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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.
[1] [2] Well-known examples of protostomes are arthropods, molluscs, annelids, flatworms and nematodes. They are also called schizocoelomates since schizocoely typically occurs in them. Together with the Deuterostomia and Xenacoelomorpha , these form the clade Bilateria , animals with bilateral symmetry , anteroposterior axis and three germ ...
In deuterostomes, the original dent becomes the anus, while the gut eventually tunnels through the embryo until it reaches the other side, forming an opening that becomes the mouth. [1] It was originally thought that the blastopore of the protostomes formed the mouth, and the anus formed second when the gut tunneled through the embryo.
Dissection of human embryo The cephalic end of the neural groove exhibits several dilatations that, when the tube is closed, assume the form of the three primary brain vesicles , and correspond, respectively, to the future forebrain (prosencephalon), midbrain (mesencephalon), and hindbrain (rhombencephalon) (Fig. 18).
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 ...
In protostomes, the coelom forms by a process known as schizocoely. [6] The archenteron initially forms, and the mesoderm splits into two layers: the first attaches to the body wall or ectoderm , forming the parietal layer and the second surrounds the endoderm or alimentary canal forming the visceral layer .
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 ...
The terms "deuterostomes" and "protostomes" originally defined distinct ways of forming the mouth from the blastopore, a depression that appears in an early stage of the embryo. However, some "protostomes" form the mouth using a process more like that typical of deuterostomes.