Q.Segmentation in the body is first observed in which of the following:

The correct option is (c) Annelida.

• In annelids the body surface is clearly divided into a series of rings or segments called metameres, a pattern known as metameric segmentation, and this repetition of segments both outside and inside the body first appears in this phylum.
• Platyhelminthes (flatworms) and Aschelminthes (roundworms) do not show this segmentation. …

The question asks in which of the listed phyla segmentation of the body is seen for the first time as we move through the animal kingdom. Segmentation here means metameric segmentation — the body being built out of a series of repeated ring-like units, both externally and internally, with organs repeated in a regular series from one segment to the next.

Looking at the options in order:

• Platyhelminthes are dorso-ventrally flattened flatworms and do not have a segmented body.
• Aschelminthes are roundworms, circular in cross-section, and again are not segmented.
• Annelida are marked precisely by having the body surface divided into a series of rings, or metameres — this is their defining feature, and the phylum's name comes from a word meaning little ring. This is where segmentation first appears. …

Method: Sequential phylum-order elimination

Rather than recalling directly that annelids show segmentation first, walk through the four options in the same order the animal kingdom is normally surveyed — from simpler to more complex phyla — and stop at the first one that qualifies.

• Platyhelminthes: dorso-ventrally flattened flatworms. Checking their body plan shows no repeated ring-like units at all, external or internal — no segmentation here. Move on.
• Aschelminthes: roundworms, circular in cross-section. Again, the body is a single continuous tube with no repeated segments — no segmentation here either. Move on.
• Annelida: the very name comes from "annulus," meaning little ring, and the body surface is visibly divided into a series of rings (metameres), matched internally by repeated organs. This is the first point in the sequence where true metameric segmentation appears.
• Arthropoda: also segmented, but this phylum comes AFTER Annelida in the sequence, so even though it qualifies too, it is not where segmentation is first seen. …