Problem 257 - Objective Biology for NEET

Question

Assertion: Coelenterates, Ctenophores and adult Echinoderms are said to be radial symmetrical. Reason: Their body can be dived into two equal halves in any plane passing through central axis of the body.

Step-by-Step Solution

Answer

Yes, the assertion is true and the reason is a correct explanation. Coelenterates, Ctenophores, and Echinoderms are radially symmetrical based on their body structure which can be divided into two identical parts in any plane passing through their central axis.

Step 1: Understand Radial Symmetry

Understanding the concept of 'radial symmetry' is crucial in solving this exercise. In biology, organisms with radial symmetry can be sliced into two identical parts in any plane along its central axis. This typically applies to animals that are usually round or cylindrical in shape.

Step 2: Apply the Concept

In the case of coelenterates, ctenophores, and adult echinoderms, their body structure does indeed allow them to be divided into two identical halves in any plane passing through the central axis. This agreement with the concept of radial symmetry confirms the validity of the reasoning.

Step 3: Confirm the Assertion

Since the reason correctly explains why they are radially symmetrical, and considering that the characteristic applies to coelenterates, ctenophores, and adult echinoderms, the initial assertion is therefore supported by the reasoning.

Key Concepts

Coelenterates

Coelenterates are a fascinating group of aquatic animals that belong to the phylum Cnidaria, which includes jellyfish, corals, and sea anemones. These creatures are primarily found in marine environments, although some can be found in freshwater habitats as well. One of their distinct features is their body structure, which displays radial symmetry.
This means they can be divided into equal halves along any plane that passes through their central axis.

• Body Composition: Coelenterates have a simple body structure consisting of two layers of cells: the outer epidermis and the inner gastrodermis, with a jelly-like substance called mesoglea in between.
• Tentacles with Cnidocytes: They are equipped with tentacles that contain specialized stinging cells called cnidocytes, used for capturing prey and defense.
• Feeding Mechanism: Most coelenterates are carnivorous and use their tentacles to capture small marine organisms for nutrition.
  

Their radial symmetry not only aids in predation by allowing them to sense the environment from all directions but also assists in their reproductive and feeding processes.

Ctenophores

Ctenophores, commonly known as comb jellies, are unique marine creatures that belong to the phylum Ctenophora. Like coelenterates, they exhibit radial symmetry, which is evident in their spherical or cylindrical bodies. This symmetry allows them to easily navigate through water as they can be split into two identical halves through any plane passing through their central axis.

• Body Characteristics: Ctenophores possess eight rows of ciliary plates called ctenes, which they use for locomotion.
• Bioluminescence: Many ctenophores exhibit bioluminescence, producing light that creates enchanting displays in the ocean.
• Feeding Habits: They primarily feed on zooplankton, utilizing sticky cells called colloblasts to capture their prey.
  

Their radial symmetry and unique locomotion make them efficient swimmers, helping them avoid predators and capture food. Additionally, their ability to produce light is both a defensive mechanism and a means to attract prey. Ctenophores play a crucial role in marine ecosystems and contribute to the ocean's biodiversity.

Echinoderms

Echinoderms are marine animals classified under the phylum Echinodermata, representing starfish, sea urchins, and sand dollars, among others. These fascinating organisms are notable for their radial symmetry, which is particularly evident in their adult form. This symmetry allows their bodies to be divided into identical portions around a central point.

• Water Vascular System: A unique feature of echinoderms is their water vascular system, which aids in movement, food collection, and respiration.
• Regenerative Abilities: Many echinoderms can regenerate lost parts, such as starfish regrowing arms.
• Calcareous Skeleton: They possess an internal skeleton made up of calcareous plates, providing support and protection.

Their radial symmetry not only facilitates movement along the sea floor but also aids in their interaction with the environment. As a group, echinoderms play critical roles in marine environments, partaking in processes that maintain the ecological balance of ocean habitats.

Biological Symmetry

Biological symmetry refers to the balanced distribution of duplicate body parts or shapes in an organism. This characteristic can significantly affect how an organism interacts with its environment. Radial symmetry, in particular, is observed in organisms that have body structures organized around a central axis.

• Types of Symmetry: The main types of biological symmetry are radial symmetry, bilateral symmetry, and asymmetry.
• Function and Evolution: Radial symmetry, as seen in coelenterates, ctenophores, and echinoderms, allows for the equal exposure of all body parts to the surrounding environment, facilitating functions like prey capture and interaction.
• Evolutionary Significance: Radially symmetrical organisms tend to be sessile (stationary) or slow-moving, enabling them to fulfill ecological roles such as filter feeders or predators within their habitats.
  

Understanding biological symmetry, especially radial symmetry, offers insights into the evolutionary adaptations of organisms, by reflecting their lifestyle and survival strategies. This concept illustrates the fascinating ways in which life has evolved, adapting form and structure to ecological niches.