Ecdysozoa

Ecdysozoa is a major clade of protostome animals uniting the arthropods (insects, crustaceans, arachnids and other chelicerates, and myriapods), the roundworms (Nematoda), and several smaller, less familiar phyla. The grouping was first proposed by Eernisse and colleagues in 1992 from a phylogenetic analysis of 141 morphological characters, then formally named by Aguinaldo and colleagues in 1997 based primarily on 18S ribosomal RNA gene sequence data; a large 2008 study by Dunn and colleagues strongly supported the clade's monophyly using much broader molecular evidence. Ecdysozoa is defined chiefly by growth through ecdysis, the periodic moulting of a rigid outer cuticle without accompanying cell division in the underlying epidermis, a process controlled by the hormone ecdysone, together with internal fertilisation; the grouping was initially contested by biologists favouring more traditional taxonomic schemes.