The
embryological origin of the mouth and anus is an important characteristic, and forms the morphological basis for separating
bilateriaThe bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back. The bilateralia are a subregnum of animals, including the majority...
n animals into two
natural groupingA clade is a term used in modern alpha taxonomy, the scientific classification of living and fossil organisms, to describe a monophyletic group, defined as a group consisting of a single common ancestor and all its descendants.[The term "monophyletic group" is used in this article...]
s: the
protostomeProtostomia are a clade of animals. Together with the deuterostomes and a few smaller phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers...
s and
deuterostomeDeuterostomes are a superphylum of animals. They are a subtaxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes...
s.
In animals at least as complex as an
earthwormEarthworm is the common name for the largest members of Oligochaeta in the phylum Annelida. In classical systems they were placed in the order Opisthopora, on the basis of the male pores opening posterior to the female pores, even though the internal male segments are anterior to the female...
, a dent forms in one side of the early,
spheroidalA sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...
embryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
. This dent, the
blastoporeA blastopore is an opening into the archenteron during the embryonic stages of an organism. The distinction between protostomes and deuterostomes is based on the direction in which the mouth develops in relation to the blastopore...
, deepens to become the
archenteronThe primitive gut that forms during gastrulation in the developing blastula is known as the archenteron. It develops into the digestive tract of an animal.-Formation of the Archenteron:[See Gastrulation]...
, the first phase in the growth of the gut.
In
deuterostomeDeuterostomes are a superphylum of animals. They are a subtaxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes...
s (including humans), 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
protostomeProtostomia are a clade of animals. Together with the deuterostomes and a few smaller phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers...
s formed the mouth, and the the anus was formed second when the gut tunnelled through the embryo.
The
embryological origin of the mouth and anus is an important characteristic, and forms the morphological basis for separating
bilateriaThe bilateria are all animals having a bilateral symmetry, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical animals like jellyfish have a topside and downside, but no front and back. The bilateralia are a subregnum of animals, including the majority...
n animals into two
natural groupingA clade is a term used in modern alpha taxonomy, the scientific classification of living and fossil organisms, to describe a monophyletic group, defined as a group consisting of a single common ancestor and all its descendants.[The term "monophyletic group" is used in this article...]
s: the
protostomeProtostomia are a clade of animals. Together with the deuterostomes and a few smaller phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers...
s and
deuterostomeDeuterostomes are a superphylum of animals. They are a subtaxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes...
s.
In animals at least as complex as an
earthwormEarthworm is the common name for the largest members of Oligochaeta in the phylum Annelida. In classical systems they were placed in the order Opisthopora, on the basis of the male pores opening posterior to the female pores, even though the internal male segments are anterior to the female...
, a dent forms in one side of the early,
spheroidalA sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...
embryoAn embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
. This dent, the
blastoporeA blastopore is an opening into the archenteron during the embryonic stages of an organism. The distinction between protostomes and deuterostomes is based on the direction in which the mouth develops in relation to the blastopore...
, deepens to become the
archenteronThe primitive gut that forms during gastrulation in the developing blastula is known as the archenteron. It develops into the digestive tract of an animal.-Formation of the Archenteron:[See Gastrulation]...
, the first phase in the growth of the gut.
In
deuterostomeDeuterostomes are a superphylum of animals. They are a subtaxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes...
s (including humans), 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
protostomeProtostomia are a clade of animals. Together with the deuterostomes and a few smaller phyla, they make up the Bilateria, mostly comprising animals with bilateral symmetry and three germ layers...
s formed the mouth, and the the anus was formed second when the gut tunnelled through the embryo. More recent research has shown that our understanding of protostome mouth formation is somewhat less secure than we had thought. The edges of the dent appear to move together and close up in the middle, leaving openings at the ends which become the mouth and anus.
However, this idea has been challenged, because the flatworms, which form a sister group to the rest of the bilaterian animals, have a single mouth which 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 around the protostomes', which suggests that the two structures are equivalent.
Evolutionary origin
Bilaterians evolved from a jellyfish-like ancestor, which was radially symmetrical. Because the development of the embryo typically reflects lineages' evolutionary origin, this has led to suggestions that the blastopore started out as the digestive surface on a radial organism, which became elongated (and thus bilaterially 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
flatwormThe flatworms, known in scientific literature as Platyhelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...
s, which are basal to protostomes and deuterostomes. The story is a little more complex, because the blastopore itself does not go on to become the mouth. However, flatworms, the sister group to the rest of the bilaterians, branched off from the ancestral stock before the bilaterian group originated, and have a mouth at one end and a blind gut. 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.
Monophyly
The origin of the digestive tract separates the protostomes from the deuterostomes, and genetic data backs up the premise that the monophyletic deuterostomes form a sister group to the monophyletic protostomes. This confirmation is important because it paints a picture of the ancestral bilaterian having a blind gut, with two alternative methods of anus-development leading to the two groups of organisms that dominate the diversity of life today. In other words, the deuterostomes did not arise from within the protostomes by modifying the method of gut formation, but are an unrelated lineage; the significance of developing an anus is reflected by the success of these two clades.