Question

Most animals have a body that is ___________. a. radially symmetrical b. bilaterally symmetrical c. asymmetrical

Short answer

b. bilaterally symmetrical.

Step-by-step solution

Define Symmetry Types

Understanding the types of symmetry is important. Radial symmetry means an organism can be divided into similar halves by passing a plane at any angle along a central axis. Bilateral symmetry means there is a single plane that divides the body into two equal halves that are mirror images. Asymmetry means the organism cannot be divided into similar segments.

Determine Common Symmetry in Animals

Most animals in the animal kingdom exhibit bilateral symmetry. This form of symmetry allows for active movement and the development of a head (cephalization), which is advantageous for organisms that move in a specific direction.

Choose the Correct Answer

Considering the defined types of symmetries and the common symmetry in animals, the correct option is: b. bilaterally symmetrical.

Key concepts

Radial Symmetry

Radial symmetry is a fascinating body structure found in certain groups of animals, notably within the oceanic environment. When an organism has radial symmetry, it means their body structure can be split into similar sections both ways through multiple planes that pass through their central axis.

This kind of symmetry is most commonly found in animals like jellyfish, sea anemones, and starfish. These creatures don't have a left or right side, instead, they have a top and a bottom.

• Allows interaction with the environment from all directions.
• Organisms usually sessile or move slowly.

Radial symmetry is particularly advantageous for organisms living in aquatic environments. It allows for feeding from multiple directions without needing a specific orientation. The downfall is that it's less suitable for organisms that need to move quickly in a certain direction.

Bilateral Symmetry

Bilateral symmetry is the most common form of body symmetry encountered amongst higher-order animals, including humans. This type of symmetry divides the body into two identical halves with a single plane.

Animals with bilateral symmetry have a distinct "head" and "tail" end, and usually a "back" and "belly" side too. This orientation supports forward movement and is linked to a process called cephalization, where sensory organs and nerve tissues concentrate at the front.

• Facilitates directed movement.
• Allows for streamlined, more efficient movement.
• Associated with complex behaviors and interactions.

Bilateral symmetry has proven evolutionarily advantageous as it enables the development of specialized body parts and more refined motor skills, necessary for survival and adapting to various environments.

Asymmetry

Unlike radial or bilateral symmetry, asymmetry refers to organisms that lack any form of symmetry in their body plan. This means they cannot be divided into mirror images even with multiple attempts.

This is relatively uncommon in the animal kingdom but does appear in certain species such as sponges. These organisms don't rely on physical symmetry because they often lead a sessile (non-moving) lifestyle.

• Often found in simpler organisms.
• Reflects an evolutionary path that diverges from more organized forms of symmetry.

While asymmetrical organisms may appear less complex, their existence underscores the fascinating diversity of body structures evolved to suit a myriad of environmental niches.