What is a key characteristic of the Phylum Annelida?

The key characteristic of the Phylum Annelida is segmentation of bodies. Annelids, commonly known as segmented worms, exhibit a body plan that is divided into repeating segments or metameres. Each segment contains structures like muscles, nerves, and blood vessels, allowing for greater complexity and specialization of body functions compared to non-segmented organisms.

This segmentation is fundamental to their movement and physiology, as it enables a coordinated contraction of segments that facilitates more efficient locomotion through burrowing or swimming. Segmentation also allows for redundancy in biological systems, meaning that if one segment is damaged, others can potentially take over some of its functions.

The other characteristics mentioned in the choices relate to different groups of organisms. For example, hard external shells are typical of mollusks, while exoskeletons made of chitin are found in arthropods. Radial symmetry is a characteristic primarily associated with organisms in the Phylum Cnidaria, such as jellyfish and sea anemones. Therefore, segmentation stands out as the defining trait of Annelida, distinguishing them from these other groups.