Question

Radial symmetry is shown by (a) Coelenterate (b) Platyhelminthes (c) Adult echinodermates (d) Both (a) and (c)

Short answer

The correct answer is (d) Both (a) and (c).

Step-by-step solution

Understanding Radial Symmetry

In biology, organisms with radial symmetry can be divided into roughly equal halves by any plane of symmetry that passes through the central axis. It's a kind of symmetry wherein the organism can be cut into identical pieces in various ways, like a pie.

Identifying Organisms

(a) Coelenterates or cnidarians which include jellyfish, sea anemones and coral are radially symmetrical. (b) Platyhelminthes are flatworms, they exhibit bilateral symmetry, that is, they have a definite right and left side. (c) Adult echinoderms, such as starfish, exhibit radial symmetry but their larvae have bilateral symmetry.

Choosing the Correct Options

From the above information, it is clear that options (a) and (c) are the organisms that show radial symmetry. So, the correct answer is (d) Both (a) and (c)

Key concepts

Coelenterates

Coelenterates, also known as cnidarians, encompass a diverse group of aquatic organisms that are renowned for their radial symmetry. This defining characteristic means that similar parts of their bodies are arranged symmetrically around a central axis, comparable to the spokes of a wheel.

Representatives of coelenterates include jellyfish, sea anemones, and coral. These creatures might look simple, yet their radial structure is a sophisticated adaptation to their environment. The radial symmetry enables them to interact with their surroundings from all directions, a useful trait for stationary or slow-moving species that feed on organisms passing by their reach.

Their bodies are composed of two primary layers: the outer layer (ectoderm) and inner layer (endoderm), with a jelly-like substance called mesoglea sandwiched between them. They possess tentacles with specialized cells called cnidocytes, which contain stingers that can inject toxins into prey or predators.

Platyhelminthes

In contrast to coelenterates, Platyhelminthes, commonly referred to as flatworms, exhibit bilateral symmetry. This means their bodies have a distinct front and back, as well as a well-defined top (dorsal) and bottom (ventral) surface, enabling direction-oriented movement.

Morphology and Adaptation

Platyhelminthes' flattened body design is not accidental; it helps these worms with gas exchange and nutrient distribution, as they lack specialized respiratory and circulatory systems. This particular body shape increases the surface area relative to body volume, making diffusion of oxygen and nutrients more efficient.

These worms are often parasites or free-living organisms found in various environments such as marine, freshwater, and even damp terrestrial habitats. Parasitic flatworms, like tapeworms, have complex life cycles and can significantly impact host organisms.

Echinoderms

Adult echinoderms are fascinating creatures that present radial symmetry as adults, although they go through a bilateral larval stage during their development—a unique evolutionary twist. This group includes organisms such as starfish, sea urchins, and sea cucumbers.

Echinoderm Features

Their name 'echinoderm' means 'spiny skin,' which accurately reflects their appearance. They are known for their hard calcium carbonate structures beneath their skin that form various shapes like the striking arms of a starfish.

Echinoderms possess a water vascular system, used for movement, food and waste transportation, and respiration. Their ability to project parts of their stomach out of their body to aid in the external digestion of food shows an exceptional adaptation to their environment.

Despite their radial appearance, echinoderms' internal systems are not always radially symmetrical, indicating their bilateral ancestry. Their unique combination of radial and bilateral features makes echinoderms a compelling study subject for understanding evolutionary symmetry.