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The Bilateria are all animals having a bilateral symmetry
Symmetry (biology)

Symmetry in biology is the balanced distribution of duplicate body parts or shapes. The body plans of most multicellular organisms exhibit some form of symmetry, either radial symmetry or bilateral symmetry or glide symmetry....
, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical
Symmetry (biology)

Symmetry in biology is the balanced distribution of duplicate body parts or shapes. The body plans of most multicellular organisms exhibit some form of symmetry, either radial symmetry or bilateral symmetry or glide symmetry....
 animals like jellyfish
Jellyfish

Jellyfish are free-swimming members of the phylum Cnidaria. They have several different morphologies that represent several different cnidarian classes including the Scyphozoa , Staurozoa , Cubozoa , and Hydrozoa ....
 have a topside and downside, but no front and back. The bilateralians are a subregnum (a major group) of animal
Animal

Animals are a major group of multicellular, eukaryotic organisms of the Kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life....
s, including the majority of phyla; the most notable exceptions are the sponge
Sea sponge

The sponges or poriferans are animals of the phylum Porifera . Their bodies consist of an outer thin layer of cells, the pinacoderm and an inner mass of cells and skeletal elements, the choanoderm....
s and cnidaria
Cnidaria

Cnidaria Cnidarians were for a long time grouped with Ctenophores in the phylum Coelenterata, but increasing awareness of their differences caused them to be placed in separate phyla....
ns.






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Symmetryoflifeformsonearth
The Bilateria are all animals having a bilateral symmetry
Symmetry (biology)

Symmetry in biology is the balanced distribution of duplicate body parts or shapes. The body plans of most multicellular organisms exhibit some form of symmetry, either radial symmetry or bilateral symmetry or glide symmetry....
, i.e. they have a front and a back end, as well as an upside and downside. Radially symmetrical
Symmetry (biology)

Symmetry in biology is the balanced distribution of duplicate body parts or shapes. The body plans of most multicellular organisms exhibit some form of symmetry, either radial symmetry or bilateral symmetry or glide symmetry....
 animals like jellyfish
Jellyfish

Jellyfish are free-swimming members of the phylum Cnidaria. They have several different morphologies that represent several different cnidarian classes including the Scyphozoa , Staurozoa , Cubozoa , and Hydrozoa ....
 have a topside and downside, but no front and back. The bilateralians are a subregnum (a major group) of animal
Animal

Animals are a major group of multicellular, eukaryotic organisms of the Kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life....
s, including the majority of phyla; the most notable exceptions are the sponge
Sea sponge

The sponges or poriferans are animals of the phylum Porifera . Their bodies consist of an outer thin layer of cells, the pinacoderm and an inner mass of cells and skeletal elements, the choanoderm....
s and cnidaria
Cnidaria

Cnidaria Cnidarians were for a long time grouped with Ctenophores in the phylum Coelenterata, but increasing awareness of their differences caused them to be placed in separate phyla....
ns. For the most part, Bilateria have bodies that develop from three different germ layer
Germ layer

A germ layer is a group of cell s, formed during animal embryogenesis. Germ layers are particularly pronounced in the vertebrates; however, all animals more complex than sea sponge produce two or three primary tissue layers ....
s, called the endoderm, mesoderm, and ectoderm. From this they are called triploblastic. Nearly all are bilaterally symmetrical, or approximately so. The most notable exception is the echinoderm
Echinoderm

Echinoderms are a Phylum of Marine animals . Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone.Aside from the problematic Arkarua, the first definitive members of the phylum appeared near the start of the Cambrian period....
s, which achieve near-radial symmetry as adults, but are bilaterally symmetrical as larvae.

Except for a few highly reduced forms, the Bilateria have complete digestive tracts with separate mouth
Mouth

The mouth, buccal cavity, or oral cavity is the first portion of the alimentary canal that receives food and begins digestion by mechanically breaking up the solid food particles into smaller pieces and mixing them with saliva....
 and anus
Anus

The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to expel feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest, such as coprolite ; food material after all the nutrients have b...
. Most Bilateria also have a type of internal body cavity, called a coelom
Body cavity

By the broadest definition, a body cavity is any fluid filled space in a multicellular organism. However, the term usually refers to the space, located between an animal?s outer covering and the outer lining of the gut cavity, where internal organs develop....
. It was previously thought that acoelomates gave rise to the other group, but there is some evidence now that in the main acoelomate phyla (flatworm
Flatworm

The flatworms, known in scientific literature as Platyhelminthes are a Phylum of relatively simple bilaterian, Segmentation , soft-bodied invertebrate animals....
s and gastrotrich
Gastrotrich

The gastrotrichs are a phylum of microscopic animals abundant in fresh water and marine environments. Most fresh water species are part of the periphyton and benthos....
s) the absence could be secondary.

Evolution

The hypothetical last common ancestor of all bilaterians is termed the "Urbilaterian
Urbilaterian

The urbilaterian is the hypothetical last common ancestor of the bilaterian clade, i.e., all animals having a bilateral symmetry.Its appearance is a matter of debate, for no representative has been identified in the Fossil; the reconstructed morphology it would display largely depends on whether the acoelomorpha are included within the b...
". There is some debate about its appearance. The first evidence of bilateria in the fossil record comes from trace fossils in Ediacaran
Ediacaran

The Ediacaran Period is the last geological period of the Neoproterozoic Era and of the Proterozoic Eon, immediately preceding the Cambrian Period, the first period of the Paleozoic Era and of the Phanerozoic Eon....
 sediments, and the first bona fide bilaterian fossil is Kimberella
Kimberella

Kimberella is a genus of fossils known only from rocks of the Ediacaran period, and only one species, Kimberella quadrata, has been recognized....
, dating to . Earlier fossils are controversial; The fossil Vernanimalcula
Vernanimalcula

Vernanimalcula guizhouena is a fossil believed by some to represent the earliest known member of the Bilateria . It is known from deposits dating to ....
 may be the earliest known bilaterian, but may also represent an infilled bubble. Fossil embryos
Fossil embryos

Many fossils of the Doushantuo formation have been interpreted as fossil embryos; embryos are also common throughout the Cambrian fossil record....
 are known from around the time of Vernanimalcula , but none of these have bilaterian affinities.

Phylogeny


There are two or more superphyla (main lineages) of Bilateria. The deuterostome
Deuterostome

Deuterostomes are a superphylum of animals. They are a taxon of the Bilateria branch of the subregnum Eumetazoa, and are opposed to the protostomes....
s include the echinoderm
Echinoderm

Echinoderms are a Phylum of Marine animals . Echinoderms are found at every ocean depth, from the intertidal zone to the abyssal zone.Aside from the problematic Arkarua, the first definitive members of the phylum appeared near the start of the Cambrian period....
s, hemichordates, chordate
Chordate

Chordates are a group of animals that includes the vertebrates, together with several closely related invertebrates. They are united by having, at some time in their life cycle, a notochord, a hollow dorsal nerve cord, pharyngeal slits, an endostyle, and a post-anal tail....
s, and possibly a few smaller phyla. The protostome
Protostome

Protostomia are a clade of animals. Together with the deuterostomes and a few smaller phylum, they make up the Bilateria, mostly comprising animals with symmetry #Bilateral symmetry and triploblastic germ layers....
s include most of the rest, such as arthropod
Arthropod

Arthropods are animals belonging to the Scientific classification Arthropoda , and include the insects, arachnids, crustaceans, and others....
s, annelid
Annelid

The annelids, collectively called Annelida , are a large Scientific classification of animals comprising the segmented worms, with about 15,000 modern species including the well-known earthworms and leeches....
s, mollusks, flatworm
Flatworm

The flatworms, known in scientific literature as Platyhelminthes are a Phylum of relatively simple bilaterian, Segmentation , soft-bodied invertebrate animals....
s, and so forth. There are a number of differences, most notably in how the embryo
Embryo

An embryo is a multicellular organism ploidy eukaryote in its earliest stage of development, from the time of first cell division until birth, Egg , or germination....
 develops. In particular, the first opening of the embryo becomes the mouth in protostomes, and the anus in deuterostomes. Many taxonomists
Taxonomy

Taxonomy is the practice and science of classification. The word comes from the Greek language ', taxis and ', nomos .Taxonomies, or taxonomic schemes, are composed of taxonomic units known as taxa , or kinds of things that are arranged frequently in a hierarchical structure....
 now recognize at least two more superphyla among the protostomes, Ecdysozoa
Ecdysozoa

The Ecdysozoa are a grouping of protostome animals, including the Arthropoda , roundworm, and several smaller phylum . They were first defined by Aguinaldo et al. in 1997, based mainly on trees constructed using 18S ribosomal RNA genes....
 (molting animals) and Lophotrochozoa
Lophotrochozoa

The Lophotrochozoa are one of three major groupings of protostome animals. The taxon was introduced in 1995 in a paper by Kenneth M Halanych et al based on molecular data....
. Some taxonomists also recognize another protostome superphylum, Platyzoa
Platyzoa

The Platyzoa are a group of protostome animals proposed by Thomas Cavalier-Smith in 1998. Cavalier-Smith included in Platyzoa the Phylum flatworm or flatworms, and a new phylum, Acanthognatha, into which he gathered several previously described phyla of microscopic animals....
, while others would include the Platyzoans in Lophotrochozoa or not include them in any superphylum. The arrow worms (Chaetognatha
Chaetognatha

Chaetognatha is a Phylum of predatory marine worms that are a major component of plankton worldwide. About 20% of the known species are benthic and can attach to algae or rocks....
) have proven particularly difficult to classify, with some taxonomists placing them among the deuterostomes and others placing them among the protostomes. The two most recent studies to address the question of chaetognath origins (Helfenbein et al. 2004 and Papillon et al. 2004) support protostome affinities.

A proposed phylogeny of the Bilateria after Nielsen is as follows. An alternate phylogeny suggests a basal group called the ecdysozoa
Ecdysozoa

The Ecdysozoa are a grouping of protostome animals, including the Arthropoda , roundworm, and several smaller phylum . They were first defined by Aguinaldo et al. in 1997, based mainly on trees constructed using 18S ribosomal RNA genes....
.

See also

Embryological origins of the mouth and anus
Embryological origins of the mouth and anus

The embryological origin of the mouth and anus is an important characteristic, and forms the morphological basis for separating bilaterian animals into two clades: the protostomes and deuterostomes....


Other references

  • Helfenbein, Kevin G., H. Matthew Fourcade, Rohit G. Vanjani, and Jeffrey L. Boore (2004). The mitochondrial genome of Paraspadella gotoi is highly reduced and reveals that chaetognaths are a sister group to protostomes. Proceedings of the National Academy of Sciences of the United States of America 101(29), July 20 2004: 10639-10643.
  • Papillon, Daniel, Yvan Perez, Xavier Caubit, and Yannick Le Parco (2004). Identification of chaetognaths as protostomes is supported by the analysis of their mitochondrial genome. Molecular Biology and Evolution 21(11), November 2004: 2122-2129.


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