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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]
Deuterostomes are therefore known as enterocoelomates. Examples of deuterostome coelomates belong to three major clades: chordates ( vertebrates , tunicates , and lancelets ), echinoderms ( starfish , sea urchins , sea cucumbers ), and hemichordates ( acorn worms and graptolites ).
The deuterostomes traditionally include the echinoderms, hemichordates, chordates, and the extinct Vetulicolia. The protostomes include most of the rest, such as arthropods, annelids, molluscs, and flatworms. There are several differences, most notably in how the embryo develops. In particular, the first opening of the embryo becomes the mouth ...
Vetulicolia is a group of bilaterian marine animals encompassing several extinct species from the Cambrian, [2] and possibly Ediacaran, [1] periods. As of 2023, the majority of workers favor placing Vetulicolians in the stem group of the Chordata, [3] but some continue to favor a more crownward placement as a sister group to the Tunicata. [2]
(These together sometimes are called the lower deuterostomes. [ 4 ] ) Whether the Xenacoelomorpha clade is the sister group to the Ambulacraria remains a contentious issue, with some authors arguing that the former should be placed more basally among metazoans , [ 5 ] [ 6 ] [ 1 ] and other authors asserting that the best choices of phylogenetic ...
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).
A hollow neural tube exists among some species (at least in early life), probably a primitive trait that they share with the common ancestor of chordata and the rest of the deuterostomes. [8] Hemichordates have a nerve net and longitudinal nerves, but no brain. [9] [10] Some species biomineralize in calcium carbonate. [11]