceans, and myriapods), Nematoda, and several smaller phyla. The grouping of these animal phyla into a single clade was first proposed by Eernisse et al. (1992) based on a phylogenetic analysis of 141 morphological characters of ultrastructural and embryological phenotypes.^[5] This clade, that is, a group consisting of a common ancestor and all its descendants, was formally named by Aguinaldo et al. in 1997, based mainly on phylogenetic trees constructed using 18S ribosomal RNA genes.^[6]

Quick Facts Scientific classification, Phyla ...

Ecdysozoa Temporal range: Cambrian - Recent PreꞒ Ꞓ O S D C P T J K Pg N Molecular clock indicates a possible Ediacaran origin[1]
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Subkingdom:	Eumetazoa
Clade:	ParaHoxozoa
Clade:	Bilateria
Clade:	Nephrozoa
(unranked):	Protostomia
Superphylum:	Ecdysozoa Aguinaldo et al., 1997
Phyla
Scalidophora Kinorhyncha Priapulida Loricifera †Palaeoscolecida Cryptovermes[1] Nematoida Nematoda Nematomorpha Panarthropoda †"Lobopodia" Onychophora Arthropoda Tardigrada †Saccorhytus?[2] †Beretella †Acosmia[3]

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A large study in 2008 by Dunn et al. strongly supported the monophyly of Ecdysozoa.^[7]

The group Ecdysozoa is supported by many morphological characters, including growth by ecdysis, with moulting of the cuticle – without mitosis in the epidermis – under control of the prohormone ecdysone, and internal fertilization.^[8]

The group was initially contested by a significant minority of biologists. Some argued for groupings based on more traditional taxonomic techniques,^[9] while others contested the interpretation of the molecular data.^[10][11]

The name Ecdysozoa is "scientific" Greek, derived from ἔκδυσις (ékdusis) "shedding" + ζῷον (zôion) "animal".

The most notable characteristic shared by ecdysozoans is a three-layered cuticle (four in Tardigrada^[12]) 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 and produce mostly amoeboid sperm, and their embryos do not undergo spiral cleavage as in most other protostomes. Ancestrally, the group exhibited sclerotized teeth within the foregut, and a ring of spines around the mouth opening, though these features have been secondarily lost in certain groups.^[13][14] A respiratory and circulatory system is only present in onychophorans and arthropods (often absent in smaller arthropods like mites); in the rest of the groups, both systems are missing.

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.^[15] 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.^[7] The non-panarthropod members of Ecdysozoa have been grouped as Cycloneuralia but they are more usually considered paraphyletic in representing the primitive condition from which the Panarthropoda evolved.^[16]

A modern consensus phylogenetic tree for the protostomes is shown below.^{[17][18][19][20][21][22]} It is indicated when approximately clades radiated into newer clades in millions of years ago (Mya); dashed lines show especially uncertain placements.^[23]

The phylogenetic tree is based on Nielsen et al.^[24] and Howard et al.^[25]