bilaterios

Bilaterians are a major group of animals that exhibit bilateral symmetry. This means their bodies can be divided into two mirror-image halves along a single plane, often referred to as the sagittal plane. This fundamental body plan distinguishes them from radially symmetrical animals like jellyfish and sea anemones, and from asymmetrical animals like sponges. The vast majority of known animal species, including insects, fish, amphibians, reptiles, birds, and mammals, belong to the Bilateria.

The evolution of bilateral symmetry is considered a significant event in animal evolution, allowing for directional movement and the development of a distinct head end (anterior) and tail end (posterior). This anterior end typically bears the animal's sensory organs, leading to cephalization, the concentration of nervous tissue and sensory structures at the front of the body. Bilaterians also possess a through-gut, meaning they have a distinct mouth and anus, facilitating more efficient digestion and waste removal.

Bilateria are further divided into two main clades: protostomes and deuterostomes. This classification is based on differences in their embryonic development, particularly the pattern of cleavage in early cell divisions and the fate of the blastopore, the first opening to form in the embryo. Protostomes include a wide array of invertebrates such as mollusks, annelids, and arthropods, while deuterostomes encompass echinoderms and chordates, the group that includes vertebrates. Understanding bilaterian phylogeny is crucial for comprehending the diversity and evolutionary history of animal life.