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Early deuterostomes and their modern counterparts. Bilateria, one of the five major lineages of animals, is split into two groups; the protostomes and deuterostomes. Deuterostomes consist of chordates (which include the vertebrates) and ambulacrarians. [20] It seems likely that the Kimberella was a member of the protostomes.
All chordates are deuterostomes. This means that, during embryonic development, the anus forms before the mouth does. All chordates are based on a bilateral body plan. [15] All chordates are coelomates, and have a fluid-filled body cavity with a complete serosal lining derived from mesoderm called mesothelium (see Brusca and Brusca). [16]
The chordates are dominated by the vertebrates (animals with backbones), [149] which consist of fishes, amphibians, reptiles, birds, and mammals. [150] The deuterostomes also include the Hemichordata (acorn worms).
The deuterostomes traditionally include the echinoderms, hemichordates, chordates, and the extinct Vetulicolia. The protostomes include most of the rest, such as arthropods, annelids, mollusks, flatworms, and so forth. There are several differences, most notably in how the embryo develops. In particular, the first opening of the embryo becomes ...
Hemichordata (/ ˌ h ɛ m ɪ k ɔːr ˈ d eɪ t ə / HEM-ih-kor-DAY-tə) is a phylum which consists of triploblastic, eucoelomate, and bilaterally symmetrical marine deuterostome animals, generally considered the sister group of the echinoderms. They appear in the Lower or Middle Cambrian and include two main classes: Enteropneusta (acorn worms ...
Though they are considered deuterostomes, the dorsoventral axis of hemichordates retains features of both protostomes and chordates. For example, enteropneusts have an ectodermally -derived dorsal nerve cord in the collar region which has been proposed to be homologous to the chordate neural tube.
One theory is that the three-part body originates from an early common ancestor of all the deuterostomes, and maybe even from a common bilateral ancestor of both the deuterostomes and protostomes.
Most living animal species belong to the infrakingdom Bilateria, a highly proliferative clade whose members have a bilaterally symmetric and significantly cephalised body plan, and the vast majority of bilaterians belong to two large superphyla: the protostomes, which includes organisms such as arthropods, molluscs, flatworms, annelids and ...