Bilateria

Bilateria (/ˌbaɪləˈtɪəriə/) is a large clade of animals that have bilateral symmetry when they are very small and growing. This means their bodies are laid out along a line with a front ("head") and a back ("tail") end, as well as a left–right–symmetrical belly (ventral) and back (dorsal). Almost all bilaterians keep this symmetry as adults; the main exception is the echinoderms, which have a different shape as adults but still have bilateral symmetry as embryos.

Bilaterians are one of the five main groups of animals. The other four are Porifera (sponges), Cnidaria (jellyfish, hydrozoans, sea anemones and corals), Ctenophora (comb jellies) and Placozoa. They became very common in the late Ediacaran and the Cambrian periods, and now they are the most successful animal group, with most known animal species. Bilaterians are usually divided into two groups: deuterostomes or protostomes, depending on whether a certain hole in their early development becomes the anus or the mouth.

The group Xenacoelomorpha, once thought to be flatworms, was added in 2011. This added some new questions to how scientists classify bilaterians, since these animals probably do not fit into either of the two main groups. Bilaterians include many different animals, like insects, fish, birds, and humans, showing how important they are to life on Earth.

Body plan

Animals with a bilaterally symmetric body plan usually move in one direction. They have a head end and a tail end, and a back and a belly. This helps them find food and grow sense organs in their heads. Most of these animals have a gut that runs from their mouth to their anus, while some have a simpler body plan.

Some of these animals have simple nerve networks. Others have nerve cords on their backs, bellies, or both.

Evolution

Main article: Urbilaterian

The most recent common ancestor of all animals with bilateral symmetry is called the 'urbilaterian'. Scientists are not sure what this first bilaterian looked like.

The first evidence of these animals appears in fossils from about 555 million years ago, with Kimberella being the earliest clear example. Bilateria today includes most known animal species, split into two main groups: deuterostomes like echinoderms and chordates, and protostomes like arthropods and molluscs. These groups differ mainly in how their embryos develop.

Taxonomic history

The group Bilateria was named by an Austrian scientist named Berthold Hatschek in 1888. He used this name for animals that develop with bilateral symmetry. This means they have a head and tail end and matching left and right sides.

Later, in 1910, another Austrian scientist, Karl Grobben, changed some of Hatschek's names. He renamed Zygoneura to Protostomia and created a new group called Deuterostomia for other animals.