Ecdysozoa /ˌɛkdɪsɵˈzoʊ.ə/ is a group of protostome animals, including Arthropoda (insects, chelicerata, crustaceans, and myriapods), nematoda, and several smaller phyla. They were first defined by Aguinaldo et al. in 1997, based mainly on phylogenetic trees constructed using 18S ribosomal RNA genes. A large study in 2008 by Dunn et al.