Deuterostome

Deuterostomes (taxonomic term: Deuterostomia; meaning 'second mouth' in Greek) comprise a superphylum of animals. It is a sister clade of Protostomia, with which it forms the Nephrozoa clade. Deuterostomia is a subtaxon of the Bilateria branch of the subkingdom Eumetazoa, within Animalia, and are distinguished from protostomes by their deuterostomic embryonic development; in deuterostomes, the first opening (the blastopore) becomes the anus, while in protostomes, it becomes the mouth. (There are some occurrences of deuterostomy among protostomes.) Deuterostomes are also known as enterocoelomates because their coelom develops through enterocoely. Many groups of organisms originally thought to have belonged to this group (e.g. Lophophorata, Chaetognatha) have been placed elsewhere, to the point where the possibility of deuterostomes being obsoleted is considered. The main extant group in the deuterostome clade is the Chordata, i.e. the vertebrates and their kin. The other possible deuterostome extant group is the Ambulacraria: Echinodermata (starfish, sea urchins, sea cucumbers) + Hemichordata (acorn worms and graptolites). In 2019 it was assessed that the Ambulacraria are sister to the Xenacoelomorpha together forming the Xenambulacraria, probably as basal Bilateria or deuterostome. Without Ambulacraria, Deuterostomes are a junior synonym to the Chordata. Previously, Deuterostomia also included the phyla Brachiopoda, Bryozoa, Chaetognatha, and Phoronida based on embryological characteristics. However, Superphylum Deuterostomia was redefined in 1995 based on DNA molecular sequence analyses when the lophophorates were removed from it and combined with other protostome animals to form superphylum Lophotrochozoa. The phylum Chaetognatha (arrow worms) may belong here, but molecular studies have placed them in the protostomes more often.