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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.
The two competing hypotheses as to the phylogeny of Xenoturbella are shown in red. One suggests it is a deuterostome (right), while the other suggests it is a basal bilaterian (left). When it was discovered, X. bocki was placed in a new genus Xenoturbella. Above the genus level, the classification of this animal is controversial.
The embryological origin of the mouth and anus is an important characteristic, and forms the morphological basis for separating bilaterian animals into two natural groupings: the protostomes and deuterostomes. In animals at least as complex as an earthworm, a dent forms in one side of the early, spheroidal embryo.
A chordate (/ ˈ k ɔːr d eɪ t / KOR-dayt) is a deuterostomal bilaterian animal belonging to the phylum Chordata (/ k ɔːr ˈ d eɪ t ə / kor-DAY-tə).All chordates possess, at some point during their larval or adult stages, five distinctive physical characteristics (synapomorphies) that distinguish them from other taxa.
Numerous placements have been proposed for vetulocystids, without any consensus emerging. These include stem-group ambulacrarians, [8] stem-group echinoderms, [9] stem-group deuterostomes, [10] and even a phylogenetic study that found weak support for a placement at the base of the chordate stem. [6]
Pharyngeal slits are filter-feeding organs found among deuterostomes. Pharyngeal slits are repeated openings that appear along the pharynx caudal to the mouth. With this position, they allow for the movement of water in the mouth and out the pharyngeal slits.
The egg cell is generally asymmetric, having an animal pole (future ectoderm).It is covered with protective envelopes, with different layers. The first envelope – the one in contact with the membrane of the egg – is made of glycoproteins and is known as the vitelline membrane (zona pellucida in mammals).
Stereom was the first form of biomineralization to evolve in deuterostomes, predating the evolution of spicules in tunicates and bone in vertebrates. [3] Stereom likely evolved before other distinctive traits of echinoderms, such as radial symmetry, as it is present in basal echinoderms with bilaterally symmetric or asymmetric body plans. [4]