A major division within the animal kingdom containing the majority of animal species and comprising the phyla Priapulida, Kinorhyncha, and Loricifera (no common names), Arthropoda (insects, crustaceans, spiders, etc.), Onychophora (velvet worms), Tardigrada (water bears), and Nematoda (roundworms). These forms include animals with disparate morphologies, but their close relationship is suggested by molecular DNA sequence comparisons—especially of both large and small subunit RNA molecules—and is consistent with myosin heavy-chain sequence comparisons and with a variety of uniquely shared genomic and morphological features. Ecdysozoans were named after their habit of molting (ecdysis), unique among invertebrates, and presumably all are descended from a common molting ancestor.