The tube feet on the under surface of the arms are swollen when fluid is pushed into them. Suction cups at the end of each tube foot attaches to the ground while the muscles contract to shorten the foot. These organisms eat by attaching their tube feet to the shells of the clam and wrap around it,holding it tightly. The clam's muscles fatigue allowing the shell to be pulled open. The sea star everts its stomach into the clam and secrete digestive juices onto its soft tissues.
Sea stars have a limited ability to regenerate.
A single arm cannot regenerate an entire body. Brittle stars differ from the sea stars in many ways. They also typically have five arms, but these arms are slender and distinctly set off from the central disc. Echinoids have a compact body encased in test, or shell. The dermal ossicles, which are modified to be close-fitting plates, make up the test.