Bilateria

Bilaterians Temporal range: Ediacaran—Present, 555–0 Ma PreЄ Є O S D C P T J K Pg N
A bowfin, Amia calva
Scientific classification
Kingdom:	Animalia
Subkingdom:	Eumetazoa
Clade:	Bilateria Hatschek, 1888
Phyla
Orthonectida Dicyemida Proarticulata † Xenacoelomorpha Nephrozoa Deuterostomia Chordata Hemichordata Echinodermata Saccorhytida † Protostomia Ecdysozoa Scalidophora (unranked) Kinorhyncha Loricifera Priapulida Nematoda Nematomorpha Panarthropoda (unranked) Lobopodia Dinocarida † "Xenusia" † Onychophora Tardigrada Arthropoda Platyzoa Platyhelminthes Gastrotricha Rotifera Acanthocephala Gnathostomulida Micrognathozoa Cycliophora Superphylum Lophotrochozoa Sipuncula Hyolitha † Nemertea Phoronida Bryozoa Entoprocta Brachiopoda Mollusca Annelida Chaetognatha

The Bilateria /ˌbaɪləˈtɪəriə/ or bilaterians, or triploblasts, are animals with bilateral symmetry, i.e., they have a head ("anterior") and a tail ("posterior") as well as a back ("dorsal") and a belly ("ventral"); therefore they also have a left side and a right side. In contrast, radially symmetrical animals like jellyfish have a topside and a downside, but no identifiable front or back.

The bilateria are a major group of animals, including the majority of phyla but not sponges, cnidarians, placozoans and ctenophores. For the most part, bilateral embryos are triploblastic, having three germ layers: endoderm, mesoderm, and ectoderm. Nearly all are bilaterally symmetrical, or approximately so; the most notable exception is the echinoderms, which achieve near-radial symmetry as adults, but are bilaterally symmetrical as larvae.

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 pseudocoel) or secondary cavities (that appear de novo, for example the coelom).