Problem 18 - Campbell Biology Concepts & Connections

Question

Name two phyla of animals that are radially symmetric and two that are bilaterally symmetric. How do the general lifestyles of radial and bilateral animals differ?

Step-by-Step Solution

Answer

Radial: Cnidaria, Echinodermata. Bilateral: Chordata, Arthropoda. Radial animals are often stationary or drifters, while bilateral animals are more mobile.

Step 1: Identify Radially Symmetric Phyla

Radial symmetry means the body plan is arranged around a central axis. Examples of radially symmetric phyla are Cnidaria (e.g., jellyfish and corals) and Echinodermata (e.g., starfish and sea urchins).

Step 2: Identify Bilaterally Symmetric Phyla

Bilateral symmetry means the body plan is divided into left and right halves that are mirror images. Examples of bilaterally symmetric phyla are Chordata (e.g., mammals, birds, reptiles) and Arthropoda (e.g., insects, spiders, crustaceans).

Step 3: Describe Lifestyles of Radial Symmetry

Animals with radial symmetry often have a lifestyle that revolves around a central point. They are usually sessile (attached and immobile) or planktonic (drifting in water currents).

Step 4: Describe Lifestyles of Bilateral Symmetry

Bilaterally symmetric animals typically have a more active lifestyle. They usually have a head and tail (anterior and posterior ends) and move in a directed fashion.

Step 5: Summarize the Differences

Radially symmetric animals are often stationary or drifters, taking advantage of their ability to interact with the environment from all sides. Bilaterally symmetric animals are typically more mobile, engaging in complex movements and interactions.

Key Concepts

Radial Symmetry

Radial symmetry is a type of symmetry where the body plan is arranged around a central axis. This means that any plane cut through the central axis divides the organism into roughly similar halves. Examples of animal phyla that exhibit radial symmetry include:

• **Cnidaria**: This phylum includes animals like jellyfish, corals, and sea anemones. They have a simple body plan that radiates out from a central point.
• **Echinodermata**: This phylum includes starfish, sea urchins, and sand dollars. Although their larvae are bilaterally symmetric, the adults exhibit radial symmetry.

Radial symmetry is especially beneficial for organisms that are sessile (non-moving) or planktonic (drifting in current). This type of symmetry allows them to interact with their environment from all sides equally, which is ideal for animals that do not move much.

Bilateral Symmetry

Bilateral symmetry refers to a body plan in which the left and right halves of an organism are mirror images. In this kind of symmetry, there is usually a distinct head (anterior) and tail (posterior) end. Examples of animal phyla that show bilateral symmetry include:

• **Chordata**: This phylum includes vertebrates like mammals, birds, reptiles, amphibians, and fishes. These animals have complex body structures and move in a purposeful direction.
• **Arthropoda**: This is the largest animal phylum, including insects, spiders, and crustaceans. They also exhibit bilateral symmetry, which helps in coordinated and directed movement.

Bilateral symmetry is advantageous for animals that are more active in their environment. It supports the development of a head, which often contains sensory organs and a brain, aiding in efficient movement and interaction with the surroundings.

Animal Phyla

Animal phyla are broad classifications of animals that share fundamental body plans and other key characteristics. Some of the major animal phyla include:

• **Cnidaria**: This phylum possesses radial symmetry and includes jellyfish and corals.
• **Echinodermata**: Known for their radial symmetry in adult form, examples include starfish.
• **Chordata**: Exhibiting bilateral symmetry, this phylum includes vertebrates like humans and other mammals.
• **Arthropoda**: This phylum represents the largest group of animals, including insects and crustaceans, all characterized by bilateral symmetry.

Understanding the characteristics of these phyla helps in categorizing animals based on their structural and functional similarities.

Lifestyles of Symmetry

The lifestyle of an animal can greatly depend on its type of symmetry. Radial symmetry is often associated with a more sedentary lifestyle. Animals with this type of symmetry, like jellyfish and starfish, can interact with their environment from all sides, making them well-suited for stationary or drifting lifestyles.

On the other hand, bilateral symmetry allows for a more active lifestyle. Bilaterally symmetric animals, such as mammals and insects, usually have a head and tail, which supports directed movement. This type of symmetry benefits animals that need to hunt, evade predators, or interact with their environment in complex ways.

By understanding these lifestyles, it's easier to see why certain animals have evolved with specific types of symmetry. Radial symmetry supports a sessile or planktonic life, while bilateral symmetry aids in an active, mobile existence.