Bilateria

Bilateria (/ˌbaɪləˈtɪəriə/) is a large clade or infrakingdom of animals called bilaterians, 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. Nearly all bilaterians maintain a bilaterally symmetrical body as adults; the most notable exception is the echinoderms, which achieve secondary pentaradial symmetry as adults, but are 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"). Most so-called "higher-order" animals are bilaterians. 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).