The tube feet are located on the bottom of the five arms radiating out from a central disc. A sea star can exert enough prolonged suction on a clam or oyster to open its shell. It then inverts its stomach inside the clam and digests the soft tissue. Sea urchins and sand dollars are, respectively, spherical or flattened animals that possess movable spines. Urchins usually live on firm bottoms and scrape food off the surface.
Sand dollars are burrowers and feed on detritus found in the sediments.
This is the largest group of all animals, containing some three-quarters of a million described species. The major unifying trait within the group is the presence of jointed legs. Included within the group are crustaceans, spiders, insects and several less familiar organisms. Spiders and insects are found primarily on land and in fresh water. Crustaceans, on the other hand, are most common to marine and estuarine environments.