Bilateria refers to animals that possess bilateral symmetry.
In biology, the term describes a major group of animals characterized by having a body plan with distinct left and right sides, a head and tail (anterior-posterior axis), and a back and belly (dorsal-ventral axis). This symmetry allows for cephalization, the concentration of sensory organs and nervous tissue in the head region, which is advantageous for directional movement and hunting.
Characteristics of Bilateria
- Bilateral Symmetry: The most defining feature. A single plane (the sagittal plane) can divide the body into two mirror-image halves.
- Three Germ Layers (Triploblastic): Bilaterians develop from three primary germ layers: the ectoderm, mesoderm, and endoderm. These layers give rise to different tissues and organs.
- Cephalization: The concentration of sensory and neural structures in the anterior (head) region.
- Body Cavity (Coelom): Many bilaterians possess a fluid-filled body cavity called a coelom, which provides space for organ development, supports circulation, and acts as a hydrostatic skeleton in some species.
- Bilateral Symmetry: It allows for the development of specialized appendages and structures on either side of the body, promoting efficient movement and interaction with the environment.
Examples of Bilaterians
The vast majority of animal species are bilaterians. Examples include:
- Humans
- Insects
- Worms
- Snails
- Fish
- Birds
In essence, Bilateria encompasses a wide range of animal species unified by the fundamental characteristic of bilateral symmetry, setting them apart from other animal groups like sponges (no symmetry) and cnidarians (radial symmetry).
