Q.Which of the following phyla shows metameric segmentation for the first time in the classification sequence followed in this chapter?

Segmentation, or metamerism, is the division of an animal's body — both externally and internally — into a series of repeated units called segments, often carrying a serial repetition of organs. It first appears as a defining feature in Annelida, where each segment closely resembles its neighbours and is internally separated from them by partitions called septa; the coelomic fluid within each segment acts as a hydrostatic skeleton that, together with circular and longitudinal body-wall muscles, powers locomotion.

In Arthropoda, segmentation is retained but becomes specialised: unlike the annelid pattern of near-identical repeating units, arthropod segments look different from one another and are grouped into functionally distinct regions (typically head, thorax and abdomen), each performing different jobs. This shift from uniform to specialised (tagmatised) segments is a hallmark of the more complex arthropod body plan compared to the simpler, more uniform segmentation of annelids.