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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 majority of animals more complex than jellyfish and other cnidarians are split into two groups, the protostomes and deuterostomes, the latter of which contains chordates. [43] It seems very likely the 555 million-year-old Kimberella was a member of the protostomes.
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.
Bilateria has traditionally been divided into two main lineages or superphyla. [27] 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.
The presence of pharyngeal slits in hemichordates led to debates of whether this structure was homologous to the slits found in chordates or a result of convergent evolution. [10] With the placement of hemichordates and echinoderms as a sister group to chordates, a new hypothesis has emerged-suggesting that pharyngeal gill slits were present in ...
A cell can only be indeterminate (also called regulative) if it has a complete set of undisturbed animal/vegetal cytoarchitectural features. It is characteristic of deuterostomes—when the original cell in a deuterostome embryo divides, the two resulting cells can be separated, and each one can individually develop into a whole organism.
Two extinct taxa of uncertain placement, Vetulocystida and Yanjiahella, have each been proposed as either stem-group echinoderms [9] [10] or stem-group ambulacrarians. [11] [12] Vetulocystids have also been tentatively proposed as the basal-most stem-group chordates, [13] while Yanjiahella has also been proposed to be a stem-group hemichordate ...
In deuterostomes, the anus forms first while the mouth develops secondarily. [140] [141] Most protostomes have schizocoelous development, where cells simply fill in the interior of the gastrula to form the mesoderm. In deuterostomes, the mesoderm forms by enterocoelic pouching, through invagination of the endoderm. [142]