How do sea stars differ from brittle stars in body shape and arm structure?

Sea stars and brittle stars, both members of the phylum Echinodermata, exhibit distinct differences in their body morphology and arm structure:

• Body Shape: In sea stars, the central disk is indistinct, as the arms merge gradually into the body. Conversely, brittle stars possess a small, central disk that is sharply distinct from their arms.123
• Arm Structure: Sea stars have broad, hollow arms that contain extensions of the body cavity. These arms feature grooves on their underside that house rows of tube feet, which are often used for movement and attachment. In contrast, brittle stars have long, slender, and solid arms. These arms lack the grooves found in sea stars, and their tube feet are not used for locomotion; instead, they serve primarily as sensory organs. Additionally, while sea star arms are typically five in number, brittle star arms are also usually five but are often spiny and can be forked or branched, as seen in basket stars.132