Bilateria

Bilateria (/ˌbaɪləˈtɪəriə/) is a large clade of animals characterised by bilateral symmetry during embryonic development. This means their body plans are laid around a longitudinal 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 have pentaradial symmetry as adults, but bilateral symmetry as embryos.

Bilaterians constitute one of the five main lineages of animals, the other four being Porifera (sponges), Cnidaria (jellyfish, hydrozoans, sea anemones and corals), Ctenophora (comb jellies) and Placozoa. They rapidly diversified in the late Ediacaran and the Cambrian, and are now by far the most successful animal lineage, with over 98% of known animal species. Bilaterians are traditionally classified as either deuterostomes or protostomes, on the basis of whether the blastopore becomes the anus or mouth.

The phylum Xenacoelomorpha, once thought to be flatworms, was erected in 2011, and has provided an extra challenge to bilaterian taxonomy, as they likely do not belong to either group. Bilaterians are incredibly diverse, including animals like insects, fish, birds, and humans, making them a crucial part of life's story on Earth.

Body plan

Animals with a bilaterally symmetric body plan usually move in one direction and have a head end and a tail end, as well as a back and a belly. This setup helps them find food and develop 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, while others have nerve cords on their backs, bellies, or both.

Evolution

Main article: Urbilaterian

The hypothetical most recent common ancestor of all animals with bilateral symmetry is called the 'urbilaterian'. Scientists debate what this first bilaterian looked like. Some think it had a solid body, while others believe it had a body cavity.

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. Recent studies suggest some simpler worm-like animals might represent the original body plan of bilaterians.

Taxonomic history

The group Bilateria was named by an Austrian scientist named Berthold Hatschek in 1888. He used this name to describe animals that develop with bilateral symmetry, meaning 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.