Why are brittle star arms more sharply separated from the central disk than sea star arms?

The distinction between the arms and the central disk in brittle stars (class Ophiuroidea) and sea stars (class Asteroidea) is a fundamental anatomical difference between these two groups of echinoderms.

In sea stars, the arms are not sharply separated from the central body; instead, the arms and the disk are continuous, with the arms radiating from an indistinct central area. Conversely, brittle stars are characterized by a small, disk-shaped body from which their long, slender arms are distinctly set off.123

This structural difference is further reflected in their internal anatomy and movement. While sea star arms are broad and hollow, containing extensions of the body cavity and organs, the arms of brittle stars are solid and lack the open grooves for tube feet found in sea stars. These distinct physical arrangements represent different evolutionary paths within the subphylum Asterozoa, with brittle stars having evolved a body plan where the central disk is clearly demarcated from the flexible, independent arms.413