The members of the phylum do, however, have some distinctive or special features. First, the animals are primarily radially symmetrical. However, the opening of the water vascular system is somewhat off-center . Consequently, some zoologists refer to the symmetry as biradial. Finally, echinoderms have a water vascular system which, in starfish, provide the primary means of movement.
All echinoderms are marine, many live in tide pools.
The starfish was used as the primary example of the phylum. Starfish are carnivores, and they eat clams, other mollusks, and an occasional fish. In feeding, a starfish plants itself on top of a clam and begins to pull apart the shells using its tube feet. Once the shell is partly open, the starfish inserts its cardiac stomach into the interior of the clam and proceeds to release digestive enzymes into the soft body of the mollusk. As the clam is digested within its shell, the adductor muscles holding together the two halves of the shell become digested to the point where the hinged clam shell springs open.