Q.What is metamerism? Give an example of an animal having this structure.

Picture a train made of identical carriages coupled one behind another. Each carriage is a repeated unit, and though the train as a whole is long, it is built from the same block over and over. Some animals are constructed on just this principle — their bodies are assembled from a series of repeated units called segments.

When an animal's body is divided, both on the outside and on the inside, into such a series of segments, and at least some of the internal organs are repeated in a regular series from one segment to the next, the arrangement is called metameric segmentation, and the phenomenon itself is known as metamerism. The key point is that the repetition is not skin-deep: the external ring-like divisions are matched by a genuine serial repetition of organs running along the length of the body.

The standard example is the earthworm. Its body is marked externally into ring-like segments, and this outward segmentation is mirrored inside by organs that repeat, segment after segment, along its length. This same feature defines the whole phylum Annelida, whose very name comes from the Latin word annulus, meaning little ring — a direct nod to their ringed, segmented bodies.

• The segments are called metameres.
• The repetition appears both outside (the visible rings) and inside (the serially repeated organs).
• Genuine metameric segmentation is a defining character of the annelids. …