29. Radial symmetry is the characteristic feature of which of the following phyla?

(A) Arthropoda

(B) Mollusca

(C) Cnidaria

(D) Chordata

Radial Symmetry Is the Characteristic Feature of Phylum Cnidaria

Detailed Explanation of the Correct Answer

Radial symmetry is a characteristic feature of phylum Cnidaria. Therefore, option (C) is the correct answer.

Body symmetry is an important characteristic used in the classification of animals. It describes how the body of an organism can be divided into similar or approximately equal parts by imaginary planes passing through its central axis. Animals may show asymmetry, radial symmetry, biradial symmetry, or bilateral symmetry depending on their body organization.

In radial symmetry, the body parts are arranged around a central longitudinal or oral-aboral axis. Multiple planes passing through this central axis can divide the body into similar halves. This type of organization is particularly suitable for animals that are sessile, attached to a surface, drifting, or capable of interacting with the surrounding environment from several directions.

Members of phylum Cnidaria, such as hydra, jellyfish, sea anemones, and corals, characteristically show radial symmetry. Their body is organized around a central gastrovascular cavity, with structures such as tentacles often arranged around the mouth. This body plan allows them to detect and capture food or respond to environmental stimuli coming from different directions.

The other phyla listed in the question—Arthropoda, Mollusca, and Chordata—are fundamentally characterized by bilateral symmetry. Therefore, Cnidaria is the correct answer.

What Is Body Symmetry in Animals?

Body symmetry refers to the arrangement of body structures in relation to an axis or plane. It is an important feature of animal body plans and provides information about an organism’s mode of life, movement, feeding behavior, and evolutionary organization.

An animal is considered symmetrical when its body can be divided into similar parts according to a definite geometrical pattern. The major types of symmetry observed in animals include asymmetry, radial symmetry, and bilateral symmetry.

In asymmetrical animals, no plane passing through the body can divide it into two similar halves. Many sponges are classic examples of asymmetrical animals.

In radially symmetrical animals, body structures are arranged around a central axis. Several planes passing through this axis may divide the animal into similar sections.

In bilaterally symmetrical animals, only one major plane divides the body into approximately equal right and left halves. Most actively moving animals show this type of symmetry.

What Is Radial Symmetry?

Radial symmetry is a type of body organization in which body parts are arranged around a central axis. The organism does not have distinct right and left sides in the same way as a bilaterally symmetrical animal.

In a typical radially symmetrical organism, multiple planes passing through the central axis can divide the body into similar parts. This arrangement is commonly found in animals that interact with their environment from all sides.

For example, a sea anemone remains attached to a surface but can receive food particles and respond to stimuli arriving from different directions. Its tentacles are arranged around the mouth, allowing it to capture prey from the surrounding water.

Similarly, a jellyfish has a body organized around a central axis, making radial symmetry suitable for its floating or swimming mode of life.

Why Is Cnidaria Radially Symmetrical?

Body Parts Are Arranged Around a Central Axis

The body of a cnidarian is organized around an oral-aboral axis. The oral end contains the mouth, while the opposite region represents the aboral end.

Structures such as tentacles are commonly arranged around the mouth. This organization creates a body plan in which the animal can interact with its surroundings from multiple directions.

Because several planes passing through the central axis can divide the body into similar portions, cnidarians are considered radially symmetrical.

Radial Symmetry Supports Their Mode of Life

Many cnidarians are sessile or relatively slow-moving. Polyps such as hydra, sea anemones, and many corals remain attached to a substrate.

For an attached animal, a body with a distinct forward-facing direction may provide less advantage than a body capable of responding to stimuli from all sides. Radial symmetry allows the organism to detect food, predators, and environmental changes from multiple directions.

Medusae such as jellyfish are free-swimming or drifting forms, but their body organization also allows them to interact with the surrounding water from different directions.

Tentacles Surround the Mouth

A characteristic feature of many cnidarians is the presence of tentacles arranged around the mouth. These tentacles contain specialized stinging structures associated with cnidocytes.

The circular or radial arrangement of tentacles is highly effective for capturing prey approaching from different directions.

This feeding arrangement is closely associated with the radial body plan of cnidarians.

Major Characteristics of Phylum Cnidaria

Cnidarians are aquatic animals, and most species are marine. The group includes hydra, jellyfish, sea anemones, and corals.

Their bodies are generally organized around a central gastrovascular cavity. This cavity is involved in digestion and typically communicates with the exterior through a single opening that functions as the mouth.

One of the most distinctive features of the phylum is the presence of specialized stinging cells called cnidocytes. These cells contain structures that can be used for prey capture and defense.

Cnidarians generally occur in two major body forms: the polyp and the medusa. The polyp is usually cylindrical and attached to a substrate, whereas the medusa is generally free-swimming and umbrella-shaped.

Despite differences between these body forms, radial symmetry is a major characteristic of the cnidarian body plan.

Option (A): Arthropoda

Option (A), Arthropoda, is incorrect.

Arthropods are fundamentally bilaterally symmetrical animals. Their bodies can be divided into approximately equal right and left halves by a single longitudinal plane.

Phylum Arthropoda includes insects, crustaceans, spiders, scorpions, centipedes, and millipedes. Members of this phylum are characterized by jointed appendages, a segmented body, and an external skeleton or exoskeleton.

Bilateral symmetry is closely related to active directional movement. Arthropods usually move forward in a definite direction, and their sensory structures are often concentrated toward the anterior region of the body.

Therefore, radial symmetry is not a characteristic feature of Arthropoda.

Hence, option (A) is incorrect.

Option (B): Mollusca

Option (B), Mollusca, is incorrect.

The basic body plan of molluscs is bilaterally symmetrical, although some adult molluscs may show secondary asymmetry because of developmental modifications.

Phylum Mollusca includes snails, slugs, clams, mussels, oysters, squids, and octopuses. A typical molluscan body consists of a muscular foot, a visceral mass, and a mantle.

Many molluscs are actively mobile and show a clear anterior-posterior orientation. Their fundamental organization is therefore bilateral rather than radial.

Gastropods can develop asymmetry because of torsion during development, but this does not make radial symmetry the characteristic feature of the phylum.

Therefore, option (B) is incorrect.

Option (C): Cnidaria

Option (C), Cnidaria, is the correct answer.

Members of phylum Cnidaria characteristically show radial symmetry. Their body structures are arranged around a central oral-aboral axis.

Examples include hydra, jellyfish, sea anemones, and corals. Their tentacles are commonly positioned around the mouth, allowing them to capture food and respond to stimuli from different directions.

This radial arrangement is especially suitable for sessile, floating, or slow-moving organisms.

Therefore, option (C) is correct.

Option (D): Chordata

Option (D), Chordata, is incorrect.

Chordates are fundamentally bilaterally symmetrical animals. Their bodies can be divided into approximately equal right and left halves by a single plane.

Phylum Chordata includes fishes, amphibians, reptiles, birds, mammals, and several other chordate groups. Members of the phylum possess important features such as a notochord, a dorsal hollow nerve cord, pharyngeal slits or pouches at some stage of development, and a post-anal tail.

The bilateral body plan of chordates is associated with directional movement and the development of a distinct anterior region containing important sensory and nervous structures.

Therefore, radial symmetry is not the characteristic feature of Chordata.

Hence, option (D) is incorrect.

Difference Between Radial and Bilateral Symmetry

Radial and bilateral symmetry represent two major forms of animal body organization.

In radial symmetry, body structures are arranged around a central axis. Multiple planes passing through the axis can divide the body into similar parts. Cnidarians are important examples of animals with radial symmetry.

In bilateral symmetry, the body has distinct right and left sides. Only one major plane divides the organism into approximately equal mirror-image halves. Arthropods, molluscs, and chordates fundamentally show bilateral symmetry.

Radial symmetry is commonly associated with sessile, floating, or slow-moving animals, whereas bilateral symmetry is strongly associated with active directional movement.

Relationship Between Symmetry and Mode of Life

The type of symmetry shown by an animal is closely connected with how it interacts with the environment.

A radially symmetrical animal can receive environmental information from multiple directions. This is advantageous for an organism that remains attached to a surface or drifts in water.

For example, a sea anemone cannot actively chase prey over long distances. Instead, its tentacles surround the mouth and can capture organisms approaching from different directions.

In contrast, a bilaterally symmetrical animal generally moves forward in a particular direction. This promotes the development of a distinct anterior end where sensory organs and nervous tissue may become concentrated.

Thus, the radial symmetry of cnidarians is closely related to their ecological and functional way of life.

Examples of Radially Symmetrical Cnidarians

Hydra is a freshwater cnidarian with a cylindrical polyp body. Its tentacles are arranged around the mouth, creating a radial organization.

Jellyfish represent the medusa body form. Their umbrella-shaped bodies and surrounding tentacles clearly demonstrate radial symmetry.

Sea anemones remain attached to a substrate and possess numerous tentacles around the mouth. Their radial organization allows them to capture food from the surrounding water.

Corals are also cnidarians and generally exist as polyps. Many coral species form colonies, but individual polyps retain the basic radial body organization characteristic of the phylum.

These examples demonstrate why radial symmetry is considered an important characteristic of Cnidaria.

Final Answer

Radial symmetry is the characteristic feature of phylum Cnidaria. Members of this phylum have body parts arranged around a central axis, allowing them to interact with the environment from multiple directions.

Therefore, the correct answer is:

(C) Cnidaria