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The embryological origin of the mouth and anus is an important characteristic, and forms the morphological basis for separating
bilateria The Bilateria or bilaterians are animals with bilateral symmetry as an embryo, i.e. having a left and a right side that are mirror images of each other. This also means they have a head and a tail (anterior-posterior axis) as well as a belly and ...
n animals into two natural groupings: the
protostome Protostomia () is the clade of animals once thought to be characterized by the formation of the organism's mouth before its anus during embryonic development. This nature has since been discovered to be extremely variable among Protostomia's me ...
s and deuterostomes. In animals at least as complex as an
earthworm An earthworm is a terrestrial invertebrate that belongs to the phylum Annelida. They exhibit a tube-within-a-tube body plan; they are externally segmented with corresponding internal segmentation; and they usually have setae on all segments. Th ...
, a dent forms in one side of the early,
spheroidal A spheroid, also known as an ellipsoid of revolution or rotational ellipsoid, is a quadric surface obtained by rotating an ellipse about one of its principal axes; in other words, an ellipsoid with two equal semi-diameters. A spheroid has circ ...
embryo. This dent, the
blastopore Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of cells), or in mammals the blastocyst is reorganized into a multilayered structure known as the gastrula. Be ...
, deepens to become the
archenteron The primary gut that forms during gastrulation in a developing embryo is known as the archenteron, the gastrocoel or the primitive digestive tube. It develops into the endoderm and mesoderm of an animal. Formation in sea urchins As primary mesenc ...
, the first phase in the growth of the gut. 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. It was originally thought that the blastopore of the
protostome Protostomia () is the clade of animals once thought to be characterized by the formation of the organism's mouth before its anus during embryonic development. This nature has since been discovered to be extremely variable among Protostomia's me ...
s formed the mouth, and the anus was formed second when the gut tunneled through the embryo. More recent research has shown that our understanding of protostome mouth formation is somewhat less secure than we had thought.
Acoelomorpha Acoelomorpha is a subphylum of very simple and small soft-bodied animals with planula-like features which live in marine or brackish waters. They usually live between grains of sediment, swimming as plankton, or crawling on other organisms, suc ...
, which form a sister group to the rest of the bilaterian animals, have a single mouth that leads into a blind gut (with no anus). The genes employed in the embryonic construction of the flatworm mouth are the same as those expressed for the protostome and deuterostome mouth, which suggests that the structures are equivalent homologous, and that the older ideas about
protostome Protostomia () is the clade of animals once thought to be characterized by the formation of the organism's mouth before its anus during embryonic development. This nature has since been discovered to be extremely variable among Protostomia's me ...
mouth formation were correct. An alternative way to develop two openings from the blastopore during gastrulation, called amphistomy, appears to exist in some animals, such as
nematode The nematodes ( or grc-gre, Νηματώδη; la, Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant- parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a bro ...
s. In humans the perforation of the mouth and anus happen at 4 weeks and 8 weeks respectively.


Evolutionary origin

Bilaterians likely evolved from an ancestor that was radially symmetrical. There have been suggestions that the blastopore started out as the digestive surface on a radial organism that became elongated (and thus bilaterally symmetrical) before its sides closed over to leave a mouth at the front and an anus at the rear. This matches with the "flaps-folding-over" model of gut formation, but an alternative view is that the original blastopore migrated forwards to one end of the ancestral organism before deepening to become a blind gut. This is consistent with living Xenacoelomorpha, which are the sister taxon to protostomes and deuterostomes.Hejnol, A., Obst, M., Stamatakis, A., Ott, M., Rouse, G. W., Edgecombe, G. D., et al. (2009). Assessing the root of bilaterian animals with scalable phylogenomic methods. Proceedings of the Royal Society, Series B, 276, 4261–4270. The story is a little more complex, because the blastopore itself does not directly give rise to the mouth of these worms. This suggests that the last common ancestor of bilaterians had a similar gut configuration, and that the anus evolved after the mouth. Exactly how a through gut formed from this blind gut is somewhat harder to tell. The genetic mechanisms responsible for anus formation are quite variable, which might suggest that the anus evolved several times in different groups. Scientists are currently looking into this matter to generate a more complete picture. In humans (a deuterostome), the development proceeds differently. The
buccopharyngeal membrane The region where the crescentic masses of the ectoderm and endoderm come into direct contact with each other constitutes a thin membrane, the buccopharyngeal membrane (or oropharyngeal membrane), which forms a septum between the primitive mouth and ...
is created in the
foregut The foregut is the anterior part of the alimentary canal, from the mouth to the duodenum at the entrance of the bile duct. Beyond the stomach, the foregut is attached to the abdominal walls by mesentery. The foregut arises from the endoderm, devel ...
and it is perforated during the 4th week of human development, creating the primitive mouth, whereas the cloacal membrane is created in the
hindgut The hindgut (or epigaster) is the posterior ( caudal) part of the alimentary canal. In mammals, it includes the distal one third of the transverse colon and the splenic flexure, the descending colon, sigmoid colon and up to the ano-rectal junc ...
and it is perforated during the 8th week of human development, creating the primitive anus after the mouth opening has already been created.http://www.columbia.edu/itc/hs/medical/humandev/2004/Chapt18-Endoderm.pdf ENDODERMAL DERIVATIVES, FORMATION OF THE GUT AND ITS SUBSEQUENT ROTATION, Gershon, M. pages 18-3 - 18-13


See also

*
Embryology Embryology (from Greek ἔμβρυον, ''embryon'', "the unborn, embryo"; and -λογία, ''-logia'') is the branch of animal biology that studies the prenatal development of gametes (sex cells), fertilization, and development of embryos an ...
* Gastrulation


References

{{Reflist Embryology of digestive system Mouth Digestive system