In starfish the arms and the interradial regions are continuous and merge gradually into each other, whereas in ophiuroids the arms and disc are clearly defined. The building blocks of ophiuroid arms are fused central plates known as vertebrae , each connected to the adjacent by muscles. The freeing of the arms and disc combined with this arm internal structure enable ophiuroid arms to be highly mobile. Consequently ophiuroids are able to wriggle their arms and move rapidly, and also rely on their arms to feed. Basket-stars show highly branched arms which extend into the water column to feed on suspended material.
Arms form a fan-like structure which maximises filtration area.
Food particles are collected by hooks or caught on mucus nets spun between the arms. Other ophiuroids use the tube feet alone to either collect suspended food or to scour the sediment surface for detritus. In contrast asteroid arm ossicles are unfused and motion is restricted by often large interradial regions, causing asteroids to rely on tube feet for locomotion and feeding. Suckered tube feet enable the asteroid Asterias to grip bivalve shells, and pull apart the valves by tube foot contraction. This achieved, the starfish is able to extrude its stomach through its own mouth cavity and into the bivalve, which is then digested.