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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.
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.
[4] [1] It is now known that the fate of the blastopore among protostomes is extremely variable; while the evolutionary distinction between deuterostomes and protostomes remains valid, the descriptive accuracy of the name protostome is disputable. [1] Protostome and deuterostome embryos differ in several other ways.
In deuterostomes, the anus forms first while the mouth develops secondarily. [144] [145] 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. [146]
The distinction between protostomes and deuterostomes is based on the direction in which the mouth (stoma) develops in relation to the blastopore. Protostome derives from the Greek word protostoma meaning "first mouth" (πρῶτος + στόμα) whereas Deuterostome's etymology is "second mouth" from the words second and mouth ...
In deuterostomes, the blastopore becomes the anus while the gut eventually tunnels through to make another opening, which forms the mouth. In the protostomes, it used to be thought that the blastopore formed the mouth (proto– meaning "first") while the anus formed later as an opening made by the other end of the gut. More recent research ...
Bilaterians can be divided, based on events that occur very early in embryonic development, into two groups called protostomes and deuterostomes. [20] Deuterostomes include vertebrates as well as echinoderms and hemichordates (mainly acorn worms). Protostomes, the more diverse group, include arthropods, molluscs, and numerous types of 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 ...