Bilateria

Bilaterians; Temporal range: Ediacaran–Present,
	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 (/ˌbaɪləˈtɪəriə/ BY-lə-TEER-ee-ə)^[2] is a large clade or infrakingdom of animals called bilaterians (/ˌbaɪləˈtɪəriən/ BY-lə-TEER-ee-ən),^[3] characterized by bilateral symmetry (i.e. having a left and a right side that are mirror images of each other) during embryonic development. This means their body plans are laid around a longitudinal axis (rostral–caudal axis) with a front (or "head") and a rear (or "tail") end, as well as a left–right–symmetrical belly (ventral) and back (dorsal) surface.^[4] Nearly all bilaterians maintain a bilaterally symmetrical body as adults; the most notable exception is the echinoderms, which extend to pentaradial symmetry as adults, but are only bilaterally symmetrical as an embryo. Cephalization is also a characteristic feature among most bilaterians, where the special sense organs and central nerve ganglia become concentrated at the front/rostral end.

Bilaterians constitute one of the five main metazoan lineages, the other four being Porifera (sponges), Cnidaria (jellyfish, hydrae, sea anemones and corals), Ctenophora (comb jellies) and Placozoa (tiny "flat animals"). 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).