Final answer:
Sea stars have an external ambulacral groove with two rows of tube feet on each arm, which assist in locomotion, feeding, and sensation.
Step-by-step explanation:
Sea stars have an external ambulacral groove on the oral side of each arm. This groove runs along two rows of tube feet, which are part of the echinoderm's water vascular system. The tube feet assist in locomotion, feeding, and sensation, and they also help attach the sea star to surfaces.