In animal classification, which of the following phyla is characterised by animals that possess a truly segmented body with clear internal metamerism?

Introduction / Context:
This biology question checks understanding of basic animal classification and body organisation patterns, specifically the idea of segmentation. Many introductory zoology topics compare phyla based on features such as body cavity, symmetry and segmentation.

Given Data / Assumptions:

• The question is about the presence of a truly segmented body with internal metamerism.
• Four phyla are listed: Mollusca, Arthropoda, Annelida and Coelenterata.
• We must select the phylum that is classically associated with true segmentation throughout the body.

Concept / Approach:
Segmentation or metamerism refers to the division of the body into repetitive segments that may contain repeated sets of organs. Phylum Annelida, which includes earthworms and leeches, shows clear internal and external segmentation. Arthropods have externally segmented bodies, but they are usually highlighted for jointed appendages and exoskeleton. In basic school level questions on true body segmentation, Annelida is commonly emphasised.

Step-by-Step Solution:
Step 1: Recall that annelids such as earthworms have ring like segments repeated along the body length, each segment containing similar structures.Step 2: This pattern of repeated segments with a high degree of internal repetition is a key feature of Phylum Annelida.Step 3: Phylum Arthropoda also has segmented bodies, but the segments are often grouped into specialised regions like head, thorax and abdomen, and the focus is on jointed limbs and exoskeleton.Step 4: Phylum Mollusca, which includes snails and octopus, generally has unsegmented soft bodies.Step 5: Phylum Coelenterata, which includes jellyfish and corals, has a simple body plan without segmentation.Step 6: Therefore, among the given options, Annelida is the best answer for animals with a truly segmented body and internal metamerism.

Verification / Alternative check:
Standard zoology charts of animal classification place examples like earthworm, leech and Nereis under Annelida and specifically highlight segmentation as a defining feature. This helps confirm that Annelida is the correct phylum for this question.

Why Other Options Are Wrong:
Option A is wrong because Mollusca is mainly characterised by a muscular foot, visceral mass and mantle, not true segmentation.
Option B, Arthropoda, does show segmentation but is not usually chosen as the classic example of true internal metamerism in basic classification questions, which focus on Annelida instead.
Option D is wrong because Coelenterata has a simple sac like body without segmentation.

Common Pitfalls:
The main confusion arises between Arthropoda and Annelida because both show some form of segmentation. In exam questions that emphasise true internal metamerism and repeated segments throughout the body, Annelida is preferred as the classical answer.

Final Answer:
The phylum that is classically known for a truly segmented body with internal metamerism is Phylum Annelida.