Bilateria

Bilaterians Temporal range: Ma[1] PreꞒ Ꞓ O S D C P T J K Pg N
Bilaterian diversity
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Subkingdom:	Eumetazoa
Clade:	ParaHoxozoa
Clade:	Bilateria Hatschek, 1888
Subdivisions
Proarticulata? † Xenambulacraria? Xenacoelomorpha Ambulacraria? Nephrozoa ? Ikaria? † Deuterostomia ? Chordata Xenambulacraria? Xenacoelomorpha? Ambulacraria Protostomia Ecdysozoa Spiralia
Synonyms
Triploblasts Lankester, 1873

Bilateria (

Bilaterians constitute one of the five main

Body plan

Some of the earliest bilaterians were wormlike, and a bilaterian body can be conceptualized as a cylinder with a gut running between two openings, the mouth and the anus. Around the gut it has an internal body cavity, a coelom or pseudocoelom.^[a] Animals with this bilaterally symmetric body plan have a head (anterior) end and a tail (posterior) end as well as a back (dorsal) and a belly (ventral); therefore they also have a left side and a right side.^[4][2]

Having a front end means that this part of the body encounters stimuli, such as food, favouring

The hypothetical

One hypothesis is that the original bilaterian was a bottom dwelling worm with a single body opening, similar to Xenoturbella.^[3] Alternatively, it may have resembled the planula larvae of some cnidaria, which have some bilateral symmetry.^[10] However, there is evidence that it was segmented, as the mechanism for creating segments is shared between vertebrates (deuterostomes) and arthropods (protostomes).^[11]

Fossil record

The first evidence of bilateria in the fossil record comes from trace fossils in

A modern consensus phylogenetic tree for Bilateria is shown below, although the positions of certain clades are still controversial (dashed lines) and the tree has changed considerably since 2000.^{[27][25][28][29][30]}

ParaHoxozoa

Cnidaria Placozoa  Bilateria  Proarticulata? †  Xenacoelomorpha  Xenoturbellida  Acoelomorpha    Nemertodermatida Acoela  Nephrozoa    Deuterostomia    Ambulacraria    Echinodermata Hemichordata † Cambroernida   Chordata   Cephalochordata  Olfactores    Tunicata Vertebrata   † Saccorhytus coronarius   † Vetulocystids † Vetulicolians   Protostomia    Ecdysozoa     Nematoida    Nematoda Nematomorpha Loricifera  Panarthropoda  Onychophora  Tactopoda    Tardigrada Arthropoda  Scalidophora    Priapulida Kinorhyncha > 539 Mya  Spiralia   Gnathifera    Rotifera and allies Chaetognatha  Platytrochozoa  Platyhelminthes and allies  Lophotrochozoa    Mollusca Annelida and allies ¿†Kimberella? 550 mya 580 Mya Kimberella? † 610 mya 650 Mya

A different hypothesis is that the Ambulacraria are sister to Xenacoelomorpha together forming the

ParaHoxozoa

Cnidaria Placozoa  Bilateria  †Proarticulata  Xenambulacraria   Xenacoelomorpha  Xenoturbellida  Acoelomorpha    Nemertodermatida Acoela  Ambulacraria    Echinodermata Hemichordata † Cambroernida  Centroneuralia    Chordata   Cephalochordata  Olfactores    Urochordata Vertebrata   † Saccorhytus coronarius   † Vetulocystids † Vetulicolians   Protostomia    Ecdysozoa     Nematoida    Nematoda Nematomorpha Loricifera  Panarthropoda  Onychophora  Tactopoda    Tardigrada Arthropoda  Scalidophora    Priapulida Kinorhyncha > 539 Mya Spiralia  Gnathifera    Rotifera and allies Chaetognatha  Platytrochozoa  Platyhelminthes and allies  Lophotrochozoa    Mollusca Annelida and allies ¿†Kimberella? 550 Mya 580 Mya ¿†Kimberella? 610 Mya 650 Mya

See also

• Embryological origins of the mouth and anus

Notes

References