Protostomia and Deuterostomia: Do They Exist?

Molecular phylogenetics has transformed the systematics of triploblastic Bilateria (=Triploblastica). Nevertheless, most phylogenetic trees retain the division into two main clades that coincide with those proposed by Grobben (1908): Protostomia and Deuterostomia. However, the fate of the mouth cannot be a reliable feature that characterizes the clades Protostomia and Deuterostomia. Deuterostomy is observed in the embryogenesis of many protostomes. Among representatives of Deuterostomia, typical deuterostomy is characteristic only of Echinodermata. The common ancestor of Triploblastica was a coelomic metameric organism with two circles of appendages (marginal circle and labial circle) around a slit-like blastopore. The nervous system of Triploblastica has a chimeric origin from two sources: the nervous plexus around the slit-like blastopore and aboral nervous plexus around the aboral organ. Amphistomy is the initial way of formation of the through gut for all representatives of Triploblastica. Protostomia have an extensive ectodermal pharynx with sclerites, which develop from the cuticular lining (jaws, radula, gastric mill, mastax, etc.). We propose the name Ectopharyngeata for the clade Protostomia. Deuterostomia retain an endodermal pharynx; therefore, the name of Endopharyngeata is proposed for this clade. Synapomorphies of Endopharyngeata (= Deuterostomia) are gill slits and the division of the pharynx into the branchial and digestive zones. Morphological synapomorphies of the main taxa of Ectopharyngeata (Ecdysozoa and Lophotrochozoa) and Endopharyngeata (Chordata and Ambulacraria) are discussed. The synapomorphy of Ecdysozoa is a molting chitinous cuticle, which can be interpreted as a chitinous theca attached to the body. Appendages of the marginal and labial circles gave rise to the primary biramous limbs of Ecdysozoa. Chaetae developing on the appendages of the marginal circle are the most important synapomorphy of Lophotrochozoa. Lophotrochozoa have lost the appendages of the labial circle (except circumoral tentacles), but have retained the neurotroch, which gives rise to the ciliated creeping sole, e.g., the foot of mollusks. The larvae of Lophotrochozoa have protonephridia draining the blastocoel. Protonephridia are retained in progenetic lophotrochozoans, such as Gastrotricha, Rotifera, Kamptozoa, and Plathelminthes, in their adult state. In Endopharyngeata, the metameric appendages were fused into longitudinal folds, which gave rise to fin folds in Chordata and to genital wings in Enteropneusta. The synapomorphy of Chordata is the development of a notochord (based on the digestive zone of the pharynx of the ancestor of Deuterostomia) and a neural tube (based on the neural plate along the slit-like blastopore). The main synapomorphy of Ambulacraria is an axial complex that performs the functions of a head kidney. Planktotrophic larvae of Ambulacraria have a coelom that is drained by the larval metanephridium opening via a hydropore.