Bilateria

Bilaterians; Temporal range: Ediacaran–Present,
	Diversity of bilaterians.
Scientific classification
Kingdom:	Animalia
Subkingdom:	Eumetazoa
Clade:	ParaHoxozoa
Clade:	Bilateria; Hatschek, 1888
Phyla
	Proarticulata? †; Xenacoelomorpha?; Nephrozoa: Ikaria †; Superphylum Deuterostomia Chordata; Xenacoelomorpha?; Ambulacraria (unranked) Hemichordata; Echinodermata; Cambroernida †; ; ; Protostomia (unranked) Superphylum Ecdysozoa Saccorhytida? †; Scalidophora (unranked) Kinorhyncha; Loricifera; Priapulida; ; Nematoida (unranked) Nematoda; Nematomorpha; ; Panarthropoda (unranked) Onychophora; Arthropoda; Tardigrada; ; ; Spiralia (unranked) Chaetognatha; Mesozoa (unranked) Monoblastozoa; ; Superphylum Platyzoa Acanthocephala; Cycliophora; Gastrotricha; Gnathostomulida; Micrognathozoa; Platyhelminthes; Rotifera; ; Superphylum Lophotrochozoa Annelida; Brachiopoda; Bryozoa; Dicyemida; Entoprocta; Hyolitha †; Mollusca; Nemertea; Orthonectida; Phoronida; ; ; ; ;
Synonyms
	Triploblasts Lankester, 1873

Bilateria (/ˌbaɪləˈtɪəriə/) is a group of animals, called bilaterians, 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 a back (ventral–dorsal axis).^[2] Nearly all are bilaterally symmetrical as adults as well; the most notable exception is the echinoderms, which achieve secondary pentaradial symmetry as adults, but are bilaterally symmetrical during embryonic development.

Most animals are bilaterians, excluding sponges, ctenophores, placozoans and cnidarians. For the most part, bilateral embryos are triploblastic, having three germ layers: endoderm, mesoderm, and ectoderm. Except for a few phyla (i.e. flatworms and gnathostomulids), bilaterians have complete digestive tracts with a separate mouth and anus. Some bilaterians lack body cavities (acoelomates, i.e. Platyhelminthes, Gastrotricha and Gnathostomulida), while others display primary body cavities (deriving from the blastocoel, as pseudocoeloms) or secondary cavities (that appear de novo, for example the coelom).

^ Martin, M. W.; Grazhdankin, D. V.; Bowring, S. A.; Evans, D. A.; Fedonkin, M. A.; Kirschvink, J. L. (5 May 2000). "Age of Neoproterozoic bilatarian [sic] body and trace fossils, White Sea, Russia: implications for metazoan evolution". Science. 288 (5467): 841–45. Bibcode:2000Sci...288..841M. doi:10.1126/science.288.5467.841. PMID 10797002. S2CID 1019572.
^ Brusca, Richard C. (2016). "Introduction to the Bilateria and the Phylum Xenacoelomorpha: Triploblasty and Bilateral Symmetry Provide New Avenues for Animal Radiation" (PDF). Invertebrates. Sinauer Associates. pp. 345–372. ISBN 978-1-60535-375-3.

[1] Martin, M. W.; Grazhdankin, D. V.; Bowring, S. A.; Evans, D. A.; Fedonkin, M. A.; Kirschvink, J. L. (5 May 2000). "Age of Neoproterozoic bilatarian [sic] body and trace fossils, White Sea, Russia: implications for metazoan evolution". Science. 288 (5467): 841–45. Bibcode:2000Sci...288..841M. doi:10.1126/science.288.5467.841. PMID 10797002. S2CID 1019572.

[Brusca2016-2] Brusca, Richard C. (2016). "Introduction to the Bilateria and the Phylum Xenacoelomorpha: Triploblasty and Bilateral Symmetry Provide New Avenues for Animal Radiation" (PDF). Invertebrates. Sinauer Associates. pp. 345–372. ISBN 978-1-60535-375-3.

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