Coelenterates, ctenophores and echinoderms show which basic type of body plan or symmetry in their overall organisation?

Introduction / Context:
Body symmetry is an important feature used to classify animals. It describes how body parts are arranged around a central axis or plane. Some animals can be divided into similar halves in multiple planes, while others show only one plane of symmetry or no symmetry at all. This question asks which type of symmetry is characteristic of coelenterates, ctenophores, and echinoderms.

Given Data / Assumptions:

• Groups mentioned are coelenterates cnidarians, ctenophores, and echinoderms.
• The options include radial symmetry, bilateral symmetry, asymmetry, and references to body plans of Annelida and Platyhelminthes.
• We consider adult body form for echinoderms.
• We assume standard zoological definitions of symmetry.

Concept / Approach:
Radial symmetry means that the body can be divided into similar halves by several planes passing through the central axis, like slices in a cake. Coelenterates such as jellyfish and sea anemones, and ctenophores such as comb jellies, show radial or biradial symmetry, which suits their sessile or free floating lifestyles. Echinoderms such as starfish show radial symmetry in the adult form, usually pentaradial, even though their larvae are bilaterally symmetrical. Bilateral symmetry, by contrast, is typical of most higher animals, including annelids and platyhelminths, where only one plane divides the body into mirror image halves.

Step-by-Step Solution:
Step 1: Recall that cnidarians and ctenophores are mostly marine animals with a central mouth and tentacles arranged around it, suggesting radial body organisation. Step 2: Understand that radial symmetry allows these animals to interact with the environment in all directions, which suits their floating or attached lifestyle. Step 3: Remember that echinoderms such as sea stars and sea urchins also exhibit radial symmetry in adult stages, often in multiples of five. Step 4: Note that annelids and platyhelminths show clear head to tail orientation and only one plane of symmetry, which corresponds to bilateral symmetry. Step 5: Recognise that asymmetrical animals, like sponges, do not have ordered body symmetry at all and are not mentioned in the question. Step 6: Since all three groups in the question share radial type organisation in their adult form, the correct choice is radial symmetry.

Verification / Alternative check:
Study of zoology charts and diagrams shows coelenterates with cylindrical or umbrella shapes and tentacles arranged around a central axis. Ctenophores have rows of comb plates arranged radially. Adult echinoderms have arms or tube feet arranged around a central disc. All these patterns conform to radial symmetry. In contrast, when you look at earthworms or flatworms, you can see only one symmetrical division, confirming that bilateral symmetry belongs to these other phyla and not to the groups mentioned.

Why Other Options Are Wrong:
The segmented body plan of Annelida is associated with bilateral symmetry and a distinct head end, not seen in coelenterates, ctenophores, or adult echinoderms. Bilateral symmetry is typical of platyhelminths and many higher animals but not of the groups listed in the question. The body plan typical of Platyhelminthes again refers to bilateral flatworms. Asymmetrical body plans are found mostly in sponges, which are not mentioned here.

Common Pitfalls:
Students may recall that echinoderm larvae are bilaterally symmetrical and become confused about the symmetry of adults. Exam questions about echinoderms usually refer to the adult form, which is radial. It is important to read the groups given together and remember that coelenterates, ctenophores, and adult echinoderms are classic examples used to illustrate radial symmetry in basic zoology.

Final Answer:
Coelenterates, ctenophores, and adult echinoderms show Radial symmetry as their basic body plan.