Bilateraalse

Bilateria, also known as bilaterian animals, is a large clade that includes most animal life. They are characterized by bilateral symmetry at some life stage, a triploblastic body plan with three germ layers (endoderm, mesoderm, ectoderm), and typically a centralized nervous system and a complete digestive tract with a mouth and an anus. Most bilaterians exhibit cephalization, the concentration of sensory organs at the front end.

Taxonomically, Bilateria is divided into two major divisions: Protostomia and Deuterostomia. Protostomes encompass ecdysozoans (such as arthropods and nematodes) and lophotrochozoans (such as mollusks, annelids, and flatworms). Deuterostomes include chordates (vertebrates and their relatives), echinoderms, and hemichordates. The two groups are traditionally distinguished by embryological development: in protostomes the blastopore typically becomes the mouth, development is usually spiral and determinate, and coelom formation occurs by schizocoely; in deuterostomes the blastopore usually becomes the anus, development is radial and indeterminate, and coelom formation is often enterocoelic.

Evolution and fossil record: Bilaterians likely originated in the Neoproterozoic era, with the urbilaterian posited as a common ancestor to protostomes and deuterostomes. The Cambrian period saw a rapid diversification of bilaterian lineages, though precise timing and relationships remain subjects of ongoing research.

Diversity and ecology: Bilaterians occupy nearly every habitat and include representatives from microscopic worms to large vertebrates. Some lineages later evolve specialized body plans or secondary losses of certain features, but the bilateral body plan remains a defining and widespread characteristic of the group.