Learning Outcomes
i. Define and differentiate between radial and bilateral symmetry in animals.
ii. Identify examples of animals with radial and bilateral symmetry.
iii. Explain the relationship between symmetry and movement patterns of animals.
iv. Discuss the evolutionary significance of radial and bilateral symmetry.
Introduction
The animal kingdom exhibits a remarkable diversity of shapes and forms, each reflecting adaptations to specific environments and lifestyles. Among these diverse body plans, symmetry plays a crucial role in animal development, movement, and behavior. Symmetry refers to the arrangement of an organism's body parts around a central axis or point.
i. Radial Symmetry
Radial symmetry is characterized by the arrangement of body parts around a central axis, allowing for equal distribution of sensory organs and movement along any radius. Animals with radial symmetry often inhabit sessile or sedentary lifestyles, such as:
Cnidarians: Jellyfish, anemones, corals
Echinoderms: Starfish, sea urchins, sand dollars
The radial symmetry of these animals allows them to filter food from the surrounding water or capture prey with tentacles extending in all directions.
ii. Bilateral Symmetry
Bilateral symmetry is characterized by the division of the body into two mirror-image halves along a central axis. This symmetry is particularly common in animals that move actively, as it allows for efficient locomotion and coordination. Examples of animals with bilateral symmetry include:
Vertebrates: Fish, amphibians, reptiles, birds, mammals
Arthropods: Insects, spiders, crustaceans
The bilateral symmetry of these animals allows them to have distinct head and tail ends, specialized sensory organs on each side, and paired appendages for movement.
iii. Relationship between Symmetry and Movement Patterns
The type of symmetry an animal possesses is often closely linked to its movement patterns. Radial symmetry is well-suited for sessile or slow-moving animals that rely on capturing prey or filtering food from their surroundings. Bilateral symmetry, on the other hand, is advantageous for active, directional movement, as it allows for efficient coordination and balance.
iv. Evolutionary Significance of Symmetry
The development of radial and bilateral symmetry in animals represents significant evolutionary milestones. Radial symmetry allowed early animals to efficiently capture food and adapt to sessile lifestyles, while bilateral symmetry paved the way for the evolution of active locomotion and the diversification of movement patterns.
Symmetry plays a fundamental role in the development, movement, and behavior of animals. Radial and bilateral symmetry represent two distinct adaptations that have shaped the evolution of diverse animal body plans and lifestyles. Understanding the relationship between symmetry and animal form and function provides insights into the remarkable diversity and adaptability of the animal kingdom.
