Bilateral Symmetry:
Bilaterians have a body plan that can be divided into mirror images along a single plane. This symmetry is evident in their external appearance, internal anatomy, and organ systems.
Anterior-Posterior Axis:
Bilaterally symmetrical animals have a well-defined anterior (front) and posterior (back) end. This axis determines the direction of movement and orientation of the animal.
Cephalization:
Most Bilaterians exhibit cephalization, which is the concentration of sensory organs, nerve centers, and feeding structures at the anterior end. The head region houses essential organs such as the brain, mouth, and sense organs.
Coelom:
With few exceptions, bilaterians possess a coelom, a fluid-filled body cavity that lies between the body wall (ectoderm) and the gut (endoderm). The coelom provides support, allows for organ movement, and facilitates efficient organ system functioning.
Metamerism:
Some bilaterians, such as earthworms and arthropods, exhibit metamerism, a body plan characterized by a series of repeating units or segments called metameres. Each segment typically has similar structures, contributing to the modular organization of the body.
Organ System Complexity:
Bilaterians have evolved increasingly complex organ systems compared to radially symmetrical animals. They possess specialized digestive, circulatory, respiratory, nervous, and reproductive systems to carry out various functions efficiently.
Closed Circulatory System:
Most bilaterians have a closed circulatory system, where blood is contained within vessels (blood vessels) and pumped by a heart. This system ensures efficient transport of nutrients, oxygen, and waste products throughout the body.
Complete Digestive System:
Bilaterally symmetrical animals possess a complete digestive system with a mouth, pharynx, esophagus, stomach, intestines, and anus. This arrangement allows for sequential digestion, absorption of nutrients, and elimination of waste.
Centralized Nervous System:
Bilaterians have a centralized nervous system, often consisting of a brain and nerve cords. The brain serves as the main control center, processing information from sensory organs and coordinating motor responses.
Reproduction:
Bilaterians exhibit diverse reproductive strategies, including both asexual and sexual reproduction. Sexual reproduction involves the production of specialized reproductive cells (gametes) and the fusion of these gametes to form a zygote, leading to the development of a new individual.
These characteristics collectively define the features associated with animals that exhibit bilateral symmetry during their life cycle. Bilateral symmetry has enabled these animals to evolve a wide range of body plans, organ system complexities, and ecological adaptations that have contributed to their success and dominance in various environments.